Overview and Essentials of Biomass Gasification Technologies and

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Overview and Essentials of Biomass Gasification Technologies and their Catalytic Cleaning Methods. Vincent Claude, Claire Courson, Martina Köhler, and Stéphanie D. Lambert Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01642 • Publication Date (Web): 07 Sep 2016 Downloaded from http://pubs.acs.org on September 7, 2016

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Overview and Essentials of Biomass Gasification

2

Technologies

3

Methods.

4

Vincent CLAUDEa*, Claire COURSONb, Martina KÖHLERc, Stéphanie D. LAMBERTa

and

their

Catalytic

Cleaning

5

6

7

8

9

10

a

11

University of Liege, B6a, 4000 Liège, Belgium

12

b

13

7515, University of Strasbourg, 25 rue Becquerel, 67087 Strasbourg, France

14

c

15

Universität Mainz,76726 Germersheim, Germany

: Departement of Chemical Engineering - Nanomaterials, Catalysis and Electrochemistry,

: Institut de Chimie et Procédés pour l’Energie, l’Environnement et la Santé, UMR CNRS

:Fachbereich für Translations-, Sprach- und Kulturwissenschaft der Johannes Gutenber

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KEYWORDS: Biomass, Gasification, Syngas, Catalysts, Tar reduction

2

3

ABSTRACT:

4

Obtaining a tar free bio-syngas from biomass gasification processes has been the

5

subject of many studies in the last two decades, and it still remains the major technologic and

6

economic challenge. Unfortunately, the countless publications about gasification technologies

7

and different techniques permitting to reduce the tar present at the outlet of gasifier reactors

8

usually confuse inexperienced persons who attempt to further research this subject. More than

9

presenting the basis of biomass gasification technologies and positioning them among other

10

bio-energies, this work mainly aims at reviewing and comparing the different methods

11

developed in order to produce a tar free bio-syngas. In this way, bio-syngas quality

12

improvement can be obtained through different operating processes such as reactor designs,

13

gasifying ratios, feedstock, temperature, and space ratio. Since catalytic destruction has

14

proved to be one of the most convenient and efficient way to eliminate undesirable tars, an

15

important part of this work also highlights the catalytic and deactivating phenomena involved.

16

Furthermore, this work takes inventory of numerous studies conducted to understand the

17

influence of different properties, especially supports and active site compositions, on the tar

18

reforming activities and lifetime of catalyst materials. Thus, this review aims at summarizing

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basic and more recent improvements applied to biomass gasification processes and catalytic

20

syngas purification.

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1.

Introduction

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Adapted from old coal gasification technologies developed during the industrial

3

revolution, the biomass gasification appears nowadays as an interesting and versatile way to

4

take advantage of different sources (e.g. agricultural and urban wastes, energy crops, food and

5

industrial processing residues). If managed conscientiously, these processes can therefore lead

6

to the sustainable and renewable production of a bio-syngas which can either be used directly

7

as combustible or converted into storable and high valuable chemical compounds such as

8

methanol

9

viable energy and that some industrial plants are already in action, bio-syngas technologies

10

still encounter some technical problems which seriously hinder their commercial development

11

3–5

12

degradation of cellulose aromatic rings, still remains a major problem that the industry has to

13

face. Modifications of the gasifier reactor design and the gasification operating conditions

14

(temperature, space ratio, gasifying reagent) have proved to substantially reduce the tar

15

concentration

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gasifier reactors also appears as a convenient and economical solution to obtain a clean bio-

17

syngas, which explains the numerous studies conducted during the last two decades on this

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topic. The catalytic tar reforming has been carried out with numerous types of catalysts, the

19

researchers modifying several aspects such as the material structures, the crystallinity or the

20

elementary composition of the supports and active sites. Furthermore, catalytic operating

21

conditions such as the temperature, space ratio or gas mixture also proved to be of significant

22

importance for the catalysts’ sustainability and performances.

1,2

. However, despite the fact that bio-syngas is predicted to be an economically

. In this way, the tar presence at the gasifier outlet, which results from the uncomplete

6–10

. Furthermore, the catalytic reforming of tars at the inside or outside of the

23

The first part of this review aims at situating the biomass gasification processes among

24

other biomass derived technologies. Thereafter, the economic and technical advantages as 3 ACS Paragon Plus Environment

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well as the drawbacks of different gasifier reactors and operating conditions are described.

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Finally, among the several existing syngas cleaning operations, this review emphasizes the

3

basic notions and recent progresses achieved in the catalytic tar reforming field.

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2. Biomass valorization processes

5

The term “biomass” covers the raw (wood, energy crops, agricultural residues …) or

6

processed (effluents, food processing residues, green wastes …) organic matter which can

7

either be of vegetal or animal origin. Depending strongly on its origins, biomass materials are

8

generally composed of cellulose, hemicellulose, lignin, lipids, proteins, simple sugars and

9

starches. Among these compounds, cellulose, hemicellulose and lignin are the three main 11

10

constituents

. The renewable aspect that is attributed to biomass can be explained by the

11

carbon cycle: the carbon dioxide emitted during its use is compensated by the carbon stock

12

accumulated during its growing stage. Therefore, the biomass can only be considered as a

13

clean and renewable energy if obtained in a sustainable way. In case the CO2 emission is not

14

compensated by its natural growth (for example an overexploited forest) during its utilization,

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the biomass cannot and must not be considered as a clean and renewable energy 12. The term

16

“bio-energies” refers to all the processes (industrial or not) which can produce energy from

17

biomass. Figure 1 gives a general view of the part of bio-energy in the worldwide energy

18

consumption in 2013 13.

19

Figure 1 : Total wordlwide energy consumption by source in 2013 13.

20

Most of the biomass is used for “traditional use” i.e. heating or cooking (9 %). Despite

21

the fact that the energy production from biomass becomes more interesting in developed

22

countries, its actual part in the total global energy consumption is still very low (~3.3 %). In

23

this way, in the European Union, the “bio-electricity” (issued from solid or liquid biomass, 4 ACS Paragon Plus Environment

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biogas or wastes) has generated 121 TWh in 2010, which corresponds to 3.6 % of the total

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electric production of the E.U. 14. According to a report from the International Energy Agency

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(IEA), the part of bio-electricity has been growing at an average rate of 2.5% per year over the

4

2000-2010 decade

5

agricultural or urban residues are collected and transported over only short distances.

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However, expenses can quickly arise, when raw materials are imported (ex: wood pellets) 15.

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Following these observations, the biomass valorization sector is more inclined to develop

8

itself into small interconnected installations rather than huge centralized complexes.

11

. From an economical aspect, the supplying costs can be low, when

9

Biomass can be converted into energy or by-products via versatile transformation

10

technologies: Figure 2 gives an overview of the principal processes. Basically, two different

11

operations of biomass transformation are realized: physico-chemical or thermo-chemical.

12

Figure 2 : Overview of main biomass valorizations adapted from literature 16.

13

Although gasification technologies have been developed from coal gasification and are 17

14

commercially available, many economic and technological reviews

agree that more

15

progress has to be achieved in terms of research and development. Indeed, merely 370 MWh

16

of large-scale industrial plants were in use in 2010, with only two additional projects planned

17

for the period of 2016, totaling 30 MWh 15.

18

More than investing in a sustainable development, adding other renewable resources to

19

the biomass gasification, could allow Europe to emancipate from fuel imports. Indeed,

20

according to a previous study

21

saving up to 5 billion crude oil barrels each year, which approximately corresponds to 10 % of

22

the E.U. imports in 2012 19. According to Figure 3, biomass gasification technologies provide

23

power with relatively low costs compared to other renewable resources 20,21. Figure 4, obtained

18

, the installation of 300 fluidized bed reactors would allow


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from a study conducted by the International Energy Agency in 2010, shows a cost estimate

2

for different fuels: gasoline from petroleum, conventional and advanced biodiesel, bio-

3

synthetic gas and ethanol from different sources

4

being predicted to increase, some renewable solutions such as conventional or advanced bio-

5

diesel do not seem to be economically interesting alternatives. In comparison, bio-synthetic

6

gas obtained from gasification is on the third position below ethanol, which means that bio-

7

syngas could really be an efficient alternative to gasoline.

20

. Despite the conventional gasoline price

8

Seeing all these promising studies, one could wonder why gasification technologies

9

are not more developed. The Achilles’ heel of gasification technology lies in its cleaning 3,4,17

10

processes. Indeed, according to previous studies

, the gas cleaning operations for a

11

fluidized bed make up for 65-85 % of the total costs. These cleaning costs, mainly due to the

12

elimination of tars, are the main obstacles to the commercialization of these technologies.

13

Thus, explaining why they are less interesting in comparison to the low costs of oil, gas and

14

coal 15,17.

15

Figure 3: Typical cost ranges for renewable power generation technologies 21.

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Figure 4: Projected costs of biofuels compared to petroleum gasoline, 2010-2050

20

.

17

2.1. Physico-chemical transformations

18

Three main technologies of physicochemical transformations can be distinguished: oil

19

20

extraction, fermentation and anaerobic digestion.

The oil extraction from plants is realized through mechanical processes

16

. A typical

21

example is the colza oil extraction which can then be converted into biodiesel via

22

transesterification reactions using an alcohol (usually methanol or ethanol)

22

. In the case of 6

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fermentation, the starch and the cellulose are converted into primary alcohols thanks to

2

bacteria. The alcohols are then extracted via distillation processes

3

digestion, also called “bio-methanization”, differs from the classic fermentation since the

4

process takes place without oxygen. The gaseous product obtained is therefore called “biogas”

5

which is a mixture of methane, CH4 (50 to 75 %), and carbonic gas, CO2 (25 to 50 %). The

6

biogas can be used as a substitute for natural gas after purification. This is currently the most

7

developed industrialized bio-energy technology since in the year of 2011, 8000 Bio-

8

Methanizer installations were in function in the E.U. 15.

9

23,24

. The anaerobic

2.2. Thermo-chemical transformations

10

The thermo-chemical processes are generally more efficient than the physicochemical

11

ones. The reasons are as follows: (1) a lower reaction time (a few seconds or minutes for

12

thermo-chemical processes vs. several days, weeks or even longer for bio-chemical/biological

13

processes); (2) a higher ability to destroy most of the organic compounds. Lignin materials are

14

typically considered to be non-fermentable and thus cannot be completely decomposed via

15

biological processes, whereas they can be fully converted thanks to thermo-chemical

16

transformations

17

similar weight basis. In fact, the heating value of biomass ranges from 15-19 GJ/t compared to

18

20-30 GJ/t for coals. Furthermore, the bulk density, also known as energy density, is only 10-

19

40 % of most fossil fuels. However, in comparison to fossil fuels, biomass contains much

20

higher volatile matter contents (80 % in biomass instead of 20 % for fossil fuels), which

21

means that biomass has a high ignition stability and can easily be thermo-chemically

22

processed towards other higher value fuels (syngas)

23

processes are summarized in Figure 5. They can be classified into three different paths:

24

combustion, pyrolysis and gasification processes. The main difference lies in the amount of

11,25

. In comparison to fossil fuels, biomass has a lower heating values on a

11

. The thermo-chemical transformation


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air used during the operations, thus modifying the thermo-chemical reactions and reaction

2

products 2,12.

3

Figure 5 : Scheme of the three different thermo-chemical ways adapted from the literature 2.

4

2.2.1. Combustion processes

5

Combustion is the most obvious and the oldest method to create energy from biomass.

6

During combustion, material is burned in the presence of oxygen and converted into CO2 and

7

H2O while releasing energy. By definition, combustion reactions are always exothermic.

8

During these processes, three main steps occur: drying, pyrolysis and combustion. During the

9

drying step, the humidity of the samples is released using temperatures between 120 and

10

200°C. At temperatures of up to 600°C, pyrolysis and reduction steps decompose the organic

11

matter into carbonaceous residues and volatile compounds. Finally, between 700°C and

12

1400°C, the combustion steps consist in the oxidation of the combustible gas and the

13

carbonaceous residues into CO2 and H2O 12,15,25.

14

2.2.2. Pyrolysis processes

15

Pyrolysis is realized at high temperatures (400°C-700°C) without the use of oxygen.

16

Thus, the biomass is converted into char, liquids (bio-oils) and gas 11. The obtained products

17

strongly depend on the process operating parameters (temperature and time). One major

18

advantage of these methods is the storage and carrying capacity in case of solid and liquid

19

products. From a technical point of view, these processes can be considered as mature and are

20

currently being developed at an industrial scale 26,27.

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The first stage of the pyrolysis consists in a pre-pyrolysis step (between 120 and 200°C)

22

during which the biomass undergoes internal rearrangements such as the breaking of chemical 8 ACS Paragon Plus Environment

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bonds, appearance of free radicals, formation of carbonyls groups, water evaporation and

2

production of CO and CO2. The pyrolysis step can be divided into two paths: (1) the “slow

3

pyrolysis” is used to create solid chars, which can be used for household purposes (cooking,

4

heating), raw carbon for the chemistry field and as active charcoal for adsorption uses. The

5

“slow pyrolysis” operating parameters are characterized by a low temperature (400°C) and

6

high residence times (hours or days)

7

characterized by higher operating temperatures (450-550 °C) and very short residence times

8

(seconds). These operating parameters lead to the production of bio-oils. These bio-oils,

9

usually showing a high calorific power (15-20 MJ/kg), are generally mixed with classic fuel

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for “liquid combustible” purposes 27. Other valorizations of these bio-oils become growingly

11

interesting: the bio-oils can be washed with water in order to separate the water soluble and

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non-soluble compounds. The molecules present in the washing solutions (mainly acetic acid

13

and acetone) are then converted into syngas via classic steam reforming processes, whereas

14

the non-soluble phases, mainly composed of phenolic compounds, are used as raw materials

15

for the synthesis of phenolic resins 26,27.

12,15

; (2) the “fast pyrolysis” or “flash-pyrolysis”, is

16

2.2.3. Gasification processes

17

The biomass gasification is a sum of complex thermo-chemical processes which

18

include the biomass drying, pyrolysis, char gasification and reforming of gaseous products

19

formed by pyrolysis

20

which is mainly composed of H2 and CO; and whose lower heating value (LHV) is situated

21

between 5 and 20 MJ/Nm3 (depending on the biomass and gasification vector: air, water

22

vapor or O2)

23

steam and air or oxygen, with the amount of oxygen being generally one-fifth to one-third of

24

the amount theoretically required for complete combustion. The necessary heat for

12,25

. Its final product is a combustible gas, called syngas or bio-syngas,

11,25

. Common gasifying agents used in industrial gasifiers include a mixture of


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gasification is produced by a partial combustion of the biomass in the same reaction chamber.

2

In addition to the typical biomass products and wastes described above, it is important to note

3

that the gasification operations can also be realized with plastics, coal or a mixture of

4

plastics/coal/biomass, thus increasing the versatile aspect of the gasification 28–30.

