| The environmental pollution from harmful trace elements,such as arsenic(As),selenium(Se)and lead(Pb),has been widely worried around the world due to the high volatility and strong accumulation during the process of coal conversion and utilization.Coal combustion technology for generating heat and electricity is one of the main sources for anthropogenic trace element emissions.In this dissertation,a novel method for stable digestion,sufficient extraction and accurate measurement of As,Se and Pb within coal and its combustion by-products is firstly developed by optimizing the microwave-enhanced digestion route and combining with hydride generation-atomic fluorescence spectrometry.The speciation distribution characteristics of As,Se and Pb in coal and its combustion byproducts are detected by the sequential extraction method.Then,the content and speciation distribution of As,Se or Pb in raw coal,cleaned coal,middling coal,coal gangue and coal slime are measured for understanding the migration and transformation of As,Se and Pb during coal washing process.The content and speciation distribution of As,Se or Pb in feed coal and its combustion by-products are obtained to identify the migration and transformation characteristics of the above trace elements in ultra-low emission coal-fired power units.Moreover,the enrichment behaviors of As,Se and Pb in fly ash are investigated to compare the difference of fly ash characteristics between CFB and PC units.Finally,the migration and transformation routes of As,Se and Pb in the co-combustion process of sewage sludge and coal are explored to support the CFB coal-fired power unit for co-firing sewage sludge.The main contents and conclusions are as follows:Compared with traditional wet digestion method,microwave-enhanced digestion method proposed in this dissertation has the advantages of less element loss,simpler operation step and higher experimental safety.The suitable acid dissolution system is HNO3-HCl-HF mixed acid solution with the volume ratio of 6:2:2 for the samples of raw coal and its washery products form coal washing units,sewage sludge from sewage treatment plants,feed coal and its combustion by-products from coal-fired power units.The As,Se and Pb of samples can be stably digested,fully extracted and accurately measured by using microwave-enhanced digestion method combined with hydride generation-atomic fluorescence spectrometry.The As,Se and Pb in coal and its combustion by-products can be divided into exchangeable state(F1),carbonate+sulfate+oxide bound state(F2),silicate+siloaluminate bound state(F3),sulfide bound state(F4)and organic bound state(F5)by using the sequential chemical extraction method.The recovery of As,Se or Pb in each sample is between 82%-111%,which is within the error range of 70%130%for mass balance of trace element by U.S.Environmental Protection Agency(EPA).The speciation distribution characteristics of As,Se and Pb have certain correlations with sulfur in raw coal and its washery products.The results show that inorganic states(F2,F3 and F4)are the main speciation with more than 60%of As,Se,Pb or S in raw coal.Compared to raw coal,the removal ratio of As,Se,Pb or S in cleaned coal or coal slime is between 30%and 65%.The proportions of organic bound state(F5)As,Se,Pb and S are increased,which can be obviously released together with water and volatiles from sample when the combustion temperature is below 500℃.However,41%-73%of As,Se,Pb and S are migrated from raw coal to coal gangue,whose enrichment ratios are in the range of 57%-215%.Furthermore,As,Se,Pb and S in coal gangue are mainly existed in silicate+silicoaluminate bound state(F3)and sulfide bound state(F4),which are mainly released with the decomposition of pyrite.sulfate and other minerals when the combustion temperature ranges from 500℃ to 1000℃.The above experimental results show the synchronous release between As,Se,Pb and S during the combustion of raw coal or its washery products.The migration and transformation characteristics of As,Se and Pb are investigated in four circulating fluidized bed(CFB)units with capacity between 25 MW to 350 MW and five pulverized coal(PC)units with capacity between 350 MW to 600 MW.The results show that almost all Se in feed coal is released to the gas phase,while the proportions of As and Pb remained in the bottom slag are higher than Se because of lower volatility.The volatilized As,Se and Pb are mainly enriched in fly ash and existed in inorganic bound states(F2,F3 and F4),among which the proportions of As,Se and Pb in silicate+aluminosilicate bound state(F3)are more than 60%.However,more than 70%of As,50%of Se and 90%of Pb are migrated to the solid phase in wet flue gas desulfurization system,and the proportions of exchangeable(F1)and organic bound state(F5)As,Se and Pb are higher than those in fly ash and bottom slag.Although the contents of As,Se and Pb are similar in the feed coal of the two types of coal-fired units,the contents of As,Se and Pb in the bottom slag of CFB units are higher than those of PC units largely due to the difference in combustion temperature.Furthermore,the relative enrichment factors of As,Se and Pb in fly ash of CFB units are generally higher than those of PC units.Besides,the environmental risks of As,Se and Pb in combustion by-products are assessed through three leaching methods.Under the acidic leaching environment of multiple extraction procedure(MEP),Pb in combustion by-products is in the range of low risk,while As and Se are in the range of medium risk.The leaching ratios of As,Se and Pb in gypsum are higher than those in fly ash and bottom slag.The difference of intrinsic characteristics of fly ash between CFB and PC units is the main reason for enriching As,Se and Pb in fly ash.For the same size particle,the unburned carbon(UBC)content of fly ash from CFB units is higher than that from PC unit.The content of As,Se or Pb is increased with the increase of UBC content for fly ash from CFB unit,whilst their correspond content is decreased with the increase of UBC content for fly ash from PC unit.The fly ash of CFB units is irregular shape and rough surface with more honeycomb pores,while the fly ash of PC units is regular and spherical with smooth surface.Therefore,the specific surface area,pore volume and pore diameter of fly ash with different particle sizes from CFB units are higher than the corresponding values of fly ash from PC units.As,Se and Pb contents in fly ash of these two types of units and their leaching ratios under acidic environment are increased with the decrease of fly ash particle size.Taking the same capacity of 330 MW units as example,the leaching ratios of As,Se and Pb in fly ash of CFB4 unit increase from 17.43%,24.01%and 9.82%to 23.91%,29.74%and 12.30%,respectively,while the values of which in fly ash of PC1 unit increase from 22.98%,26.99%and 9.10%to 25.89%,29.84%and 11.10%,respectively.The release characteristics of As,Se and Pb are significantly affected by the content of sewage sludge for co-firing coal and sewage sludge.The experimental results show that the release ratios of As,Se and Pb are increased with the increase of sewage sludge mixing ratio during the co-combustion process.The chlorine content in sewage sludge is generally higher than that in coal,which can promote the release of As,Se and Pb.The mass fraction ratio of acid oxides to basic oxides(A/B)in ash of fuel is of importance to influence the co-combustion characteristics,resulting in the different release characteristics and chemical stability of As,Se and Pb.The release ratios of As and Se decrease with the decrease of the A/B,while the release ratios of Pb reach the lowest when A/B is 1.The leaching ratios of As,Se and Pb in combustion products reach the lowest at A/B equal to 1.The samples were collected from two CFB units with capacity of 135 MW and 350 MW coal-fired units blending with sewage sludge.It can be found that the migration and transformation behaviors of As,Se and Pb could not be considered when the content of sewage sludge mixing is less than 10%,which are consistent with the experimental data in lab-scaled equipment. |
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