Migration And Transformation Of Alkali Metals And NO_x Emission Characteristics During The Combustion Of High Alkali Coal/gasification Fly Ash

The high alkali Zhundong(ZD)coal has good reactivity and large reserves,the reserves are estimated to be more than 3900 billion tons.However,because of the high sodium oxide(Na2O)in coal ash,the slagging and contamination are seriously generated during the direct combustion of high alkali ZD coal in a boiler.Furthermore,at present,the NOx emission value of the national key areas is limited to 50 mg/m3,but the characteristics of the NO,emission during the combustion of high alkali ZD coal are still unclear.The circulating fluidized bed boiler(CFB)is characterized by low operating temperature and low pollutant emission concentration,so it has a good prospect in the clean and efficient use of high alkali ZD coal.The high alkali ZD coal step utilization technology based on CFB provides a new possibility for the clean and efficient utilization of high alkali ZD coal,that is,the most alkali metals in the high alkali ZD coal are precipitated in gaseous form by gasification process,and then the alkali metals in the gas are removed by physical means,and finally the purified gas is used as the industrial raw material,and the gasification fly ash produced is used as fuel in the CFB.The high alkali ZD coal CFB gasification fly ash(ZDf)is characterized by low volatile content,high ash content and small particle size and so on.Na content in ZDf is as high as 1-4%,and there is a significant difference between the content and occurrence of Na in ZDf and the Na in ZD coal.New problems of ash deposition and fouling will be produced in the process of high alkali ZDf re-combustion in a CFB.Aiming at the ash deposition and fouling problems that are caused by migration and transformation of alkali metal during the fluidized combustion of high alkali ZD coal and ZDf and the NO,emission problem during the fluidized combustion of ZD coal,the migi’ation and transformation,removal and recovery characteristics of alkali metals and the effects of alkali metals on the NOx emission during the combustion of high alkali ZD coal and ZDf are studied with the help of tube furnace and constant temperature water bath.And the thermochemical conversion and the low NOx emission basic tests of high alkali ZD coal and ZDf are carried out with the help of CFB test device.The main results are as follows:1)The contents of alkali metals in high alkali ZDf are much higher than those in high alkali ZD coal,Na is mainly in the form of water-soluble NaCl,Na2SO4 and low-melting sodium aluminosilicate,and K is mainly in the form of water-soluble K.The content of K(0.3-0.7 mg/g)in high alkali ZD coal is much lower than that of Na(4.2-9.1 mg/g),but the content of K in high alkali ZDf is higher(0.8-3.1 mg/g)and cannot be ignored compared with that of Na(4.3-25.3 mg/g).The content of Na in various forms of ZD coal does not change substantially with the change of the particle size.However,the content of Na in various forms in the ZDf is closely related to the particle size of the fly ash.2)The bed temperature,equivalent air ratio(ER),coal type,and cooling mode of the heating surface have important influences on the proportion of the total Na in the system to the recoverable soluble Na in fly ash during the gasification of high alkali ZD coal.As the bed temperature rises,more soluble Na is precipitated from the fly ash into the gas phase,and the ratio of soluble Na in the fly ash is reduced.As the ER increases,the proportion of soluble Na converted to fly ash increases,when the ER is 0.5,the ratio of soluble Na is the highest(22.6%).The yield of gasification fly ash and soluble Na in gasification fly ash increases when the heating surface is cooled,so that the proportion of total Na converted to the soluble Na in fly ash increases.The proportions(of recoverable soluble Na in the gasification fly ash of Shaerhu(SEH)coal and Tianchi(TC)coal are higher(19.5%and 14.1%),but it is lower in the Shenhua(SH)coal gasification fly ash(11.4%),so the Na in the gasification fly ash of the SEH coal and TC coal is more suitable for recycling.3)The bed temperature has important influences on the migration and transformation of Na during the combustion of high alkali ZD coal and ZDf,but it has no obvious influences on the migration and transformation of K.During the combustion of SH coal,most of the Na has precipitated into the gas phase at a lower bed temperature(<860℃),and Na is mainly distributed in the gas phase.During the combustion of SH coal gasification fly ash(SHf),Na is mainly distributed in fly ash and gas phase,and most of Na begins to precipitate at a higher bed temperature(>910℃).The surface of the ZD coal gasification fly ash is covered with a layer of calcium oxides and sulfates,which inhibits the precipitation of Na into the gas phase,thus the release and conversion of Na in the combustion process of SHf is weaker than that in SH coal combustion process.The Na in the gasification process of ZD coal is more likely to be retained in the bottom ash than the Na in the combustion process,and less Na is released into the gas phase in the gasification process.4)The ash deposition during the SH coal and gasification fly ash of TC coal(TCf)combustion occurred primarily as a result of the agglomeration and bonding of the ash particles that are rich in NaCl,Na2SO4 and CaSO4,and the ash deposition propensity is higher.However,the ash deposition during the combustion of SHf resulted primarily as a result of the accumulation of fine fly ash particles with a low Na content,and the ash deposition propensity is lower.The ash deposition during the combustion and gasification of the SEH coal is mainly as a result of the condensation of gas phase NaCl and the deposition of fly ash rich in NaCl on the wall,but the combustion presents stronger ash accumulation characteristics than gasi fication,and the corrosion of ash on the wall is more serious.5)The water-soluble Na in the fly ash of high alkali ZD coal can be effectively removed by water washing,the removal rate of Na is 3 1-93%.Increasing the washing temperature,washing time,solid-liquid ratio and acid solution concentration can improve the removal rate of Na in ZD coal fly ash.The recovered products of Na are NaCl and Na2SO4,and the economic analysis shows that it is economical to recover Na from ZD coal fly ash by water washing.The recovery rate of Na in the SEH coal combustion fly ash(SEHcf),SEHf and TCf reach 49.4-81.2%,while that in the SHf is only 16.6%.Therefore,it is more suitable for recovering Na from SEHf,SEHf and TCf,while SHf can be directly used for combustion.6)NaCl-based water-soluble Na in ZD coal has a significant inhibitory effect on the NOx emission during combustion,but the inhibition decreases with the increase of combustion temperature.In the temperature range of 700-900℃,and the NaCl addition ratio range of 2-3%,the inhibitory effect of Na on NO.formation is stronger.The NO.emission concentration during ZD coal conventional combustion in a CFB is 254-601 mg/m3.The lowest NOx emission concentration is 93 mg/m3 during the post-combustion of ZD coal,which is 67.9%lower than the NO.emission concentration of conventional combustion.The post-combustion process has a tiny effect on the release of NO.,it mainly plays a role in reducing the carbon content of fly ash and improving the combustion efficiency.When the furnace excess air ratio is in the range of 0.8-0.9,the post-combustion wind speed is higher than the flue gas speed,and the post combustion wind enters from the cyclone exit horizontal section,the overall NOx emissions are lower.