Co-Gasification of Wood and Lignite in a Dual Fluidized Bed Gasifier

Article pubs.acs.org/EF

Co-Gasification of Wood and Lignite in a Dual Fluidized Bed Gasifier Stefan Kern,* Christoph Pfeifer, and Hermann Hofbauer Vienna University of Technology, Institute of Chemical Engineering, Getreidemarkt 9/166, 1060 Vienna, Austria ABSTRACT: Co-gasification tests were carried out with wood pellets and lignite at lignite ratios ranging from 0 to 100% in the dual fluidized bed pilot plant at Vienna University of Technology. The operating parameters for all gasification and co-gasification tests were kept constant; the input fuel power was chosen as 90 kWth while the gasification temperature was held at 850 °C. Olivine was used as the bed material. In addition to standard online measurements of the permanent gas components of the product gas, the tar content was also measured. NH3 and H2S were detected in the product gas for gasification of the pure fuels. It turned out that increasing lignite ratios in the fuel mix with biomass improved the quality in terms of reducing tar load and the possibility to adjust the main components of the product gas. The beneficial effects of lignite can be reached already at low proportions of lignite in the fuel mix with wood which will be presented in this paper.

1. INTRODUCTION The world’s energy demand is growing constantly, and the greater part of this growth is covered by oil and gas.1 Up to the past few years, coal-based gasification processes for power generation did not prove themselves to have an economic advantage compared to combustion power plants using coal or other fossil fuels. However, there is a current focus on renewable energy, and biomass gasification has been studied in detail in recent years. Since carbon dioxide emissions from biomass are perceived as being carbon neutral,2 and since coal is a fuel with high availability and is less expensive than oil, gasification of mixtures of these two oil alternatives is a natural consequence in the medium-term perspective. The combination of biomass and coal offers the opportunity to build larger plants with higher feedstock flexibility and, moreover, allows for adjusting the product gas composition according to the utilization route. Generally, co-firing is the use of different fuels at the same time in the same plant for combustion. For example, fuels that cannot be burned alone because of their low energy content (such as sewage sludge) can be burned together with other high caloric fuels, such as natural gas or coal with good performance. In industrial coal-fired power plants, co-firing can be used to reduce CO2 emissions without any loss in efficiency and with only minor changes of the plant’s settings. Therefore, only low percentages of biomass are usually used.3 Co-combustion of biomass with coal is a matter of intensive research for different applications, and several comprehensive studies exist on this topic.3−6 Co-firing can be accomplished via three different ways: direct, indirect, and parallel co-firing.6 For direct co-firing, a mixture of the standard fuel and the additional fuel is burned together in the boiler, while for parallel co-firing, a separate boiler where only the additional fuel is burned would be required. For indirect co-firing, a separate gasification unit is required. In the case of co-gasification of biomass and fossil fuels at different ratios, there have been fewer investigations on fluidized bed reactors under atmospheric and pressurized conditions, fixed bed gasifiers, and entrained flow gasifiers. An overview (synopsis) of the research carried out in this area is © 2013 American Chemical Society

given in Table 1. Particularly for the gasification of lignite with pure steam and a coal ratio range from 0 to 100%, no experience is available in the open literature. The combination of biomass and coal in the gasification reactor can be beneficial for the performance of the process. Synergistic effects during co-gasification of coal and biomass were found by Collot et al.25 and were also successfully accomplished in an industrial scale dual fluidized bed gasification plant up to a coal ratio of 22% in terms of energy.12 It was found that the process was stabilized due to the lower reaction rate of coal as well as to the reduced level of devolatilization. This effect was explained by the increased char content on the bed material that reduces the amount of recycled product gas to be burned in the combustion reactor on the one hand and the lower heat demand in the gasification reactor (as char gasification is slower compared to devolatilization) on the other hand. Co-gasification of hard coal and wood pellets with steam in a dual fluidized bed gasifier was successfully carried out for coal ratios ranging from 0 to 100% at a fuel power of 78 kW.7 Also, the influence of the gasification temperature on the performance of co-gasification with a coal ratio of 20% in terms of energy at a steam to fuel ratio of 0.8 kgH2O/kgfuel,daf8 and 1.0 kgH2O/kgfuel,daf9 was studied. Ruoppolo et al.17 reported that a coal ratio of 30% in terms of mass led to higher elutriation of fine particles with respect to wood pellets by the higher ash and fixed carbon content and lower reactivity of coal. Kumabe et al.23 found, for fluidized bed co-gasification of coal and wood, the highest hydrogen content for the gasification of pure coal, whereas the highest gas yield and cold gas efficiency were reached with a feedstock of 100% wood. Collot et al.25 performed co-pyrolysis and co-gasification of Polish coal and wood. These experimental results showed that the volatile yields matched the values calculated from the behavior of the pure fuels in a linear correlation. The change to a fuel with a low content of volatile components, such as coal, can reduce the tar amount effectively as Abu El-Rub et al.26 Received: November 2, 2012 Revised: January 15, 2013 Published: January 15, 2013 919

dx.doi.org/10.1021/ef301761m | Energy Fuels 2013, 27, 919−931

920

pressurized BFB (4−5 bar)

BFB

BFB

de Jong et al.

Li et al.

Ruoppolo et al. McLendon et al.

Kumabe et al. Hernández et al.

