Combustion of coal as a source of N 2O emission

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Coal combustion is examined as a source of nitrous oxide pollution and a review of relevant research is presented. The role N 2O plays in global warming and in stratospheric ozone depletion is explained and comparisons are made between N 2O and other greenhouse gases. A balance of known N 2O sources and sinks is also reported. Nitrous oxide emerges as a powerful and long-lived greenhouse gas with atmospheric concentration increasing at an annual rate of 0.3%. Most anthropogenic sources of N 2O are of comparable strength and all should be subject to control. The fate of fuel-bound nitrogen is discussed in detail starting with coal devolatilisation and pyrolysis, through gas-phase and heterogeneous N 2O/NO x formation and destruction mechanisms, to conclude with the relevant side reactions. Kinetic data are provided where available and compiled in tables. The main nitrogenous products of coal pyrolysis are HCN and NH 3, and both can act as gas-phase N 2O/NO precursors. Nitrous oxide is preferentially formed from cyano species, whereas NH 3-based compounds tend to react mainly to NO. Gas equilibrium calculations show that N 2O concentrations in flue gas are several orders of magnitude above their equilibrium values. Gas-phase formation of N 2O is competitive with respect to NO formation. As temperature decreases, more N 2O is formed at the expense of NO. Heterogeneous N 2O/NO formation probably involves a similar trade-off and underlying mechanisms are discussed. Only up to 10% of charbound nitrogen has been found to form N 2O. Destruction mechanisms of N 2O/NO on char surface are important under fluidised-bed combustion (FBC) conditions, especially in the presence of CO. NO reduction on char is believed to be a negligible source of N 2O. Temperature has been identified as the most important parameter that controls N 2O levels, high temperature leading to reduced N 2O emissions. As a result, N 2O emission is low (<20 ppm) in gas- and oil-fired boilers, pulverised-coal burners and most conventional combustors, whereas fluidised-bed combustion poses a threat of increased N 2O emissions (20–250 ppm). Further augmentation of N 2O presence in the atmosphere may result from the use of catalytic convertors in cars and from some NO x control technologies (e.g. selective non-catalytic reduction). Limestone addition tends to reduce N 2O levels and to increase NO emission, whereas increased SO 2 levels in the combustor have the opposite effect. The nature of these interactions is discussed. Due to different design, different N 2O/NO formation and destruction pathways may be important in bubbling and circulating FBC's. In view of uncertainties surrounding the issue of global warming, cautious but prompt N 2O control measures are advocated: (a) improvements in the existing plant (operating conditions, process control, etc.); (b) research directed to understand N 2O/NO chemistry in an FBC which would lead to innovative design of new combustors and their operating régimes. Staged combustion, gas reburning and catalytic enhancement of N 2O decomposition seem to be attractive options in N 2O control. In view of the existing interactions among individual pollutants and pollution control measures, an integrated approach to SO x /NO x /N 2O abatement is emphasised.