Pressurized Steam Torrefaction of Biomass - American Chemical Society

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Pressurized steam torrefaction of biomass: focus on solid, liquid and gas phases distribution Paola Brachi, Francesco Miccio, Giovanna Ruoppolo, and Michele MICCIO Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02845 • Publication Date (Web): 02 Oct 2017 Downloaded from http://pubs.acs.org on October 3, 2017

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Industrial & Engineering Chemistry Research

Pressurized steam torrefaction of biomass: focus on solid, liquid and gas phases distribution P. Brachi1, F. Miccio2,*, G. Ruoppolo1, M. Miccio3 1. Istituto Ricerche sulla Combustione CNR, P.le V. Tecchio 80, 80125 Napoli I 2. Istituto Scienza e Tecnologia dei Materiali Ceramici CNR, via Granarolo 64, 48018 Faenza I 3. Dipartimento di Ingegneria Industriale DIIn, Università di Salerno, via Giovanni Paolo II, 84084 Fisciano I * corresponding author: [email protected]

Abstract Torrefaction is a thermal pre-treatment of biomass feedstocks aimed at their conversion into a commodity solid fuel with more uniform and standard properties. Pressurized torrefaction – a novel concept – deserves consideration because of possible advantages with respect to the atmospheric process, in particular the establishment of favorable conditions for generation of valuable condensed products. The objective of this research are: i. an investigation on the feasibility of the pressurized, steam-assisted, batch torrefaction of some agro-industrial biomass residues; ii. the conversion of these latter into solid and liquid products to be used in energy production or chemical processes, with improved characteristics with respect to the raw biomass. The results reported in the article prove that the operation under pressure allows to maintain high water vapor pressure in the system, enhancing the solid biomass conversion to liquid products. The recovery of valuable liquid compounds from the solid residue proved to be further boosted by a subsequent stage of solvent extraction. The condensed liquid fraction resulting from torrefaction turns out much higher (i.e., up to 3 times ) under pressurized condition than under the atmospheric one. The influence of the water-to-fuel feed ratio on the distribution between solid and liquid fractions is also noticeable. Thermodynamic computations demonstrate a decrease of the heat duty required for the torrefaction at highest pressure (P=40 ata) due to the presence of liquid water in the final system.

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1. Introduction Nowadays, significant barriers still exist to the deployment of biomasses for energy and chemicals production and are closely related to some drawbacks of biomass compared to fossil fuels, first of all the low energy density1. Furthermore, biomass feedstocks, being of biological origin, can be subjected to biodegradation of the solid matrix, even with release of gas emissions2 during long-term storage. In most cases biomass originates from agriculture (e.g., food-like biomass, energy crops, dry and wet residues) or related industry (residual - dry and wet - biomass such as wood mill residues, agro-industrial wastes, etc.) 3, 4

, therefore it suffers of non-uniform and time-variable properties 5. Problems may be also experienced in

storage and handling of biomass particles/powders, which exhibit in some cases the tendency to be cohesive with consequent loss in flow-ability 6. Thus, a thermal process for biomass-to-energy conversion, such as combustion, gasification or pyrolysis, can be negatively affected by the poor biomass characteristics; in addition, the thermal conversion plant must be located in a site close to the biomass source and must operate in the narrow temporal window of biomass availability. On this basis, one of the most challenging aspects for developing biomass-to-energy projects is to overcome the operating and logistical limitations related to the use of raw biomass, by adopting effective pre-treatments, like pelletization and torrefaction7. These latter aim at improving the quality of the fuel as a solid and, consequently, the performance during the subsequent conversion process8. Torrefaction is a relatively new thermal pre-treatment (at around 200−300 °C) of biomass that, over the past ten years, has proved to be a technically feasible method for converting lignocellulosic feedstocks into a derived solid fuel i. with high energy density, ii. hydrophobic, iii. grindable, iv. compactable by pelletization, and v. biochemically stable 9, 10. It is well known that the main biomass components (e.g., hemicellulose, cellulose, lignin and extractives) decompose to a different extent and with a different rate in the torrefaction temperature window11, thus providing a different contribution to the mass and energy yield of the converted biomass. Actually, torrefied biomass can be produced from a variety of feedstocks different from lignocellulosics (e.g., tomato peels, sugar beet pulp and orange skins)12-15, while yielding similar product properties; this would allow the torrefied biomass to become a commodity fuel, with rather uniform properties. Techno-economic analyses of an integrated torrefaction process are being developed for evaluating the distance from autothermal operation depending on biomass moisture content16. Thus, the potential of torrefaction in next decades is expected to significantly increase 17, in particular if appropriate product standards will be adopted in a near future18. The products of pyrolysis and torrefaction may also have application in fields different from energy production: for instance biochar can be conveniently used for soil remediation 19, as well as water retention and nutrients release in agriculture20. Furthermore, the use of torrefied feedstock turned out beneficial in gasification as far as process efficiency and reduced tar yield are concerned21. ACS Paragon Plus Environment