Fermoso et al.

fixed bed (downdraft) atmospheric and pressurized at 15 atm Fixed bed (downdraft) EF

BFB BFB

BFB

Japanese cedar, Mulia coal dealcoholized grape marc, coal-coke (50 wt%)

BFB

Mastellone et al. Pinto et al. Vélez et al.

pine wood, black coal, Sabero coal daw mill coal, straw, miscanthus Shenmu coal, pine sawdust, rice straw wood pellets, German lignite subbituminous coal, bituminous coal, sawdust low-sulfur lignite, highsulfur lignite, wood pine wood, PE, coal coal, rice husk, coffee husk, sawdust coal, petcoke, pine sawdust, olive pulp

DFB DFB DFB

Aigner Pfeifer et al. Vreugdenhil et al. Pan et al.

pressurized BFB (3 bar)

wood pellets, Polish coal wood pellets, Polish coal wood pellets, Polish coal pine wood, German lignite wood pellets, Polish coal wood chips, Polish coal beech wood, lignite

solid fuels

DFB DFB DFB DFB

reactor

Kern et al. Kern et al. Kern et al. Miccio et al.

research group

n.a. n.a.

n.a.

n.i. n.i.

quartz sand

n.i.

quartzite, dolomite, Ni-alumina

100 wt% silica sand, 95 wt% silica sand + 5 wt% limestone n.i.

n.i.

olivine olivine olivine 100 wt% silica sand, 75 wt% silica sand +25 wt% Ni-γ-alumina olivine olivine olivine

bed material

12 mmolcarbon/min 2.0−2.4 kg/h

3.3−5.5 kgdaf/h 3.9−8.1 kg/h

n.i.

31.8−43.1kg/h

2.6 kg/h

3 kg/h

1.5 MWth

0.6−2.4 kg/h

78 kW 8 MW 4.44−5.86 kg/h

78 kW 110 kW 78 kW 3.9−4.3 kg/h

fuel power fuel feeding rate

Table 1. Publications Involving Experimental Research on the Co-Gasification of Biomass and Coal

0−100 wt% 0−100 wt %

45, 90, 95 wt%

60 wt% (20 wt% wood + 20 wt% PE) 85, 94 wt%

60 wt%

65−100 wt%

30 wt%

66−100 wt%

0, 15, 25, 75, 100 energy-%

0−100 wt% (0, 20, 40, 60, 80, 100 wt%)

0−100 energy-% 12, 18, 22 energy-% 0 wt%, 28 wt%, 55 wt%

0−100 energy-% 20 energy-% 20 energy-% 30 wt%, 50 wt%

coal ratio

air + H2O air

O2 + H2O (+ N2)

O2 + H2O air + H2O

air

air + H2O

air + H2O

O2 + H2O

air + H2O

air + H2O

H2O H2O H2O air, air + H2O H2O H2O H2O

gasification agent

16 17

940−1045 °C 780 °C, 850 °C

20 21 22

23 24

750−900 °C 797−846 °C 950−1000 °C 900 °C 750−1150 °C

19

15

764−908 °C

850 °C

14

840−910 °C

18

11 12 13

870 °C 870 °C 776−801 °C

-

7 8 9 10

reference

870 °C 750−870 °C 830−870 °C 663−901 °C

gasification temperature

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process is shown in Figure 1, and a schematic drawing of the pilot plant is shown in Figure 2.

summarized; char also has the important potential to act as a catalyst for tar reduction in the gasification process. Thus, when the feedstock has a lower content of volatile components, more char is left in the reactor to be gasified and can be beneficial for tar adsorption and reduction. Fluidized bed gasification turned out to be the most suitable reactor design for biomass gasification regarding fuel particle size and ash melting behavior.27 For coal gasification, the most common technology is entrained flow gasification with oxygen and steam as the gasification agent. The disadvantage is the huge amount of pure oxygen that is required to drive the process autothermally; this is an expensive medium and, moreover, its use is ecologically questionable. The drawback of using air instead of oxygen is that a lot of nitrogen is introduced, which dilutes the product gas and lowers the heating value of the product gas to about 3−6 MJ/Nm3db.28,29 The utilization of oxygen produces a higher quality gas, but increased operating costs as the production of oxygen requires 0.25−0.30 kWh/kgO230 for an O2 purity of 99.5%. When steam or CO2 are used as the gasification agent, the product gas is also free of nitrogen and the calorific value of the gas is quite high; for steam gasification, values between 10 and 18 MJ/Nm3db can be reached.31,32 The advantage of using steam instead of CO2 is that the reactivity of steam is, on average, about four times higher than that of CO2,33 so residence times of the char in the gasification section would have to be longer, and gasification efficiency would suffer. With H2O or CO2 as the gasification agent, the process becomes allothermal, so the heat for the endothermic gasification reactions has to be provided externally. A solution for the external introduction of the heat for gasification at an industrial scale can be provided by dual fluidized bed gasification (DFB). This technology separates the combustion reactor, which provides the energy for gasification, from the gasification reactor and pure steam is used as the gasification agent. Circulating bed material between these two reactors carries the heat from the combustion reactor to the gasification reactor.34 This gasification technology has been successfully demonstrated, in Güssing and Oberwart, Austria, on the 8 and 10 MWth scale, since 2001 and 2008, respectively.35−37 Further plants in Villach, Austria,38 Klagenfurt, Austria, Gothenburg, Sweden,39 and Senden, Germany, are currently under construction or in the startup period, and will gain a fuel power of 15 MW (Villach, Senden), 25 MW (Klagenfurt), and 32 MW (Gothenburg). The Institute of Chemical Engineering operates a 100 kW DFB pilot plant for research where investigations concerning different types of feedstock,7,40−43 operating conditions,8,9 and bed material properties44,45 has been carried out for more than a decade. To gain knowledge about the influence of co-gasification of wood and lignite on the performance of the DFB system, as well as on the gas quality, gasification tests were carried out and the results are presented in this paper.

Figure 1. Basic principle of the dual fluidized bed gasification process.

Figure 2. Schematic of the dual fluidized bed gasification pilot plant at VUT.

