Effects of Chemical Forms of Alkali and Alkaline Earth Metallic Species on the Char Ignition Temperature of a Loy Yang Coal under O2/N2 Atmosphere

Abstract

Experiments were performed to determine the effects of alkali and alkaline earth metallic (AAEM) species on the ignition behavior of char under O2/N2 conditions. The char ignition temperatures of a Loy Yang brown coal were investigated using a wire-mesh reactor, where the secondary reactions of the evolved volatiles were minimized. Water, ammonium acetate, and sulfuric acid sequential extractions were employed to prepare the coal samples of different chemical forms of AAEM compounds from raw coal. In comparison to that of raw coal, the increases in the average char ignition temperature of 33 and 55 °C were observed for ammonium-acetate-extracted coal and sulfuric-acid-extracted coal (H-form coal), respectively, indicating the strong catalytic effects of various chemical forms of AAEM species on the char ignition behavior of brown coal. To find out which chemical form of sodium has a higher catalytic effect on the char ignition behavior, combustion experiments were also carried out for Na-exchanged coals and NaCl-loaded coals, which were prepared from H-form coal. The average char ignition temperature of 0.05 M Na-exchanged coal is 48 °C lower than that of 0.05 M NaCl-loaded coal in air, although those two samples have a close content but different chemical forms of sodium. It was found that sodium in char as carboxylates (−COONa) or NaCl can both improve the ignition characteristics of coal samples but carboxylates (−COONa) show a more significant catalytic effect on the average char ignition temperature in air combustion.