Gasification reactions

5

•

6

At temperatures between 800 and 1200 °C, several parallel gasification reactions take

7

place inside the gasifier (Table 1). The produced tars are converted by further partial

8

oxidation, reforming, hydrogenation and thermal cracking with highly endothermic reaction

9

enthalpies comprised between +200 to 300 kJ/mol (Eq.1 - Eq.5). The char and volatile

10

compounds (CO, H2, CH4) combustion via partial or complete oxidation reactions occurs in

11

the presence of air or oxygen (Eq.6 - Eq.7 and Eq. 13- Eq. 16). These reactions are highly

12

exothermic and allow generating the necessary heat for the drying, pyrolysis and gasification

13

reactions. The produced H2O and CO2 molecules are thereafter consumed during the char

14

gasification (Eq.8 - Eq.12). Reactions of water-gas shift (Eq.17) and methanation (Eq.18 -

15

Eq.19) take place in either direction, depending on the specific temperature, pressure and the

16

reactants concentrations. Water-gas shift is of great importance since it plays a significant role

17

for the generation of hydrogen, and therefore for the LHV of the syngas. The methanation

18

reactions occur slowly at low temperatures and in the absence of any catalysts

19

Gibb’s energy for the Boudouard (Eq.8) and water-gas shift (Eq.17) reactions are negative at

20

temperatures above 720°C and up to 820 °C, respectively 34,35.

(32,33)

.The

21 22 23 24

Table 1: Main reactions involved in the biomass gasification process 31–33.


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Syngas empowering

1

•

2

The gaseous products obtained from the gasification may be further processed to a variety

3

of useful products (Figure 6) 1. Some products can be used immediately without further

4

processing, while others require simple or complex conditioning and/or processing before use

5

in special applications.

6

Currently, most of the syngas obtained from the biomass gasification is used for the

7

generation of power. The cogeneration method is the more current method to convert syngas

8

into energy: thanks to a few technical adjustments, the syngas can replace gasoline or natural

9

gas in internal or external combustion machines normally used for electricity production. At

10

the same time, the heat emitted during these processes is collected and used for household

11

heating or for additional electricity production thanks to steam turbines 36,37.

12

The generation of fuel and various chemicals thanks to the Fischer-Tropsch and methanol

13

synthesis processes is another important syngas application 1 . The Fischer-Tropsch process

14

(Eq.20) is known for a long time. It was developed during the last century for the production

15

of oils from the syngas obtained by coal gasification

16

chemical reactions realized on heterogeneous catalysts at relatively low temperature (150 –

17

300 °C) that converts a mixture of carbon monoxide and hydrogen into liquid hydrocarbons.

18

The liquid product can be further refined to different types of fuels, from crudes and diesel to

19

kerosene.

1,25

. The process is a collection of

(20)

20

21

Figure 6 : Gasification output pathways 1.


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The methanol, which is the easiest product formed by the Fisher-Tropsch process,

2

becomes growingly interesting since it could be a practical alternative to gasoline and diesel

3

38,39

4

converted into different fuels or chemicals commonly used in industrial and commercial

5

applications. The synthesis of methanol is realized on heterogeneous catalysts at relatively

6

low temperature (150 – 300 °C) and consists in CO (Eq.21) or CO2 hydrogenation (Eq.22).

. More than being a liquid fuel easy to store, the methanol is also a reagent which can be

7

(21)

8

(22)

9

3. Technological aspects of biomass gasification

10 11

3.1. Overview of the processes implied in the fabrication and purification of bio-syngas

12

The global processes involved in biomass gasification are presented in Figure 7. At

13

first, the biomass undergoes upstream processes such as milling and drying. Thereafter, the

14

biomass is converted into gaseous products during the gasification step.

15

Figure 7 : Main processes involved in biomass gasification 40.

16

The primary methods consist in modifications inside the gasification reactor, allowing 12,40

17

obtaining a cleaner syngas with more interesting general composition (higher LHV)

18

theory, if the primary methods were perfect, the syngas exiting the gasifier would be tar-free,

19

thus eliminating the need of downstream secondary methods. An optimization of the

20

gasification can be reached through three different technical modifications

21

done by modifying the gasification conditions, such as temperature, pressure, gasifying

22

medium (air/steam/O2), residence time, and equivalence ratio. The selection of these

40

. In

: (1) it can be


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12,40,41

1

parameters also depend on the type of gasifier

2

profile and a well-functioning bed fluidization are of utmost importance in case of a fluidized-

3

bed gasifier; (2) it can be performed with the use of bed additives, also known as “primary

4

catalysts”. These catalysts, located inside the reactor, promote the different gasification

5

reactions via different catalytic paths, allowing obtaining a tar-free syngas with higher LHVs.

6

Furthermore, the addition of primary catalysts also prevents the agglomeration of solid

7

products such as slag and subsequent choking of the bed

8

strongly influences the gasification processing parameters, resulting in an important tar

9

reduction and a modification of the final gas composition 40,42.

10

. A homogeneous bed temperature

40

(3) the kind of reactor also

At the gasifier outlet, the syngas usually undergoes various downstream processes in order

11

to be purified from undesirable compounds.

12

categories:

These processes can be divided into two

13

(1) The secondary methods, which aim at removing the remaining tars at the reactor

14

outlet. These processes can be conducted at low temperatures and consist of mechanical

15

methods such as the use of a cyclone, baffle filter, ceramic filter, rotating particle separator,

16

electrostatic filter and scrubber. Although these methods are reported to be very effective, in

17

most cases they are neither convenient, nor economically viable

18

operations are realized with physical separations, which result in the problematic creation of

19

large amounts of toxic condensates. Therefore, instead of transferring the tars into a

20

solid/liquid phase, it is largely preferred to destroy them by catalytic reforming methods,

21

which take advantage of the high temperatures of the exiting syngas (500 – 700 °C). These

22

techniques result in an increase of efficiency and lower operational costs

23

than destroying the tars, using a catalyst permits the improvement of several useful reactions

24

such as the reverse methanation or Water-Gas shift reactions, thus permitting to obtain a

40,42 .

Indeed, most of these

12,43

. Indeed, more


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syngas with a high LHV 11,43. Some other tar cleaning methods such as for instance the use of

2

plasma arcs could also be used, however this technology remains only at a laboratory scale

3

11,43

.

4

(2) The gas clean up methods, which aim at purifying the syngas from undesirable

5

gaseous (mainly H2S) and solid (dusts, ashes) compounds. The elimination of H2S and other

6

sulfide compounds is usually done at lower temperatures (150 – 300 °C) thanks to the

7

adsorption on specific metallic oxide (for example ZnO) doped ceramic filters. Once these

8

filters are saturated, they can undergo air regeneration 44. The solid separation is finally done

9

thanks to mechanical methods similar to the ones presented above: cyclones, filters or

10

scrubber. Due to a tar-free syngas at this stage, the mechanical cleaning methods are no

11

hindrances 12.

12

3.2. Types of gasifier reactors

13

The type of reactor has a strong impact on gasification processes and on the final gas

14

composition. In 2012, the European Biomass Industry Association made an inventory of the

15

50 manufacturers identified in the U.S.A, Canada and Europe offering “commercial”

16

gasification plants, resulting in the following occurrence of gasifier kinds 45:

17

-

75% of the designs were using a downdraft fixed bed;

18

-

20% of the designs were using fluidized bed systems (bubbling/ circulating or dual);

19

-

2.5% of the designs were using an updraft fixed bed;

20

-

2.5% were of various other designs.

21

Each type of reactor has its own economic and technologic strengths and weaknesses. As

22

a general point, air-based gasifiers are relatively cheap and typically produce a syngas with a 14 ACS Paragon Plus Environment

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Energy & Fuels

1

high nitrogen content, resulting in a low energy content (5 - 6 MJ/m3 on a dry basis). In

2

comparison, oxygen or steam-based gasifiers tend to produce a syngas with a high

3

concentration of CO and H2 resulting in a much higher energy content (9 - 19 MJ/m3) 15. The

4

gasifier capacities and their typical particulate/tar loadings (Figure 8) are the two major

5

technical aspects taken into account in a gasifier design 41,43.

6

Figure 8: Technological gasifier capacity range and tars/particulate loadings, adapted from the literature 41,43.

7

Details about the main commercial and pilot gasifier types, processes, advantages and

8

drawbacks are given in the Figure 9.

9

Figure 9: Overview of the main gasifier types; and their advantages and drawbacks.

10

3.3. Gasification products

11

3.3.1. General composition of gas at the outlet of gasifiers

12

Table 1 presents the composition of outlet gas which can be obtained depending on the

13

gasifying agents (air, steam, O2). Plasma gasifiers are added in Table 1 because their operating

14

parameters are different from the other gasifiers. The exiting gas compositions depend on

15

various operating parameters, making it unique for each installation

16

CO concentrations are obtained under steam conditions, whereas the gasification made with

17

air, the most common for fixed-bed reactors, leads to weak CO and H2 concentrations.

18

Table 1: General composition of gasification gas 6,16.

19

20

6,16

. The highest H2 and

3.3.2. Undesirable gasification compounds

3.3.2.1.

Tars


Energy & Fuels

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Page 16 of 80

1

The major issue in biomass gasification is dealing with the tar formed during the

2

process. Milne & Evans came up with a good definition of tar in the biomass gasification 47:

3

“Tar is a complex mixture of condensable hydrocarbon, which includes single ring to multiple

4

ring aromatic compounds along with other oxygen containing hydrocarbons and complex

5

polycyclic aromatic hydrocarbon.”

6

Tars are formed during the biomass gasification by a series of complex reactions

7

which depend on gasification conditions such as the gasifying reagent (air, O2, steam), the

8

kind of reactor, the temperature of the gasification, the raw material moisture and the biomass

9

feedstock

41,42,47–49

. Elliott et al.

48

was the first to establish a scheme of the formation and 47

10

evolution of tar composition with the temperature. Milne & Evans

improved this scheme

11

and showed the influence of residential time on the tar formation. They also introduce an

12

important concept: the tar ranking. Figure 10 is inspired by these two major studies and

13

describes the different tar families as function of temperature 47,48.

14

At low temperatures (around 400 °C), primary tars, derived from the biomass

15

decomposition through pyrolysis and gasification, are present. They are composed of

16

molecules rich in oxygen such as alcohols, aldehydes, ketones, carboxylic acids, phenols and

17

furans. With increasing temperatures (above 500 °C), the primary tars further decompose into

18

secondary tars: aromatic compounds with one ring, two rings, three rings and a small

19

concentration of compounds with more than three rings. Gasification ranging from 700-900

20

°C produces tertiary tars which are polyaromatic compounds with four and five rings 47–49.

21

Figure 10: Tar ranking as function of temperature adapted from the literature 41,47.


Page 17 of 80

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1

Energy & Fuels

According to Coll et al.

50

(Figure 11), the majority of the hydrocarbons present are

2

toluene and other one ring aromatics (46 wt. %), followed by naphthalene and two ring

3

aromatic hydrocarbons (28 wt. %).

4

Figure 11: Typical composition of biomass tars (wt. %) 50,51 .

5

A study conducted by the Energy Research Center of the Netherlands (ECN), the 52 ,

6

Toegepast Natuurwetenschappelijk Onderzoek (TNO) and the University of Twente (UT)

7

proposed a more elaborated classification in order to rank the multitude of unconverted tar

8

compounds present at the exit of a gasifier. Tars were classified according to their molecular

9

weight, solubility and condensability. Table 2 presents the five different classes of tars 50

10

established, their properties and representative compounds. Coll et al.

11

reforming of five typical biomass tars with two commercials catalysts (UCI G90-C and ICI

12

46-1), both containing about 15 wt. % of nickel on alumina. The results show that the order of

13

reactivity for the reforming of tars was the following: benzene > toluene >> anthracen >>

14

pyrene > naphthalene. Therefore, according to this study, since it has the lowest reactivity,

15

naphthalene appears to be the most adequate molecule as biomass tar model.

16

Table 2: Classification of tars based on molecular weight (wt %) 49.

17

3.3.2.2.

studied the catalytic

Other contaminants

18

A variety of other gas contaminants may cause technical and environmental problems

19

(Table 3). Nitrogen may lead to the formation of toxic compounds such as ammonia (NH3) or

20

cyanide (HCN). Hydrogen sulfide (H2S) is a major problem since it can provoke corrosion of

21

the pipes and cause acid rain if emitted into the atmosphere. This is also the case for other

22

elements such as alkali metals or chlorine 15,17,53.


Energy & Fuels

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1 2 3

Page 18 of 80

Table 3: Examples of other gas contaminants 15,17,53.

3.4. Influences of experimental parameters on the final bio-syngas composition

4

3.4.1. Feedstock influence

5

6

Nuno Couto et al. 42 evaluated the gasification of biomass residues of different origins (rice

7

husk, nut shell, vine pruning, eucalyptus and pine) and concluded that the biomass feedstock

8

does not have a strong influence on the final syngas composition.

9

Paradoxically, Hanaoka et al.

54

showed that the air-steam gasification of the main woody

10

biomass compounds (cellulose, xylan and lignin) does neither show the same ability to

11

convert nor the same final syngas composition. Cellulose was the easiest convertible

12

compound compared to xylan and lignin, with conversion reaching 98, 92 and 53 %

13

respectively. However, the H2/CO ratio was the highest for xylan and lignin (1.3 and 1.2),

14

whereas cellulose was showing the lowest ratio (0.8). Barneto et al.

15

composting of biomass enables converting a part of the lignocellulosic compounds into lignin,

16

which resulted in an increase of H2 production up to 20 % during the gasification. Pinto et al.

17

30,56

18

polyethylene). According to the authors, the addition of coal or polyethylene did not cause

19

any technical problems. The addition of PE apparently favored the formation of gaseous

20

hydrocarbons and tars whereas the addition of coal leads to the release of higher NH3

21

contents. In both cases, the association of coal or PE with biomass permitted to decrease the

22

hydrocarbons’ concentration (up to 63 % less), increase H2 production (up to 70%) and

23

therefore produce a syngas with a higher LHV 56.

55

observed that the

studied the co-gasification of mixtures of different feedstock (pine wastes, coal and


Page 19 of 80

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Energy & Fuels

1

Based on similar technologies, the gasification of raw plastic wastes is a field becoming

2

growingly interesting since it would constitute an appropriate method to take advantage of

3

undesirable plastic wastes

4

before commercialization.

1,28,29

. However, this subject needs to be studied more thoroughly

5

6

3.4.2. Gasification temperature

7

Many studies showed that increasing the temperature tends to decrease the global tar

8

amount 7,56,57. Nevertheless, at high operating temperatures, the remaining tars are essentially

9

heavy polyaromatic hydrocarbons which are very difficult to reduce

58

. Indeed, compounds

10

containing oxygen such as phenol exist in significant amounts only at temperatures below 800

11

°C, whereas the destruction of aromatic hydrocarbons occurs only above 850 °C

12

Furthermore, one can notice that a decrease of up to 40 % of tar formation was reported when

13

the temperature was raised from 700°C to 900°C 59.

57

.

14

Narvaez et al. 7 studied the effect of temperature (700-850 °C) in a bubbling fluidized

15

bed on the final syngas. They reported that when the temperature increased from 700 to 850

16

°C, the H2 concentration increased from 5 to 10 %, the CO concentration increased from 12 to

17

18 % and the CO2 concentration decreased from 16 to 14 %. It was also reported that the LHV

18

of the produced gas was slightly increased thanks to the increase of the H2 and CO content.

19

Moghtaderi et al.

20

below that temperature the CH4 production is much more favored than the generation of H2.

