Coal Combustion in Fuel Rich Flames: A Review of Experimental Behavior

Page 2

SUMMARY Reaction efficiencies (11) of cokes and coals in fuel-rich flames are shown generally to obey the simple mass-balance equation: 11 = 2 (R C S) [ 1 + E%/lOO]; over the range -80 to -30% excess air, and for an RCS factor of O.S. This RCS factor is for carbon conversion to CO with no CO; it implies very fast 2 conversion of any primary CO. The data used were obtained in different experiments using a plug-flow furnace, a jet-mix reactor, and a high-intensity furnace. The trend of reaction efficiency above -30% excess air then depended on the reactor used. With the jet-mix reactor, the reaction effi­ciency peaked at about the stoichiometric equivalence ratio for an enhanced VM yield (at a Q-factor of 2). Temperatures also peaked at about that equiva­lence ratio for both the jet-mix reactor and the high-intensity furnace. The flame mechanism appears to be combustion primarily or totally in volatiles at this fuel-rich optimum, with heterogeneous reaction providing an increasing contribution as excess air increases. With decreasing excess air, however, as discussed in the paper, this argument appears to lead to the conclusion that the extent of pyrolysis may be governed by the availability of oxygen to react with it but there are obvious difficulties with this conclusion. These results show that a reasonably clear pattern of experimental behavior still presents major difficulties in mechanistic interpretation.