Modeling Of Radiative Heat Transfer And Experimental Study On Lignite-blended Coal Combustion In Boilers

When the coal powder blended with high content of lignite burned in the bituminous boilers with bin-feeder coal pulverizing systems, which is extensively being used in the power plants, it is very common to see the problems of the coal powder exploration, the insufficient drying output and the undercapacity caused by the low-heat value of the blended coal. In order to enable this combustion technology with low-heat value lignite which is cheaper and abundant in northeastern part of China, based on the systematic analysis and theoretical calculation, a new method to mix some hot flue gas from the reversing chamber into the coal pulverizing system by the local pressure difference without any extra pumping equipment is proposed in the industrial application. In the modeling of the radiant heat transfer in the combustion chamber with zone method, a relatively thorough analysis is conducted, in which the isotropic scattering media is well considered.Within all the state-of-art mathematical models of the radiation heat transfer, the zone method is the most accurate and robust, which is commonly being used as the reference to verify the other models. In this study, based on the radiant exchange area of the zone method, a 1-D analytical expression is deducted for the direct exchange area which subjects to the scattering participant media. This formula is much convenient to accurately analyze the radiant heat transfer in scattering media. The integration functions of the radiant exchange area in the 2-D and 3-D systems are summarized into four categories, which is helpful to the structural programming, program simplification and program debug. In the regular 2-D or 3-D system, an effective calculation method is proposed based on the principles of general symmetry of the matrix of direct exchange areas and the equal distances of the geographic center, which can avoid the huge calculation of the direct exchange areas.Based on both the superposition method and the radiation heat transfer mechanism, the Scattering Heat Flux Density is proposed and defined in the radiant exchange area calculation. The matrix expression is applied in the calculation of the total heat exchange areas. The individual matrix equations and total matrix equations of total exchange areas are derived in an enclosure with isotropic scattering media. Due to the clear physics and the simple deduction, this Scattering Heat Flux Density is very useful in the analysis of total radiant heat exchange area.With the volume scattering resistance and the equivalent volume emissivity in the three-element system, the radiant heat transfer in scattering media is analyzed qualitatively based on the radiation network method and the surface analysis method. The analytical results show that, when the scattering media is involved, the radiant heat transfer between the volume unit and surface unit is degraded because of the scattering thermal resistance of the volume unit. It is also shown that, when the scattering media is presented, the volume-to-volume total radiant exchange area becomes greater, while the surface-to-volume total exchange area becomes smaller, compared with those without the scattering media involved. The stronger the scattering effect of the participant media, the larger error from the calculation without scattering effect considered.The radiant heat transfer with zone method in isotropic scattering media is established to simulate the temperature profile of the HG-670/140-9 boiler with natural circulation in Qing-He Electric Power Generation Co. Ltd. The numerical simulations show that the low-temperature combustion with blending lignite and recirculating hot flue gas can relieve the slag growing. The feasibility of this new combustion solution is verified. The measurement results show that, the oxygen content of the wet gas is below 16% at the outlet of the coal pulverizing system, which is safe in terms of coal powder explosion. Through the grey relation analysis, it is feasible to blend the coal powder with large ratio of lignite in the bituminous boiler with bin-feeder coal pulverizing system. With this new combustion method, the other common problems in the similar types of the boilers, such as the lower temperature of reheated steam and the insufficient heat absorption of the superheater, can be solved too.