The system physically separates gasification and combustion, with two fluidized bed reactors connected by loop seals. Hot bed material, circulating between these two reactors, carries the required heat for gasification from the combustion reactor, where residual char together with some other fuel for combustion, if required, is burned and heats up the bed material. By the loop seals, any leakage of product gas from the gasification reactor to the combustion reactor, which would cause a loss of the valuable gas, can be effectively avoided. Also, any flow of air (the fluidization agent of the combustion reactor) or flue gas through the loop seals from the combustion reactor to the gasification reactor can be eliminated as it would cause a dilution of the product gas, mainly with nitrogen. The process yields two separate gas streams at high temperatures: a high quality product gas and a conventional flue gas. The product gas is generally characterized by a relatively low

2. MATERIALS AND METHODS 2.1. Dual Fluidized Bed Pilot Plant at Vienna University of Technology. The experiments were carried out at the 100-kW dual fluidized bed gasification pilot plant located at Vienna University of Technology. The dual fluidized bed gasification system provides the heat necessary for the gasification reactions via a separate combustion reactor, with circulating bed material carrying the heat to the gasification reactor. The basic principle of the dual fluidized bed gasification 921


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content of condensable higher hydrocarbons (2−16 g/Nm3 of tars, heavier than toluene) and a high H2 content of 30−50 vol %db. The basic geometry data of the dual fluidized bed reactor system are summarized in Table 2.

separate hoppers for the pure fuels and the possibility to adjust the speed of the screw conveyor continuously, the mass flow rates and fuel ratios can be adjusted in an accurate way. The decision for feeding lignite into the bubbling bed and wood pellets onto the bed was chosen as it turned out in a previous study46 that on-bed feeding of wood pellets produces a higher amount of product gas and improves cold gas efficiency but comes with the drawback of a higher tar content. Therefore with this combination the high gas yield should be maintained while a low amount of tar is produced. 2.2. Analytics. 2.2.1. Measurement of the Main Product Gas Composition. The compositions of the product and flue gases were measured after their exit from the reactors, with the permanent gas components CH4, H2, CO, CO2, and O2 analyzed using a Rosemount NGA 2000, and N2, C2H4, and C2H6 measured via an online gas chromatograph (PerkinElmer Clarus 500). A detailed description of the measurement setup and the gas preparation for the NGA 2000 and the PerkinElmer Clarus 500 can be found in ref 46. 2.2.2. Tar Measurement. Tar was sampled isokinetically using impinger bottles, with both gravimetric and GC/MS tar subsequently determined. Tar sampling was applied discontinuously by condensing and dissolving the tar components. The measurement method was based on the tar protocol according to CEN/TS 15439,47 focusing on tars originating from biomass gasification. The applied method differs in the solvent since toluene was employed instead of isopropanol (IPA), as recommended by CEN/TS 15439. The choice of toluene allowed simultaneous detection of the product gas water content, since the latter could be measured as a separate phase in the impinger bottles. This was decided as the product gas made by steam gasification often contains a high content of unused steam, especially when wet biomass is gasified the water content in the product gas is can reach values up to 50%. The separate water phase in the impinger bottles is an uncomplicated and accurate way to quickly get information about the product gas quality and the performance of the gasification plant. Although this meant that tar components with a boiling point lower than that of toluene, such as benzene, toluene, and xylene (BTX), could not be detected, when using toluene, the separation performance for tar components larger than BTX is higher than for IPA. A schematic drawing of the arrangement of the tar sampling line is shown in Figure 3. In summary, the described measurement method produced the following data:

Table 2. Basic Geometry Data and Main Operating Conditions of the Dual Fluidized Bed System unit

gasification reactor

mm

conical bottom section with square-shaped upper freeboard section 304 (equivalent cylindrical diameter) 2.35 650−870

geometry reactor inner diameter reactor free height operable temperature range fluidization agent fluidization regime steam-to-fuel ratio bed material particle size (applicable)

m °C

steam bubbling fluidized bed μm

combustion reactor cylindrical 98 3.9 750−920 air fast fluidized bed

0.5−2.0 200−800

The pilot plant is equipped with three different hoppers in order to enable fuel feeding at different positions into the gasification reactor, as well as for the possibility of the cogasification of pure substances at any mixing ratio which was carried out for the investigations described here. • Hopper 1: For feeding of solid fuels into the bubbling bed of olivine particles. The screw conveyor introduces the fuel about 0.3 m below the splash zone of the bubbling bed. In most of the cases, this hopper is used. • Hopper 2: For feeding of solid fuels from the side into the freeboard of the gasifier. The screw conveyor introduces the fuel about 0.3 m above the splash zone of the bubbling bed. • Hopper 3: For feeding of solid fuels from the top of the gasifier (top-down feeding). This feeding position is designed for fuels with a low melting point like plastics and makes sure that the fuel particles do not come into contact with hot surfaces (free falling into the reactor) before they come in contact with the hot bed material. During the experiments carried out in the present study, hopper 1 was used to feed the lignite while hopper 2 was used to feed wood pellets, so lignite was fed into the bubbling bed while wood pellets were fed onto the bubbling bed. By using

• Gravimetric tar content • GC-MS tar content

Figure 3. Sampling line for tar, water, and entrained particulate matter (char and dust). 922


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Table 3. Chemical Composition and Mechanical Properties of the Olivine Bed Material

GC-MS tar composition Water content Char load (organic matter) Dust load (inorganic matter)

