Engineering bed models for solid fuel conversion process in grate-fired boilers

A comparison between two numerical models describing the thermo-chemical conversion process of a solid fuel bed in a grate-fired boiler is presented. Both models consider the incoming biomass as subjected to drying, pyrolysis, gasification and combustion. In the first approach the biomass bed is treated as a 0D system, where the thermo-chemical processes are divided in two successive sections: drying and conversion. Phenomenological laws are written to characterize the syngas release as a function of the main governing parameters. The second model is an empirical 1D approach. Temperature, species concentrations and velocity of the syngas provided by the two models are compared. Sensitivity analyses with respect to the drying agent mass flow rate, the initial moisture content and the composition of the biomass are performed. The relative error between the mean values of the temperature and velocity of the syngas predicted by the two models is equal to about 7%. The application to different types of biomass shows that the difference in the predictions increases as the carbon content grows. The phenomenological model, in fact, generally considers higher conversion rates of this element to volatiles with respect to the analogy model.