Study On Transformation And Enrichment Behaviour In Particulate Matter Of Selenium In Combustion Flame

Coal combustion process has been the largest anthropogenic emission source of selenium in the world.After being burned,almost all the selenium would be emitted directly into the atmospheric environment in vapor phase,or show enrichment behaviour in fine particulate matters.Since the bio-accumulative effect of selenium could have great negative impact to human health and ecological environment,this kind of typical volatile trace element has attracted increasing worldwide attention,and study on transformation and emission control measures of selenium in combustion process has become an emerging and frontier area.At present,most study on transformation and enrichment behaviour of selenium in combustion process concentrated on experiments which are carried out in large boilers in coal-fired power plants,or in other kind of coal-fired devices,while study on transformation of selenium directly in combustion flame is relatively short.Based on the fact above mentioned,experimental study on transformation and enrichment behaviour of selenium directly in combustion flame was carried out in this paper,in order to exclude the influence of other factors that may affect the transformation of selenium during coal combustion process.Besides,Three reaction mechanisms of selenium in the combustion flame were preliminarily discussed through quantum chemistry calculation methods.First,an experimental system was built creatively,in which an inverse diffusion flame burner was set as the main body,using ethylene as fuel.To simulating the actual environment of boilers,selenium was brought into the combustion flame in vapor phase under the condition of reducing atmosphere.Then in the flame,selenium of different speciation(gaseous selenium and particulate selenium)were sampled through the U.S.EPA method 29,and were determined by hydride generation atomic fluorescence spectrometry(HG-AFS),in order to study the mechanism of transformation and enrichment behaviour of selenium during the reaction process.The experimental results indicated that selenium was mainly released in vapor phase from the flame into the air,which means gaseous state is the main transformation form of selenium during combustion process.With the increase of flame temperature,the concentration of selenium both in gaseous state and in particulate state would increase,namely high temperature is beneficial to the release of selenium.It was also observed that selenium tended to be enriched in fine particulate matters with small size and high specific surface area.In the actual combustion process,the air-fuel ratio,namely the fuel oxidation degree,is an important factor that could affect the combustion result.Besides,other gases like sulfur dioxide may affect the transformation process of selenium.Therefore in the latter experiments,the air-fuel ratio of the flame burner was adjusted to different value,and sulfur dioxide gas of different concentration was added into the combustion atmosphere,in order to compare the distribution of selenium under different conditions,and to analyse what influence would have to the transformation and enrichment behaviour of selenium since the change of the air-fuel ratio and sulfur dioxide concentration.According to the experimental results,under conditions of different air-fuel ratio and different sulfur dioxide concentration,gaseous selenium is still the main transformation form of selenium from the flame into the air.When adjusting the air-fuel ratio,with an increasing air-fuel ratio,both the generation rate of gaseous selenium and particulate selenium would increase.High flame temperature and large air-fuel ratio could increase the concentration of particulate selenium,and further improve the removal efficiency of selenium in the post-combustion process.When adding sulfur dioxide gas of different concentration,the result indicated that low concentration sulfur dioxide will both increase the concentrations of gaseous selenium and particulate selenium,and accelerate the release of selenium from the flame,while on the contrary,high concentration sulfur dioxide would decrease the emission amount of selenium.The study on microcosmic reaction mechanism of selenium in combustion flame is helpful to predict its transformation after leaving the flame,and also helpful to put forward targeted emission control measures.But so far,studies about reaction mechanism and their corresponding reaction kinetic parameters(such as activate energy,pre-exponential factors,and reaction rate constant),is relatively short.Based on the above-mentioned fact,reaction mechanism of three reactions in Se/O2 reaction system has been studied by abinitio calculations of quantum chemistry.Applying MP2/GEN method and basis set,the geometry optimizations of reactant,transition state,intermediate and product of three reactions have been made.Then at the same level,the reaction potential barriers were calculated,and the zero-point energies are corrected.Kinetic parameters including activate energy,pre-exponential factors,and reaction rate constant under different temperatures,were also calculated.The calculating results of microcosmic reaction mechanism and kinetic parameter could be a reference for the work of other researchers.