Study On Wide Oxygen Concentration Operation Strategy Of Oxy-CFB Boiler And Conceptual Design Of A 350MWe Supercritical Oxy-CFB Boiler

As a new concept of clean coal technology,Oxy-fuel combustion in Circulating Fluidized Bed?Oxy-CFB?boiler is a promising technology with low-cost CO₂ capture,which is of great significance for China to control air pollution and achieve low-carbon development.However,it is still in the stage of pilot test and industrial demonstration.The conceptual design of a350MWe supercritical oxy-fuel CFB boiler was accomplished in the paper,which solves the key problem of heating surface layout and heat load distribution of CFB boiler under high oxygen concentration.It is important to the development of oxy-CFB technology in the future.Firstly,based on the ring-core flow model,the solid concentration distribution model of the CFB boiler in the dilute phase under air and oxygen-rich atmosphere is constructed.It is found that increasing the fluidization velocity can significantly increase the solid concentration and heat transfer coefficient in the dilute phase.When conventional air combustion is switched to a 21%O₂/79%CO₂ atmosphere,the change in solid concentration and heat transfer coefficient in the dilute phase is small.On the basis of simulating the material concentration distribution in the dilute phase,considering the effect of oxy-combustion atmosphere on the heat transfer in the furnace,the classic particle cluster renewal theory is adopted to predict the axial heat transfer coefficient distribution under the air,21%O_2/79%CO₂ and 30%O_2/70%CO₂ atmosphere of the CFB boiler.The results show that when the conventional air-fired CFB boiler is switched to the high oxygen concentration oxy-fuel combustion,with constant coal feeding rate,the fluidization velocity is reduced.The material concentration and heat transfer coefficient of the dilute phase are significantly decreased.By optimizing the particle size of the bed material,the material concentration in the dilute phase can be increased.It leads to the increase of total heat transfer coefficient and heat load in the dilute phase.It shows that the conventional air-fired CFB boiler can be operated under a high oxygen concentration oxy-fuel atmosphere without complicated modification.If the CFB boiler is oxy-fuel design,when the input heat power is constant,the total gas volume required in theory decreases with the increase of the oxygen concentration.In order to match up the similar hydrodynamics with a traditional air-fired boiler,the cross-sectional area of the furnace must be reduced,thus,the area of the heated surface that can be arranged is reduced.When the oxygen concentration exceeds 30%,it is generally necessary to arrange an external heat exchanger.The conceptual design of 350MWe supercritical oxy-fuel CFB boiler has been completed here.Specific dimensions,heating surface layout and heat load distribution of CFB boiler under 30%O_2/70%CO₂ atmosphere are given.According to the actual size and location of each component,the three-dimensional map of the 350MWe supercritical oxy-fuel CFB boiler has been finished.The design is compared with a CFB boiler with supercritical air combustion of the same grade in terms of boiler geometry,heating surface area and heat load distribution.It is found that the furnace cross-sectional area of the 350MWe supercritical oxy-fuel CFB boiler is reduced by 30.0%compared with the air-fired boiler under high oxygen concentration atmosphere.The tail flue section area is reduced by 27.8%.The area of heated surface that can be placed in the furnace is reduced by 28.6%,so the external heat exchanger must be used in the 350MWe supercritical oxy-fuel CFB boiler.The internal heat load of the furnace accounts for 47.55%,heat load of the cyclone and the tail flue accounts for 38.38%,and heat load of the external heat exchanger accounts for 14.07%.The ratio of the three is about4.5:4:1.5.The study can provide references for the further scale up of the Oxy-CFB technology.