Oxy-fuel combustion of millimeter-sized coal char: Particle temperatures and NO formation

In this work, differences in particle temperature and NO yield during char oxidation in O-2/N-2 and O-2/CO2 atmospheres, respectively, have been examined. A laboratory scale fixed bed reactor, operated isothermally at 1073 K, was used for combustion of millimeter-sized lignite and bituminous coal char particles in 5-80 vol.% O-2. Experiments were carried out with both single particles of different sizes (1.3-543 mg) and multiple particles (30-50 mg). Particle temperatures and structural changes were recorded by a Charged Coupled Device (CCD) camera during the experiments. The particle surface temperatures increased with mass loading, by as much as 700 K above the furnace set point. The formation of NO from lignite char was not influenced by the change from N-2 to CO2 whereas the NO yield from bituminous coal char was considerably lower in O-2/CO2 compared O-2/N-2. For both chars the conversion to NO decreased as the O-2 concentration or the particle size increased. However, for the bituminous coal char, a peak in NO yield was observed at an intermediate particle size of 0.1-0.2 g. The differences in the effect of gas atmosphere, O-2 concentration, and particle mass on the NO yield from oxidation of bituminous coal char and lignite char, respectively, cannot be fully explained. Char/NO interactions appear to be quite complex, and mineral catalysis and release to the gas-phase of volatile N-species such as HCN, either from secondary pyrolysis or as a product of the char-N + O-2 reaction, may play a role. (C) 2013 Elsevier Ltd. All rights reserved.