Experimental Study On Partitioning Behaviors Of Trace Elements During Circulating Fluidized Bed Combustion Of Coal

Due to the current energy structure and the coal resources reserved in China, coal will still be the dominant energy sources in the forseenable future. Although their content in coal is small, the emission of trace elements of coal is quite big regarding the huge consumption of coal. Controlling the trace element pollution has become one of the top priorities in the environmental protection area in China. As a promising clean coal technology, circulating fluidized bed （CFB） technology is widely applied due to its wide fuel flexibility, high combustion stability, good load regulation, as well as effective emission control of SO₂ and NOx. In addition, CFB combustion under O₂/CO₂ atmosphere can also realize the CO₂ reduction. Therefore, a detailed study of the transformation behavior of trace elements during coal combustion in CFB can help understanding the enrichment mechanism of trace elements in different products and can also contribute to the development of new technologies to control trace elements emission.In this paper, coal combustion experiments were conducted using a 2.5 MWa, pilot scale CFB combustor at Southeast University and a 6 kWth bench-scale CFB, respectively. Different factors that may influence the partitioning behavior of trace elements （As, Ba, Cd, Cr, Cu, Mn, Pb in this study） were investigated including coal type, atmosphere as well as additives.The results of coal combustion experiments in a 2.5 MWth pilot scale CFB combustor indicate that when burning the anthracite, the mass balance rates of trace elements are between 77% and 115%, except for Mn and Cd. When burning the bituminous coal, the mass balance rates of trace element are between 85% and 104%, except for Ba, Cr and Mn. Most of the trace elements are partitioned in bottom ash and fly ash, only parts of As, Cd, Pb that can easily volatile are distributed in gas stream. Compared with the bituminous coal, a smaller share of trace elements in the anthracite coal eventually distributes in bottom ash. Along with the increase of the particle size of ash, the specific surface area decreases, and the general trend of the enrichment of trace elements in different particles is as:bag filter fly ash> cyclone fly ash> particles of in-bed heat exchanger> bottom ash.The results of coal combustion experiments using 6 kWth CFB show that:compared to air combustion, the temperature of dense-phase zone reduces duing oxy-fuel combustion. With the elevation of oxygen concentration, bed temperature increases in the dense-phase zone. For both anthracite and bituminous coal, the mass balance rates of the trace elements in different atmosphere are achieved between 70% to 130%. Compared with air atmosphere, O₂/CO₂ atmosphere promotes the evaporation of Ba, Cd, Cr and Mn, and inhibits the volatility of As, Cu and Pb.Under O₂/CO₂ atmosphere, the enrichment of oxygen concentration inhibits the volatilization of Ba, Cr and Cu, but promotes the volatilization of Cd. When NH₄Br is blended with the raw coal, larger portion of As, Cd, Cu and Pb is distributed in fly ash. The modeling results using Factsage software validate the mechanism that existence of Br in the system improves the volatility of the element like Pb and Cu.The effects of different particle size of the fly ash on element concentration show that:compared to air combustion, oxy-fuel combustion can significantly decrease the mass share of trace element in the fine particles. With the improvement of oxygen concentration, the amount of trace elements in fine particles increases. O₂/CO₂ atmosphere promotes the migration of As, Ba and Cd to the coarse particle, thus inhibiting the volatilization. However, the inhibitory effect on volatilization will be weakened with the increase of oxygen concentration. Oxy-fuel combustion enhances the concentrations of Cr、Cu、Mn and Pb in fine ash, but decreases the amount of the total mass of them in fine ash. Combustion atmosphere do not change the enrichment mechanism of trace elements.