| As an alternative renewable energy,biomass has been the fourth energy resource after the coal,petroleum and natural gas.It can be thermally converted into biofuels,gases,and chemicals via combustion,gasification and pyrolysis.Among these routes,the gasification has been considered as one of the most effective ways to convert the energy embedded in the biomass as it can typically achieve superior efficiency and significantly reduce the pollutants generated in the combustion process.This paper adopts the Multiphase Particle-in-cell(MP-PIC)method suitable for gas-solid reaction,coupled with pyrolysis model,coke conversion model,homogeneous/heterogeneous reaction model,and other sub-models.These methods are used to study the gas-solid flow,heat transfer,and chemical reactions in several fluidized beds.The first part:the essay adopts the MP-PIC method to simulate the three-dimensional spouted bed,and studies the effect of the bed height and vapor/biomass and bed temperature and particle size on the gasification performance.Secondly,the effects of macroscopic gas-solid flow characteristics and particle transport characteristics(particle residence time,particle Reynolds number and slip velocity,particle temperature,particle flux,etc.)in the spouted bed are analysed.The research shows that biomass particles have a larger Reynolds number and slip velocity than the bed material in the three areas.The maximum Reynolds numbers of sand and biomass in the spout area are 1.32 and 9.47,and the maximum slip velocity is0.72m·s-1and1.17m·s-1.The thermal conductivity of the entire gas phase in the spout area,the annular gap boundary area,and the fountain area is 0.09(J·(s·m·k)-1)?0.12(J·(s·m·k)-1)?0.16(J·(s·m·k)-1).The average density of gas phase in spouting area,annulus area and fountain area is 0.21kg·m-3?0.196kg·m-3and 0.12kg·m-3,respectively.The average viscosity of the spouting area,the annulus area and the fountain area are3.97×10-5(kg·(m·s)-1)?3.68×10-5(kg·(m·s)-1)?3.34×10-5(kg·(m·s)-1).The average specific heat capacity values of the nozzle area,the annulus area and the fountain area are 1928.7(J·kg-1·k-1)?2356.8(J·kg-1·k-1)?2376.1(J·kg-1·k-1),respectively.The heat transfer coefficients of biomass and sand in the spout area,the annulus boundary area,and the fountain area are 102.4(W·(m2·k)-1)?75.2(W·(m2·k)-1)?101.7(W·(m2·k)-1),117.4(W·(m2·k)-1)?64.4(W·(m2·k)-1)?131.5(W·(m2·k)-1).The second part:via the full-loop three-dimensional numerical simulation of the laboratory-scale dual fluidized bed,the macroscopic gas-solid flow characteristics and particle movement characteristics of the dual fluidized bed are explored(particle Reynolds number and slip velocity,particle temperature,particle flux,etc.).The MP-PIC method is verified.The effects of biomass flow rate,water vapor/biomass ratio on gasification performance are studied.Subsequently,the influence of operating parameters on the heat transfer coefficient of the main components(e.g.,gasifier and combustion chamber)of the dual fluidized bed is studied.The study showed that:within the range allowed by the numerical simulation,the dry N2 free synthesis gas obtained at the outlet of the twin-body fluidized bed is composed of 37.5%H2?25.5%CO?21%CO2?and 16%CH4.The gas density of the full loop of the double-body fluidized bed ranges from 0.2kg·m-3 to 0.4 kg·m-3,and the gas density in the U-shaped tube is almost twice that in the standpipe.The HTC of the solid phase in the DFB gasifier ranges from 40 to 80 W·(m2·k)-1,and the HTC of the solid phase in the combustion chamber is 180 to 280W·(m2·k)-1.Biomass energy is an important part of my country's energy structure,and the development of biomass pyrolysis and gasification technology can improve the utilization of biomass resources,which is of great significance to the energy s tructure,reducing greenhouse gas emissions,and developing a green and low-carbon economy. |
PDF Full Text Request