Research On Fluid Dynamics And Radiative Heat Transfer In Pressurized Oxygen-enriched Fluidized Bed Combustion

Pressurized oxygen-enriched combustion power cycle is one of the most promising carbon capture and sequestration technologies in power plant. In this system, the development of pressurized oxygen-enriched combustor is a key issue. Until now, the most ideal and mature technology is the widely used pressurized bubbling fluidized bed in chemical industry which can satisfy the new system just by further rising operation pressure. Knowledge of the fundamental behaviors of high pressure fluidization combustion and radiative heat transfer is mandatory for optimal design and operation of the high pressure fluidized bed reactors employed in new system. So a pressurized fluidized bed test facility was established, in which the cold-state and hot-state tests were done and theoretical analysis was given.In cold-state test, an experimental study of the influence of pressure on fluidization characteristics was carried out over the range of operating absolute pressure0.1-4.5MPa with Geldart A and B particles. The results show that for B particle minimum fluidization velocity is found to decrease obviously, but there is no influence of pressure for A particle. It is found that the bed expansion height increases with the increasing of pressure for fixed value of gas velocity whilst average bed voidage at minimum fluidization velocity is unaffected from B to A particles. For A particle minimum bubbling velocity average bed voidage at minimum bubbling velocity, and dense phase voidage are found to increase obviously with pressure. Furthermore, the existing correlations which are developed for predicting high pressure fluidization were examined based on the experimental data. The influence of pressure on bubble properties in bed was studied with CCD cameras. It is found that bubble diameter and velocity decrease obviously with the increasing of pressure, but bubble frequency and fraction increase with increasing of pressure. Correlations of bubble diameter and allocation of fluidization gas between two phases in pressurized fluidized bed were proposed based on other worker’s data and experimental results. The results show that calculated result using correlations show good agreement with observed values and the degree of dispersion is with18%.In hot-state test, the minimum fluidization velocity of Geldart B and D particles were measured in the range of operating absolute pressure0.1-0.6MPa and temperature20～800℃.The results show that at a given pressure, the effect of temperature on minimum fluidization velocity is dependent on the material. Based on Ergun equation and bed stress analysis, an equation about minimum fluidization velocity was proposed, and then a reasonable calculation steps about minimum fluidization velocity at elevated pressure and temperature was given. The discrete degree between predicted minimum fluidization velocity and experimental results is small. The influence of pressure on combustion characteristics of bituminous coal was carried out in bed. Under the circumstances of constant bed temperature and coal feeding rate, the CO concentration and excess air coefficient corresponding to minimum CO concentration in flue gas are found to decrease with the increasing of pressure. A new theoretical heat transfer model of dense zone in pressurized fluidized bed was established based on the high pressure experimental results of fluidization and combustion.A part spectrum k-distribution model is developed to calculate the radiative properties of sparse zone in pressurized fluidized bed under oxygen-enriched combustion mode, characterized by possibly high concentration of water vapor and carbon dioxide compared to air-fired boilers. In order to verify the model, calculation and analysis are carried out in coal-fired boiler of flue gas with soot under the pressurized oxygen-enriched combustion mode by coupling with discrete ordinate method, results of radiation and wall flux are compared with that of line by line calculation, full spectrum correlated k distribution and weighted sum of grey gases model. The results show that the deviation of weighted-sum-of-grey-gases model is more than35%, because of grey approximation. The deviation of full spectrum correlated k distribution model is also big, because the correlated k hypothesis is invalid when temperature changes quickly. However the relative error of the part spectrum k-distribution model is within12%and much smaller than that of other models when comparing with benchmark line-by-line calculations.