Independent components models incorporate the notion of ‘pseudo-component’. Fig­ure 11.2d gives a typical example of pyrolysis with multiple decomposition levels. The combination of ‘pure’ components decomposing in the same system at differ­ent temperatures would be equivalent to the reaction pattern in Fig. 11.2b. As the temperature of the biomass particles increases, the stability of the solid macromolec­ular matrices changes accordingly. This variation is mostly due to the release of low molecular weight compounds as previously explained. The reactivity of the particu­late material will thus vary during pyrolysis and this behaviour is similar to having different components decomposing with specific reaction kinetics.

The pyrolysis of wood, for example, can be described by the combination of hemicellulose, cellulose and lignin pyrolysis kinetics, which all show single or two steps decomposition [37]. In this case, the combination of the decomposition steps for each of the three individual components can accurately model the overall pyrolysis behaviour of wood. However, ideal cases like wood are rare and the concept of ‘pseudo-component’ may not be useful for other biomass such as manure and MSW, for example, which are highly heterogeneous. One major disadvantage of this second

Temperature/ C

type of models is its complexity and the large number of reaction parameters involved. With materials having a variable composition from one provider to another, the value of these parameters will change as well as the weighting of each ‘pseudo-component’ species.