Nitrous oxide emissions in fluidized-bed combustion: Fundamental chemistry and combustion testing

Abstract

Growing scientific and public concern over global warming and ozone layer depletion has led researchers to study nitrous oxide (N 2O) emissions from natural and anthropogenic sources. N 2O 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 N 2O 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 N 2O that have been investigated include biomass burning, fertilization, groundwater release through irrigation and fossil fuel combustion. Until recently, pulverized-coal combustion was implicated as the main source of N 2O emissions due to errors encountered in sampling. Emissions from these units are typically less than 10 ppm N 2O. 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 (N 2O) emissions than their pulverized-coal counterparts. This fact is related to lower combustion temperatures used to enhance SO x /NO x control, which, as a consequence, increases N 2O emissions. Nitrous oxide emissions from FBCs range from 50–200 ppm. At N 2O concentrations of 100 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 N 2O 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°F and 3.5% O 2), fuels such as wood, peat and lignite generally will have N 2O 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 N 2O/°F) or decreasing excess air levels (7–21 ppm N 2O/% O 2). Possible N 2O abatement strategies include afterburning of a gaseous fuel in the freeboard or cyclone, increasing bed temperatures, decreasing excess air, or catalytic reduction of N 2O by metal oxides.