General

Whatever the actual mechanisms of biomass pyrolysis, many reactions take place and many products are formed. The older biomass pyrolysis processes were carried out in the batch mode over long periods of time for char produc­tion. As the technology developed, other processes were designed to operate in the batch or continuous modes over shorter residence times at moderately higher temperatures. Depending on the pyrolysis temperature, the char fraction contains inorganic materials ashed to varying degrees, any unconverted organic solids, and carbonaceous residues produced on thermal decomposition of the organic components. The liquid fraction is a complex mixture of water and organic chemicals having lower average molecular weights than the feedstock components. For highly cellulosic biomass feedstocks, the liquid fraction usu­ally contains acids, alcohols, aldehydes, ketones, esters, heterocyclic deriva­tives, and phenolic compounds. The tars contain native resins, intermediate carbohydrates, phenols, aromatics, aldehydes, their condensation products, and other derivatives. The pyrolysis gas is a low — to medium-energy gas having a heating value of about 3.9 to 15.7 MJ/m3 (n) (100 to 400 Btu/SCF). It contains carbon dioxide, carbon monoxide, methane, hydrogen, ethane, ethylene, minor amounts of higher gaseous organics, and water vapor.

It is apparent that if one wishes to obtain pure chemicals by biomass pyrolysis, further processing to separate the reaction mixture is necessary. As will be shown later, this did not hinder commercial use of biomass pyrolysis for the manufacture of specific chemicals. The slow, destructive distillation of biomass was commercial technology for the production of several commodity chemicals long before fossil fuels became the preferred feedstocks. Hardwood pyrolysis once served as an important commercial source of methanol, acetic acid, ketones, and other chemicals.