Pyrolysis is the cracking of hydrocarbon molecules into smaller gas molecules without any major reaction with air or any other gasifying medium. The kinetic information of pyrolysis is crucial for the design and scale-up of any gasification process. Extensive investigations have been done on the kinetics of biomass de­volatilization in an inert atmosphere. Table 10.2 shows the most widely used kinetic schemes of biomass pyrolysis.

Earlier kinetic models consisted of simple, single first-order reaction schemes to describe the total volatile yield. Later, more complicated two — and three-step reaction networks containing parallel and series reactions were introduced by different authors [22, 24]. These are empirical models whose parameters are calculated by fitting experimental data generally derived from thermo-gravimetric measurements. Since most of the biomass is composed of cellulose, hemi-cellulose, and lignin, the most accurate models are reported to be a three independent parallel reactions model [25-29].

Most modeling efforts with the three independent parallel reactions model have been conducted using experimental data from a single heating rate [25, 30-33]. The effect of heating rate on the pyrolysis yield is, however, significant because the kinetic parameters derived from a single heating rate cannot be confidently extrapo­lated to other heating rates. Radmanesh and Chaouki proposed an improved model for biomass pyrolysis, which is applicable to different heating rates [27]. The ki­netic parameters were calculated from experimental data obtained at relatively low heating rates (maximum heating rate was 50 ° C/min). Therefore the extrapolation of these kinetic models to actual gasification process conditions yields significant

uncertainties since, in reality, biomass temperature increases very rapidly from am­bient temperature to about 800-1,000 ° C (in less than 1 s) as it is fed into the gasifier [27]. Consequently, the actual heating rates applied to biomass particles in pyrolysis systems are significantly higher than 50 ° C/min or even 100 °C/min.

There are only a few studies on pyrolysis at high heating rates (1,000 °C/s) and the resulting gas production [34-37]. Although it is very important to have knowl­edge about pyrolysis kinetic gasification design and optimization, it is very difficult to obtain reliable data for kinetic constants that can be used for a wide range of biomass at different heating rates. Most models are derived from cellulose pyrolysis experiments, and the available models in the literature are only applicable to specific conditions.