2.2.3. NH3 and H2S Measurement. For ammonia measurements, the gas was sampled in a similar way to tar, i.e. using impinger bottles, although in this case the solvent employed was diluted sulfuric acid at a temperature of about −2 °C. To avoid tar condensation in the pump, a bottle containing toluene was added after the impinger bottles filled with sulfuric acid to sample NH3. Following this procedure the concentration of ammonium ions in the sulfuric acid could be detected via a photometric method according to DIN 38 406 part 5 and ISO 7150. Hydrogen sulfide was again sampled first using impinger bottles filled with an aqueous potassium hydroxide solution at a temperature of about −2 °C. H2S values were subsequently determined potentiometrically. 2.2.4. Flue Gas Measurement. The flue gas composition (CO, CO2, and O2) was measured with a Rosemount NGA 2000. Further details regarding the measurement methods can be found in ref 46. 2.3. Balance of the Pilot Plant. All online values measured during each gasification test were recorded using a process control software program. These data could then be analyzed to calculate the mass and energy balances of the dual fluidized bed system. For this purpose, the balance tool IPSEpro was employed. IPSEpro is a stationary, equation-oriented flow sheet simulation tool that has been developed for power systems.48 Further detailed information about the program, its mode of operation, and utilization for biomass-based energy systems can be found in Pröll et al.49 2.3. Bed Material. Olivine is a naturally occurring mineral composed of silicate tetrahedra which also contains irons and magnesium in the form (Mg1‑x, Fex)SiO2, although the content of these latter two elements varies with mining location. The catalytic tar reduction effect caused by the use of olivine as a bed material has been reported by Koppatz et al. 44 Precalcination of the olivine can considerably improve catalytic activity,50,51 so in the investigations done for this study calcined olivine was used. In the present study, the olivine used in the tests was provided by the Austrian manufacturer Magnolithe GmbH. The results of XRF analysis and the mechanical properties of the olivine bed material are shown in Table 3. Due to its high hardness and high heat capacity, olivine is considered ideal for fluidized bed applications. Based on the sieve analysis of the bed material the particle sauter diameter was calculated to be dsv = 370 μm. This size corresponds to particle group B classified by Geldart.52 2.5. Feedstock. For the processing of biomass in a power plant, wood chips are mostly the designated fuel, but for the pilot plant, the pieces have to be smaller, and the quality of the fuel has to be held constant for the entire test campaign. Therefore, instead of wood chips, wood pellets according to the Austrian standard Ö NORM M 7135 were used for the tests, as it was found during previous tests that wood pellets behave like wood chips in the gasifier and the results can be compared.41 Lignite from the Rhenish lignite mining region (Germany) was used as the second solid fuel for the tests. The feedstock was provided with a particle size of 2−6 mm. This type of lignite is characterized by a relatively low content of sulfur, nitrogen, and ash compared to other types of lignite. In addition to the utilization of the pure substances, two mixtures of lignite ratios

composition

unit

olivine

Na2O MgO Al2O3 SiO2 P2O5 SO3 K2O CaO Cr2O3 MnO Fe2O3 NiO Cl others hardness particle density Sauter diameter, dsv

wt% wt% wt% wt% wt% wt% wt% wt% wt% wt% wt% wt% wt% wt% Mohs scale kg/m3 μm

0.43 46.76 0.40 39.84 0.03 0.06 0.32 0.90 0.28 0.15 10.32 0.31 0.10 0.11 6−7 2850 370

with 33% and 66% in terms of energy were tested. The proximate and ultimate analyses of these two fuels are listed in Table 4. Table 4. Proximate and Ultimate Analyses of the Feedstock lignite unit water content ash content C H N O S Cl volatile matter fixed carbon LHV

wt% wt% wt% wt% wt% wt% wt% wt% wt% wt% MJ/kg

wood pellets

dry basis

as used

dry basis

as used

4.23 65.53 3.75 0.84 25.22 0.38 0.05 51.8 48.2 24.31

18.63 3.44 53.32 3.05 0.68 20.53 0.31 0.04 42.15 39.22 19.33

0.29 50.23 6.04 0.05 43.38 0.005 0.003 86.45 13.55 18.75

6.11 0.27 47.16 5.67 0.05 40.73 0.005 0.003 81.17 12.72 17.46

3. RESULTS AND DISCUSSION 3.1. Operating Conditions. Co-gasification of lignite and wood pellets with lignite ratios ranging from 0 to 100% was performed at four operating points. Key parameters of these four tests are summarized in Table 5. For each test, a fresh batch of bed material (olivine) was used. The gasification temperature was set to 850 °C in the bubbling bed, while the input fuel power of solid feedstock was chosen as 90 kW. As the energy density of the two used fuels was different, the mass flow rates of the feedstock differed from each other for the different fuel blend ratios and also for the pure fuels. The mean temperature in the combustion reactor is a result of the required heat demand in the gasification reactor and the circulation rate. The influence of the amount of steam as gasification agent in the gasification reactor is essential for system performance and product gas quality,44 so it is mandatory to maintain the same amount of steam for the gasification of solid carbon for all operating points. During the gasification of carbonaceous solid feedstock, several heterogeneous reactions take place to convert 923


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constant between 1.2 and 1.3 kgH2O/kgC. The change in the steam-to-fuel ratio (φSF) toward higher values for higher ratios of lignite was a consequence of the different feedstock mass flow rate to maintain the same input fuel power for all tests and the different carbon content of the fuel blends. Tar was sampled at every operating point, whereas H2S and NH3 were only sampled for the gasification of the pure fuels, as their release to the product gas is relatively constant with the introduced amount of sulfur as well as nitrogen, if the gasification temperature is not changed and could, therefore, be calculated. 3.2. Product Gas Composition. The dual fluidized bed gasification process yields two separate gas streams, a product gas stream (gasification reactor) and a conventional flue gas stream (combustion reactor). The product gas primarily consists of the gas components hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), ethylene (C2H4), ethane (C2H6), and unconverted water (H2O). The difference in the sum of these main components to 100% of gas components is made up by the following components or impurities: • A nitrogen (N2) content of 0 (1)

Equation 1 reveals that for each mole of carbon in the feedstock, one mole of water is required for stoichiometric fuel conversion. However in reality, the main elements of a feedstock include not only carbon and hydrogen but also oxygen. The oxygen content of wood pellets is 44 wt%daf, while that of the coal used in the present study is 27 wt%daf. Including this fact in the overall steam gasification reaction results in eq 2: y ⎛ ⎞ Cx HyOz + (x − z) ·H 2O → x·CO + ⎜x + − z⎟ ·H 2 ⎝ ⎠ 2 for (x > z)

ΔHR ,850 > 0

(2)

Based on eq 2, theoretically, the stoichiometric demand of steam is (x − z) if complete conversion of the solid fuel (CxHyOz) is assumed. By the equations above, it can be estimated that a different amount of steam present in the system can push the reaction toward to its products or reactants. Therefore, the amount of steam introduced into the gasification reactor is referred to the introduced amount of fuel or to the introduced amount of carbon by the solid fuel. Those ratios are called the steam-to-fuel ratio (φSF) and the steam-tocarbon ratio (φSC). ṁ steam + νH 2O·ṁ fuel φSF,wt = (1 − νH2O − νash) ·ṁ fuel (3) φSC,wt =

Figure 4. Main gas components in the product gas vs lignite ratio.

