NO-NO_2 EMISSIONS FROM THE COMBUSTION OF SOLID FUELS

摘要

This paper examines the nitrogen oxide (NO_x = NO + NO_2) emissions from the combustion of various conventional and alternative solid fuels. The fuels burned included a bituminous coal, tire-derived fuel, poly(styrene) and poly(ethylene) in powder form. Streams of particles were injected and burned in a drop-tube furnace in air, at a gas temperature of 1500 K. The bulk equivalence ratio1 in the furnace was controlled by varying the amount of fuel injected, while keeping the air flow rate constant. The range of equivalence ratios 0.5 < φ < 2 was examined, which is of interest to conventional and 'low-NO_x' boilers. Sampling was conducted at the exit of the drop-tube furnace and the sampling conditions were identical in all experiments. Results showed that the NO_x combustion emissions of coal were up to four times higher than those of tire and more than ten times higher than those of the two plastics. For all fuels examined the NO_x emissions declined drastically with increasing equivalence ratio, especially at fuel-rich conditions. However, for all fuels the fraction of NO_2 to NO_x appeared to increase with equivalence ratio in the fuel-rich region. The NO_2 emissions were sensitive to the temperature of the exhaust duct and sampling lines, especially, the temperature of the Permapure drier (dehumidifier) used. When sampling of the furnace exhaust was conducted with unheated sampling lines and drier (or without a drier or with the drier repaced by a chiller) large NO_2 partial pressures were recorded at fuel-rich conditions. When the entrance of the drier was heated to a temperature in excess of the 'dew point' temperature of the exhaust the effectiveness of the dehumidifier decreased and condensate was found in the downstream lines. This decreased the measured NO_2, as well as the total NO_x concentration, most likely, because of its dissolution in the water. Thus, these experiments indicate that substantial NO_2 can be present in the effluent of fuel-rich combustion. Since NO_2 is far more toxic than NO, this observation could be of concern in the case of 'low-NO_x' burners, since their primary stage is operated at fuel-rich conditions.