Chemical Looping Combustion of a Typical Lignite with a CaSO₄–CuO Mixed Oxygen Carrier

Abstract

Calcium sulfate (CaSO4) has attracted a great amount of attention as a potential oxygen carrier (OC) to be applied in chemical looping combustion (CLC) due to its high oxygen transfer capacity, wide distribution, and easy accessibility, but its low reactivity and sulfur emission from side reactions of CaSO4 should be well resolved. In this research, the CaSO4–CuO mixed OC was prepared using the template method combined with the sol–gel combustion synthesis (SGCS). Its reaction characteristics with a selected lignite (designated as YN) were investigated and the greatly enhanced reactivity of this mixed OC was confirmed relative to the single CaSO4 and CuO. Meanwhile, the comprehensive heat effect showed the desired exothermic characteristics for this mixed OC reaction with YN when the mass ratio of CaSO4 to CuO was fixed as 6:4. Furthermore, morphological analysis indicated that the solid products from YN reaction with the CaSO4–CuO mixed OC were porous without discernible sintering, mainly because the CaSO4 included not only provided the lattice oxygen involved for oxidation of YN coal, but also acted as the temporary inert support to improve the resistance of the reduced Cu to sintering. Finally, the gaseous and solid products formed were systematically investigated and clearly indicated that the gaseous sulfur species formed from the side reactions of CaSO4 were effectively fixed with solid Cu2S formation; as such the potential harms incurred could be eliminated. Overall, this preliminary research revealed the greatly enhanced reactivity of this mixed OC as well as its good fixation capacity for sulfur emitted from the side reaction of CaSO4, which is much desired in the real CLC system.

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

“…From the related TG and DTG curves, after dehydration of absorbed water in the CaSO 4 substrate below 200 °C, although there still existed three identifiable weight loss stages during the reaction of YN with the reference CaSO 4 −CuO or CaSO 4 −CaO, similar to single CaSO 4 , the inherent reaction characteristics changed a lot. According to our previous research, 21 with regard to the reduction of reference CaSO 4 −CuO by YN, its first reaction stage occurred at the peak temperature of 432.7 °C, much higher than that of the CaSO 4 −CuO reaction with H 2 , as shown in Figure 3 above, which was mainly attributed to oxidation of the gaseous primary pyrolysis products of YN by doped CuO. The resulting CO 2 was formed at the first distinct CO 2 peak, as shown in Figure 5a.…”

“…From Figure 2a for the prepared CaSO 4 −CuO reference oxide, from point 1 on SEM in Figure 2a, it could be observed that many agglomerated particles were formed below the microsize, which was mainly determined as CuO by the related composition analysis shown in the corresponding EDX pattern on the right side, while, from the point 2 in Figure 2a, CaSO 4 was observed as a platelike shape, which was consistent with our previous research. 21 The CuO particles were found to encapsulate around the exterior surface of the CaSO 4 substrate. Far different from reference CaSO 4 −CuO, from Figure 2b for prepared CaSO 4 −Ca 2 CuO 3 , in combination of the SEM microstructure on the optionally selected points with the corresponding composition analysis by EDX, little white nanosized grains were found to disperse on the surface, as shown in point 1 across the CaSO 4 −Ca 2 CuO 3 mixed OC, which was mainly ascribed to the trace of separate CuO.…”

“…To determine the main components present in the as-prepared CaSO 4 −Ca 2 CuO 3 mixed OC and reveal the possible interaction occurring among the Ca−Cu−O multiphases across the substrate CaSO 4 during its preparation, crystalline phases involved in the mixed OC were identified via XRD analysis and shown in Figure 1. From Figure 1a for the reference CaSO 4 −CuO mixed OC, according to our previous research, 21 it mainly consisted of the CaSO 4 substrate and separate CuO grains dispersing around the substrate, while for the CaSO 4 −CaO mixed OC, as shown in Figure 1b, besides CaSO 4 and CaO, as a result of the easy hygroscopic preference of CaO, 38 trace Ca(OH) 2 was found to form via hydration of produced CaO with the moisture in the atmosphere [CaO + H 2 O(g) → Ca(OH) 2 ]. With regard to CaSO 4 −Ca 2 CuO 3 , as shown in Figure 1c, three main crystalline components were detected, including the substrate CaSO 4 and a little separate CuO and Ca 2 CuO 3 was wellidentified and determined as the only stable phase in the Ca− Cu−O system during the preparation stage, as depicted above, which was verified to easily form through the interaction between CaO and CuO at a high temperature above 900 °C.…”

“…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.…”