A cost-effective approach to reducing carbon deposition and resulting deactivation of oxygen carriers for improvement of energy efficiency and CO2 capture during methane chemical-looping combustion

摘要

The objective of this research is to investigate the effect of steam on reducing carbon deposition during CH4 chemical looping combustion (CLC) on a fixed bed reactor. Steam proved to be significantly effective not only for the fuel utilization efficiency of CLC but also for the stability of the oxygen carrier (OC). The evaluation tests were performed with CuO/gamma-Al2O3 OCs prepared with three different methods. The results showed that OC prepared using co-precipitation method performed the best with the highest reactivity for CH4 combustion at low temperature (700 degrees C) compared to that prepared using mechanical mixture and impregnation methods. It was also found that the carbon deposition was the main reason for the low combustion efficiency and the deactivation of OCs in redox cycles. For OCs at 800 degrees C, after 10 cycles without steam, 25-44% CO2 selectivity reductions were observed, accompanied by Barrett-Joyner-Halenda (BJH) pore volume decreases of 31-47%. With the steam introduction, the CO2 selectivity consistently achieved similar to 100% in 10-cycle CLC, and the pore volumes of OCs decreased by only 11-22%, which was mostly attributed to the carbon-steam gasification reaction. The OCs for CH4 CLC with and without steam were characterized by using different methods, including surface area and pore analysis, thermal gravimetric analysis (TGA), X-ray diffraction (XRD), H-2 temperature programming reduction (TPR), and scanning electron microscope (SEM). The results demonstrated that steam could easily gasify the carbon deposition, greatly intensify the redox degree of OCs at 800 degrees C, and strengthen the redistribution of Cu on the surface of Al2O3, contributing to the fine and uniform distribution of CuO. Therefore, steam can be successfully used for considerable enhancement of the overall performance of CH4 CLC. (C) 2017 Elsevier Ltd. All rights reserved.