Experimental Study On Gasification Of Coal Preheated In Circulating Fluidized Bed

Coal is the important basic energy and chemical material in China.Coal accounts for about 94.22%of China's total primary energy resources,and oil and natural gas account for less than 6%.Gasification is an important technology for the clean and efficient utilization of coal and is the front-end technology in coal to gas,coal to liquid fuel and coal to chemical products.As an important part of gasification technologies,the fluidized bed gasification is characterized by uniform temperature distributions and broad fuel adaptability.However,its application is limited by the low carbon conversion because the fluidized bed gasifier has to be operated below 1000 ? to ensure the fluidization of particles in the reactor.The entrained bed gasification is characterized by an extremely high operating temperature though the grindability and slurry character of coal are strictly required.The high operating temperature contributes to an extremely high carbon conversion.On the basis of characters of fluidized bed gasification and entrained bed gasification,the melting gasification of coal preheated in circulating fluidized bed(CFB)is proposed by Institute of Engineering Thermophysics,Chinese Academy of Sciences.In this technology,the gasification is divided into preheating and melting gasification.Requirements of coal ranks and particle sizes are reduced by using a CFB and the gasification efficiency is improved with the advantage of an entrained bed.As a preliminary study of the technology,gasification of coal preheated in CFB(dry slagging)was investigated.A CFB reactor is used as the preheater and a down flow bed reactor is used as the gasifier.An exploration study was carried out on a rebuilt 0.15 t/d test-rig to verify the feasibility of the process.Then a 15 kg/h test-rig was designed and built according to the design gist obtained from the exploration study.Characteristics of the preheating process were investigated on the test-rig.Then the influences of preheater operations and gasifier operations on the process property and the fuel adaptability of the process were studied experimentally.Furthermore,the equilibrium model of gasification of coal preheated in CFB was established and methods to enhance gasification reactions were predicted through the simulation.The main conclusions are as follows:(1)In the exploration study,stable operation was obtained in the preheater at 900 ? and in the gasifier at 1100 ?.When coal was converted into preheated char in the preheater,the gasification reactivity increased due to the improvement of specific surface area,total pore volume and intrinsic reactivity.Effective gasification of the preheated char was achieved in the gasifier as the gasifier temperature was significantly elevated above the preheating temperature,resulting in increases in the CO+H2 yield and carbon conversion.(2)The calculation method of the process design and structural design were established for gasification of coal preheated in CFB.According to the method,a 15 kg/h test-rig was completed.Smooth operations using air,oxygen enriched air-steam and oxygen-steam as the gasifying agent were realized.(3)The improvement of gasification reactivity in preheated char was obtained in the preheating process by breaking bridged bonds between macromolecular compounds and increasing disordered active carbon structure in coal.Due to that the disordered active carbon structure can be both contributed and consumed by the steam gasification reaction,900 ? was the best preheating temperature and 0.87 kg/kg was the best preheating steam-to-coal ratio to increase the gasification reactivity of the preheated char in O2-steam preheating,and 50%was the best O2 concentration in the primary gasifying agent in O2 enriched air-steam preheating.When the particle size of coal increased from 0-0.5 mm to 0-1.0 mm,the improvement of gasification reactivity of preheated char was weaker because diffusion rates of gases in particles were slowed.(4)The preheated char was gasified via the steam gasification reaction and the combustion reaction which generated CO in the gasifier.When air was replaced by O2-steam in the primary gasifying agent or the preheating temperature increased from 692 ? to 879 ?,the gasification reactivity of preheated char and the carbon conversion of the system were improved.Higher carbon conversion of the system was obtained at the steam-to-coal ratio of 0.81 because higher gasification reactivity of preheated char was achieved.The steam gasification reaction was enhanced by the increased gasifier temperature and the combustion reaction was enhanced by the increased O2 concentration in the secondary gasifying agent and the increased oxygen-to-coal ratio.The carbon conversion of the system was improved by the enhancement of both reactions.When the preheater product was inj ected into the top of the gasifier tangentially or axially,the average temperature of the gasifier was improved by staging the secondary oxygen,so that the carbon conversion and cold gas efficiency of the system were improved.Effective gasification of Shenmu bituminite,Shenmu semicoke and Suqian coal gasified fine char were achieved in this technology so a good fuel adaptability was verified.(5)The equilibrium model of gasification of coal preheated in CFB was established and verified by experimental results.When the gasifier temperature was adjusted from 1200 ? to 1350 ? and the system carbon conversion was adjusted from 70%to 99%by changing the oxygen-to-coal ratio and heat dissipation,CO2 yield was only influenced by the heat dissipation.When the system carbon conversion and gasifier temperature were improved by increasing the oxygen-to-coal ratio at constant heat dissipation,CO yield and H2 yield were improved by the enhancement of steam gasification reaction and the combustion reaction that generates CO.The enhancement of the combustion reaction showed first.When the system carbon conversion and gasifier temperature were improved by decreasing the heat dissipation at a constant oxygen-to-coal ratio,CO yield and H2 yield were improved by the enhancement of steam gasification reaction and Boudouard reaction,and the enhancement of Boudouard reaction showed first.