60

confirmed that gasification below 600 °C was absolutely useless, since

21

More generally, it has been proved that the reaction of coke with water and CO2 is

22

thermodynamically more favorable at higher temperatures, producing CO and H2. However, a

23

too high temperature may have a negative effect on the H2 yield due to the inversion of the 19 ACS Paragon Plus Environment

Energy & Fuels

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Page 20 of 80

42

1

Water-Gas shift reaction up to 850 °C, thus decreasing the final syngas heating value

.

2

Therefore, the internal maximum temperature for a fluidized bed gasifier seems to be situated

3

around 850 °C. The complete conversion of tars into gas without any catalysts would require

4

operating temperatures of up to 1100 °C 47.

5

6

3.4.3. Gasifying reactants

7

8

Two major operating ratios are to be taken into account for a defined biomass flow

9

rate: the equivalence ratio (Eq.23) for the air gasification applications and the steam to

10

biomass ratio (Eq.24) for the steam gasification applications.

(23)

11

12

The equivalence ratio (ER) (Eq.23) is defined as the actual air to biomass weight ratio

13

divided by the stoichiometric air to biomass weight ratio needed for complete combustion 7. A

14

high degree of combustion occurs at high ER which supplies more air into the gasifier and

15

improves the burning of char and tars to produce CO2 instead of combustible gases such as

16

CO, H2, CH4 and CnHm. A too high ER value results in a lower concentration of H2, CO and

17

higher CO2, thus decreasing the LHV of the gas 6. However, studies have shown that a too

18

small ER is also unfavorable for biomass gasification because the combustion reactions are

19

not favored, thus decreasing the heat which is needed for other endothermic reactions

20

Narvaez et al. 7 showed that the amount of tars can be divided by 3 when increasing the ER

21

from 0.25 to 0.50, however, the LHV of the gas was decreased from 6 to 4 MJ.m3. Therefore,

61

.


Page 21 of 80

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Energy & Fuels

1

an ER optimum value in biomass gasification ranges from 0.2–0.4 6,7. When the produced gas

2

is burnt in downstream furnaces, tar compounds are not a serious issue and the gas should

3

have a high heating value. Therefore, the gasifier can be operated at a minimum ER of about

4

0.2. In case of temperatures lower than 850 °C, the tar yield is high and the ER should be

5

increased to about 0.3–0.4 to compensate such negative effects 6,7.

(24)

6

7

The steam to biomass ratio (SB) (Eq.24) is defined as the flow rate of the steam fed

8

into the gasifier divided by the biomass flow rate. It is one of the most important process

9

parameters involved in steam gasification 62. Raising the SB ratio increases H2 and CO2 yields

10

and decreases the CO and CH4 yields, thus, raising the LHV value of the exit gas. These

11

results can be explained as a consequence of the reactions involved in this process, mainly

12

Water-Gas shift and steam reforming of tars and methane 8,9.

13

More than allowing a significant tar reduction, addition of water also strongly

14

modifies the tar composition. Indeed, raising the SB ratio is known to decrease the formation

15

of high aromatic compounds 63 and to increase the formation of phenolic compounds. This is a

16

huge advantage since these compounds are much easier to reform with catalysts

17

way, Narvaez et al. 7 showed that a strong decrease of the tar content (from 20 to 5 g/m3) can

18

be obtained when the SB ratio is increased from 1.6 to 2.2 at 800 °C.

19

All optimum SB ratios advised in literature are situated between 1.3 and 4

47

. In this

7,10

. In the

20

case of a SB > 4, the LHV and carbon conversion are decreased due to the low reaction

21

temperature caused by the addition of steam 61. According to some economic evaluations 10,

22

the best SB ratio tends to be situated around 2.5. 21 ACS Paragon Plus Environment

Energy & Fuels

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1

3.4.4. Influence of gasifying pressure

2

According to the review carried out by Couto et al.

Page 22 of 80

42

, pressurized gasification

3

operations usually create more problems than they solve. Indeed, in that case biomass feeding

4

becomes more complex and very costly, and requires a high inert gas for purging. Although

5

equipment sizes are much smaller, pressurized gasification systems can cost up to four times

6

more than atmospheric systems

7

installations (30-50 MWh) were the exiting gas does not need to be compressed for further

8

combustion in turbines

9

LHVs. Although the total amount of tars decreases, raising the pressure favors the formation

10

42

. These processes become only relevant in case of large

42

. Increasing the pressure leads to similar gas compositions, and

of stable PAH compounds 5.

11

3.4.5. Influence of gasification time

12

According to Corella et al.

64

, the residence time has little influence on the tar yield,

13

but it significantly influences the tar composition. Indeed, it was found that increasing the

14

contact time results in less O2 containing compounds, less 1- and 2- ring compounds but more

15

3- and 4- ring compounds in the total tar fraction. A study conducted by the Energy research

16

Centre of the Netherlands concludes that increasing the gas residence time in a hot zone has

17

similar effects as raising the temperature 58.

18

4. Bio-syngas catalytic purification

19 20

4.1. Tar reforming

21 22

4.1.1. Catalytic reforming mechanisms and thermodynamic approach


Page 23 of 80

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Energy & Fuels

1

Figure 12 presents the different mechanisms of catalytic steam and dry reforming of

2

hydrocarbons (methane, polyaromatic hydrocarbons, alkanes, …) with a catalyst based on

3

ceramic support (Al2O3, SiO2, …) with metallic active sites (Ni, Co, Pt, …)

4

metallic active site, tar molecules are adsorbed and hydrocracking, catalytic thermal cracking,

5

hydrodealkylation and various hydrogenation reactions take place to decompose the tar

6

molecules into active surface species: carbon C*, hydrogen H* and tar-derived fragments

7

CxHy*. H2O(g) and CO2(g) molecules are dissociated on the support/metal sites and generate

8

active species such as H*, HO* and O*. CO2(g) molecules can also react with the adsorbed H* to

9

produce CO(g) and HO*. The active species generated on the support surface can migrate

10

towards the metallic active sites and decompose the activated carbon C* and the tar fragments

11

CxHy* into CO(g) and H2(g) via several oxidation reactions. Finally, CO(g) and H2(g) molecules

12

resulting from these several reactions are desorbed from the metallic active sites. The

13

mechanisms presented in Figure 12 are not the only mechanisms occurring: for example, active

14

surface species (C*, O*, HO* and H*) can reassemble according to reverse activations to

15

produce again H2O(g) and CO2(g) molecules. Furthermore, H* species can also react to form

16

H2(g) molecules.

17

Figure 12: General scheme of catalytic tar steam and dry reforming inspired from the literature 65–67.

18

33,65–69

. On the

4.1.2. Kinetic approach

19

Numerous studies about toluene steam- and dry- reforming conducted at laboratory scale

20

use some catalysts with a particles’ diameter of 300-800 µm. It is assumed that heat and mass

21

transfer limitations do not take place, and that the reactor is a plug flow without any gas

22

expansion (the flow rate is constant). It was shown by different authors

23

of the catalyst for the tar removal can be expressed with respect only to the toluene

24

concentration (Eq.25):

70–73

that the activity


Energy & Fuels

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Page 24 of 80

(25)

1

2

With r being the reaction rate, kapp the apparent kinetic constant and CToluene the concentration

3

of toluene. It is also assumed that the decomposition of tar is a first order (n=1), which leads

4

to (Eq.26):

(26)

5

6

Under plug flow conditions, using the residence time t and the conversion Xc, the apparent

7

rate constant kapp becomes (Eq.27):

8

(27)

9

Combined with the Arrhenius law, the activation energy is obtained in function of the

10

residence time and the conversion (Eq.28):

11

(28)

12

It is important to note that the hypothesis of the first order reaction for the

13

decomposition of tar is only possible when the CO2/C and H2O/C ratios are higher than the

14

stoichiometric ratio

15

aromatic compound increases. Indeed, the decomposition of large aromatic species takes

16

place according to multiple reaction pathways 73–75.

70

. Moreover, this assumption is less accurate when the size of the


Page 25 of 80

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1

Energy & Fuels

The comparison of activation energies and apparent constants is an important criterion 49,73,74,76

. Li et al.

49

2

for the determination of the catalyst performances

3

activation energy required for the steam reforming of a tar mix at the outlet of a biomass

4

gasifier reactor studied from different authors. The activation energies were comprised

5

between 70 and 100 kJ/mol depending on the type of gasifier (air, steam) and the type of

6

catalysts used. As element of comparison, the thermal conversion of naphthalene, toluene and

7

benzene without any catalysts was respectively 350, 247, 443 kJ/mol .

8

Table 4 : Activation energies and constant rates from different studies

49

compared the

.

9 10 11

4.2. Influence of operating parameters on the catalyst performances 4.2.1. Influence of the temperature

12

Though promoting the gasification reactions, increasing the temperature in gasifiers or in

13

unities situated at the outlet of the gasifiers also has significant drawbacks: (1) in the case of

14

catalysts being situated inside the gasifier (primary catalyst), a temperature increase up to

15

very high temperatures (T is above 950 °C) can damage and reduce the reactor lifetime.

16

Furthermore, increasing the temperature will directly impact the biomass reactions and the

17

final gas composition. As explained previously, in a reactor without any catalysts, a

18

temperature increase leads to a higher tar yield, but with more stable tars (HAP) that remain.

19

Generally, the optimum temperature for primary catalysts is situated at around 850 °C, which

20

matches with the bed reactor temperature 70,77–79; (2) in the case of catalysts being situated at

21

the outlet of the gasifier (secondary catalyst), heating the catalytic cleaning unities include

22

additional costs. Therefore, efficient unities with secondary catalysts are expected to work at

23

the lowest temperature possible (500-650 °C) in order to be economically interesting 46.


Energy & Fuels

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Page 26 of 80

1

The temperature has a strong effect on the kinetic parameters. Narvaez et al. 73 studied the

2

influence of different operating parameters (temperature, space time, equivalence ratio, steam

3

to carbon ratio) on the kinetic constant for the degradation of a tar mixture at the exit of a

4

fluidized gasifier with a commercial nickel-based catalyst. They showed that the apparent

5

kinetic constant, kapp, was multiplied by 5 between 650 and 800 °C. This trend was confirmed

6

by Aznar et al.

7

catalysts as function of the temperature and the space ratio.

76

who conducted a kinetic comparison between different nickel based

8

In many publications concerning the development of secondary catalysts used at low

9

temperatures (500-650 °C), a pre-reduction step is usually undertaken at higher temperatures

10

(750 °C, 1 h, H2) in order to reduce the metallic nanoparticles 3. For a classic commercial

11

catalyst with 10 wt. % of Ni on alumina at temperatures of up to 650 °C, the nickel oxide

12

particles are reduced to Ni(0) by the H2 and CO molecules that are naturally present in the

13

exiting syngas 3,10. However, below 650 °C, even if the catalyst has been previously reduced,

14

deactivation can occur due to the presence of oxidant molecules in the syngas (CO2 and H2O)

15

which convert active Ni(0) particles into less efficient NiO particles

16

temperature of 650 °C appears to be the minimum working temperature for catalysts at the

17

exit of a gasifier.

69

. Therefore, a

18

4.2.2. Influence of the gas composition

19

Usually the tar concentration is situated around 500-10000 ppmv, whereas the reagents

20

(H2O and CO2) are present in a much higher concentration ranging from 9 to 36 %. So, the

21

H2O/C and CO2/C ratios are always much higher than the stoechiometric ratios. However,

22

some authors showed that the degradation reactions depend strongly on the gas compositions.

23

Laosiripojana et al.

80

studied the effect of H2O/C and CO2/C on the degradation of


Page 27 of 80

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Energy & Fuels

1

naphthalene with Ni- and Ni–Fe bimetallic based catalysts supported on palygorskite

2

(magnesium aluminum phyllosilicate). H2O/C and CO2/C ratios were increased from 0 to 3 at

3

700 °C. The results were similar for each catalyst and for both oxidants. Indeed, in both cases,

4

the maximum H2 yield, which is linked to the general conversion of naphthalene, reached a

5

maximum value corresponding to H2O/C and CO2/C ratios situated between 1.0 and 1.5. The

6

authors also noticed that the carbon deposition was higher after dry-reforming (5.7 mmol/gcat)

7

than after steam-reforming (3.9 mmol/gcat). According to the author, the water present in the

8

feed gas reacts with CH4 to form CO and H2, and with the CO to form H2 and CO2, thus

9

reducing the selectivity of the Boudouard and methane decomposition reactions. Moreover,

10

as explained above, H2O molecules are dissociated into OH* radicals on the surface of the

11

catalysts, allowing the degradation of tars into CO and H2, whereas the dissociation of CO2

12

molecules is always associated with a risk of carbon deposition. Narvaez et al.

13

performance of a nickel-based (BASF G1-25S) commercial catalyst for the catalytic

14

conversion of tar mixtures at the outside of a bubbling fluidized bed reactor. The authors

15

studied the influence of the H2O/C ratio (1.4 - 2.4) on the kinetics apparent reaction rate (kapp).

16

The results show that for two different temperatures (750 and 800 °C), the apparent kinetic

17

constants are multiplied by a factor of around 2 when the H2O/C ratio increases from 1.4 to

18

2.4.

73

studied the

19

4.2.3. Space velocity influence

20

Space velocity (1/s) is defined as the inverse of the residence time (s) and therefore

21

corresponds to the quotient of the incoming reagents divided by the reactor/catalyst bed

22

volume. Generally, as the residence time increases, reactions in the catalytic reformer have a

23

greater opportunity to proceed, and the system tends towards thermodynamic equilibrium.


Energy & Fuels

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Page 28 of 80

1

Kinoshita et al. 79 highlighted the importance of the residence time for the composition

2

of the produced gas and the tar degradation with a commercial nickel-based catalyst. The

3

authors showed that increasing the residence time has similar effects as increasing the

4

temperature: a rise of the volume percentage of H2 and CO and a decrease of the CO2

5

concentration. It is noteworthy that the tar concentration at 750 °C was divided by 10 (from

6

20 g/m3 to 2 g/m3) when the residence time was increased from 0.7 to 1.7 s. Furthermore, Li

7

et al.

8

mayenite (calcium aluminate). They showed that the H2 yield and toluene conversion was

9

increasing with the residence time until reaching a maximum value where an equilibrium was

10

reached. By comparing the results obtained at 650 and 800 °C, they also confirmed that the

11

optimum residence time varies for every temperature (when T increases, the optimum space

12

ratio decreases).

13

81

found similar trends when studying the decomposition of toluene on Ni-doped

4.3. Catalyst deactivation and regeneration

14 15

4.3.1. Coking mechanisms

16

Deactivation by coking is the most significant problem that catalysts have to face since

17

the carbon deposits reduce the activity of the metallic active sites and can block the support

18

pores that are responsible for the high active surface. Furthermore, the carbon deposits

19

increase the pressure drop in the reactor, resulting in frequent regeneration or early shutdown

20

of the reactor.

21

•

Carbon deposit properties

22

Carbon deposits result from the dissociation of hydrocarbons or reverse reactions

23

between gaseous species and the catalyst surface. The carbon deposits are of different types 28 ACS Paragon Plus Environment

Page 29 of 80

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Energy & Fuels

1

(Figure 13): adsorbed hydrocarbons, atomic carbons adsorbed on the surface, graphitic

2

carbons, metal carbides or carbon whiskers 50,82–85.

