Energy Fuels 2010, 24, 46–52 Published on Web 08/06/2009
: DOI:10.1021/ef9004797
Thermal Decomposition of Wheat Straw and Mallee Residue Under Pyrolysis Conditions† Cara J. Mulligan,‡,§ Les Strezov,§ and Vladimir Strezov*,‡ ‡
Graduate School of the Environment, Department of Environment and Geography, Macquarie University, Sydney NSW 2109, Australia, and §The Crucible Group, PO Box 183, Mayfield NSW 2304, Australia Received May 18, 2009. Revised Manuscript Received July 12, 2009
The pyrolysis behavior of wheat straw and mallee residue and resulting gases, liquids, and chars were examined. The specific heat and thermal conductivity of both species were measured using computer-aided thermal analysis at heating rates of 10 and 100 °C/min to a temperature of 1000 °C. The sample decomposition was also measured by thermogravimetry. Gas chromatography detected evolved gases, and the bio-oils were characterized using GC-MS. Chars were examined using FTIR, proximate, and ultimate analysis. Both species initially displayed endothermic behavior, followed by rapid decomposition and fluctuating specific heat and thermal conductivity between 250 and 500 °C. Oxides of carbon were the primary gases evolved, with small amounts of hydrocarbons and hydrogen. The bio-oils predominantly contained oxygenated aromatics and organic acids, and the chars had high fixed carbon and low sulfur. In all instances approximately half of the product output was liquid. Straw produced 14% gas and 32% solid at 500 °C, whereas mallee produced 13% gas and 36% solid. At 1000 °C the proportions of solid decreased and gas increased. The efficiency of pyrolysis to 500 °C, assuming no losses, was around 96% for both species. At 1000 °C the efficiency decreased, with pyrolysis of mallee slightly more efficient than for straw. Previous measurement of the thermal behavior of wheat straw has been conducted using differential thermal analysis and differential scanning calorimetry.4-7 A study using differential scanning calorimetry found the overall energy requirement to heat wheat straw to 500 °C to be 558 kJ/kg.4 Thermogravimetric analyses of wheat straw have shown that the highest rate of decomposition generally occurs between 250 and 350 °C, with around 20-25% residual mass at 500 °C for a heating rate of 10 °C/min.6-9 A study of a woody mallee species found that high rates of mass loss occur between 250 and 400 °C with 20% residual mass at 500 °C when heating at 10 °C/min.10 Biomass decomposition occurs at higher temperatures for higher heating rates.9-11 Analysis of gases evolved during the constant heating of wheat straw has shown that carbon dioxide, carbon monoxide, and methane were the major gases, with respective outputs in the range of 9.5-14, 4.5-5.7, and 0.4-0.5 wt % produced during heating to 500 °C.12,13 Another study heated wheat straw to 900 °C and obtained yields of 10.5 wt % CO2,
Introduction Biomass pyrolysis for the production of carbon-neutral oil, gas, and char is a technology that has the potential to reduce our dependence on fossil resources by providing fuels and specialty chemicals. One of the greatest challenges for the successful realization of large-scale biomass pyrolysis is the availability of suitable feedstock. Appropriate and accessible biomass may come in the form of crop residues, forestry residues ,and biomass energy crops and plantations. In Australia, bioenergy feed opportunities exist in the form of wheat straw and coppiced mallee residue, with 4-40 Mt of harvestable wheat straw produced each year1 and a potential annual mallee biomass production in the range of 8-39 Mt (dry).2 A biomass resource of this scale could potentially provide up to 30% of Australia’s current electricity generation using existing technology. A current use of coppiced oil mallee involves steaming the leaves to extract valuable eucalyptus oil, leaving residual leaves, twigs, and wood. The availability of the biomass is a result of the growing trend toward alley farming of wheat straw and mallee trees to protect the soil and prevent dryland salinity.3 The Australian pyrolysis industry has recognized the potential for the production of bio-oil and biochar using wheat straw and mallee feedstocks.
