**3.1 Development of Ca-based CO2 sorbents**

*Global Warming and Climate Change*

reaction stage.

needed of 1.3 [17].

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

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

4CuO + CH4 → 4Cu + CO2 + 2H2O Δ*H*<sup>298</sup> <sup>K</sup> = −158.3 kJ/mol (4)

CuO + H2 → Cu + H2O Δ*H*<sup>298</sup> <sup>K</sup> = −85.8 kJ/mol (5)

CuO + CO → Cu + CO2 Δ*H*<sup>298</sup> <sup>K</sup> = −126.9kJ/mol (6)

Three functional materials are needed for running the Ca-Cu process, namely, the CaO-based CO2 sorbent, the Cu-based material and the reforming catalyst. Their proportion in bed will be determined by, on the one hand, the energy balance in the calcination/reduction stage in the case of Cu-based material/sorbent and, on the other hand, the CH4 space velocity that a system is able to convert for the sorbent/catalyst ratio. In any case, it is important to maximize the active phase in every material, as the presence of inert in the reactor would negatively affect the efficiency of the process. In this section, a revision on the recent developments of CaO and Cu-based materials suitable for this process is included. As for the catalyst, conventional Ni-based reforming catalysts have been typically proposed for this process [15, 17], which have been successfully tested under suitable conditions for

**3. Development of materials suitable for the Ca-Cu process**

**94**

In the recent years, intense work has been carried out in the field of synthetic CaO-based sorbents with the objective of overcoming the decay in CO2 capture capacity that presents CaO-based sorbents derived from natural limestones and dolomites (see e.g. recent reviews [20–22]). Among the different strategies followed to produce materials resistant to sintering, the incorporation of the active phase (i.e. CaO) into an inert matrix is the most extended and validated synthesis method [23, 24]. The performance of the materials (referred to as CO2 carrying capacity in gCO2 absorbed per unit of CaO or sorbent weight) is commonly related to their pore structure/surface area and its evolution with the reaction cycles. The decay in sorption capacity is mainly a result of a sintering phenomenon that consists of the agglomeration of small CaO grains and the evolution of the pore structure towards higher pore diameters. It is important to highlight that it is not possible to directly compare the behaviour of materials tested under diverse reaction conditions as these will affect the materials performance in the long term (high number of reaction cycles) [25–27]. Anyway, there are valid trends that can be extracted from the results in the literature, as for example, that the maximum CO2 carrying capacity of a sorbent is directly proportional to its CaO load [20] and that a minimum amount of inert matrix is required to maintain the pore structure and so reduce sintering.

A wide variety of synthesis methods (i.e. wet mixing, spray pyrolysis, sol-gel, co-precipitation, etc.) and inert supports (i.e. Al2O3, MgO, ZrO2, SiO2, Ca12Al14O33, etc.) have been studied in the literature for preparing synthetic CaO-based sorbents (see detailed reviews [20, 22] for an extended list of synthetic CaO-based sorbents). A recent paper by López et al. [24] evaluated the effect that sorbent inert support has on CO2 carrying capacity and reactivity towards carbonation reaction. For this purpose, materials with different CaO amounts were prepared using two different inert supports (i.e. MgO and Ca12Al14O33). CaO/MgO materials were prepared through co-precipitation (with CaO contents between 100 and 40%wt.), whereas materials with Ca12Al14O33 as inert support were prepared via mechanical mixing and later calcination. The results indicated that a minimum amount of inert species was required to stabilize and to improve the CO2 carrying capacity of the materials beyond the capacity of pure CaO. In **Figure 2,** the CO2 carrying capacity of the different synthetic CaO-based materials prepared in [24] is depicted. A minimum amount of 10%wt. MgO improves the CO2 carrying capacity of the material with respect to the performance of the co-precipitated CaO. Moreover, reducing the amount of CaO in the material diminishes the decay in the CO2 sorption capacity along the initial cycles that is typical of naturally derived CaO-based materials. Taking into account that operation of the Ca-Cu process is thought to be carried out in fixed-bed reactors, the different functional materials should be in particle or pellet form. López et al. [24] prepared particles through an agglomeration process from the synthetized powder and demonstrated that the agglomeration process affected the textural properties of the materials, reducing the BET surface area and porosity with respect to the properties of the powder. Synthetic dolomites with a CaO/MgO molar ratio of 2:1 and a particle size cut of 0.6–1 mm were obtained, which showed a CO2 carrying capacity of about 0.28 gCO2 /g calcined material after 100 reaction cycles performing calcination under realistic conditions for the Ca-Cu process (i.e. at 900°C and 70%vol. CO2).

