Low temperature oxidation studies of dried New Zealand coals

Abstract

The chemical and thermal responses of six dried coals on exposure to a flow of oxygen or air were studied using isothermal differential thermal analysis (DTA) and temperature-programmed in situ diffuse reflectance infrared Fourier Transform spectroscopy (DRIFTS). A range of samples varying from lignites to bituminous coals and reaction temperatures from 30 to 180 °C were used. The major DTA response was an immediate sharp exotherm which for each coal increased with preset oxidation temperature and was always smallest for the coal least prone to spontaneous combustion. These exotherms may prove useful as indicators of spontaneous combustion propensity. The first product signal detected by DRIFTS was assignable to the carbonyl-stretch frequency of a carboxylic acid/aldehyde functionality. At higher temperatures the rate of increase of this signal correlated closely with the thermal response profiles suggesting that, regardless of coal rank, the reactions responsible for coal heating across the 30–180 °C range involve the formation and breakdown of the peroxy-precursors of these carbonyl-containing products and that susceptibility to spontaneous combustion is strongly influenced by the uncontrollable factors that govern such reactions. Oxidations were also carried out under a static blanket of O 2/argon at each temperature and the gas mixture analysed after five hours. At the lower temperatures no changes were seen. At 90 °C carbon oxides were first detected and their yields increased with temperature. An oxidation mechanism is proposed.