Kinetic models for biomass pyrolysis

Biomass is a versatile material. It can be directly combusted to produce heat; however, by first subjecting it to a thermal or biological degradation, it can also be converted to products with a higher heating value or to materials with properties superior to those of the raw material [1]. When heated in the absence of oxygen, biomass decomposes into a range of products; including char (solid), tar (liquid) and gas; each with properties that differ from the raw biomass. Pyrolysis is considered to occur in two stages: primary decomposition of biomass and secondary reactions of the products generated in the primary decomposition. In order to maximize tar production, the volatiles released from the primary decomposition should be removed from the reaction zone and condensed before the secondary reactions occur, otherwise the volatiles will react further to form more gas and char. The thermal conversion of biomass is affected by heat and mass transfer, reactor configuration and the operating conditions that define the process environment, but the underlying reaction kinetics are key to describing, optimizing and scaling-up the process. Many studies have been undertaken to understand the kinetics of biomass pyrolysis; however, due to the heterogeneity of biomass and the complexity of the chemical and physical changes that occur during pyrolysis, it is difficult to develop a simple kinetic model that is applicable in every case. As a result, this field is still an active area of research. In this review, different methods to describe biomass pyrolysis and different types of kinetic mechanisms are discussed. Other up to date reviews on the subject with different scopes can be found in the literature [2,3].