Progress in the development and application of CaO-based adsorbents for CO2 capture—a review

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Abstract

Carbon dioxide (CO2) capture using CaO-based adsorbents has recently attracted intense attention from both academic and industrial sectors in the last decade due to the high theoretical capacity of CO2 capture, low cost, and potential use in large scale. However, the successful development of CaO-based adsorbents is limited by significant sintering of adsorbent particles over a number of cycles of CaO carbonation/calcination. In this work, a systematic understanding of fundamental aspects of the cyclic carbonation/calcination of CaO-based materials is reviewed. A number of efforts have been discussed to improve the sintering-resistant properties of CaO-based adsorbents, such as decreasing the particle size and increasing the surface area, dispersing CaO on inert support, as well as surface modification. In particular, severe process conditions such as carrying out material calcination under pure CO2 atmosphere were considered for the development of CaO-based materials. In addition, important process parameters for CaO-based carbon capture such as CO2 partial pressure, carbonation temperature, carbonation time and the presence of contaminants have been reviewed, as well as the reactivation of spent sorbents. Synthetic CaO-based adsorbents have better performance than natural adsorbents due to the improved porosity and the presence of nanosized particles. The promising capacity and stability of CO2 capture can be obtained when the synthetic adsorbents have high surface area and mesopores and/or CaO particles are stabilized using inert materials such as Ca12Al14O33. With an increase of steam concentration in the process, the decay of CO2 capture capacity was mitigated due to the formation of a stable pore structure. The calcium looping technology has been demonstrated in pilot scale combining with catalytic reforming or gasification process. However, the reduction of sorbent costs and the optimization of process conditions (e.g. carbonation and calcination time) still need to be tested in larger scale to reduce the overall costs and enhance the overall energy efficiency. Material attrition is a key challenge in large-scale demonstration of calcium looping process. Novel technology could be developed to avoid the transportation of solid sorbents. For example, by integrating with CO2 conversion to methane, the capture of CO2 and the regeneration of bifunctional sorbents can be carried out at the same temperature in a fixed bed reactor. This can be fulfilled by introducing hydrogen to the stage of sorbent regeneration. However, much more fundamental understanding is required in this area, such as the exploration of synergies between sorbent regeneration and catalytic conversion of CO2.

Introduction

CO2 is one of the major anthropogenic greenhouse gases in the atmosphere. The accumulation of CO2 leads to serious climate changes [1], [2]. In the last half century, the concentration of CO2 has exhibited a dramatic growth from about 310 to over 401 ppm owing to the large-scale utilization of fossil fuels and various chemical refinery processes [3], [4], [5]. With the growth of global economics, more fossil fuels will be consumed to feed the global activity, especially in developing countries. Thus, worldwide efforts have been devoted to developing new technologies for reducing CO2 emissions, and most of the United Nations members have approved Paris Climate Agreements for greenhouse gas reduction [6], [7], [8], [9]. CO2 capture and storage (CCS) technology is one of the most promising solutions and has been commercially demonstrated. CCS consists of three main steps including CO2 capture, transportation, and storage. Among these steps, CO2 capture is the most critical technology as it takes more than 70% of the whole operating costs of CCS [10]. Several methods based on various sorbents have been developed for CO2 capture such as liquid absorption, solid adsorption, and membrane absorption. Among them, liquid absorption and membrane absorption have challenges in large-scale operation such as solvent degradation, high cost, and the corrosive nature of sorbents [11], [12], [13], [14].

Recently, CO2 capture using solid adsorbents attracts a great attention due to the technology that it can be operated at a wide range of temperature window (from ambient temperature to 700 °C). In addition, the spent solid sorbents can be disposed with less environmental precautions [15]. According to the range of reaction temperature, solid CO2 adsorbents can be classified into three types: low-temperature (<200 °C) [16], intermediate-temperature (200–400 °C) [17], and high-temperature (>400 °C) adsorbents [18]. High-temperature CaO-based materials have been largely applied for CO2 capture because of the high reactivity with CO2 (theoretical capture capacity of CaO is 17.8 mmol g−1) and the availability of low cost natural CaO precursors [19], [20], [21], [22], [23]. Recent studies showed that the cost of using CaO-based adsorbents for carbon capture was $ 16–44 per ton CO2 [24], [25], which is very competitive than the current amine scrubbing technologies costing about $ 32–80 per ton CO2.

