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

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.