(4) He, F.; Yi, W.; Bai, X. Energy Convers. Manage. 2006, 47, 2461– 2469. (5) Stenseng, M.; Jensen, A.; Dam-Johansen, K. J. Anal. Appl. Pyrolysis 2001, 58-59, 765–780. (6) Stenseng, M.; Zolin, A.; Cenni, R.; Frandsen, F.; Jensen, A.; DamJohansen, K. J. Therm. Anal. Calorim. 2001, 64, 1325–1334. (7) Lopez-Capel, E.; Abbott, G. D.; Thomas, K. M.; Manning, D. A. C. J. Anal. Appl. Pyrolysis 2006, 75, 82–89. (8) Ghaly, A. E.; Ergudenler, A. Appl. Biochem. Biotechnol. 1991, 2829, 111–126. (9) Stenseng, M.; Jensen, A.; Dam-Johansen, K. In Progress in Thermochemical Biomass Conversion; Bridgwater, A. V., Ed.; Blackwell Science: Oxford, UK, 2001, pp 1061-1075. (10) Garcia-Perez, M.; Wang, X. S.; Shen, J.; Rhodes, M. J.; Tian, F.; Lee, W.; Wu, H.; Li, C. Ind. Eng. Chem. Res. 2008, 47, 1846–1854. (11) Williams, P. T.; Besler, S. Renew. Energ. 1996, 7, 233–250. (12) Di Blasi, C.; Signorelli, G.; Di Russo, C.; Rea, G. Ind. Eng. Chem. Res. 1999, 38, 2216–2224. (13) Bassilakis, R.; Carangelo, R. M.; W ojtowicz, M. A. Fuel 2001, 80, 1765–1786.
† Presented at the 2009 Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies. *To whom correspondence should be addressed. E-mail: vstrezov@ gse.mq.edu.au. (1) Dunlop, M.; Poulton, P.; Unkovich, M.; Baldock, J.; Herr, A.; Poole, M.; O’Connell, D. Assessing the Availability of Crop Stubble as a Potential Biofuel Resource. In Proceedings of the 14th Australian Society for Agronomy Conference: Adelaide, South Australia, 2008. (2) Bartle, J.; Olsen, G.; Cooper, D.; Hobbs, T. Int. J. Global Energy Issues 2007, 27, 115–137. (3) Wu, H.; Fu, Q.; Giles, R.; Bartle, J. Energy Fuels 2008, 22, 190– 198.
r 2009 American Chemical Society
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Energy Fuels 2010, 24, 46–52
: DOI:10.1021/ef9004797
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compositions of the samples are given in Table 1, where the values were measured using AS 1038 parts 1, 3, 5, 6.3.3, and 6.4. The wheat straw (Triticum aestivum) was grown in the Maryborough region of Victoria and was received as bags of chaffed wheat straw. The sample included some leaf material as well as the stalk of the plant. The oil mallee (Eucalyptus Kochii subsp. Plenissima) was grown in the Kalannie region of Western Australia and had been harvested, shredded, and steamed for eucalyptus oil removal. The mallee residue sample contained the steamed leaf, twigs, and a small amount of bark from the plant. Further grinding of both biomass species to approximately 500 μm mean diameter was achieved using a blender. The samples were dried in a vacuum oven at 50 °C and -100 kPa for three hours prior to conducting experiments. Thermal Analysis. Computer-aided thermal analysis (CATA) was used to measure the thermal properties of the biomass during unsteady state heating conditions. The technique and apparatus has previously been described in more detail.20 The equipment was calibrated and checked using copper, potassium sulfate, magnesium hydroxide,20 and cellulose.21 Pyrolysis was conducted with 1-2 g of sample packed at a density of around 450 kg/m3 for straw and 650 kg/m3 for mallee. Samples were heated to 1000 °C at rates of 10 and 100 °C/min. Thermogravimetric analysis was performed in a Mettler Toledo TGA/DSC. The sample mass was continually measured during heating from ambient temperature to 1000 °C at rates of 10 and 100 °C/min for samples initially weighing between 10 and 25 mg. The effect of sample size was not investigated in this study. The aluminum oxide crucibles were predried, and an experiment with the empty crucible was conducted before each biomass experiment, with the resulting curve subtracted from the biomass experimental curve to account for buoyancy effects. All analyses