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Atmospheric torrefaction of biomass feedstocks has been reviewed9, 17, 22 providing general indications about the influence of main operating variables. The yield in solid, liquid and gas fractions is strongly dependent on biomass type, particle size, temperature and residence time, with condensed liquids exceeding 20% of the initial biomass weight22. Findings of research on torrefaction in atmospheric conditions also proved that fluidized bed reactors are very effective for the torrefaction of some agroindustrial residues12. Pressurized torrefaction – a novel concept – deserves a new consideration because of the lack of investigations (with respect to the atmospheric case) and possible advantages, which are enabled by the more favorable conditions for formation of condensed products (solids and liquids). Furthermore, the use of pressurized reactors in industrial processes can improve efficiency and economics. Agar et al.23 studied the influence of elevated pressure on the torrefaction of wood showing that the decomposition reactions were accelerated with pressure so that a given mass yield was realized in a shorter time. Xiao et al.24 investigated the torrefaction of rice straw under three different conditions: i. atmospheric fixed bed under N2 purge, ii. N2-pressurized autoclave, and iii. piston-operated fixed bed under N2 purge. They concluded that the pressurized torrefaction was the best option for the quality of the produced char and the large reduction of tars in a subsequent stage of gasification. Wet or steam torrefaction of biomass has also been studied for energy applications (e.g., combustion, gasification and pyrolysis) with a focus on the use of wet feedstocks (e.g., cow and swine manure); since the wet process employs water in subcritical conditions as the reaction medium, the energy-intensive drying stage of a feedstock containing a high amount of moisture is eliminated25, 26. It should be mentioned that, even though the terms of wet torrefaction (WT) and hydrothermal conversion (HTC) have been sometimes used interchangeably27, there is a significant difference between them, which mostly lies in the solid product application. In fact, while wet torrefaction aims at producing upgraded solid fuels for energy applications only, HTC processes are employed mainly for producing a carbon-rich and lignite-like solid product (hydrochar)28, which can be used not only as a fuel, but also as activated carbon, soil enhancer, fertilizer, etc29. Accordingly, the energy efficiency is more critical for the steam or wet torrefaction process than for the HTC. Anyway, due to the similar operating conditions, studies on hydrothermal carbonization process (HTC) available in the literature can be beneficial and complementary to further research and development in WT of biomass for energy applications. Swine manure was subjected to hydrothermal conversion in a pressurized batch reactor30 with the aim of maximizing the oil yield from the volatile fraction of the feedstock, thus achieving a value as high as 76% by mass. Conversely, Sabio et al. 31 focused on the solid fraction obtained by HTC carbonization of tomato peel residues in the torrefaction temperature window, determining the influence of the main operating variables (i.e., temperature, residence time and water/biomass ratio) on yield and properties the produced solids. The use of water at high pressure has also been investigated with fossil fuels: H2O at hydrothermal conditions facilitates the extraction of asphaltene- and kerogene-rich components from lignite and oil ACS Paragon Plus Environment


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shales 32. Studies comparing the production of a liquid fuel through pyrolysis and the extraction in water at subcritical and supercritical conditions concluded that the presence of water has a positive impact on the yield and the quality of the extracted oil 33, 34. Apart from the improvements in energy density, hydrophobicity and grindability of biomass due to wet torrefaction, the presence of liquid water or water vapor as a reaction medium, during the thermal treatment of biomass, allows removing a portion of the incombustible material responsible for the formation of ash during combustion, in particular potassium (K) that is one of the leading contributors to clinkering during biomass combustion35. Furthermore, wet torrefaction also decreased the nitrogen and sulfur content of duckweed36 and paper sludge samples37, resulting in a low formation of sulfur dioxide and nitrogen oxides upon combustion. Instead, the results are contrasting found in the literature about the effect of liquid water or water vapor as a reaction medium on textural properties of the solid product such as surface area, average pore size, and pore volume35. The focus of this paper is on the potential of the pressurized steam torrefaction of wet agro-industrial residues as a means of converting them into solid and liquid fractions with improved characteristics with respect to the raw biomass, in view of their end-use as chemicals or fuels. The authors extended their previous research on atmospheric torrefaction by taking into account the system pressure as a further operating variable, as well as by adding a post-torrefaction step of solvent washing of solids. Applied pressure was kept at moderate values for a reasonable feasibility of a practical process application. The solvent washing with acetone of the carbonaceous solid products disclosed a route to the recovery of valuable chemicals from the condensable organic compounds deposited on the char surface. The technique and results of the experiments are discussed in the article.

