2008 Volume 41 Issue 9 Pages 915-922
The deactivation mechanism of a commercial Ni/Al2O3 catalyst used during coal volatile decomposition was investigated by transmission electron microscopy–energy dispersive X-ray spectroscopy (TEM-EDS), X-ray diffraction (XRD) and nitrogen adsorption. The existence of carbonous species in the reaction system promoted nickel particle growth during coal volatile decomposition, and subsequent coking from volatile cracking. Throughout the catalyst deactivation tests, coke deposits were observed as encapsulating carbon in the spent catalyst, and nickel particles doubled in size from around 10 to 20 nm. The spent catalyst was regenerated in oxygen at relatively moderate conditions by removing the coke deposits. As a result, the catalyst activity was restored remarkably; 1.7 times the surface area and double pore volume were present in the regenerated catalyst compared to the spent catalyst. Also, the regenerated catalyst showed high activity for coal volatile decomposition. Under catalysis of the regenerated Ni/Al2O3, the tarry material in coal volatile matter could transform much more completely, gaining both high gas yields and high carbon balance.
We also found that methanation is structure sensitive to nickel particles. Under the action of the regenerated catalyst, CO formed during coal volatile decomposition could not be further converted into methane, and the product gases provided a higher CO concentration. Noticeably, tar decomposition was confirmed to be less structure sensitive to the nickel particles than CO-methanation.