Experimental Study On Process Intensification Of Circulating Fluidized Bed Coal Gasification

Circulating fluidized bed(CFB)gasifiers operate at a mild temperature,which means that the operation temperature is limited by ash fusion characteristics.Furthermore,the decay of temperature and solid concentration along the riser leads to the decrease of gasification reaction rate and a relatively low carbon conversion.To optimize the temperature field and gas-solid flow field inside the riser and promote gasification reaction,theoretical analysis and experimental study were carried out in this paper.First,based on industrial practice and theoretical analysis,the methods of gasification agent staging and the riser equipped with enlarged top structure were proposed.Then,the cold model test rig was designed and cold model test was carried out to study the influences of the methods on the operation of CFB system and gas-solid flow behavior inside the riser.According to the results of cold model test,a CFB gasifier was designed and built.The study on gasification agent staging was firstly carried out.Based on experimental results,the intensification effects of gasification agent staging on gasification process were proved,the matching between mass and heat transport around the nozzle was analyzed and the intensification mechanism and its realization conditions were determined.After that,the gasification experiments on the gasifier equipped with enlarged top structure was implemented to analyze the effects of operating and structure parameters on gasification process.The main conclusions are as follows:(1)In the cold model test,the influence of jet on main gas-solid flow is characterized by the competition between the re-entrainment of solid near the wall and the cut-off effect on upward particles in the core region.The re-entrainment phenomenon prevents solid near the wall from back mixing into the dense phase zone and realizes the transport of solid from wall to the core region,improving the uniformity of axial solid holdup profile.Under gasification agent staging conditions,the increase of pressure drop of the cyclone may cause a failure of the pressure balance of the circulation loop.When the nozzle height is 0.1 of the riser height,the CFB system runs stably.The improvement of riser by enlarging the top diameter promotes the back-mixing of solid,and solid amount and solid concentration in the riser increase;it also decreases the inlet solid concentration and the pressure drop of the cyclone,improving the stability of the system under gasification agent staging condition.(2)The re-entrainment phenomenon is realized during gasification experiments.It strengthens the heat and mass transport and helps to increase the temperature and solid concentration in the dilute phase zone,which are beneficial for char gasification reaction.At the same coal feeding rate and oxygen to coal ratio,the gasification performance of the gasifier increased with increasing secondary gasification agent ratio(SGAR).For Shenmu coal,compared with the condition without staging,the cold gas efficiency increased by 14.8%,the carbon conversion increased by 14.3%and the gas yield increased by 7%when the SGAR was 30%.The gasification agent staging can also promote the gasification of low-reactivity coal.The intensification effect of gasification agent staging is limited by the oxygen concentration of the secondary gasification agent and solid concentration around the nozzle.With increasing oxygen concentration,the combustion rate of char increased and the combustible material was excessively consumed,leading to the decrease of char reactivity and the restriction of intensification effect.Under experimental conditions,when the oxygen concentration of the secondary gasification agent was 45%,the gasifier had the best gasification performance.When the oxygen concentration reached 65%,it caused slagging.The intensification effect was strengthened by increasing the secondary gasification agent flow rate until the cut-off effect occurred when the jet velocity reached 30 m/s.The increase of nozzle number can strengthen re-entrainment of solid which is beneficial for intensification effect.(3)The solid concentration increases and temperature decreases inside the riser after enlarging the diameter of the top structure of the riser.The intensification effect on gasification process is mainly reflected in promoting the steam decomposition reaction and increasing H2 content in gas.The intensification effect on cold gas efficiency gradually disappears with increasing oxygen to coal ratio.The top structure with enlarged diameter has limited effects on the gasification process of low-reactivity coal.For the CFB gasifier equipped with enlarged top structure,with the increase of oxygen to coal ratio,the carbon conversion,cold gas efficiency and gas yield gradually increase,and the heat value of gas first increases and then decreases.With the increase of steam to coal ratio,the carbon conversion,cold gas efficiency and gas yield first increase and then basically remain unchanged,and the heat value of gas gradually decreases.(4)During the oxygen-enriched gasification of Shenmu coal,the tar content in gas is in the range of 11.7-73.3 mg/m3 under experimental conditions and it is sensitive to operation conditions.With increasing oxygen to coal ratio,tar content decreases and the tar compositions become heavy.With increasing steam to coal ratio,tar content decreases and the tar compositions first become light and then become heavy when steam to coal ratio reaches 0.31 kg/kg.Gasification agent staging can promote the conversion of tar.When the SGAR was 30%,the tar content decreased by 54.6%.(5)The carbon content distribution with particle size of bottom ash obtained from an industrial CFB gasifier appears as a single peak.The high yield and high carbon content of particles with about 2 mm in diameter are responsible for the high carbon content in bottom ash.Compared with raw coal and fly ash,the combustion reactivity of bottom ash is low.Its ignition type is heterogeneous and the ignition and burn-out temperatures are 599? and 756?,respectively.Bottom ash particles with different sizes have the same combustion behavior and their combustion reactivity is mainly related to carbon content.With increasing oxygen concentration,the weight loss peak moves towards low-temperature region,the peak height becomes high and the peak width becomes narrower,indicating the improvement of the combustion process of bottom ash.