Biochar Modification And Its Experimental Study Of Adsorption Properties Of Mercury In Flue Gas

The mercury emission control technology has been a research hot topic after emission control of SO2and NO during coal combustion. The combination technology of sorbent injection and conventional air pollution control devices (APCD) is a promising mercury emission control technology for coal-fired power stations. One of the key problems for successful application of the technology is how to prepare a high efficiency and cheap mercury sorbent. Because biomass has the characteristics of abundant reserve, low sulfur and nitrogen content, high ash reactivity, and zero CO2net emission, it is very important to deal with the preparation of biomass chars and their mercury adsorption characteristics over different biomass chars.In this paper, four kinds of biomass, named mulberry twig (MT), nut shell (NS), wheat straw (WS) and rice straw (RS) were used to prepare the biomass-pyrolysis chars in a self-made fixed bed pyrolysis/activation experimental facility. The steam activated biomass chars were obtained by steam activation on the biomass-pyrolysis chars. The modified biomass chars were gained through treating the steam activated biomass chars in different modification reagents (including H2O2, ZnCl2, NaCl and NaBr). The physical and chemical characteristics of biomass, biomass-pyrolysis chars, activated biomass chars and modified biomass chars were characterized by means of TGA analyzer, surface area analyzer, SEM-EDS analyzer and FTIR analyzer. The results indicated that:1) The surface areas of biomass-pyrolysis chars presented the trend of increase at first and a slight decrease later when the pyrolysis temperature increased from400℃to800℃. For the same kind of biomass, biomass-pyrolysis chars prepared at600℃showed the best pore structure and surface chemical structure. For the different kind of biomass, the pore structure of NS char is the best, the pore structures of MT and WS char are the second, and the pore structure of RS char is the worst.2) Steam activation can improve the pore structure of biomass chars significantly, and also increase the oxygen content in the char surface.3) The surface area and the quantity of surface hydroxyl groups of biomass chars increased when activated biomass chars were modified by H2O2solution. However, the pore structures of biomass chars modified by halides got worse than that of the activated chars, the quantities of halogen elements and halogen groups in the char surfaces were increased significantly.The mercury adsorption over biomass-pyrolysis chars were carried out in a self-made fixed bed adsorption reactor. The effects, such as adsorption temperature, initial mercury concentration and pyrolysis temperature, etc. on mercury adsorption characteristics were analyzed. The results indicated that:1) At the adsorption temperature of60℃～120℃, the unit mercury adsorption capacity of biomass-pyrolysis chars decreased with increasing the adsorption temperature significantly. The unit adsorption capacity of biomass chars was highest when the adsorption temperature was60℃.2) The unit adsorption capacity of biomass-pyrolysis chars decreased with the increasing of initial mercury concentration.3) For biomass chars pyrolized from four tested biomass, the adsorption capacity of NS char was the highest, MT and WS chars were the second, and the adsorption of RS char was the lowest.4) At the pyrolysis temperatures of400℃～600℃, the biomass chars pyrolyzed at600℃showed the best adsorption characteristics, and biomass chars pyrolyzed at400℃performed the worst adsorption characteristics.The mercury adsorption characteristic of steam activated chars and modified biomass chars were carried out in a self-made fixed bed reactor. The effects, such as steam activation, reaction temperature, modification agents and simulated flue gas composition, etc. on mercury adsorption were analyzed. The results indicated that:1) Steam activated can improve the mercury adsorption characteristics of activated biomass chars significantly.2) The unit mercury adsorption capacity and adsorption efficiency of biomass chars modified by H2O2decreased with increasing the adsorption temperature, and the optimal adsorption temperature was60℃based on the experiments. The unit mercury adsorption capacity of biomass chars modified by ZnCl2showed the trend of increased first and decreased later with increasing the adsorption temperature, and the suggested adsorption temperature was90℃based on the experiments.3) The mercury adsorption capacity of the biomass chars modified by H2O2, ZnCl2, NaCl and NaBr improved significantly. Comparing with other biomass chars, MT600-A-ZC(5%) impregnated with ZnCl2solution showed the best mercury adsorption characteristics, whose unit mercury adsorption was29.55μg/g, and was3.37times than that of steam activated biomass chars.4) The mercury adsorption capacity of the modified chars decreased with the increasing of SO2concentration in the simulated flue gas. NO concentration in the simulated flue gas had no significant effect on mercury adsorption capacity over modified biomass chars. SO2and NO in the simulated flue gas can inhibit the mercury adsorption.