The structure of coal-air-CH4 laminar flames in a low-pressure burner: CARS measurements and modeling studies

An experimental study is described of the structure of a flat, premixed, la minar, coal-air flame, with some methane added for flame stability. A low-p ressure burner, at a combustion pressure of 30.4 kPa, was employed, in orde r to extend the reaction zone. Gas temperatures were measured by the CARS t echnique and the C-2 emissions observed with the laser diagnostics were fou nd to depend upon the laser power. Concentration profiles of permanent spec ies also were measured over a range of equivalence ratios. Measured values are compared with those predicted by a mathematical model, which assumes th at CH4 and HCN devolatilize from the coal and react in the gas phase. Allow ance also is made for reactions of char and radiative heat transfer. The mo del gives good predictions of the temperature and oxygen concentration prof iles, while predictions of NO are somewhat higher than those measured. Form ation of NO is favored by OH and removal of it by NH2 and NH. The sensitivi ty of the modeled results to various activation energies and pre-Arrhenius constants is examined and optimal values of these are in line with other va lues in the literature. The principal limitation in the model is the overpr ediction of CO concentration. An explanation of this lies in the formation, neglected in the model, of tarry structures of high molecular mass, follow ed by the generation of soot. This interpretation is supported by the measu red profiles of C-2 emission intensity and their dependence upon the laser power, in contrast to the weaker emissions from rich CH4-air flames, which show no such dependence and are less persistent. (C) 2001 by The Combustion Institute.