Experimental And Mechanism Study Of In-Duct Mercury Removal By Modified Sorbent Injection

Mercury is highly toxic pollutant with volatility, bioaccumulation and persistence in the environment. Coal combustion is the largest source of anthropogenic mercury emissions. Therefore, mercury emission control from coal combustion has been an important research topic in the field of energy and environment. Up to now, activated carbon injection (ACI) has been recognized as the most promising control technology for removing mercury from the coal combustion flue gas. However, there are lots of unknown problems that exist in the application of this technology into coal-fired power plants due to the fact that in-duct mercury removal by sorbent injection is a complex two-phase flow process combined with flow, heat transfer, mass transfer and chemical reaction. The present work is devoted to solving some of these scientific problems related to the mercury adsorption species and mechanism, mercury adsorption kinetics, thermodynamics and equilibrium, the factors that affect the in-duct mercury removal efficiency, the potential of synergistic SO2 and NO removal by sorbent injection. In addition, a mathematical model for the in-duct mercury removal is established to predict mercury removal efficiency by sorbent injection.Four sorbents for mercury removal were prepared and characterized, including raw activated carbon (R-AC), NH4Br impregnated activated carbon (NH4Br-AC), NH4Cl impregnated activated carbon (NH4Cl-AC) and fly ash (FA). The effects of flue gas components, inlet Hg0 concentration, flue gas temperature, particle diameter on the static mercury adsorption were investigated in a fixed-bed reactor. The mercury adsorption species on sorbent were studied by the temperature programmed desorption (TPD) technology and the mercury adsorption mechanism of different sorbent were also analyzed. The results show that NH4Br and NH4Cl remain on activated carbon surface or mesoporous in the amorphous form after modification. The static mercury adsorption capacity of the four sorbent is NH4Br-AC>NH4Cl-AC>R-AC>FA. Physisorption is the main form of Hg0 adsorption on R-AC and little chemisorption also occurs with the production of HgO. Chemisorption is the main form of Hg0 adsorption on NH4Br-AC with the production of HgBr2.O2 in the flue gas is beneficial for promoting mercury adsorption on R-AC and NH4Br-AC with the mercury mechanism unchanged. SOt has different effects on mercury adsorption on R-AC and NH4Br-AC. SO2 in flue gas leads to the reduction of HgBr2 generation, but little HgS formation on the NH4Br-AC. NO promotes the mercury adsorption on R-AC and NH4Br-AC due to the generation of Hg(NO3)2.The kinetics of gas-phase mercury adsorption on activated carbon and fly ash were studied by using kinetic models. Based on the kinetic analysis, the mercury adsorption activation energy and the initial mercury adsorption rate were calculated. Besides, the mercury adsorption thermodynamics and equilibrium were analyzed. The results show that the mercury adsorption process can be divided into two stages:surface adsorption and intraparticle diffusion adsorption. Although mercury adsorption is limited by external mass transfer and intraparticle diffusion, chemisorption at active sites is the rate controlling step of mercury adsorption on R-AC and NH4Br-AC. However, external mass transfer is the rate controlling step of mercury adsorption on FA. The activation energy of mercury adsorption on R-AC and NH4Br-AC are-10.066 kJ/mol and -28.068 kJ/mol respectively, indicating that mercury adsorption is the combination of physisorption and chemisorption. The initial mercury adsorption rate is positively correlated with the mercury adsorption capacity of a sorbent. The thermodynamic analysis indicates that mercury adsorption on R-AC and NH4Br-AC is a spontaneous and endothermic process. The mercury adsorption process increases confusion and complexityof the gas-solid phases. Mercury adsorption on R-AC can be described by Temkin and Langmuir equations while mercury adsorption on NH4Br-AC and FA can be described by Freundlich equation.An entrained flow reactor for the in-duct mercury removal of simulated flue gas by sorbent injection was set up for the first time in China. The in-duct mercury removal capacity of R-AC, NH4Br-AC, NH4Cl-AC and NH4Br impregnated fly ash (NH4Br-FA) were evaluated in this reactor. The effects of inlet mercury concentration, sorbent residence time, flue gas temperature, sorbent particle diameter and sorbent feeding rate on the in-duct mercury removal efficiency were explored. In addition, the in-duct mercury removal mechanism of the four sorbents was analyzed. The results show that high inlet mercury concentration, long sorbent residence time and small sorbent particle are beneficial for the in-duct mercury removal efficiency and the mercury adsorption capacity of R-AC and NH4Br-AC. The increase of sorbent feeding rate promotes the in-duct mercury removal efficiency, but reduces the mercury adsorption capacity. The increase of flue gas temperature reduces the in-duct mercury removal efficiency of R-AC, but promotes the in-duct mercury removal efficiency of NH4Br-AC. The enhancement of oxidation and adsorption on Hg0 by NH4Br modification is stronger than that by NH4Cl modification. In the in-duct mercury capture process, the Br or Cl group on NH4Br-AC or NH4CI-AC can oxidize gas-phase Hg0 to generate HgBr2 or HgCl2, which is easy to be adsorbed compared to Hg0. The in-duct mercury removal efficiency of NH4Br-FA is low and Hg0 oxidation is the main form for the mercury removal by NH4Br-FA, due to the poor specific surface area and pore structure of raw fly ash.An entrained-flow reactor equiped with a 6kWth coal-fired circulating fluidized bed combustor was set up to evaluate the in-duct mercury removal by NH4Br-AC injection under coal combustion flue gas. The mercury emission and distribution of Guizhou anthracite combustion in the 6kWth circulating fluidized bed combustor was explored. Also, the potential of synergistic SO2 and NO removal by NH4Br-AC injection was investigated. The results show that particle-bound mercury is the main mercury emission form when Guizhou anthracite is burned in this combustor. The proportion of particle-bound mercury is 77.34% while that of gas-phase mercury is 22.65%, in which Hg0 is 10.27% and Hg2+ is 12.38%. The in-duct mercury removal efficiency of NH4Br-AC increases from 70.7% to 90.5 as the sorbent residence time increases from 0.59s to 1.79s. Br group on NH4Br-AC increases the affinity for mercury chemisorption. The SO2 removal efficiency of NH4Br-AC reaches 30.6% resuting from chemisorption, capillary condensation and part of SO2 oxidation to SO3. The NO removal efficiency of NH4Br-AC reaches 38% because of chemisorption and part of NO oxidation to NO2.A new mathematical model was proposed to predict the in-duct mercury capture efficiency by sorbent injection. The model was based on external film mass transfer and surface adsorption assumption, including mass balance and adsorption isotherm. The results show that this model can provide a rational prediction result and be used to estimate the activated carbon consumption cost. The model parameters, including sorbent concentration, particle size, equilibrium constant K, external film mass transfer coefficient and residence time have important effects on the in-duct mercury capture rate by sorbent injection.