Modeling biomass particle pyrolysis with temperature-dependent heat of reactions

An accurate formulation of energy conservation to model pyrolysis of a biomass particle needs to account for variations in the heat of reaction with temperature, usually neglected in most past studies. It is shown that by including this effect in a pyrolysis model with three parallel reactions yielding char, gas and tar, a wide range of experimental data can be accurately predicted. In particular, through comprehensive comparisons of the simulation results with various measurements, a consistent and single value of 25 kJ/kg is obtained for enthalpy of pyrolysis, which represents a lumped heat of volatiles and char formation at a reference temperature. It is found that the kinetic parameters of Chan et al. [W.C. Chan, M. Kelbon, B.B. Krieger, Fuel 64 (1985) 1505-1513] and Thurner and Mann [F. Thurner, U. Mann, Ind. Eng. Chem. Process Des. Dev. 20 (1981) 482-488] provide reasonable agreement between the model predictions and experiments compared to other reported kinetic constants. These comparisons also indicate that inclusion of tar cracking reactions to yield additional light gases does not give a better prediction of the process parameters. The presented thermo-kinetic model is capable of successfully predicting various experimental observations such as the internal temperature peak reported in some past studies. It is shown that the sensible heat released due to the conversion of virgin biomass to the reaction products is responsible for this phenomenon. Simulation results reveal that a temperature peak at an internal location of the particle may occur when the corresponding local temperature reaches the particle surface temperature while the local biomass conversion is not finalized yet.