Pyrolysis and Combustion Behavior of Coal Gangue in O2/CO2 and O2/N2 Mixtures Using Thermogravimetric Analysis and a Drop Tube Furnace

Combustion of coal gangue is extensively used for power generation in China. In this paper, pyrolysis and combustion characteristics of a low-rank coal gangue have been investigated under oxy-fuel (O 2 /CO 2 ) and air (N 2 /O 2 ) conditions using a drop tube furnace and thermogravimetric analysis. Pyrolysis experiments were carried out in N 2 and CO 2 environments, which are the main diluting gases of air and oxy-fuel environments, respectively. The burnout and yields of volatile matter were analyzed during DTF experiments. At lower temperatures, the weight loss rate of coal gangue during pyrolysis in N 2 was higher than that in CO 2 . However, further weight loss took place in CO 2 atmosphere at temperatures above 800 °C due to CO 2 gasification of chars. The thermogravimetric analysis (TGA) results confirmed that the pyrolysis in CO 2 environment can be divided into three stages: moisture release, devolatilization, and char gasification by CO 2 in a higher temperature zone. Combustion experiments were carried out in four different atmospheres: air, an oxygen-enriched air environment (30% O 2 –70% N 2 and 40% O 2 –60% N 2 ), an oxy-fuel environment (21% O 2 –79% CO 2 ), and an oxygen-enriched oxy-fuel environment (30% O 2 –70% CO 2 and 40% O 2 –60% CO 2 ). Coal gangue reactivity under oxy-fuel conditions differed from that under air combustion conditions. The combustion rate of coal gangue increased with increasing O 2 concentration while the ignition and burnout points shifted to lower temperatures and complete combustion was achieved at lower temperatures and shorter times. Comparison of the combustion performance of coal gangue in N 2 /O 2 and CO 2 /O 2 environments for equivalent O 2 concentrations indicated that the combustion characteristics of coal gangue in a CO 2 /O 2 environment were similar to those in a N 2 /O 2 environment at 21 vol % O 2 oxygen concentrations. However, increasing O 2 partial pressures resulted in a higher combustion rate under oxy-fuel conditions. Kinetic constants for the samples were calculated by using the isoconventional method. The activation energy decreased with increasing O 2 partial pressure under oxy-fuel combustion conditions and reached lower values compared to air combustion cases. The mineral matter reactions during coal gangue combustion were investigated by means of XRD analysis. During combustion at 1200 °C, mineral phases in coal gangue were transformed to ash comprised of kaolinite, quartz, mullite, magnetite, hematite, and anhydrite.