Movement And Combustion Characteristics Of Biomass Particles In The Pressurized Oxy-fuel Fluidized Bed

Oxy-fuel fluidized bed was employed since it represents a promising new carbon capture technology,which can realize negative CO₂ emission when burning biomass.Pressurized oxy-fuel combustion（POFC）,a second generation oxy-fuel combustion technology,permits the operation of the combustor under pressure,which can improve the net efficiency of system.With the change of pressure and atmosphere,the combustion characteristics of biomass in the fluidized bed will be changed.The combustion process of fuel particles in the fluidized bed is closely coupled with the movement behavior of particles.Therefore,combining the particle movement process with the combustion process and analyzing the influence of each parameter on the particle movement and combustion will be helpful to further understand the combustion reaction process of biomass particles in the furnace.A series of studies on the combustion and movement characteristics of rice hull particles in O₂/N₂,O₂/CO₂,and O₂/H₂O were carried out on the existing visualization pressurized fluidized bed experiment platform.The main results are as follows:When the combustion atmosphere changes from O₂/N₂ to O₂/CO₂,the ignition time of char surface is delayed and the ignition mode is changed.Meanwhile,the volatile flame temperature and the peak temperature of char surface are reduced,while the devolatilization time and particle burnout time are prolonged.When the concentration of O₂ is 10%,the volatile flame temperature and the char surface peak temperature of rice hull particles in O₂/H₂O atmosphere are lower than that in O₂/N₂ atmosphere,the corresponding devolatilization time and particle burnout time are longer than that in O₂/N₂ atmosphere.When the concentration of O₂ is more than 21%,the volatile flame temperature and char surface peak temperature of particles in O₂/H₂O atmosphere are higher than those in O₂/N₂ atmosphere,and the burnout time of particles is lower than that in O₂/N₂atmosphere.In the devolatilization stage,fuel particles are more likely to appear in the emulsion phase due to the high particles density.With the completion of drying and devolatilization,the density of particles decreased and the probability of floating on the bed surface increased.The high fluidization velocity increases the voidage of bed and bubbles fraction,which significantly reduces the influence of buoyancy,and increases the bed disturbances and the probability of the fuel particles to be trapped by bubbles.As a result,the probability of finding the particles in the emulsion phase and the splash zone increase.With the increase of oxygen concentration,the probability of the particle to be found floating on the bed surface increases while that in the emulsion phase decreases correspondingly.At the same oxygen concentration,the probability of the particles being found at any given location under the O₂/CO₂ atmosphere is similar to the situations under the O₂/N₂ atmosphere.Compared with the lignite particle,the rice hull particle is found to be more likely to float on the bed surface throughout the combustion process due to its low density.In the drying and devolatilization process,the change of pressure and atmosphere does not affect the probability of particles being found at any given location.In the char combustion process,the probability of a rice hull particle being found floating on the bed surface increases with pressure.At the same pressure,the probability of finding a rice hull particle at a given location（emulsion phase,bed surface,and splash zone）had a negligible relationship with the atmosphere.However,when the N₂ was replaced by CO₂,the temperature of the volatile flame and char particle both decreased,whereas the burnout time was obviously prolonged.At elevated pressure,the temperature of the volatile flame and char particle increased whereas the burnout time of particle decreased significantly,which is caused by the increase in the oxygen partial pressure and the high probability of a rice hull particle being found floating on the bed surface strengthen the diffusion of oxygen.The results of this study will provide reference for understanding the reaction process of biomass particles in the pressurized oxy-fuel fluidized bed.