Chars, gases, light and heavy liquids, and water are formed in varying amounts on pyrolysis of biomass. The yields depend particularly on the feed composi­tion, dimensions of the feed particles, heating rate, temperature, and reaction time. When hardwoods are heated in the absence of air, they decompose and are converted into charcoal and a volatile fraction that partly condenses on cooling to a liquor called pyroligneous acid, which separates into a dark heavy oil as the lower layer in about 10 wt % yields, and an upper aqueous layer. Dry distillation of softwoods such as pine yield similar products in about the same amounts as well as lighter pine oils and terpene liquids such as turpen­tines. The supernatant layer contains methanol, acetic acid, traces of acetone, allyl alcohol, and other water-soluble compounds. Methanol is formed from the lignin components that bear methoxyl groups. The heavy oil contains tars, higher viscosity pitches, and some char. The wood tars and pitches are complex mixtures in which hundreds of organic compounds have been identified, pri­marily acidic and heterocyclic compounds.

The data in Table 8.3 show how the gas, pyrolytic oil, and char yields vary with pyrolysis temperature and different biomass feedstocks (Epstein, Kostrin, and Alpert, 1978). Extensive depolymerization of the celluloses starts at about 300°C and usable charcoal formation (carbon content about 75 wt %) starts at about 350°C (Zaror and Pyle, 1982). Higher temperatures and longer residence times promote gas production, while higher char yields are obtained at lower temperatures and slow heating rates. The product slate is similar for each feedstock at a given temperature, although the yields of gas, pyrolytic oil, and char can be quite different. The cellulosics and hemicellulosics are the main sources of volatiles in biomass feedstocks, but yield only about 8 to 15% of their weight as charcoal under conventional pyrolysis conditions (i. e., slow heating rate, atmospheric pressure, and maximum temperatures of 400 to 450°C). The lignins yield nearly 50% of their weight as charcoal under these conditions (Zaror and Pyle, 1982). A more detailed distribution of specific products on long-term pyrolysis of three woody biomass feedstocks, birch, pine, and spruce wood, to a final temperature of 400°C is shown in Table 8.4 (Nikitin et al, 1962; Bagrova and Kozlov, 1958). Both the individual product distributions and the yields of carbon, pyroligneous distillate, and gases are similar for each wood species. It is evident that the product mixture is complex and that selectivities for specific chemicals are low. The order of decreasing yield on a weight basis by product group from highest to lowest is pyroligneous distillate, charcoal, and gaseous products. This might be expected because of the relatively low pyrolysis temperatures and the 8-h period over which these experiments were performed. However, if water is excluded from the yield calculations, the order of decreasing yield is charcoal, pyroligneous distillate, and gaseous products.

The pyrolysis of the combustible fraction of MSW at higher temperatures is illustrated by the data in Table 8.5. These data show how temperature affects product yields and gas and char compositions on pyrolysis at temperatures up to 900°C (Hoffman and Fitz, 1968). Gas yield increases as the temperature is increased from 500 to 900°C. Although the heating value of the product gas remains about the same, significant increases in gas yield on a weight percent and energy yield basis and in hydrogen occur with increasing tempera­ture. Interestingly, as the temperature increases, the char yields and volatile matter content of the chars decrease as expected, but the energy value of the chars is relatively constant.

"Epstein, Kostrin, and Alpert (1978). The feedstock was pyrolyzed in a 0.5-m ID fluid-bed reactor containing sand and an inert gas generated from compressed air-natural gas combustion with a slight excess of air (about 0.2 to 0.6%). The fluidizing velocities were 0.3 to 1 m/s. The products were low-energy gas (3.89-11.78 MJ/mJ (n)), pyrolytic oil (23.3-27.9 MJ/kg), and charcoal. Feed rates were 50-200 kg/h. The moisture contents of the feedstocks were not specified. The balance of the yield for each feedstock is water.

Since biomass pyrolysis product mixtures are very complex and selectivities are low for specific products, considerable effort has been devoted to improving selectivities. Selectivities can sometimes be increased by addition of coreactants or catalysts, or by changing the pyrolysis conditions (cf. Nikitin et al, 1962). For example, the pyrolysis of maplewood impregnated with phosphoric acid increased the yield of methanol to 2.2 wt % of the wood as compared to 1.3 wt % obtained on dry distillation of the untreated wood. Addition of sodium carbonate to oak and maple increased the yield of methanol by 100 and 60%, respectively, compared to pyrolysis yields without sodium carbonate. Other weakly alkaline reagents exhibited a similar effect. Pyrolysis of wood in a stream of benzene, xylene, or kerosine increased the yields of acetic acid, aldehydes, and phenols and reduced the yield of tars. Optimization of pyrolysis conditions will be shown later to have large effects on product distributions and yields.