An experimental investigation into the fate of fuel-bound N-species during fluidised bed combustion of a dried coal slime in O2/Ar mixtures

Abstract

The formation of NO, NO2, N2O and N2 from fuel-bound nitrogen species during the combustion of a coal slime (CS) and its char (CS char) were investigated in a laboratory bench-scale fluidised bed reactor by tracking the N-containing gas species using a gas chromatograph and an electrochemical gas analyser. Quartz sand with a particle size fraction of 0.3–0.38 mm was used as the bed material. A stream of an Ar/O2 mixture of varying O2 concentration was used as the oxidant and fluidisation gas. The effect of operating temperature (900–1300 K), initial O2 concentration (3–8%), fuel feed rate (0.0041 g min−1 for CS, 0.0036 g min−1 for CS char) and C/O2 mole ratio (0.14–0.78) on carbon burnout, NOx, N2O and N2 formation were systematically studied. It was found that the oxidation process of CS in the fluidised bed is controlled by mass diffusion under the tested conditions. N2, N2O and NO concentrations in the flue gas during CS combustion were higher than during CS char combustion under the same experimental conditions due to the presence of the greater N content in CS (1.56%) than in CS char (1.34%). At low combustion temperatures of 900 K and 1000 K, the fuel-N was mainly converted to NO and N2O. However, as the operating temperature increased, NO and N2 became the dominating species in both the CS and CS char combustion experiments. The NO concentration increased with increasing initial O2 concentration due to enhanced oxidation of char-N. Comparing the N2 formation data during CS and CS char combustion, it is inferred that in the presence of volatile, fuel-N was preferentially converted to N2.