Mercury emission is an important issue during chemical looping combustion (CLC) of coal. The aim of this work is to explore the effects of different flue gas components (e.g., HCl, NO, SO 2 , and CO 2 ) on mercury transformation in the flue gas cooling process. A two-stage simulation method is used to reveal the reaction mechanism of these gases affecting elemental mercury (Hg 0 ) oxidation. Furthermore, using this method, Hg 0 oxidation by eight oxygen carriers (Co 3 O 4 , CaSO 4 , CeO 2 , Fe 2 O 3 , Al 2 O 3 , Mn 2 O 3 , SiO 2 , and CuO) commonly used in CLC are investigated and their Hg 0 oxidation efficiencies were compared with the existing experimental results. The results show that HCl, NO, and CO 2 promote Hg 0 oxidation during flue gas cooling, while SO 2 inhibits Hg 0 oxidation. The stronger the oxygen release capacity of oxygen carriers, the higher the oxidation efficiency of Hg 0 becomes. The order of Hg 0 removal efficiency from high to low is Co 3 O 4 , CuO, Mn 2 O 3 , CaSO 4 , Fe 2 O 3 , CeO 2 , Al 2 O 3 , and SiO 2 , and this sequence is in good agreement with the existing experimental results. Different flue gas components directly or indirectly affect the O 2 content, thus affecting the content of gaseous oxidized mercury (Hg 2+ ). Different oxygen carriers have different oxygen release capacities and different Hg 0 oxidation efficiencies. Therefore, O 2 is the core species affecting the mercury transformation in CLC.
Several preparation methods for current spinel compounds are briefly introduced. The effects of temperature and flue gas components on the removal of Hg 0 over spinel compounds are reviewed. The reaction processes and mechanisms of Hg 0 adsorption, oxidation, and desorption on the surface of spinel compounds are analyzed deeply. The current methods for improving Hg 0 catalytic oxidation performance by spinel-based photocatalytic materials are also introduced. There are two main types of spinel compounds for Hg 0 removal: conventional spinel and loaded spinel compounds. Loaded spinel compounds are favorable for the adsorption and oxidation of Hg 0 due to more active sites and a larger surface area than ordinary spinel compounds. Both O 2 and HCl contribute to Hg 0 removal with Hg 0 oxidation efficiency being sensitive to both O 2 and HCl concentrations. The presence of SO 2 has a detrimental effect on the removal of Hg 0 due to the competition for the active adsorption sites on the surface of catalysts and adsorbents. The impact of temperature is bidirectional, and an optimum temperature exists. Based on the analysis of three stages (adsorption, oxidation, and desorption) included in the Hg 0 catalytic oxidation process, it was discovered that for most catalysts the Hg 0 oxidation process is the rate-limiting step for the reaction.
Acid gas compression and purification are an effective way for the treatment of NO x in oxy-fuel combustion flue gas due to the reduced volume flow rate of flue gas in oxy-fuel combustion in comparison with air combustion. The design of reactors for NO x removal is particularly important for future CO2 utilization and storage and is also rather challenging due to the impact of pressure on both kinetic as well as mass-transfer parameters. To obtain the sensitivity as well as pressure dependence of different parameters, NO x removal in water under high pressure in both bubbling and film modes was investigated based on both experimental and modeling methods. Six uncertain parameters, including mass-transfer coefficients of NO and NO2 (kNO and kNO2), NO oxidation rate constant k1, liquid surface area S1, liquid film surface area S2, and liquid film volume VL, were identified and optimized to minimize relative errors between measurements and simulations. Three most important parameters affecting the simulated NO x absorption results are kNO2, k1, and S1, followed by kNO. While S2 and VL only affect NO2, these two parameters have little effect on NO, HNO3, and HNO2. In the gas bubbling reactor system, the liquid phase plays the main absorption role in NO x and the liquid film plays a supplementary absorption role. The effective removal of NO x in the high-pressure system can also be achieved by using only liquid film absorption rather than gas bubbling absorption as gas absorption by the liquid film can generate high concentrations of HNO3, which is beneficial for subsequent utilization.
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