2. Materials and methods 2.1. Materials Two agro-industrial residual biomass fuels have been used: 1. Tomato peels (TP) from a tomato processing industry located in Salerno (Italy) and 2. virgin olive husk (OH) from an oil mill located in Benevento (Italy). Both raw materials have moisture content higher than 50% by weight, corresponding to a water/drybiomass ratio much larger than 1. Prior to use, they were air-dried in a ventilated fume hood through 48 hours of exposure to fresh air. After air-drying and before further processing, the moisture content of samples was checked by means of a Kern DBS Halogen Moisture analyzer, which heats the sample up to 120 °C. Air-dried biomass samples were then subjected to grinding in a batch knife mill (Grindomix GM 300 by Retsch) for about 20 s at a speed as high as 3200 rpm. The milled samples were finally sieved and the 1.0-2.0 mm size fraction was selected for the torrefaction tests. Photographs of the dry biomass samples used for the experimental campaign are displayed in Figure 1. Their properties are reported in Table 1.


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Figure 1. Photographs of the air-dried biomass under investigation: tomato peels (left) and olive husks (right) Proximate analysis was carried out in a TGA 701 LECO thermogravimetric analyzer. Carbon, hydrogen, and nitrogen content was determined by using a CHN 2000 LECO analyzer. The oxygen content was finally calculated by subtraction of the ash and CHN content from the total. The higher heating value (HHV, MJ kg-1, dry basis) of raw and torrefied samples was estimated based on ultimate analyses results by means of following correlation by Channiwala and Parikh38:

HHV = 0.3491 C% + 1.1783 H% + 0.1005 S% - 0.1034 O% -0.0151 N% – 0.0211 ASH%

(1)

where C%, H%, S%, O%, N% and ASH% are weight percent of carbon, hydrogen, sulfur, oxygen, nitrogen and ash as determined by ultimate analysis on dry basis. The HHV was worked out for calculating the low calorific value (LHV, MJ kg-1, dry basis):

LHV = HHV – 2.442(8.936∙H%/100)

(2)

The higher heating values (HHV) estimated by means of the correlation of Channiwala and Parikh (Eq. 1) were compared with those obtained using a different correlation proposed by Friedl et al39. A good agreement was found between the calculated data , which confirms the reliability of the LHV values presented in Table 2. The two biomasses have very similar chemical composition, but differ for density and particle shape (Fig. 1), TP having a typical flat shape whilst OH being in form of stones.



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Table 1. Properties of dried biomass fuels. Particle size, mm

Olive Husks (OH) 1.0 – 2.0

Tomato Peels (TP) 1.0 – 2.0

Tap density, g cm-3 Low heating value, MJ kg-1 Residual moisture, %

0.350 22.5 6.2

0.091 23.8 6.0

Proximate analysis (dry basis) Volatiles, % Fixed C, % Ash, %

78.4 18.9 2.7

84.3 13.4 2.3

Ultimate analysis (dry basis) C, % H, % N, % O, % (by diff.)

54.4 6.8 0.8 35.9

58.4 7.7 1.5 30.6

(from Vlyssides et al.40) 20.5 15.9 34.9 28.7

(from Brachi et al.12)

Components (dry basis) Lignin, % Hemicellulose, % a-cellulose, % Others

24.9 52.4 17.5 5.2

2.2. Experimental Apparatus and Procedure Torrefaction tests were performed in a batch pressurized reactor of 48 mL internal volume (AISI 316 stainless steel, 21 mm inner diameter and 200 mm height), containing an ordinary glass test tube, as schematically depicted in Figure 2. A pressure indicator (PI) and a K-type thermocouple (TT) are connected to the reactor, allowing to measure the temperature (T) at the top of the glass tube and the pressure (P) inside the chamber. During each test, a dry biomass sample of weight m0 (typically 1.0 or 2.0 g) is placed on the bottom of the glass tube and a preset amount of distilled water is added to the solid in order to achieve a prefixed value (about 1 or 2) of the water-to-dry-biomass mass ratio (Y). The glass tube, the top of which is filled by a wad of ceramic fiber wool, is inserted into the steel tube and then the reactor is closed. To ensure an initial inert atmosphere, the reactor is purged with argon by injecting and discharging pressurized Ar (2-3 bar) 4-5 times through the access valve. Afterwards, the stainless steel reactor is inserted into an electric tubular furnace controlled by a temperature programmer (CARBOLITE 1200°C). Upon heat-up, the oven temperature is set at a fixed value in the range of interest for torrefaction while the temperature and pressure inside the reactor pressure vessel are continuously monitored. The holding time is counted from the moment at which the reactor temperature approaches the oven set point (± 10°C). The reactor is kept closed during the whole reaction time (1 h) allowing an increase in pressure, as ACS Paragon Plus Environment

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consequence of water evaporation and the release of volatile matter from the biomass, in parallel with gas compression by thermal effect. After the prefixed holding time (1 h) is passed, the reactor is quickly extracted from the tubular furnace and cooled in a water bath. In case of residual pressure after cooling, the gas is expanded in a 100 mL syringe by opening the exit valve for measuring the volume Vg of the generated gas. Afterwards, the reactor is opened and the glass tube recovered for subsequent treatments. Two alternative paths were followed during the experimental campaign: i) the residual solid in the tube was recovered, oven-dried and weighted, before analytical determinations, or ii) extraction of soluble compounds retained in the solid sample was accomplished by solvent washing (2-5 mL acetone addition), followed by filtration and oven-drying of the solids. In the second case, the produced liquor was subjected to analysis by an Agilent GC 7890A gas-chromatograph with a MSD 5975A detector using a DB 624 column.

