Mercury Oxidation And Adsorption Characteristics Of Chemically Modified Coal Char

With the development of economy, the problem of mercury pollution is increasingly rigorous, gaseous elemental mercury has attracted worldwide attention,and strengthening the control of mercury from coal-burning has become very important in China. According to the previous study, active carbons injection upstream of a particulate control system has the potential of providing a low cost option for control of mercury emission from flue gas,however the enormous cost of the active carbon powder prevents the commercial popularize in China. To reduce the cost of carbon injection process, either a more efficient adsorbent that can apply at a lower C/Hg ratio, or a lower cost adsorbent is urgent required.As a source of carbonaceous matter, with the advantages of abundant, easy obtained and low cost, the coal char is proposed as a promising alternative mercury removal absorbent. Therefore, the study of mercury removal efficiency and mechanism has important significance in developing new adsorbent material. In this study,the Inner Mongolia lignite was used to produce coal char sorbent, and modified chars were studied with an attempt to produce economical and effective sorbent for capturing mercury.The effect of BET and pore volume, adsorption temperature, surface functional groups, atmosphere components on gasphase Hg0 adsorption by fresh and modified coal char was evaluated on a fixed bed reactor using an online gas-mercury analyzer.Coal char was prepared by the tube furnace under a series of conditions to explore the effect of pyrolysis conditions, and the removal experiments for gaseous elemental mercury were carried out in bench-scaled reactor. Under the temperature of 700? and 520?, coal chars were produced to study the influence of temperature; Coal chars were produced under the conditions of slow heating and rapid heating respectively to study the influence of heating rate;The pyrolysis preservation time under 700? and 520? were set as 5, 30 and 60 min respectively to study the influence of retention time. The BET and porous examination showed that the higher temperature, slow heating rate and longer preservation time were helpful for coal chars to acquire higher BET and larger pore volume. According to the sorption experiments, the coal char with well-developed microstructure showed higher mercury removal performance, coal cahr pyrolyzed at 700 ? for 1h showed optimal capturing capabilities for mercury. For further study of effects of microstructure of coal char on the mercury removal efficiency, chemical activation with KOH was used to modifiy the coal char. Surface area and pore structure of coal char were well developed after KOH modification, mercury adsorption rate and adsorption capability of the coal char increased effectively, and the higher the KOH/coal char was, the higher adsorption rate was.The results showed that mercury removal by fresh coal char depended mainly on physical adsorption, and its adsorption capacity was unsatisfied and stability was weak. To obtain costeffective sorbents for mercury removal, some oxidizing reagents and acid solution were used to modify coal char. Due to the corrosive action of acid solution,surface area and pore structure of modified coal char slightly increased. Meantime,the modification were found to functionalize the coal char. The modified coal chars' s oxygen-containing group? nitrogencontaining functional groups, carboxyl group, lactone? increased obviously after modification by nitric acid,and the hydrochloric acid modified coal char had much more Cl-C-Cl functional groups. The Hg0 adsorption capacity and stability of acid modified coal char improved significantly,which indicated functional groups provided active sites for gaseous Hg0 chemical adsorption. KCl and KI were used to produce the halogenated sorbents, and the removal experiments for gaseous elemental mercury were carried out in bench-scaled reactor to study the influence on mercury removal; CuCl₂, FeCl₃ and Cu?NO₃?₂ were used to modify coal char to study the effects of Cu²⁺ and Fe³⁺on the mercury sorption rate. Both the pore volume and the BET surface area of the impregnated samples decreased comparing to the raw char, attributing to the blockage of internal porosity. According to the fixed bed experiments, the marshalling sequence of mercury removal efficiency was shown as following:CuCl₂> KI> FeCl₃>KCl>fresh coal char>Cu?NO₃?₂.This thesis has made thorough research into the mercury adsorption mechanism of most improved CuCl₂ modified coal char. According to the results of fixed bed experiments of CuCl₂ modified char, the adsorption characteristics of modified chars got worse with the temperature increasing. Both the pore volume and the BET surface area of the CuCl₂ impregnated samples decreased with more CuCl₂ loaded, attributing to the blockage of internal porosity by incorporated CuCl₂ molecules, which hindered mercury atoms from diffusing into pore internal. According the results of sorption experiments,the optimal concentration of Cu impregnated onto the coal char was 1.08%,and the 1.08%-CuCl₂ modified coal char's sorption rate remained above 95% for a long time. O₂ and HCl strongly promoted mercury adsorption, owing to heterogeneous oxidation and chemisorptions reactions, and Cu exhibited catalytic effects to the reaction between HCl and elemental Hg0. Additionally,S02 presence weakened CuCl₂ modified coal char's mercury adsorption. The x-ray photoelectron spectroscopy?XPS? was used to characterize the CuCl₂ modified sorbents, and the results reported that Hg0 was oxidized by consuming CuCl₂, generating CuCl and HgClx.