Numerical Simulation And Experimental Verification Of High Temperature Gasification Of Biomass Using Enriched Air

The excessive consumption of fossil fuels has caused energy crisis and environmental problems.Biomass is considered to be the ideal alternative energy because of its richness,low pollution and renewability.Gasification is an important development direction for the utilization of biomass.At present,biomass gasification system has the problems of low gasification efficiency,low heating value of syngas and high tar content,which leads to poor economy of biomass gasification technology.In response to these series of problems,a two-stage high-temperature gasification technology using enriched air as gasifying agent is introduced.It is the combination of low-temperature gasification in the fluidized bed and high-temperature gasification in the melting furnace.In this paper,the numerical simulation of the two-stage gasification process was performed to obtain the biomass gasification characteristics.In this paper,a self-built two-stage gasification system was regarded as the research object to simulate the low temperature fluidized bed gasification and high temperature melting furnace gasification.In the modeling process,based on the Euler-Euler model,the k-s turbulence model was selected for the gas phase and the kinetic theory of granular flow was used for the solid phase.The Gidaspow model and Guun empirical formula were used to describe the gas-solid two-phase drag force and heat transfer,respectively.At the same time,the model of heterogeneous reaction and homogeneous reaction was established and the pyrolysis of tar was taken into consideration.All chemical reaction kinetic parameters suitable for biomass gasification were selected.Finally,a three-dimensional comprehensive calculation model of biomass oxygen-rich gasification was obtained.The numerical simulation results of the low temperature fluidized bed gasification reveal that the temperature was relatively uniform in the dense phase but it was low in the middle area of the dilute phase.At the bottom of the fluidized bed,the concentrations of CO,CO2,H2 and CH4 were very low.But the CO2 concentration decreased and the concentrations of CO and H2 increased along with the increase of the height.With the increase of the equivalent ratio,the gasification temperature increased.The concentrations of CO and H2 first increased and then decreased,the concentration of CO2 decreased gradually,and the CH4 concentration increased.The heating value of gas first increased up to the 6.4 MJ/Nm3 and then decreased.At the same time,the gas yield and carbon conversion rate increased,but they were significantly lower than those obtained from gasification under the high equivalent ratio.At low equivalence ratios,the carbon conversion was only 50%and the tar content exceeds 13 g/Nm3.With the increase of oxygen percentage,the concentrations of CO and CO2 increased rapidly,the concentration of H2 decreased slightly,and the concentration of CH4 remained almost unchanged.The heating value and tar content increased while the gas yield and carbon conversion rate decreased.With the increase of fluidizing velocity,the concentration of CO increased,the concentration of CO2 decreased,the concentration of H2 increased slightly,and the concentration of CH4 remained almost unchanged.The heating value,gas yield and carbon conversion ratio increased gradually,while the tar content first decreased and then increased.The numerical simulation results of high temperature melting furnace gasification show that the concentrations of CO,CO2,H2 and CH4 were low at the top of the melting furnace.With the decrease of height,the concentrations of CO and H2 increased,the concentration of CO2 decreased,and the concentration of CH4 increased slightly.Compared with the fluidized bed gasification,the gasification efficiency of the two-stage gasification was significantly improved.CO concentration increased 1.9 times,H2 concentration increased 2.2 times,and CO2 concentration decreased by 45%.The heating value and gas yield could be increased to 1.5 times and the tar content reduced to 1 g/Nm.The overall gasification efficiency and carbon conversion rate could increase to about 75%and 90%,respectively.With the increase of the total equivalent ratio,the concentrations of CO and CO2 increased slightly,the concentration of H2 decreased slightly,and the concentration of CH4 decreased.The heating value,tar content and gasification efficiency decreased gradually while the gas yield first increased and then decreased.The heating value was in the range of 9-11 MJ/Nm3 and the gas yield was above 1.1 Nm3/kg.As the total oxygen percentage increased,the concentrations of CO and H2 increased while the concentrations of CO2 and CH4 increased slightly.The heating value,gasification efficiency and tar content increased,but the gas yield decreased.On the whole,the total oxygen percentage of the two-stage gasification system was preferably in the range of 50%to 60%.The experimental results show that they were in good agreement with the simulation results,which could prove that the gasification model in this paper was reliable.