Investigation On The Formation Process And Enhanced Removal Of Particulate Arsenic During Coal Combustion

Arsenic is a highly toxic and easily volatile element that is widely distributed in coal.As one of the main anthropogenic sources of arsenic emission in China,it is essential to gain knowledge on the fate of arsenic during coal combustion and search for possible arsenic control strategies.In this study,arsenic-enriched particulates were collected or prepared from different Chinese coal-fired power plants,with objectives of studying the arsenic distribution in coal combustion by-products,especially the formation mechanism of fine granular arsenic.It is expected to provide reference for the emission control of arsenic during coal combustion.The key issues to be solved include:(1)The formation mechanism of granular arsenic at different size fractions during combustion remain unclear.(2)The difference of the dust removal efficiency removal device for different forms of particulate arsenic.(3)Organic combination of in furnace and post combustion control to enhance arsenic removal.In view of the above issues,the main conclusions of this study are as follows:Investigation on arsenic enrichments in coal fly ash and bottom ash:Samples from four typical power plant were analyzed to discuss the distribution characteristics in coal combustion by-products.The results showed that arsenic was found to enrich in fly ash rather in bottom ash.Meanwhile,arsenic distributed more in fly ash(78.6-97.1%)in pulverized coal boiler compared to fluidized bed boiler.Then,the arsenic concentration and speciation in fly ash from different ash hoppers were compared.It was found that the concentrations of arsenic in fly ash from the first two ESP hoppers(ESP1/ESP2)were at similar level while were significantly lower than that of ash samples from rear hoppers(ESP3).More than 90%of arsenic in fly ash was in the form of As⁵⁺,indicating that gaseous arsenic was mainly transformed to particulate arsenic through reactions with Ca/Fe/Al-based minerals.The sequential extraction procedures results showed that Fe/Al-bound arsenic increased while Ca-bound arsenic decreased as the number of dust collector stages increased.The difference of arsenic speciation at different fractions and the removal efficiency of different forms of arsenic might be the main reason for the above phenomenon.Study on the distribution characteristics of arsenic in fine particulate matter(PM₁₀):The particle classification device was used to separate fly ash from different coal-fired power plant into PM₁,PM_1-2.5,PM_2.5-10.During coal combustion,arsenic was easily enriched in sub-micron fine particles(PM₁)and the concentration in PM₁ was about 2-6 times that of PM_2.5-10.Compared with pulverized coal boiler,arsenic was less enriched in PM₁ in fluidized bed boiler.Furthermore,the application of calcium injected in furnace could promote arsenic enrichment in coarse particles.According to the result of sequential extraction procedures,57-88%of arsenic in PM₁ existed in the form of Ca-bound arsenic.However,the combination of Ca and S in the formation of sub-micron particulates may affect the generation of Ca-As.Investigation on the capture characteristics of different forms of arsenic by electrostatic precipitator:The result indicated that the proportion of Ca-As in PM₁,PM_1-2.5,PM_2.5-10 of ESP3 was less than that of ESP1 while the proportion of Fe/Al-As was more than that of ESP1.Specifically,based on the ash resistivity calculation,arsenic-bearing fine particles with certain amount of Fe had suitable conductivity and favored the capture of particulate arsenic by.Consequently,Ca-As and Fe-As were easier to be removed by electrostatic precipitator than Al-As.Furthermore,the enhanced capture of arsenic by fly ash adsorbent,hybrid particulate collector and agglomeration technology was discussed.It was found that hybrid particulate collector at 900? could not only promote the transformation of gaseous arsenic to coarse-grained arsenic,but also promote the formation of Ca-As,which was easier to be captured by ESP.In addition,the fly ash adsorbent had great resistance to SO₂/NO.The hybrid particulate collector could effectively capture the easily escaped sub-micron fine particulate arsenic.