Effect of steam addition during carbonation, calcination or hydration on re-activation of CaO sorbent for CO₂ capture

Highlights

• The effect of steam to re-activate the spent sorbent for CO₂ capture was studied.

• Steam addition during carbonation improved sorbent structure and its reactivity.

• The effect of steam during calcination was influenced by the concentrations.

• Steam hydration enhanced the formation of cracks for increased particle surfaces.

• Injecting steam during carbonation was a more favourable option to be recommended.

Abstract

Calcium looping (CaL), based on cyclic carbonation/calcination using lime-based sorbents, is a promising technology for post-combustion CO₂ capture. An important obstacle of this technology is the decay of sorbent over multiple cycles. Steam re-activation is a potential approach to improve the sorbent reactivity, however, the effects of both in-situ high-temperature steam (in carbonator or/and calcinator) and ex-situ low-temperature steam (steam hydration in hydrator) are still a matter of debate. Here, a bubbling fluidised bed reactor was used to investigate three steam addition options: steam addition during carbonation stage, or during calcination stage, or intermediate steam hydration after each CaL cycle. A CaO sorbent was tested under varying steam concentrations up to 40 vol.% for 10 CaL cycles (carbonation: 650 °C, calcination: 900 °C). Compared to the dry condition, steam addition during carbonation and intermediate steam hydration were both found effective for CaO re-activation. For the former, the improved sorbent reactivity was mainly attributed to an increased pore volume enhancing the extent of carbonation in kinetic regime; while for the latter, the formation of cracks accelerated the rate of diffusion. Nevertheless, decreasing the hydration temperature was detrimental to sorbent reactivity due to enhanced sintering upon cooling. In case of steam addition during calcination, the sorbent was affected negatively or positively depending on the concentration of steam. At higher steam concentrations (20, 40 vol.%), the sorbent reactivity was significantly decreased due to sintering associated with larger pores and lower surface areas. Overall, steam addition during carbonation was recommended for sorbent re-activation.

Introduction

Recognizing the continuing global reliance on fossil fuels, CO₂ emission from fossil fuels combustion, such as coal-fired power plants, has become one major source of greenhouse gas emissions. The alarming impact of climate change becomes a powerful driving force for the developing guidelines, such as Kyoto Protocol and The Paris Agreement, to lessen the related environmental loads [1].

In the context of reducing CO₂ emission, a potential approach is to capture it from combustion systems followed by sequestration in underground geological formations or on the ocean floor, named, carbon capture and storage (CCS) [2,3]. Calcium Looping (CaL), based on reversible carbonation/calcination cycles of CaO-based sorbents, is one of the promising CCS technologies in post-combustion systems. The process involves the reaction of CaO with CO₂ from combustion flue gas or gasification syngas in a carbonator (see Eq. 1 forward), followed by re-generation of sorbent in a calcinator (see Eq. 1 backward). The CaO sorbent is repeatedly cycled between the two reactors, which exhibits a number of advantages, such as: (1) high theoretical CO₂ capture capacity of 0.78 g(CO₂)/g(CaO) and fast reaction kinetics; (2) recycling of sorbent at lower energy penalty and operating cost; (3) relatively cheap and abundant CaO sorbent which could be derived from limestone [4,5].

$${\text{CaO}}_{\text{(s)}}+{{\text{CO}}_2}_{\text{(g)}}\text{⇌}{\text{CaCO}}_{\text{3(s)}}\text{\hspace{0.17em}ΔH=-178\hspace{0.17em}kJ/mol}$$

$${\text{CaO}}_{\text{(s)}}+{{\text{H}}_2\text{O}}_{\text{(g)}}\text{⇌}{\text{Ca(OH)}}_{\text{2(s)}}\text{\hspace{0.17em}ΔH=-109\hspace{0.17em}kJ/mol}$$

However, one drawback of the CaO sorbent is the decrease of CO₂ capture capacity upon iterated carbonation/calcination cycles. This has resulted in additional consumption of fresh sorbent to compensate for sorbent de-activation and accordingly, an increased overall cost of the CaL process [6,7]. Aspects which deserve consideration when designing CaL therefore, requires the improvement of sorbent reversibility for its extended use in industrial plants.

The use of steam is a potential method to periodically re-activate the spent CaO sorbent. This can be achieved through two potential approaches: (i) by adjusting the concentration of steam in the carbonator or/and calcinator (see Fig. 1(a)); and (ii) by adding an external vessel between the carbonator and calcinator, within which steam hydration occurs (see Fig. 1(b)). The former approach considers that steam is usually present in the combustion flue gas (5–10 %) or gasification syngas (∼20 %) to be treated in the carbonator [8], and also as a combustion product of the supplementary fuel in the calcinator [6]. This arises the possibility to utilise the high-temperature steam as an in-situ re-activation approach during the CaL process. While the temperature and pressure of the carbonator and calcinator are such that the formation of Ca(OH)₂ would not be thermodynamically favoured in these reactors, in the latter approach, a hydration reactor is installed for ex-situ re-activation of the spent sorbent retrieved from the calcinator (Eq. 2 forward). This is followed by Ca(OH)₂ de-hydration as the sorbent from the hydrator is reinjected into the carbonator (Eq. 2 backward), the recrystallisation of the sorbent has the potential to increase its reactivity [9].

