Development Of Cao-Based Mixed Oxides As Stable Sorbents For Sorption Enhanced-Chemical Looping Steam Methane Reforming
Authors: Lemonidou Angeliki A 
Published: November 2013

Methane steam reforming is a carbon-intensive process with high energy requirements. The addition of a CO2 sorbent in the reformer has been proposed as an alternative process to reduce heat demands (since the carbonation reaction is exothermic) and at the same time capture CO2, drive the thermodynamic equilibrium and obtain high-purity hydrogen. In a QNRF funded project, AUTH and TAMUQ are investigating sorption enhanced steam reforming combined with chemical looping. The process is called Sorption Enhanced-Chemical Looping Steam Methane Reforming and in addition to the sorbent, the reformer contains material which serves as an oxygen transfer material (OTM). The basic principle is shown in Fig.1. During the reduction step (reforming), the OTM is reduced by methane into metallic nickel, the active catalyst for endothermic methane reforming, and the reaction proceeds under near autothermal conditions due to heat released by sorbent carbonation. During the regeneration/oxidation step, the reduced OTM is re-oxidized, releasing significant amount of heat which drives the endothermic regeneration of the saturated sorbent. The key for successful commercialization of the process is development of stable sorbents and OTMs that can undergo multiple cycles of reaction/regeneration without deterioration in their performance. Herein, we report results from our studies on development of CaO-based mixed oxides CO2 sorbents. Mixed calcium oxides with alumina, zirconia, magnesia and lanthana were synthesized via the sol-gel autocombustion method using citric acid as a combustion agent. The mixed oxides were prepared with constant 66wt% concentration of free CaO and were characterized by BET and XRD. Results showed that CaO is formed in all cases, accompanied by formation of mixed Ca3Al2O6 and CaZrO3 in Ca-Al and Ca-Zr sorbents, respectively. On the contrary no mixed phase was formed between CaO and Mg or La. The sorption capacity and stability of the sorbents were tested in a TGA instrument (or unit) for 100 consecutive sorption-desorption cycles under 15% CO2 in N2 for 30 minutes at 650°C and 100% N2 for 5 minutes at 850°C. The CO2 capacity of all the synthesized sorbents as a function of a number of cycles is shown in Fig.2. The highest CaO conversion was obtained with Ca-Al (99.8%), followed by Ca-Zr, Ca-Mg and Ca-La. The most stable material proved to be Ca-Zr with only 13.7% loss of capacity after 100 cycles. Ca-Al also had a promising performance with 20.7% loss, while Mg and La both exhibited significant deactivation (i.e.  30% loss in sorption capacity). The two most promising materials (Ca-Al and Ca-Zr) were also synthesized using a different combustion agent, triethanolamine, to investigate the effect of an organic fuel. The use of TEA was beneficial for Ca-Zr,which achieved higher CaO conversion (96.9%) and better stability (13.4% loss), while the opposite trend was observed for Ca-Al. Overall among all investigated CO2-sorbents, Ca-Al and Ca-Zr prepared with both citric acid and triethanolamine as combustion agents, exhibited excellent initial capacity and stability, with a sorption capacity higher than 9 mol CO2/kg of sorbent after 100 sorption-desorption cycles under relatively mild operating conditions.