The study of tar behaviors in underground coal gasification (UCG) is essential for pollution control, system safety and conversion efficiency; however, existing studies have only focused on tar in products without revealing tar evolution in the reaction zone, and the experimental conditions in reported work are far from those in the real situation. In this work, tar behaviors were studied with a self-developed apparatus to simulate the UCG process. During the experiments, the sampling method along the gasification channel was used to collect tar at different positions; the gasification object was a large raw coal block 460 mm × 230 mm × 230 mm in size, and the flow rate of the inlet gas was adjusted according to the composition of products. The tar samples were not only taken from the outlet, but also from the reaction zone, and then analyzed using gas chromatography mass spectrometry. For all the tar samples, C15H13N and its isomer were the most abundant compounds, with a total percentage greater than 14%. Most of the top five chemicals contained more than nine carbon atoms in their molecular formulae, indicating that more heavy tar than light tar is formed by low-temperature pyrolysis. Compared with the upstream tar, the downstream tar had fewer PAHs and a lower boiling point, due to the decomposition of the heavy tar. The downstream tar contained more of the element fluorine (F) than upstream and outlet tars, indicating that tar pollution remaining in the reaction zone cannot be evaluated by monitoring the outlet tar.
MILD combustion is featured with a uniform heat flux and low NOx emission, and is thus a promising technology for clean coal utilization. Establishing the theoretical criteria of coal combustion modes is essential to guide the design and organization of MILD coal combustion. In this work, different coal combustion modes were classified theoretically based on the analysis of time scale, heterogeneous ignition, heat transfer, and flue gas entrainment. The predicted coal combustion modes agreed well with the experimental results from the literature. The effects of the structural and operational parameters on coal combustion modes were also discussed. As the nozzle diameter increases, the critical inlet Reynolds number switching from traditional combustion to MILD combustion increases, while the corresponding jet speed decreases. When the size of the furnace cross section increases, the critical inlet Reynolds number increases. As the coal particle diameter increases, the critical inlet Reynolds number decreases. As the secondary air temperature increases, the critical inlet Reynolds number decreases. The classification method of coal combustion modes was further applied to instruct the organization of MILD coal combustion on a Hencken burner. The experimental results proved that the theoretical analysis is reasonable.
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