More recently a reaction scheme has been proposed by Evans and Milne as shown in Figure 2.5 (16). Similar to the Copper Mountain reaction scheme, the influence of "pyrolysis severity" is considered as the increase in temperature, heating rate and vapour residence time. Under high pressure conditions, the direct formation of a liquid product is due to the wood deforming to a "plastic" state. This has been confirmed by Diebold (49) and Lede et al. (50, 51,52) who have shown that under condition where ablation occurs, the wood may exhibit the properties of a "molten plastic state”. At low pressure, however, it is not clear whether a liquid phase exists, after the primary decomposition of the biomass.

It can be seen that the initial models for the pyrolysis of the main components of wood have developed through to an overall reaction scheme as proposed by Diebold, and Evans and Milne. Evans and Milne do, however, consider a discrete tertiary reaction stage. The process of pyrolysis is complex, but a recent theory is that primary vapours are first produced, the characteristics of which are most influenced by heating rate. These primary vapours then further degrade to secondary tars and gases, the proportions and characteristics of which are a function of temperature and time (53). Yields of liquids from pyrolysis can thus be influenced by the rate of reaction, with fast or flash pyrolysis at lower temperatures of typically 450-650°C giving the highest liquid yields.