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Wood: Chemicals
Wood: Chemicals$ WE Mabee, Queen’s University, Kingston, ON, Canada L Cabral, Université de Sherbrooke, Sherbrooke, QC, Canada DN Roy, University of Toronto, Toronto, ON, Canada r 2016 Elsevier Inc. All rights reserved.
1 2 3 4 References Further Reading
1
Introduction Principal Chemical Components of Wood Chemical Products through Thermochemical Conversion Chemical Products through Biochemical Processes
1 1 2 3 4 4
Introduction
In addition to supplying lumber and fiber, wood from trees has been used as a source of chemicals for many applications throughout history. Naval stores, tanning agents, rubber, and methanol are all commonly known products. Today, concern over the cost, environmental impacts, and accessibility of petroleum products and other fossil fuels has brought about a resurgence of interest in the use of renewable biomass resources, including wood from trees, as a source of the fuels and chemicals that are so vital to our economy.
2
Principal Chemical Components of Wood
Wood, the structural component of a tree, is a complex matrix of three major polymers commonly labeled as cellulose, hemicellulose, and lignin, which together can be referred to as ‘lignocellulose.’ Table 1 shows the proportions and subgroups associated Table 1
Principle components of wood
Main group
% of Dry wood mass Hardwood (angiosperm) Acer saccharum
Cellulose Hemicellulose Glucomannan Glucuronoxylan Other polysaccharides Lignin
Extractives Phenolics Fats Terpenoids
Softwood (gymnosperm) Eucalyptus globulus
40.7 51.3 30.8 25.2 3.7 1.4 23.6 19.9 3.5 3.9 25.2 21.9 Guaiacyl–syrigyl lignin (ratios from 4:1 to 1:2, depending upon species) 3.3 1.6 Stilbenes Tannins Palmitic Oleic Turpentine Pinene Rubber
Pinus radiata
Juniperus communis
37.4 33.2 20.4 8.5 4.3 27.2 Guaiacyl lignin
33.0 30.3 16.4 10.7 3.2 32.1
2.2 Lignans Flavonoids Stearic Linoleic Limonene Resin acids
4.6
Source: Sjöström, E., 1993. Wood Chemistry: Fundamentals and Applications, second ed. Toronto, ON: Academic Press.
☆
Change History: February 2015. W.E. Mabee, L. Cabral, and D.N. Roy carried out significant edits to the text. Primary sections remain the same, but text in each section has been expanded and brought up-to-date. In-line citations have been introduced. Table 1 has been updated to reflect the text. May 2015: W.E. Mabee has carried out edits to the reference list based on the editor’s comments.
Reference Module in Materials Science and Materials Engineering
doi:10.1016/B978-0-12-803581-8.02212-8
1
2
Wood: Chemicals
with each major polymer for two ‘hardwood’ (angiosperm) and two ‘softwood’ (gymnosperm) species. As shown in the table, the greatest component of wood is cellulose (properly speaking alphacellulose) which is made up of β-D-glucose units connected in a straight chain through 1–4 linkages, with a degree of polymerization (DP) of between 10 000 and 15 000 glucose units. Cellulose is the structural component of wood. Hemicellulose is a branched polymer made up of both 5- and 6-carbon sugars, including xylose and arabinose (5-C) as well as glucose, galactose, mannose, and rhamnose (6-C); hemicellulose molecules have a DP of 200 or less. It serves to connect bundles of cellulose strands into microfibrils, which in turn are a principle building block of wood fibers. Lignin is made up of a number of monomeric structures, including p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol; the latter two dominate, with coniferyl units polymerized into so-called guaiacyl lignin and sinapyl alcohol forming syringyl lignin. Guaiacyl lignin dominates in softwood species, while a combination of syringyl and guaiacyl lignin is found in hardwoods. The DP of lignin is difficult to determine because the polymer essentially acts as the ‘glue’ within the lignocellulosic structure, and extraction is very difficult if not impossible. Wood also contains a number of other chemicals, in small portions, known collectively as the extractives, which were historically the most important group for chemical production. Chemicals derived from extractives include naval stores (i.e., turpentine and pitch), resin, and rosins. This group contains many interesting compounds that, even today, are becoming more important in the global quest for pharmaceutical and chemical goods. Contrasting the proportions of chemicals found in each species provides an illustration of the variation found between different species; it should also be noted that chemical composition varies between different trees of the same species, and indeed between wood found in different locations within the tree. Generally speaking, hardwoods have a greater proportion of cellulose and lower proportion of lignin than softwoods; hemicellulose in hardwoods is dominated by xylans (5-C sugars), which are found in much lower proportions in softwoods; and hardwoods have significant proportions both syringyl and guaiacyl lignin, while softwood lignin is dominated by the guaiacyl form (Biermann, 1996; Sjöström, 1993; Goldstein, 1981).
