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

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

**95**

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

space velocity that a system sorbent/catalyst is able to convert.

the Ca-Cu process [18, 19]. It is important to assess the effect that the redox cycles have on catalyst activity and to determine the operational window in terms of CH4

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

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

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

cycles performing calcination under realistic conditions for the Ca-Cu process (i.e.

absorbed per unit of CaO or sorbent weight) is commonly related

/g calcined material after 100 reaction

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

capacity in gCO2

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

pore structure and so reduce sintering.

a CO2 carrying capacity of about 0.28 gCO2

at 900°C and 70%vol. CO2).

the Ca-Cu process [18, 19]. It is important to assess the effect that the redox cycles have on catalyst activity and to determine the operational window in terms of CH4 space velocity that a system sorbent/catalyst is able to convert.
