Effect of heat treatment on the combustion characteristics of a lignite

In this study, the ignition and combustion behavior of raw and heat-treated single particles of lignite were experimentally investigated, with a focus on the effect of heat treatment temperatures. The lignite particles were heat treated to various final temperatures (473, 623 and 773 K) in nitrogen and characterized using proximate, ultimate, and Fourier transform infrared spectroscopy (FTIR) analysis. A single lignite particle of 2 or 3 mm in diameter was suspended on a silicon carbide fiber and burned in air in a horizontal tube furnace operating at 1123 K. The ignition and combustion process of the particle was record using a color CCD camera at 25 fps. The ignition mechanism, ignition delay time, volatile flame duration, and burnout time of the single particles were examined by processing the recorded images. The proximate and ultimate analysis results indicated that the volatile matter and oxygen contents decreased, while the carbon content increased with increasing temperature of heat treatment. This trend was consistent with observations in the FTIR analysis, in which the intensity of oxygen-containing functional groups decreased with increasing the heat treatment temperature. The ignition of raw and heat treated lignite particles followed a joint hetero-homogeneous mechanism under all conditions studied. The ignition delay time, volatile flame extinction time, and the total combustion time decreased with increasing heat treatment temperature up to 623 K. A further increase in the heat treatment temperature to 773 K resulted in prolonged key ignition and combustion characteristic times.