**2. Ca-Cu looping process: the concept**

*Global Warming and Climate Change*

source as in conventional SMR.

and methanol production [3]. However, fossil fuels represent the principal feedstock for hydrogen production worldwide, with around 96% of the global hydrogen produced from natural gas, fuel oil and coal. Hydrogen production with low carbon footprint has a great potential in fulfilling the stringent CO2 emission cuts needed in the energy sector [4]. Therefore, the development of large-scale hydrogen production technologies including CO2 capture that enable a reduced cost as well as an improved efficiency would greatly contribute to the climate change mitigation route [5].

In this context, the steam methane reforming (SMR) coupled with in situ CO2 separation is gaining importance as a method for obtaining high-purity hydrogen in one single step [5]. This sorption-enhanced reforming (SER) proposes carrying out the reforming of methane in the presence of a CO2 sorbent that reacts with the CO2 as soon as it is formed, pushing the reaction equilibrium towards hydrogen production [6, 7]. Due to its good performance and favourable kinetics, CaO-based materials have been typically proposed as CO2 sorbents in the SER process [7]. According to SER equilibrium (see Eqs. (1)–(3)), using CaO as a CO2 sorbent allows reaching H2 contents as high as 96 vol.% (dry basis) in a single step for temperatures about 650–700°C [6]. No water-gas shift (WGS) reactors are needed downstream the SER process since the CO content in the syngas produced is very low thanks to the presence of the CO2 separation process. Moreover, since all the reactions occur in a single reactor, the energy released by the exothermic CaO carbonation reaction and the WGS reaction compensates the energy required for the reforming of CH4, resulting in an almost neutral system that does not need from an external energy

CH4 + H2O ↔ CO + 3H2 Δ*H*<sup>298</sup> <sup>K</sup> = 206.2 kJ/mol (1)

CO + H2O ↔ CO2 + H2 Δ*H*<sup>298</sup> <sup>K</sup> = −41 kJ/mol (2)

CaO + CO2 ↔ CaCO3 Δ*H*<sup>298</sup> <sup>K</sup> = −178.8 kJ/mol (3)

One of the main issues of the SER process is the CaCO3 regeneration step, which is a high endothermic reaction that needs to be performed continuously to allow for a cyclic operation. Several alternatives have been proposed in the literature for supplying the large amount of energy needed in the CaCO3 regeneration step. Commonly, the direct combustion of a fuel in the same reactor in the presence of an O2-rich atmosphere has been proposed [8, 9], which will allow producing a CO2 stream that is not diluted with N2 and so easy to be purified and compressed for its final storage. Other options have been proposed as an alternative to the high energyconsuming air separation unit (ASU) needed for supplying the pure O2 required in this direct combustion option. For instance, the introduction of a high-temperature solid stream coming from a combustor in the calciner [10] or the use of an integrated high-temperature heat exchanger in the regenerator for transferring the heat indirectly from a high-temperature fluid [11, 12] have been proposed, but both options have not reached a sufficient development stage due to their limitations. As an alternative method for solving the problem of CaCO3 regeneration in the SER process, the Ca-Cu looping process emerged [13]. This process proposes carrying out the calcination of the CaCO3 by coupling in the same reactor the exothermic reduction of CuO with a gaseous fuel (i.e. containing CH4, H2 and CO). In this way, the coupling of the endothermic and exothermic reactions in a single step allows to supply directly the heat needed for CaCO3 calcination without the need of costly heat exchange surfaces or energy-demanding units like the ASU, resulting in this

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way in a high-efficiency process.

The Ca-Cu looping process was originally proposed in 2009 by Abanades and Murillo [13], and its basic scheme is based on the three main reaction stages shown in **Figure 1** [14]. The reactor configuration that fully exploits the advantages of the proposed concept is a series of fixed-bed reactors that operate in parallel at different pressure and temperature. Each fixed reactor passes through each stage of the Ca-Cu process in a sequential manner when changing the feed gas. Three functional materials are needed for operating this process: (i) a Cu-based material, (ii) a Ca-based CO2 sorbent and (iii) a reforming catalyst (typically Ni-based).