3

Figure 13: Carbon formation mechanisms proposed by Trimm et al. 82.

4

Adsorbed hydrocarbons do not deactivate the catalysts, but they can be transformed

5

into more problematic species. When Thermal Programmed Oxidation (TPO) or

6

Thermogravimetric (TG) measurements are carried out under air, adsorbed hydrocarbons are

7

generally oxidized below 400 °C.

8 9

Due to their important activity, most of atomic carbons adsorbed on the surface of the catalyst (Cα*) are gasified into CO and CH4.

10

Polymerization and rearrangements of Cα* can lead to graphitic carbons (Cβ*). They

11

are less reactive, but they can still be gasified. If not, they can either combine to form

12

polymeric amorphous films which encapsulate the metallic active sites and clog the substrate

13

pores, or dissolve into the metallic active site to form metal carbides or carbon whiskers 82,83.

14

Using air, TPO or TG, amorphous carbon deposits are generally oxidized around 500 °C

15

whereas filamentous carbons are removed at around 600 °C.

16

Graphitic carbon can interact with the lattice of the metal active site to form metal

17

carbides (Ni3C, Fe3C, Co2C…). More than altering the efficiency of the active site, the

18

metallic carbides can become nucleation points for the growth of amorphous and filamentous

19

carbons. They are very difficult to re-gasify (their combustion under TPO or TG takes place at

20

up to 600 °C).

21

The carbon species dissolved into the active site can also diffuse through the metallic

22

particle in order to nucleate and precipitate at the interface crystallite/support 82,83. This effect 29 ACS Paragon Plus Environment

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Page 30 of 80

1

results in the formation of carbon whiskers, which lift the metallic nanoparticles from the

2

surface and lead to the fragmentation of the catalysts. The ejection of metal nanoparticles due

3

to the formation of carbon whiskers is usually present in catalysts with low

4

nanoparticles/support interactions. The formation of carbon whiskers is known to be the most

5

deactivating coking mechanism 82,83.

6

In order to quantify the catalysts’ resistance towards coking, Farnood et al.

86

defined the

7

“coke formation rate” parameter, which is the weight loss of the used catalysts measured by

8

TGA analysis divided by the testing time. More thorough studies have permitted the

9

establishment of certain kinetic laws of deactivation. Bain et al. 87 investigated the reforming

10

and coking kinetics during the steam degradation of benzene, toluene and light alkane using a

11

Ni-alkali-Al2O3 catalyst. First order rate equations were found to represent both the kinetics of

12

reforming and deactivation. A similar study was conducted by Dou et al.75. The authors

13

established a kinetic model of the deactivation through coking during the steam reforming of

14

1-methylnaphtalene on Ni-Mo/Al2O3 catalysts.

Attempts to reduce coking

15

•

16

The formation of coke results from a destabilization of the balance between the rate of the

17

hydrocarbon decomposition and the gasification rate of the carbon species on the metallic

18

active site 69. To prevent the coking of catalysts, two main improvements can be carried out:

19

decreasing the hydrocarbon adsorption/decomposition or increasing the gasification rate of

20

the carbon species 69.

21

Increasing the oxidants’ (H2O, CO2) concentration of the feedstock enables a higher

22

gasification rate of the carbon species on the catalyst surface, resulting in a lower carbon

23

deposition

50,73,80

. According to several authors

50,84,88

the most significant factors for carbon 30


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Energy & Fuels

1

deposition are the concentration and type of hydrocarbon in the gas phase. It was shown that

2

the carbon deposit formation increases with the unsaturation, molecular weight and

3

aromaticity of the hydrocarbons since the compound structure thus becomes more similar to

4

the carbon deposit composition. Coll et al.50 highlighted that the minimum H2O/C ratio to

5

avoid the formation of coke was increasing with the tar compound aromaticity (from 2.5 for

6

toluene to 8.4 for pyrene at 780 °C).

7

Low temperatures (500-700 °C) favor the formation of coke on the catalysts and the 73,76,88

8

catalyst deactivation

. Indeed, the thermal equilibrium of the Boudouard reaction is

9

achieved at approximately 700 °C. At temperatures higher than 700 °C, the adsorbed or 35,67

10

deposited carbon reacts with CO2 to form CO

. Therefore, a temperature increase has

11

generally more impact on the gasification rate of the carbon species than on the hydrocarbon

12

decomposition on the active site 88,89.

Regeneration

13

•

14

The catalysts can be regenerated by removing the carbon deposit thanks to gasification

15

reactions with O2, H2O, CO2 and H2. Thanks to its rate and simplicity, air regeneration is the

16

most common regeneration process: first, the catalysts are purged with inert gas (N2) in order

17

to evacuate H2 and CH4, thereafter the temperature is maintained at a high level (400 – 600

18

°C) and a special gas mix containing N2/O2 or atmospheric air is injected to burn the carbon

19

deposits. One major drawback is the deactivation of the catalyst due to the formation of hot

20

spots during the burning of carbon

21

recommend operating the regeneration at temperatures lower than 450 °C in order to avoid the

22

conversion of γ- to α-alumina and formation of nickel aluminate spinels 90.

89

. That is why some authors such as McCulloch


Energy & Fuels

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1 2

Page 32 of 80

Steam regeneration is also a common regeneration technique, however the sensitivity of supports such as SiO2 or Al2O3 towards hydrolysis is its major drawback 69,89.

3

4.3.2. Poisoning of metal active sites

4

According to Forzatti et al.

91

, poisoning is the loss of activity due to the strong

5

chemisorption on the active sites of impurities present in the feed stream. A poison may act

6

simply by blocking an active site (geometric effect), or it may alter the sorption capacity of

7

other species by an electronic effect. Poisons can also modify the chemical nature of the

8

active sites or result in the formation of new compounds so that the performances of the

9

catalyst are strongly altered.

Sulfide compounds:

10

•

11

As showed in Table 3, sulfur compounds such as sulfur-organic species (thiophene, thiols

12

…) or simple sulfur species (H2S, SO2, SO42-) are usually present in the bio-syngas.

13

Following the availability of electrons in the molecular orbital of sulfur compounds, the order

14

in which the poisonous ability increases is the following: SO42- < SO2 800 °C) improve the tar conversions and prevent the

11

catalytic deactivation by H2S. However, the use of catalysts located at the outlet of the

12

gasifiers (secondary catalyst) remains the only resort for cleaning syngas out of

13

fixed/entrained bed. Since the working temperatures of secondary catalysts are lower (550-

14

700 °C), the complete conversion of tars is more difficult to achieve. Moreover, catalytic

15

deactivations through coking and sulfidation are more threatening. The ideal case would

16

consist in the presence of a primary catalyst inside the reactor (ex.: olivine), in order to

17

convert the high aromatic tars into smaller molecules such as toluene or benzene, followed by

18

a secondary catalyst (ex.: Ni/Al2O3) in order to completely eliminate the remaining tars

19

This work therefore focuses on the description of basic notions and recent advances of

20

secondary catalysts.

21 22

23

, studies on obtaining the

106

. The best results

(11)

.

Numerous reviews agree on ranking the following properties required by bio-syngas catalysts 46,51,107–109:

1) Effective tar reforming capacity, even at low temperatures (550-700 °C);


Page 39 of 80

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Energy & Fuels

1

2) High CH4 reforming capacity;

2

3) Resistance towards deactivation by coking, sulfidation and sintering;

3

4) Easy regeneration;

4

5) Cheap materials (especially for primary catalysts);

5

6) Strong resistance against attrition (especially for primary catalysts);

6

7) High H2 production, notably through the Water-Gas Shift reaction;

7

8) Made of non-toxic materials (especially for primary catalysts).

8

Richardson introduced a global overview of the usual catalyst properties (Figure 14).

9

The concept is the following: the catalyst design can be operated through three main parts: the

10

morphological and mechanical properties (meaning size, shape and strength); the

11

chemicophysical properties (meaning surface area, porosity, acidity, composition and density)

12

and the catalytic properties (meaning activity, selectivity and stability) 110.

13

Figure 14: Catalyst design triangle introduced by Richardson 110.

14

Yung et al.

107

made an exhaustive literature review in order to list and classify the

15

different elements used for catalytic tar reforming (Figure 15). The authors ranked the

16

different active site promoter and support performances in either good, moderate or poor.

17

Some elements such as P, S and Cl were classified as biomass derived poisons.

18

Figure 15: Overview of the different elements used for bio-syngas purification catalysts 107.

19

In addition to this overview, a classification of the types of catalysts used for tar

20

elimination has been introduced by several authors (Figure 16) 46,51,107,111. The catalysts are

21

divided into two groups: synthetic catalysts and minerals. Synthetic catalysts are usually more

22

performant due to a high control of their properties (porosity, surface area, metal dispersion


Energy & Fuels

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Page 40 of 80

1

and interactions with the support …) but they are usually too expensive and fragile to be used

2

in primary conditions. As for natural catalysts, their biggest advantages resides in their

3

extremely low costs, but also sometimes in their mechanical strengths (example: olivine). In

4

both cases, the supports are usually doped with metallic elements.

5

Figure 16: Classification and types of catalysts used for tar elimination 111.

6

4.4.1. Mesostructure and catalysts shaping

7

The micro- (50 nm) porosity of a catalyst are

8

strongly influencing its performance. An adequate pore repartition is essential for a good

9

diffusion of the reagent molecules. Indeed, since the common tar molecule size lies at around

10

0.5-1 nm (toluene: 0.68 nm; naphthalene: 0.72 nm), it has been proved that catalysts with a

11

pore opening below 0.7 nm are inefficient for this application

12

hierarchical pore distribution favors a better metal particle dispersion and therefore a higher

13

metal surface

14

metal nanoparticles. This effect has been illustrated by the study conducted by Tian et al. 113

15

who managed to block Ni nanoparticles in alumina nano-honeycombs, thus permitting to

16

avoid the migration and coalescence mechanisms at high temperatures.

53,109

. In addition, a good

109,112

. A controlled pore structure may also reduce the sintering ability of the

17

The catalyst macroscopic shaping is also known to have an influence. In this way, it

18

was proved that the estimated rate of deactivation ranks as follows: nets > pellets > monoliths

19

10

20

.

4.4.2. Support composition influence

21 22

4.4.2.1.

Carbon supports


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Energy & Fuels

1

Activated carbons, i.e. non-graphitic and microcrystalline forms of carbon can be used

2

as catalyst supports. Their neutral surface properties make them more resistant than acid

3

supports 114 towards the deactivation by coke deposition. Their hierarchical pore structure is a

4

strong asset since it greatly improves the dispersion of metallic particles and especially

5

facilitate the transport of reactant molecules 109.

6

Fuentes-Cano et al. 71 studied the kinetics of toluene and naphthalene steam reforming

7

at different temperatures (750 - 950 °C) over commercial coconut, coal chars, and char

8

prepared in a lab. They observed similar conversion trends for every type of char they used.

9

Naphthalene and toluene were completely removed at temperatures up to 850 °C.

10

El-Rub et al.

115

compared the activity of biomass char to other common catalysts

11

(dolomite, olivine, nickel (70 wt. %) /Al2O3 and sand) for the reforming of phenol and

12

naphthalene at temperatures from 700 to 900 °C and in the presence of CO2 and H2O. At 700

13

°C, the phenol conversion was in the following order: Ni/Al2O3 > dolomite > char > olivine >

14

sand. At 900 °C the degradation conversions were as follows: Ni/Al2O3 > char > dolomite >

15

olivine > sand.

16

Despite their interesting properties, chars do not seem to be the most appropriate

17

support for long term operations since they are slowly consumed by the CO2 and H2O present

18

in the syngas composition 46,109.

19

4.4.2.2.

Basic supports

20

By definition, a basic solid catalyst shows the ability to extract a proton from a

21

molecule. Alkaline earth metallic oxides, alkali ion-added zeolites, alkali metallic ions

22

supported on silica or alumina and some clay minerals are examples for basic solids 53. It is 41 ACS Paragon Plus Environment

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Page 42 of 80

1

known that the support basicity plays an important role by increasing the ability of the catalyst

2

to chemisorb CO2, thus improving the surface carbon oxidation 3,116,117.

Pure calcium and magnesium oxides

3

•

4

CaO and MgO are the most commonly used basic catalysts for tar reforming. The basicity

5

of MgO and CaO oxides is attributed to their O2- ions: the sites with the highest basicity are

6

located at the morphological defects of the polycrystalline solid, where each anion is

7

coordinated to only three or four metallic cations 53. CaO has a higher basicity because of its

8

larger lattice constant. In this way, the electrons around O2- ions in CaO are more labile than

9

the electrons around O2- ions in MgO, favoring their overlap with the orbitals of the incoming

10

molecules 118,119. The ability of CaO and MgO to adsorb CO2 and H2O reagents is defined by

11

their carbonation (Eq.35) and the hydration (Eq.36)

12

and CaO are possible only below 700 °C 121,122.

120

. The carbonation reactions of MgO

13

(35)

14

(36)

15

Despite the most famous natural mixture of these oxides being dolomite

16

(CaMg(CO3)2), various synthetic mixes have also been synthesized. Apparently, an addition

17

of only 1 wt. % of dolomite in the biomass feed is enough to reduce 40 % of tars. Despite its

18

interesting reforming properties, dolomite is not convenient as a primary catalyst due to its

19

low attrition resistance, leading to dust formation

20

feedstock has also proved to have an important influence on the LHV of the syngas (the H2

21

concentration went from 23 to 55 vol. % when CaO was added) 123.

40

. An addition of pure calcite into the


Page 43 of 80

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Energy & Fuels

According to Delgado et al.

1

124

, the activity towards tar reforming is the following:

2

MgO-CaO > CaO > MgO. Li et al. 125 studied the toluene reforming of Ni supported on MgO-

3

CaO with different weight percentages of magnesium (30 / 50 / 70). Their results showed that

4

the MgO70%CaO30% support mix was showing the highest activity. According to Zhao et al.

5

126

6

of smaller aromatics such as benzene or naphthalene. The authors also demonstrated that in

7

situ CO2 capture by CaO favors the Water-Gas shift reaction beyond equilibrium limitations,

8

resulting in a syngas with high hydrogen and low CO content. These trends were also

9

confirmed by Guan et al.

, CaO degrades heavy tars very effectively, but is less efficient for cracking and reforming

66

. A remarkable study on the reforming catalytic activity of iron

10

and nickel based catalyst supported over CaO, MgO and calcined dolomite was conducted by

11

Felice et al. 127: the authors highlighted the fact that Ni-CaO was more efficient than Ni-MgO

12

mostly because MgO reacts with Ni to form a solid solution, thus decreasing the catalytic

13

activity. A similar effect was also shown for Fe-CaO materials, on which Fe was less active

14

due to the formation of a Ca2Fe2O5 solid solution. Lu et al. 128 carried out a comparative study

15

of CH4 steam reforming with nickel supported catalysts and showed that the deactivation rates

16

of four catalysts was in the following order: Ni/MgO < Ni/γ-Al2O3 < Ni/SiO2 < Ni/α-Al2O3.

17

The long term stability of Ni/MgO was attributed to the low sintering of nickel crystallites,

18

thus limiting carbon formation.

