The formation of N 2O during the reduction of NO by NH 3

Abstract

The process of selective non-catalytic reduction of NO, SNCR, is important for limiting emissions of nitrogen oxides from coal-fired power plants. Such a process has been studied for many years, both in the laboratory and under practical conditions. This work was an attempt at elucidating some of the problems associated with the method when used under circulating fluidized bed (CFB) conditions and in particular, the formation of the N 2O by-product. The NO + NH 3 reaction has been studied in the laboratory, over quartz sand in a heated fixed bed flow reactor. In comparison with a combustion environment, the composition of the gas phase was drastically simplified and limited to NO and NH 3, in nitrogen as the carrier gas, with O2 added in some experiments. The product gases were analyzed for NO, N 2O and NH 3. The effects the following parameters were studied: temperature inside the reactor between 850 and 1250 K, height of the sand bed, NH 3/NO molar ratio over the range 0.54–2.0 and the addition of 1 or 2% of O 2 in volume. Baseline tests with an empty reactor were also made. With no sand in the reactor, the results were both qualitatively and quantitatively different. The sand helped to increase the efficiency of NO reduction, particularly at lower temperatures, but N 2O formation also appeared to be strongly enhanced, except at the highest temperatures. Higher molar NH 3/NO ratios favored NO reduction and N 2O production, both with and without sand. The reduction of NO did not appear to require the presence of O 2, but the introduction of 1% or 2% of O 2 gave some benefit. The results confirmed that under practical conditions more attention should be paid to the role of the bed solids in the SNCR process.