Other work has been carried out on the pyrolysis of hemicellulose but this has received less attention due to its lower abundance, variety of constituents, high reactivity and rapid degradation at low temperatures (150-350°C). It is believed that the intermediate levoglucosan is replaced by a furan derivative (16, 19). This may be due its lack of crystallinity (22). Soltes and Elder (23) suggest a two step degradation process where the first step is depolymerisation to water soluble fragments subsequently followed by decomposition to volatile components.

2.2.2 Lignin Pyrolysis

The complex structure of lignin has led to a lack of understanding of the pyrolysis of this component. Lignin is the most thermally stable component but its structure varies according to its source and the method of isolation. To date therefore, most detailed work in lignin pyrolysis has been obtained from model compounds. Minor decomposition begins at 250°C but most significant lignin pyrolysis occurs at higher temperatures (13,18, 22). High molecular weight compounds such as

2-6

coniferyl alcohol and sinaptyl alcohol are formed during the initial stage of pyrolysis by the formation of double bonds in the alkyl side chain of the lignin structure.

Low temperature pyrolysis of lignin (< 600°C) has been carried out by a large number of researchers (16,18, 19, 24, 25, 26, 27, 28, 29, 30). Detailed work using Kraft lignin has also been carried out by Jegers and Klein (31, 32) who identified and quantified 33 products (12 gases, water, methanol, and 19 aromatic compounds such as phenol, creso! and guaiacol) at a range of temperatures from 300 to 500°C. latridis and Gavalas (33) studied the pyrolysis of kraft lignin at 400- 700°C using a captive sample reactor, obtaining a total volatiles yield of 60 wt%. Nunn et al. (34) have also carried out work in this area obtaining a maximum of 53 wt% liquid at 625°C, again in a captive sample reactor.

High temperature pyrolysis of lignin (> 600°C) leads to complex cracking, dehydrogenation, condensation, polymerisation and cyclisation reactions resulting in the formation of products such as СО, CH4, other gaseous hydrocarbon, acetic

acids, hydroxyacetaidehyde and methanol. Polyaromatics, benzene, phenylphenols, benzofurans and naphthalenes are formed by other secondary reactions (9, 16, 18, 19, 25, 34, 35).

Other work has been carried out with model compounds and mathematical models to obtain reaction mechanisms and reaction kinetics for lignin pyrolysis. Klein and Virk have proposed a reaction mechanism derived from the pyrolysis of the model compound phenethylphenyl ether (18, 32, 33).

Mathematical modelling of lignin pyrolysis has been attempted using the Monte Carlo technique (36, 37). The overall simulation is comprised of two Monte Carlo simulations: one comprising the lignin structure and the second the degradation of its oligomers. The simulation contained model compound reaction pathways and kinetics in a Markov-chain based simulation of the reaction of lignin polymers which subsequently produced yields of various hydrocarbons and oxygenated compounds. Other mathematical models have been developed by Solomon (25, 38) to predict the molecular weight distribution of the tars and Anvi (24, 39) who predicted the rate of evolution of lignin pyrolysis gases.