Modeling of a Bubbling AFBC with Volatiles Release

A comprehensive model for continuous combustion of lignite with wide size distribution burning in its own ash in an atmospheric bubbling fluidized bed combustor (ABFBC) has been presented and used to correlate the data from a 0.3 MW ABFBC test rig. The model consists of submodels for hydrodynamics, volatiles release and combustion, char combustion, particle size distribution, entrainment and elutriation and is based on conservation equations for energy and chemical species. The volatiles release model is based on a particle movement model for estimation of portion of volatiles released in bed and a devolatilization kinetics model described by the distributed activation energy model for determination of time-resolved devolatilization profile of lignite particles. Uniform in-bed volatiles release is assumed. The overall model was applied to a 0.3 MW ABFBC test rig fired with lignite with VCM/FC ratio of 2.16. The fuel was fed 0.22 m above the distributor plate and the expanded bed height was 1 m. Predictions of the model were compared with measured concentration and temperature profiles and good agreement was obtained. Bed/freeboard combustion split predicted by the model was found to be 80/20 as opposed to the value 84/16 determined from measured species profiles.