Study On Mercury Migration And Mercury Adsorption Characteristics Of Fly Ash In CFB Boiler In Southwest China

The fundamental realities of county determine that the current energy consumption is still dominated by coal while the coal consumption has brought serious pollution to the environment.Mercury,as the fourth largest pollutant of coal burning except dust,SO₂ and NO_x,has long been the focus of global attention due to its special physical and chemical properties and toxicity.Based on field experiments,this paper studied the mercury migration and transformation characteristics of an industrial CFB boiler burning high mercury coal in southwest China,and characterized the fly ash of several representative CFB boilers,so as to understand the mercury enrichment characteristics in fly ash.In this paper,mercury on-line detection system was used to analyze the mercury in flue gas of 130 t/h CFB boiler at each flue measuring point during operation,and the solid and liquid by-products were sampled and analyzed.Then,the uniformity test of cold air distribution was carried out to compare and analyze the composition of flue gas and mercury concentration at the section of flue duct behind the air preheater.The results show that:The mercury in the flue gas before the dust collector is more than 87%enriched in fly ash,and the mercury in the bottom slag only accounts for 0.07%.The mercury removal efficiency of the fabric filter and wet flue gas desulfurization with the boiler is 91.19%and 51.49%respectively,and the final mercury concentration of flue gas discharged to the atmosphere is only 3.43μg/m³.The mercury emission of high mercury coal combustion in CFB boiler meets the national emission requirements.For the primary air,secondary air and coal seeding air,there are different degrees of inhomogeneity.Under the combined action of all air volumes and input coal,there are obvious differences in oxygen content and pollutant concentration in the flue before the dust collector.There are higher secondary air,coal seeding air and coal quantity in the furnace on the right side,and the flue of the furnace on the right side has higher concentrations of NO_x,SO₂ and mercury.The influence of CFB boiler combustion on the distribution of total mercury concentration in the flue is almost the same as that of other pollutants,and the combustion characteristics will affect the valence distribution of mercury more.Secondly,based on 130 t/h boiler fly ash,the original samples and screened samples of CFB boiler fly ash of with different capacities（75 t/h,300 MW and 600 MW）were characterized.Mercury content,particle size distribution and UBC content of fly ash samples and sieving samples were measured respectively.Nitrogen adsorption experiment and SEM-EDS test were carried out to explore the enrichment rule of mercury in CFB boiler fly ash.The results show that the median fly ash particle size,UBC content,specific surface area and total pore volume of four CFB boilers with different capacities decrease with the increase of boiler capacity.The mercury content and mercury enrichment factor in fly ash have the highest values in 130 t/h CFB boiler.The mercury content and enrichment factors of 75 t/h,300 MW and 600 MW boilers decrease with the increase of capacity due to the change of fly ash characteristics.After particle size screening of all samples,except 130 t/h CFB boiler fly ash,the mercury enrichment factor of fly ash increases with decreasing particle size.Particle size,UBC content,specific surface area and other factors jointly determine the adsorption of mercury on fly ash.According to SEM-EDS scanning results,most of Hg in fly ash is enriched in the surface of fly ash,and more Hg will exist in the form of HgO and Hg⁰.