well as the lower heating value. The most important change in the gas composition was observed for H2, as it increased from 32.8 vol%db for the gasification of pure wood, nearly linearly up to 49.4 vol%db for lignite. Consequently, all of the other permanent gas components decreased with higher lignite ratios. CO decreased from 34.7 to 29.5 vol%db, CO2 decreased from 14.6 to 12.9 vol%db, and the CH4 content went from 10.3 to 4.4 vol%db. Also, C2H4 decreased from 2.7 to 0.7 vol%db while C2H6 was nearly unaffected by the different feedstock as its content in the product gas was low, around 0.1 vol%db for all lignite ratios. The significant increase of the H2 content and the simultaneous

ṁ steam + νH 2O·ṁ fuel νC·ṁ fuel

(4)

During the test series for the co-gasification of lignite and wood pellets, the steam-to-carbon ratio (φSC) was kept nearly 924


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of the fuel particles in the gasification reactor and therefore better carbon and water conversion rates, as well as an increased amount of product gas. However, more external fuel to the combustion reactor is needed. Inorganic (dust) as well as organic (char) matter entrained with the product gas were also measured. The results are shown in Figure 6. It can be seen that no constant trend of those

Figure 5. Ethane and ethylene in the product gas and LHV of the product gas vs lignite ratio.

decrease of CO, CO2, and the higher hydrocarbons in the product gas can be explained with the composition of the fuel respective to the fuel mix. As can be seen in Table 4, the carbon content of lignite is higher than the carbon content of the wood pellets. Further, the content of volatile matter of the wood pellets is around twice as high compared to the value for lignite. This leads to the assumption that by higher lignite ratios in the fuel mix the contribution of devolatilization to the product gas formation process decreases while char gasification tends to play the leadership role for product gas formation. As shown by eq 1 and eq 2 the main gas products by char gasification are H2 and CO, so theoretically their production can be forced by a higher lignite ratio in the fuel mix. Nevertheless, the limited effect for CO is caused by the high oxygen content of the wood pellets which influences the equilibrium of CO to higher values for low lignite ratios. The net effect of these changes on the lower heating value (LHV) was a constant decrease from 14.23 to 10.95 MJ/Nm3db. These values for the co-gasification of lignite and wood show the significant influence of the feedstock on the product gas composition. A suitable mixture of these fuels can be chosen to obtain the required product gas composition. For any downstream utilization of the product gas beyond direct application in a boiler or gas engine for heat and power production, such as methanation, Fischer−Tropsch, or mixed-alcohol synthesis, the H2-to-CO ratio is essential. With a fuel mix of wood and lignite, H2/CO ratios between 0.9 and 1.7 can be reached. As explained before, for the tests carried out in this test campaign, the lignite was fed into the gasification reactor at the half height of the bubbling bed while the wood pellets were fed into the freeboard above the splash zone of the bubbling bed. The point of fuel feeding has an effect on the product gas composition and the performance of the system, which was studied earlier46 and must not be neglected. The feeding position above the surface of the bubbling bed caused a reduced contact time of the devolatilization products with the hot bed material which acts as a catalyst and promotes homogeneous reactions of the gaseous products such as steam reforming, water-gas shift, tar reforming, and tar cracking reactions.53 Goméz-Barea et al.54 concluded that, compared to in-bed feeding, a fuel feeding position above the bubbling bed leads to a product gas that shows more characteristics of a pyrolysis gas. This is also one of the reasons for the comparably low content of hydrogen and (as discussed later) the high tar levels for wood pellet gasification. The benefit of a feeding position above the bubbling bed is the increased residence time

Figure 6. Entrained dust and char in the product gas vs lignite ratio.

values was found with the lignite ratio. The values for dust as well as for char were found to be in the same range between 7 and 17 g/Nm3db. The amounts of H2S and NH3 were measured during gasification of the pure fuels (lignite ratios of 0% and 100%) and the results are summarized in Table 6. These impurities are Table 6. Detected Concentrations of Ammonia and Hydrogen Sulfide in the Product Gas value

unit

wood pellets

lignite

NH3 H2S

ppmv ppmv

856.5 21.5

4709.5 762

formed from the sulfur and nitrogen present in the solid fuel. Previous research showed that nearly all of the introduced nitrogen is released in the gasification reactor mainly as ammonia and the release of H2S in the product gas is between 50 and 90% of the amount of sulfur introduced with the feedstock; the rest of sulfur leaves the system as SO2 in the combustion reactor or is captured in the ash.7,8 The values of NH3 and H2S are increasing with increasing ratios of lignite in the fuel mixture as the content of S and N are much higher in lignite compared to wood pellets. The lignite ratio also influenced the amount of product gas, as shown in Figure 7. The product gas flow rates are calculated by a mass and energy balance of the system. The total amount of product gas stayed more or less at the same level (around 19.5 N m3db/h), but the product gas-to-fuel ratio differed significantly for the applied lignite ratios. The product gas-tofuel ratio increased constantly with increasing the lignite ratios from 1.15 to 1.47 N m3db/kgdaf. The reason for this is mainly caused by the lower fuel feeding rate for higher lignite ratios maintaining the fuel power of 90 kW which is a consequence of the higher heating value and higher carbon content of lignite. Nevertheless, it demonstrates the suitability of lignite for steam gasification. While, as explained before, the steam-to-carbon ratio was kept constant for the tests, the water content of the product gas showed significant changes (Figure 8). A steady decrease with 925


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The conversion of carbon in the gasifier to gaseous products can also be used as a key figure for the performance of the gasification process. This value is the carbon conversion. For a dual fluidized bed gasifier, one has to distinguish between the carbon conversion to product gas in the gasification reactor itself and the conversion of carbon of the whole system to product gas and flue gas. In the first case, the carbon conversion is the ratio of carbon leaving the gasification reactor in the form of gaseous products in the product gas stream to the amount of carbon introduced by the feedstock (eq 6). ⎛ ṁ CPG ⎞ XC = ⎜ ⎟ ·100 ⎝ νC·ṁ fuel ⎠

(6)

XC can be used as a kind of parameter for the determination of the amount of char that leaves the gasification reactor to the combustion reactor, neglecting the small amount of char present in the product gas stream. The trends, plotted in Figure 7, strengthen the data in Figure 8 and Figure 9. With increasing lignite ratios in the fuel mix, the water-consuming char gasification reactions were enhanced while fuel devolatilization decreased by the lower content of volatile matter in the fuel mix. Mainly the lower energy and gas density of H2, compared to the typical devolatilization products (CH4, C2H4, C2H6) led to a higher volume flow rate and therefore higher specific product gas yields. Also more water was used by the enhanced char gasification reaction. 3.3. Tar Content. Tar was sampled according the methodology described in Section 2.2.2, so gravimetrically and GC/MS detectable tar components can be listed separately. The results of the tar measurements for the cogasification tests can be seen in Figure 10.

