Formation and measurement of N 2O in combustion systems

Abstract

Direct N2O emissions from fossil fuel combustion have previously been reported to be equivalent to 25–40% of the NOx levels. At these levels, fossil fuels have been suggested to be a major anthropogenic source of N2O. Recent tests have shown these measurements to be in error, most of the N2O having been formed by reaction between NOx, SO2, and H2O in the sample containers. Time resolved measurements of gas samples stored in Tedlar bags, supported by chemical kinetic calculations, indicate that the majority of N2O forms over a time period of 6 hours. The conversion of NOx to N2O in the sample containers is shown to depend on the amount of SO2 present. This sampling artifact raises questions about the validity of the existing data base, collected by grab sampling methods. As a result, a continuous infrared analyzer, developed primarily for characterization of N2O emissions from full scale combustion sources, was used to perform on line N2O measurements at several full scale utility combustion systems. A variety of conventional and advanced utility combustion systems (firing pulverized coal, oil, and gas) were tested. The measurements from conventional systems (natural gas, oil, and pulverized coalfired) indicate that the direct N2O emission levels are generally less than 5 ppm and are not related to the NOx levels in the flue gas. However circulating fluidized bed units produced elevated N2O emissions. At one circulating fluidized bed combustor firing a bituminous coal, N2O levels ranged from 84 to 126 ppm as the load was varied from 100% to 55%, respectively. The N2O emissions from the circulating fluidized bed appeared to be inversely related to the bed temperature. However, temperature is not the only parameter affecting N2O emissions from fluidized beds; all three of the units studied operated at similar temperatures during full load operation, but the N2O emissions ranged between 25 and 84 ppm. N2O emissions were also elevated at a full-scale boiler using selective non-catalytic NOx reduction with urea; 11–13% of the reduced NOx was converted to N2O.