Study On Release And Transformation Of Alkali Metals And Trace Elements During Biomass Combustion

Since the beginning of the new century,economy has been increasingly developed,shortage of energy resource and environmental pollution problems have become increasingly severe.In China,the efficient and clean utilization of biomass plays an important role in ensuring energy security,protecting environment and developing rural economy.This paper investigated the scientific problems of the release and transformation of alkali metals and trace elements in the field of biomass combustion and power generation technology.First,a kind of typical biomass in China,rice straw,was seleted as studied fuels.The release and transformation behavior of alkali metals during rice straw combustion was investigated.The effect of torrefaction was also studied.A detailed analysis of the fuel samples and the solid residues from combustion was conducted.The results indicated that pretreatment of torrefaction can significantly reduce chlorine?Cl?in straw,improve the heat value.However,torrefied rice straw still contains high amount of alkali,and during combustion,the release of alkali metal is similar to raw rice straw.As temperatures increased from 700? to 800?,the release of potassium?K?,sodium?Na?,Cl of both the raw rice straw and the torrefied rice straw increased significantly.K and Na evaporated in the form of chloride.At 900?,the decomposition of potassium carbonate ocuured.Most of K is in the form of silicate and aluminosilicate in the ash obtained at 900?.At 800-900?,the atmosphere of oxygen/carbon dioxide inhibited the release of K and Cl because of the presence of more carbonates in the atmosphere and the increased ash fusion point.The release and migration of alkali metals in micro-spatial distribution during rice straw pyrolysis and combustion was studied.Scanning Electron Microscope/Energy-Dispersive Spectrometry was used to analyze the morphology and distribution of inorganic matters in raw rice straw,the char and the ash,both upon the epidermis and the ground tissue.The particles dispersedly distributed on the cell walls in the ground tissue were proven to be inorganic salt,rich in K and Cl.K and Cl diffuse from the ground tissue to the epidermis and they might be partly restricted to the epidermis during rice straw pyrolysis.The higher the pyrolysis temperature is,the less K and Cl are in the obtained solid residues.At 900?,the potassium in the obtained solid residues is mostly in the form of sulfate or aluminosilicate.During rice straw combustion at 700? and 800?,the morphology of the epidermis was unchanged and the char matrix formed from cell walls in the ground tissue would be consume by oxygen.The ash samples obtained after combustion at 700? and 800?were presented as flocculent and net structure,respectively.For the ash samples obtained after combustion at 900?,fusion phenomenon was found and the epidermis and the ground tissue were fused together.To our knowledge,this study is the first to directly reveal the exact region rich in K and Cl and the morphology of K-and Cl-rich particles in rice straw.The transformation behavior of K involving the change of water-insoluble K and K₂CO₃ during biomass pyrolysis and combustion was studied.KCl-loaded cellulose samples were used as fuels with the aim to determine the key reactions involved during K transformation.The quantification results indicated that,during the pyrolysis of the KCl-loaded cellulose,the reactions between KCl and active functional groups which are produced from organic matter in cellulose during pyrolysis lead to a certain amount of water-insoluble K formed above 300 ? and the reactions were governed by both the availability of active functional groups produced during cellulose pyrolysis and the amount of KCl.The water-insoluble K was transformed into K₂CO₃ above 600?.During combustion,the presence of O2 promoted the generation of K₂CO₃.The transformation of KCl into water-insoluble K and K₂CO₃ should be emphasized for the further research of K transformation during the thermal conversion of biomass.On the basis of the mechanism of K release obtained from the above experiment studies,the kinetic model of potassium release during combustion of rice straw was established.Model calculations include K release due to the decomposition of organic K,the evaporation of KCl and the decomposition of K₂CO₃.The quantitative description of the release of K element during combustion process was obtained.The comparison between the calculated value and the experimental measured value showed that the amount of K release obtained from the model calculations at different combustion temperature and different heating rate conditions are in good agreement with the experimental measured value.It indicated that the main mechanisms of alkali metal release during the combustion in the model are reasonable.The descriptions of the change of alkali metal in the model basically reflect the main process of the actual reaction.The total amount analysis mass balance and chemical sequential extraction procedure were used to investigate the contents,mode of occurrence of trace elements of maize straw,rice straw and eucalyptus bark and trace elements release and transformation during pyrolysis and combustion.For comparision,a kind of bituminous coal was selected.Compared with the bituminous coal,the contents of As and Cd in the maize straw,the rice straw and the eucalyptus bark are lower,Zn is higher,and Cu and Pb are similar.The mobility of the five trace elements is greater than that of the bituminous coal.Considering the two aspects of element content and mobility,the potential harm to environment of Zn and Cu in biomass ash is higher than that of bituminous coal ash.During pyrolysis and combustion at 600-1000 ?,for the maize straw,the rice straw and the eucalyptus bark,the release amount of As was higher than 60%,the release amount of Cu was less than 22%.Cd,Pb and Zn is obviously released,and it increased with the increase of temperature.Different elements and different forms have different release and migration behavior.In addition,the release of Cd,Cu and Zn of pyrolysis was higher than that of combustion.The main reason is that more oxides are formed in oxidizing atmosphere.At lase,coal bottom ash?CBA?obtained from a pulverized coat-fired power plant was used to replace all the regular bed materials?quartz sand?in a biomass fired circulating fluidized bed?CFB?boiler,attempting to solve its HTC problem.Two kinds of mature deposits on the high-temperature superheater with regular bed materials and with the CBA as a bed material were obtained and analyzed in detail.The deposit formation mechanisms with regular bed materials were discussed.Our results show that the CBA can capture 22%of potassium in fuels and reduce the K and Cl in combustion area during biomass combustion in the CFB boiler.CBA can decrease the deposit thickness from 5-7mm to 4-5mm.The results indicate that the deposit formation speed has been inhibited.