were carried out in a nitrogen atmosphere at a flow rate of 20 mL/min. Analysis of Pyrolysis Products. An MTI Activon M200 series micro gas chromatograph was used to analyze the gases evolved from the biomass sample every 90 s during heating from ambient temperature to 1000 °C. The instrument was calibrated prior to the experiment using a calibration gas mixture of 3% CO2, 3% CO, 1% CH4, 1% C2H4, 1% C2H6, and 1% H2 in ultrahigh purity argon. A 50 mg mass of biomass sample was packed in a quartz tube and heated at the rate of 10 °C/min in an infrared furnace. Helium gas at 50 mL/min was used to continually transport the evolved gases from the quartz tube and through a glass wool filter and an ice condensing coil, before reporting to the micro GC. A molecular sieve 5A column at 90 °C was used to measure H2 and CO, and a Poraplot U column at 55 °C was used to measure CO2, CH4, C2H4, and C2H6. Both columns were at atmospheric pressure. Oil samples were produced by heating approximately 1.5 g of biomass in a quartz tube inside an infrared furnace. Oils were collected for heating rates of 10 and 100 °C/min, where the vapors were continually removed and condensed during heating to 1000 °C. Argon gas flowed through the tube at 20 mL/min, allowing the vapors to flow and condense into
Table 1. Composition of Raw Biomass Samples analysis
moisture % dry basis: ash % volatile matter % fixed carbon % total sulfur % calorific value (gross) MJ/kg
wheat straw Proximate Analysis 8.9 5.2 76.3 18.5 0.15 19.96
mallee residue
10.5 2.9 77.9 19.2 0.08 20.42
carbon hydrogen nitrogen oxygen
Ultimate Analysis (dry basis) 46.0 5.92 1.42 41.3
51.2 5.84 0.51 39.5
silicon (SiO2) aluminum (Al2O3) iron (Fe2O3) calcium (CaO) magnesium (MgO) sodium (NaO2) potassium (K2O) titanium (TiO2) manganese (Mn3O4) sulfur (SO3) phosphorus (P2O5) barium (BaO) strontium (SrO) zinc (ZnO)
Ash Composition 63.6 0.93 0.78 3.3 3.5 1.8 16.6 0.07 0.19 2.6 6.0 0.04 0.04 0.05
22.3 9.3 2.6 27.4 7.8 8.1 6.5 0.4 3.1 4.2 2.6 0.11 0.16 0.67
6.6 wt % CO, and 0.9 wt % CH4.14 A wood sample of a related mallee species produced approximately 7 wt % CO2, 1.3 wt % CO, and 0.5 wt % CH4 during heating to 500 °C.10 Previous accounts of the bio-oil produced from the pyrolysis of wheat straw15 and woody mallee16 have illustrated the presence of organic acids, aromatic compounds and phenols, ketones, aldehydes, and other oxygenated compounds typical of biooils from biomass.17,18 The exact chemistry of the bio-oil was found to be mainly influenced by the pyrolysis conditions.15 Char analysis for both agricultural residues and woody species have shown decreases in the volatile matter present as pyrolysis temperature increased.10,19 The purpose of this study was to comprehensively describe the wheat straw and oil mallee pyrolysis processes and resultant products and establish a mass and energy balance. The results can be used to calculate the energy requirements of a pyrolysis process utilizing these materials and predict the product chemistry and energy value. This has importance in evaluating the value of wheat straw and mallee residues as large scale biomass resources. Experimental Section Biomass Samples. Wheat straw and steamed oil mallee biomass were the samples selected for the study. The (14) Serio, M. A.; Chen, Y.; W ojtowicz, M. A. ACS Fuels Preprints Fall Washington DC 2000, 45, 446–474. (15) Ates, F.; Isikdag, M. A. Energy Fuels 2008, 22, 1936–1943. (16) Garcia-Perez, M.; Wang, S.; Shen, J.; Rhodes, M; Lee, W. J.; Li, C. Energy Fuels 2008, 22, 2022–2032. (17) Zhang, Q.; Chang, J.; Wang, T.; Xu, Y. Energy Fuels 2006, 20, 2717–2720. (18) Deng, L.; Fu, Y.; Guo, Q. Energy Fuels 2008, 23, 564–568. (19) Demirbas, A. J. Anal. Appl. Pyrolysis 2004, 72, 243–248.