Olivine

19

•

20

Thanks to its non-toxic, cheap and high attrition resistance properties, olivine has proved 46,107

21

to be the best natural support for primary catalyst application

. Its general formula is

22

(Mg,Fe)2SiO4, but its composition varies depending from where it is extracted

23

this support exhibits poor surface areas (SBET < 5 m2/g).

129

. However,


Energy & Fuels

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Page 44 of 80

1

Virginie et al. 77 studied the reforming activity of olivine doped with iron and calcined at

2

different temperatures (400 – 1400 °C). The calcination temperature strongly influenced the

3

iron metallic state (0, +II, +III) and its diffusion through the olivine support, thus resulting in

4

a different reforming activity. It was shown that the highest activities were obtained after

5

calcination at 1000 °C. In the same laboratory, Swierczynski et al.

6

influence on Ni/olivine catalysts. The interactions of Ni with the intrinsic Fe and MgO

7

elements naturally present in olivine were supposed to explain the high resistance towards

8

carbon formation. These trends were confirmed by other studies using Ni and olivine

9

The association of CaO or Ca12Al14O33 with olivine was studied by Zamboni et al.

70

studied this calcination

130,131

.

121

:

10

carbonation-decarbonation cycles highlighted a better stability of the CO2 sorption ability of

11

Ca12Al14O33 compared to CaO. The authors also showed that the dispersion of the iron

12

naturally present in the olivine structure was improved by the Ca12Al14O33 phase 121.

Other natural supports

13

•

14

Guan et al. 66 investigated the utilization of scallop shell as a raw support for tar catalysis

15

applications. This cheap support composed of basic element (CaO) presents interesting

16

macro- and micro-porosity. Doped with iron or nickel salts, and promoted with potassium 132,

17

this support was efficient for the tar reforming at 650 °C. However, its fragility made it

18

inappropriate for an application as primary catalyst.

4.4.2.3.

19

20

Acid supports

Acid catalysts are materials which are able to provide a proton.

Many chemical

21

processes (ex.: cracking) are catalyzed by solid acids, usually using their ability to break C-C

22

bonds. Most materials used are zeolites, alumina and various pure and sulfated metal oxides

23

53,69,91

. 44 ACS Paragon Plus Environment

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Energy & Fuels

Silica

1

•

2

Pure silica supports are always avoided in steam reforming applications

69

. Indeed, the

3

hydrolysis of SiO2 leads to volatile Si(OH)4 species which cause various technical problems

4

in downstream unities. This effect can be countered by the addition of alkali which is known

5

to reduce the volatility of SiO2 supports

6

proved to be valuable for the destruction of CH4, C2+ and NH3. The magnesium-silicate

7

synergy was supposed to explain the good resistance to H2S during hydrocarbons reforming.

8

However, even though the catalyst seems to have interesting properties, one should consider

9

that the authors carried out the tests at high temperature (900 °C) and with a low H2S content

10

69

. Magnesium doped nickel silicates have also

(20 ppm) 133.

Zeolites

11

•

12

Zeolites are alumina-silica materials commonly used for catalytic applications

53,69

. They

13

are either natural or synthetic. It is easy to modify their Al/Si ratios, their amounts of acid

14

sites and their mesostructures thanks to surfactant assisted synthesis (SBA, MCM-41…)

15

Unfortunately, zeolite materials do not seem to be appropriated for tar reforming purposes. In

16

fact, according to Forzatti et al. 91, the Brønsted acid sites stabilize the carbon intermediates

17

(carbonium ions), which can condensate and form polyaromatic compounds. This fact was

18

confirmed by Buchireddy et al. 135 who researched the reforming of naphthalene at 750 °C

19

with seven different types of zeolites impregnated with 7.5 %wt. of nickel. It was observed

20

that the tar yield of zeolite containing Na (chabazite) was low due to the disappearance of

21

acids sites. The authors highlighted the fact that the acidic nature of zeolites increases in the

22

naphthalene conversion via cracking reactions and that the more acidic the zeolites were, the

23

faster they were deactivated through coking. Nevertheless, when doped with other elements,

134

.


Energy & Fuels

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Page 46 of 80

1

zeolites can achieve very interesting results. Tao et al. 136 developed a Ni (3 wt. %)-CeO2 (0-3

2

wt. %)/SBA-15 catalyst which showed very good results for the catalytic reforming of toluene

3

between 700 and 850 °C. The positive anti-coking results were attributed to CeO2 doping.

4

Iron oxide supports

5

•

6

The catalytic activity of iron depends on its oxidation state (Fe2O3, Fe3O4 or Fe0). When

7

iron is in a metallic state (Fe(0)), it achieves its maximum tar destruction yield thanks to the

8

ability of the Fe0 species to break the C-C and C-H bonds 66. The magnetite (Fe3O4) is also

9

known to enhance the Water-Gas Shift reaction (Figure 17).

10

Figure 17 : Scheme of the tar decomposition mechanism and the Water-Gas Shift enhancement over an iron

11

oxide catalyst 137.

12

Azharuddin et al. 137 studied the degradation of Fe2O3 and Al2O3 mixes concerning the

13

destruction of tars which formed during the cedar wood gasification at 850°C. It was found

14

that Al2O3 permitted to achieve a higher surface area, without decreasing the Fe2O3 activity.

15

The Fe2O3; 50 wt. % /Al2O3; 50 wt. % composition showed the more interesting results (up to 90 %

16

of converted tars at 850°C). Other Fe-minerals were studied, such as natural iron ore

17

ilmenite (FeTiO3) 139. Generally, iron based catalysts showed interesting properties when used

18

at high temperatures (> 800 °C), i.e. inside the reactor. Due to low temperatures at the gasifier

19

outlet, the iron cannot be reduced to its metallic state and therefore enables a sufficient tar

20

yield.

21

•

138

and

Al2O3 supports


Page 47 of 80

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1

Energy & Fuels

Various reviews agree on the fact that γ-Al2O3 seems to be the best support for secondary 46,69

2

catalyst applications

3

Miyazawa et al.

4

pyrolysis of cedar wood with different supports. They found out that the order of activity at

5

600 °C was the following: Ni/γ-Al2O3 > Ni/ZrO2 > Ni/TiO2 > Ni/CeO2 > Ni/MgO > no

6

catalyst.

140

. Interesting catalytic properties of γ-Al2O3 were highlighted by

who studied the steam reforming of a tar mixture derived from the

7

γ-Al2O3 appears to be an interesting support because of its large specific surface area

8

(200-500 m2/g), high mechanical strength and good sintering resistance. However, if the

9

temperature is too high (> 800 °C), α-Al2O3 phase formation can occur. The catalytic activity

10

is much lower for α compared to the γ phase principally because of the drastic pore volume

11

and surface area reduction (SBET.α-Al2O3 > 20 m2/g)

12

with transition metals (Co, Ni, Cu …), the calcination and operating temperatures become

13

critical factors due to the formation of strong metal-aluminate interactions which occurs at

14

temperatures higher than 800 °C, thus reducing the reducibility and activity of the active

15

species. However, these strong metal-alumina interactions can be interesting since they reduce

16

the metallic nanoparticle sintering and therefore the catalyst coking, thus resulting in a longer

17

lifetime of the catalyst 10.

4.4.2.4.

18

3,128

. When γ-Al2O3 supports are doped

γ- Al2O3 promoted supports

19

As γ-Al2O3 is one of the most adapted supports, many doping attempts have been

20

conducted in order to optimize its properties (acidity/basicity) and its resistance towards

21

sintering.

22

•

Calcium doped alumina supports


Energy & Fuels

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Page 48 of 80

1

Used as pure material, CaO particles tend to agglomerate and can be evacuated by the

2

stream. In contrary, adding CaO to γ-Al2O3 allows keeping a high dispersion of CaO particles

3

and provides a synergy of the interesting properties of calcium oxides and alumina

4

Furthermore, when the calcination temperature is sufficiently high (around 900 °C

5

mayenite structure can be formed. Mayenite shows a very good anti-coking and anti-sulfur

6

properties thanks to the presence of “free oxygen” in its structure

7

property provided by calcium was confirmed by Engelen et al. 143, who washcoated Ni and Ca

8

salts on an α-Al2O3 support used for the benzene steam reforming with H2S concentrations of

9

0 / 50 / 100 ppm. At 900 °C and with 100 ppm of H2S, the samples doped with 0.5/1/1.5 wt.

10

% of CaO exhibited reforming activity losses of 40 % less compared to pure Ni-Al2O3

11

samples.

12

•

13

In addition to the interesting properties of magnesium (Ni-MgO interactions, CO2

141

121

.

), a

142

. The sulfur-resistance

Magnesium doped alumina supports

144,145

14

adsorption), under the adequate calcination temperature (around 800°C

), the formation

15

of magnesium aluminate spinels (MgAl2O4) considerably increases mechanical strength and

16

sintering resistance of the catalyst 65. Li et al. 146 studied the degradation of rice straw derived

17

tars on nickel catalysts (Ni from 2 to 20 wt. %) with different supports (CaO, TiO2, γ-Al2O3

18

and γ-Al2O3 +0.5/1/2 wt. % of MgO). The best results were obtained with 7.5 wt. % of nickel

19

supported on γ-Al2O3 + 1 wt. % of MgO.

Potassium doped alumina supports

20

•

21

Among a series of different additives (Na, K, Mg, Ca, Ba, La, Zr, Ce) deposited on a γ-

22

Al2O3 support, Lee et al. 147 showed that potassium was the most efficient coking. This effect

23

was researched thoroughly by Gálvez et al.

72

who suggested that after calcination at a 48


Page 49 of 80

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1

temperature up to 650 °C, the formation of K2O was possible. Under a syngas atmosphere,

2

K2O is immediately transformed into K2CO3 or KOH, which favors the formation of reactive

3

O* and HO* species. Hou et al. 148 showed that the combination of 0.5 wt. % of K and 0.5 wt.

4

% of Ca on nickel-alumina catalysts had a beneficial influence against the coke deposition

5

during the dry reforming of methane. Zhenissova et al.

6

promote the CO2 sorption ability of both CaO and MgO in hydrotalcite supports.

117

observed that K doping can

Lanthanum doped alumina supports

7

•

8

The beneficial influence of lanthanum on γ-Al2O3 against sintering was early highlighted

9

by Novakovic et al.

104

who compared the sintering at 1000 °C of different γ-Al2O3 supports

10

doped with different La2O3 percentages. Wei et al. 149 confirmed this effect and showed that

11

when comparing different elements (La, Ce, Y and B), lanthanum was the element allowing to

12

keep the best structural properties at high temperatures (1000-1200 °C). The high specific

13

surface area preservation was attributed to the fact that La permitted to delay the phase

14

transition from γ to α-Al2O3.

15

Even if Ni-La-Fe/γ-Al2O3 catalysts developed by Li et al.

150

showed interesting results

16

towards the reforming of cellulose tars at 800 °C, Sricharoenchaikul et al. 151 proved that an

17

addition of La decreased the toluene steam reforming activity of Ni-Mg-La tri-metallic

18

catalysts supported on α-Al2O3 and favored the formation of carbon whiskers. Moreover,

19

Gallego et al. 152 showed that the lanthanum element appears to be sensitive towards sulfur

20

deactivation during the dry reforming of methane over LaNiO3 perovskite. Despite the very

21

good long-term results expressed by the catalyst (100 h without carbon deactivation), a low

22

amount of H2S (25 ppm) was sufficient to deactivate it in 5 hours. The XRD spectra proved

23

the presence of a La2O2S phase, which appeared to be very difficult to regenerate.


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Page 50 of 80

Cerium doped alumina supports

1

•

2

Despite its high price, cerium oxide appears to be the most promising support doping

3

element discovered in the last two decades. Indeed, cerium oxide can easily stock and destock

4

oxygen, which increases its redox properties and its oxygen lability, thus strongly inhibiting

5

any surface carbon formation. Furthermore, CeO2 is known to facilitate the H2O and CO2

6

surface dissociation (Eq.37). The O* radicals are then easily stabilized and integrated into the

7

CeO2 structure until they can react with adsorbed hydrocarbon species (Eq.38). This high

8

oxygen lability is apparently also efficient for protecting the catalyst from sulfur deactivation

9

by converting H2S compounds into less threatening SO2 species 153,154.

10

(37)

11

(38)

12

Li et al. 153 experimented the reforming of a tar mixture at 550 °C on different supports

13

(γ-Al2O3, ZrO2, TiO2, CeO2, MgO) doped with 4 wt. % of Ni. The catalytic activity was found

14

to be ranked accordingly: Ni/γ-Al2O3 > Ni/ZrO2 > Ni/TiO2 > Ni/CeO2 > Ni/MgO. The pure

15

cerium support showed a remarkable anti-coking effect since the carbon deposition was as

16

follows: Ni/ZrO2 > Ni/MgO > Ni/γ-Al2O3 > Ni/TiO2 > Ni/CeO2.

17

Roh et al.

155

and Chang et al.

154

both compared the efficiency of Ni/ZrO2, Ni/CeO2

18

and Ni/Ce0.8-ZrO2.0.2 for the dry reforming of methane. In both studies, the best catalysts were

19

the ones combining CeO2 and ZrO2. Same trends were obtained by Park et al. 156 for the steam

20

reforming of benzene with different supports (γ-Al2O3, ZrO2, CeO2, ZrO2.0.25-CeO2.0.75) doped

21

with 15 wt. % of Ni. According to these authors, the fact that a cubic Ce-ZrO2 support 50 ACS Paragon Plus Environment

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Energy & Fuels

1

provides more stable catalysts than raw CeO2 or ZrO2 for Ni-based catalysts is due to the fact

2

that ZrO2 stabilizes the cubic structure at high temperatures and increases the CO2 and H2O

3

dissociation by increasing the oxygen storage capacity.

4

Weng et al. 157 investigated the dry reforming of methane with Ni catalysts supported

5

on γ-Al2O3, CeO2 and CeO2-γ-Al2O3. The catalytic performances were the best for the

6

composition Ni/ CeO2-γ-Al2O3, followed by Ni/γ-Al2O3 and finally Ni/CeO2. The authors

7

highlighted the fact that adding CeO2 to γ-Al2O3 generates strong metal-support interactions,

8

resulting in a better dispersion of Ni nanoparticles and a better resistance against sintering.

9

More recently, Quitete et al.

158

investigated the catalytic performances of different

10

supports (LaAl11O18, La0.8Ce0.2Al11O19 and CaAl12O19) doped either with 6 or 14 wt. % of Ni

11

for the steam reforming of toluene. The composition Ni (14 wt. %.)/La0.8Ce0.2Al11O19,

12

followed by Ni (6 wt. %)/La0.8Ce0.2Al11O19, showed the longer lifetimes with almost no

13

deactivation after 18 h of catalytic tests. In comparison, the composition Ni (6 wt.

14

%)/LaAl11O18 was almost completely deactivated after 10 h, thus exhibiting the beneficial

15

synergy of La and Ce doping.

16

4.4.3. Metallic active sites

17 18

4.4.3.1.