3
Chemical Products through Thermochemical Conversion
Transforming the polymers of wood into chemical products can follow several paths. Thermochemical processes use heat in a controlled fashion to break down the lignocellulosic matrix into products such as bio-oil and char, or to completely deconstruct the wood to produce synthesis gases (H2, CO, and CO2) which can then be reformed into new products through catalytic conversion (Base, 2010). At relatively low temperatures of between 250–380 °C, a liquid bio-oil may be generated through hydrothermal liquefaction. In this process, solid biomass together with water are pressurized to 5–30 MPa and heated at 250 to 380 °C. The resulting reaction dissolves the biomass and produces bio-oil, as well as hot gasses and water containing dissolved organic compounds. The bio-oil obtained from this process has a relatively high heating value, but still requires further processing (described below) to produce a fuel that is compatible with existing infrastructure, such as generators or engines. Pyrolysis occurs when wood is heated in the absence of external air or oxygen. During pyrolysis, the biomass is heated in a sealed vessel at varying rates to temperatures of between 300–650 °C. In ‘slow’ pyrolysis, the period of heating to moderate temperatures of between 300–500 °C is extended, as is the residence time at full temperature. The primary products from slow pyrolysis are biochar, a solid form of ‘fixed’ carbon, as well as bio-oil and synthesis gases; the exact proportions of each depend upon residence time and temperature employed. Biochar is currently being explored as an option for carbon sequestration, as in this form carbon will remain fixed for potentially thousands of years. It also has potential utility as a soil amendment and as an agent for wastewater treatment (Sohi et al., 2010). ‘Fast’ pyrolysis, which involves similarly moderate temperatures of approximately 500 °C, differs from the previous process in that the heating period and residence times are shortened, and in that the reaction is stopped via rapid cooling at the end in order to increase bio-oil production by condensing gas outputs. As total reaction time drops, the proportion of bio-oil (also labeled biocrude) generated from pyrolysis increases, and the proportions of biochar and synthesis gas decrease. Bio-oil, as stated previously, has high heating values, but is not compatible with existing infrastructure due to a relatively high oxygen content and the corrosive nature of the liquid, as well as a tendency to form oligomers. These issues can be addressed through hydrotreatment (a practice used with petroleum products to remove impurities) or via hydrodeoxygenation (a catalyzed version of hydrotreatment which is currently being developed). As options for the cost-effective upgrading of bio-oil are explored, it is increasingly being looked at as a source of fuel for transportation and small-scale power generation (Wang et al., 2013; Shemfe et al., 2015). At relatively high temperatures of above 800 °C, pyrolysis is often referred to as gasification. At these temperatures and with long residence times, approximately 85% of lignocellulose may be converted to synthesis gas (also labeled syngas), which is primarily hydrogen, carbon monoxide, and carbon dioxide. Gasification sometimes differs from true pyrolysis in that it may be carried out with controlled access to external air, steam, or oxygen; this addition influences the products obtained, allowing nitrogen-rich syngas (with air) or the formation of CH4 (with water). The syngas produced via biomass gasification may be enriched with other gases (such as hydrogen) or before further processing is carried out. A common goal of biomass gasification is the production of liquid fuels for transport or energy generation. To produce liquid fuel, synthesis gases (potentially enriched) are passed over a metal catalyst in order to produce alcohols, including methanol, ethanol, and larger mixed alcohols. For methanol and ethanol, the catalyst used may be copper, aluminum, or zinc oxides; for longer-chain alcohols, catalyst types that have been researched include sulfide-, oxide-, and rhodium-based systems. Typically, synthesis gases are reacted over the catalyst at temperatures and pressures in the range of 300 °C and 10 MPa respectively. Longer-
Wood: Chemicals
3
chain hydrocarbon fuels may be created using iron or cobalt catalysts via the Fischer–Tropsch process. This reaction occurs at 200– 350 °C and 1.5–4.0 MPa; varying reaction conditions within this band determines the chain length (from C4 to C33) and the proportion of byproducts (unsaturated hydrocarbons and alcohols) in the final product (Demirbas, 2007). Fischer–Tropsch fuels have been made commercially for many years using coal or natural gas as feedstock; commercial F-T operations based on biomass have not (as of 2015) been successfully operated. In addition, a number of chemical products (e.g., 2-ethylhexanol (a plasticizer used in the manufacture of PVC) and isobutene (a precursor for gasoline oxygenates)) may be generated using biomass-based synthesis gas at higher temperatures and pressures than typically seen in liquid fuel production, and using relatively exotic catalysts (such as rhodium or thorium). The primary focus of most research in this area, however, remains on liquid fuel generation (Spath and Dayton, 2003).
