The calcium looping cycles method has been identified as an attractive method for CO2 capture during coal combustion and gasification processes. However, it is well-known that the capture capacity of CaO undergoes a rapid decrease after mutiple cycles. In order to improve the stability of CO2 capture capacity in CaO, this paper focuses on the development and performance of the synthetic CaO/La2O3 sorbents for calcium looping cycles.The sorbents were synthesized by three different methods: dry physical mixing, wet chemistry, and sol−gel combustion synthesis (SGCS). Their multicyclic CO2 capture capacity and the effect of the additive La2O3 were investigated in a fixed bed reactor system. The results indicate that the additive of La2O3 plays a positive role in the carbonation/calcination reactions, and the SGCS-made synthetic sorbent is composed of ultrafine well-dispersed hollow structured particles which are beneficial to the gas-phase diffusion on the surface area and can prevent small CaO particles from agglomeration effectively. As a result, the novel synthetic sorbent with the molar ratio of Ca to La of 10:1 made by the SGCS method provides the best performance of a carbonation conversion of 72% under mild calcination conditions and a carbonation conversion of 36% under severe calcination conditions (high temperature and high CO2 concentration) after 20 cycles.

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To improve the stability of CaO adsorption capacity for CO 2 capture during multiple carbonation/calcination cycles, modified CaO-based sorbents were synthesized by sol-gel-combustion-synthesis (SGCS) method and wet physical mixing method, respectively, to overcome the problem of loss-in-capacity of CaO-based sorbents. The cyclic CaO adsorption capacity of the sorbents as well as the effect of the addition of La 2 O 3 or Ca 12 Al 14 O 33 was investigated in a fixed-bed reactor. The transient phase change and microstructure were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FSEM), respectively. The experimental results indicate that La 2 O 3 played an active role in the carbonation/calcination reactions. When the sorbents were made by wet physical mixing method, CaO/Ca 12 Al 14 O 33 was much better than CaO/La 2 O 3 in cyclic CO 2 capture performance. When the sorbents were made by SGCS method, the synthetic CaO/La 2 O 3 sorbent provided the best performance of a carbonation conversion of up to 93% and an adsorption capacity of up to 0.58 g-CO 2 /g-sorbent after 11 cycles.

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.

Calcium looping cycle for post-combustion CO2 capture has gained increasing attention worldwide. However, CaO-based sorbents derived from natural sources for calcium looping cycle experience rapid loss of capacity during high-temperature cyclic carbonation/calcination reactions. Synthesizing sintering-resistant CaO-based sorbents by adding a support material has been extensively studied as an effective method of combating the problem. The support material in the synthetic sorbents plays an important role in retaining the capacity, and various support materials have been tested in the literature. In practical reactors, sulfur is present and it has been reported that sulfation of sorbents will also reduce the CO2 capture capacity. However, thus far, it is not clear whether/how support material would affect sulfation of sorbents and, thus, CO2 capture. In this paper, four different support materials, such as Ca2MnO4, La2O3, Ca12Al14O33, and MgO, were studied. The cyclic CO2 capture performance of the synthetic sorbents made from CaO and the support materials were investigated in detail, in the presence of SO2 and steam. The results showed that a mass ratio of 20–25% support material would be optimum for synthesizing sorbents with high cyclic CO2 capture capacity, and Ca12Al14O33 and MgO seem to be more effective than Ca2MnO4 and La2O3. The 80:20 wt % CaO/MgO synthetic sorbent achieved the highest CO2 capture capacity under ideal conditions over 100 cycles. However, the CaO/MgO sorbent had a strong affinity to SO2 capture during cyclic reactions, especially in the presence of steam. Under realistic conditions (i.e., both SO2 and steam are present during carbonation), the CaO/MgO sorbent showed the highest cumulative SO2 capture capacity, whereas the CaO/Ca12Al14O33 sorbent obtained the highest CO2 capture capacity after 10 cycles. The smaller average crystallite size of MgO in the sorbent was responsible for the strong SO2 affinity of the CaO/MgO sorbent as well as its stable cyclic CO2 capture abilities under ideal conditions.