(20) Strezov, V.; Lucas, J. A.; Strezov, L. J. Therm. Anal. Calorim. 2003, 72, 907–918. (21) Strezov, V.; Moghtaderi, B.; Lucas, J. J. Therm. Anal. Calorim. 2003, 72, 1041–1048.
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: DOI:10.1021/ef9004797
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Figure 1. Apparent specific heat (Cp) and thermal conductivity (k) of wheat straw and mallee residue pyrolyzed at a heating rate of 10 °C/min.
Figure 3. TGA and DTG curves for wheat straw and mallee residue decomposition at a heating rate of 10 °C/min.
Figure 2. Apparent specific heat (Cp) and thermal conductivity (k) of wheat straw and mallee residue pyrolyzed at a heating rate of 100 °C/min.
Figure 4. TGA and DTG curves for wheat straw and mallee residue decomposition at a heating rate of 100 °C/min.
glass wool packed into the end of the quartz tube held at ambient temperature. Dichloromethane was used to extract the oils from the tube and glass wool packing. Analysis of the oil fraction was conducted by a Shimadzu QP2010 GC-MS with a 30 m long 0.25 μm diameter SGE-BP1 column. Calibration with perfluorotributylamine was conducted before the analysis, with dichloromethane purges between each sample. A dichloromethane standard was analyzed every four experiments. Samples were heated to 150 at 5 °C/min, then heated to 300 at 10 °C/min. Samples were diluted in helium carrier gas at 35 cm/s with a split ratio of 20. Analysis was by Shimadzu Lab Solutions GC-MS software. Char was created during the production of oil samples and collected at the end of each experiment following heating to 1000 °C. Char samples heated to 500 °C were obtained separately using the same method. Analysis was conducted using a Thermoscientific Nicolet 6700 series FTIR analyzer and Omnic software. Other char analysis included proximate, ultimate, sulfur, and calorific value tests.
underwent a second endothermic reaction at around 210 °C. Between 250 and 400 °C both species showed similar fluctuations in the specific heat and thermal conductivity, likely to be from the combined decomposition of the lignin, cellulose, and hemicellulose in the biomass since the breakdown of these components has been shown to occur between 250 and 450 °C.21 A discrete endotherm additionally occurred at 750 °C for the mallee sample. The specific heat and thermal conductivity tended to slowly increase for both species when temperatures approach 1000 °C. The slower heating rate of 10 °C/min showed greater clarity of the individual reactions compared to the heating rate of 100 °C/min, where the peaks were larger and smoother. At slow heating rates the reactions occurred in a range close to the activation temperature, and as such the individual reactions were more clearly expressed by peaks of increased or decreased specific heat. Figures 3 and 4 show the residual mass (TGA) and rate of mass loss (DTG) superimposed for temperatures up to 1000 °C for wheat straw and steamed mallee at heating rates of 10 and 100 °C/min. The increased mass loss rate of both species around 100 °C was a result of the evaporation of strongly bound water remaining after drying and corresponded to the first endothermic reaction detected by the CATA. The mallee lost slightly more mass than straw, explaining the higher magnitude of the endothermic reaction at 100 °C. During heating at 10 °C/min, rapid decomposition of both samples occurred between 200 and 400 °C, with mallee decomposing at slightly higher temperatures than straw. The phase of greatest decomposition occurrred in the approximate temperature range of lignin breakdown.21 Mallee maintained a slightly greater residual mass of 35.6% at 500 °C, compared with the wheat straw retaining 32.2%. At 1000 °C wheat straw produced 26%
Results and Discussion Thermal Behavior During Pyrolysis. The apparent specific heat and thermal conductivity of wheat straw and steamed mallee during heating at rates of 10 and 100 °C/min are shown in Figures 1 and 2, respectively. The apparent specific heat is the measured specific heat as a function of temperature and includes reaction heat. The thermal conductivity includes heat transfer by conductivity, convection, and radiation and is an overall value. The specific heat in Figures 1 and 2 are given on a volumetric basis. At the heating rate of 10 °C/min, both species displayed a maximum of specific heat close to 100 °C, indicating an endothermic reaction likely to be from the evaporation of bound water not removed during drying. The mallee residue 48
Energy Fuels 2010, 24, 46–52
: DOI:10.1021/ef9004797
Mulligan et al. Table 2. Total Gases Produced from Wheat Straw and Mallee Residue Pyrolysis wheat straw
mallee residue
gas
mass % at 500 °C
mass % at 1000 °C
mass % at 500 °C
mass % at 1000 °C
H2 CO2 CO CH4 C2H4 C2H6
0 75.0 18.5 4.4 0.8 1.2
3.2 55.6 33.8 5.9 0.6 0.9
0.1 70.2 23.6 4.7 0.7 0.7
3.4 56.2 33.9 5.5 0.5 0.6
Figure 5. Gases evolved from wheat straw decomposition at a 10 °C/min heating rate.