The nature of metallic active sites

19

Among the entire metallic elements commonly used, transition metals are the more

20

recurrent. By definition, a transition metal is a metal that forms one or more stable ions which

21

have not completely filled their d orbitals. This particularity enables them to change their

22

oxidation states and therefore to easily lend or take electrons from other molecules. These


Energy & Fuels

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Page 52 of 80

1

active element states possess a greater capacity to decompose tars in the produced gas in

2

comparison to their oxidized states 159.

Noble metal-based catalysts

3

•

4

Experimental results showed that group VIII metals exhibit a good resistance against

5

carbon formation during CH4 steam 160 and dry reforming 161. Nishikawa et al. 162 investigated

6

the influence of different noble metals (Pt, Pd, Rh, Ru) on the performance of Ni (12 wt. %t)

7

/CeO2 (30 wt. %)/γ-Al2O3 catalysts during the steam reforming of gasifier tar mixture. The

8

noble metal loading was very small (0.01-0.1 wt. %). The conversion yields and the carbon

9

deposit did not drastically differ between the different catalysts. The authors nevertheless

10

highlighted the fact that the Pt-doped catalyst was showing the similar performance with and

11

without the pre-reduction step. TPR and EXAFS characterizations suggested that the Pt

12

species can alloy with Ni species much easier than with other noble metals, thus enabling an

13

easier reduction of the Ni species. Furusawa et al.

14

benzene/naphthalene with either Pt (1 wt. %) or Ni (20 wt. %t) or Co (20 wt. %t) supported

15

on MgO or γ-Al2O3. Their conclusions were that Pt (1 wt. %) and Ni (20 wt. %) catalysts

16

showed similar results and in both cases higher conversions than Co (20 wt. %) catalysts.

17

Moreover, γ-Al2O3 seemed to be the most adapted support, limiting the sintering of Pt and Ni

18

nanoparticles.

19

Miyazawa et al.

163

and Asadullah et al.

164

88

studied the steam reforming of

showed the interesting tar destructing

20

properties of rhodium supported on CeO2-SiO2. According to the authors, Rh catalysts

21

exhibited higher yields, were more stable and had lower carbon deposits than Ni based

22

catalysts. Nevertheless, the authors admitted that the high price of Rh made these catalysts

23

difficult for large scale applications. Furthermore, Asadullah et al.165 also investigated the


Page 53 of 80

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Energy & Fuels

1

order of cellulose conversion with noble metals supported on CeO2 and concluded that the

2

performances were the following: Rh > Ru > Pd > Pt > Ni.

3

Recently, Dagle et al. 166 researched the synergic influence of iridium doped Ni/Al2O3

4

catalysts during the naphthalene steam reforming. More than simply increasing the catalyst

5

activity, the addition of Ir also permitted to counter the coke formation (12 % of activity loss

6

after 80 h of test for 2.5 wt. % of Ir + 15 wt. % of Ni on Al2O3 and > 40 % of activity loss

7

after 80 h for 2.5 wt. % Ir or 15 wt. % Ni / Al2O3).

Ni-based catalysts

8

•

9

Although widely used industrially, nickel is not the most effective catalyst for steam

10

reforming. However, it is the element showing the most interesting activity/price ratio

11

compared to other more precious metals such as Ru or Rh 3. The optimum nickel loading is

12

situated around 15 wt. % for impregnation and up to about 20 wt. % for precipitated catalysts

13

10

14

10,69

15

doped with an alkali (see previous parts), or alloyed with another metallic element. These

16

alloys avoid the formation of aromatic carbon precursors at the surface, the carbon dissolution

17

inside the nickel structure, or decrease the Ni-S bonds energy, in order to facilitate the

18

catalysts regeneration 98.

. Nickel active sites are extremely sensible to deactivation via coking and sulfur poisoning . In order to counter these drawbacks, Ni nanoparticles are either dispersed on a support

19

The synergy between nickel and cobalt elements for the steam reforming of toluene at 600

20

°C were investigated by Wang et al. 167. Different loadings of Co, Ni and Ni-Co supported on

21

γ-Al2O3 were tested. Surprisingly, pure Co catalysts (12 wt. %) were more efficient than pure

22

Ni catalysts (12 wt. %). Characterizations (EXAFS, XRD) highlighted the formation a

23

homogeneous Ni-Co solid solution alloy. The addition of Co to Ni showed interesting results 53 ACS Paragon Plus Environment

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Page 54 of 80

1

of up to a doping of 25 wt. %, after which a lower activity was attributed to an important

2

decrease of the Ni surface area. The synergic effects of Ni-Co were also highlighted by Zhao

3

et al. 126 who studied the steam reforming of cellulose on Ni-Co supported on SBA-15, either

4

or not doped with CaO. According to the authors, a CaO addition is very useful for the

5

cracking of large tar compounds, whereas Ni-Co bimetallic catalysts are only highly effective

6

for the cracking and steam reforming of small fragments and water-gas shift reaction.

7

Alstrup et al. 98 showed that copper and nickel easily form a solid solution which appeared

8

to reduce the carbon deposit during methane reforming. A Cu addition would hinder the

9

aromatic carbon formation at the surface and make the carbon diffusion through the Ni-Cu

10

alloy more difficult compared to pure Ni. Furthermore, several studies showed that a Cu

11

addition favors the reduction of the Ni nanoparticles by the hydrogen spillover effect. Thus, it

12

was possible to obtain metallic Ni nanoparticles and therefore a better activity at lower

13

temperatures 168.

Guan et al.

14

66

studied the steam reforming of biomass tars over Ni-Fe catalyst supported

15

on scallop shell. According to the authors, the activity of Ni-Fe/Al2O3 catalysts were high due

16

to the synergy between the activation of tar on Ni active sites and the oxygen atoms supplied

17

by the carbonaceous intermediates (CO32- ) available on neighboring Fe atoms. Ashok et al.

18

169

19

and 1:2 Fe/Al ratio. Three calcination temperatures (500, 700 or 900 °C) were applied for the

20

pure supports (Fe2O3-Al2O3) before adding Ni by wet impregnation. The authors highlighted

21

the formation of Ni-Fe alloys and explained the good results by the fact that Fe species

22

increase the coverage of oxygen species during the reforming reaction, thus enhancing the

23

degradation of toluene and suppressing coke formation. The calcination temperature

24

influenced the Ni-Fe2O3 interactions since 500/700 °C calcined catalysts showed the highest

also investigated the toluene reforming over Ni-Fe/Al2O3 catalysts with a 10 wt. % of Ni


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Energy & Fuels

1

tar yields, whereas supports calcined at 900 °C showed a high carbon deposit and lower tar

2

conversions.

3

VIB metal groups such as Mo and W are known to prevent the deactivation of Ni-based 170

4

catalysts in sulfur containing conditions

. It was found that the addition of Mo to Ni

5

decreased the interactions between Ni species and alumina, thus increasing the reducibility of

6

the Ni species. Mo species were also assumed to serve as barrier, thus preventing the growth

7

of Ni nanoparticles. Furthermore, the incorporation of a small amount of Mo permits to

8

greatly enhance the resistance of Ni catalysts towards coke formation. Indeed, Mo species are

9

supposed to decrease the coking rate and also prolong the induction period of the coke 88

10

formation

. Finally, some authors presume that MoOx species can prevent coking by

11

undergoing redox cycles during the catalytic test, thus contributing to oxygen mobility on the

12

catalyst surface

13

from MoOx species to Ni, leading to an increase in the electron density of metallic Ni, and

14

hence, the catalytic activity. Sato et al.72 researched the sulfuring resistance of Ni (25 wt.

15

%)/CaO-MgO catalysts doped with different tungsten concentrations (0 - 23 wt. %) for the

16

reforming of a toluene/naphthalene mix at 850 °C. Ni (20 wt. %)/γ-Al2O3 and Ru (2 wt. %)/γ-

17

Al2O3 catalysts were also tested under the same conditions. The authors highlighted the fact

18

that W enables the most remarkable stability of Ni against sulfur poisoning, even after 10 h of

19

testing with a 500 ppmv H2S concentration. The tungsten was supposed to promote the

20

dissociation of the Ni-S compounds and accelerate the evacuation reactions (Eq.39).

171

. Furthermore, according, to Maluf et al.

172

, an electron transfer occurs

(39)

21

22

Chaiprasert et al. 173 studied the reforming of a biomass gasifier tar mixture with Pt, Fe

23

or Co (1 wt. %) doped Ni (10 wt. %)/Dolomite catalysts. According to the author, an addition 55 ACS Paragon Plus Environment

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Page 56 of 80

1

of Pt increases the steam reforming and the Water-Gas shift yield. Addition of Fe only

2

influenced the Water-Gas shift reaction. A co-promoted catalyst was more effective for the

3

methane reforming reaction. The carbon deposition increased as follow: Ni+Pt (6.5 wt. %)
3 100 g/m )

Circulating fluidizedbed

Bubbling fluidized bed

c 189

, Skive (Danemark)

43

,

ECN (Netherlands)

p 186

, VTT (Finland)

c 43

* Details about primary and secundary tars are given in the next parts p : pilot installation c: commercial installation

Ex.: Gussing (Austria)

c 188

The system has two chambers: a gasifier and a combustor. Biomass is fed in the gasification chamber and converted into nitrogen-free syngas and char using steam. The char is burnt in air in the combustion chamber and provides heat to the inert particle bed (sand, olivine). The particles are fed back to the gasification chamber, providing the heat. A cyclone remove the flue gas (N2) whereas the syngas is evacuated upward the gasifier. Temperature are below 900 °C to avoid ashes melting 10. ⇒ - : Equipment erosion due to attrition. ⇒ + : Adapted for high installation (up to 100 MWh) / Nitrogen-free Syngas with high 187 c

Ex.: Varnamo (Sweden)

Fine bed of inert material (ex.: sand, olivine) is suspended throughout the reactor with air/oxygen/steam. Biomass is fed from the sides and burns to provide heat or reacts to form Syngas. The mixture of syngas and particles is separated using a cyclone at the top. Temperatures are below 900 °C to avoid ashes melting 10,11,43 . ⇒ - : Heat transfer less efficient than in bubbling reactor / Equipment erosion due to attrition. ⇒ + : Adapted for large installations (up to 100 MWh)

Ex.: Gothenburg (Sweden)

Fine bed of inert material (ex.: sand) at the bottom is agitated with air/oxygen/steam. Biomass is fed from the sides, mixed and combusted at temperatures below 900 °C to avoid ashes melting. Syngas is evacuated at the top 10,11,43 . ⇒ - : Biomass must be finely milled / Injection of pure oxygen. ⇒ + : Able to process high quantity by day / Very high quality syngas.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Dual fluidizedbed

Page 65 of 80 Energy & Fuels

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Page 66 of 80

1 2

Figure 10: Tar ranking as function of temperature adapted from the literature 41,47.

3 4

Figure 11: Typical composition of biomass tars (wt. %) 50,51 .

5

6 7

Figure 12: General scheme of catalytic tar steam and dry reforming inspired from the literature 65–67.

8 9


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Energy & Fuels

1 2

Figure 13: Carbon formation mechanisms proposed by Trimm et al. 82.

3

4 5

Figure 14: Catalyst design triangle introduced by Richardson 110.

6

7 8 9

Figure 15: Overview of the different elements used for bio-syngas purification catalysts 107.


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Page 68 of 80

1 2

Figure 16: Classification and types of catalysts used for tar elimination 111.

3

4 5 6

Figure 17 : Scheme of the tar decomposition mechanism and the Water-Gas Shift enhancement over an iron oxide catalyst 137.

7 8

9

10

11

12


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Energy & Fuels

1

TABLES

2 3

Table 1: Main reactions involved in the biomass gasification process 31–33. Heat of reaction (kJ/mol) at 298 K Tars general equations + . → . + . 2 2 + . → 2. + . 2 + . → 2. + + . 2 + 2 − . → . 2 → − . + . 4 4 Char combustion + → 1 + . → 2 Char gasification + ↔ 2. + ↔ + 2. + 2. ↔ + + 2. ↔ + 2. + 2. ↔ Homogeneous volatile oxidation 1 + . → 2 1 + . → 2 + 2. → + 2. 1 + . → + 2. 2 + ↔ + Methanation reactions + 3. ↔ + 2. + 2. ↔ +

4 5 6 7 8

Reaction name

Equation reference

Tars partial oxidation

1

Tars dry reforming

2

Tars steam reforming

3

Tars hydrogenation

4

Tars thermal cracking

5

-394 -111

Complete combustion Partial combustion

6 7

+173 +131 +16 -91 -75

Boudouard reaction Water-Gas reaction 1 Water-Gas reaction 2 Water-Gas reaction 3 Hydrogasification reaction

8 9 10 11 12

-283

13

-242

Carbon monoxide combustion Hydrogen combustion

-283 -35

Methane combustion Methane partial oxidation

15 16

-41

Water-Gas Shift reaction

17

-204 -248

Methanation reaction 1 Methanation reaction 2

18 19

Highly endothermic + (200 to 300)

14

Table 2: General composition of gasification gas 6,16. Gasifying agent Air Steam Steam+O2 Plasma

H2 5-16 38-56 14-32 16-52

CO 10-22 17-32 43-52 26-47

Gas product (vol. %) CO2 CH4 9-19 2-6 13-17 7-12 14-36 6-8 1-37 0

N2 42-62 0 0 3-16

H2 O 11-34 52-60 38-61 0-4

9 10 11 12 69 ACS Paragon Plus Environment

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1 2 3

4 5 6 7 8 9

Table 3: Classification of tars based on molecular weight (wt %) 49. Tar class 1 2 3

Class name

Property

GC-undetectable Heterocyclic aromatics Light aromatic (1 ring)

4

Light PAH (2-3 rings)

Very heavy tars Highly water-soluble Do not pose a problem regarding condensability and solubility Condense at low temperature, even at very low concentrations

5

Heavy PAH (4-7 rings)

Representative compounds

Condense at high temperature and very low concentration

Pyridine, phenol Toluene, benzene Naphthalene, anthracene, fluorine, indene Fluoranthene, chrysene, coronene, pyrene

Table 4: Examples of other gas contaminants 15,17,53. Contaminant Concentration Potential problem

10 11 12 13 14 15

Page 70 of 80

NH3 (ppmv) 1000-14000 Emissions

H2 S (ppmv) 20-200 Corrosion + Emissions

HCN (ppmv) 5-500

Alkali metals (Na/K) #

Emissions

Chlorine # Corrosion + Emissions

Corrosion

#: not available

Table 5 : Activation energies and constant rates from different studies 49. Commercial nickel-based catalyst

Ea, kJ/mol 3

kapp,0, (m dry/(kg/h))

Calcined dolomite

No catalysts

Tar generated in a gasifier with air

Tar generated in a gasifier with steam

Tar generated in a gasifier with air

Naphtalene

Toluene

Benzene

72

84

97

350

250

440

5

1.4 x 10

3

2.6 x 10

6

1,2.10

14

1.7 x 10

10

3.3 x 10

2. x 1016

16

17

18

19

20 70 ACS Paragon Plus Environment

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1

AUTHOR INFORMATION

2

Corresponding Author

3

* Vincent Claude, Nanomaterial Catalysts and Electrochemistry, B6a, University of Liege,

4

B-4000 Liège, Belgium. E-mail address: “[email protected]”. Tel: +32 4 366 35

5

40. Fax: +32 4 366 35 45.

6

Author Contributions

7

The manuscript was written through contributions of all authors. All authors have given

8

approval to the final version of the manuscript.