4
Chemical Products through Biochemical Processes
Biochemical processes can also be used to make fuels and chemicals from wood. These processes may be subdivided into aerobic and anaerobic approaches; these approaches may be used separately or combined depending upon the desired end products. Aerobic processes include hydrolysis and fermentation, which are primarily utilized to modify the polysaccharide components of wood. The complexity of lignocellulosic substrates usually requires that a pretreatment take place, which is designed to open up the matrix of chemicals and improve the accessibility of the material to subsequent process steps. A number of different pretreatments, including steam explosion (the injection of steam at high temperature and pressure, followed by rapid release), organosolv pulping (the use of ethanol or other organic solvents to dissolve lignin), and treatment with acid and water at elevated temperatures have been considered. Depending upon the subsequent processes, it may be beneficial to remove lignin completely (as with acid pretreatments or organosolv pulping), or lignin may remain in a modified state within the substrate (as with steam explosion). The development of pretreatment technology is therefore typically integrated with design of hydrolysis operations in mind (Chandra et al., 2007). The hemicelluloses are the easiest of the three major macromolecules to hydrolyze, due to a relatively low molecular weight. A variety of products can be produced from hemicelluloses, but the principle chemicals yielded are glucose, mannose, arabinose (from softwoods), and xylose (from hardwoods). Mannose and glucose can both be fermented in commercial processes, yielding ethanol. For the five-carbon sugars found in hemicelluloses, fermentation with new yeast strains may be an option, and several bacterial strains have been developed for the conversion of compounds such as xylose to alcohols such as butanol or butenediol. Xylose may be combined to make furfural under highly acidic conditions. Glucose, arabinose, and xylose may also be hydrogenated to sorbitol, xylitol, and arabinitol respectively. These are three biomass chemicals singled out by the US Department of Energy as being very promising value-added chemicals and building blocks to other commercially important chemicals (Werpy and Petersen, 2004). Lignin may be isolated from wood through conventional pulping processes (e.g., in black liquor from Kraft pulping, or in spent sulfite liquor) or via new pretreatment processes (such as organosolv pulping). The principle chemical products from lignin are phenolic compounds, which are obtained through hydrogenation of the cell wall. In an initial screening of potential chemical products from lignin, the Pacific Northwest National Laboratory in the US considered current and future feasibility; market volumes, value, and risk; and the utility of the product as a building block. One important finding was that many of the lignin products with the best combined potential were likely to be recovered through thermomechanical processes, including heat and power as well as syngas-derived products such as Fischer–Tropsch liquids, methanol, and ethanol. Important structures that might be derived through chemical means and which had a combination of positive technical and market feasibility included polyelectrolites, aromatic acids, cyclohexanol, aliphatic acids, nutraceuticals, and formaldehyde-free adhesives and binders (Holladay et al., 2007). In the past, cellulose has been used in the production of textiles or modified cellulose products. To achieve this, cellulose is treated under alkali conditions, until the long filaments of the polymer have been broken down. The resulting solution is then dissolved in carbon disulfide and treated with sodium hydroxide, producing a solution known as viscous. Finally, this material is placed in a bath containing sulfuric acid, which causes regenerated cellulose to be precipitated. This material, after a series of washing and bleaching stages, can then be further processed. At one time, this material was spun into rayon fibers, or extruded into cellophane films. Today, cellulose is more likely to be used as a feedstock for new materials such as cellulose nanocrystal (CNC). Cellulose is treated to a hydrolysis step (either acid or enzymatic) which removes hemicelluloses from microfibrils; the pure cellulosic material is recovered through centrifugation and purified with a dialysis stage to remove any acids, then treated to achieve a uniform suspension which can be concentrated and dried. CNC may be used in polymer production, as a component in foams and aerogels, and as a basis for advanced membranes (Brinchi et al., 2013). Cellulose may also be hydrolyzed to D-glucose and then fermented to produce ethanol (Humbird et al., 2011). Enzymatic hydrolysis, using enzymes expressed from wood rot fungi such as Trichoderma reesei or Aspergillus niger, can occur under relatively mild conditions of 45–50 °C. The challenge with enzymatic hydrolysis is generally the presence of inhibitors, including lignin, as well as hemicellulose sugars in the lignocellulosic matrix that remain after pretreatment. A number of enzyme ‘cocktails’ that contain both cellulases and a variety of accessory enzymes, capable of breaking down various hemicellulosic structures, have been developed. In commercial practice, acid hydrolysis has been superseded by enzymatic hydrolysis for biofuel production based on starch, and cellulosic ethanol based on wood will likely also utilize enzymatic hydrolysis (Sims et al., 2010).