There are several review articles focusing on developing CaO-based sorbents. For example, Kierzkouska et al. [26] discussed fundamental aspects of the carbonation of CaO including the associated changes in the material’s morphology and kinetic parameters. Erans et al. [27] reviewed the decay of adsorbent reactivity and the attrition of sorbents in fluidized bed reactors. The authors also discussed the progress on the modification of sorbents over extended numbers of cycles of CO2 capture. System integration and pilot-scale testing of calcium looping technology have also been reported [22], [28]. In this review work, we extensively reviewed the fundamental aspects of carbon capture using CaO-based material. The improvement of CaO-based adsorbents for CO2 capture (e.g. surface modification and dispersing on inert supports) has also been discussed in detail. In particular, severe process conditions such as generating CaO-based sorbents under 100% CO2 were discussed for CO2 capture. A systematic explanation was conducted to study the effects of operation conditions (e.g. CO2 partial pressure, carbonation temperature, carbonation time, and contaminants) on the cyclic performance of carbonation and calcination. The comprehensiveness of this review work extends to the discussion of the applications of calcium looping technology in steam-reforming and gasification process, as well as the recent studies in large-scale demonstration. The attrition problem of CaO-based materials in real applications was further addressed, followed by an economic analysis of the carbon capture technology using CaO-based sorbents.

Section snippets

Fundamental understanding of the adsorbent reactivity

CaO reacts with CO2 in a temperature range of 600–800 °C and is regenerated at a temperature higher than 800 °C [26]. The carbonation and calcination reactions with CO2 are as follows:

Carbonation:CaO(s)+CO2(g)CaCO3(s),ΔHr,298K=178kJmol1

Calcination:CaCO3(s)CaO(s)+CO2(g),ΔHr,298K=+178kJmol1

The carbonation of CO2 is an exothermic reaction that consists of an initial fast reaction stage followed by a substantially slower second reaction stage [29]. The two-stage mechanisms for the carbonation

Improvements of CaO-based adsorbents

The deactivation of CaO-based materials for CO2 capture is well known and represents one of the key challenges for the deployment of the technology. In addition, developing novel CaO materials with high CO2 capture capacity and fast conversion is a promise for industrial applications. CaO-based materials' improvement is reviewed in this section, while the capacity of CO2 capture (Eq. (7)) and the conversion of CaO carbonation (Eq. (8)) as well as carbonation rate (Eq. (9)) are used to

Influence of CO2 partial pressure on carbonation rate

The partial pressure of CO2 in the process of CaO-based carbon capture dynamically affects the efficiency of the process [123]. For example, Grasa et al. [131] carried out the carbonation of CaO at 650 °C for 20 min by varying CO2 partial pressure from 2 kPa to 100 kPa and found that the carbonation reaction was strongly affected by the partial pressure of CO2 during the multiple cycles of CO2 capture as shown in Fig. 5a and b. The first-order reaction of CaO with respect to CO2 capture was

Reactivation of spent CaO adsorbent

The rapid degradation of the capacity of CO2 capture using CaO-based materials during the cycles of carbonation/calcination is a big challenge for practical applications of the technology [157], [158]. Reactivating the degraded adsorbents has been proposed as a good solution. Besides, the requirement of low-cost adsorbents promotes the development of reactivation technology [159].

Calcium looping for precombustion carbon capture

A typical process diagram for the application of calcium looping for precombustion carbon capture is shown in Fig. 9a. The CaL cycle provides a number of benefits when it is coupled with reforming or gasification as precombustion capture process. Carbonation of the limestone is an exothermic process and provides heat for endothermic in situ steam reforming or gasification reactions, leading to an overall autothermic reaction. Another benefit derived from the presence of CaO is to shift the

Economic assessment of the calcium looping process

The economics of calcium looping process used in postcombustion system has been investigated by Abanades et al. [24]. The authors studied a system consisting of three main components: a combustion power plant, an oxy-fired fluidized bed calciner, and a fluidized bed carbonator. They estimated a capture cost around 15 USD/tCO2 for the proposed system in comparison to 23.8 USD/tCO2 for a standard oxyfuel circulating fluidized bed combustion process (2007 estimate). It was explained that despite a

Summary and outlook

In this article, the most recent research progress in the development of CaO-based adsorbents for high-temperature CO2 capture has been reviewed and discussed. Despite the simplicity of the reaction, it still lacks a systemic fundamental understanding of the adsorbent reactivity for CO2 capture. A two-stage model is usually used to investigate the kinetics of CaO carbonation including a fast stage limited by kinetics and a substantially slow stage controlled by the diffusion of CO2. However,

Declaration of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The financial supports from National Natural Science Foundation of China (no. 201706050) and the China Scholarship Council (no. 201606450016) are greatly acknowledged.

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