Figure 7. Total and specific product gas flow rate vs lignite ratio.

Figure 8. Product gas H2O content vs lignite ratio.

increased lignite ratios was observed. This leads to the conclusion that more water was consumed for the gasification and steam reforming reactions for lignite than for wood pellets. This goes along with the converted carbon in the gasification reactor. Both values, carbon conversion (XC) and relative water conversion (XH2O,rel) are shown in Figure 9. Relative water conversion is defined as the amount of water consumed per mass unit of fuel fed into the gasifier (eq 5). ṁ H2O,con X H2O,rel = (1 − νH2O − νash) ·ṁ fuel (5)

Figure 10. Gravimetric and GC/MS tar content in the product gas vs lignite ratio.

The data show that the tar contents decreased with higher lignite ratios. In the case of gravimetrically detectable tars, the tar content in the product gas decreased from 9.7 g/Nm3db for the gasification of wood pellets to 0.8 g/Nm3db for the gasification of pure lignite, which is a reduction of 92.0%. The situation was similar for the GC/MS detectable tars where a reduction of 82.2% was reached; from 16.8 to 3.0 g/Nm3db. Taking a closer look at the trends in the tar content in Figure 10, it can be seen that the most rigorous abatement of gravimetric and GC/MS tars occurred with an increase in the lignite fraction from 0 to 33%. During this step, the GC/MS tars decreased by about 59.1% (down to 6.9 g/Nm3db at a

Figure 9. Carbon conversion and relative water conversion vs lignite ratio. 926


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The super ordinated groups of the GC/MS tar are plotted in Figure 11. There it can be seen that, with the exception of

lignite fraction of 33%) and the gravimetric tars went down even more, by about 74.5% (down to 2.5 g/Nm3db for a lignite ratio of 33%), so even a low amount of lignite in the fuel mix can significantly increase the gas quality with regard to tar levels. This can be explained due to several reasons: • The feeding position of the wood pellets (on bed feeding) causes in general higher tar levels. In a previous study, the feeding positions were compared for wood pellets under the same operating conditions. It was found that if the wood pellets were fed into the bed, the gravimetric tar content was about 1.5 g/Nm3db and the GS/MS tar was found to be 7.2 g/Nm3db.46 Nevertheless, the tar values for lignite are many times lower than for wood pellets, even if they would also be fed into the bubbling bed like the lignite in this study. • The fuel analysis shows that the lignite has a lower content of volatile components and a higher value for fixed carbon. Therefore, lignite causes less gaseous products to be made by devolatilization and more made by the gasification of carbon. As, in general, many tar components are identified to be caused by devolatilization products of the solid fuel in the fluidized bed,55 it is a logical effect that lignite causes less condensable hydrocarbons in the product gas. • The higher content of fixed carbon in the lignite compared to wood pellets causes a higher char load in the fluidized bed. Abu El-Rub et al.26 summarized that char particles can act as a catalyst in the gasification reactor as they are capable of adsorbing higher hydrocarbon tars and promote their cracking and reforming. • A very interesting aspect is the catalytic effect of the fuel ash. It was found that certain ash components can act as a catalyst if they are present in the system in a sufficient amount. In a previous investigation into the gasification of lignite, it was identified that the ash of lignite is catalytically active and the main components were found to be mainly calcium, magnesium, iron, and sodium.56 In the case of co-gasification described here, a lignite ratio of 33% in terms of energy seems to provide enough ash (about 0.2 kg/h) for a contribution of the beneficial effect of tar reduction. By GC/MS analysis the individual tar components were detected. A classification of these tar components can be made using several factors. Milne et al.57 categorized the components into primary, secondary, and tertiary tar components, depending on their temperature of formation. Another classification can be made when the individual components are dedicated to their super ordinated groups.58 These groups are phenolic compounds, furans, aromatic compounds, and polyaromatic hydrocarbons (PAH). Naphthalene, as the most dominant and most stable tar component, would belong to the group of PAH. To make the content of naphthalene visible, it will be removed from the group of PAH and treated separately. A third, widely used, classification is the characterization according to ECN.59 Here, tars are classified into five tar classes. Class I contains tars that are nondetectable by GC/MS as they condense at high temperatures (gravimetric tar). These tars have a high molecular weight. The other tar classes (II to V) are lighter hydrocarbons that are present in the tar. The complete list of the tar components and their affiliation to the groups mentioned before can be found in 44.

