**4. Ca-Cu process lab-scale testing**

The feasibility of the reaction steps of the Ca-Cu looping process has been experimentally confirmed in packed-bed reactors at laboratory scale during the recent EU-FP7 Project ASCENT [32]. Grasa et al. [18] focused the investigation on the SER stage using a commercial Ni-based catalyst and a CaO-Ca12Al14O33 sorbent. After 200 reduction/oxidation cycles, the sorbent/catalyst system produced a gas with more than 90 vol.% H2 on a dry basis (i.e. close to the maximum equilibrium value), operating with space velocities up to 2.5 kg CH4 h<sup>−</sup><sup>1</sup> kg cat<sup>−</sup><sup>1</sup> (i.e. a gas velocity of 0.53 m/s inside the bed). The maximum space velocity at which the CH4 is totally converted during the SER operation is determined by the CaO carbonation reaction. Sorbent carbonation reaction rates up to 4.42 × 10<sup>−</sup><sup>2</sup> kmol h<sup>−</sup><sup>1</sup> kg sorbent<sup>−</sup><sup>1</sup> were calculated in the experiments.

The feasibility of the Cu oxidation stage was experimentally demonstrated by Alarcón et al. [45] under relevant conditions for the Ca-Cu looping process. Oxygen in the feed was diluted to 3%vol. with N2 simulating the recirculation of a large fraction of the product gas from the oxidation stage outlet. The maximum temperature in the bed was kept below 800°C during the oxidation, which should prevent the agglomeration or sintering of the Cu-based material and highly reduce the loss of CO2 by the partial calcination of the sorbent. Even at low starting temperatures in the reactor (of about 400°C), the oxidation of Cu occurred very fast taking place in sharp reaction fronts throughout the reactor. During the pre-breakthrough period, complete conversion of O2 was observed despite of the very low O2 content in the feed.

Fernández et al. [46] demonstrated at TRL4 the viability of the calcination of CaCO3 by the in situ reduction of CuO with H2 giving rise to a product gas composed of virtually pure CO2 (after the condensation of H2O). Tests were carried out in a fixed-bed reactor (1 m long and inner diameter of 38 mm) operating close to adiabatic conditions, loaded with commercial CaO- and Cu-based materials in pellet form (particle size of about 3 mm). The fixed-bed contained a Cu/CaO molar ratio of about 1.8, which is the theoretical value to accomplish the reduction/calcination with H2 under neutrally thermal conditions. A fast and complete reduction of CuO with H2 was observed even at relatively low initial solid bed temperatures (i.e. 400°C). However, only temperatures in the solid bed higher than 700°C allowed a simultaneous reduction/calcination operation, leaving uncalcined material in those zones at lower temperatures. Alarcón et al. [45] evaluated the effect of the fuel gas composition on the CuO reduction/CaCO3 calcination operation. Different Cu/Ca molar ratios were used for this purpose to maintain neutral conditions in the reduction/calcination front. Mixtures of CO and H2 showed high reactivity with the CuO-based material, resulting in the complete reduction of CuO to Cu in a sharp reaction front and the total oxidation of the gaseous fuel to CO2 and H2O. The Cu-based material was able to catalyse the reverse WGS reaction, favoured by the high temperature and the high CO2 content in the atmosphere. Moreover, combined Ca-Cu oxides formed because of the multicycle operation at

**99**

h<sup>−</sup><sup>1</sup>

kg Ni<sup>−</sup><sup>1</sup>

**5. Process analysis**

**5.1 Reactor design and modelling**

formation of hot spots inside the reactors.

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

material was significantly lower than that measured with H2.

between the different solids loaded in the reactor.

high temperature, which slightly modified the chemical composition of the starting materials. These oxides carbonated in the presence of CO2, affecting the CO2 sorption capacity of the solid bed. Recently, Fernández et al. [47] studied the reduction/ calcination stage using pure CH4 as reducing gas. The effect of the initial bed temperature and the inlet gas flow rate was evaluated. CuO reduction was favoured when using initial bed temperatures higher than 800°C, resulting in the complete oxidation of inlet CH4 and the calcination of a large fraction of CaCO3. A low flow rate (i.e. 3 lN/min of CH4) allowed a sufficient residence time of the CH4 inside the reactor to be almost completely converted to CO2 and H2O. Temperature profiles higher than 900°C were measured, and large amounts of CO2 resulting from CH4 oxidation and CaCO3 decomposition were observed. The relatively long breakthrough periods demonstrated that the reactivity of the CH4 with the CuO-based

