Vaporization Characteristics Of Arsenic During Isothermal Pulverized Coal Combustion And A Novel Determination Method Of The Kinetic Parameters Of Arsenic Compounds

With increasing public awareness of the environmental impact of coal combustion,serious concerns have been raised regarding the emissions of various hazardous trace elements,such as mercury,arsenic,lead,and cadmium from power plants.Among the more harmful elements,arsenic(As)is garnishing greater attention due to its toxicity,volatility,bioaccumulation in the environment,and potential carcinogenic propensities.As the basis of transformation and emission control of arsenic,the vaporization behavior of arsenic was studied in this work.The effects of temperature,coal type,gas composition and coal blending were studied,and the related mechanisms were investigated.Furthermore,the vaporization kinetics of arsenic with time were studied and a novel vaporization model of arsenic considering various arsenic compounds with determined reaction kinetics were developed and analyzed based on single coal particle combustion.The main content and conclusions are listed below:(1)Isothermal combustion experiments of coals in three ranks were carried out in a high-temperature tube furnace in the temperature range of 600-1500? to study the vaporization characteristics of arsenic.The effects of temperature,coal type,atmosphere and coal blending were considered.The mass loss rate of arsenic with respect to temperature was defined to better characterize the vaporization behavior of arsenic.Experiment results show that,the volatilization proportion of arsenic increases with temperature.A staged vaporization phenomenon with temperature is observed within 800-900? the first peak and>1000? the second peak.A typical coal was chosen to study the difference of arsenic vaporization between air and oxy fuel combustion.Compared to 2102/79N2 atmosphere,the second mass loss peak of arsenic at 21O2/79CO2 atmosphere occurs at higher temperature while with a larger peak value.Furthermore,a high arsenic lignite was blended with various bituminous coals to study arsenic vaporization of blended coal combustion cases.The blended results were compared to the weighted average data of single coal combustion.(2)Vaporization mechanism study of arsenic during coal combustion.For the phenomena observed in part(1),sequential chemical extraction method was applied to measure arsenic speciation in parent coal and coal ashes,with which to explain the first mass loss peak of arsenic.ICP-AES analysis on the mineral content of coal ash was carried out to study the correlation between arsenic and minerals.Thermodynamic calculation was also used to study the transformation of arsenates,these are applied to explain the second mass loss peak of arsenic.FTIR and SEM instruments were used to characterize the coal ash to see the transformation difference of minerals under air and oxy fuel combustion condition,and further explain the difference of arsenic vaporization under the two atmospheres and the influence of CO2.The mass loss ratio and mass loss peak of arsenic in mixed coals were monitored to study collaborative mass loss and arsenic vaporization of mixed coals.In addition,the vaporization ratio of arsenic during co-combustion of lignite and bituminous coal was predicted.(3)Simultaneous release behavior of arsenic and sulfur was first obtained.Based on the final vaporization ratios obtained from part(1)and part(2),through changing the residence time of coal sample,the vaporization process of arsenic with time was further studied.Six coals that with different rank and sulfur contents were chosen for the isothermal combustion experiments at 900? and 1300?.The mode of occurrence of arsenic and sulfur in coal was tested by sequential chemical extraction.SEM-EDS analysis was also used to explore the possible As-S compounds.The curves of arsenic vaporization,SO2 emission and mass loss of coal were compared together to study the simultaneous vaporization characteristics of arsenic and sulfur.The simultaneous releasing behavior of arsenic and sulfur is observed at 900? due to the moderate mass loss of coal and at 1300? due to the decomposition of As-bearing sulfides.(4)A new determination method of the vaporization kinetic parameters of arsenic compounds was proposed.Based on the experiments in part(3),the author tried to develop the vaporization model of arsenic.Due to the toxicity of arsenic,the reaction kinetics of arsenic compounds have not been reported yet.Thus,the primary task of this part is to determine the kinetic parameters related to arsenic vaporization.Based on the experiments in part(1)and speciation analysis and thermodynamic analysis in part(2),the vaporized yield models of various arsenic compounds were established through S type curve fitting of experimental data.Combined with the single coal particle combustion model,it is found that the order of magnitudes of the parameters could be obtained precisely when they have to meet various coal samples and temperature conditions.(5)Analyses of the vaporization model of arsenic based on single coal particle combustion.With the kinetic parameters determined in part(4),a novel vaporization model of arsenic considering reaction kinetics was developed.The vaporization ratio as well as the As2O3(g)concentration with time and section were obtained.Model results were compared to experimental results and validated.The effects of temperature and coal type were discussed to show the inner vaporization process of arsenic to better understand the vaporization behavior of arsenic.