Figure 11. GC/MS tar components according to Wolfesberger et al.58 vs lignite ratio.

naphthalene and a small amount of furans, the groups are decreasing in their share of the total amount with increasing lignite ratios. This trend goes along with the allocation of the GC/MS tar components to the ECN groups, provided in Figure 12, where class II and in particular class IV tar

Figure 12. GC/MS Tar classification according to ECN59 vs lignite ratio.

components raise with higher lignite ratios. The classification into primary, secondary, and tertiary tars, given in Figure 13, shows a slight decrease of tertiary tars while secondary tar components rise with more lignite in the fuel blend. However, it has to be kept in mind that some of the tar components with a significant share, such as naphthalene, belong to secondary and tertiary tars. As mentioned in Section 2.2.2, tar components with a boiling point lower than that of toluene, such as benzene, toluene, and xylene (BTX), could not be detected with the used measurement method. Nevertheless, to give an insight about the typical values of benzene, toluene, and xylene can be given for an operating point of pure wood pellets gasification at a gasification temperature of 850 °C and a steam-to-fuel ratio (φSF) of 0.8 kgH2O/kgdb.60 In this previous study the benzene content in the product gas was found to be between 65 and 105% of the GC/MS tar (detected by the employed method here) while the value for toluene varied from 10 to 20%, and 927


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calculation of this value should take into account the fact that pilot plants do not usually reach the low ratio of heat losses seen in industrial plants. In the case of the present dual fluidized bed pilot plant, heat losses amounted to nearly 20% of fuel input. Stidl61 calculated the heat losses occurring via radiation for the main components of the 10 MWth dual fluidized bed gasification plant in Oberwart, Austria.37 Based on these data, radiation heat loss for a typical industrial plant can be assumed to be 2% of input fuel power. Effective use of cold gas efficiency data can be achieved based on the calculations for an industrial-scale plant, as per eq 7. ηC = Figure 13. GC/MS tar classification according to Milne ratio.

57

(Pfuel,G

̇ ·LHVPG VPG + Pfuel,C − Q̇ PP + Q̇ IP) ·3600

(7)

The cold gas efficiency was, in general, higher for lower lignite ratios. Nevertheless, a constant trend could not be found with the data obtained here. The highest efficiency was reached with the addition of 33% of lignite in terms of energy to the gasification of wood pellets. This highlights the fact that, as discussed before, the addition of a low amount of lignite is beneficial to the whole system. In general, for steam gasification, the water−gas shift reaction is favored toward taking place in the reactor as it increases H2 yield and promotes water conversion. Analysis has proven that, as a result of this reaction, other reactions that promote the decomposition of tar compounds also take place.53 To quantify the distance to equilibrium, the model parameter can be expressed as the logarithm of the ratio of the actual partial pressure product to the equilibrium constant (eq 8). If

vs lignite

xylene was found to be in a range from 2.5 to 20% of the total GC/MS tar. Thus, if IPA would have been used as a tar solvent, the GC/MS values would have been higher as BTX would be included. 3.4. Specific Data of the Tests. To sum up the results and some specific numbers of gasification and co-gasification of wood pellets and lignite, characteristic values are listed in Table 7. In the upper section of this table, characteristic values, such as the product gas volume flows, specific gas yields, and heating values, are summarized from the data discussed in Section 3.2. An evaluation of the efficiency of the gasification system was carried out by determining the cold gas efficiency. The Table 7. Specific Data of the Accomplished Tests value total product gas yield H2O content in product gas dry product gas yield specific product gas yield lower heating value product gas product gas powerexcl. tar specific H2 yield specific CO2 yield specific CO yield specific CH4 yield specific C2H4 yield cold gas efficiency, ηc logarithmic deviation from CO-shift equilibrium, pδeq,CO‑shift abs. water conversion, XH2O relative water conversion, XH2O,rel relative water conversion, XH2O,rel stoichiometric steam demand stoichiometric steam demand ratio of water conversion to stoichiometric steam demand specific tar content, GC/MS specific tar content, grav. specific tar content, GC/MS specific tar content, grav. tar intensity per kWh of syngas, GC/MS tar intensity per kWh of syngas, grav. superficial velocity gasification reactor, Ug superficial velocity combustion reactor, Uc mean gas residence time freeboard gasifier, τf mean gas res. time combustion reactor, τc

unit Nm3/h vol% Nm3db/h Nm3db/kgfuel,daf MJ/Nm3db kW Nm3H2/kgfuel,daf Nm3CO2/kgfuel,daf Nm3CO/kgfuel,daf Nm3CH4/kgfuel,daf Nm3C2H4/kgfuel,daf % % kgH2O/kgfuel,dry kgH2O/kgfuel,daf,N,S,Cl free kgH2O/kgfuel,daf,N,S,Cl free molH2O/kgfuel,daf,N,S,Cl free kgH2O/kgH2O g/kgfuel,daf g/kgfuel,daf g/kgcarbon g/kgcarbon g/kWhsyngas g/kWhsyngas m/s m/s s s 928

0% lignite

33% lignite

66% lignite

100% lignite

30.7 35.8 19.9 1.1 14.2 78.6 0.43 0.16 0.37 0.12 0.05 69.4 −0.49 20.29 0.14 0.14 0.27 14.76 0.52 19.34 11.18 38.39 22.19 4.26 2.46 0.47 9.0 4.1 0.9

29.0 28.6 20.7 1.3 13.5 75.8 0.56 0.16 0.42 0.11 0.03 75.2 −0.34 36.58 0.28 0.28 0.39 21.89 0.72 8.97 3.23 16.21 5.83 1.88 0.68 0.43 8.5 3.9 0.9

30.5 28.5 21.8 1.5 12.0 72.9 0.74 0.20 0.44 0.09 0.02 76.8 −0.22 42.4 0.46 0.46 0.55 30.47 0.84 8.08 2.32 13.20 3.79 1.61 0.46 0.41 9.3 4.1 0.8

26.0 19.1 21.8 1.7 11.0 66.8 0.82 0.26 0.49 0.06 0.01 77.3 0.16 55.47 0.55 0.55 0.74 41.06 0.75 4.96 1.29 7.25 1.88 0.98 0.25 0.41 8.9 4.1 0.9


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pδeq,CO−shift < 0, the actual state is still on the side of the reactants, and so further reaction is thermodynamically possible. If pδeq,CO−shift = 0, the water−gas shift equilibrium is fulfilled by the product gas composition, whereas if pδeq,CO−shift > 0, the actual state is on the side of the products. The latter scenario cannot be reached thermodynamically by means of the water−gas shift reaction, but is instead as a result of the products produced by devolatilization and the other gasification reactions, as well as the water content of the product gas. ⎡ ⎤ ∏i pi ⎥ pδeq,CO − shift(pi , T ) = log10⎢ ⎢⎣ K p ,CO − shift(T ) ⎥⎦

extent with one-third of lignite in the fuel mix. This strengthens the fact that primary co-gasification has positive effects on the system and opens an additional degree of freedom for influencing the product gas composition by simple adjustment of the fuel mixture.

