All biomass resources are composed primarily of cellulose (typically 30-40% of dry weight), hemicellulose (25-30% of dry weight), and lignin (12-30% of dry weight), but the percent of each compound differs significantly among biomass resources. This heterogeneity creates variability in the yields of pyrolysis products.
Cellulose is converted to char and gases (CO, CO2, H2O) at low temperatures (< 300oC), and to volatile compounds (tar and organic liquids, predominantly levoglucosan) at high temperatures (> 300oC) (Funakuzuri, 1986). The yield of light hydrocarbons (i.e., C1 - C4) is negligible below 500°C, but increases substantially at high temperatures (Scott et al., 1988). At temperatures above 600°C, tar yields drop, gas yields increase, and the pyrolysis of cellulose is complete (Hajaligol, 1982; Bradbury, 1979; Funazukuri, 1986; and Piskorz, 1986).
Hemicellulose is the most reactive component of biomass and decomposes between 200 and 260oC (Koufopanos, 1989). The decomposition of hemicellulose is postulated to occur in two steps - the breakdown of the polymer into water soluble fragments, followed by conversion to monomeric units and decomposition into volatile compounds (Soltes and Elder, 1981). Hemicelluloses produce more gases and less tar than cellulose, and no levoglucosan. They also produce more methanol and acetic acid than cellulose.
Lignin is a highly linked, amorphous, high molecular weight phenolic compound which serves as cement between plant cells and is the least reactive component of biomass. The time required to pyrolyze biomass resources is controlled by the rate of pyrolysis of lignin under operating conditions. Decomposition of lignin occurs between 280°C and 500°C, although some physical and/or chemical changes (e.g., depolymerization, loss of some methanol) may occur at lower temperatures (Koufopanos, 1989). At slow heating rates, lignin loses only about half of its weight at temperatures below 800°C (Wenzel, 1970). Pyrolysis of lignin yields more char and tar than cellulose (Soltes and Elder, 1981).
For wood, the decomposition of the major components occurs separately and sequentially, with the hemicellulose decomposing first and the lignin last. Up to 200°C, moisture is removed, volatile products such as acetic acid and formic acid are released, and non-condensable gases such as CO and CO2 are produced. Between 200 and 280°C, further decomposition of the char and wood occur, resulting in the release of pyroligneous acids, water, and non-condensable gases. Separation of tar also occurs. Between 280 and 500°C, release of combustible volatile products (CO, CH4, H2, formaldehyde, formic acid, methanol, and acetic acid) occurs. Char formation decreases and the carbon content of the char increases. Condensable tar is released. Above 500°C, carbonization is complete. Secondary reactions begin if the materials are not removed from the reaction zone as quickly as they form.
When cooled, some volatile compounds produced during the pyrolysis of biomass resources condense to form a liquid called bio-oil. Bio-oil consists of 20-25% water, 25-30% pyrolytic lignin, 5-12% organic acids, 5-10% non-polar hydrocarbons, 5-10% anhydrosugars, and 10-25% other oxygenated compounds. Due to large amounts of oxygenated compounds, bio-oil is polar and does not mix readily with hydrocarbons (such as petroleum-derived fuels). It contains less nitrogen than petroleum, and almost no metal or sulfur. Bio-oil is acidic (pH of 2 to 4), due to the creation of organic acids (e.g., formic and acetic acid) when biomass degrades, and is corrosive to most metals except stainless steel. Hydrophilic bio-oils contain 15 to 35% water by weight, which can not be removed by conventional methods like distillation. High water content decreases its viscosity, which aids in transport, pumping and atomization, improves stability, and lowers the combustion temperature, which reduces NOx emissions. Some additional water can be added, but only up to a point before phase separation occurs - which prevents bio-oils from being dissolved in water. Bio-oil is relatively unstable compared to fossil fuels, due to the presence of more polymeric compounds. Table 1 summarizes select properties of bio-oil derived from the pyrolysis of wood.
A number of studies have examined factors that affect the kinetics of biomass pyrolysis reactions. Studies that have examined temperature and heat rate interactions include Scott, 1988 (maple wood); Aarsen, 1985 (wood); Ayll’on, 2006 (meat and bone meal); Koufapanos, 1989 (sawdust); Nunn, 1985 (wood and cellulose); Utioh, 1989 (wheat grain); and Sadakata, 1987. These studies indicate that temperature is more important than rate of heating with respect to the mix of products, and that at any given temperature and heat rate, bio-oil and char are the dominant products. Bio-oil yields increase up to temperatures between 550°C and 680°C and then decline. As temperatures increase, char production decreases (to a steady level above 650°C) and the carbon content of the char increases. Hydrocarbon gas yields (e.g., C2H6, C3H6) increase up to about 660°C and then decline, probably due to thermal cracking. The time required to obtain a given conversion level decreases with increasing temperature.
Biomass weight loss is higher at lower pressures (Ward and Braslaw, 1985). At any given temperature, char residues increase pressure. Cellulose displays the strongest pressure dependency and lignin the lowest; the pressure effect is observable at temperatures above 350°C. The higher pressure increases the residence time of the volatile compounds, resulting in higher yields of low molecular weight gases and lower yields of tar and liquid products (Blackadder and Rensfelt, 1985).
The presence of inorganic materials (either as additives or as the natural ash content of the biomass resource) affects the mix of pyrolysis products. The impacts are measured using thermogavimetry (TG), thermal evolution analysis (TEA), and differential thermal analysis (DTA). Alkaline compounds have a more pronounced effect than do acidic compounds. Alkaline catalysts increase gas yields and char production, and decrease tar yields; reduce the decomposition temperature; increase weight loss; and increase reaction rates (Utioh, 1989; Roberts, 1970; Tsuchiya and Sumi, 1970). Acid catalysts cause transglycosylation reactions in small quantities, and dehydration of the anhydrosugars in larger quantities. Acidic catalysts enhance the condensation of intermediate compounds and affect char oxidation. Inorganic salts reduce CO, H2, and hydrocarbon gases - but increase CO2,, decrease tar, increase water yields, and increase char yields (Nasser, 1986). The presence of catalysts are most significant for wood and cellulose pyrolysis, but negligible for lignin pyrolysis (Nassar and MacKay, 1986).