THE AMOUNTS OF NOX AND N2O FORMED IN A FLUIDIZED-BED COMBUSTOR DURINGTHE BURNING OF COAL VOLATILES AND ALSO OF CHAR

Three quite different coals were burned batchwise in electrically heat ed beds of sand fluidized with O-2 and N-2 at either 800 or 900 degree s C. The coal was injected into the bed either as a single piece (1 or 2 cm in size) or as 1.0 g batches of smaller particles (either 1.4-1. 7 or 2.0-2.4 mm in diameter). The concentrations of NOx (i.e., [NO] [NO2]), N2O, CO, and CO2 in the off-gases were measured as functions o f time whilst the coal was burning. It proved relatively easy to separ ate the emission of any one of these gases into that from the initial, devolatilization stage of combustion and that from the subsequent bur ning of the char. The first stage of coal combustion, when the volatil e matter burns, turns out to be very important. For a low rank coal, 7 0%-90% of all the N2O produced appears while the coal is undergoing de volatilization. The fraction drops to 40% for a coal with a low volati le content. The figures for NOx emissions range from 55% of the NOx ar ising from the volatiles in a low rank coal to 25% for a coal of high rank. In general, the effects of bed temperature and coal size were mu ch less important than a coal's volatile content. Interestingly, chang ing the bed temperature altered the ratio [N2O]/[NOx] in the off-gases and not the total quantity of oxides of nitrogen emitted. Lower conce ntrations of O-2 resulted in slightly less N2O and NOx being produced. The rates of production of NOx and N2O during combustion of the volat iles were found to be proportional to one another. This seems to deriv e from a lack of mixing of the volatiles. The fact that large (> 1 cm) coal particles float on top of a fluidized bed during devolatilizatio n is important and, e.g., can result in large particles producing less NOx and N2O during devolatilization than tiny particles. The observat ions made during devolatilization conform to effectively all the fuel- nitrogen in the volatile matter being converted to HCN. Nitric oxide i s then produced most probably heterogeneously on the sand from CN radi cals, which alternatively can yield NCO radicals. N2O is generated dur ing devolatilization by the reaction NCO + NO --> N2O + CO occurring, probably in the gas phase. As for the burning of char in a fluidized b ed, the particle size (> 1 mm) here is large enough for there to be co ntrol by mass transfer of oxygen to a particle. Oxidation is thus conf ined to the exterior of the char. It appears that CO, from burning cha r, is important, together with reaction between NO and solid carbon, i n converting NO to N-2. This is why the yield of NO from char-N is les s than that from volatile-N.