Oxygen-Induced Elemental Mercury Oxidation in Chemical Looping Combustion of Coal

Abstract

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.

Review and Outlook of Theoretical Understanding on Pollutants Reaction Mechanism during Chemical Looping Combustion

“…Additionally, the reducing gases such as CO, H 2 , and NH 3 will consume lattice oxygen over oxygen carriers, which is unfavorable for Hg 0 oxidation. 97 However, the active substances generated from H 2 S, HCl, and Cl 2 can further promote Hg 0 oxidation. 94,102 The oxidation mechanism of Hg 0 in the presence of H 2 S is shown in Figure 8a.…”

“…95,96 In addition, SO 2 , H 2 S, HCl, Cl 2 , and other flue gas components will further affect the reaction performance of Hg 0 over oxygen carriers. 97 Therefore, the reaction mechanism of mercury with the oxygen carrier is systematically discussed through DFT calculations in this section.…”

“…Mercury in the fuel reactor mainly exists in the form of gaseous Hg 0 , and its content rises with the increase of temperature . Currently, Hg 0 is mainly converted to Hg 2+ or is adsorbed on solid substances to become Hg p , and then mercury removal is carried out by the existing equipment. , The experimental results show that oxygen carriers can promote the Hg 0 adsorption and oxidation. , In addition, SO 2 , H 2 S, HCl, Cl 2 , and other flue gas components will further affect the reaction performance of Hg 0 over oxygen carriers . Therefore, the reaction mechanism of mercury with the oxygen carrier is systematically discussed through DFT calculations in this section.…”

“…Based on thermodynamic modeling and fixed bed experiment, Ni et al found that the oxidation efficiency of metal oxides for Hg 0 was as follows: CaSO 4 > Co 3 O 4 (Mn 2 O 3 ) > Fe 2 O 3 > CuO (CeO 2 ) > SiO 2 (Al 2 O 3 ). Additionally, the reducing gases such as CO, H 2 , and NH 3 will consume lattice oxygen over oxygen carriers, which is unfavorable for Hg 0 oxidation . However, the active substances generated from H 2 S, HCl, and Cl 2 can further promote Hg 0 oxidation. , …”

Enhancement of Elemental Mercury Oxidation by Core–Shell CuCo2O4@SiO2 Oxygen Carrier with Strong Oxygen Release and Weak Adsorption Capacity in Chemical-Looping Combustion

“…The removal efficiency of Hg 0 (g) by CuO@TiO 2 @Al 2 O 3 with oxygen uncoupling ability in CLC was 60.71%. Previously, when evaluating oxygen carriers, it was found that the oxidation efficiency of metal oxides for Hg 0 in a HCl atmosphere was in the following order: Co 3 O 4 (Mn 2 O 3 )>Fe 2 O 3 >CuO­(CeO 2 )>SiO 2 (Al 2 O 3 ), , with the lattice oxygen release in the oxygen carrier playing an important role in the removal of mercury. , Oxygen carriers can effectively reduce the emission of Hg 0 , and the conversion of Hg 0 to Hg 2+ is still possible under high-temperature conditions . Therefore, oxygen carriers with good mercury removal performance need to be further developed to improve the efficiency of Hg 0 oxidation in fuel reactors.…”