Enhanced Performance of Chemical Looping Combustion of CO with CaSO₄-CaO Oxygen Carrier

Abstract

CaSO4 is an interesting alternative oxygen carrier for chemical looping combustion (CLC). Utilization of a CaSO4 oxygen carrier suffers deactivation caused by sulfur loss. To prevent sulfur loss, a small amount of CaO particles were mixed with the CaSO4 oxygen carrier. The predominant regions of CaSO4, CaS, and CaO have been obtained with consideration of SO2 and COS emissions. The predominant region of CaO is increasing with the temperature and CO2 partial pressures. Isothermal as well as nonisothermal kinetics of CaSO4-based oxygen carrier reduction, and reduction/oxidation cycle tests, have been carried out. The effects of reaction temperature, CO concentration, and molar ratio of CaO to CaSO4 and cycle number on sulfur emission and CO2 generation efficiency are taken into account. XRD, XRF, and gas analyses were performed to investigate the variations of solid phase change, elements composition in solid residual, and sulfur release with reaction time. The results show that the use of fresh CaO additive not only enhances CO2 yield but also captures SO2 and COS, which are identified by XRD and XRF analysis. During cycle tests, the use of CaO additive also improves CO2 yield. CO2 yields drop a little after the first cycle, but they can maintain at high levels subsequently in cases with the use of CaO additive. SO2 released from the fuel reactor (SO2-FR) is dominant over COS released from the fuel reactor (COS-FR) and SO2 released from the air reactor (SO2-AR). With the use of CaO additive, the releases of COS-FR and SO2-AR decrease, but SO2-FR emission increases a little. The comprehensive roles of CaO additive on CO2 generation and gas sulfide releases are evaluated by the environmental factor. The CLC with the CaSO4-CaO oxygen carrier with CO, ensuring a low environmental impact during the early 1–6 cycles, could achieve a high CO2 generating efficiency. In subsequent cycles, there was no obvious drop in the E-factor of SO2.

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

“…To well address the initiation of CaSO 4 as an OC by inhibition of the sulfur release and amelioration of its reactivity over the cycles, two approaches have been applied. One practice is the addition of limestone, 11,15 CaCO 3 , 16 or CaO 17,18 as the sorbent to capture sulfur and decrease its release during the reduction/oxidation process. Zheng et al 18 investigated the performance of the CaSO 4 −CaO OC during CLC, and the results showed that the CaO additive could not only improve the CO 2 yield but also capture SO 2 and COS.…”

“…What is worse is that increasing the operating temperature would singly exacerbate the emission of sulfur from CaSO 4 via the side reactions of eqs – below. , Sulfur released not only led to atmospheric pollution but also tarnished the purity of outlet CO 2 and declined the reactivity of CaSO 4 . To well address the initiation of CaSO 4 as an OC by inhibition of the sulfur release and amelioration of its reactivity over the cycles, two approaches have been applied. One practice is the addition of limestone, , CaCO 3 , or CaO , as the sorbent to capture sulfur and decrease its release during the reduction/oxidation process. Zheng et al investigated the performance of the CaSO 4 –CaO OC during CLC, and the results showed that the CaO additive could not only improve the CO 2 yield but also capture SO 2 and COS.…”

“…One practice is the addition of limestone, , CaCO 3 , or CaO , as the sorbent to capture sulfur and decrease its release during the reduction/oxidation process. Zheng et al investigated the performance of the CaSO 4 –CaO OC during CLC, and the results showed that the CaO additive could not only improve the CO 2 yield but also capture SO 2 and COS. Simultaneously, deactivation of the CaSO 4 OC was partly suppressed.…”

Characteristics of a CaSO4 composite oxygen carrier supported with an active material for in situ gasification chemical looping combustion of coal

“…The CaSO 4 –CaS–CaO phase diagram predicts that CaS is stable at the high partial pressure of sulphurous gas and reducing gas H 2 /CO using gaseous fuel. 32,33 Tian et al 33 observed that the solid product was almost pure CaS when the partial pressure of CO or H 2 was higher than 40 kPa. However, the side reactions generating sulphurous gas are unavoidable in the fuel reactor due to the aim of minimizing the unconverted products such as H 2 , CO and CH 4 .…”

Characteristics of CaS–CaO Oxidation for Chemical Looping Combustion with a CaSO4-Based Oxygen Carrier

“…The SO 2 liberation from CaS oxidation may be trapped by CaO additive via reaction . In CLC with a CaSO 4 -based oxygen carrier, such as CaSO 4 , ,− CaSO 4 –Fe 2 O 3 , , CaSO 4 –NiO, CaSO 4 –CuO, and CaSO 4 –CaO oxygen carriers, the problem of sulfur emission occurs. In the literature, it has been proposed that the sulfur release should be captured by adding lime and limestone sorbent in the fuel reactor of the CLC system.…”