Figure 7. GC-MS spectra of oils produced by wheat straw at a heating rate of 10 °C/min.
Figure 6. Gases evolved from mallee residue decomposition at a 10 °C/min heating rate.
char, with mallee maintaining 27.5% char. The temperature region of highest decomposition rate is similar to that previously reported for wheat straw,6-9 however the residual char mass was slightly higher in this work. For both species the higher heating rate of 100 °C/min caused a delay in the sample mass loss with respect to temperature and reduced char formation for final temperatures above 680 °C. At 1000 °C straw retained 24.2% and mallee retained 26.1% char. The shift in the thermogravimetric curves at higher heating rates has been explained to be due to different heat transfer and kinetic rates delaying the sample decomposition.11 Pyrolysis Products. The evolution rate of gas species during heating to 1000 °C is shown by Figure 5 for wheat straw and Figure 6 for mallee. Total gases produced at 500 and 1000 °C are given in Table 2. Both species primarily produced carbon dioxide and carbon monoxide during heating to 500 °C. Methane was released between 300 and 800 °C, with ethane and ethene evolved between 300 and 600 °C. These light hydrocarbons are usually favored for their energy value, although they only comprise a small proportion of the total gas. Hydrogen, also favored for its high energy value, was evolved above 500 °C. Gas evolution profiles were similar for both species, with the most notable difference being the release of oxides of carbon over three distinct temperature regions for mallee compared to one region for straw. For both species high volumes of gas were released between 200 and 400 °C, where the TGA showed high rates of decomposition and the specific heat indicated a series of reactions. The amount of carbon monoxide evolved from wheat straw during heating to 500 °C was lower than values reported in the literature for similar heating conditions, although carbon dioxide evolutions compared well.12,13
Figure 8. GC-MS spectra of oils produced by wheat straw at a heating rate of 100 °C/min.
