Study On Effect Of Modified Mineral Adsorbents On Arsenic Volatilization Characteristics During Coal Combustion

As coal remains an important fuel and raw material for industry in China,the harmful trace elements such as arsenic,selenium and lead released during the combustion process of coal have attracted more and more attention.Arsenic has received special attention due to its high volatility,high toxicity,and biological accumulation.In the context of green low-carbon development,it is of far-reaching significance to study the control methods of arsenic emissions in coal-fired power plants.Gongyi anthracite（GYA）,Datong bituminous coal（DTB）and Yunnan lignite（YNL）were selected as representatives to study the effect of four typical mineral compositions（Fe₂O₃,CaO,SiO₂and Al₂O₃）on arsenic volatilization characteristics during coal combustion.Thus,based on the effect of mineral compositions on the retention efficiency of arsenic during coal combustion,modified calcium-based adsorbents were respectively prepared by the dipping method,wet mixing method,and sol-gel method to achieve better capture performance of the adsorbents at high temperatures.Finally,three adsorbents were prepared based on limestone,kaolin,rice husk ash and dolomite,and arsenic retention efficiency and mechanism of them were explored.The main contents of the paper are as follows:（1）The arsenic volatilization characteristics of three typical coal samples were studied by the tube furnace combustion experiment.X-ray diffraction was used to analyze the mineral transformation in three typical coal ashes with temperature increasing.Aluminosilicate-rich GYA coal ash was gradually transformed into mullite and then enstatite phase at high temperatures.And iron-rich DTB coal ash was gradually transformed into magnetite due to the change of the local atmosphere during the combustion process.Besides,the anhydrite in calcium-rich YNL coal ash reacted with volatile arsenic to form the corresponding calcium arsenate.During coal combustion,the arsenic volatilization ratio increased with temperature increasing,which was related to the occurrence modes of arsenic in the raw coal.Anthracite GYA had a higher proportion of residual arsenic with high thermal stability,and thus the volatilization ratio was relatively low.The proportions of reducible and oxidizable arsenic in bituminous coal DTB and lignite YNL were relatively high,which resulted in high volatilization ratios during coal combustion.（2）The effect of typical mineral compositions on arsenic volatilization characteristics was studied.The arsenic retention efficiencies of mineral compositions below 1000℃were in the following order:CaO>Fe₂O₃>Al₂O₃>SiO₂,of which CaO showed excellent arsenic capture performance.However,the arsenic retention efficiencies of all mineral compositions decreased significantly over 1000℃.In addition,SiO₂and Al₂O₃showed better arsenic capture efficiency than CaO in high-calcium lignite,owing to the formation of calcium silicates or calcium aluminates with better sintering resistance.The thermodynamic software HSC 6.0 was adopted to analyze the phase equilibrium in the systems of Ca-As-O,Fe-As-O and Al-As-O.The reactions between arsenic and mineral compositions at certain temperatures contributed to form the corresponding arsenates,such as Ca₃（As O₄）₂,Fe As O₄and Al As O₄),to achieve the retention of gas-phase arsenic.（3）Based on the arsenic retention effect of mineral compositions,calcium-based materials were adopted as active mineral components,while SiO₂,Al₂O₃and MgO were used as anti-sintering frameworks.Three types of modified calcium-based adsorbents（CaO/Al₂O₃,CaO/SiO₂and CaO/MgO）were prepared by wet-mixing or dipping methods.Compared with CaO,all modified adsorbents had achieved better arsenic capture performance at high temperatures,which was mainly due to the composite oxide calcium aluminates(Ca₁₂Al₁₄O₃₃and Ca Al₄O₇),calcium silicates（Ca₂SiO₄,Ca SiO₃）and MgO in the modified calcium-based adsorbents,which exerted a positive effect on alleviating the sintering growth of calcium oxide grains at high temperatures and decreasing the gas-solid reaction resistance.In addition,the specific surface areas of the adsorbents had been greatly improved and the pore structures were further improved,which provided more active sites for the gas-solid reaction.Combined with the SEM analysis of the coal ash samples,compared with the coal ash sample with the addition of CaO,the apparent morphologies of coal ashes with the addition of the modified calcium-based adsorbents became loose and porous and the pore structures were more abundant,which could lead to the greatly improved arsenic retention efficiency of modified calcium-based adsorbents at high temperatures.Three modified natural mineral adsorbents（limestone-kaolin,limestone-rice husk ash and limestone-dolomite）showed a certain increase in arsenic retention efficiency at1000-1200℃compared to the original limestone,which was mainly due to high specific surface areas and excellent pore structures of modified mineral adsorbents,and anti-sintering carriers formed during the modification process such as calcium aluminates,calcium silicates and magnesium oxide,which may alleviate the sintering phenomenon of calcium-based minerals at high temperatures.