Study On The Caking-increasing Characteristics And Hydrogen Transfer Regular During Lignite Modified In Subcritical H₂O-CO System

China is rich in low rank coal resources,but coking coal resources are short.If low rank coal can be modified into caking coal and used as coking coal blending,it will play an important role in expanding coking coal resources and realizing efficient utilization of low rank coal resources such as lignite.In this paper,caking-increasing characteristics and hydrogen transfer regular during lignite modified in subcritical H₂O-CO system were studied,and the effects of catalyst type,catalyst addition,reaction temperature,water coal ratio,constant temperature time,initial CO pressure and other process conditions on caking index,group component yield,and the generation and consumption of active hydrogen from subcritical water gas shift are investigated.The structural evolution of caking components in modified coal and the distribution and occurrence of active hydrogen in group components were studied by Soxhlet extraction,hydrogen isotope tracing,FTIR,IRMS and ¹H-NMR.The main research conclusions are as follows.（1）The effect of each catalyst on caking-increasing was as follows:basic Na/Ca based catalyst>neutral Na/Ca based catalyst>Fe based catalyst;The amount of Na₂CO₃ added,the CO conversion(X_CO)and the production of active hydrogen（9）)were increased.The consumption of active hydrogen increased（9）)from 0 wt%to 0.8 wt%,and then remained unchanged;Increasing the initial pressure of CO,X_CO、9)、9)increases first and then decreases,and reaches the maximum at 4.5 MPa of CO initial pressure;X_CO、9)、9)increases with the raise of constant temperature time,However,the utilization rate of active hydrogen increased continuously within 0～60 min of constant temperature,and decreased continuously after 60 min.More active hydrogen H*did not participate in the modification reaction,but produced H₂,which increased the content of H₂ in the gas component Increasing water coal ratio,X_CO、9)、9)were increasing;Increasing the reaction temperature,X_CO、9)is increasing,while 9)increases from 280℃to 320℃,and tends to be flat when it is higher than 320℃.The optimized process conditions were as follows:catalyst additive amount 1.25 wt%,constant temperature time 60 min,water coal ratio 1.25:1,reaction temperature 340℃,CO initial pressure 4.5 MPa,the caking index of modified lignite reached 97.（2）Na₂CO₃ can improve the utilization of active hydrogen by 30%by promoting the depolymerization of lignite macromolecule structure and hydrolysis of ether bond and carbonyl group（especially ether bond）;Increasing the initial pressure of CO will promote the forward transformation of subcritical water gas shift reaction,making the hydrogen partial pressure rising in the reactor,which is conducive to the diffusion of active hydrogen into the coal molecular structure,so as to stabilize the free radicals generated by coal cracking and generate more cohesive substances.The longer the constant temperature time is beneficial to the water gas transformation reaction,and provides a large amount of hydrogen source for the modified reaction;Increasing the ratio of water to coal can increase the free radical dispersion and hydrogen supply capacity of the system,which provides more opportunities for the reaction of the two;The appropriate reaction temperature can improve the water gas ratio,increase the equilibrium conversion rate and reaction speed of WGSR,match with the free radical generation rate,and facilitate the hydrogenation of lignite to increase caking.（3）The caking index(G_RI)of modified coal decreased by 2.6～6.1%,13.3～30.3%and66.4～89.8%respectively after the removal of N-hexane soluble（NS）,benzene soluble（BS）and tetrahydrofuran soluble（TS）.G_RI increased rapidly with the increase of total BS+TS,and increased slowly after the total amount reached 26.5%(G_RI=88.9).When the reaction temperature is 280～320℃,the BS formation rate is the fastest,with an increase of 122%.The content of long chain aliphatic group increased,the degree of branching decreased,and the caking of BS was weak.However,BS has the greatest contribution to the caking property of modified coal at this stage,which is attributed to the rapid increase of its production;When the reaction temperature was between 320℃and 350℃,the BS production continued to increase,while the TS production remained constant.However,the caking of TS increases with the rapid increase of hydrogenation degree,hydrogen bond association,condensation degree and branching degree of aliphatic hydrocarbon.In this stage,TS has the greatest contribution to the caking property of the modified coal,which is attributed to the enhancement of viscosity due to the change of molecular structure.（4）The yield of n-hexane soluble（NS）,benzene soluble（BS）and tetrahydrofuran soluble（TS）in the modified coal increased by 1,4 and 9 times respectively by increasing the conversion degree of coal cracking,stabilizing the macromolecular free radicals and inhibiting the further cracking of coal by the active hydrogen provided by subcritical water gas shift reaction;There is a good linear correlation between the amount of active hydrogen and the reaction temperature.Among the consumed active hydrogen,the proportion of active hydrogen added to coal first increases and then decreases with the increase of reaction temperature,reaching the maximum value of 97%at 340℃;The results of tracing active hydrogen with isotope deuterium show that with the increase of reaction temperature,the total deuterium content in NS decreases and then increases,while the total deuterium content in BS and TS increases continuously,reaching the maximum at 340℃,and then decreases with increasing temperature.Active deuterium enters into the H_αof NS and BS by binding with free radicals which formed by the cleavage of aromatic carbon.The addition amount increases with the increase of reaction temperature;And active deuterium enters into the H_βof TS by binding with free radicals which formed by breaking the side chain of substituted alkanes in the aromatic ring of TS.However,the addition amount does not change with the reaction temperature.Deuterium was added to the benzene ring of the three soluble substances,but the amount of deuterium was NS<BS<TS.