4
Wood: Chemicals
Once extracted from the cellulose, glucose can be fermented through traditional means using Saccharomyces cervisiae (i.e., Baker’s yeast) or other commercially-available preparations; the fermentation process converts glucose into ethanol and carbon dioxide. Often, fermentation is conducted in different reactors than the enzymatic hydrolysis step (separate hydrolysis and fermentation – SHF), which allows both processes to be conducted at their respective optimum temperatures; the downside to this is that enzymatic activity in the first step is inhibited as glucose content rises. This can be solved using simultaneous saccharification and fermentation (SSF), which allows both steps to be performed in a single reactor, yielding more ethanol in a shorter process time. Ethanol does inhibit enzymes, but less so than glucose. The resulting ethanol can be recovered and concentrated through distillation; ethanol currently produced from sugarcane or corn starch is already used widely as a biofuel, and cellulosic ethanol may add to the volumes of this product in coming years. As with hemicellulose, sugars recovered from cellulose may also be converted to a wide variety of products through variations in the fermentation process; new aerobic and anaerobic organisms are being constantly identified with potential to produce a range of alternative products, including (but not limited to) acetic, butyric, citric, succinic, and lactic acids; acetone; glycerin; and higher alcohols such as butanol and isopropanol. While most research has concentrated upon ethanol as the most viable chemical product from this process, there is great interest in developing value-added chemicals from wood (Werpy and Petersen, 2004). Anaerobic processing may also be employed to dispose of wood residues and to generate methane. Given time, degradative bacteria will decompose the lignocellulosic matrix, producing simple molecules of carbon dioxide, hydrogen, and acetate. Methanogenic bacteria can then be harnessed to convert these materials to methane. A large amount of carbon dioxide remains in the methane gas produced, resulting in a mixture termed biogas that contains anywhere from 50 to 80% methane. Biogas has been considered for use in heat and electricity generation, but is typically not suitable as a synthesis gas.
References Base, P., 2010. Biomass Gasification and Pyrolysis − Practical Design and Theory. Burlington, MA: Academic Press. Biermann, C.J., 1996. Handbook of Pulping and Paper Making, second ed. San Diego, CA: Academic Press. Brinchi, L., Cotana, F., Fortunati, E., Kenny, J.M., 2013. Production of nanocrystalline cellulose from lignocellulosic biomass: Technology and applications. Carbohydrate Polymers 94 (1), 154–169. Chandra, R.P., Bura, R., Mabee, W.E., et al., 2007. Substrate pretreatment: The key to effective enzymatic hydrolysis of lignocellulosics? Advances in Biochemical Engineering/ Biotechnology 108, 67–93. Demirbas, A., 2007. Converting biomass derived synthetic gas to fuels via Fisher−Tropsch synthesis. Energy Sources, Part A: Recovery, Utilization, and Environmental Effects 29 (2), 1507–1512. Goldstein, I.S. (Ed.), 1981. Organic Chemicals from Biomass. Boca Raton, FA: CRC Press. Holladay, J.E., Bozell, J.J., White, J.F., Johnson, D., 2007. Top Value-Added Chemicals from Biomass Volume II − Results of Screening for Potential Candidates from Lignin. Batelle, OR: Pacific Northwest National Laboratory. Humbird, D., Davis, R., Tao, L., et al., 2011. Process Design and Economics for Biochemical Conversion of Lignocellulosic Biomass to Ethanol. Golden, CO: National Renewable Energy Laboratory. Shemfe, M.B., Gu, S., Ranganathan, P., 2015. Techno-economic performance analysis of biofuel production and miniature electric power generation from biomass fast pyrolysis and bio-oil upgrading. Fuel 143, 361–372. Sims, R.E.H., Mabee, W.E., Saddler, J.N., Taylor, M., 2010. An overview of second generation biofuel technologies. Bioresource Technology 101 (6), 1570–1580. Sjöström, E., 1993. Wood Chemistry: Fundamentals and Applications, second ed. Toronto, ON: Academic Press. Sohi, S.P., Crull, E., Lopez-Capel, E., Bol, R., 2010. A review of biochar and its use and function in soil. In: Sparks, D.L. (Ed.), Advances in Agronomy 105. Burlington, MA: Academic Press, pp. 47–82. Spath, P.L., Dayton, D.C., 2003. Preliminary Screening − Technical and Economic Assessment of Synthesis Gas to Fuels and Chemicals with Emphasis on the Potential for Biomass-Derived Syngas. Golden, CO: National Renewable Energy Laboratory. Wang, H., Male, J., Wang, Y., 2013. Recent advances in hydrotreating of pyrolysis bio-oil and its oxygen-containing model compounds. ACS Catalysis 3 (5), 1047–1070. Werpy, T., Petersen, G. (Eds.), 2004. Top Value-Added Chemicals from Biomass Volume I − Results of Screening for Potential Candidates from Sugars and Synthesis Gas. Batelle, OR: Pacific Northwest National Laboratory.