9

ACKNOWLEDGMENT

10

V. Claude thanks the F.R.S.-F.N.R.S. for his doctoral grant obtained with the “Fonds

11

de Recherche collective” n° 2.4541.12. S. D. L. is also grateful to F.R.S.-F.N.R.S for her

12

Research Associate position. The authors also acknowledge the Ministère de la Région

13

Wallonne Direction Générale des Technologies, de la Recherche et de l’Energie (DG06) for

14

financial supports.

15

16

17

18


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1

REFERENCES

2

(1)

Gasification of Plastics From Municipal Solid Waste In the United States; GBB report; 2013.

3 4

Page 72 of 80

(2)

Le bioraffinage ou valorisation optimale de la biomasse pétrole brut biomasse; Valbiom report; 2010.

5 6

(3)

Chan, F. L.; Tanksale, A. Renew. Sustain. Energy Rev. 2014, 38, 428–438.

7

(4)

Qiu, M.; Li, Y.; Wang, T.; Zhang, Q.; Wang, C.; Zhang, X.; Wu, C.; Ma, L.; Li, K. Appl. Energy 2012, 90 (1), 3–10.

8 9

(5)

Bridgwater, A. V. Fuel 1995, 14 (5), 631–653.

10

(6)

Alauddin, Z. A. B. Z.; Lahijani, P.; Mohammadi, M.; Mohamed, A. R. Renew. Sustain. Energy Rev. 2010, 14 (9), 2852–2862.

11 12

(7)

Narvaez, I.; Orio, A.; Aznar, M. P. Ind.Eng.Chem.Res. 1996, 35 (7), 2110–2120.

13

(8)

Simell, P. A.; Bredenberg, J. Fuel 1990, 69, 1219–1225.

14

(9)

Salvador, M. L.; Arauzo, J.; Bilbao, R. Energy and Fuels 1999, 13, 851–859.

15

(10)

Albertazzi, S.; Basile, F.; Brandin, J.; Einvall, J.; Hulteberg, C.; Sanati, M. Clean Hydrogen-rich Synthesis Gas; Chrisgas report; 2004.

16 17

(11)

Zhang, L.; Xu, C. (Charles); Champagne, P. Energy Convers. Manag. 2010, 51 (5), 969–982.

18 19

(12)

Kumar, A.; Jones, D. D.; Hanna, M. a. Energies 2009, 2 (3), 556–581.

20

(13)

Total World Energy Consumption by Source 2013.

21

(14)

Renewable Energy in Europe, European Renewable Energy Council report; 2010.

22

(15)

Biomass for power generation, International Renewable Energy Agency report; 2012.

23

(16)

Total; La Recherche 2011, No. 454, 85–91.

24

(17)

Asadullah, M. Renew. Sustain. Energy Rev. 2014, 29, 201–215.

25

(18)

Wild, P. J. De.; Tar measurement in biomass gasification , standardisation and supporting R & D; Energy research Centre of the Netherlands report; 2006.

26 27

(19)

oil-imports.

28 29

(20)

32

Adam, B.; Simon, M.; Dobrotková, Z. Renewable Energy Market and prospect by technology, International Energy Agency report; 2011.

30 31

Crude oil imports in EU; Web Site: https://ec.europa.eu/energy/en/statistics/eu-crude-

(21)

Renewable Power Generation Costs in 2012: An Overview; International Renewable Energy Agency report; 2012.


Page 73 of 80

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

(22)

Leung, D. Y. C.; Wu, X.; Leung, M. K. H. Appl. Energy 2010, 87 (4), 1083–1095.

2

(23)

Aydemir, E. J. Bioprocess. Biotech. 2014, 4 (7), 1–8.

3

(24)

Khan, Z.; Dwivedi, A. K.; Engineering, C.; College, U. E. Univers. J. Environ. Res. Technol. 2013, 3 (1), 1–13.

4 5

(25)

P.Basu. Biomass gasification and pyrolysis; Elsevier, Burlington, USA, 2010.

6

(26)

Effendi, A.; Gerhauser, H.; Bridgwater, A. V. Renew. Sustain. Energy Rev. 2008, 12 (8), 2092–2116.

7 8

(27)

Bridgwater, A. V. Therm. Sci. 2004, 8, 21–49.

9

(28)

Brems, A.; Dewil, R.; Baeyens, J.; Zhang, R. Nat. Sci. 2013, 5 (6), 695–704.

10

(29)

Leppdahti, J.; Koljonen, T. Fuel Process. Technol. 1995, 43, 1–45.

11

(30)

Pinto, F.; Lopes, H.; André, R. N.; Gulyurtlu, I.; Cabrita, I. Fuel 2007, 86 (14), 2052– 2063.

12 13

(31)

Gómez-Barea, a.; Leckner, B. Prog. Energy Combust. Sci. 2010, 36 (4), 444–509.

14

(32)

Diepen, A. E. Van; Moulijn, J. A. Effect of Process Conditions on Thermodynamics of Gasification; Springer-Verlag Berlin Heidelberg, 1998.

15 16

(33)

Simell, P. A.; Hepola, J. O.; Krause, A. O. I. Fuel 1997, 76 (12), 1117–1127.

17

(34)

Marin, L. S. Treatment of biomass-derived synthesis gas using commercial steam reforming catalysts and biochar, PhD Thesis, Oklahoma State University, 2011.

18 19

(35)

C.Picard. Thermochimie: exercices corrigés, De Boeck, Bruxelles; 1998.

20

(36)

Balat, M.; Balat, M.; Kirtay, E.; Balat, H. Energy Convers. Manag. 2009, 50 (12), 3158–3168.

21 22

(37)

Sridhar, G.; Paul, P. J.; Mukunda, H. S. Biomass and Bioenergy 2001, 21 (1), 61–72.

23

(38)

Huang, P.; Ju, H.; Tan, S.; Wang, H.; T.Zhao. The future of methanol fuel; The Franke Institute for Humanities report; 2015.

24 25

(39)

carbon dioxide; European Parliamentary Research Service report; 2014.

26 27

Faberi, S.; Paolucci, L. Methanol : a future transport fuel based on hydrogen and

(40)

Devi, L.; Ptasinski, K. J.; Janssen, F. J. J. . Biomass and Bioenergy 2003, 24 (2), 125– 140.

28 29

(41)

Brown, M. D.; Mudge, L. K. Biomass 1986, 11, 255–270.

30

(42)

Couto, N.; Rouboa, A.; Silva, V.; Monteiro, E.; Bouziane, K. Energy Procedia 2013, 36, 596–606.

31 32

(43)

Review of Technologies for Gasification of Biomass and Wastes; E4Tech report; 2009.

33

(44)

Vamvuka, D.; Arvanitidis, C.; Zachariadis, D. Environ. Eng. Sci. 2004, 21 (4), 525–

34

547. 73 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

(45)

European Biomass Industry Association ; Website: http://www.eubia.org/

2

(46)

Anis, S.; Zainal, Z. a. Renew. Sustain. Energy Rev. 2011, 15 (5), 2355–2377.

3

(47)

Milne, T. a; Evans, R. J. Biomass Gasifier “ Tars ”: Their Nature , Formation , and

Page 74 of 80

Conversion; U.S.A National Renewable Energy Laboratory report; 1998.

4 5

(48)

Elliott, D. C. ACS Symp. Ser. 376, pyrolysis oils from biomass 1988, 55–65.

6

(49)

Li, C.; Suzuki, K. Renew. Sustain. Energy Rev. 2009, 13 (3), 594–604.

7

(50)

Coll, R.; Salvado, J.; Farriol, X.; Montane, D. Fuel Process. Technol. 2001, 74, 19–31.

8

(51)

Abu El-Rub, Z.; Bramer, E. a; Brem, G. Ind. Eng. Chem. Res. 2004, 43 (22), 6911– 6919.

9 10

(52)

Kiel, J.; Paasen, S. Van; Neeft, J. Primary measures to reduce tar formation in

11

fluidised-bed biomass gasifiers; Energy research Centre of the Netherlands report;

12

2004.

13

(53)

Torres, W.; Pansare, S. S.; Goodwin, J. G. Catal. Rev. 2007, 49 (4), 407–456.

14

(54)

Hanaoka, T.; Inoue, S.; Uno, S.; Ogi, T.; Minowa, T. Biomass and Bioenergy 2005, 28 (1), 69–76.

15 16

(55)

957.

17 18

Barneto, A.; Carmona, J.; Galvez, A.; J.Conesa. Energy and Fuels 2009, 111 (1), 951–

(56)

Pinto, F.; Franco, C.; André, R. N.; Tavares, C.; Dias, M.; Gulyurtlu, I.; Cabrita, I. Fuel 2003, 82 (15-17), 1967–1976.

19 20

(57)

Wu, C.; Yin, X.; Ma, L.; Zhou, Z.; Chen, H. Biotechnol. Adv. 2009, 27 (5), 588–592.

21

(58)

Rabou, L. P. L. M.; Zwart, R. W. R.; Vreugdenhil, B. J.; Bos, L. Energy & Fuels 2009, 23 (12), 6189–6198.

22 23

(59)

Yu, Q.; Chen, G. J. Anal. Appl. Pyrolysis 1997, 41, 481–489.

24

(60)

Moghtaderi, B. Fuel 2007, 86 (15), 2422–2430.

25

(61)

Lv, P. M.; Xiong, Z. H.; Chang, J.; Wu, C. Z.; Chen, Y.; Zhu, J. X. Bioresour. Technol. 2004, 95 (1), 95–101.

26 27

(62)

Campoy, M.; Gómez-Barea, A.; Vidal, F. B.; Ollero, P. Fuel Process. Technol. 2009, 90 (5), 677–685.

28 29

(63)

Qin, Y.-H.; Feng, J.; Li, W.-Y. Fuel 2010, 89 (7), 1344–1347.

30

(64)

Gil, J.; Caballero, M. A. Energy & Fuels 1999, 1122–1127.

31

(65)

Garcia, L.; French, R.; Czernik, S.; Chornet, E. Appl. Catal. A Gen. 2000, 201 (2), 225–239.

32 33 34

(66)

Guan, G.; Chen, G.; Kasai, Y.; Lim, E. W. C.; Hao, X.; Kaewpanha, M.; Abuliti, A.; Fushimi, C.; Tsutsumi, A. Appl. Catal. B Environ. 2012, 115-116, 159–168. 74 ACS Paragon Plus Environment

Page 75 of 80

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

(67)

Ito, M.; Tagawa, T.; Goto, S. Appl. Catal. A Gen. 1999, 177 (1), 15–23.

2

(68)

Duprez, D. Appl. Catal. 1992, 82, 111–157.

3

(69)

Rostrup-Nielsen. In Catalysis Science and Technology; J.Anderson and M.Boudouart, Springer Berlin Heidelber New York Tokyo, 1984; pp 3–110.

4 5

(70)

74 (3-4), 211–222.

6 7

Świerczyński, D.; Libs, S.; Courson, C.; Kiennemann, a. Appl. Catal. B Environ. 2007,

(71)

Fuentes-Cano, D.; Gómez-Barea, A.; Nilsson, S.; Ollero, P. Chem. Eng. J. 2013, 228, 1223–1233.

8 9

(72)

Sato, K.; Fujimoto, K. Catal. Commun. 2007, 8 (11), 1697–1701.

10

(73)

I. Narvaez, J. Corella, A. O. Ind.Eng.Chem.Res. 1997, 3, 317–327.

11

(74)

Devi, L.; Ptasinski, K. J.; Janssen, F. J. J. G. 2005, 9096–9104.

12

(75)

Dou, B.; Pan, W.; Ren, J.; Chen, B.; Hwang, J.; Yu, T.-U. Energy Convers. Manag. 2008, 49 (8), 2247–2253.

13 14

(76)

2668–2680.

15 16

Aznar, P.; Caballero, M. A.; Gil, J.; Martı, J. A. Ind.Eng.Chem.Res. 1998, 5885 (97),

(77)

Virginie, M.; Courson, C.; Niznansky, D.; Chaoui, N.; Kiennemann, A. Appl. Catal. B Environ. 2010, 101 (1-2), 90–100.

17 18

(78)

Ferella, F.; Stoehr, J.; Michelis, I. De; Hornung, Fuel 2013, 105, 614–629.

19

(79)

Kinoshita, C. M.; Wang, Y.; Zhou, J. Ind.Eng.Chem.Res. 1995, 2949–2954.

20

(80)

Laosiripojana, N.; Sutthisripok, W.; Charojrochkul, S.; Assabumrungrat, S. Fuel Process. Technol. 2014, 127, 26–32.

21 22

(81)

Li, C.; Hirabayashi, D.; Suzuki, K. Fuel Process. Technol. 2009, 90 (6), 790–796.

23

(82)

Trimm, D. L. Catal. Today 1997, 37 (3), 233–238.

24

(83)

Rostrup-Nielsen, J.; D.Trimm. J. Catal. 1977, No. 48, 155–165.

25

(84)

Mendiara, T.; Johansen, J. M.; Utrilla, R.; Jensen, A. D.; Glarborg, P. Fuel 2011, 90 (4), 1370–1382.

26 27

(85)

Luo, J. Z.; Yu, Z. L.; Ng, C. F.; Au, C. T. J. Catal. 2000, 194 (2), 198–210.

28

(86)

Azadi, P.; Farnood, R. Int. J. Hydrogen Energy 2011, 36 (16), 9529–9541.

29

(87)

Bain, R. L.; Dayton, D. C.; Carpenter, D. L.; Czernik, S. R.; Feik, C. J.; French, R. J.; Magrini-bair, K. A.; Phillips, S. D. Ind.Eng.Chem.Res. 2005, 7945–7956.

30 31

(88)

Furusawa, T.; Saito, K.; Kori, Y.; Miura, Y.; Sato, M.; Suzuki, N. Fuel 2013, 103, 111–121.

32 33

(89)

Argyle, M.; Bartholomew, C. Catalysts 2015, 5 (1), 145–269.

34

(90)

McCulloch, D. . Catalytic hydrotreating in pretoleum refining; Applied Industrial 75 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Catalysis, 1983.

1 2

(91)

Forzatti, P.; Lietti, L. Catal. Today 1999, 52, 165–181.

3

(92)

Rodriguez, Ä. A.; Hrbek, J. A. N. Acc. Chem. Res. 1998.

4

(93)

Macleod, N.; Fryer, J. R.; Stirling, D.; Webb, G. Catal. Today 1998, 46, 37–54.

5

(94)

Bakker, W. J. W.; Rossen, J. C. P. Van; Janssens, J. P.; Moulijn, J. A. Hot Gas

6

Cleaning, Sulfiding Mechanisms in Absorption of H2S by Solids; Springer-Verlag

7

Berlin Heidelberg, 1998.

8

(95)

Rodriguez, J.; Romeo, E.; Fierro, J.; Santamaria, J.; Monzon, A. Catal. Today 1997, 37, 255–265.

9 10

(96)

Albertazzi, S.; Basile, F.; Brandin, J.; Einvall, J.; Fornasari, G.; Hulteberg, C.; Sanati, M.; Trifiro, F.; Vaccari, Biomass and Bioenergy 2008, 32 (4), 345–353.

