Numerical modeling of combustion and pyrolysis of cellulosic biomass in thermogravimetric systems

A two-dimensional axisymmetrical numerical model for pyrolysis and combustion of cellulosic biomass in a thermogravimetric system is presented. Both chemical reactions and physical phenomena are considered, using fundamental principles. This mathematical model has combined the transient mass, species and energy conservation equation, Darcy momentum equation and a global Arrhenius decomposition for a porous reacting biomass. It can predict weight loss rate, ignition temperature and gas composition. The results show that the biomass ignition is preceded by pyrolysis and subsequent release of volatiles. The secondary ignition of char occurs after pyrolysis is complete. The volatile and char ignition temperature increases with the increase in the heating rate and biomass volume, and decrease in biomass porosity. A comparison of weight loss calculations with experimental data indicates acceptable agreement at different operating conditions. The model can be used for expanding limitations of thermogravimetric furnaces. The sensitivity analysis, carried out for selected combustion parameters, was useful for setting up a pilot-scale combustor.