The numerical modeling of the combustion of air-dried Victorian brown coal in O 2 /N 2 and O 2 /CO 2 mixtures with 21−30% O 2 has been conducted via the use of computational fluid dynamics (CFD), ANSYS FLUENT 13.0, with the refined weighted-sum-of-gray-gases model (WSGGM), the single-film model with multiple surface reactions (i.e., char-O 2 , char-CO 2 , and char-H 2 O) for char particle, and the refined two-step mechanism for the oxy-firing of methane to mimic the volatile oxidation. The purpose of this study is to verify the experimental observations in a lab-scale drop-tube furnace (DTF) and to promote the understanding on the details underpinning the combustion characteristics of Victorian brown coal, the youngest coal in the world and the single largest source for power generation in Victoria, Australia. As confirmed, the modeling results show good agreement with the experimental measurements on the particle temperature, coal ignition delay photographed by a high-speed camera, carbon burnout rate, and particle velocity. The air-dried Victorian brown coal bears an extremely high reactivity for devolatilization and char-O 2 and char-CO 2 reactions. The inherent moisture in coal was released with volatile matter simultaneously rather than as that predicted by the CFD wet combustion module for pulverized coal with surface moisture. Increasing the secondary gas temperature greatly narrowed coal ignition delay caused by the substitution of 21% O 2 balanced in CO 2 for air. At the furnace temperatures of 1073 and 1273 K, the contribution of char-CO 2 to coal burnout reached approximately 10 and 25% in the oxy-fuel mode, respectively, which, in turn, reduced the coal particle temperature by a maximum of 300 K. To achieve an identical flue gas temperature with the air-firing case, the use of 30% O 2 in CO 2 is essential. However, the radiation heat flux match can be achieved by the substitution of 27% O 2 in CO 2 for air.

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The influence of external clay additive and inherent minerals on the ignition of a Xinjiang lignite and its volatile flame propagation in air versus oxy-fuel combustion have been clarified in this work, through the use of a flat-flame burner reactor (FFBR) coupled with in-situ optical diagnosis tools. As has been confirmed, ignition of the lignite studied in this paper was initiated by homogeneous oxidation of a tarry volatile cloud. The removal of HCl-soluble metals shifted coal devolatilization toward higher temperatures in air and 21% O 2 in CO 2 . The mixing of external clay with coal had little effect on the ignition time. However, it enhanced the decomposition of volatiles, leading to a larger volatile cloud shielding on the particle surface. The oxygen fraction in the bulk gas was found to be most influential. Increasing the oxygen fraction to 30% eliminated all of the discrepencies between raw lignite, acid-washed lignite, and a mixture of raw lignite and clay.

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