| Mercury is one of trace elements in coal,it is emitted into the atmosphere during the coal combustion process.Mercury control is arosed great attention because of its high physiological toxicity and bioaccumulation.Coal is the major energy source in China,so the control of mercury emission from coal-fired power plants demands immediate actions.This research is focused on activated carbons which are the major sorbents used for mercury removal in coal-fired power plants currently.The oxygen-containing functional groups on activated carbon surfaces were studied.They could be removed by heating in nitrogen(1000℃).After heat treatment,carboxyl and phenol groups were added separately to the surface of activated carbons by chemical immersion method.The effect of physical-chemical properties for original activated carbons and modified activated carbons was studied by adsorption experiment,and simple mathematic models were developed to describe mercury adsorption on fixed-bed activated carbon system.The major contents and results of this research were as follow:(1) Preparation of modified samplesThe oxygen-containing functional groups on activated carbon surfaces were removed by heating in nitrogen(1000℃).After heat treatment,carboxyl and phenol groups were added separately to the surface of activated carbons by chemical immersion method.The origin materials were sprayed with some deionized water to prepare samples containing different contents of moisture,and then the effect of moisture on mercury adsorption was studied.(2) Mercury adsorption experiment on fixed-bed activated carbon system at room temperatureThe effect of physical-chemical properties of activated carbons on mercury adsorption was studied at room temperature.The results show that the mechanism of mercury adsorption on activated carbons is a combination of physical and chemical adsorption.At the first phase of adsorption,physical reaction is more competitive than chemical reaction.Compared to the mesopore structure,micropore structure on the surface of activated carbons is in favor of capturing mercury from carrier gas.The phenol groups may have no effect on mercury adsorption,while the carboxyl groups are beneficial to the mercury adsorption.When the modification ratio is 28.89 mg carboxyl groups per gram activated carbon,the mercury adsorption capacity reaches the best result.The presence of moisture on the surface of activated carbons can enhance their mercury adsorption capacities,and when the moisture content is 14.49%,the adsorption capacity reaches the best result.(3) Mercury adsorption experiment on fixed-bed activated carbon system at high temperatureThe capacity of modified samples on mercury adsorption was studied at 140℃, and then samples which show a good adsorption capacity were chosen for studying the effect of temperature and mercury inlet concentration on mercury adsorption.The results show that the mechanism of activated carbons on mercury adsorption is a combination of physical and chemical adsorption.As the adsorption temperature increases,physical adsorption is weaken,while the chemical adsorption is reinforced. The phenol groups inhibit mercury adsorption,while the carboxyl groups are beneficial to the mercury adsorption.When the modification ratio is 28.89 mg carboxyl groups per gram activated carbon,the mercury adsorption capacity reaches the best result.The capacity of mercury adsorption on carboxyl-modified samples decreases as the reaction temperature increases.And the degressive amounts of mecury adsorption are increased as the reaction temperature increases.There is a positive relation between mercury inlet concentration and mercury adsorption capacity in a certain range.When mercury inlet concentration is over the best point, the negative effect is stronger than positive effect.When mercury inlet concentration is 48μg/m3,the mercury adsorption capacity for BPL samples reaches the best result. While mercury inlet concentration is 60μg/m3,the mercury adsorption capacity for BPL-s4 samples(the modification ratio is 28.89 mg carboxyl groups per gram activated carbon) reaches the best result. (4) Kinetic models for Hg0 adsorption on activated carbonsSimple kinetic models were developed based on literatures and the results of this research,and then kinetic properties of mercury adsorption onto BPL-s4 sample was studied.The results show that the process of mercury adsorption onto activated carbons could be occured easily,but compared to high temperature,the process is more beneficial at lower temperature.The adsorption datum fit the first order reaction,and the adsorption rate constant is shown to be positively correlated with the adsorption temperature.The activation energy is 25.86 KJ/mol,and the kinetic constant is 22.16 min-1.
|
PDF Full Text Request