Study On Carbon Conversion Characteristics Of Oxy-fuel Coal Combustion In Pressurized Fluidized Bed

As one of the CO₂ capture technologies in combustion,oxy-fuel combustion can effectively capture the CO₂ produced from industry in order to storage or utilization.However,if the oxy-fuel combustion system is under the atmospheric pressure,the pressure of CO₂ capture system undergoes a process of lifting and lowering,because the air separation unit and CO₂ compression and purification unit is under high pressure,resulting in the overall economic efficiency of the system reduced,so the pressurized oxy-fuel combustion technology came into being.Increasing the combustion pressure can increase the heat load of combustion and the condensation temperature of water vapor in the flue gas,bring about recovering more heat from the exhaust gas to improve the power generation efficiency,moreover it can reduce the power consumption of the CO₂ compression and purification unit.The pressurized fluidized bed oxy-fuel combustion technology has the advantages of the fluidized bed reactor which own wide fuel adaptability,compact device and good load regulation performance,and it also improves the economy of the oxy-fuel combustion CO₂capture system,so it has a good industrial application prospect.Oxy-fuel combustion experiments were carried out on pressurized fluidized bed oxy-fuel combustion pilot test bench for three kinds of coal.The effect of combustion pressure and O₂/CO₂ atmosphere on carbon conversion law of oxy-fuel combustion in pressure fluidized bed were studied.With the increase of pressure,the CO₂ generation ratio of oxy-fuel combustion in pressurized fluidized bed is obviously improved,and the higher the coal rank,the more obvious the increase of CO₂ generation ratio.With the increase of combustion pressure,the unburned carbon in the fly ash showed a downward trend.The surface of the fly ash gradually changed from a loose porous structure to a compact sheet structure,and the flocculent material on the surface of the fly ash was significantly reduced.Compared with the air atmosphere,the condition of coal oxy-fuel combustion in pressurized fluidized bed is obviously deteriorated under the 21%O₂/79%CO₂ atmosphere.The CO₂ generation ratio of coal oxy-fuel combustion in pressurized fluidized bed at the O₂/CO₂ atmosphere is close to the air atmosphere when the oxygen concentration between 25%and 30%.As the oxygen concentration of the O₂/CO₂ atmosphere increases,the CO emission gradually decreases,but even if the oxygen concentration of the O₂/CO₂ atmosphere reaches 30%,the CO emission is still higher than the air atmosphere.Oxy-fuel combustion experiments were carried out in a pressurized fluidized bed oxy-fuel combustion pilot test bench for anthracite at 30%O₂/70%CO₂ atmosphere,aiming at studying effect of the operating parameters on carbon conversion.With the increase of excess oxygen coefficient,the CO₂ generation ratio of oxy-fuel combustion in pressurized fluidized bed increases,and the increase rate gradually decreases.However,the higher the combustion pressure,the weaker the influence of excess oxygen coefficient on CO₂ generation ratio.The unburned carbon in the fly ash decreases with the increase of the excess oxygen coefficient,but as the bed temperature increases,the decrease rate slower.With the increase of the temperature in fluidized bed,the CO₂ generation ratio of coal oxy-fuel combustion in pressurized fluidized bed increases,which is consistent with the influence of reaction temperature on carbon chemical reaction rate.The average particle size range of quartz sand in the oxy-fuel combustion pressurized fluidized bed should not exceed 1.5mm,and the height-diameter ratio of the bed should be controlled in the range of 1.0².6.Based on the CO₂ generation ratio of coal oxy-fuel combustion in the pressurized fluidized bed,the CO₂ generation model was discussed about combustion pressure,excess oxygen coefficient and bed temperature.The combustion pressure has the most significant effect on the CO₂ generation ratio of coal oxy-fuel combustion in pressurized fluidized bed,followed by the excess oxygen coefficient,while the least obvious is the bed temperature.Combined with the structure of carbon combustion rate equation,an empirical formula for calculating the CO₂ generation ratio of coal oxy-fuel combustion in pressurized fluidized bed was proposed.The coal pressurized oxy-fuel combustion model based on Aspen Plus was established to study the CO emission characteristics of oxy-fuel combustion in pressurized fluidized bed from the aspect of chemical reaction equilibrium.The kinetic model of coal oxy-fuel combustion in pressurized fluidized bed based on Aspen Plus was established to study the mechanism of unburned carbon from coal combustion in pressurized fluidized bed from the aspect of chemical reaction kinetics.