Catalytic Effect of Metal Chloride Additives on the Volatile Gas Release Characteristics for High-Temperature Lignite Pyrolysis
2019
Wang, Zhihua | Tan, Jiaxin | He, Yong | Yuan, Ye | Liu, Longlong | Zhu, Yanqun | Cen, Kefa
The comparative effects of five metal chlorides (KCl, CaCl₂, NiCl₂, MnCl₂, and ZnCl₂) on the catalytic pyrolysis, especially the volatile gas release characteristics of lignite over a high-temperature range of 800–1200 °C, were investigated via a tubular pyrolyzer. Additionally, relevant volatilization of metal additives and their catalytic mechanisms inherently were investigated along with energy-dispersive spectrometry and X-ray diffraction results. The results demonstrated that the conversion of organic matters into lightweight volatile gases was inhibited by KCl and MnCl₂, while NiCl₂ and CaCl₂ promoted the conversion. The addition of NiCl₂ promoted the highest volatile gas yield increase by 7.3% compared with raw coal at 1200 °C. It significantly improved the production of CO as well as the transformation of CO₂ to CO. CaCl₂ advanced the maximum releasing rate of up to 8 mL/(g s), mainly facilitating the yields of CO₂ and CH₄. All additives imposed a positive effect on the yield of H₂ in the order of ZnCl₂ > MnCl₂ > NiCl₂ > KCl > CaCl₂. The volatile–char interactions led to the volatilization levels of all metal species increasing with the pyrolysis temperature. It was noteworthy that metal additives with better volatilization, such as MnCl₂, ZnCl₂, and KCl, would benefit by minimizing the secondary decomposition of tar. In the catalytic mechanism, alkali and alkaline earth metals tended to bond with oxygen-containing groups and repeatedly connected with the coal/char matrix, while transition metals may work during the process of transformation to stable metallic states.
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