The mass yields of solid, gas and liquid fractions (G) were calculated according to the following equations:

G = G =

m m

(3)

m

G = 1 − G − G

(4)

(5)

where ms, Vg, Mg and R are the mass of the dry solid residue, the evolved gas volume, the average molecular weight of the gases and the universal gas constant, respectively.



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Figure 2. Experimental apparatus for pressurized steam-assisted torrefaction

3. Results and discussion 3.1 Torrefaction behavior A typical time history of temperature and pressure for a steam-assisted torrefaction test is shown in Figure 3. The transient time to approach the prefixed test temperature is about one half (e.g., 1 h) of the overall test time, the holding time being 1h. It clearly appears that the rise of the temperature is much faster than that of pressure, the dynamics of temperature being dominated by the external heat transfer rate, the ACS Paragon Plus Environment

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change of pressure being determined by both the water evaporation rate and the kinetics of chemical conversion reactions. The above finding indicates that the kinetics of fuel conversion is rather slow and the chemical reactions take a time to achieve a final stable conversion degree (chemical equilibrium) that is much longer than the transient approach to a steady state temperature (thermal regime). The release of extractives from the biomass (both OH and TP) occurs at a relatively low temperature, about 150 °C11, 41, in series/parallel with the pyrolysis of hemicellulose and cellulose starting from 250±30 °C and 285±45 °C, respectively42. It is likely that the rather long time spent at moderately high temperature (about 280 °C) also favored the depolymerization of the lignin43 under the action of the steam, with formation of gaseous compounds: this may explain the slow but continuous increase of the pressure in Fig. 3. Therefore, it is confirmed that the residence time plays a role in the process, as already highlighted by Brachi et al.12, and may affect the products distribution (permanent gases, condensable vapors and torrefied solid) as well as their specific composition.

300

20

15 200 150

10

100 5

T 50

pressure, bar (abs)

250

temperature, °C

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


P

0

0 0

20

40

60 80 time, min

100

120

140

Figure 3. Values of temperature (diamonds) and pressure (triangles) in the reactor over the time for a typical steam-assisted torrefaction test (OH-11)

Figure 4 (panel A) shows a sample of torrefied olive husks as retrieved from the reactor, which exhibits the typical brown-black color as consequence of the undergone carbonization. In the same figure, samples of unconverted OH (Fig. 4-B) and torrefied OH (Fig 4-C) both washed with acetone are also shown. The darker color of solution in Fig. 4C clearly demonstrates that pressurized steam torrefaction of biomass generates organic condensed fractions, which can be valuably recovered, and that solids washing with acetone is an effective method for such a recovery; in comparison, the acetone solution appears only slightly colored by yellow (Fig. 4B) in the case of untreated OH, indicating a limited release of soluble compounds.



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A

B

C

Figure 4. Visual appearance of olive husks (OH) samples: Torrefied sample of OH (A); raw OH after treatment in acetone (B); torrefied sample after treatment in acetone (C)

The main results of torrefaction tests carried out with both olive husks (OH series) and tomato peels (TP series) are summarized in Table 2. The average value of the final pressure recorded at the end of the onehour test i.e., the steady state part at constant temperature, is reported in the 5th column, whereas the yields of solid, gas and liquid phases are reported in columns 6-8. These latter are normalized with respect to the amount of the dry biomass sample initially charged into the reactor. The results of the chemical analyses carried out on the solid samples were worked out in terms of O/C ratio (column 9), H/C ratio (column 10), LHV (column 14), along with the content of volatiles by proximate analysis (column 13). The tests listed in Table 2 are sorted in increasing order of temperature and H2O/dry fuel ratio; the underlined values in Table 2 indicate the changes in the operating conditions between consecutive tests. OH-06 and TP-05 tests were carried out at atmospheric pressure as reference case, by allowing the gases/vapors to expand in an analytical sack downstream the reactor. OH-09 and TP-01 tests were carried out without addition of distilled water (Y = 0) as a base case. In a few tests (i.e. OH-03, TP-01 and TP-02), the extraction with acetone was skipped, and, consequently, the solid yield accounts for both the actual residual solids and the condensable organic compounds deposited on their surface during the cooling step. In these cases, in fact, the measured values are much higher than those expected for the adopted operating conditions as a consequence of the fact that a fraction of the condensable matter remains deposited over the solid phase. A further confirmation comes from the high volatile content of these torrefied solids (column 11). Correspondingly, the liquid yield is not available in Table 2. In general, the volatile content in the torrefied solid (col. 11) is lower at the higher temperatures. Furthermore, the samples obtained under atmospheric conditions (OH-06 and TP-05) exhibit the largest volatile content and lowest liquid yield, indicating that the pressure as a direct effect on the liquid yield, i.e., converting the biomass to liquid fraction. In fact, moving from OH-06 to OH-11, the second one differing for P only, there is a marked decrease of the volatiles in the sample and increase of extracted liquids. ACS Paragon Plus Environment

