Study On Mechanism Of Serial Composite Process For Biomass And Coal Co-gasification

In this paper,a new process of biomass gasification is developed:serial composite process for biomass and coal co-gasification,in order to utilize biomass energy efficiently and economically.The process uses fluidized beds as gasifiers to operate intermittently.It means this process is divided into combustion stage and gasification stage.The two stages constitute a complete cycle gas production process.This process can solve many shortcomings of biomass in fluidized bed gasification process,such as stability of operation,tar,quality of gas produced,etc.It provides a new way to utilize biomass.A thermodynamic equilibrium model and a comprehensive mathematical model based on reaction kinetics,hydrodynamics,theory of heat and mass transfer are established for serial composite process for biomass and coal co-gasification.The accuracy of the model is verified by comparing the simulation results with the test results.In this paper,the two-zone temperature correlation of coal gasification zone and biomass gasification zone is proposed,and the thermodynamic equilibrium model of biomass and coal composite serial gasification process is established.Applying the two-zone temperature correlation to the thermodynamic equilibrium model can significantly improve the model simulation accuracy.The model studies the effects of gasification operating parameters on the composition and yield of gasification products,such as the gasification temperature,the ratio of biomass and coal and the ratio of steam and coke.This model also can study the optimization of the gasification operation parameters from the perspectives of energy utilization and carbon conversion.The thermodynamic equilibrium model is more suitable for engineering applications because it has higher prediction accuracy and less consumption of computing resources.In the comprehensive mathematical model of biomass and coal co-gasification,the hydrodynamics in the furnace is taken into account,and the gasifier is divided into a combustion sub-model and a gasification sub-model.These two sub-models are divided into dense phase and dilute phase to simulate.The dense phase of the gasification sub-model is divided into a coal gasification model and a biomass gasification sub-model.The dense phase is simulated by a three-phase bubble bed theory to distinguish the dense phase into bubble phase and bubble cloud phase and emulsion phase.This can consider the flow of gas and solid between the phases and the exchange of mass respectively.The dilute phase is simulated by Wen-Chen's entrainment elutriation model combined with the core-annulus model,and the flow of gas and solid is also considered.The pyrolysis model uses Merrick model to calculate,which is widely used.The temperature in the gasifier is calculated using the multi-zone temperature model proposed in this paper.The model includes multiple temperature correlations of different reaction zones based on measured parameters.The combustion reaction model and the gasification reaction model were established by using the kinetics equations of homogeneous and heterogeneous chemical reactions.The most important process is char gasification,which adopts the JM model.Finally,the above models are combined to establish a mass balance model and an energy balance model to complete the establishment of the entire mathematical model.By solving the model,the state parameters in the gasifier under any operating parameters of the gasifier can be obtained,including the composition of each substance at different gasification times and different gasifier heights.The main adjusting parameters of the gasifier are gasification temperature,S/B and B/C.The model can analyze the effects of these various adjusting parameters on the fluidization state and the gasification reaction in the gasifier.It can not only provide reference for the operation of the gasifier,but also play a guiding role in the optimization and design of the gasifier.Compared with the thermodynamic equilibrium model,the comprehensive mathematical model needs more computing resources,so it is more suitable to be applied in the field of scientific research and gasification process design.Through the study of this paper,in this process,with the increase of gasification temperature,the concentration of H₂ and CO increases,the concentration of CO₂decreases,and the concentration of CH₄ decreases slightly in the hydrogen-rich gas.With the increase of S/B,the concentration of H₂ changes a little,the concentration of CO decreases,and the concentration of CO₂ and CH₄ increases,while the steam in the hydrogen-rich gas increases with the increase of S/B.The choice of S/B and gasification temperature should be determined according the use of the gas produced,depending on whether the users need higher calorific value of the gases or higher concentration of H₂ to adjust the gasifier operating parameters.The increase of B/C not only can make the concentration of H₂ increase,but also can improve the capacity of biomass treatment.Therefore,the B/C value should be increased as much as possible.