The GC-MS analysis of oils produced by wheat straw and mallee residue at heating rates of 10 and 100 °C/min are listed in Table 3, with the GC-MS spectra shown in Figures 7-10. The top fifteen peaks are numbered in ranked order of area percent, which corresponds to the ranked order of mass percent. Acetic acid, a common breakdown product of cellulose in biomass, was the largest proportion in each of the bio-oils. The remainder of the compounds were primarily oxygenated aromatic compounds, regardless of the biomass species or heating rate, and are likely to have been formed from lignin decomposition. Other key compounds present in the oils were alcohols (including phenols) and ketones. The oils produced at heating rates of 10 and 100 °C/min showed only minor differences. The higher heating rate tended to produce slightly higher proportions of the top fifteen compounds present as indicated by the area percent of the peaks. The difference in oils was greater between the species, with mallee producing several compounds containing benzene while straw favored producing phenolic compounds. The proximate and ultimate analysis, the sulfur content, and calorific value of the chars produced at 500 and 1000 °C for wheat straw and mallee are shown in Table 4. Chars prepared at 1000 °C had lower volatile matter and higher fixed carbon than the chars produced at 500 °C. High 49
Energy Fuels 2010, 24, 46–52
: DOI:10.1021/ef9004797
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Table 3. Compounds Contained in Oils from Wheat Straw and Mallee Pyrolysed at 10 and 100 °C/min wheat straw 10 °C/min compound
steamed mallee 100 °C/min
area %
acetic acid 2-furanmethanol
9.0 4.1
phenol, 4-methyl-
3.4
phenol
3.3
cyclopropyl carbinol
2.7
2-cyclopenten-1-one, 2-hydroxy-3-methylphenol, 2-methoxy2-propanone, 1-(acetyloxy)2-propanone, 1-hydroxy2-methoxy-4vinylphenol phenol, 2,6dimethoxy-
compound acetic acid 2-propanone, 1-hydroxy2-furanmethanol phenol
10 °C/min
100 °C/min
area %
compound
area %
compound
area %
14.1 7.8
acetic acid phenol, 2,6-dimethoxybenzene, 1,2,3trimethoxy-5-methylphenol, 2,6dimethoxy-4(2-propenyl)1,2,4-trimethoxybenzene phenol, 2-methyl-
7.3 6.3
acetic acid phenol, 2,6-dimethoxy-
9.9 8.1
5.2
30 ,50 -dimethoxyacetophenone phenol, 2,6-dimethoxy4-(2-propenyl)-
5.5
4.3 4.1
3.7
4.0
2.5
2-methoxy-4vinylphenol phenol, 2-methoxy-
2.4
phenol, 4-methyl-
3.2
2.3
2-cyclopenten-1-one
3.0
2.2
phenol, 2,6-dimethoxy-
2.9
2.1
phenol, 2-methyl-
2.6
2.0
2-propanone, 1-(acetyloxy)-
2.5
phenol, 2-methyl-
2.0
2.3
phenol, 4-ethyl-2methoxy-
2.0
phenol, 4-ethyl-
1.9
2.0
1.9
2.0
2-furanmethanol, tetrahydrovitamin E acetate
1.8
docosane
1.5
1,4:3,6-dianhydro-. alpha.-D-glucopyranose
1.8
1,4:3,6-dianhydro.alpha.-D-glucopyranose phenol, 2-methoxy4-methylphenol, 4-ethyl-2methoxyeicosane
1.9
phenol, 2-methoxy-
1.4
3.8
2-pentanone, 1-(2,4,6-trihydroxyphenyl) 2-propanone, 1-hydroxyphenol phenol, 2-methoxy4-methylcyclopenta-decanone
3.3 2.9 2.8 2.4 2.0 2.0 2.0
1,2,4-trimethoxybenzene benzene, 1,2,3-tri methoxy-5-methyl2-propanone, 1-hydroxy-
5.1 4.2 4.1 3.6
2-methoxy-4vinylphenol phenol, 2-methoxy-
2.7
2-cyclopenten-1-one, 2-hydroxy-3-methyl2-pentanone, 1-(2,4,6-trihydroxy phenyl) phenol, 4-methyl-
2.5
phenol, 2,6-dimethoxy4-(2-propenyl)phenol, 2-methoxy4-methylfurfural
2.7
2.4 2.2 2.1 2.0 1.9
Table 4. Analysis of Wheat Straw and Mallee Chars Produced at 500 and 1000 °C wheat straw char 500 °C
analysis
1000 °C
mallee residue char 500 °C
1000 °C
Proximate Analysis moisture % dry basis: ash % volatile matter % fixed carbon % total sulfur % calorific value (gross) MJ/kg
Figure 9. GC-MS spectra of oils produced by mallee residue at a heating rate of 10 °C/min.