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Table 2. Test conditions and results of steam-assisted torrefaction of olive husks (OH) and tomato peels (TP). 1

2

3

4

5

6

7

8

9

10

11

12

13

14

Fuel

Sample

Y

T

P

Solid

Gas

Liquid

O/C

H/C

N

Ash

Volatiles***

LHV

weight

H2O/

(°C)

(ata)

Yield

Yield

Yield

(-, db)

(-, db)

(%wt., db)

(%wt., db)

(%, db)

(MJ kg-1, db)

(g, db)

biomass

(-)

(-)

(-)

(-) OH-06*

1.98

1.02

248

1.0

0.710

0.049

0.241

0.460

1.36

0.61

3.52

72.1

21.8

OH-11*

1.01

1.03

277

16.5

0.444

0.097

0.459

0.273

1.19

1.45

5.57

65.5

25.7

OH-03**

1.00

1.99

255

15.5

0.750

-

-

0.322

1.44

1.39

-

71.7

26.5

OH-10*

1.03

2.12

277

17.5

0.411

0.096

0.493

0.232

1.15

1.46

6.95

62.9

26.5

OH-12*1

1.01

2.06

275

26

0.349

0.179

0.472

0.268

1.13

1.36

7.38

60.8

25.2

OH-09*

1.00

0.00

406

31.0

0.430

0.112

0.458

0.078

1.06

0.75

5.89

44.7

33.2

OH-08*

0.98

1.16

403

27.5

0.382

0.125

0.493

0.133

4.54

0.84

-

-

31.5

OH-07*

1.00

2.04

431

28.5

0.338

0.105

0.557

0.073

5.50

0.83

5.43

32.8

31.7

TP-05*

1.00

1.02

248

1.0

0.763

0.105

0.132

0.284

1.55

0.99

2.22

83.4

27.8

TP-01**

1.00

0.00

277

12.0

0.740

-

-

0.186

1.48

2.10

-

82.1

31.2

TP-02**

1.00

1.00

282

18.5

0.711

-

-

0.177

1.48

2.01

1.39

82.6

31.4

TP-06*

1.01

1.00

403

25.5

0.303

0.111

0.586

0.060

1.15

1.70

-

75.3

36.2

*extraction in acetone; **no extraction in acetone; ***proximate analysis; 1initial pressure of 2 ata; db = dry base



As far as the main operating variables are concerned, the liquid fraction increases with increasing temperature (T) and water-to-dry biomass ratio (Y). Since there is an obvious increment of the pressure at higher temperature, the increase in T exerts a double effect of enhancing the kinetics of conversion reactions and increasing the partial pressure of H2O in the vessel. The gas yield is quite low and always the smallest with respect to solid and liquid phases, ranging between 0.05 and 0.12 of the initial sample dry mass. For a couple of tests carried out with TP and OH, the composition of the collected gas was determined with a multiple gas analyzer (mod. GEIT GAS 3160), after dilution of the collected gas in 1L of Ar. that is taken into account in the data elaboration reported on dry, Ar-free basis. The results of the analysis take into account such a dilution and are reported on dry, Ar-free basis (see Fig. 5). They point out a largely prevailing concentration of CO2, being the main constituent of the mixture (75-78 % vol. on dry, Ar-free basis). Light combustible gases (CO, CH4 and H2) account for around 20% vol., whereas heavier hydrocarbons are present at around 1% vol. Only a small difference in composition can be appreciated between the two biomass fuels. It is worth noting that the gas yield calculations in Tab. 2 were based on an average molecular weight of the sampled gas, which was assumed to be 37.7 and 38.1 g/mol for OH and TP, respectively, as calculated from the data of Fig. 5.

80 70

concentration, % vol. (Ar free)

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

60 50

CO2 CO

40

CH4 H2

30

CnHm

20 10 0 OH

TP

Figure 5. Composition of the gas collected after two tests with TP and OH (T=380-390 °C, dry, Ar free)


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3.2 Solid fraction The solid yield upon steam-assisted torrefaction (Table 2 column 6) ranges between 0.76 and 0.30 as further shown in Fig. 6, being strongly influenced by the temperature and weakly by the water to biomass ratio 12, 41. For a temperature higher than 300 °C, the solid yield corresponds roughly to the ashes and some residual lignin (aromatic polymer) of the raw feedstock in agreement with the total decomposition of the carbohydrate polymers (cellulose, hemicellulose), already observed in previous works12, 41.

