Study On The Induction Of Highly Mercury Reactive Groups On Polyvinyl Chloride Heat-Treated Activated Carbon

Mercury has a huge impact on humans and the environment,the coal-fired power plants are an important part of anthropogenic mercury emission sources.Elemental mercury（Hg⁰）in coal-fired flue gas is difficult to remove due to its insolubility and high volatility.Activated carbon injection（ACI）technology is considered a promising methods for Hg⁰ removal.However,the raw activated carbon has poor Hg⁰ removal performance.In order to obtain high efficiency,a high C/Hg ratio is required,which greatly increases the cost of mercury removal.Scholars used chemical impregnation to modify activated carbon to improve its Hg⁰removal performance.However,this method has a complicated process flow and uses a large number of chemical reagents,increasing the cost of sorbent preparation.In view of these problems,this dissertation proposes to use high-chloride plastic waste polyvinyl chloride（PVC）heat-treated activated carbon to induce the formation of high mercury reactive groups on the surface of activated carbon.Its not only greatly improves the Hg⁰ removal performance of activated carbon but also reduces the chlorine emission during the PVC pyrolysis process.At the same time,the preparation cost of the sorbent is also reduced.Firstly,PVC heat-treated activated carbon was used to prepare chlorinated activated carbon.The functional group structure and other properties of activated carbon before and after modification was characterized.PVC heat-treated activated carbon would not affect the type of adsorption and desorption isotherm,but it blocked part of the pores of the activated carbon.PVC and activated carbon reacted interactively during the heat treatment process.The chlorine in PVC was loaded on the activated carbon in the form of C-Cl active groups,which reduced the emission of chlorine in PVC.The Hg⁰ removal performance of chlorinated activated carbon imcreased by more than 50%,it showed a trend of first decreasing and then increasing with the increase of heat treatment temperature.It had the optimum removal efficiency for Hg⁰ with an adsorption temperature of 140°C,which is suitable for the actual application environment of coal-fired power plants.NO,O₂,and HCl could promote the removal of Hg⁰,while SO₂ would inhibit the removal of Hg⁰.Secondly,The penetration experiment showed chlorinated activated carbon still had an excellent Hg⁰ removal performance for a long time and was not easily penetrated.The physical pore structure of the sorbent did not change significantly before and after Hg⁰removal.The physical adsorption had little effect on the Hg⁰ adsorption process.The Hg⁰adsorption process was mainly controlled by chemical adsorption.The C-Cl active group generated during the heat treatment provided the main chemisorption active sites to oxidize Hg⁰ to Hg²⁺.The mechanism of PVC inducing highly mercury reactive groups at low temperature and high temperature is different.Embodied in:at low temperature（300℃）,some organic forms of Cl in PVC reformed and combined to form highly mercury reactive groups C-Cl bond,adhered to the sorbent surface under the influence of the thermoplasticity of PVC.As the temperature increased,the unstable C-Cl active groups were thermally decomposed.However,the remaining C-Cl groups were adsorbed on the sorbent surface to form mercury removal active site.At a higher temperature（800℃）,the activated carbon undergoed denitrification and deoxidation reactions,the physical pore structure became more developed,a large number of carbon atom vacancies were formed on the surface.Meanwhile,the HCl released by PVC combined with the carbon atom vacancies to adsorb on the micropores or surface of activated carbon,a highly mercury reactive group C-Cl bond was formed,which provided the main chemisorption active site for Hg⁰ removal.Finally,the stability of mercury/halogen in the sorbent was investigated according to the standard toxicity characteristic leaching method.The stability of Hg in the sorbent prepared by heat treatment was relatively high,the Hg concentration of the leached liquid in the TCLP experiment were all lower than the limit of 200μg/L specified by the EPA.The concentration of Hg in the leaching solution increased significantly under extremely acid and alkaline environments.The leaching rate of Hg in the sorbent increased as the liquid-to-solid ratio and the leaching time increased.The leaching rate and amount of Hg under the column leaching experiment were much higher than those in batch leaching experiment.The stability of Cl in heat-treated carbon-based sorbents were higher than that of non-carbon-based sorbents.Only in the extremely acid environment,more Cl was leached out of the sorbents.It increased with the increase of the liquid-solid ratio,and was not affected by the leaching time.The leaching process would not change the type of functional groups and the valence of the elements on the sorbent surface,but it would change its relative content.