4.6
4.8
4.6
4.8
13.3 11.7 75.0 0.15 28.1
14.9 2.6 82.5 0.07 27.7
8.9 15.0 76.1 0.04 29.9
11.6 3.0 86.0 0.19 29.3
78.7 2.48 1.18 8.7
84.5 0.25 0.84 2.6
Ultimate Analysis (dry basis) carbon hydrogen nitrogen oxygen
73.6 2.34 2.58 8.0
80.4 0.19 1.19 3.2
fixed carbon is a desirable property of char, although heating to 1000 °C only increased the fixed carbon by around 9% more than heating to 500 °C. Hydrogen and oxygen were removed from the char at high temperatures as reflected by the elemental analysis. The gas analysis indicated that hydrogen evolved above 500 °C, whereas at lower temperatures bound hydrogen was released in hydrocarbon gases and biooils. The majority of oxygen was removed below 500 °C with
Figure 10. GC-MS spectra of oils produced by mallee residue at a heating rate of 100 °C/min.
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Energy Fuels 2010, 24, 46–52
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Table 5. Distribution of Mass and Energy of Pyrolysis Products Created at a 10 °C/min Heating Rate wheat straw 500 °C
mallee residue 1000 °C
500 °C
1000 °C
product
mass %
energy MJ/kg
mass %
energy MJ/kg
mass %
energy MJ/kg
mass %
energy MJ/kg
gas solid liquid (by difference)
13.5 32.2 54.3
5.1 28.05 18.9
21.6 26.0 52.5
10.9 29.3 19.2
12.8 35.6 51.5
5.5 29.9 17.6
18.5 27.6 53.9
10.8 29.3 19.2
Pyrolysis products from wheat straw and mallee residue were similar, with wheat straw yielding slightly higher gas and oil fractions at both 500 and 1000 °C. Liquid yields remained consistent at around half of the product output in all cases. Both species produced a greater proportion of gas, with a higher energy value, and lower proportion of solid when heated to 1000 °C due to the further removal of hydrogen, oxygen, and some carbon from the char. The total energy requirement to heat the biomass at 10 °C/ min was determined by integrating the apparent specific heat curves in Figure 1. Wheat straw required 900 kJ/kg to pyrolyse to 500 °C and 1860 kJ/kg to pyrolyse to 1000 °C. Mallee residue required 740 kJ/kg to heat to 500 °C and 1370 kJ/kg to heat to 1000 °C. The energy inputs and outputs were used to calculate a theoretical efficiency of the process assuming no thermal losses.
Figure 11. FTIR spectra for raw biomass, char produced at 500 °C and char produced at 1000 °C for wheat straw (left) and mallee residue (right).
the bio-oils, water, and gaseous oxides of carbon. The sulfur content of the biomass chars was low, which is a preferred characteristic of fuels. Unlike mallee, the ash percentage of straw char only marginally increased between the 500 and 1000 °C chars. The moisture present in the chars is likely to have adsorbed to the chars during handling. The FTIR spectra for wheat straw and mallee biomass and their chars produced by heating to 500 and 1000 °C are shown in Figure 11. The raw biomass samples both displayed a wide peak at 1030 cm-1 that could have been from primary alcohols and ethers in the lignin structure, and stretching of C-O and C-C bonds. The same peak was weakly expressed in the 500 °C chars of both biomass species. The raw biomass samples also showed absorbance for aromatic-OH groups at around 3300, 1420, and 1368 cm-1, which were degraded or removed from the char during heating. Peaks at 1620 and 1514 cm-1 were likely to be from aromatic rings in the raw biomass, and asymmetric and symmetric CH2 bonds were shown at 2920 and 2848 cm-1, respectively. A small CdO vibration at 1730 cm-1 existed for both biomass samples, with the small peaks between 1400 and 1200 cm-1 likely to have been from stretching of C-O, C-O-C, aromatic-O, and C-N bonds. The majority of bonding had degraded during heating to 500 °C. The chars of both species displayed CdO, C-C, and CdC bonds at 1570 cm-1, and mallee char additionally expressed potential C-O, CdC-aromatic, C-O-H, and O-CdO bonds at 1412 cm-1. Chars produced by heating to 1000 °C had no remaining peaks, with the large amounts of black carbon causing a high level of absorbance throughout the spectrum. Mass and Energy Balance. The proportions of solid, liquid, and gaseous products for pyrolysis of wheat straw and mallee at a 10 °C/min heating rate for maximum temperatures of 500 and 1000 °C are given in Table 5. Table 5 also indicates the estimated energy value of each of the outputs. Oil mass and energy values were calculated by difference and include water.
