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Investigation into the Cleavage of Chemical Bonds Induced by CO2 and Its Mechanism during the Pressurized Pyrolysis of Coal

Gao, Songping, Zhai, Lingrui, Qin, Yuhong, Wang, Zhiqing, Zhao, Jiantao, Fang, Yitian
Energy & fuels 2018 v.32 no.3 pp. 3243-3253
active sites, adsorption, aromatic hydrocarbons, carbon dioxide, chemical bonding, cleavage (chemistry), coal, ethylene, hydrogen, methane, nitrogen, pyrolysis, temperature
A Huo linhe coal sample and its N₂-devolatilized char have been pyrolyzed in a pressurized fixed-bed reactor under N₂ and 50% CO₂/50% N₂ atmospheres, respectively. With a view to avoiding any disturbance from the devolatilization process, N₂-devolatilized char was used to study the effect of CO₂ on the cleavage of chemical bonds. The results show that CO₂ intensifies the consumption of H in the char before its release in the form of H₂. More H in the char is transferred into gaseous aliphatic hydrocarbons and condensable volatiles, leading to a decrease in the yield of H₂ under 50% CO₂ atmosphere compared with that under N₂ at elevated pressure in the range 550–900 °C. A CO₂ atmosphere is the dominant factor in the release of H from char below 700 °C, such that more H in char induced by CO₂ is transferred into volatiles at 550 °C than that at 800 °C, above which temperature controls the release of H from char. Thus, CO₂ induces the decomposition of more H-containing organic groups to form CH₄, C₂H₆, C₂H₄, and H-rich low-molecular-weight tar at elevated pressure and 550 °C. Dissociative adsorption of CO₂ on coal/char can generate adsorbed active C(O) and active Cf at elevated pressure. The electronegativity of the O in active C(O) induces cleavage of adjacent bonds, and the formation of active Cf may induce new structural defects or active sites (Cf) capable of inducing bond cleavage, resulting in the cleavage of chemical bonds in the char at lower temperatures than under N₂ atmosphere at elevated pressure, such as those of aromatic rings, aliphatic chains of aromatic hydrocarbons, and ether bonds.