Reaction Characteristic Investigation of the Combined Template-Method-Made CaSO₄–Mn₃O₄ Mixed Oxygen Carrier with Lignite

Abstract

Application of the disposed CaSO4 waste of different industrial processes as a potential oxygen carrier in chemical looping combustion (CLC) has attracted great attention as a result of its high oxygen content, abundant reserve, and low cost. However, both the inferior reactivity and deleterious SO2 gas emitted via the CaSO4 side reactions hinder its wide application in CLC. To heighten the reactivity of CaSO4 and increase its potential to inhibit the emission of harmful sulfur species, the mixed CaSO4 oxygen carrier (OC) decorated with Mn3O4 was produced using the tailored synthesis route by combining the template method with the sol–gel combustion synthesis (SGCS) method developed in our group before. Reaction of the as-prepared CaSO4–Mn3O4 mixed OC with lignite (abbreviated as YN below) was systematically evaluated, and the remarkably enhanced reactivity was reached for YN coal oxidation through sequential oxygen transfer from Mn3O4, recovered oxygen from reduced Mn3O4 via metastable CaSO4 at the interface, and CaSO4. Thus, oxygen involved was effectively used. The solid products formed after the mixed CaSO4–Mn3O4 reaction with YN coal were collected and further characterized. Morphological observation of the solid products indicated the good sintering resistance for the prepared CaSO4–Mn3O4 OC in CLC. Furthermore, both comprehensive analysis of the gaseous and solid sulfur species by integration of the experimental means with the thermodynamic simulation indicated that the main solid sulfur compounds were dominated as CaS, MnS, and MnSO4 with their total fraction above 98%, and the optimized content of Mn3O4 in the prepared mixed OC was around 30–50% for effective fixation of the gaseous sulfur emission. Overall, the combined template-method-made CaSO4–Mn3O4 mixed OC not only obtained the better reactivity for coal conversion but also owned the potential to control the release of sulfur in the CaSO4 side reaction, which is quite attractive in future CLC application.

Chemical Looping Combustion of Coal in China: Comprehensive Progress, Remaining Challenges, and Potential Opportunities

“…The sulfur-suppressing effect of hematite on the CaSO 4 -based oxygen carrier was further verified by Ding et al in a lab-scale fluidized bed reactor. 111 Synthetic CaSO 4 −CuO 112 and CaSO 4 −Mn 3 O 4 113 mixed oxygen carriers were also prepared via the sol−gel combustion synthesis (SGCS) method and further tested with a Chinese lignite in a TGA. A desirable exothermic characteristic during the conversion of the lignite was attained for the CaSO 4 −CuO oxygen carrier when the mass ratio of CaSO 4 /CuO was 6:4, and greatly enhanced reactivity of the mixed oxygen carrier was achieved in comparison to that of individual CaSO 4 or CuO.…”

Chemical Looping Combustion Characteristics of Coal with a Novel CaSO4–Ca2CuO3 Mixed Oxygen Carrier

“…First, according to our previous research, 21,22 for the reaction of YN with reference CaSO 4 , after the fast dehydration of adsorbed water from CaSO 4 at 116.9 °C, three reaction stages could be observed from Figure 4b, with the peak temperatures residing at 455.0, 723.7, and 895.5 °C, respectively, which were in sequence ascribed to disintegration of various covalent bonds, cracking the functional groups of low thermal stability during pyrolysis of YN coal and reductive decomposition of CaSO 4 by YN at the latter two reaction stages. Correspondingly, from Figure 5a, during the YN reaction with reference CaSO 4 below ∼650 °C, only a trace of CO 2 was produced through single pyrolysis of YN by disintegration of carboxylic salts and lactones of different sites involved, 46 while at the higher reaction temperature region above 650 °C, from Figure 5a, reductive decomposition of CaSO 4 by YN occurred with remarkable CO 2 produced.…”

“…In addition, a calcium-based sorbent was known to effectively catalyze the gasification of coal and promote its conversion, 19 but it should be noted that a high temperature would lead to sintering of the calcium-based compounds, lowering of the reactivity of the applied OC, and reduction of the conversion rate of fuel during its long-term operation. 20 Another practice is to add some active transition metal oxides to modify CaSO 4 , such as Fe 2 O 3 , 10,12,13 CuO, 21 Mn 3 O 4 , 22 or CoO. 23 As known, to inhibit the occurrence of the side reactions listed above and prevent detrimental SO 2 released, lowering the reaction temperature seems to be a good option, 24 but the conversion efficiency of coal is unavoidably decreased.…”

“…Another practice is to add some active transition metal oxides to modify CaSO 4 , such as Fe 2 O 3 , ,, CuO, Mn 3 O 4 , or CoO . As known, to inhibit the occurrence of the side reactions listed above and prevent detrimental SO 2 released, lowering the reaction temperature seems to be a good option, but the conversion efficiency of coal is unavoidably decreased.…”

Insight into the Oxidation Mechanism of a Cu-Based Oxygen Carrier (Cu → Cu2O → CuO) in Chemical Looping Combustion

“…More than 700 OC materials have been examined in the literature . Ni-, Fe-, Cu-, Mn-, and Co-based OC materials were widely tested in CLC. − As a common OC in CLC, CuO is reduced to Cu in the FR and then regenerates itself via Cu → Cu 2 O → CuO in the AR, and the copper oxidation process (strong exothermic reaction) usually leads to an undesired sintering/agglomeration of Cu-based OC particles. − A fundamental understanding of the oxidation features will help to design high-performance and robust Cu-based OCs.…”