Based on existing works there is a consensus that the presence of steam in the CaL process may significantly impact the sorbent performance; however, the effects and mechanisms of steam on the reactivity of CaO sorbent are still a matter of debate. Concerning the effect of steam addition during carbonation, most studies reported an improved sorbent performance, but there are studies to the contrary. For example, Manovic et al. [10] found that the addition of 15 % steam during carbonation has a beneficial effect on CO₂ capture of a CaO-based synthetic sorbent. However, Lu et al. [11] reported a deleterious effect of steam based on similar reaction conditions. In some studies, the enhancement of sorbent reactivity was attributed to the increase of carbonation rate in fast-kinetic stage [[12], [13], [14], [15], [16]]. This was explained by the steam catalysis effect between CaO and CO₂ [[12], [13], [14]], or the formation of transient Ca(OH)₂ which is more reactive than CaO [15,16]. The findings were contrary to another group of studies showing that steam just enhanced carbonation at the diffusion-controlled stage, whereas there was little influence on the kinetic stage [[17], [18], [19]]. The enhanced carbonation by steam addition could also probably be ascribed to OH⁻ formation from H₂O dissociation [20].

Regarding the effect of steam addition during calcination, several studies reported an improvement of the sorbent reactivity [8,[21], [22], [23]]. Donat et al. [8] showed that steam injection during calcination enhanced CO₂ capture at concentration up to 1% with no significant improvement thereafter. However Chou et al. [21] found that a higher steam concentration up to 80 % continued to enhance the sorbent reactivity. The increased capture capacity was due to a shift from smaller to large pores, possibly eliminating the trigger time retard in the fast-kinetic stage and increasing carbonation rate in the diffusion regime. These findings contrast with a few works of a negative impact of steam which enhanced sorbent sintering during calcination, resulting in a decreased specific surface area for CO₂ capture [6,13,24]. Yancheshmeh et al. [13] in turn suggested that the positive effect of enhanced CO₂ diffusion through large pores and the negative effect of lower surface area can reach a trade-off. Only at a higher steam concentration (e.g. 9.5 %), the intensified sintering was sufficient to develop a stable sorbent structure for increased CO₂ capture.

Otherwise, steam hydration is also proven to be a feasible method of re-activating the CaO sorbent. The mechanism of enhancement has been under discussion: most studies suggested the development of particle pore structure [23,25,26], while Anthony et al. [27] mentioned that H₂O molecules penetrated the product layer more readily over that of CO₂ and resulted in a large increase of the reaction surface. Based on existing studies, the effect of steam hydration is dependent on several operating parameters, such as hydration temperature, duration and steam concentration. Blamey et al. [28] found that, at temperature ranges of 200−400 °C, decreasing hydration temperature favoured the increase of hydration extent, which subsequently led to a higher carbonation conversion. Instead, Rong et al. [24] reported that the sorbent hydrated at 300 °C was more reactive over that conducted at 200 °C, probably because the latter underwent a longer cooling residence time that led to more severe sintering. The authors also claimed that hydration temperature of 500 °C would worsen the sorbent reactivity than the case without hydration, due to thermodynamic constraint of Ca(OH)₂ formation at higher temperature. As can be summarised from the existing works [9,25,29,30], the optimal steam hydration temperature should consider a trade-off between kinetics constraints on one hand, which pushes towards a higher temperature; and on the other hand, the thermodynamic constraint calling for a lower temperature. Regarding the effect of hydration duration, Coppola et al. [25] reported that increasing the hydration duration was associated with larger degree of hydration and more extensive cold sintering of the sorbent. These two competitive effects led to non-monotonic trend of sorbent properties versus hydration duration: the sample hydrated for 30 min was the most micro- and mesoporous; while the sample hydrated for 60 min was more reactive towards CO₂ capture and was less prone to fracture. The hydration steam concentration also has a strong effect on re-activation performance. Often the largest improvement of sorbent reactivity came at the highest concentration of steam added [24,31]. However this has to be balanced with the cost of steam consumption based on economic grounds [31].

To facilitate industrial applicability of the CaL process, there is a need to recommend an optimal option for re-activating the spent sorbent. However such works have received only limited attentions. Champagne et al. [32] studied the influence of steam injection during both calcination and carbonation and found that steam had a greater impact on sorbent reactivity for carbonation than for calcination. Li et al. [23] claimed that steam activation in calcination and carbonation processes and hydration treatment were all effective, and they suggested the use of these three ways in an improved CaL process. Overall current studies are not sufficient and more work is required to better address this aspect.

Accordingly, this study reports on re-activation of spent sorbent by steam addition. CaL experiments were carried out using a CaO sorbent in a lab-scale fluidised bed reactor. The influence of three steam addition options was examined, which in detail included: steam addition during carbonation stage, steam addition during calcination stage, and intermediate steam hydration after each iterated CaL cycle. The aim is to clarify the relationships among exposure to steam, sorbent microstructural properties, morphology changes and CO₂ capture. The results can help further develop CaL process and improve reactivity of calcium-based sorbents.