Consecutive cycles of the three main reaction stages of the Ca-Cu looping process were made by Díez-Martín et al. [19] in a lab-scale fixed-bed reactor (L = 0.2 m, I.D. = 18 mm) under relevant conditions of this process at a large scale. The solid bed contained the three functional materials required to run the process (i.e. a commercial Ni-based catalyst, a CaO/Ca12Al14O33 sorbent and a CuO/Al2O3 material). The system was able to operate with space velocities of up to 13.5 kg CH4

during the SER stage at 675°C and 10 bar producing a gas with more

than 93 vol.% H2 (on a dry basis). The Cu-based solid exhibited fast reduction and oxidation kinetics, but it did not showed any appreciable reactivity towards CH4 reforming during SER operation. Total O2 conversion was observed during the Cu oxidation stage. Slightly higher amounts of CO2 than those predicted by the CaO/ CaCO3 equilibrium were measured in the product gas during oxidation due to the carbon deposited during the breakthrough period of the previous SER step. The results obtained along several cycles were highly reproducible, demonstrating the proper chemical stability of the materials. Only a slightly decrease of the CO2 sorbent capacity was observed. No mixed phases from the different active materials were detected, indicating the absence of any significant chemical interaction

The Ca-Cu looping process was mainly envisaged to be performed in several adiabatic packed-bed reactors operating in parallel. Fixed-bed reactors do not require solid filtering systems downstream since the formation of fines by attrition is avoided, and they allow the operation to take place in a more compact design at a high pressure. Moreover, H2 and N2 can be produced at a suitable pressure to be subsequently used in industrial applications and/or power generation. However, pressurized fixed beds require adequate heat management strategies in order to achieve the complete conversion of the solids and at the same time to avoid the

The first conceptual design of the overall Ca-Cu process was presented by Fernández et al. [17] in which a quite simple reactor model assuming narrow reaction fronts was used to describe the dynamic performance of every stage of the process. An ideal plug flow model with negligible axial dispersion was considered. Precise operating conditions for the process (i.e. temperature, pressure, steam-to-carbon (S/C) ratio, etc.) and material properties were defined. More rigorous reactor models were latterly developed to describe more precisely the

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

*Ca-Cu Chemical Looping Process for Hydrogen and/or Power Production DOI: http://dx.org/10.5772/intechopen.80855*

high temperature, which slightly modified the chemical composition of the starting materials. These oxides carbonated in the presence of CO2, affecting the CO2 sorption capacity of the solid bed. Recently, Fernández et al. [47] studied the reduction/ calcination stage using pure CH4 as reducing gas. The effect of the initial bed temperature and the inlet gas flow rate was evaluated. CuO reduction was favoured when using initial bed temperatures higher than 800°C, resulting in the complete oxidation of inlet CH4 and the calcination of a large fraction of CaCO3. A low flow rate (i.e. 3 lN/min of CH4) allowed a sufficient residence time of the CH4 inside the reactor to be almost completely converted to CO2 and H2O. Temperature profiles higher than 900°C were measured, and large amounts of CO2 resulting from CH4 oxidation and CaCO3 decomposition were observed. The relatively long breakthrough periods demonstrated that the reactivity of the CH4 with the CuO-based material was significantly lower than that measured with H2.

Consecutive cycles of the three main reaction stages of the Ca-Cu looping process were made by Díez-Martín et al. [19] in a lab-scale fixed-bed reactor (L = 0.2 m, I.D. = 18 mm) under relevant conditions of this process at a large scale. The solid bed contained the three functional materials required to run the process (i.e. a commercial Ni-based catalyst, a CaO/Ca12Al14O33 sorbent and a CuO/Al2O3 material). The system was able to operate with space velocities of up to 13.5 kg CH4 h<sup>−</sup><sup>1</sup> kg Ni<sup>−</sup><sup>1</sup> during the SER stage at 675°C and 10 bar producing a gas with more than 93 vol.% H2 (on a dry basis). The Cu-based solid exhibited fast reduction and oxidation kinetics, but it did not showed any appreciable reactivity towards CH4 reforming during SER operation. Total O2 conversion was observed during the Cu oxidation stage. Slightly higher amounts of CO2 than those predicted by the CaO/ CaCO3 equilibrium were measured in the product gas during oxidation due to the carbon deposited during the breakthrough period of the previous SER step. The results obtained along several cycles were highly reproducible, demonstrating the proper chemical stability of the materials. Only a slightly decrease of the CO2 sorbent capacity was observed. No mixed phases from the different active materials were detected, indicating the absence of any significant chemical interaction between the different solids loaded in the reactor.
