Energy from Biomass
Logging and sawmill residues and other biomass are alternative fuels in the current crisis of fossil fuel shortage. The higher the hydrogen content of a material the higher is the heat value of the material. Table 1 compares the heat values of many important fuels including wood, showing that hydrogen gas has a much higher heat value than gasoline and diesel and that highly oxygenated materials (biomass) such as coal and wood contain low heat values. Because of similarity in chemical composition, the heat value of crop residues is essentially the same as that of wood. Although having low values, biomass energy is cost effective. For example, the current gasoline price is approximately $2.00/kg ($3.75/Gal), which gives a heat value of 21 mega joules (MJ) for each dollar spent (21MJ/US$). Assuming a cost of $40 per metric ton (includes costs of harvesting and drying), each dollar-worth of crop residue would be able to provide 375 MJ/kg heat energy (1000 kg/$40= 25 kg/US$; 15 MJ/kg x 25 = 375 MJ/kg/US$).
Table 1. Comparative heat values of some important fuels
Heat of Combustion
Fuel MJ/kg BTU/pound kJ/mol
Hydrogen (H2) 141.6 61,000 286
Gasoline 47.3 20,400 ---
Diesel 44.8 19,300 ---
Ethanol 29.7 12,800 1,300
Butane 48.6 20,900 2,800
Heating oil 42.7 19,580
Wood (m.c.=20%) 15 6,500 ---
Coal 15-27 8,000 to 14,000 ---
The heat values of oven-dried hardwoods and softwoods are estimated to be 19.7 MJ/kg (8,500 Btu/lb) and 20.9 MJ/kg (9,000 Btu/lb), respectively; the difference is due to a higher lignin content in softwoods than hardwoods. The dry heat values of cellulose, hemicellulose and lignin respectively are 17 MJ/kg (7,320 Btu/lb), 16.63 MJ/kg (7165 Btu/lb) and 21.13 MJ/kg (9,105 Btu/lb) 1, while wood extractives such as resins, waxes, oils, tannins, and other phenolics have much higher heat values than cellulose and hemicelluloses. The heat value of wood with 20% moisture is about 15 MJ/kg (6,465 Btu/lb) because some heat is consumed as heat of evaporation and heat of wetting during the combustion process.
Potential Uses of Biomass Pyrolysis Oils
Underground pressure and heat convert biomass into crude oil, coal and natural gases, and distillation of oil-rich coal obtains more oils, coal tars and natural gases. Oil tar is a final by-product of crude oil refinery. Oil tar together with coal tar is formulated into creosote, the first used wood preservative. Pyrolysis of biomass yields flammable gases, oils (tars) and charcoal, which can all to be used as fuels.
Biomass pyrolysis oil contains hundreds of chemicals, which can be a source for chemicals in addition to be used as a fuel. Fast pyrolysis biomass meals (about 40 mesh particle size) at 500 oC with pilot scale reactors yields about 70% wet oil, which contains about 50% water, and 10% flammable gases (Table 2). Chemicals of significant amounts in the anhydrous oil are listed in Table 3. Another significant chemical not listed in Table 3 is methanol (CH3OH), and it has been shown that fast pyrolysis of wood yields close to 1% methanol 2. Methanol is an important byproduct during crude oil refining, which is a raw material to manufacture formaldehyde for wood adhesive resins. Softwood Pyrolysis oils contain higher amounts of simple phenolics than hardwood and crop residue pyrolysis oils because of higher lignin content. Similarly, because tree bark contains much more polyphenolic compounds including lignin than wood, tree bark pyrolysis oil should contain a much higher amount of simple phenols than wood pyrolysis oil.
