Performance Of Hg~0 Removal From Coal-fired Flue Gas Over Coal Gasification Slag

Mercury pollution has become a great public concern,as the largest coal consumption,coal-fired power plants are facing a prominent issue of mercury pollution control.As a major coal consumption,China is facing strict mercury pollution issue.Under this circumstance,sorbents removal technologies proved to be effective approaches to remove Hg⁰,although the preparation of sorbents is complicated and money-consuming.At present,activated carbon injecting（ACI）is a relatively mature mercury removal technology for mercury pollution control in coal-fired power plants.Coal gasification slag（CGS）,a solid waste produced in coal gasification,urgently needs to seek a green-efficient processing mode,the residual carbon inside is loose and porous with potential for carbon-based sorbent.Using CGS as raw material for mercury removal sorbents,its conducive to reduce the cost of mercury pollution control for coal-fired power plants,as well as provide a new way for high-value utilization of CGS.The Hg⁰ removal performance of CGS and its sorted product was investigated on fixed-bed reactor and entrained flow reactor.In fixed-bed Hg⁰ removal evaluation,the effects of adsorption temperature,gas hourly space velocity（GHSV）,flue gas components for Hg⁰removal performance of sorbents were investigated,the characterization methods such as N₂adsorption-desorption,X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）,scanning electron microscopy（SEM）,Hg-temperature programmed desorption（Hg-TPD）were performed to analyze the mechanism of Hg⁰ removal over sorbents and Hg⁰ removal performance of different types of CGS was evaluated.In entrained flow bed Hg⁰ removal evaluation,the effects of flue gas temperature,sorbents residence time,S/Hg ratio（θ）,flue gas components for Hg⁰ removal performance of sorbets were investigated.The Hg⁰ adsorption discrepancy of sorbents in two reactors was explored.The main conclusions of this paper were shown as follows:（1）As shown in the evaluation of fixed-bed Hg⁰ removal,the Hg⁰ removal efficiencies of origin slag（OS）and high carbon production（HCP）could maintain more than 90%at 60-120℃,N₂+4%O₂,GHSV was 1×10⁵ h^-1 after 2 h reaction,which occurred evident drop as adsorption temperature increased for high ash production（HAP）.While increasing GHSV could decrease Hg⁰ removal activity of three sorbents.The presence of SO₂ inhibited Hg⁰ removal activity of three sorbents and weak inhibitory appeared in the presence of NO.HCP showed a certain tolerance to SO₂ and NO.There was adsorption capacity for Hg⁰ removal over different types of CGS.（2）There were two steps for the adsorption of Hg⁰ by three sorbents:The Hg⁰ capture was mainly achieved through physical adsorption,first,Hg⁰_（g）was adsorbed on the surface of sorbents into Hg_（ad）,then Hg_（ad）was oxidized to Hg²⁺by O_βwith forming Hg O.The specific surface area and morphology of sorbents,which were great impacts to physical adsorption,the larger specific surface area and flat surface were conductive to provide stronger physical adsorption.（3）As shown in the evaluation of entrained flow bed Hg⁰ removal,the Hg⁰ removal activities of OS and HCP were relatively stable when the flue gas temperature was 60-90℃,as the flue gas temperature increasing,Hg⁰ removal activities decreased.The Hg⁰ removal activities promoted with residence time of sorbents extended andθincreased.The presence of SO₂ and NO inhibited Hg⁰ removal activity of three sorbents.The highest Hg⁰ removal efficiencies of OS,HCP and HAP in air were 100%,82%and 45%,respectively.The Hg⁰removal activity of sorbent in flowing state was not as good as sorbent in stable state.In the mass-transfer process of Hg⁰ from flue gas to sorbent,gas film resistance of outer layer for sorbent was the main resistance.