During auto-thermal gasification air provides thermal
energy by partial (C + 0.5O2?CO, 111 kJ mol1) and complete
(C + O2?CO2, 394 kJ mol1) oxidation of the char produced from
pyrolysis. This thermal energy is needed for the endothermic
gasification reactions. Char gasification is the slowest of the
gasification reactions and governs the overall conversion rate.
The reactivity of the char varies greatly, depending on porosity,
crystal structure of the carbon and catalytic effects of ash components
(Higman and van der Burgt, 2007). We estimated the amount
of oxygen required to provide sufficient thermal energy for gasification
according to calculations by Basu (2010c). Heat is required
for drying the feedstock, to raise the feedstock to the reaction temperature
and drive the water-gas gasification reaction (C + H2O?
CO + H2, +131 kJ mol1). Furthermore, the gasifier must provide
energy to compensate for the heat lost through the reactor walls.
According to Basu (2010c) the amount of oxygen required was
9.7 kg h1; however, in our case, the air oxygen input to the reaction
zone was only 5.0 kg h1. Taking into account the high amount
of unconverted char and the low gasification performance parameters,
we reason that there was only enough energy provided for
pyrolysis. In fact, ‘‘autothermal pyrolysis” might be a more appropriate
classification.