### **Figure 2.**

*Evolution of CO2 carrying capacity with the number of cycles for different CaO-based materials (adapted from the information published in* [24]*). White symbols correspond to materials tested in powder form (<75 μm) and black symbols to materials in particle size cut of 0.6–1 mm.*

Promising results have been reported in the literature for materials with Ca12Al14O33 as inert support prepared through diverse synthesis routes under relevant calcination conditions for the Ca-Cu process (i.e. temperatures above 900°C in presence of CO2 and steam). Pacciani et al. [28] reported a CO2 carrying capacity of 0.17 gCO2 /g calcined sorbent after 110 reaction cycles for a 85%wt. CaO, 15% wt. Ca12Al14O33 sorbent prepared by co-precipitation. In another study, Koirala et al. [29] prepared different Ca-based sorbents with different Al/Ca molar ratios via singlenozzle flame spray pyrolysis. A CO2 carrying capacity of 0.25 gCO2 /g calcined sorbent was demonstrated after 100 calcination/carbonation cycles for a material with an Al/Ca molar ratio of 3:10 under severe calcination. Radfarnia and Sayari [30] used a citrate-assisted sol-gel technique followed by a two-step calcination method to produce an Al-stabilized sorbent that presented a CO2 carrying capacity of 0.33 gCO2 /g sorbent after more than 30 reaction cycles calcined at 930°C and 100% CO2. An effort has been done by Kazi et al. [31] to produce efficient and stable CaO-Ca12Al14O33 sorbents via a cost-effective and easy scalable hydrothermal synthesis route, starting from low-cost hydroxide precursors. Through this method a highly stable sorbent presenting 0.21 gCO2 /g calcined sorbent was synthesized, whose production has been recently scaled up within the framework of the FP7 ASCENT project [32].

### **3.2 Development of Cu-based materials**

There is an important number of works focused on the development of Cu-based materials due to their application in chemical looping processes as oxygen carriers [16]. Different synthesis routes have been reported in the literature for these materials, as freeze granulation, impregnation, extrusion, spray drying, co-precipitation or mechanical mixing, using different inert supports (i.e. Al2O3 as the most common, but also MgAl2O4, ZrO2, CeO2, TiO2 and SiO2 have been proposed), as widely reviewed by Adánez et al. [16]. Cu-based materials with high Cu loads (i.e. about 60%wt. Cu) highly resistant to agglomeration and deactivation are those suitable for the Ca-Cu process. The recent interest of combusting solid fuels through CLOU process speeded up the development of materials with higher oxygen transport capacity (OTC) and therefore higher Cu contents [33]. However, despite some works reporting stable OTC along a reduced number of cycles

**97**

*Ca-Cu Chemical Looping Process for Hydrogen and/or Power Production*

to 65% wt. onto Al2O3 and MgAl2O4 from co-precipitated powders.

**3.3 Development of combined CaO-CuO materials**

maintained a CO2 carrying capacity of 0.18 gCO2

the highest CO2 uptake of 0.13 gCO2

In every case, the Cu phase reacted over 98%. Also these authors explored the effect that inert species might have on the chemical stability of the combined material, in this way they prepared via sol-gel materials supported on to Al2O3, MgO and MgAl2O4 with CuO and CaO molar ratio of 1.3:1 and 3.3:1 [43]. As found by other authors, the presence of Mg in the support stabilized the CO2 uptake and minimized carbon deposition. CuO-MgAl2O4 with a proportion of 1.3:1 was the material with

/g material after 15 reaction cycles.

/g material after 15 cycles of repeated carbonation/

operated in fluidized bed under CLOU conditions for materials containing 80%wt. CuO on to MgAl2O4 [34], there are not many works published so far about highly loaded Cu materials specifically prepared for operation in fixed-bed reactors in pellet or large-particle form. A recent paper from Díez-Martín et al. [35] evaluated the maximum CuO load onto different inert supports (Al2O3, MgAl2O4, ZrO2) that allowed chemically and mechanically stable materials along representative conditions for the Ca-Cu process. According to the results from this work, it was possible to produce chemically and mechanically stable pellet materials with Cu contents up

