Many simplified pyrolysis global reaction models were proposed in the scientific lit­erature. Babu [36] proposed to regroup these conceptualizations of pyrolysis kinetics into three categories: (1) single-step models, (2) independent components models and (3) parallel and series reactions models. Figure 11.2 illustrates these models.

11.3.2.1 Single Decomposition Step Models (1-Step Models)

The simplest pyrolysis models consider a single decomposition step (Fig. 11.2a). Biomass decomposition directly yields a stream of bio-char, bio-oil and non­condensable gas. These models have the advantage of simplicity and possess a limited number of parameters. These models can be accurate for a limited range of pyrolysis conditions where the temperature is constant (isothermal system) or relatively low (<450 °C, thus conventional pyrolysis) and the product composition does not vary significantly.

In reality, biomass decomposition is more complex. In the case of non-isothermal systems at high heating rates, it may appear as if different components are reacting at different temperatures with their specific reaction kinetics. The notion of ‘pseudo­component’ emerges from that behaviour. Even if a system is operated isothermally at high temperature, pyrolysis will happen during the heating step where different products composition will be obtained. Single decomposition step models are thus only suitable in specific pyrolysis situations and are generally inadequate to reproduce industrial fast pyrolysis behaviour.