11 12

(97)

Rostrup-Nielsen, J. J. Catal. 1984, 85, 31–43.

13

(98)

I. Alstrup. Mater. Corros. 1998, 372, 367–372.

14

(99)

Sinquefield et al. Sulfur and alkali resistant catalyst. Patent US 2007/0169412 A1,

15

Page 76 of 80

2007.

16

(100) Westmoreland, P. R.; Harrison, D. P. Environ. Sci. Technol 1976, 10 (7), 659–661.

17

(101) Kul Ryu, C.; Wong Ryoo, M.; Soo Ryu, I.; Kyu Kang, S. Catal. Today 1999, 47 (1-4),

18

141–147.

19

(102) Pérez-Martínez, D.; Giraldo, S. a.; Centeno, A. Appl. Catal. A Gen. 2006, 315, 35–43.

20

(103) Moulijn, J. A.; Diepen, A. E. Van; Kapteijn, F. Appl. Catal. A Gen. 2001, 212, 3–16.

21

(104) Novakovic, B.; Z.Vukovic; N.Jovanovic. Adv. Sci. Technol. Sinter. 1999, 347–352.

22

(105) Sehested, J. J. Catal. 2004, 223 (2), 432–443.

23

(106) Oh, G.; Park, S. Y.; Seo, M. W.; Kim, Y. K.; Ra, H. W.; Lee, J. G.; Yoon, S. J. Renew.

24 25 26

Energy 2016, 86, 841–847. (107) Yung, M. M.; Jablonski, W. S.; Magrini-Bair, K. a. Energy & Fuels 2009, 23 (4), 1874–1887.

27

(108) Shen, Y.; Yoshikawa, K. Renew. Sustain. Energy Rev. 2013, 21, 371–392.

28

(109) Xu, C.; Donald, J.; Byambajav, E.; Ohtsuka, Y. Fuel 2010, 89 (8), 1784–1795.

29

(110) Richardson, J. T. Principles of Catalyst Development; Springer, Ed.; Springer,

30

Houston, 1989.

31

(111) El-rub, Z. A.; Bramer, E. A.; Brem, G. Ind. Eng. Chem. Res. 2004, 6911–6919.

32

(112) Jung, Y.-S.; Yoon, W.-L.; Seo, Y.-S.; Rhee, Y.-W. Catal. Commun. 2012, 26 (6), 103–

33 34

111. (113) Tian, H.; Li, S.; Zeng, L.; Ma, H.; Gong, J. Sci. China Mater. 2015, 58 (1), 9–15. 76 ACS Paragon Plus Environment

Page 77 of 80

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

(114) Xu, C. (Charles); Hamilton, S.; Ghosh, M. Fuel 2009, 88 (11), 2097–2105.

2

(115) Abu El-Rub, Z.; Bramer, E. a.; Brem, G. Fuel 2008, 87 (10-11), 2243–2252.

3

(116) Xu, L.; Song, H.; Chou, L. Int. J. Hydrogen Energy 2013, 38 (18), 7307–7325.

4

(117) Zhenissova, A.; Micheli, F.; Rossi, L.; Stendardo, S.; Foscolo, P. U.; Gallucci, K.

5 6 7 8 9

Chem. Eng. Res. Des. 2014, 92 (4), 727–740. (118) Paganini, M. C.; Chiesa, M.; Martino, P.; Giamello, E.; Torino, I.; Giuria, V. J. Phys. Chem. B 2002, 106, 12531–12536. (119) Pacchioni, G.; Ricart, J. J. M.; Illas, F. J. Am. Chem. Soc. 1994, 116 (100), 10152– 10158.

10

(120) Sciences, G. Am. Mineral. 1979, 64, 32–40.

11

(121) Zamboni, I.; Courson, C.; Niznansky, D.; Kiennemann, a. Appl. Catal. B Environ.

12

2014, 145, 63–72.

13

(122) Brice, W.; Chang, L. Am. Mineral. 1973, 58, 979–985.

14

(123) Acharya, B.; Dutta, A.; Basu, P. Int. J. Hydrogen Energy 2010, 35 (4), 1582–1589.

15

(124) Delgado, J.; Aznar, M. P.; Corella, J. Ind. Eng. Chem. Res. 1997, 36 (5), 1535–1543.

16

(125) Li, C.; Hirabayashi, D.; Suzuki, K. Bioresour. Technol. 2010, 101 Suppl , S97–S100.

17

(126) Zhao, M.; Yang, X.; Church, T. L.; Harris, A. T. Int. J. Hydrogen Energy 2011, 36 (1),

18 19 20

421–431. (127) Di Felice, L.; Courson, C.; Foscolo, P. U.; Kiennemann, A. Int. J. Hydrogen Energy 2011, 36 (9), 5296–5310.

21

(128) Wang, S. Appl. Catal. B Environ. 1998, 16 (3), 269–277.

22

(129) Kuhn, J. N.; Zhao, Z.; Felix, L. G.; Slimane, R. B.; Choi, C. W.; Ozkan, U. S. Appl.

23 24 25 26 27

Catal. B Environ. 2008, 81 (1-2), 14–26. (130) Kuhn, J. N.; Zhao, Z.; Senefeld-Naber, A.; Felix, L. G.; Slimane, R. B.; Choi, C. W.; Ozkan, U. S. Appl. Catal. A Gen. 2008, 341 (1-2), 43–49. (131) Zhao, Z.; Lakshminarayanan, N.; Kuhn, J. N.; Senefeld-Naber, A.; Felix, L. G.; Slimane, R. B.; Choi, C. W.; Ozkan, U. S. Appl. Catal. A Gen. 2009, 363 (1-2), 64–72.

28

(132) Guan, G.; Hao, X.; Abudula, A. J. Adv. Catal. Sci. Technol. 2014, 1, 20–28.

29

(133) Long, R. Q.; Monfort, S. M.; Arkenberg, G. B.; Matter, P. H.; Swartz, S. L. Catalysts

30 31 32 33 34

2012, 2 (2), 264–280. (134) Michael Stöcker, Karge, J. C.; Jansen, J. W. Advance in zeolite science and applications; Elsevier, 1994. (135) Buchireddy, P. R.; Bricka, R. M.; Rodriguez, J.; Holmes, W. Energy & Fuels 2010, 24 (4), 2707–2715. 77 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2

Page 78 of 80

(136) Tao, J.; Zhao, L.; Dong, C.; Lu, Q.; Du, X.; Dahlquist, E. Energies 2013, 6 (7), 3284– 3296.

3

(137) Azharuddin, M.; Tsuda, H.; Wu, S.; Sasaoka, E. Fuel 2008, 87 (4-5), 451–459.

4

(138) Huang, Z.; He, F.; Feng, Y.; Zhao, K.; Zheng, A.; Chang, S.; Wei, G.; Zhao, Z.; Li, H.

5 6 7 8 9 10 11

Energy & Fuels 2013, 38 (34), 131115124133006. (139) Min, Z.; Asadullah, M.; Yimsiri, P.; Zhang, S.; Wu, H.; Li, C.-Z. Fuel 2011, 90 (5), 1847–1854. (140) Miyazawa, T.; Kimura, T.; Nishikawa, J.; Kado, S.; Kunimori, K.; Tomishige, K. Catal. Today 2006, 115 (1-4), 254–262. (141) S. Nwamaka-Ude. The Synthesis and Crystal Chemistry of Ca12Al14O33 doped with Fe2O3, Master’s Thesis, University of Tennesse, 2010.

12

(142) Li, C.; Hirabayashi, D.; Suzuki, K. Appl. Catal. B Environ. 2009, 88 (3-4), 351–360.

13

(143) Engelen, K.; Draelants, D. J.; Baron, G. V. Improvement of sulphur resistance of a

14

nickel-modified catalytic filter for tar removal from biomass gasification gas, Vrije

15

Universiteit Brussel report; 2001.

16 17 18 19

(144) Zhou, S.; Zhou, Y.; Zhang, Y.; Sheng, X.; Zhang, Z.; Kong, J. J. Mater. Sci. 2013, 49 (3), 1170–1178. (145) Li, J.; Ikegami, T.; Lee, J.; Mori, T.; Yajima, Y. J. Eur. Ceram. Soc. 2001, 21, 139– 148.

20

(146) Li, Q.; Ji, S.; Hu, J.; Jiang, S. Chinese J. Catal. 2013, 34 (7), 1462–1468.

21

(147) Seok, S. J. Catal. 2002, 209 (1), 6–15.

22

(148) Hou, Z.; X.Zheng; T.Yashima. React.Kinet.Catal.lett 2005, 84 (2), 229–235.

23

(149) Wei, Q.; Chen, Z.-X.; Wang, Z.-H.; Hao, Y.-L.; Zou, J.-X.; Nie, Z.-R. J. Alloys Compd.

24

2005, 387 (1-2), 292–296.

25

(150) Li, J.; Xiao, B.; Yang, R.; Liu, J. Bioresources 2005, 4 (4), 1520–1535.

26

(151) Thassanaprichayanont, S.; Atong, D.; Sricharoenchaikul, V. Adv. Mater. Res. 2011,

27 28 29

378-379, 614–618. (152) Gallego, J.; Batiot-Dupeyrat, C.; Barrault, J.; Mondragón, F. Energy & Fuels 2009, 23 (10), 4883–4886.

30

(153) Li, D.; Nakagawa, Y.; Tomishige, K. Chinese J. Catal. 2012, 33 (4-6), 583–594.

31

(154) Chang, J.-S.; Hong, D.-Y.; Li, X.; Park, S.-E. Catal. Today 2006, 115 (1-4), 186–190.

32

(155) Roh, H.-S.; Potdar, H. S.; Jun, K.-W. Catal. Today 2004, 93-95, 39–44.

33

(156) Park, H. J.; Park, S. H.; Sohn, J. M.; Park, J.; Jeon, J.-K.; Kim, S.-S.; Park, Y.-K.

34

Bioresour. Technol. 2010, 101 Suppl (1), S101–S103. 78 ACS Paragon Plus Environment

Page 79 of 80

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

(157) Wang, S.; Lu, G. Q. Appl. Catal. B Environ. 1998, 19 (3-4), 267–277.

2

(158) Quitete, C. P. B.; Bittencourt, R. C. P.; Souza, M. M. V. M. Appl. Catal. A Gen. 2014,

3

478, 234–240.

4

(159) Nordgreen, T.; Nemanova, V.; Engvall, K.; Sjöström, K. Fuel 2012, 95, 71–78.

5

(160) Jones, G.; Jakobsen, J. G.; Shim, S. S.; Kleis, J.; Andersson, M. P.; Rossmeisl, J.;

6

Abild-Pedersen, F.; Bligaard, T.; Helveg, S.; Hinnemann, B.; Rostrup-Nielsen, J. R.;

7

Chorkendorff, I.; Sehested, J.; N??rskov, J. K. J. Catal. 2008, 259 (1), 147–160.

8

(161) Rostrup-Nielsen, J.; Bak Hansen, J.-H. Journal of Catalysis. 1993, pp 38–49.

9

(162) Nishikawa, J.; Nakamura, K.; Asadullah, M.; Miyazawa, T.; Kunimori, K.; Tomishige,

10 11 12 13 14

K. Catal. Today 2008, 131 (1-4), 146–155. (163) Miyazawa, T.; Kimura, T.; Nishikawa, J.; Kunimori, K.; Tomishige, K. Sci. Technol. Adv. Mater. 2005, 6 (6), 604–614. (164) Asadullah, M.; Miyazawa, T.; Ito, S.; Kunimori, K.; Koyama, S.; Tomishige, K. Biomass and Bioenergy 2004, 26 (3), 269–279.

15

(165) Asadullah, M.; Tomishige, K.; Fujimoto, K. Catal. Commun. 2001, 2, 63–68.

16

(166) Dagle, V. L.; Dagle, R.; Kovarik, L.; Genc, A.; Wang, Y. G.; Bowden, M.; Wan, H.;

17

Flake, M.; Glezakou, V. A.; King, D. L.; Rousseau, R. Appl. Catal. B Environ. 2016,

18

184, 142–152.

19 20 21 22

(167) Wang, L.; Li, D.; Koike, M.; Watanabe, H.; Xu, Y.; Nakagawa, Y.; Tomishige, K. Fuel 2013, 112, 654–661. (168) Juszczyk, W.; Colmenares, J. C.; Śrębowata, A.; Karpiński, Z. Catal. Today 2011, 169 (1), 186–191.

23

(169) Ashok, J.; Kawi, S. ACS Catal. 2014, 4, 289–301.

24

(170) Hepola, J.; Simell, P. Appl. Catal. B Environ. 1997, 14 (3-4), 287–303.

25

(171) Malaibari, Z. O.; Croiset, E.; Amin, A.; Epling, W. Appl. Catal. A Gen. 2015, 490, 80–

26

92.

27

(172) Maluf, S. S.; Assaf, E. M. Fuel 2009, 88 (9), 1547–1553.

28

(173) Chaiprasert, P.; Vitidsant, T. Korean J. Chem. Eng. 2010, 26 (6), 1545–1549.

29

(174) Koike, M.; Ishikawa, C.; Li, D.; Wang, L.; Nakagawa, Y.; Tomishige, K. Fuel 2013,

30

103, 122–129.

31

(175) Baker, E.; Mudge, L. Catalysis in Biomass Gasification; Batelle, 1984.

32

(176) Lercher, J. A.; Bitter, J. H.; Hally, W.; Niessen, W.; Seshan, K. Stud. Surf. Sci. Catal.

33 34

1996, 101, 463–472. (177) Xu, S.; Zhao, R.; Wang, X. Fuel Process. Technol. 2004, 86 (2), 123–133. 79 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

(178) Kim, J.; Jin, D.; Park, T.; Kim, K. Appl. Catal. A Gen. 2000, 197, 191–200.

2

(179) Xia, W.-S.; Hou, Y.-H.; Chang, G.; Weng, W.-Z.; Han, G.-B.; Wan, H.-L. Int. J.

3

Page 80 of 80

Hydrogen Energy 2012, 37 (10), 8343–8353.

4

(180) Zhang, Y.; Xiong, G.; Sheng, S.; Yang, W. Catal. Today 2000, 63, 517–522.

5

(181) Zhang, L.; Wang, X.; Tan, B.; Ozkan, U. S. J. Mol. Catal. A Chem. 2009, 297 (1), 26–

6

34.

7

(182) Richardson, Y.; Blin, J.; Julbe, A. Prog. Energy Combust. Sci. 2012, 38 (6), 765–781.

8

(183) Biomass gasification plants; Volund report; 2011.

9

(184) Xylowatt Company Website: http://www.xylowatt.com/.

10

(185) Hudol Company Website http://www.hudol.co.uk/.

11

(186) Biomass gasification in the Netherlands, Energy Research Center of the Netherlands

12 13 14 15 16 17

2013. (187) Gussing renewable energy Website: http://www.gussingrenewable.com/htcms/en/werwas-wie-wo-wann/wie/thermische-vergasungficfb-reaktor.html. (188) Stahl, K.; NeerGaard, M.; Nieminen, J. Varnamo gasification plant; Skydraft report; 1999. (189) Hansson, J.; Leveau, A.; Hulteberg, C. SGC Report; 2011.

18

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