Research on the Key Issue of Coal Partial Gasification Technology

Author:Ye Chao

Supervisor:wang qin hui zuo ke fa luo zhong zuo


Degree Year:2018





China’s primary energy is mainly coal at present and for a long time to come.However,faced with the increasing energy demand and the increasingly sever air pollution situation,how to realize the clean and efficient use of coal has become an urgent issue.As a highly efficient energy conversion technology,the cascading utilization of coal has been paid more and more attention.According to the different properties of different components in coal,the combination of pyrolysis,gasification and combustion can be used to realize the cascading utilization of coal resource and improve the utilization efficiency of coal.It is one of the solutions to achieve clean and efficient utilization of coal in China.In view of the problem of single utilization of coal and low efficieny,Zhejiang University has put forward the technology of dual fluidized bed of coal partial gasification and char combustion,which combines CFB coal gasification and CFB char combustion.In the process,the easily converted part of coal is gasified into the syngas,the syngas can be used for gas turbine power generation or chemical materials.The char is used for power generation by combustion.First of all,the partial gasification characteristics of typical coal char are studied on the modified thermal gravimetric analyzer.The experimental results of kinetic characteristics of gasification reaction in CO2,H2O and the mixture of CO2 and H2O.The Homogeneous reaction model and Langmuir-Hinshelwood(L-H)model are adopted to predict the reaction rate of char gasification,and the effects of temperature and reactant gas on the gasification rate are studied.In the meantime,the common active sites machanism and the separate active sites mechanism are verified.The results show that when the temperature is less than 900℃,the separated active sites mechanism accord to the experimental results,and when the temperature is more than 900℃,the common active sites mechanism accord with the experimental results.According to the test results of specific surface area of different carbon conversion,the reaction rate first increases and then decreases.There is a maximum value during the char gasification,and the specific surface area has important influence on the reaction rate.Secondly,the coal partial gasification experiments of typical are carried out on the small-scale CFB.And the O2/steam and O2/CO2 are adopted as the reactant gas,respectively.The effects of oxygen-coal ratio,steam-coal ratio and CO2-coal ratio on the gasfication characteristics are researched.The results show that the combustible gas composition,lower heat value of gas,char yield and tar yield decrease with the increase of gasification temperature.All of the parameters above increase with the increase of steam-coal ration,except the gasification share.When in the O2/steam atmosphere,the maximum percentage of combustible composition woul reach almost 70%.At this time,the oxygen-coal ratio and steam-coal ratio are 0.46 and 0.31,respectively.The Raman Spectrometer and Fourier Infrared Analyzer are adopted to obtain the structure of char.The results show that a significant reduction in carbon structure occurs as the gasification temperature increases.And the degree of graphitization has a overall improvement.While in the O2/CO2 atmosphere,the combustible gas composition first increases and then decreases.When the combustible gas composition reaches the maximum value which is 40%,when the oxygen-coal ratio is 0.57.By using the Scanning Electron Microscope and Raman Spectrometer,the mechanism of char gasification is obtained.It shows that the pore inside the char opens gradually as the reactant gas enters the char.The formation of O-contained functional group and aromatic rings causes the char to become increasingly disordered.The coal-air partial gasification experiments are conducted on the 1MWt dual CFB facility,of which the gasification temperature are 790℃,850℃ and 920℃.Each run lasts 3 hours.The experimental system operates stably under various operating conditions and is easy to adjust.The gasifier and combustor can operate normally and harmoniously.The coal is gasified with air in the gasifier and the syngas and tar are produced.The char is combusted completely in combustor in stable condition.The experimental results show that with air as gasification medium can gain high yield gas during the partial gasification.With the improvement of gasification temperature,the gas volume flowrate per kilogram is increasing accordingly,while the gas production of 920℃ run is the highest.Under the three operating conditions,the obtained tar yield is very low,especially at the 920℃ run.The char is combusted stably and reliably in the combustor which comes from the gasifer.According to the ash accumulation on the condensed tube,there is no ash accumulation in each run,which can greatly reduce the pollution of ash to the heating surface.Thirdly,the process simulation of coal partial gasification cascading conversion system is carried out on Aspen Plus software.The atmospheric coal partial gasification combined cycle power generation system(APGCC),the pressured coal partial gasification combied cycle power generation system(PPGCC),and Poly-generation of methanol and electricity based on coal partial gasification are established,of which the coal feed is 4320 kg/h.The technical scheme is as follows.Coal is firstly gasified in the CFB gasifier with O2/H2O to produce syngas.The syngas is sent to the gas turbine for power generation of rmethanol syrnthesis after purification.The ungasified char is sent to the CFB combustor for combustion to obtain high temperature and high pressure stealm,which is supercritical parameter.And finally the steam is used for steam turbine power generation.Through the simulation,the influence of gasification pressure,gasification share,steam coal ratio,oxygen coal ratio and methanol synthesis temperature on system efficiency and equipment investment performance are obtained.It is compared with Integrated Gasification Combined Cycle Generation system(IGCC).The results show that with the increase of gasification share,the power generation efficiency increases and then decreases,when the carbon conversion is around 80%,the system efficiency is the highest.While the system efficiency of PPGCC is the highest,reaches 55.96%,and the system efficiency ofAPGCC is 54.43%.Both of them are higher than IGCC(53.11%).Witih the higher temperature of methanol synthesis.the methanol yield and system efficiency decreases.With the higher percentage of cyclic exhaust gas,methanol yield increases and the system efficiency decreases.When the methanol synthesis temperature is 220℃ and cyclic exhaust gas ratio of 0.5,the methanol yield is 76220kg/h,and the system efficiency is 56.8%,respectively.In the aspect of economic performance,the Internal Rate of Return(IRR)of APGCC is 15.2%,far higher than the 3.4%of IGCC.The Dynamic Payback Period(DPP)is 7.24 years,far less than 18.06 years of IGCC.The IRR of the ploy-generation system is as high as 24.1%,and the DPP is only 4.14 years.All of the system based on coal partial gasification with high efficiency,also have greater advantages in economic aspenct,which is a worthy of promotion and utilization of coalFinally,the coal air partial gasification combined cycle system and co-production of gas,ecletricity and heat system based on coal air partial gasification are established by using the Aspen Plus,of which the coal capacity is 4320t/h.The technical scheme of combined cycle system is as follows.The coal is gasified in the gasifier with air in the CFB gasifier to produce gas.After purification,it is fed to the low-calorific gas turbine for power generation.The ungasified char is sent to the CFB combustor for high parameter steam production.The technical scheme of co-production system is as follows.The gas from the gasifier is supplied to the industrial users directly after purification.The ungasified char is used for power generation.Through the simulation,effects of the air-coal ratio,carbon conversion,and gas calorific value efficiency on the system efficiency are obtained.Two economic performance of the system are analyzed at the same time,such as IRR and DPP.By thermaldynamic analysis,the system efficiency of combined cycle system is 53.56%,while the co-production system is 69.19%.Both of the systems based on coal partial gasification are higher than IGCC and the existing conventional pulverized coal fire power plant.By the economic analysis,the IRR of the combined cycle is 17.1%,and the DPP is 5.79 years.While the IRR of co-production is 30.2%and the DPP is 3.29 years.According to the calculation results,the two systems have strong risk resistance capacity and broad market application prospects.