Calcium looping is a promising CO2 capture technology for either pre or post-combustion systems. The technology has received a great deal of attention during the past decade, owing to its relatively low energy penalty compared with other CO2 capture technologies (e.g., amine scrubbing). However, the economic advantage of calcium looping is partially offset by the existing limitations in itself, including the problems of loss-in-reactivity, attrition of CaO-based sorbents in fluidized beds, and the employment of an energy-intensive O2 plant. Therefore, there is scope to improve the calcium looping process by eliminating the makeup flow of fresh sorbent and/or oxy-fuel combustion for sorbent regeneration.
First of all, this work has confirmed the effectiveness of periodic water hydration on reactivating the spent limestone and synthetic CaO/cement sorbent. Also, the effects of the control factors (i.e., particle size, hydration duration, hydration temperature, and pre-calcination temperature) on water hydration have been investigated. It was found that the synthetic CaO sorbent showed strong dependence on those factors. And, the hydration level of the synthetic CaO sorbent can be significantly enhanced by ultrasonic hydration. This is the first work reported in the literature to study the effect of controlling factors on water hydration and the use of ultrasonic bath to enhance the hydration of CO2 sorbents.
Secondly, this work has studied the influence of hydration by steam/superheating on the CO2 capture performance and, more importantly, physical properties of pelletized CaO-based sorbent particles. Both hydration by steam and hydration by superheating were effective in reactivating the CaO-based sorbent particles. Also, the physical properties of particles were significantly weakened by steam hydration, but were largely maintained via hydration by superheating. The effect of hydration on the mechanical strength of synthetic CaO pellets has been rarely studied and the results are useful to minimize or eliminate the makeup flow of fresh sorbent in calcium looping.
Thirdly, a modified calcium looping process (referred to as HotPSA) was proposed to avoid the need of oxy-fuel combustion. The operating conditions of the new process also favour the chemical reversibility and mechanical stability of CaO-based sorbents. The preliminary results have confirmed that the problem of loss-in-capacity was solved in the new process. Then, the operating temperature window of the process was determined to be 750-840 oC by investigating the quasi-equilibrium and kinetics of carbonation/calcination of synthetic CaO-based sorbents. Following that, the process was demonstrated in a fixed-bed reactor by examining the temperature variations in the synthetic CaO-based sorbent particles in 20 vol % CO2 (N2 balance). The effects of operating variables (i.e., operating temperature, switching time, and sample type) on the cyclic temperature variation were also investigated. It was observed that the process operated in a highly stable manner over 100 cycles at 770 oC with the switching time of 1 min both in carbonation and calcination modes. Such a process is not yet available in the literature and has a potential to significantly reduce the CO2 capture cost.
Finally, a mathematical model coupled with mass and heat transfers was applied in this work to investigate the mechanism of the effect of steam in the calcination rate and to provide in-depth understanding of the decomposition of CaCO3 in two different applications (i.e., the typical calcium looping and the HotPSA process). The results show that the physical effect of steam addition on the calcination of CaCO3 was negligible. In the typical calcium looping process, the decomposition of CaCO3 was affected by particle size, gas temperature, and CO2/H2O concentrations in the bulk gas, but independent of the initial solid temperature. Also, the enhancement in the calcination rate was not obvious when H2O concentration was over 5% at 950 oC. In the HotPSA process, the decomposition of CaCO3 was also significantly affected by particle size and gas temperature. The analysis of energy loss shows that the energy loss rate in the calcination mode associated with convection was severe compared to the heat consumption rate. This is the first work in the literature that numerically models the decomposition of CaCO3 taking into account the participation of H2O in the reaction. The results are significant for optimizing the operating conditions for the calcium looping process and better understanding the feasibility of decomposing CaCO3 in low temperature H2O in the HotPSA process.
In summary, this thesis thoroughly investigated the hydration-based methods for reactivating and/or reutilizing the spent CaO-based sorbent in calcium looping. The results are helpful to reduce the makeup flow of fresh sorbent, so that the CO2 capture cost of calcium looping can be reduced. In addition, the HotPSA process is envisaged to reduce the CO2 capture cost by avoiding the makeup flow of fresh sorbent and oxy-fuel combustion. Finally, the better understanding of CaCO3 decomposition in the presence of H2O favours the design of a compact calciner in the typical calcium looping process and the operation of the HotPSA process.