Experiment And Numerical Simulation Of Oxy-fuel Co-Combustion Of Coal/Biomass In A Pressurized Fluidized Bed

Fluidized bed oxy-fuel combustion is the representative of solid fuel non-air combustion technology,and also an important technical route for CO2 capture in the new generation of combustion.Among them,the fluidized bed oxy-fuel combustion of coal and biomass mixed fuel,which integrates the absorption of CO2 during biomass growth process and the capture of CO2 by oxy-fuel combustion,is known as a novel clean combustion technology that is expected to achieve negative CO2 emissions,and has received important attention in the field of multiphase combustion science and technology.So far,there have been many practices on atmospheric oxy-fuel combustion of coal/biomass in fluidized beds.However,the combustion under pressurized conditions is still at the initial exploration stage,the complex combustion laws are not fully mastered,and the operating experience of pressurized fluidized bed is quite lacking.Therefore,this paper attempts a new research exploration-co-firing of coal and biomass under pressurized oxy-fuel combustion mode in a fluidized bed,which couples pressurized fluidized bed and oxy-fuel co-combustion technology.By constructing a pressurized test device,grasping the characteristics of combustion process,explaining the rules of pollutant formation and transformation,and developing a three-dimensional numerical simulation method,it provides useful basic theory,research methods,and data support for the development of pressurized fluidized bed oxy-fuel combustion technology to capture CO2.First,a 10 kWth pressurized fluidized bed coal/biomass oxy-fuel co-combustion test system is constructed,and the dynamic characteristics of the start-up,combustion mode switching,and fuel switching processes under different pressures are mastered.The temperature distribution of fluidized bed,CO2 enrichment in flue gas,incomplete combustion products(including combustible gas and solid unburnt carbon),changes in bed materials before and after operation,the surface morphology,particle size distribution and pore structure of ash(including bottom ash and fly ash)are recorded and analyzed in detail.The effects of key operating parameters such as combustion pressure,biomass blending ratio,excess oxygen coefficient on combustion characteristics,fuel carbon conversion,and fluidized bed combustion efficiency are revealed,and the range of operating parameters that is conducive to achieving higher pressurized oxy-fuel combustion performance is obtained.Further,through the joint analysis of gas-phase and solid-phase products,the effects of key operating parameters and fuel properties on the emission patterns and transformation mechanisms of nitrogen and sulfur pollutants during the pressurized fluidized bed oxy-fuel combustion process are explained,including pollutant emissions,fuel nitrogen/sulfur release and conversion,nitrogen/sulfur species distribution in flue gas and ash,mechanism and efficiency of self-desulfurization process,and synergistic,common or opposite effects between these parameters.The properties,chemical composition,mineral transformation of solid ash are analyzed,and the slagging,fouling,agglomeration propensities and relevant ash chemistry mechanisms under pressurized co-firing conditions are evaluated.On this basis,a three-dimensional mathematical model suitable for pressurized oxy-fuel co-combustion of coal/biomass in fluidized beds is constructed.By adopting different chemical reaction models for different fuel,using the refined JL four-step mechanism to modify the COCO2 homogeneous reaction subset,improving the heterogeneous reaction model of char nitrogen,adding char nitrogen to generate NO and N2O equations,and considering the selfdesulfurization effect caused by CaO in fuel,the accurate simulation of the two fuel particle system under Eulerian-Lagrangian framework is successfully achieved.A series of numerical simulations of coal/biomass pressurized oxy-fuel co-combustion are carried out,and the particle motion characteristics,gas-solid flow structure,and combustion parameter field distribution and evolution in the fluidized bed that are difficult to understand by experimental methods are mastered.Compared with previous studies,this paper takes the lead in successfully practicing the pressurized oxy-fuel co-firing of coal/biomass on a 10 kWth fluidized bed,and comprehensively reveals the characteristics of pressurized oxy-fuel fluidized co-combustion,CO2 enrichment,gas/solid pollutant emission laws and transformation mechanisms,which enrich the knowledge system of fluidized bed pressurized oxy-fuel combustion.In addition,the numerical simulation method of multi-fuel particle reaction system under the Eulerian-Lagrangian framework has been significantly developed,which makes up for the shortcomings of experiments,enriches the research methods,provides the basis for the future research on reactor structure and parameter optimization by numerical simulation,and also lays the foundation for the further development of simulation of dense multiphase reaction systems with more than three fuels.