Fundamental Study On Fluidized Bed Two-Stage Gasification Of Low-Rank Crushed Coal

China has a huge demand for industrial gas and the annual gas production is up to 756 billion cubic meters.At present,single and two-stage fixed bed gasifier are mainly adopted in the domestic industrial gas technology,whose main technical limitations lie in limited capacity,poor feasibility for crushed coal,and high tar within the produced gas.The gas purification process is easy to produce phenol-containing water that cause secondary pollution to the environment severely.The entrained flow bed gasifier is rarely employed due to higher operating temperature,pressure,cost and poor adaptability to coal particle with relatively wide particle size distribution.Fluidized bed gasification technologies,represented by bubbling fluidized bed and circulating fluidized bed gasification technologies,are more attractive and economical for fuel gas production due to the good heat and mass transfer effect,large processing intensity,easy amplification,suitability for handling small-particle-size materials.However,the above gasification technologies are still faced with the challenges of low carbon conversion,relative low gas calorific value and high tar content within the generated gas.Based on the huge demand for industrial gas and the shortcomings of traditional production technologies,a new type of fluidized bed two-stage gasification process for low rank coal was proposed by Institute of Process Engineering,Chinese Academy of Sciences to generate industrial gas with low tar content.The so-called fluidized bed two-stage gasification technology decouples the whole coal gasification process into two sub-processes,including the coal pyrolysis and char gasification,which are carried out in the fluidized bed(FB)pyrolyzer and transport fluidized bed(TFB)gasifier respectively.Firstly,coal is pyrolyzed autothermally and oxidized partially in the FB pyrolyzer.Then,all the products(pyrolysis gas,char and tar)from the FB are introduced into the TFB gasifier to conduct char gasification and pyrolysis gas upgrading.In gasifier,by thermal cracking and especial the catalytic reforming from hot char bed layer,the tar will be removed as far as possible during passing through the hot char bed layer,thus realizing low-tar-containing industrial gas production.The key reaction parameters were systematically studied and the technological feasibility was verified by using the laboratory fluidized bed apparatus and the pilot-scale test platform respectively.The main research contents and results were summarized as follows:(1)A laboratory fluidized bed reactor was applied to investigate the pyrolysis behavior of Xiaolongtan lignite under different conditions,including reaction temperature,moisture content and reaction atmosphere.The pyrolysis products distribution,char gasification activity,gas and tar composition were analyzed and discussed.Under the inert atmosphere,the devolatilization rate is accelerated resulting in the increased gas yield with raising the pyrolysis temperature,while the yield of char and tar decreased.With the volatile releasing,the specific surface area and gasification reactivity of char increased,but the higher pyrolysis temperature(900?)would cause the char lattice structure trended to be in order and gasification activity reduced.The active atmosphere(oxygen or water steam)could further improve the gas yield and the char gasification activity.The presence of oxygen leaded to an increase of the heavy tar component content and a decline in the overall tar yield,while a certain external steam atmosphere could promote the light tar content and reduce the overall yield of tar.The inherent moisture of the coal could increase the light tar component to some extent,but it also caused the volatile release incompletely,resulting in a low degree of coal pyrolysis.Thus,the coal inherent moisture content should be controlled within an appropriate range.With the same water-coal ratio,the external steam was more effective on increasing gas yield than the inherent moisture content.On account of the results of the fundamental experiments,the reaction temperature of 800?,excessive air ration of 0.14,external water-coal ratio of 0.11 and the inherent moisture-coal ratio lower than 0.11 were the optimum conditions for the upstream fluidized bed pyrolyzer in the fluidized bed two-stage gasification process.(2)To verify the technical feasibility and features of the fluidized bed two-stage gasification technology for clean fuel gas production,an autothermal pilot platform with a capacity of 100 kg/h had been designed and built up to test the gasification characteristics of Inner Mongolia Shengli lignite.The running state as well as the product quality were compared systematically under different typical operational conditions.Keeping the reaction temperatures of pyrolyzer and gasifier at around 840? and 1000? respectively,the corresponding tar content in the produced gas was 365 mg/Nm3 and the heating value was close to 1100 kcal/Nm3.Compared with the tar from pyrolyzer,the heavy oil fraction of tar from gasifier reduced significantly,while the light oil components increased sharply simultaneously,showing an effective tar thermal cracking and catalytic reforming in the high-temperature char bed of gasifier.