COMBUSTION OF COAL AS A SOURCE OF N2O EMISSION

Coal combustion is examined as a source of nitrous oxide pollution and a review of relevant research is presented. The role N2O plays in global warming and in stratospheric ozone depletion is explained and comparisons are made between N2O and other greenhouse gases. A balance of known N2O 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 N2O 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 N2O/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 NH3, and both can act as gas-phase N2O/NO precursors. Nitrous oxide is preferentially formed from cyano species, whereas NH3-based compounds tend to react mainly to NO. Gas equilibrium calculations show that N2O concentrations in flue gas are several orders of magnitude above their equilibrium values. Gas-phase formation of N2O is competitive with respect to NO formation. As temperature decreases, more N2O is formed at the expense of NO. Heterogeneous N2O/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 N2O. Destruction mechanisms of N2O/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 N2O. Temperature has been identified as the most important parameter that controls N2O levels, high temperature leading to reduced N2O emissions. As a result, N2O 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 N2O emissions (20-250 ppm). Further augmentation of N2O 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 N2O levels and to increase NO emission, whereas increased SO2 levels in the combustor have the opposite effect. The nature of these interactions is discussed. Due to different design, different N2O/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 N2O control measures are advocated: (a) improvements in the existing plant (operating conditions, process control, etc.); (b) research directed to understand N2O/NO chemistry in an FBC which would lead to innovative design of new combustors and their operating regimes. Staged combustion, gas reburning and catalytic enhancement of N2O decomposition seem to be attractive options in N2O control. In view of the existing interactions among individual pollutants and pollution control measures, an integrated approach to SO(x)/NO(x)/N2O abatement is emphasised.