Kinetic compensation effects in the chemical reaction-controlled regime and mass transfer-controlled regime during the gasification of biochar in O2

This study aims to investigate the kinetic compensation effects during the char-O2 reaction in a fluidised-bed reactor for two particle sizes of 0.80-1.0 mm and 2.0-3.35 mm. The rate of char-O2 reaction was determined by analysing the gasification product gas composition in a quadrupole mass spectrometer. The char-O2 reaction exhibited different kinetic compensation effects between apparent activation energy and apparent pre-exponential factor in the kinetics-controlled, diffusion-controlled and mixed regimes for both particle sizes. The same reaction mechanism is followed during the char-O2 reaction in the kinetic regime at same or at different pyrolysis temperatures as revealed by the kinetic compensation effects. In the mixed regime, higher diffusion limitations increased the m and c values in the kinetic compensation effect lnAapp = mEapp + c for any given particle size. Due to higher rates of reaction at higher char conversions, the char-O2 reaction switched from kinetics-controlled to mixed regimes, resulting in higher slopes 'm' and y-intercepts 'c' in the kinetic compensation effects. The absence of isokinetic temperature at higher conversions indicates that char properties changed significantly at higher conversions compared with those at lower conversions.