A model reactor is used to investigate the extent to which the overall performance and detailed flowfield structure of a non-premixed swirl-stabilized distributed reaction are sensitive to modest changes in inlet conditions (e.g., fuel injection angle, inlet geometry, and swirl vane solidity). Measurements of combustor performance are based on exhaust plane species concentration profiles (HC, CO2, CO, O2), combustion efficiency, and visual observation of combustor stability. The detailed flowfield structure is established by spatially mapping the axial and azimuthal velocity fields using two-component laser anemometry, and the temperature field using a thermocouple probe. The results show that relatively modest changes in inlet conditions can dramatically affect the flowfield structure. As an example, for the model reactor evaluated the addition of a small step at the outer boundary of the swirler yields significantly lower centerline axial velocities and a more uniform thermal structure in the recirculation zone. Furthermore, a modest reduction in swirl vane solidity transforms the aerodynamic structure of the recirculation zone from an off-axis to a central, on-axis structure. These results explain, in part, the contradictory conclusions drawn from data acquired in non-premixed swirl-stabilized distributed reactions, and establish that comparisons and generalizations of such flows require, at a minimum, (1) careful measurements and specification of the inlet conditions, (2) detailed measurements of the flow structure, and (3) an assessment of the effects of modest changes in key operating and configurational variables. © 1988 Combustion Institute.

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. © 1991 Combustion Institute.