NITROUS-OXIDE EMISSIONS IN FLUIDIZED-BED COMBUSTION - FUNDAMENTAL CHEMISTRY AND COMBUSTION TESTING

Growing scientific and public concern over global warming and ozone layer depletion has led researchers to study nitrous oxide (N2O) emissions from natural and anthropogenic sources. N2O is reportedly increasing in the atmosphere at an estimated rate of 0.7 ppb per year and was measured in the atmosphere at 307 ppb in 1988. Since N2O is a stable compound, it is transported to the stratosphere where it is photochemically oxidized to nitric oxide (NO), a contributor to catalytic ozone depletion. Potential anthropogenic sources of N2O that have been investigated include biomass burning, fertilization, ground-water release through irrigation and fossil fuel combustion. Until recently, pulverized-coal combustion was implicated as the main source of N2O emissions due to errors encountered in sampling. Emissions from these units are typically less than 10 ppm N2O. Fluidized-bed combustion (FBC) has emerged as an advanced method of energy production by utilities, but under current designs these units have been shown to emit greater N2O) emissions than their pulverized-coal counterparts. This fact is related to lower combustion temperatures used to enhance SOx/NOx control, which, as a consequence, increases N2O emissions. Nitrous oxide emissions from FBCs range from 50-200 ppm. At N2O concentrations of 10) ppm, and assuming a 2.5% growth rate in U.S. coal-derived energy production with 50% of this generated by FBCs, by the year 2000 this production would account for only 1% of the total current global N2O inventory. Nitrous oxide emissions from FBCs are strongly dependent upon fuel type, operating temperature, and excess air level. Based on current measurements at standard operating conditions (1550-degrees-F and 3.5% O2), fuels such as wood, peat and lignite generally will have N2O emissions ranging from 15-50 ppm. Subbituminous and bituminous coal combustion generate emissions ranging from 40-100 ppm and 70-200 ppm, respectively. Petroleum coke combustion is similar to bituminous coal. Actual emissions from a plant will decrease with increasing operating temperature (0.2-1.1 ppm N2O/degrees-F) or decreasing excess air levels (7-21 ppm N2O/% O2). Possible N2O abatement strategies include afterburning of a gaseous fuel in the freeboard or cyclone, increasing bed temperatures, decreasing excess air, or catalytic reduction of N2O by metal oxides.