Further Reading Zhu, Y., Biddy, M.J., Jones, S.B., Elliott, D.C., Schmidt, A.J., 2014. Techno-economic analysis of liquid fuel production from woody biomass via hydrothermal liquefaction (HTL) and upgrading. Applied Energy 129, 384–394.
1 2 3 4 References Further Reading
1
Introduction Principal Chemical Components of Wood Chemical Products through Thermochemical Conversion Chemical Products through Biochemical Processes
1 1 2 3 4 4
Introduction
In addition to supplying lumber and fiber, wood from trees has been used as a source of chemicals for many applications throughout history. Naval stores, tanning agents, rubber, and methanol are all commonly known products. Today, concern over the cost, environmental impacts, and accessibility of petroleum products and other fossil fuels has brought about a resurgence of interest in the use of renewable biomass resources, including wood from trees, as a source of the fuels and chemicals that are so vital to our economy.
2
Principal Chemical Components of Wood
Wood, the structural component of a tree, is a complex matrix of three major polymers commonly labeled as cellulose, hemicellulose, and lignin, which together can be referred to as ‘lignocellulose.’ Table 1 shows the proportions and subgroups associated Table 1
Principle components of wood
Main group
% of Dry wood mass Hardwood (angiosperm) Acer saccharum
Cellulose Hemicellulose Glucomannan Glucuronoxylan Other polysaccharides Lignin
Extractives Phenolics Fats Terpenoids
Softwood (gymnosperm) Eucalyptus globulus
40.7 51.3 30.8 25.2 3.7 1.4 23.6 19.9 3.5 3.9 25.2 21.9 Guaiacyl–syrigyl lignin (ratios from 4:1 to 1:2, depending upon species) 3.3 1.6 Stilbenes Tannins Palmitic Oleic Turpentine Pinene Rubber
Pinus radiata
Juniperus communis
37.4 33.2 20.4 8.5 4.3 27.2 Guaiacyl lignin
33.0 30.3 16.4 10.7 3.2 32.1
2.2 Lignans Flavonoids Stearic Linoleic Limonene Resin acids
4.6
Source: Sjöström, E., 1993. Wood Chemistry: Fundamentals and Applications, second ed. Toronto, ON: Academic Press.
☆
Change History: February 2015. W.E. Mabee, L. Cabral, and D.N. Roy carried out significant edits to the text. Primary sections remain the same, but text in each section has been expanded and brought up-to-date. In-line citations have been introduced. Table 1 has been updated to reflect the text. May 2015: W.E. Mabee has carried out edits to the reference list based on the editor’s comments.
Reference Module in Materials Science and Materials Engineering
doi:10.1016/B978-0-12-803581-8.02212-8
1
2
Wood: Chemicals
with each major polymer for two ‘hardwood’ (angiosperm) and two ‘softwood’ (gymnosperm) species. As shown in the table, the greatest component of wood is cellulose (properly speaking alphacellulose) which is made up of β-D-glucose units connected in a straight chain through 1–4 linkages, with a degree of polymerization (DP) of between 10 000 and 15 000 glucose units. Cellulose is the structural component of wood. Hemicellulose is a branched polymer made up of both 5- and 6-carbon sugars, including xylose and arabinose (5-C) as well as glucose, galactose, mannose, and rhamnose (6-C); hemicellulose molecules have a DP of 200 or less. It serves to connect bundles of cellulose strands into microfibrils, which in turn are a principle building block of wood fibers. Lignin is made up of a number of monomeric structures, including p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol; the latter two dominate, with coniferyl units polymerized into so-called guaiacyl lignin and sinapyl alcohol forming syringyl lignin. Guaiacyl lignin dominates in softwood species, while a combination of syringyl and guaiacyl lignin is found in hardwoods. The DP of lignin is difficult to determine because the polymer essentially acts as the ‘glue’ within the lignocellulosic structure, and extraction is very difficult if not impossible. Wood also contains a number of other chemicals, in small portions, known collectively as the extractives, which were historically the most important group for chemical production. Chemicals derived from extractives include naval stores (i.e., turpentine and pitch), resin, and rosins. This group contains many interesting compounds that, even today, are becoming more important in the global quest for pharmaceutical and chemical goods. Contrasting the proportions of chemicals found in each species provides an illustration of the variation found between different species; it should also be noted that chemical composition varies between different trees of the same species, and indeed between wood found in different locations within the tree. Generally speaking, hardwoods have a greater proportion of cellulose and lower proportion of lignin than softwoods; hemicellulose in hardwoods is dominated by xylans (5-C sugars), which are found in much lower proportions in softwoods; and hardwoods have significant proportions both syringyl and guaiacyl lignin, while softwood lignin is dominated by the guaiacyl form (Biermann, 1996; Sjöström, 1993; Goldstein, 1981).
