SULFUR RETENTION AS CAS(S) DURING COAL COMBUSTION - A MODELING STUDY TO DEFINE MECHANISMS AND POSSIBLE TECHNOLOGIES

Several mechanistic alternatives are available for the retention of co al sulphur as solid CaS(s) during pulverized coal combustion at temper atures above that at which CaSO4 (S) decomposes (i.e. 1473 K). Under f uel-rich combustion, previous experiments by other investigators and o ur thermodynamic predictions indicate that sulphur retentions of 90% c an be obtained at particle temperatures above 1500 K even with CaS(s) fully exposed to the flue gas. Coal moisture has a strong effect on th e retention level due to the formation of H2S(g). For fuel-lean combus tion, retention can occur with calcium dispersed within the burning co al due to the locally reducing atmosphere when carbon is present. The product, d(Circle)S(Circle), that characterizes this atmosphere (where do is the initial coal particle diameter and S(Circle) is the initial coal internal surface area) must be sufficiently high for reasonable retention to occur. The model calculations indicate that during the co mbustion of Illinois no. 6 at 1773 K, at least 90% sulphur can be reta ined within 90% of the coal particle volume. This sulphur retention is possible if d(Circle)S(Circle) > 30 m3 kg-1, a typical value for pulv erized coal combustion. On complete carbon burnout, CaS(s) decompositi on occurs; however, comparison of literature data indicates that the o xidation rate of CaS(s) is slower than that of most coal chars. This s uggests that coals of high reactivity may be suitable for sulphur rete ntion as CaS(s).