The Ca-Cu process can be applied as a post-combustion CO2 capture process in power plants, but the application having received more attention has been the developing of processes for the production of high-purity hydrogen and/or power [15]. At the beginning of the process, the Cu-based material and the reforming catalyst should be present in the bed in their reduced form, whereas the CO2 sorbent should be fully calcined. The first stage of the process (referred to as 'A' in **Figure 1**) consists of a SER process, and it starts when natural gas and steam are fed to the reactor. The SMR, WGS and CaO carbonation reactions (Eqs. (1)–(3), respectively) occur during this stage. Pressure proposed to operate this SER stage ranges from about 10 to 25 bar depending on the main output of the process (i.e. hydrogen or power production). A H2-rich gas is obtained at the outlet of this stage at high temperature, which should be cooled down to be used as fuel in a power production process or to be exported and used as feedstock for a downstream chemical process. SER stage finishes when all the CaO present in the solid bed is fully carbonated and there is no extra CaO to react with CO2. The Cu-based material remains in its reduced form, unreacted, through this SER stage.

The second stage of the Ca-Cu process consists of the oxidation of the Cu present in the solid bed to produce the amount of CuO needed for the calcination of the CaCO3. The oxidation stage (indicated as 'B' in **Figure 1**) starts when diluted air is fed to the fixed-bed reactor. This stage should be operated at controlled conditions of temperature and pressure to avoid temperature peaks within the reactor that lead to the prompt decomposition of the CaCO3, as well as to avoid operational problems related to the Cu-based material (i.e. agglomeration and/or loss in reactivity) [16]. The operation strategy proposed for limiting such maximum allowable temperature has been reducing the temperature and the O2 content in the diluted air stream fed to this stage. Recirculating a large fraction of the O2-depleted gas at this stage outlet dilutes the O2 content in the feed gas and increases the flow rate of the gas

**Figure 1.**

*Conceptual scheme of the Ca-Cu looping process.*

fed to this reactor. This oxidation stage is operated at high pressure for reducing the driving force towards CaCO3 decomposition. O2 contents of around 3 vol.% and inlet gas temperatures of 150–300°C for this oxidation stage have been proposed as suitable for limiting the maximum temperature reached within the solid bed at 830–850°C [17]. The operation at high pressure allows using the non-recirculated O2-depleted gas at the reactor outlet for producing electricity in a gas turbine. The Cu oxidation stage finishes when all the Cu present in the solid bed is oxidized into CuO. At this moment, all the solid bed has been left at the temperature of the inlet gas (i.e. around 300°C), which is too low for the subsequent calcination/reduction stage to begin. The recirculated O2-depleted gas exiting the oxidation stage needs to be cooled down to about 300°C to be fed to the reactor, and it is passed through the fully oxidized bed to be cooled down while transferring its sensible heat to the solids, leaving them at a temperature of around 760–800°C suitable for the next reaction stage.

The last stage of the Ca-Cu process consists of the calcination of the CaCO3 formed during the SER process by means of the energy released by the exothermic reduction of the CuO. This stage operates at atmospheric pressure to limit the maximum temperature needed to around 850–870°C. Typically, a mixture of H2, CH4 and CO is proposed as feed gas in this stage, coming from either a hydrogen purification section or a separate reforming process. Proper CaO and Cu amounts are needed in the reactor to ensure that the energy released by CuO reduction is enough for fulfilling the energy requirement from CaCO3 calcination as well as to reach the desired calcination temperature. The Cu/Ca ratio needed depends on the composition of the fuel gas used in this stage considering the reaction enthalpies of the reduction reactions with H2, CH4 or CO (see Eqs. (4)–(6)). The maximum amount of Cu is needed when using CH4 as reducing gas since it leads to the lowest exothermic reduction reaction per mole of CuO (i.e. a Cu/Ca molar ratio of 3.1 considering the reaction enthalpies at 850–870°C). On the contrary, the largest reduction enthalpy of the CuO with CO leads to the lowest Cu/Ca molar ratio needed of 1.3 [17].

$$\text{4CuO} + \text{CH}\_4 \rightarrow \ 4\text{Cu} + \text{CO}\_2 + 2\text{H}\_2\text{O} \quad \Delta H\_{298\text{ K}} = -158.3 \text{ kJ/mol} \tag{4}$$

$$\text{CuO} \star \text{H}\_2 \rightarrow \text{Cu} + \text{H}\_2\text{O} \quad \Delta H\_{298\,\text{K}} = -85.8 \text{ kJ/mol} \tag{5}$$

$$\text{CuO} + \text{CO} \rightarrow \text{Cu} + \text{CO}\_2 \quad \Delta H\_{298\,\text{K}} = -126.9 \,\text{kJ/mol} \tag{6}$$