3
Chemical Products through Thermochemical Conversion
Transforming the polymers of wood into chemical products can follow several paths. Thermochemical processes use heat in a controlled fashion to break down the lignocellulosic matrix into products such as bio-oil and char, or to completely deconstruct the wood to produce synthesis gases (H2, CO, and CO2) which can then be reformed into new products through catalytic conversion (Base, 2010). At relatively low temperatures of between 250–380 °C, a liquid bio-oil may be generated through hydrothermal liquefaction. In this process, solid biomass together with water are pressurized to 5–30 MPa and heated at 250 to 380 °C. The resulting reaction dissolves the biomass and produces bio-oil, as well as hot gasses and water containing dissolved organic compounds. The bio-oil obtained from this process has a relatively high heating value, but still requires further processing (described below) to produce a fuel that is compatible with existing infrastructure, such as generators or engines. Pyrolysis occurs when wood is heated in the absence of external air or oxygen. During pyrolysis, the biomass is heated in a sealed vessel at varying rates to temperatures of between 300–650 °C. In ‘slow’ pyrolysis, the period of heating to moderate temperatures of between 300–500 °C is extended, as is the residence time at full temperature. The primary products from slow pyrolysis are biochar, a solid form of ‘fixed’ carbon, as well as bio-oil and synthesis gases; the exact proportions of each depend upon residence time and temperature employed. Biochar is currently being explored as an option for carbon sequestration, as in this form carbon will remain fixed for potentially thousands of years. It also has potential utility as a soil amendment and as an agent for wastewater treatment (Sohi et al., 2010). ‘Fast’ pyrolysis, which involves similarly moderate temperatures of approximately 500 °C, differs from the previous process in that the heating period and residence times are shortened, and in that the reaction is stopped via rapid cooling at the end in order to increase bio-oil production by condensing gas outputs. As total reaction time drops, the proportion of bio-oil (also labeled biocrude) generated from pyrolysis increases, and the proportions of biochar and synthesis gas decrease. Bio-oil, as stated previously, has high heating values, but is not compatible with existing infrastructure due to a relatively high oxygen content and the corrosive nature of the liquid, as well as a tendency to form oligomers. These issues can be addressed through hydrotreatment (a practice used with petroleum products to remove impurities) or via hydrodeoxygenation (a catalyzed version of hydrotreatment which is currently being developed). As options for the cost-effective upgrading of bio-oil are explored, it is increasingly being looked at as a source of fuel for transportation and small-scale power generation (Wang et al., 2013; Shemfe et al., 2015). At relatively high temperatures of above 800 °C, pyrolysis is often referred to as gasification. At these temperatures and with long residence times, approximately 85% of lignocellulose may be converted to synthesis gas (also labeled syngas), which is primarily hydrogen, carbon monoxide, and carbon dioxide. Gasification sometimes differs from true pyrolysis in that it may be carried out with controlled access to external air, steam, or oxygen; this addition influences the products obtained, allowing nitrogen-rich syngas (with air) or the formation of CH4 (with water). The syngas produced via biomass gasification may be enriched with other gases (such as hydrogen) or before further processing is carried out. A common goal of biomass gasification is the production of liquid fuels for transport or energy generation. To produce liquid fuel, synthesis gases (potentially enriched) are passed over a metal catalyst in order to produce alcohols, including methanol, ethanol, and larger mixed alcohols. For methanol and ethanol, the catalyst used may be copper, aluminum, or zinc oxides; for longer-chain alcohols, catalyst types that have been researched include sulfide-, oxide-, and rhodium-based systems. Typically, synthesis gases are reacted over the catalyst at temperatures and pressures in the range of 300 °C and 10 MPa respectively. Longer-
Wood: Chemicals
3
chain hydrocarbon fuels may be created using iron or cobalt catalysts via the Fischer–Tropsch process. This reaction occurs at 200– 350 °C and 1.5–4.0 MPa; varying reaction conditions within this band determines the chain length (from C4 to C33) and the proportion of byproducts (unsaturated hydrocarbons and alcohols) in the final product (Demirbas, 2007). Fischer–Tropsch fuels have been made commercially for many years using coal or natural gas as feedstock; commercial F-T operations based on biomass have not (as of 2015) been successfully operated. In addition, a number of chemical products (e.g., 2-ethylhexanol (a plasticizer used in the manufacture of PVC) and isobutene (a precursor for gasoline oxygenates)) may be generated using biomass-based synthesis gas at higher temperatures and pressures than typically seen in liquid fuel production, and using relatively exotic catalysts (such as rhodium or thorium). The primary focus of most research in this area, however, remains on liquid fuel generation (Spath and Dayton, 2003).
