Directional Transformation,Reactivity,and Mechanism Of Catalytic Pyrolysis Of Low-rank Coal

China is the biggest producer and consumer of coal in the world with abundant coal resources;low-rank coal accounts for >55% of China's coal resources.Low-rank coal is considered as a good material for low-temperature pyrolysis,with high volatility and water content.To achieve the directional transformation of low-rank coal pyrolysis,this study mainly focused on the catalytic pyrolysis of coal.The distribution and composition of products can be controlled by the catalytic pyrolysis of coal,and the directed conversion of coal can be achieved by adding additives.In this study,Huangling coal?HL?,Hecaogou coal in Zichang?HG?region,and Datong coal in Shanxi?DT?were selected as the coal samples;three types of catalysts?KCl,CaO,and Fe₂O₃?were selected.The best additives of catalysts were determined from TG and DTG analyses.Moreover,the catalytic mechanism was elucidated in terms of the changes in gas composition and kinetic analysis.The results are given below:?1?The catalytic pyrolysis of coal was studied using a thermogravimetric analyzer.The results of TG and DTG analyses show that the pyrolysis temperature of coal,weight loss,and maximum weight loss rate clearly changed.With the addition of catalysts,the pyrolysis temperature decreased,and the weight loss and maximum weight loss rate increased.However,with excess addition of catalysts,the pyrolysis of coal was limited.Hence,there is an optimum potassium content.?2?The characteristics of pyrolysis products were evaluated by loading coal and catalysts into an aluminum steamer low-temperature carbonization furnace.Then the coal samples containing 1.5% catalyst were investigated.The results show that the three catalysts clearly affected the pyrolysis of three coals,achieving the directed conversion of coal.The alkali catalyst KCl had the most effect on the yield of tar.CaO had the most effect on the yield of gas.The transition-metal catalyst Fe₂O₃ had the most effect on the yield of char.?3?The gas composition was analyzed using a GC system,and the mechanism of the catalytic pyrolysis process was elucidated.With the addition of additives,the yields of H₂,CO,CO₂,and CH₄ for HL coal,HG coal,and DT coal increased.It can be concluded that the addition of additives can promote the condensation reaction,cracking of hydrocarbons,decomposition of carbonyl and carboxyl groups,and fracture of the side chains.?4?The mechanism of the catalytic pyrolysis was analyzed by kinetics.A dynamic model of pyrolysis was established to calculate the E and A for a certain composition.The parameters of low-rank coal were calculated using the Coats-Redfern method,and the pyrolysis mechanism for three coals was elucidated.The fitting coefficient was among 0.9.Therefore,the Coats-Redfern method was fitted to the catalytic pyrolysis of the three coals.The results show that with the addition of additives,the activation energy of HL coal decreased,temperature range of pyrolysis decreased,and the reactions made easier.At a low temperature,the activation energy of HG coal decreased from 28.46 k J/mol to 15.04 k J/mol,the activation energy decreased,and the extent of reactions increased.The pyrolysis of DT coal can be divided into two regions: 461–700 ? and 700–760 ?.With the addition of catalysts,the temperature range clearly decreased.This indicates that the catalyst promoted the pyrolysis of DT coal,and the coal pyrolysis progressed easily.