In the gasification step that follows pyrolysis, several parallel reactions occur:

• Char gasification involves the reaction between char and steam, carbon dioxide, hydrogen, and oxygen (R1, R2, R3, andR4 shown in Table 10.1). These reactions

Table 10.2 Summary of the proposed kinetics for biomass pyrolysis

are endothermic, except for those involving O2 and H2 which are exothermic. The rate of these reactions depends on the reactivity of char and the gasifying medium: Oxygen is the most reactive species followed by steam and carbon dioxide. Char oxidation reactions are so fast that most of the oxygen is used in this specific reaction step. The relative reaction rates of the gasification reactions are estimated by Walker et al. [38]:

RC+O2 ^ RC+H2O ^ RC+CO2 ^ rc+h2

• Char is usually assumed to be pure carbon for simplification. In reality, it is composed of small amounts of hydrocarbon. Biomass char is generally more

porous and reactive compared to coal char, so its reaction should be considered different [39].

• Water-gas (R2) reaction involves hydrogen, which affects char and steam reaction negatively as shown by Barrio et al. [40]. The continuous removal of hydrogen from the reactor is necessary in order to accelerate water-gas reactions.

• The gasification of char with carbon dioxide (known as Boudouard reaction—R1) is a relatively slow reaction. The rate of this reaction is negligible below 1,000k [41].

• Water-gas shift reaction (R8) is an important kinetic step in the gas phase. It controls the production and the ratio of hydrogen and carbon monoxide, which is critical for downstream processes. It is a slightly exothermic equilibrium reaction with negligible sensitivity to pressure. Above 1,000 °C, it reaches equilibrium fast but a heterogeneous catalyst is required to reach equilibrium at lower tempera­tures. Probstein and Hicks showed that at lower temperatures the reaction has a higher equilibrium constant, which means a higher hydrogen yield with low reac­tion rates [42]. Different catalysts have been tested and employed for water-gas shift reaction, like copper promoted catalysts for the temperature range of 300­510 °C, and a copper-zinc-aluminum oxide catalyst for the 180-270 °C range in commercial applications [43].

• As mentioned before, one of the products of gasification is a condensable heavy hydrocarbon, known as tar. Produced tar from the pyrolysis reactions undergoes further cracking and polymerization reactions to produce lighter or heavier hy­drocarbons. Several studies have been done on the secondary pyrolysis reactions, which involve the fate of tar and its cracking. Boronson et al. and Liden et al. have reported separately the kinetic parameters of tar cracking derived from wood [17,44]. Rath and Staudinger also studied the tar cracking kinetics of birch wood particles in a thermo-gravimetric analyser and a coupling of thermo gravimetric analysis (TGA) with a tubular reactor [45]. They showed that the extent of tar cracking is not only dependent on the conditions in the reactor (temperature and residence time) but also on the temperature at which the tar was formed [46]. Most of the kinetic models proposed for tar cracking are based on a single step, first — order reaction. Among different kinetics, the results of Boronson et al. show com­parable rates and are the most used. The kinetics of tar cracking has also been stud­ied in another approach. Due to the complexity of the tar, several researchers have studied its cracking and decomposition reactions using a model-biomass-tar com­pound, such as phenol, toluene, naphthalene, 1-methylnaphthalene, and so on. In most of the proposed kinetics, a first-order reaction for tar cracking was used.