4
Chemical Products through Biochemical Processes
Biochemical processes can also be used to make fuels and chemicals from wood. These processes may be subdivided into aerobic and anaerobic approaches; these approaches may be used separately or combined depending upon the desired end products. Aerobic processes include hydrolysis and fermentation, which are primarily utilized to modify the polysaccharide components of wood. The complexity of lignocellulosic substrates usually requires that a pretreatment take place, which is designed to open up the matrix of chemicals and improve the accessibility of the material to subsequent process steps. A number of different pretreatments, including steam explosion (the injection of steam at high temperature and pressure, followed by rapid release), organosolv pulping (the use of ethanol or other organic solvents to dissolve lignin), and treatment with acid and water at elevated temperatures have been considered. Depending upon the subsequent processes, it may be beneficial to remove lignin completely (as with acid pretreatments or organosolv pulping), or lignin may remain in a modified state within the substrate (as with steam explosion). The development of pretreatment technology is therefore typically integrated with design of hydrolysis operations in mind (Chandra et al., 2007). The hemicelluloses are the easiest of the three major macromolecules to hydrolyze, due to a relatively low molecular weight. A variety of products can be produced from hemicelluloses, but the principle chemicals yielded are glucose, mannose, arabinose (from softwoods), and xylose (from hardwoods). Mannose and glucose can both be fermented in commercial processes, yielding ethanol. For the five-carbon sugars found in hemicelluloses, fermentation with new yeast strains may be an option, and several bacterial strains have been developed for the conversion of compounds such as xylose to alcohols such as butanol or butenediol. Xylose may be combined to make furfural under highly acidic conditions. Glucose, arabinose, and xylose may also be hydrogenated to sorbitol, xylitol, and arabinitol respectively. These are three biomass chemicals singled out by the US Department of Energy as being very promising value-added chemicals and building blocks to other commercially important chemicals (Werpy and Petersen, 2004). Lignin may be isolated from wood through conventional pulping processes (e.g., in black liquor from Kraft pulping, or in spent sulfite liquor) or via new pretreatment processes (such as organosolv pulping). The principle chemical products from lignin are phenolic compounds, which are obtained through hydrogenation of the cell wall. In an initial screening of potential chemical products from lignin, the Pacific Northwest National Laboratory in the US considered current and future feasibility; market volumes, value, and risk; and the utility of the product as a building block. One important finding was that many of the lignin products with the best combined potential were likely to be recovered through thermomechanical processes, including heat and power as well as syngas-derived products such as Fischer–Tropsch liquids, methanol, and ethanol. Important structures that might be derived through chemical means and which had a combination of positive technical and market feasibility included polyelectrolites, aromatic acids, cyclohexanol, aliphatic acids, nutraceuticals, and formaldehyde-free adhesives and binders (Holladay et al., 2007). In the past, cellulose has been used in the production of textiles or modified cellulose products. To achieve this, cellulose is treated under alkali conditions, until the long filaments of the polymer have been broken down. The resulting solution is then dissolved in carbon disulfide and treated with sodium hydroxide, producing a solution known as viscous. Finally, this material is placed in a bath containing sulfuric acid, which causes regenerated cellulose to be precipitated. This material, after a series of washing and bleaching stages, can then be further processed. At one time, this material was spun into rayon fibers, or extruded into cellophane films. Today, cellulose is more likely to be used as a feedstock for new materials such as cellulose nanocrystal (CNC). Cellulose is treated to a hydrolysis step (either acid or enzymatic) which removes hemicelluloses from microfibrils; the pure cellulosic material is recovered through centrifugation and purified with a dialysis stage to remove any acids, then treated to achieve a uniform suspension which can be concentrated and dried. CNC may be used in polymer production, as a component in foams and aerogels, and as a basis for advanced membranes (Brinchi et al., 2013). Cellulose may also be hydrolyzed to D-glucose and then fermented to produce ethanol (Humbird et al., 2011). Enzymatic hydrolysis, using enzymes expressed from wood rot fungi such as Trichoderma reesei or Aspergillus niger, can occur under relatively mild conditions of 45–50 °C. The challenge with enzymatic hydrolysis is generally the presence of inhibitors, including lignin, as well as hemicellulose sugars in the lignocellulosic matrix that remain after pretreatment. A number of enzyme ‘cocktails’ that contain both cellulases and a variety of accessory enzymes, capable of breaking down various hemicellulosic structures, have been developed. In commercial practice, acid hydrolysis has been superseded by enzymatic hydrolysis for biofuel production based on starch, and cellulosic ethanol based on wood will likely also utilize enzymatic hydrolysis (Sims et al., 2010).