1.0

OH TP

0.8

Solid Yield, -

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


0.6

0.4

0.2

0.0 200

250

300

350

400

450

Temperature (°C) Figure 6. Solid yield as a function of temperature (before extraction with acetone)

The low heating value of the torrefied solids obtained under standard atmospheric torrefaction is generally higher than that of the raw materials as a consequence of the release of volatiles, which are richer in oxygen and hydrogen12. Similarly, the LHV values of torrefied solids arising from pressurized steam torrefaction tests are greater than that of the raw biomass, exhibiting values up to 36.2 MJ/kg (dry basis), in the case of tomato peels feedstock. The LHV values obtained for the tomato peels are comparable with those reported under similar condition by Sabio et al. 31. In general, a higher value of LHV, i.e., a greater densification of energy, corresponds to a lower char yield. LHV increases with both temperature and pressure of the tests in Table 2, as a consequence of the decreasing O/C dependence on such variables, as shown in Fig. 7. The LHV of the torrefied solids (OH) achieves values up to 1.2 and 1.7 times higher than that for atmospheric torrefaction and raw biomass, respectively.



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

Figure 7. Van Krevelen diagram including torrefied solids obtained from pressurized torrefaction tests of tomato peels and virgin olive husks. Solid symbols refer to raw materials.

Figure 8 displays the comparison of the yield in solid, gas and liquid fractions between OH and TP at same operating conditions, namely the tests OH-08 and TP-06. The produced gas volume is rather similar, accounting for around 10÷12% of the sample weight. An appreciable difference can be noted in liquid and solid fractions, since tomato peels produce a larger liquid yield (53%), if compared to olive husks (49%). Accordingly, the solid yield is lower for torrefaction tests performed on TP than OH. This outcome is mainly related to different composition of the raw materials, in particular the content of volatiles (Tab. 1).


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1.000 OH-08

0.800

TP-06 0.600

0.400

0.200

0.000 solid

gas

liquid

Figure 8. Mass yield in solid, gas and liquid fractions upon torrefaction for tomato peels and olive husks at T=400 °C, P=25÷27 ata and Y=1.

The trend of produced solid, gas and liquid fractions against the H2O/fuel ratio is proposed in Fig. 9 for tests carried out with olive husks at 400°C. It appears an appreciable impact of Y on the partitioning between solid and liquid fractions, whilst the gas yield remains at rather constant value of 11÷12%. The larger the H2O/fuel ratio, the higher is the liquid fraction, confirming an increased effect of the steam on the conversion of the solid biomass particles.

1.00 Y=0.0 Y=1.1

0.80

Y=2.0 0.60

0.40

0.20

0.00 solid

gas

liquid

Figure 9. Mass yield in solid, gas and liquid fractions upon torrefaction for olive husks at different H2O/fuel ratio (T=400 °C, P=27÷31 ata).



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

The chemical characteristics of the solids obtained upon steam-assisted pressurized torrefaction are proposed in terms of the H/C and O/C ratio, as reported in Table 2, and compared to each other in the Van Krevelen diagram (Fig. 7). Both O/C and H/C ratio decrease increasing the torrefaction temperature. The H2O/fuel ratio Y also influences the O/C ratio; in particular, a higher initial water concentration makes the O/C ratio to markedly decrease and the effect is more pronounced at higher temperature. The composition of the torrefied solids moves in the Van Krevelen diagram from the biomass region, where the raw biomasses points (solid symbols) are collocated, through to the low rank fossil fuels to reach that of coal for higher temperature and larger Y. It is worth noting that all data-points of TPs are collocated over the clouds of the Van Krevelen diagram, in line with their high LHV.

SEM images of solids at different magnification are displayed in Fig. 10. Panels A and B show the microstructure of a char particle from OH obtained for reference at a much higher temperature (i.e., T=800 °C) in Ar atmosphere that is compared in panels C and D with the microstructure of a torrefied particle as resulting from the solid fraction and the acetone washing of a test of the present investigation. A big difference can be observed between the two solids. The char exhibits pores (Fig. 10-A) and a network of tunnels inside the particle (Fig. 10-B), as a consequence of the large release of volatile matter and destruction of the biological tissue of the original biomass. Conversely, the torrefied particles still show the characteristic texture with ordered polygonal and adjacent cells (Fig. 10-D), as well as little evidence of open porosity in the microstructure. Furthermore, presence of cortical foils can be noted in Fig. 10-C under a lower magnification, probably due to a surface defoliation operated by the action of H2O. In conclusion, SEM analysis proves the mild transformation of the solid fraction that retains morphological characteristics of the biomass.


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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


Figure 10. E-SEM images at different magnification (100x and 1000x) of a char particle of OH (panels A and B) obtained at T=800 °C in Ar; torrefied OH (panels C and D) obtained at P=10 ata and T=280 °C after extraction in acetone.

