Pilot-scale experimental and CFD modeling investigations of oxy-fuel combustion of Victorian brown coal

摘要

This paper presents the first trial in the world for experimental and modeling investigation of oxy-fuel combustion of Victorian brown coal in a 3 MW_(th) pilot-scale facility. Two coal samples with different moisture contents, wet (40 wt% moisture) and air-dried (24 wt% moisture), were tested in air- and oxy-firing modes with oxygen level varying from 27% (vol) to 40% in furnace. The commercial software, FLUENT 13, was employed to interpret the experimental data by taking into account a series of validated refined sub-models for oxy-fuel combustion. The modeling results showed reasonable agreement with the measurements of flue gas temperature profile as well as in-situ flame photographs in furnace. As has been confirmed, a rather identical flue gas temperature profile was achieved between air-firing and oxy-27% O_2 for the dried Victorian brown coal. For the wet brown coal with 42 wt% moisture, 30% O_2 in oxy-fuel mode is however essential to match air to achieve an identical flue gas temperature profile. Even so, under above optimum retrofit conditions, the radiative heat transfer and CO emission profiles were still slightly mismatched between air-firing and oxy-firing cases regardless of the moisture content within coal. The radiative heat transfer was increased in wet coal oxy-firing because of the higher gas emissivity for the abundant CO_2 and H_2O within the flue gas. The CO emission concentration in oxy-fuel mode was raised, due to the enhanced C-CO_2 and C-H_2O gasification reactions that occurred readily at relatively low temperatures for Victorian brown coal char. The presence of no less than 30% oxygen in primary gas is also essential to minimise particle ignition delay caused by the large specific heat capacity of CO_2 and steam. For flue gas properties, it has been confirmed that, under the optimised oxy-fuel conditions, the typical CO_2 purity reached approximately 80% in dried flue gas, in which the excess oxygen remained at ～4% whereas steam reached 35%, relative to 10% in air-firing flue gas. By shifting coal combustion from air-firing to oxy-fuel mode, the flue gas recirculation ratio varied from 48% to 64% for the use of wet and dried coals, which are considerably lower than the literature results for bituminous coal.