4
Wood: Chemicals
Once extracted from the cellulose, glucose can be fermented through traditional means using Saccharomyces cervisiae (i.e., Baker’s yeast) or other commercially-available preparations; the fermentation process converts glucose into ethanol and carbon dioxide. Often, fermentation is conducted in different reactors than the enzymatic hydrolysis step (separate hydrolysis and fermentation – SHF), which allows both processes to be conducted at their respective optimum temperatures; the downside to this is that enzymatic activity in the first step is inhibited as glucose content rises. This can be solved using simultaneous saccharification and fermentation (SSF), which allows both steps to be performed in a single reactor, yielding more ethanol in a shorter process time. Ethanol does inhibit enzymes, but less so than glucose. The resulting ethanol can be recovered and concentrated through distillation; ethanol currently produced from sugarcane or corn starch is already used widely as a biofuel, and cellulosic ethanol may add to the volumes of this product in coming years. As with hemicellulose, sugars recovered from cellulose may also be converted to a wide variety of products through variations in the fermentation process; new aerobic and anaerobic organisms are being constantly identified with potential to produce a range of alternative products, including (but not limited to) acetic, butyric, citric, succinic, and lactic acids; acetone; glycerin; and higher alcohols such as butanol and isopropanol. While most research has concentrated upon ethanol as the most viable chemical product from this process, there is great interest in developing value-added chemicals from wood (Werpy and Petersen, 2004). Anaerobic processing may also be employed to dispose of wood residues and to generate methane. Given time, degradative bacteria will decompose the lignocellulosic matrix, producing simple molecules of carbon dioxide, hydrogen, and acetate. Methanogenic bacteria can then be harnessed to convert these materials to methane. A large amount of carbon dioxide remains in the methane gas produced, resulting in a mixture termed biogas that contains anywhere from 50 to 80% methane. Biogas has been considered for use in heat and electricity generation, but is typically not suitable as a synthesis gas.
References Base, P., 2010. Biomass Gasification and Pyrolysis − Practical Design and Theory. Burlington, MA: Academic Press. Biermann, C.J., 1996. Handbook of Pulping and Paper Making, second ed. San Diego, CA: Academic Press. Brinchi, L., Cotana, F., Fortunati, E., Kenny, J.M., 2013. Production of nanocrystalline cellulose from lignocellulosic biomass: Technology and applications. Carbohydrate Polymers 94 (1), 154–169. Chandra, R.P., Bura, R., Mabee, W.E., et al., 2007. Substrate pretreatment: The key to effective enzymatic hydrolysis of lignocellulosics? Advances in Biochemical Engineering/ Biotechnology 108, 67–93. Demirbas, A., 2007. Converting biomass derived synthetic gas to fuels via Fisher−Tropsch synthesis. Energy Sources, Part A: Recovery, Utilization, and Environmental Effects 29 (2), 1507–1512. Goldstein, I.S. (Ed.), 1981. Organic Chemicals from Biomass. Boca Raton, FA: CRC Press. Holladay, J.E., Bozell, J.J., White, J.F., Johnson, D., 2007. Top Value-Added Chemicals from Biomass Volume II − Results of Screening for Potential Candidates from Lignin. Batelle, OR: Pacific Northwest National Laboratory. Humbird, D., Davis, R., Tao, L., et al., 2011. Process Design and Economics for Biochemical Conversion of Lignocellulosic Biomass to Ethanol. Golden, CO: National Renewable Energy Laboratory. Shemfe, M.B., Gu, S., Ranganathan, P., 2015. Techno-economic performance analysis of biofuel production and miniature electric power generation from biomass fast pyrolysis and bio-oil upgrading. Fuel 143, 361–372. Sims, R.E.H., Mabee, W.E., Saddler, J.N., Taylor, M., 2010. An overview of second generation biofuel technologies. Bioresource Technology 101 (6), 1570–1580. Sjöström, E., 1993. Wood Chemistry: Fundamentals and Applications, second ed. Toronto, ON: Academic Press. Sohi, S.P., Crull, E., Lopez-Capel, E., Bol, R., 2010. A review of biochar and its use and function in soil. In: Sparks, D.L. (Ed.), Advances in Agronomy 105. Burlington, MA: Academic Press, pp. 47–82. Spath, P.L., Dayton, D.C., 2003. Preliminary Screening − Technical and Economic Assessment of Synthesis Gas to Fuels and Chemicals with Emphasis on the Potential for Biomass-Derived Syngas. Golden, CO: National Renewable Energy Laboratory. Wang, H., Male, J., Wang, Y., 2013. Recent advances in hydrotreating of pyrolysis bio-oil and its oxygen-containing model compounds. ACS Catalysis 3 (5), 1047–1070. Werpy, T., Petersen, G. (Eds.), 2004. Top Value-Added Chemicals from Biomass Volume I − Results of Screening for Potential Candidates from Sugars and Synthesis Gas. Batelle, OR: Pacific Northwest National Laboratory.
Further Reading Zhu, Y., Biddy, M.J., Jones, S.B., Elliott, D.C., Schmidt, A.J., 2014. Techno-economic analysis of liquid fuel production from woody biomass via hydrothermal liquefaction (HTL) and upgrading. Applied Energy 129, 384–394.