With the objective of improving heat and mass transfer phenomena within the reduction/calcination stage of the process, there is an increasing number of works evaluating the synthesis of combined functional Ca-Cu materials [36–39]. Mechanical mixing of CaO and CuO powders was the selected synthesis route followed by Manovic and Anthony [40] for synthetizing for the first time this combined material. These authors prepared pellets by mixing CaO from calcined natural limestone with commercial CuO particles and Ca-aluminate cement as binder in a proportion that resulted in 45:45:10 mass fraction. Material performance was evaluated in a TGA apparatus along successive reduction/calcination and oxidation cycles. The Cu phase was totally converted during reduction (at 800°C in a CH4 atmosphere) and oxidation in air, indicating that this could be a suitable material for the Ca-Cu process. Trying to explore the possibilities of the synthesis route, the same authors prepared core-in-shell materials with different CaO, CuO and Ca-aluminate cement proportions [41], maintaining the CuO in the inner core of the pellet. The OTC of the pellets indicated that a 25%wt. CaO in the core is sufficient to support the CuO and prevent the decay of its activity as an oxygen carrier. In other works, Quin et al. [39, 42] assessed the performance of materials composed by CaO and CuO supported on to MgO, Al2O3 or cement, prepared following diverse synthesis routes (wet mixing, sol-gel and mechanical mixing). The materials were tested in TGA and showed good reactivity along reduction and oxidation cycles using CH4 and air, respectively. The presence of inert support allowed the combined material to maintain its CO2 carrying capacity along cycles. This was especially clear for materials containing MgO on its structure, as this species greatly reduced the resistance to CO2 diffusion during the carbonation stage. In contrast, the presence of Al2O3 produced Ca12Al14O33 after reaction with CaO reducing in this way the amount of active phase for the carbonation reaction. As it happened for the sorbent and oxygen carriers, co-precipitation has been also a synthesis route explored to produce combined materials. Kierzkowska and Müller [38] prepared through this route combined materials with diverse CaO and CuO contents (CaO:CuO molar ratios of 1:1, 1.3:1 and 3.3:1) to be tested in a TGA along multiple carbonation/reduction/calcination/oxidation cycles. These cycles were performed isothermally at 750°C, carrying out carbonation in 40%vol. CO2 in air, reduction in 10%vol. CH4 in N2 and oxidation 4.2%vol. O2 in N2. According to this study, the best result was obtained for the material with a molar ratio 1:1 that

*DOI: http://dx.org/10.5772/intechopen.80855*

*Global Warming and Climate Change*

Promising results have been reported in the literature for materials with Ca12Al14O33 as inert support prepared through diverse synthesis routes under relevant calcination conditions for the Ca-Cu process (i.e. temperatures above 900°C in presence of CO2 and steam). Pacciani et al. [28] reported a CO2 carrying capacity of

*Evolution of CO2 carrying capacity with the number of cycles for different CaO-based materials (adapted from the information published in* [24]*). White symbols correspond to materials tested in powder form* 

nozzle flame spray pyrolysis. A CO2 carrying capacity of 0.25 gCO2

*(<75 μm) and black symbols to materials in particle size cut of 0.6–1 mm.*

/g calcined sorbent after 110 reaction cycles for a 85%wt. CaO, 15% wt. Ca12Al14O33 sorbent prepared by co-precipitation. In another study, Koirala et al. [29] prepared different Ca-based sorbents with different Al/Ca molar ratios via single-

was demonstrated after 100 calcination/carbonation cycles for a material with an Al/Ca molar ratio of 3:10 under severe calcination. Radfarnia and Sayari [30] used a citrate-assisted sol-gel technique followed by a two-step calcination method to produce an Al-stabilized sorbent that presented a CO2 carrying capacity of 0.33 gCO2

recently scaled up within the framework of the FP7 ASCENT project [32].

There is an important number of works focused on the development of Cu-based materials due to their application in chemical looping processes as oxygen carriers [16]. Different synthesis routes have been reported in the literature for these materials, as freeze granulation, impregnation, extrusion, spray drying, co-precipitation or mechanical mixing, using different inert supports (i.e. Al2O3 as the most common, but also MgAl2O4, ZrO2, CeO2, TiO2 and SiO2 have been proposed), as widely reviewed by Adánez et al. [16]. Cu-based materials with high Cu loads (i.e. about 60%wt. Cu) highly resistant to agglomeration and deactivation are those suitable for the Ca-Cu process. The recent interest of combusting solid fuels through CLOU process speeded up the development of materials with higher oxygen transport capacity (OTC) and therefore higher Cu contents [33]. However, despite some works reporting stable OTC along a reduced number of cycles

sorbent after more than 30 reaction cycles calcined at 930°C and 100% CO2. An effort has been done by Kazi et al. [31] to produce efficient and stable CaO-Ca12Al14O33 sorbents via a cost-effective and easy scalable hydrothermal synthesis route, starting from low-cost hydroxide precursors. Through this method a highly stable sorbent

/g calcined sorbent was synthesized, whose production has been

/g calcined sorbent

/g

**96**

0.17 gCO2

**Figure 2.**

presenting 0.21 gCO2

**3.2 Development of Cu-based materials**

operated in fluidized bed under CLOU conditions for materials containing 80%wt. CuO on to MgAl2O4 [34], there are not many works published so far about highly loaded Cu materials specifically prepared for operation in fixed-bed reactors in pellet or large-particle form. A recent paper from Díez-Martín et al. [35] evaluated the maximum CuO load onto different inert supports (Al2O3, MgAl2O4, ZrO2) that allowed chemically and mechanically stable materials along representative conditions for the Ca-Cu process. According to the results from this work, it was possible to produce chemically and mechanically stable pellet materials with Cu contents up to 65% wt. onto Al2O3 and MgAl2O4 from co-precipitated powders.
