Calcium looping (CaL), based on the reversible reaction between CaO and CO2, is a promising technology for CO2 capture.It has received considerable attention in recent years owing to the main advantages of cheap and abundant reserves and the high CO2 carrying capacity of potential sorbents, the readiness of commercialization because of the use of the well-established fluidization technology, and a wide range of potential applications in post- and pre-combustion CO2 capture systems. However, it still faces several technical challenges. The first one is the loss-in-capacity problem of CaO-based sorbents, i.e. reactivity of sorbents decreases rapidly with the number of cycles and only 8-10% of the original sorption capacity is retainedafter long-term operation.Thus, a large make-up flow of sorbents is needed to maintain the capture efficiency. The second one is inadequate physical strength of the sorbents. The solid sorbents tend to be mechanically fragile, leading to an excessively high mass loss from fluidization systems. This further increases the make-up flow of the sorbents.The third one is the requirement ofextensive amount of energy and pure oxygen for the regeneration of the sorbents in calciner. In particular, pure oxygen represents a significant part of ongoing costs for calcium looping. These problems increase the cost and operating difficulty and are among the main obstacles to the application of calcium looping process. The aim of the work is to identify a solution(s) to these problems to enhance the competitiveness of the calcium looping process.
Firstly, the work attempted to develop a method that can be used to manufacture CaO-based sorbents with good physical properties (high mechanical strength and attrition resistance). Extrusion was found to be a suitable method. Sorbent particleswithvaryingCaO content in the range of 25-75 wt. % were prepared from reagent calcium hydroxide, commercial hydrated lime, and cement using a twin-screw extruder. It was found that the extruded particles with cement as binder showed good mechanical strength and acceptable attrition resistance, although the problem of loss-in-capacity still exists.
Secondly, some efforts were made to synthesize CaO-based sorbents with stable CO2 carrying capacity.Ascreening of calcium and support precursors was carried out and 19 sorbents were prepared via a mixing method. Experimental results showed that calcium aluminate cement was a promising low-cost support material in enhancingthe cyclic CO2 uptake, especially when organic calcium precursors were used. The sorbent made from calcium lactate and cement with 75 wt. % CaO exhibited a high CO2 sorption capacity of 0.36 g-CO2/g-sorbent after 70 cycles of carbonation (30 min at 650 °C in 15 vol.% CO2) and calcination (10 min at 900 °C in N2), a few times higher than that of a natural lime as sorbent. When CaO content in the sorbents was decreased to 50 wt. %, the sorbents demonstrated an excellent stability in CO2 carrying capacityover multiple cycles. Therefore, it was confirmed that it is possible to synthesize highly stable sorbents with careful selection of precursors.
Thirdly, weconfirmed the feasibility of manufacturing CaO-based sorbents with both stable CO2 carrying capacity and good physical properties, by combining the extrusion and mixing techniques. Some sorbents were manufactured using organic calcium precursors and cement by extrusion. It was found that the synthesized pellets have good mechanical strength and attrition resistance as previously found for other extruded sorbents, whilehavingmuch higher and more stable CO2 sorption capacity compared to natural sorbents. Based on these results a large-scale sorbent production process for calcium looping could be quickly developed.
Fourthly, some work was conducted to investigate a novel calcium looping process, which was proposed to eliminate the need of pure oxygen in sorbent regeneration. The so-called “combined Ca/Cu chemical looping process” still utilizes calcium oxide to capture CO2, and the key innovation is that it utilizes the reaction heat from the reaction between a hydrocarbon fuel and CuO for calcium oxide regeneration (rather than oxy-fuel combustion). Therefore, a sorbent with calcium oxide and copper oxide is needed for this process to operate. In this work, we attempted to develop composite sorbents with CaO/CuO, and observed an accelerated loss-in-capacity of CaOwhen Cu/CuO was introduced into the sorbents, which was not reported previously.Theunderlying reason was hypothesized to bethe change of Cu/CuOdistribution in compositesas a result oftheir low melting points. Further effort was made to mitigate the accelerated loss-in-capacity of CaO. It was found that careful selection of precursors, thermal pre-treatment of copper precursors, and the addition of steam,could reduce/eliminate the negative effect of Cu/CuO on the carbonation of CaO, although more workis still needed.
In summary, the main contribution of this work is the development of a simple and easy-to-scale-up method for the synthesis of CaO-based sorbents with good chemical and physical properties for calcium looping and combined Ca/Cu chemical looping process. This work was also the first to demonstrate enhanced sintering and more rapid fall off in the sorbent’s carrying capacity caused by the existence of Cu/CuO, and to suggest ways of mitigating this effect. The results obtained in this work would be helpful in further developing the calcium looping technology.