3.3 Liquid fraction The semi-quantitative results of gas-chromatographic analyses carried out on the extracted liquids are reported in Fig. 11. The extraction was always carried out at normal conditions (T=25 °C and atmospheric pressure). The temperature at which the samples were produced is also indicated in the bottom of Fig. 11, being 25 °C only for raw biomass samples. As expected, the liquid fraction obtained upon extraction was a very complex mixture. The detected species have been grouped in five different classes based on their functionality,

i.e.,

aldehydes/ketones,

aliphatics,

aromatics,

esters/carboxylic

acids

and

oxygenates/phenols. Their relative amounts in terms of percentage of chromatographic area obtained during the analysis are compared in Fig. 11. The results of the analyses have been normalized with respect to 1 g of biomass sample and 10 mL of acetone. ACS Paragon Plus Environment


A different liquid composition was obtained for the two unconverted feedstocks (raw OH and TP), the differences being related to their different nature, in particular to the lignin-to-cellulose ratio (Table 1). A large mass fraction of aldehydes/ketones plus phenols was detected in the liquid extracted from raw OH while esters and carboxylic acids prevailed in the case of raw TPs. As concerns the OH series, which are sorted in ascending order of T, it results that, whatever is the amount of adopted water, the higher was the torrefaction temperature, the greater was the aliphatic fraction and the lower the amount of oxygenates/phenols (test OH-8 and OH-11; test OH-07 and OH-10 respectively). For OH-07 the total extracted species declined in comparison with OH-08 and OH-09, indicating a too much high torrefaction temperature. For tests OH-08 and OH-09, carried out at a temperature of around 400 °C, a high concentration of aromatics can be observed, being likely generated by steam-assisted thermal degradation of lignin44. The production of aliphatics is also enhanced by increasing the pressure (test OH-10). This suggest that high temperature and low water-to-dry biomass ratio improve the quality of the test products. As concerns the TP, higher temperatures promote the further transformation of the aliphatics and organic acids, lower concentration of aliphatics are observed working at about 400°C (TP-05) and also higher concentration of oxygenates/phenols. Looking at the bars of the total concentration, it is well evident that more than double values are obtained in the best cases (OH-09 and TP-06) when comparing the liquid fraction of the torrefied biomass with that of the raw material, i.e., raw OH vs. OH-09 and raw TP vs. TP-06.

400 aldeydhes/ketones concentration, mg L-1

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

aliphatics

300

aromatics esters/carb. acids

200

oxygenates/phenols total

100

0

T, °C: |- 25 -|----- 277 -----| ----- 400 -----|- 430 -|

|- 25 -|- 248 -|- 403 -|

Figure 11. Results of gas-chromatographic analyses of extracted liquids for OH and TP samples


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3.4 Heat duty The steam (pressurized) torrefaction is assumed to occur with the following pseudo-reaction step (R1), where BIOM and TORREF mean the raw biomass and the torrefied product, respectively, and only some molecular components are considered (H2O, CO2, CH4, H2).

a BIOM (s) + b H2O(l) x TORREF(s) + z CO2(g) + w H2(g) + s CH4(g)

(R1)

If the torrefaction is carried out in excess of water, H2O is also present in the final products, either as vapor H2O(g) or condensed water H2O(l), in thermodynamic equilibrium at given pressure and temperature. Thus, R1 becomes:

a BIOM (s) + b H2O(l) x TORREF(s) + z CO2(g) + w H2(g) + s CH4(g) + y H2O(g) + t H2O(l)

(R2)

a, b, s, t, x, y, w, z being stoichiometric coefficients. x/a represents the molar yield in torrefied products. The chemical formula of the pseudo-component BIOM was assumed to be C10H15.8O3.9, as computed on the basis of the ultimate analysis of tomato peels (Tab. 1). As far as the torrefied solid (TORREF) is concerned, the chemical formula hereinafter was derived from the analyses reported by Brachi et al.12 for the solid residue of TP atmospheric torrefaction, assuming no nitrogen in this pseudo-component, i.e., TORREF: C10H15O3.4 The standard enthalpy of formation may be calculated as difference between the combustion enthalpy of the elements and the heating value of the biomass or torrefied residue (Eq. 6). H0,i = XC,i HHVC + (XH,i/2-XO,i) HHVH - HHVi

(6)

where XC,i, XH,i, and XO,i are the mole content of the three elements C, H and O and HHVi the molar high heating value (dry basis) of the solid pseudo-component i. The computations yield: H0,BIOM = 232.7 kJ/mol H0,TORREF = 225.6 kJ/mol It is worth noting that H0 is positive, contrary to those of the main constituents of the biomass, lignin and cellulose, equal to -718 and -950 kJ/mol for lignin45 and cellulose46, respectively. It means that the formation of the torrefied solid requires energy, as the case of several organic compounds (e.g., unsaturated hydrocarbons and arenes47). The knowledge of the formation enthalpy and the specific heats of the compounds involved in R2 allows to determine the reaction (torrefaction) enthalpy (∆H) at a given temperature T and pressure P, the energy balance being expressed by Eq. 6. ∆H = x H TORREF|T,P + y H H2O(g)|T,P + z H CO2|T,P + w H H2|T,P + s H CH4|T,P + s H H2O(l)|T,P – a H0, BIOM - b H0, H2O(l)

where


(6)


