A generalised dynamic model for char particle gasification with structure evolution and peripheral fragmentation

A generalised model for the prediction of single char particle gasification dynamics, accounting for multi-component mass transfer with chemical react ion, heat transfer, as well as structure evolution and peripheral fragmenta tion is developed in this paper. Maxwell-Stefan analysis is uniquely applie d to both micro and macropores within the framework of the dusty-gas model to account for the bidisperse nature of the char, which differs significant ly from the conventional models that are based on a single pore type. The p eripheral fragmentation and random-pore correlation incorporated into the m odel enable prediction of structure/reactivity relationships. The occurrenc e of chemical reaction within the boundary layer reported by Biggs and Agar wal (Chem. Eng. Sci. 52 (1997) 941) has been confirmed through an analysis of CO/CO2 product ratio obtained from model simulations. However, it is als o quantitatively observed that the significance of boundary layer reaction reduces notably with the reduction of oxygen concentration in the flue gas, operational pressure and film thickness. Computations have also shown that in the presence of diffusional gradients peripheral fragmentation occurs i n the early stages on the surface, after which conversion quickens signific antly due to small particle size. Results of the early commencement of peri pheral fragmentation at relatively low overall conversion obtained from a l arge number of simulations agree well with experimental observations report ed by Feng and Bhatia (Energy & Fuels 14 (2000) 297). Comprehensive analysi s of simulation results is carried out based on well accepted physical prin ciples to rationalise model prediction. (C) 2001 Elsevier Science Ltd. AH r ights reserved.