Oxidative Catalytic Cracking and Reforming of Coal Pyrolysis Volatiles over NiO

Catalytic cracking of Pingshuo (PS) coal pyrolysis volatiles was investigated over NiO on a two-stage fixed bed reactor. The effect of the NiO loading amount and catalytic upgrading temperature on the products distribution was investigated, and the role of NiO during the catalytic upgrading was discussed through several characterizations. It is shown that the partial reduced NiO to Ni during catalytic cracking results in oxidative catalytic cracking and reforming of coal pyrolysis volatiles. The tar yield decreases from 14.2 wt % without using NiO to 3.6 wt % with 9.0 g NiO loading at a cracking temperature of 550 °C, corresponding to the tar conversion being 74.6 wt %. With the increase of the cracking temperature, the tar yield decreases first and then increases, and the lowest tar yield is 5.2 wt % at 550 °C over 6 g NiO loading. After catalytic cracking of coal pyrolysis volatiles, the used NiO is fully reduced to Ni, and the catalytic performance of used NiO was evaluated three times. The results indicate that the NiO-2nd (used NiO) shows high tar reforming performance in terms of the decrease of tar (10.7 wt %) and water yield (4.8 wt %), and H₂ yield increases significantly to 104 mL/g coal_daf at 550 °C compared to those without using NiO. However, the catalytic performance of used NiO decreases gradually with increase of cycle times; the tar and water yield increase to 13.3 and 7.4 wt %, respectively, and H₂ yield decreases to 31 mL/g coal_daf over NiO-4th. The gradual decrease of catalytic performance of used NiO is due to the deposition of carbon and formation of Ni₂S₃.