H i|T,P = HO,i + cv( T – T0)

(7)

is the enthalpy of the generic species i at temperature T in a closed system and H0,I is its enthalpy of formation at standard conditions. For the sake of simple calculation, the x (molar torrefaction yield) coefficient was assumed equal to 0.85, corresponding to around 0.75 by mass and not far from the experimental data (Tab. 2) when considering the sum of solid and liquid fractions. Apart from the solids, thermodynamic equilibrium composition for the gas-liquid system was assumed. The determination of the equilibrium system at fixed P and T was carried out by giving the excess moles of C, H and O resulting from the balance of elements BIOM-minus-TORREF as input data to a software tool for thermodynamic computations (CEA by NASA48), allowing to calculate the moles of each gas and liquid component in the system. The results of the calculations are reported in Figure 12 in terms of heat duty, i.e., the enthalpy required for the torrefaction of 1 kg of dry biomass (TP), at different values of pressure and temperature. The heat duty is positive because of the large presence of liquid water in the initial system (Y = 1), which implies that torrefaction is an allo-thermal process. However, a gradual, but appreciable decrease of the heat duty can be noted when increasing P up to 40 ata at T=400°C. On the other side, the calculated heat duty at T=250°C undergoes a sudden decrease at P=40 ata due to the presence of liquid water in the final system, as predicted by thermodynamics.

3000 T=250°C

T=400°C

2500

heat duty, kJ/kg

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

2000 1500 1000 500 0 1

10

30

40

pressure, ata

Figure 12. Heat duty required for the torrefaction of 1 kg of dry biomass as a function of pressure and temperature (Y = 1)

Figure 13 displays the mass increment ∆H2O of H2O upon torrefaction normalized per unit mass of dry biomass (i.e., 1 kg), as obtained from thermodynamic calculations at Y = 1. ∆H2O is always positive, with just the case of atmospheric torrefaction at 400°C as an exception, where the negative value implies the


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complete conversion of the initial water into products. In most cases, H2O is produced from the hydrogen and oxygen present in the biomass. This indicates that water vapor spontaneously formed as a product would be present in the process; however, the reactor operation with H2O excess can be beneficial for improving the yield of an organic liquid fraction (see above).

0.040 250 °C

0.030

400 °C

0.020

∆H2O, kg/kg

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


0.010 0.000 1

10

30

40

-0.010 -0.020 -0.030 -0.040

pressure, ata

Figure 13. Mass increment of H2O per 1 kg of dry biomass as a function of pressure and temperature (Y = 1)



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

4. Conclusions The pressurized steam torrefaction of two biogenic residues was investigated in a batch reactor under moderate pressure conditions up to 31 ata. The resulting organic liquid fraction in the final products was much higher for pressurized torrefaction than for atmospheric process. Actually, the operation under pressure allowed the establishment of a significant water vapor pressure in the system and determined an enhanced conversion of the residual solid fraction of the processed biomass into condensed liquids. Operating at high pressure is also beneficial in decreasing the heat duty required from an external energy source to run an actual torrefaction plant. A marked influence of the water-to-dry-biomass ratio on the partitioning between solid and liquid fractions was noticed, whilst the gas yield remained at rather constant value (11÷12% by mass). Solids washing of the torrefied solids with acetone or other solvents can be an effective method for increased recovery of the condensed liquid components, as proved in most of the experiments carried out in this research. From GC analyses, the concentration of total organic species extracted in acetone from torrefied biomass achieves values up to two times higher with respect to the case of the extraction from raw biomass, in particular in conditions of higher temperature and pressure. Among extracted species, aliphatics are of interest for production of biofuels, whereas aromatics and esters can be suitable for production of chemicals. From SEM analysis it was shown that the torrefied particles still have the characteristic texture with ordered adjacent cells of the original material, proving that a mild transformation occurred to the solid biomass upon torrefaction. As further development of the research, the adoption of homogeneous/heterogeneous catalysis in the pressurized reactor represents a challenging option for improving and maximizing the production of valuable species from pressurized steam torrefaction of biomass feedstocks. Other aspects to be investigates are related to the fate of ashes and contaminants (e.g. chlorine) that would either accumulate in the char or be loss in the gas, posing problems for utilization of such fractions.


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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


Nomenclature cv:

specific heat at constant volume

G:

mass yield in solid, liquid or gas

H:

enthalpy

HHV:

high heating value

LHV:

low heating value

m:

mass

M:

molecular weight

P:

absolute pressure

R:

universal constant of gases

T:

temperature

Vg:

volume of gases

x:

torrefaction yield

X:

mole fraction

Y:

water to biomass ratio

Subscripts g:

gas

l:

liquid

s:

solid

Acronyms ASH:

ash

BIOM: biomass OH:

olive husks

TORREF: torrefied solid residue TP:

tomato peels



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Graphical abstract


Pressurized Steam Torrefaction of Biomass - American Chemical Society

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