Radiation Heat Transfer Characteristics In Dilute Phase Region Of The Oxy-fuel Combustion Pressurized Fluidized Bed Based On Dependent Scattering

In the context of emission peak and carbon neutrality,the oxy-fuel combustion pressurized circulating fluidized bed(PCFB)has emerged as one of the most promising technologies to achieve near-zero carbon emissions for coal combustion.For the PCFB,radiation heat transfer is dominant in splash and dilution zones.It is of great theoretical importance to understand the radiation heat transfer characteristics and the intrinsic mechanism in the dilute phase zone for the design,manufacture and operation of the PCFB.Under pressurized oxy-fuel combustion conditions,the flue gas temperature,operating pressure and flue gas composition in the dilute phase zone of the fluidized bed change significantly,which leads to large differences in radiation heat transfer between the oxy-fuel combustion PCFB and the conventional fluidized bed.The radiation heat transfer of the particles has been investigated based on the gray body and the independent scattering.However,the radiation properties of the particles are strongly wavelength dependent and interact with each other as the increase of particle concentration,which results in the existing particle radiation models are difficult to accurately calculate the radiation heat transfer in the dilute phase zone of the oxy-fuel combustion PCFB.Therefore,the radiation heat transfer in the dilute phase zone of the oxy-fuel combustion PCFB is simulated and studied by a non-gray particle radiation model based on the particle dependent scattering,which reveals the thermal radiation characteristics and the intrinsic mechanism of radiation heat transfer in the dilute phase zone of oxy-fuel combustion PCFB.Firstly,a non-gray particle radiation model based on the particle dependent scattering(WSCKD)is developed by drawing on the ideas of the Full Spectral Correlation K-Distribution(FSCK)model and the Weighted-Sum-of-Gray-Gases(WSGG)model of gases,in which the particle radiation characteristics are calculated on the basis of the Mie theory and the PercusYevick approximate hard sphere model.By studying the radiation heat transfer of a onedimensional plane-parallel slab system under different conditions,it is found that the WSCKD model has the highest accuracy and acceptable computational efficiency.Then,the applicability range of the non-gray particle radiation model based on the particle dependent scattering is summarized by investigating the radiation heat transfer in particle systems with different volume fractions and particle sizes.The results show that the performance of the WSCKD model is almost the same as that of the non-gray particle radiation(WSCKI)model based on independent scattering for systems with low particle volume fraction,and the accuracy of the WSCKD model is higher at the industrial scale.For systems with high particle volume fraction and small particle size,the results of the WSCKD model differ significantly from those of the WSCKI model,indicating that the dependent scattering effect has a significant impact on particle radiation heat transfer.Next,the mechanism of gas and particle radiation affecting the total radiation heat transfer in the gas-solid mixture is revealed.For the laboratory scale,the gas and particle radiation promote each other and the total radiation heat transfer of the gassolid mixture increases relative to any component.For the industrial scale,the total radiation heat transfer of the gas-solid mixture decreases relative to the radiation heat transfer of the particles,which may be due to the shielding effect of the gas on the radiation of the particles.In addition,the radiation heat transfer in systems with high particle volume fraction is investigated in this paper.The results show the importance of particle radiation and emphasize the crucial role of particle radiation model in the high-volume fraction particle system.Subsequently,simulation studies of the radiation heat transfer in the dilute phase zone of oxy-fuel combustion PCFB were conducted based on the WSCKD model,in which the calculation accuracy and efficiency of different particle radiation models were compared,the law of particle concentration distribution on the radiation heat transfer was obtained,and the mechanism of particle concentration distribution on the radiation heat transfer was revealed.The results show that the maximum error of the WSCKD model can be reduced to within 1%for the wall incident fluxes.The particle concentration distribution has a significant impact on the radiation heat transfer in splash and dilution zones of CFB.Neither the 0D model nor the1 D model can provide satisfactory results.The 0D model is unable to capture the zero-source term in the core region.Although the 1D model accurately captures the phenomenon in the core region,the difference of the radiation source term in the annular region is more pronounced.For the radiation heat transfer in splash and dilution zones of CFB,the temperature difference is more dominant in the 0D model,while particle-to-particle radiation heat transfer is dominant in the 1D model and the 2D model.Finally,the effect of pressurized oxy-fuel combustion conditions on the radiation heat transfer in the dilute phase zone of CFB is researched.The results show that the dramatic change in flue gas composition caused by the conversion from air combustion to oxy-fuel combustion has a weaker effect on the radiation heat transfer in the dilute phase zone of CFB,while the effect of flue gas temperature is greater.For the region with dimensionless radius less than 0.75,the radiation source term of the 2D model increases with pressure and saturates when the pressure reaches 6 atm.For the region with dimensionless radius greater than or equal to 0.75,the radiation source term almost does not change with pressure.