| The control of major air pollutants from coal-fired power plant has been becoming more complete in recent years. Thus, the prevention of mercury pollution has become a key point and difficult problem in the world. The activated carbon injection method is a mature mercury removal technology. Unfortunately, the expensive cost and negatively impacts on the fly ash utilization byusing activated carbon injection. It is the main reason that this method can not be adopted in China. Therefore, the development of mercury sorbent with both high efficiency and low cost is particularly important. Coal fly ash modified with bromide compounds has a lower price and strong potential for mercury removal. However, the mechanism of modified fly ash to oxide the elemental mercury is unclear and still needed further study. Thus, this study was focused on the critical problems of mercury removal with modified fly ash. Hydrobromic acid was selected as the modifying agent to prepare the HBr-modified fly ash. Firstly, an innovative bench-scale experimental system was developed to investigate the homogeneous Hg~0 oxidation mechanism. And the homogeneous/heterogeneous Hg~0 oxidation pathways with respect to HBr-modified fly ash injected into the flue gas were studied in the experimental systems. Secondly, the mathematical model of Hg~0 adsorption and oxidation process by HBr-modified fly ash was established to explore the dynamic behavior of Hg~0 in the diffusion region and surface active site. Finally, this study developed on-site of fly ash sorbent mercury removal system with the integration of modification and injection, which could be applied to coal-fired power plant. Experimental studies were also included on raw material property of fly ash sorbent, modification preparation method in the field, injection technology, etc.The mercury sorbent was usually injected into the upstream of the electrostatic precipitator (ESP). If the bromine species could be released under the flue gas temperature, Hg~0 might go through the homogeneous oxidation. Therefore, this study focused on the fundamental research about homogeneous and heterogeneous Hg~0 oxidation mechanisms along with HBr-modified fly ash. Mainly includes four tasks:(1) A homogeneous reaction experimental system was developed, which could achieve the bromine species and Hg~0 enter into the tubular reactor simultaneously. This system can study on the homogeneous reaction between Hg~0 and bromine species. Followed by, a fixed-bed reactor was used to replace the tubular reactor. Hence, the interactions among original fly ash, bromine species and Hg~0 could be investigated to supplement the heterogeneous Hg~0 oxidation reaction. (2) Tests utilized homogeneous reaction experimental system specialized for the homogeneous Hg~0 oxidation behavior along with thermally released bromine species. Results indicated that the homogeneous Hg~0 oxidation level with respect to bromine species released from HBr-modified fly ash could increase with the heating temperature. The homogeneous Hg~0 reaction was thought to be kinetically controlled. The resident time was a significant important factor and 95% oxidation efficiency could be achieved under the resident time of 4.8s. Low reaction temperatures (50-180℃) were believed to be favoring the homogeneous Hg~0 oxidation. Combined with the characterization results, the release mechanism of bromine species and homogeneous reaction mechanism were proposed. The hydrogen atom of the HBr in the moving state was attracted to the lattice oxygen, forming neighboring active bromine atoms. Then recombined to form Br2, and subsequent desorbed and realized homogenous Hg~0 oxidation in the gas-phase. Furthermore, O2 could replenish the consumed lattice oxygen and formed an integral bromine Deacon Reaction process. Thus, the release of would be improved, resulting in facilitated the Hg~0 oxidation. (3) The interactions among original fly ash, bromine species and Hg~0 were conducted in the former improved experiment system. The bromine species released in the temperature corresponded to the flue temperature between the APH and ESP had a weak activity. However, the release species could promote the adsorption and oxidation of Hg~0 by the original fly ash. The mercury removal performance of HBr-modified fly ash was evaluated in fixed-bed reactor system. The adsorption strength of surface active sites could increase with the adsorption temperature, and then promoted the adsorption/oxidation of Hg~0 by HBr-modified fly ash. Based on the experimental studies and heat transfer performance of fly ash, the oxidation pathway of Hg~0 corresponded to situation of injection location setting in the region between SCR and APH was proposed. One was the adsorption and oxidation in the surface of HBr-modified fly ash, the other was cooperative effects among original fly ash, bromine species and Hg, the third was homogeneous reaction between Hg~0 and active bromine species. (4) In order to determine the key factor of the adsorption and oxidation process of Hg~0 by HBr-modified fly ash, the adsorption kinetic model was established including diffusion and surface adsorption, which could fit well with the experimental breakthrough results. The Hg~0 concentration profiles inside the particle could be obtained from the model. By comparing the concentration gradients of diffusion region and kinetic region, the external diffusion resistance could be determined to have greater influence on the initial stage of Hg~0 adsorption and oxidation.In the present mercury removal of the coal-fired power plant, the preparation and injection of sorbent were separated and there were many different places. This study proposed a new concenpt that the usage of the fly ash generated on site with modification and injection on line. And a series of applied researches were carried out. Mainly includes as follows:(1) According to the site conditions of a level of 300MW coal-fired power plant, a set of fly ash sorbent mercury removal system was developed for field application. In this system, the raw material of sorbent was selected the fly ashes under electric field of the power plant. Then, the fly ashes were modified on site for improving the adsorption performance. At the end, the modified fly ashes were continual injected into the flue gas to remove the mercury removal in the flue gas. Based on the results of basic research, a group of injection points were selectedbetween SCR and APH to increase the residence time and the active bromine species to promote the homogeneous Hg~0 oxidation pathway. Furthermore, the injection velocity of modified fly ash could be adjusted by supplementing the compressed air in hopes that the influence of external diffusion resistance could be reduced. (2) In order to investigate the property of raw material of fly ash sorbent, the characteristics of fly ashes under different electric fields in the ESP were studied. Results indicated that the mercury removal performances of different fly ashes were closely related to the content and activity of unburned carbon when the activities of inorganic components were weak. Therefore, for the specific power plant, along with the output of single dust hopper, fly ash in the first electric field with higher content of UBC was preferential selected as the raw material of sorbent. (3) For the continuous modification of fly ash in the field, impregnation method and mechanical mixed method were compared. In the process of preparation the sorbent with mechanical mixed method, the surface activity of fly ash could be improved dramatically due to the larger mechanical energy, which was conducive to the adsorption and oxidation of Hg~0 by the prepared sorbent. Considering factors of mercury removal efficiency, loss of modifying agent and mode of production, the mechanical mixed method was suitable for large-scale preparation of fly ash sorbent in the field near coal-fired power plant. (4) To verify the feasibility of the pneumatic conveying technology used for injecting the fly ash sorbent, the pilot-scale experiment of sorbent injection was performed. Results indicated that the self-designed pneumatic conveying system could achieve the goal of continuous injection. The distribution manifold could improve the flow characteristics of fly ash particles, as compared with the T-type injection pipeline. The uniformity of sorbent injection was achieved using this on line system. |
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