Insight into tar thermal cracking and catalytic cracking by char: Characteristics and kinetics

In this research, the non-isothermal and isothermal reaction behavior of tar catalytic conversion by char was tested by a thermogravimetric analyzer (TGA) and a micro fluidized bed reaction analyzer (MFBRA), respectively. The TGA analysis shows that the obvious catalytic action can be found at a temperature above 873 K. In the MFBRA measurement, the roles of heat and catalysis during tar catalytic conversion from char were examined by comparing tar thermal and catalytic cracking. The results show that, regardless of tar thermal and catalytic cracking, the isothermal reaction behavior can be divided into the primary reaction stage (S1) and the secondary reaction stage (S2). In S1, the reaction behaviors in terms of reaction time, tar conversion, and gas yields, were very similar, indicating the dominant role of heat. In S2, the process of tar catalytic conversion needed a longer reaction time with higher tar conversion and more gas product emission, manifesting the significant role of catalytic action. In the examined temperature range of 1023-1223 K, the temperature effect on gas components during tar catalytic cracking by char included promotion (CO, CH4, H-2, and CO2), suppression (C2H6, C3H6, and C3H8), and competition between them (C2H4). For the reaction kinetics, the activation energy of each gas component generation and the total gas product in S2 was much higher than that in S1. This was much related to the catalytic conversion of hard-to-crack components in the tar sample. The findings were beneficial to understanding tar catalytic reaction by char deeply and providing data support for the design and optimization of the tar removal unit in a gasification process.