Thermodynamics Simulation And Experimental Study Of Volatile Trace Elements In Coal During Combustion

Air pollution caused by coal combustion brings great harm on human health, especially the heavy metals and their compounds in the form of submicron particles are caught much attention, as they have relatively high toxicity. Therefore, the research on emission control of trace elements in coal combustion has been a new frontier field.In this paper, volatile elements in coal such as Hg, As, Pb are selected as research object, Jiaozuo anthracite and Pingdingshan bituminous as raw coal samples, thermodynamic equilibrium simulation analysis combined with combustion test in laboratory is used to research migration-transformation, enrichment behavior and absorption mechanism of trace elements before and after the additives are added. It lays foundation for pollution control on trace elements in coal in the further study.(1)The FactSage software was firstly applied to simulate migration-transformation and distribution of the volatile elements Hg, As and Pb in coal combustion, the effect of S and Cl on distribution was taken into consideration. Simulation result shows that solid phase of products maybe a little wider under oxidizing atmosphere: Elemental mercury is the thermodynamically stable form of mercury; arsenic transforms into gaseous oxide of arsenic totally when temperature rises to 700K; prior to 850K, the forms of Pb are solid PbCO₃ and Pb₃O₄, while at high temperature gaseous the main form is PbO. Under oxidizing atmosphere, element S and Cl have strong combination with Pb, with the increase of the ration of Cl/S, the temperature range of PbCl₄ becomes wider. The effect of element Cl on the balance distribution of As is little, element S has strong combination with As; as for Hg, with the increase of S/Cl, the temperature of elemental Hg becomes wider, too, which indicates that emission of elemental Hg is enhanced.(2)The Factsage was also applied to simulate migration-transformation and distribution of Hg, As and Pb in combustion after additives were added under oxidizing atmosphere. The result shows that Al₂O₃ and SiO₂ can inhibit the volatilization of Pb, at the temperature 400¹100K, the effect of Al₂O₃ is even better, at the same time, Cl and S have great influence on absorption effect of additives, and adding some sodium minerals can help to reach a better absorption effect. However, any additive has little influence on distribution of Hg. Additives of calcium make the distribution of As change in certain extent at 400¹300K, mainly occurring in the form of solid As(at 750¹200K, arsenic calcium). As the Ca/S ratio increases, the temperature range of arsenic calcium becomes wider, and when Ca/S=10 reaches to the maximum.(3)High-temperature tube furnace was applied to burn pulverized coal samples at the given temperature under oxidizing atmosphere. Advanced microwave digestion method was used to treat bottom ash; ICP-MS and AFS were applied to analyze the content distribution of Hg, As, Pb in bottom ash before and after the additives were added. The result shows that Hg volatilizes very easily, additives has no control on Hg form the perspective of chemistry, which accords with simulation result; Pb and As have enrichment trend in bottom ash, and with the increase of temperature, the trend becomes less. The absorption effect of four additives is different as the difference of temperature, coal and additive type. There are some differences between simulation and experiment in heavy metal control temperature.(4)Combined with simulation study and experimental research, from the perspective of chemical absorption mechanism, additives and coal chemical itself have some active sites, which react with trace elements to produce the materials that not easily break down, then fix in the ashes.