Study On Heat Transfer And Combustion Characteristics Of S-CO₂ Coal Boiler

The supercritical carbon dioxide?S-CO₂?coal-fired power generation system is considered as an advanced power circulation system with the advantages of high efficiency and compact system layout;countries around the world have made more efforts for the corresponding research.China is a country with abundant coal resources.The S-CO₂ coal-fired boiler application is still in its infancy.It is necessary to study the heat transfer characteristics between S-CO₂ in boiler pipeline and high temperature coal-fired flue gas and the combustion characteristics of pulverized coal in the S-CO₂ coal-fired furnace.The numerical simulation was performed on the heat transfer characteristics between S-CO₂ and high-temperature coal-fired flue gas during the heating process of vertical full-tube and half-tube,respectively.Under several of pressures and flue gas temperatures,the distribution of inner wall heat fluxes and its temperatures for boiler pipe,the average values of temperature,velocity,Re_b number,the wall heat transfer coefficient h,and Nu_b number for the S-CO₂ were carried out.The result shows that,Nu_b number,the inner wall temperature,the S-CO₂average velocity and Re_b number for the condition of the heated wall of the vertical full-tube decrease with the increase of pressure;and the local convective heat transfer coefficient of the wall and the heat fluxes increase with the increase of pressure.The radial velocity of the cross section is greatly reduced as the pressure increases.The pressure has little effect on the average temperature and central temperature of the S-CO₂ and the average temperature of the flue gas.The comparison of 32MPa S-CO₂ with supercritical water?S-H₂O?shows that the inner and outer wall temperatures,its average temperatures and average flue gas temperatures of S-CO₂ are higher than those of S-H₂O,while the convective heat transfer coefficient and Nu_b are lower than those of S-H₂O.The convective heat transfer coefficient along the inner wall of the axial S-CO₂ tube,the wall Nu_b,and the Re_b decrease with the increase of the flue gas temperature;and the average temperature and central temperature of the S-CO₂ and the average temperature of the flue gas,the heat flux,the inner wall temperature and the S-CO₂ average velocity increase with the increase of the flue gas temperature.The convection heat flux and total heat flux in the half-tube are lower than those in the full-tube,while the radiant heat flux is slightly higher than that in the full-tube.The inner and outer wall temperature,average temperature and central temperature,average velocity,Re_b,convective heat transfer coefficient and Nu_b of S-CO₂ are all lower than those of the full-tube.The temperature difference between the wall temperature and the S-CO₂ working medium temperature is about 50-100 K.Secondly,a coal-fired S-CO₂ boiler model is established.According to the“1/8drag reduction principle”in the S-CO₂ coal-fired boiler power generation system and the simulated vertical tube wall temperature distribution,the boiler wall temperature distribution was determined.This paper gave the flow field,temperature field,composition and NO_X distribution of the S-CO₂ boiler in different over-fire air rates,and at the same time,the temperature field and NO_X distribution of the S-CO₂ boiler?SOFA-35%?were compared with those of the steam boiler.The research results show that the over-fire air rate has a great influence on the coal combustion process and the formation characteristics of NO_X.With the increase of the over-fire air rate,the concentration of O₂ and NO_X in the main combustion zone and the gas temperature show a decreasing trend,and the CO concentration show a progressive increasing trend.The NO_X concentration at the outlet of the furnace decreases as the over-fire air rate increases.When the over-fire air rate increases to 30%,the increased over-fire air rate has little significant impact on the reduction of NO_X.Compared with the steam boiler,the S-CO₂ boiler has a slight increase in the average gas temperature at the outlet,and the NO_X mole fraction has increased by about 5%.