Fundamental Studies On Cascade Conversion Of Coal Combined Pyrolysis And Gasification

Developing the clean coal conversion technology is the main route for adjusting the energy structure and addressing the environmental problems caused by extensive ultilization.Cascade conversion of coal combined pyrolysis and gasification is the type of large-scale clean coal conversion technology.This technology combines pyrolysis and gasification processes in one fluidized bed reactor and uses pre-heated air and superheated steam as the fluidizing agent.The technology has many unique advantages such as,high CH₄ content in syngas,high carbon conversion,low O₂ consumption and investigation because of its special structure and gasification principle.However,this technology is now in the research and development stage,and revealing the related scientific issues is the foundation to technology design and development.Base on this,the paper selected five coals as research subject.Aiming the research methodology of coal structure and reactivity based on coal chemical group;pyrolysis behavior and volatile release of coal and groups;and effects of temperature,gasification way,agent type,and inherent minerals on the char gasification behavior,which were seen as the three most important levels to this technology.The obtained results are in the following:1.Separation system and method for coal chemical groupsSequential extraction of coals with cyclohexanone and NMP/CS₂ achieved the relative extraction yield in the range of 81–144%,which was much higher than the basis of lowest relative extraction yield and realized the coal solubilization.By referring to the standard method,a method for separating heptane soluble component of solvent soluble fraction was proposed that using heptane,toluene,ethyl acetate,and methanol,respectively as eluents in the silica-based column chromatography.The recovery could achieve 90%.Therefore,the solvent soluble fraction of coal was first extracted with CCl₄ and heptane,respectively,and then the heptane soluble component was leached with the above proposed method,coal could be successfully separated into saturate,aromatic,light resin,heavy resin,asphaltene,carbene,and carboid.2.Structure and composition of the groupsSaturate was mainly composed of alkanes and isoalkanes.Light arenes distributed in aromatic,light and heavy resins,during which the less substituted and condensed arenes were mainly in aromatic.Carbon numbers of these arenes were in C₁₁?C₂₂.Isoalkanes,ketones,ethers,carboxylic acid,and N-containing species were rich in light and heavy resins.Furthermore,light and heavy resins contained less amounts of aromatic structures,but more alkene structures.The compounds in heavy resin contained at least two O atoms.Carbene has higher aromaticity,but lower aromatic cluster size compared with asphaltene.In addition,Carbene has shorter substituted alkyls,but more substituents compared with asphaltene.Aromatic cluster size of carboid was similar to the raw coal,but the former had longer substituents.3.Pyrolysis behavior and kinetics of Yichun coal and its groupsCarboid contained three mass loss stages,light resin and asphaltene contained two mass loss stages,while saturate,aromatic,heavy resin,and carbene only contained one broad mass loss stage.Carboid could produce the most cokes,followed by carbene,while saturate almost did not generate coke during pyrolysis.It could give a good fitting effect that using pyrolysis rates of seven groups to construct that of the raw coal,however,contribution coefficients of saturate,aromatic and carbene were negative due to the reciprocal effect.Saturate mainly contributed to formation of CH₄,C₂H_4,and C²⁺aliphatics,while aromatic could generate more CO and arenes.CO₂ was from aromatic,light and heavy resins.Average bond energy of the compounds in the groups was according to carboid>carbene>asphaltene>heavy resin>saturate>aromatic>light resin.T_in,T_f,and T_maxax showed log increment distribution with heating rates,while R_max and D_i exhibited monotone increase with heating rates.Furthermore,heating rate could alter the reaction sequence of the bonds in the compounds,but the activation energy had no relationship with heating rate.4.Volatile composition during fast pyrolysis of Yichun coal and its groupsAlkanes produced largest amount at 600 ^oC and distributed in C₂–C₃₂.Contents of alkenes and arenes increased with rising the pyrolysis temperature,among which alkenes distributed in C₃–C₂₃,and arenes distributed in C₆–C₁₈.Phenols produced largest amount at500 ^oC and distributed in C₆–C₁₁.Increase degree of volatile content became faint after 5s duration time.With increasing the duration time,contents of both arenes and phenols in various carbon number increased.Volatile content of Yichun coal increased with decreasing the heating rate.Contents of alkenes and phenols decreased with enhancing the heating rate,while ethers and N-containing species showed increment trend.Alkanes and alkenes mainly produced from saturate and accounted for 32.54%and 59.20%,respectively.Arenes generated mainly from aromatic and its content was 15.5%,which was highest among the groups.Light and heavy resins produced little amount of alkanes,but light resin could generate more amount of alkanes,arenes,phenols,ethers,and ketones than that of the heavy resin.Relative contents of alkanes and alkene were least in the products of both asphaltene and carbene,but these two groups produced largest amount of phenols,which accounted for4.5%and 11.5%,respectively.5.Gasification behavior and kinetics of charTemperature could greatly promote the gasification rate and reduce the characteristic time of t_0.5 and t_0.95.The gasification rate decreased largely and values of t_0.5 and t_0.95increased after removing of the inherent minerals.The activation energes of Chifeng and Yichun coals during isothermal gasification with steam were 152.4 and 146.0 kJ/mol,respectively,however they increased to 169.7 and 161.7 kJ/mol,respectively after removing of the inherent minerals.The steam gasification rate increased greatly in the presence of O₂and the activation energy showed exponent decrease with the O₂ concentration.The gasification rate has a great connection with the pore structure and maturity of the carbon crystallite.The steam gasification rates decreased with increasing of the CO₂ content for the coal chars with rich pore structure,such as Chifeng,Yichun,and Hami,while it showed the opposite trend for Jining coal char due to lack of pore structures.As the heating rate increased from 5 to 1000 ^oC/min in char preparation,the reaction rate gradually increased during non-isothermal gasification with CO₂.In addition,the gasification temperature shifted to higher values,but the reactivity increased when the preparation temperature increased from 480 to 560 ^oC in the drop tube reactor.