Decomposition mechanisms during torrefaction

From the three main polymeric constituents of biomass, cellulose has received most attention considering the thermal decomposition of biomass. Especially since the cellulose fraction is large. Therefore, in pyrolysis research cellulose decomposition is very important, but as Figure 2.3 illustrates, cellulose decomposition is not the most relevant constituent in the temperature range of torrefaction (200 - 300 ?°C). During torrefaction, mass loss will predominantly come from the decomposition (devolatilisation) of particularly hemicellulose and some of lignin. Xylan-based hemicellulose generally has its peaking rate in decomposition around 250 to 280 ?°C. Lignin decomposition proceeds slower, but shows a gradual increase of decomposition rate starting from temperatures of about 200 ?°C or even lower. Note, however, that the thermal decomposition behaviour of the individual polymers of wood (and in general biomass) may be different from their strongly interacted structure in wood itself. Indications for this can be extracted from Figure 2.3, as wood on mass basis starts to decompose at a temperature higher than 200 ?°C.



Figure 2.3 Thermogravimetry of cotton wood and its constituents (taken from Shafizadeh and McGinnis, 1971). The green lines are added to indicate the torrefaction temperature regime

Hemicellulose decomposition can be well described by a two-step mechanism as was found by Di Blasi and Lanzetta (1997). The first reactions usually taking place below 250?°C (first step) are depolymerisation reactions leading to altered and rearranged polysugar structures. The decomposition of these oligosaccharides and monosaccharides at higher temperatures (250-300 ?°C) results in the formation of chars, CO, CO2 and water. The formation of light volatiles like carbonyl compounds result from the fragmentation of the carbon skeleton.

Up to 250 ?°C, thermal decomposition of cellulose is hardly accompanied with a serious mass loss. The most important mechanism occurring is found to be depolymerisation. Depolymerisation of cellulose has been observed at even 70 ?°C (Golova, 1971). From the work of Shafizadeh et. al. (1983) it is known that depolymerisation of wood is already occurring at significant rates at 150 ?°C. At 190 ?°C, the rate of depolymerisation is already seriously fast. Gaur and Reed (1998) mention that cellulose decomposition involving mass loss starts at 250?°C. A variety of permanent gases, condensable liquids and char are formed during this step.

Thermal degradation of lignin takes place over a wide temperature range. At temperatures below 200?°C, some thermal softening has been observed resulting in a small weight loss of a few percent. Char formation and the release of volatiles result from a devolatilisation process in the temperature region of 240-600?°C. Since the composition of lignin is different for hardwood and softwood, different kinetics are expected. Muller-Hagedorn et. al. (2003) concludes that coniferous lignin is thermally more stable than deciduous lignin.