Table 2. Products from fast-pyrolysis of wood at 500 oC 2
% of moisture-free wood Oak Poplar Spruce Hemlock
Wet oil (with water) 66 69 78 66
Charcoal 12 13 12 18
Gases 12 10 9.8 12
Water vapor 10 10 12 9.4
Organic liquids 56 59 67 57
Table 3. Significant chemicals in 500 oC fast-pyrolysis oil of pine and beech wood 3
Compound Pine Beech
1 Hydroxyaceldehyde 12.62 7.78
2 Acetic acid 3.22 4.65
3 Acetol 7.02 3.90
4 2,5-Dimethoxytetrahydrofuran 0.14 0.52
5 (5H)-Furan-2-one 0.77 0.67
6 2-Hydroxy-1-methyl-1-cyclopentene-3-0ne 0.27 0.16
7 Levoglucosan 5.42 3.52
8 Phenol 0.06 0.03
9 Guaiacol 0.53 0.15
10 0-Cresol 0.05
11 m-cresol 0.31
12 p-Cresol 0.02
13 4-methyl guaiacol 0.88 0.14
14 2,4- and 2,5-Dimethyl phenol 0.05 0.41
15 4-Ethyl guaiacol 0.14 0.08
16 4-Vinyl guaiacol 0.06 0.05
17 Euginol 0.22 0.06
18 5-Hydroxymethyl-2-furaldehyde 0.45
19 Syringol 0.32
20 Isoeugenol (cis) 0.25 0.07
21 Isoeugenol (trans) 0.67 0.30
22 4-Methyl syringol 0.30
23 Vanillin 0.26 0.10
24 Homovanillin 0.20 0.09
25 Acetoguaiacone 0.19 0.08
26 4-Ally- and 4-propyl syringol 0.21
27 Syringaldehyde 0.25
Total 33.79 23.86
Use of biomass pyrolysis oil in the synthesis of phenol-formaldehyde (PF) resin recently has gained some attention. It has been found that a high performance PF resin can be synthesized with about 35% replacement of phenol with raw pyrolysis oil 4, 5. Pyrolysis oil can be upgraded to a phenol-rich fraction with an ethyl acetate extraction, yielding about wt 35% of the raw oil. The phenolic resin synthesized directly from this upgraded pyrolysis oil performed about the same as neat PF resins 6.
Production of flammable gases increased dramatically when biomass is rapidly pyrolyzed at temperatures higher than 500 oC, and the process is called biomass gasification. Table 4 shows the analysis of gasification of wheat straw and birch wood. Clearly, the gaseous produced in rapid pyrolysis of biomass at high temperatures is a suitable substitution of natural gas.
The forest products industry for decades has been partially self-sufficient in energy by burning hog fuels. It has been estimated that over 400 million tons of crop residues is generated annually in the United States, and this abundant and potential renewable energy source is currently under-utilized. Development of biochemical and thermochemical technologies for fuels and chemicals from biomass is underway.
Table 4. Analysis of wheat straw and birch wood fast-pyrolysis at 800 oC and1000 oC 7
Biomass Weat Straw Birch Wood
Temperature ( oC) 800 1000 800 1000
Gas Yield (wt%) 75.8 86.0 77.7 87.0
Tar Yield (wt%) 0.9 0.1 1.1 0.2
Charcoal (wt%) 13.2 10.8 7.2 5.6
Water Vapor & Losses 10.1 3.1 14.0 7.2
Composition of gaseous (vol %)
H2 35.0 43.9 16.8 34.0
CH4 9.5 4.8 16.2 11.7
C2H2, C2H4 3.1 trace 6.2 0.5
C2H6 0.6 trace 1.2 trace
Benzene 0.6 0.1 1.2 0.6
CO2 23.7 5.0 8.3 7.5
CO 28.0 46.2 50.7 45.7
References
1. Murphy W. K., and K. R. Masters. 1978. Gross heat of combustion of northern red oak (Quercus rubra) chemical components. Wood Sci. 10:139-141.
2. Milne, T., F. Agblevor, M. Davis, S. Deutch, and D. Johnson. 1997. A review of the chemical composition of fast-pyrolysis oil from biomass. In“Developments in Thermochemical Biomass Conversion,”A. V. Bridgwater and D. G. B. Boocok, ed. Blackie Academic & Professional, London.
3. Meier, D., A. Oasmaa, and G. V. C. Peacocke. 1997. Properties of fast pyrolysis liquids: status of test methods. In“Developments in Thermochemical Biomass Conversion,”A. V. Bridgwater and D. G. B. Boocok, ed. Blackie Academic & Professional, London.
4. Chan, F. D., B. Rield, X. M. Wang, C. Roy, X. Lu, and C. Amen-Chen. 2001. Wood adhesives from pyrolsis oil for OSB. In“Wood Adhesives 2000,”Proceedings No. 7252, Forest Product Society, Madison, WI.
5. Himmelblau, D. A., G. A. Groditz, and M. D. Gibson. Performance of wood composite adhesives made with biomass pyrolysis oil. In“Wood Adhesives 2000,”Proceedings No. 7252, Forest Product Society, Madison, WI.
6. Kelley, S. S., X. M. Wang, M. D. Myers, D. K. Johnson, and J. W. Scahill. 1997. Use of biomass pyrolysis oils for preparation of modified phenol formaldehyde resins. In “Developments in Thermochemical Biomass Conversion,”A. V. Bridgwater and D. G. B. Boocok, ed. Blackie Academic & Professional, London.
7. Zanzi, R. K. Sjostrom, and E. Bjornbom. 1997. Rapid pyrolysis of straw at high temperature. In“Developments in Thermochemical Biomass Conversion,”A. V. Bridgwater and D. G. B. Boocok, ed., Blackie Academic & Professional, London.
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