Efficiency ¼
energy of products -energy of pyrolysis energy in feed
The theoretical maximum efficiency of the process for wheat straw heated to 500 °C is 95.5% and for 1000 °C was 90.7% using dry feed. Similarly for mallee the efficiency was 96.4% for heating to 500 °C and 93.3% for heating to 1000 °C. These calculated efficiencies will be reduced to different extents depending on the feed moisture and the degree of thermal losses from the particular pyrolysis system. For example, every 10% of additional moisture in the feed causes the process energy requirement to increase by 0.23 MJ/kg. Wheat straw and steamed mallee residues tended to have moisture contents below 10%, which is highly favorable for biomass residues. Freshly harvested mallee has a much higher moisture content of 40-50% and will therefore have a significantly lower theoretical efficiency if the energy cost of drying is borne by the process. The proportion of energy in the dry feed required for pyrolysis is around 3.5-4.5% for pyrolysis at 500 °C and 6.5-9.5 for pyrolysis to 1000 °C. Assuming that the feed is not dry and contains 10% moisture, the proportion of feed needed to operate the process would be 4.5-6% for temperatures up to 500 °C and 7.5-10.5% for up to 1000 °C. There is also the possibility of combusting the oil, gas, or char fraction to provide the heat energy for the process. This study indicates that while the pyrolysis behavior of the two feedstocks have some differences, they are not so significant as to preclude cofeeding into a pyrolysis reactor. Conclusions The thermal behavior, product compositions and material and energy balances were presented for the production of oils, gases, and chars through the pyrolysis of wheat straw and mallee residues. During heating at 10 °C/min, both species displayed endothermic reactions to 250 °C followed by 51
Energy Fuels 2010, 24, 46–52
: DOI:10.1021/ef9004797
Mulligan et al.
fluctuating specific heat and thermal conductivity to 400 °C. The wheat straw exhibited smaller and fewer reaction peaks compared to the mallee sample. TGA indicated that the greatest rate of decomposition was taking place in the temperature region of 200-400 °C. The wheat straw produced 32.2% char at 500 °C, with the mallee maintaining 35.6%. At 1000 °C the wheat straw and mallee maintained 26.0 and 27.5% char, respectively. The higher heating rate of 100 °C/min caused the decomposition reactions to occur over a wider temperature range and resulted in a slightly lower residual char mass for both samples. Carbon dioxide and methane were the dominant gases released during the period of high sample decomposition between 250 and 400 °C, with hydrogen in smaller quantities evolved at temperatures over 500 °C. The total mass and energy value of the gas from heating to 500 °C was 13.5% and 0.7 MJ/kg, respectively, for wheat straw and 12.8% and 0.7 MJ/kg, respectively, for mallee. The total gas produced to 1000 °C amounted to 21.6% for wheat straw and 18.5% for mallee pyrolysis. Wheat straw produced 54.3% liquid
(including water) during heating to 500 °C and 52.5% at 1000 °C, whereas mallee residue produced 51.5% at 500 °C and 53.9% at 1000 °C. The oils were primarily composed of oxygenated aromatics and organic acids. Wheat straw produced mainly phenolic aromatics, however mallee additionally produced benzene compounds. Chars produced at temperatures of 1000 °C contained 7-10% more fixed carbon and lower elemental oxygen and hydrogen than char produced at 500 °C. Volatile matter decreased as the sample was heated, with the majority of hydrocarbons removed below 500 °C. The overall net energy requirement to pyrolyse wheat straw was 898 kJ/kg for heating to 500 °C and 1856 kJ/kg for 1000 °C. For mallee pyrolysis the energy required was 735 kJ/kg for 500 °C and 1371 kJ/kg for 1000 °C. The maximum theoretical efficiency of pyrolysis to 500 °C was 95.5% for wheat straw and 96.4% for mallee residue. Acknowledgment. The authors thank Pushan Shah and Artur Ziolkowski regarding analytical equipment support.
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