Study Of Species Identification And Removal Of Mercury In Coal And During Coal Combustion

Worldwide,coal-fired power plants are known to be the largest anthropogenic source of mercury emissions.Mercury emission is a significant environmental concern due to its toxicity and high volatility.Identification of mercury occurrences in coal and mercury removal before coal utilization,mercury oxidation during coal combustion,elemental mercury capture by sorbents,and mercury speciation and distribution in full scale power plants have been studied,providing useful information for selecting/developing mercury emission control technologies.Two 200MW utilities without any differences besides burner type,low-NOx burners versus conventional burners,were studied on emission and behavior of mercury by Ontario-Hydro Method.The out/in mercury balance ranged from 113%to 125%was obtained in this study.Major mercury outgoing was fly ash in hopper and flue gas in stack as well as negligible minor amount of mercury＜2%was remained in boiler slag and economizer bottom ash.Nearly 100%of Hg_p could be removed by ESP due to its＞99% high dust removal efficiency.More than 55%of total mercury was released into the atmosphere as vapor phase,of which＞65%of total mercury was in oxidized form. Increasing operation load of boiler with low-NOx burners leads to higher Hg_p proportion in flue gas at ESP inlet and lower mercury concentration of flue gas at ESP outlet.More mercury and less chlorine were released into the atmosphere from boiler with conventional burners than with low-NOx burners.There was a weak correlation among mercury species, HCl and Cl₂.The fraction of elemental mercury decreased slightly with increasing concentration of HCl and Cl₂.Higher mercury contents were detected in fly ash from boiler with low-NOx burner than from conventional one due to higher carbon content and specific surface area.Mercury emission control technology in the combustion flue gas of coal-fired power plants has been under intensive development,but none has been implemented commercially because of the high cost.Treatment of coal before combustion provides an alternative option for Hg emission control.Samples of one anthracite,one lignite and three bituminous coals were investigated to determine the feasibility of precombustion Hg emission control. Density separation,temperature-programmed pyrolysis,HNO₃ extraction and sequential selective extraction(SSE) were used to determine the occurrence and species of mercury in the different types of coal.The results showed that the characteristic temperature range for Hg release is＜150℃for Hg⁰,150-250℃for HgCl₂/organic-bound Hg,250-400℃for HgS/silicate-bound Hg,and 400-600℃for pyrite-bound Hg.The mercury removal of different technologies are dependent on mercury occurrence in coal.HNO₃ extraction and sink-float were found to be effective for the removal of pyrite bound Hg and then the coal can be pyrolysed at 400℃to remove other forms of Hg.Experiments of rapid pyrolysis of coal showed that higher temperature led to higher rate and efficiency of mercury removal. In addition,decreasing pyrolysis pressure can increase mercury removal rate.The effects of different factors on mercury oxidation were investigated by thermodynamic calculations,kinetic calculations,and tests of coal combustion in a horizontal electrically heated tube furnace.Thermodynamic calculations suggested that HCl is a very weak oxidizer of mercury,while Cl₂ is a very strong one.The presence of H₂O may inhabit mercury oxidation.The dominating product of mercury oxidation was HgCl₂, as well as minor amount of HgO and HgSO₄.Kinetic calculations showed that HgCl,Cl₂, and HOCl formation is important in producing the oxidized mercury during combustion of coal.A suitable temperature for Hg oxidization when Cl₂ is the oxidization resource is 700-1200 K.Results of batch combustion of coal in the tube furnace showed that many factors could influence mercury oxidation.Stronger oxidizing atmosphere was propitious to mercury transformation to oxidized form.The highest degree of mercury oxidation was attained at the combustion temperature of 900℃.Extending residence time in the cooling zone can slightly promote mercury oxidation.Mercury oxidation factions during combustion of density fractionated coal are high＞medium＞light density.Na has obvious inhabitation effect on mercury oxidation via completion reaction with chlorine species. Coal leaching by hot water and ammonia got a high chlorine removal fraction,resulting in a lower oxidized mercury fraction.By cofiring CaCl₂ with coal,more elemental mercury was converted to oxidized gaseous species.Capture by sorbents is effective method to control mercury emission form coal combustion,especially for elemental mercury.A simple method combining wet-chemistry and thermal reduction was adopted to synthesize silver on the surface of carbon nano tube (CNT).The synthesized Ag-CNT was tested as sorbents for the removal of of elemental mercury from flue gases of coal-fired power plants and as a mercury trap for elementary mercury analysis.A complete capture of mercury by Ag-CNT was achieved up to a capture temperature of 150℃.The captured mercury could be quickly and completely released by simple heating at 330℃,to restore its mercury adsorption capacity.Silver on Ag-CNT were shown to be the main active component for mercury capture via amalgamation mechanism in contrast to simple physical adsorption on undoped CNT.Compared with silver-coated quartz beads(Ag-Beads) and gold-coated quartz beads(Au-Beads),which is conventionally used as mercury trap for mercury measurements,Ag-CNT showed a much higher mercury capture capacity and a minimal memory effect.With Ag-CNT as mercury preconcentration trap,calibration results showed a satisfactory linear coefficient of≥0.9998 between known amounts of standard mercury and their corresponding fluorescence signals of a Cold Vapor Atomic Fluorescence Spectrophotometry(CVAFS).The presence of SO₂,NO_x,CO₂ or O₂ showed a negligible impact on the mercury capture performance of Ag-CNT.A fixed bed and and drop tube reactor were used to test the performances of mercury capture by kaolin, zeolite,lime stone and their corresponding bromine impregnated sorbents.Bromination can obviously promote mercury adsorption efficiency.Capture efficiency of origin sorbents decreased with increasing temperature,exhibiting a physical adsorption.In contrast,capture efficiency of brominated sorbents increased with increasing temperature,exhibiting a chemisorption.