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*Tel.: +43 1 58801 166382. Fax: +43 1 58801 16699. E-mail: [email protected].

νi

Notes

The authors declare no competing financial interest.

(8)

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KP,CO‑shift is the equilibrium constant of the water−gas shift reaction and can be determined via several methods, such as HSC Chemistry.62 In the scenario of the gasification of pure wood pellets, the lower content of hydrogen and the higher amount of steam left in the product gas are responsible for the observed negative deviation from the water−gas shift equilibrium pδeq,CO−shift < 0. pδeq,CO−shift moves toward 0 with higher lignite ratios and reaches practically zero with pure lignite. This effect can be explained with the constantly decreased water content of the product while the hydrogen content increased and the carbon dioxide content decreased with higher lignite ratios. For steam gasification, the amount of introduced water consumed for gasification and steam reforming reactions is an indicator of the whole process. This value is called water conversion, as was explained before with eq 5. The absolute water conversion sets the consumed amount of water into a relation with the introduced amount of water (eq 9). ⎛ ⎞ ṁ H2O,con ⎟⎟ ·100 X H2O = ⎜⎜ ⎝ νH2O·ṁ fuel + ṁ H2O,steam ⎠

ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support granted by the European Commission as this study was carried out within the framework of the Fecundus project, funded by the Research Fund for Coal and Steel of the European Union (contract RFCR-CT-2010-00009).

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2

ABBREVIATIONS

Symbols

dP = Particle size by sieve analysis, μm dSV = Mean Sauter diameter, μm ΔHR,850 = Heat of reaction at 850 °C, kJ/mol Kp,CO−shift = Equilibrium constant of CO-shift, LHVPG = Lower heating value of the product gas (dry), MJ/ Nm3db ṁ H2O,actual = Actual mass flux of steam in the gasification reactor, kg/h ṁ H2O,stoich = Stoichiometric mass flux of steam in the gasification reactor, kg/h ṁ H2O,steam = Mass flux of steam in the gasification reactor, kg/ h ṁ fuel = Mass flux of solid fuel into the gasification reactor, kg/ h ṁ H2O,con = Amount of water that is converted to product gas, kg/h ṁ HPG = Carbon flux in the product gas stream, kg/h Pi = Actual measured gas phase partial pressure of the species i, Pa pδeq,CO−shift = Logarithmic deviation from CO-shift equilibrium, Pfuel,G = Input fuel power of solid fuel into gasification reactor, kW Pfuel,C = Input fuel power of fuel for combustion reactor, kW Q̇ PP = Heat loss of the pilot plant, kW Q̇ IP = Heat loss of an industrial size plant with the same power of the pilot plant, kW T = Temperature, °C Ug, Uc = Superficial gas velocity in gasification reactor (g) and combustion reactor (c), m/s V̇ PG = Volumetric flow rate of product gas (dry), Nm3db/h XC = Carbon conversion in the gasification reactor, % XH2O,rel = Water conversion in the gasifier, related to the fuel input, kgH2O/kgfuel,daf XH2O = Water conversion in the gasifier, related to total amount of introduced water, % x = Molarity of carbon in the fuel (dry, ash, N, Cl and S free basis), mol/kgC,H,O y = Molarity of hydrogen in the fuel (dry, ash, N, Cl, and S free basis), mol/kgC,H,O

(9)

The stoichiometric steam demand for the applied fuel mixture can be extracted from eq 2 and leads to eq 10:

ϕH O = (x − z)

AUTHOR INFORMATION

Corresponding Author

(10)

The mean gas residence times of the product gas in the freeboard of the gasification reactor and of the flue gas in the combustion reactor result from the superficial velocities in the freeboard of the gasification reactor and in the combustion reactor and the respective square sections. These values were in the same range for all tests as the operating conditions were kept constant and the total gas volume flow rates were in a similar range for the investigated fuel blends.

4. CONCLUSION Dual fluidized bed gasification is a fuel-flexible technology capable of the conversion of carbonaceous feedstock into high quality product gas. Originally designed for wood chips, it has turned out that fuel flexibility is a key issue for economic breakthrough. The combination of coal with biomass offers various advantages. This study shows that the system can handle lignite as a fuel without any drawbacks with lignite ratios up to 100%. It turned out that, despite a lower gas yield and a slightly lower efficiency, the gas quality with regard to tars was massively improved. Ammonia and hydrogen sulfide increased according to the amount of nitrogen and sulfur in the feedstock. The co-gasification tests showed that the obtained advantages with lignite as a fuel for gasification already appeared to a large 929


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z = Molarity of oxygen in the fuel (dry, ash, N, Cl, and S free basis), mol/kgC,H,O Greek Letters

δ = Deviation, ϕ = Sphericity, ϕH2O = Stoichiometric H2O demand, molH2O/kgdaf,N,S,C,l free, kgH2O/kgdaf,N,S,C,l free ηC = Cold gas efficiency, φSF,wt = Steam-to-fuel ratio, kgH2O/kgfuel,daf, φSC,wt = Steam-to-carbon ratio, kgH2O/kgC, τf = Product gas residence time in the freeboard of the gasification reactor, s τc = Gas residence time in the combustion reactor, s νash = Ash mass fraction in the fuel, νC = Carbon mass fraction in the fuel, νH2O = Water mass fraction in the fuel, Abbreviations and Subscripts

BTX = Benzene, toluene, xylene c = Carbon, cold gas (efficiency), combustion reactor CHP = Combined heat and power plant daf = Dry and ash free basis db = Dry basis DFB = Dual fluidized bed f = Freeboard g = Gasification reactor GC/MS = Gas chromatography mass spectrometry IPA = Isopropanol n.a. = Not applicable n.i. = Not indicated PAH = Polycyclic aromatic hydrocarbons PG = Product gas sv = Surface volume

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Co-Gasification of Wood and Lignite in a Dual Fluidized Bed Gasifier

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