Upscaling bifunctional materials for Ca-Cu looping: fixed bed reactor tests

Poster presented at the 10th "Trondheim Conference on CO2 Capture, Transport and Storage", June 17 - 19, 2019, in Trondheim - Norway.
Calcium-Copper Chemical Looping Technology is a hybrid concept combining Sorption-Enhanced Reforming (SER) and Chemical Looping Combustion (CLC) for hydrogen production from natural gas with integrated CO2 capture. SER involves reforming, water gas shift and CO2 capture in the same reactor vessel (reformer) making use of a CaO-based high temperature solid sorbent to capture the CO2: CH4 (g) + 2H2O (g) + CaO → CaCO3 + 4H2(g) This leads to process intensification due to avoidance of additional water gas shift steps as well as a downstream CO2 separation system, while hydrogen concentrations up to 98 vol% (dry basis) can be obtained at temperatures around 650 °C at 1 bar. To regenerate the sorbent at high temperature (900 °C, 1 bar), heat is provided via indirect heating2 or direct oxy-fuel combustion3. In the Ca-Cu Looping technology, a second Cu/CuO loop is introduced in the process and the calcination of CaCO3 is coupled and thermally sustained by the exothermic CuO reduction with H2, CO and/or CH4. In this way, expensive Air Separation Unit (ASU) or heat exchange surfaces are avoided, while fixed bed reactors are preferred to fluidized beds allowing the production of pressurized H2 without circulation of solids. Specific Energy Consumption (SPECCA) for the Ca-Cu process have been found to be in the range 1.1-1.5 MJ/kgCO2, which compares well with oxy-SER (1.6-2) and benchmark Fired Tubular Reactor / MDEA system (3.5) 4–6. The novelty proposed in this work is to combine CaO and CuO phases into one bi-functional material using relatively low-cost raw materials as well as an easy and scalable synthesis method. This configuration has the potential to a) lower the inert fraction in the reactor bed and thus limit the sensible heating requirement in the reactor, and b) to promote close contact between the CaO and CuO phases, improving the heat and mass transfer compared to a segregated particle arrangement. Mayenite, a suitable phase for increasing the stability of CaO-based synthetic sorbents7, has been used to support the Ca-Cu combined materials. In a previous work, the effect of the CuO precursors (Cu(OH)2, Cu(NO3)2 and CuO mesh powder), and the CuO-CaO loading (at constant weight ration of 2) on the hydrothermal synthesis of combined powder has been investigated in TGA for about 40 carbonation-calcination and oxidation-reduction cycles. This study has shown that 40 wt% CuO materials are stable for 40 TGA cycles while all investigated 50 wt% CuO materials deactivate, regardless of CuO precursor 8. Subsequently, the synthesis method has been upscaled to produce 100g of powders which have been agglomerated to particles with 0.5 – 0.8 mm diameter, and subjected to 40 carbonation-oxidation-reduction-calcination cycles in TGA. The agglomerates of the combined Cu(OH)2 prepared powder are stable for 40 TGA cycles, although suffer from an initial 25 wt% capacity reduction in both O2 and CO2 capacities relative to the Cu(OH)2 powder. Finally, the combined material agglomerates have been mixed with a commercial reforming catalyst and tested in a lab scale fixed bed reactor along the three main reaction stages of a Ca-Cu looping process – SER, Cu oxidation and Cu reduction. Operation conditions chosen for the process stages were representative for the scaling up of the process in terms of pressure, gas spatial velocities and gas composition. The obtained evolution of product gas composition with time, as well as the temperature profiles within the bed along each reaction stage during three complete cycles (as depicted in Figure 1 for the SER step) has validated the combined materials concept at lab scale, and shows promises in terms of increased amount of active material per reactor volume.
This work is funded by the Research Council of Norway in the framework of CLIMIT-prosjekt 254736Ca-Cu Cycles