Numerical Simulation Study Of Biomass Gasification Process In Bubbling Fluidized Bed Under The Condition Of Wide Sieve Particle Size

Biomass,as a renewable energy with great development potential,perfectly fits the development concept of "carbon neutral" in China.The bubbling fluidized bed has the characteristics of wide applicability of raw materials,high heat transfer coefficient and high gas-solid contact efficiency.It has been widely used in the area of biomass gasification,which has attracted great attention from academia and industry.However,the high coupling between the gas-solid flow in the reactor and the thermal gasification reaction has brought great challenges to the traditional experimental research.Numerical simulation method has become a trend with its powerful computing power.In this paper,the details of biomass gasification and gassolid flow in bubbling fluidized bed were studied by numerical simulation method,which provided new insights and ideas for subsequent industrial design and production.The Multiphase Particle In-Cell Method(MP-PIC)was used to establish the numerical model for the thermal gasification reaction based on the laboratory scale bubbling fluidized bed gasifier.The adaptability and accuracy of the model were verified.Based on this model,the Particle Size Distribution(PSD)in the fluidized bed reactor was used as the research variable to investigate the gasification performance and flow characteristics.The main work is summarized as follows:Based on the established model,the gasification performance,gas distribution and gas-solid mixing characteristics of fluidized bed gasifier were systematically investigated under the condition of wide particle size distribution.The results showed that:(1)Increasing the particle size distribution width can make the syngas total gas yield increased by 5%,low calorific value increased by 24%,the gasification efficiency increased by 15%,the carbon conversion rate increased by 4%,show that the increase of particle size distribution in the bed width has a promoting effect on gasification performance;(2)With the increase of the particle size distribution width,it can be observed that the gas temperature in the dense phase region decreases,while the gas temperature in the freeboard presents an opposite change;(3)The relation between the distribution of solid particles and gas shows that the combustible gas mainly exists in the dilute phase area of the reactor,while the oxygen mainly exists in the dense phase area.Based on the established solution model,the solid particles in the reactor were analyzed,and the separation characteristics of different types were discussed.The results show that :(1)The particle size distribution and chemical reactions lead to the obvious spatial separation of particles in the reactor,which leads to the uneven spatial distribution of temperature and heat transfer coefficient of particles;(2)With the increase of particle size distribution width,the total residence time of biomass particles is prolonged by 2s,the carbon content in biomass particles decreases by 5%,and the ash content increases by 5%,which proves that the wide particle size distribution has a promoting effect on gasification;(3)The change of particle properties indicated that the slip velocity,Reynolds number and temperature of biomass particles are smaller than sand particles.The increase of PSD width would lead to the decrease of related physical properties of the two particles.In order to explain the related phenomena in the bubbling bed,the dynamic and thermodynamic characteristics of the bubbles in the reactor were studied.The results show that:(1)With the increase of PSD width,the rising velocity of the bubbles decreases,but the length-width ratio along the axial direction increases,which proves that the bubbles in the reactor are slender rather than ideal spherical shape;(2)The analysis of the physical properties of bubbles shows that the pressure and density of bubbles continue to decrease along the axial direction.The temperature,thermal conductivity and specific heat capacity of the bubbles increase with enlarging the distribution width of particle size.In summary,the results provide a new perspective and insight for the study of biomass gasification under the condition of wide particle size distribution,provided a useful reference on the biomass gasification technology.