In CFB boilers, how does the change in material circulation affect combustion?

　　In CFB boilers, what is the effect on combustion when the material circulation volume changes?

　　(1) An increase in the amount of material recirculation will cause the combustion temperature to decrease, especially when the temperature of the recirculating material is low.

　　(2) The recirculation of solid material increases the residence time of the fuel in the furnace, thus increasing the combustion efficiency.

　　(3) The material circulation makes the whole combustion temperature tends to be uniform, and accordingly reduces the temperature in the combustion chamber, so that desulfurization and denitrification can be controlled at the best reaction temperature, but reduces the combustion reaction rate, and the combustion is still in the power combustion condition.

　　What is the effect of fluidization air velocity on heat transfer coefficient?

　　When the fluidization speed changes, it will cause the redistribution of combustion share, and the change of concentration field near the heating surface will be caused by the difference of ash circulation multiplier. Changes in bed temperature and circulating material gas-solid two-phase flow temperature itself can produce local fluidization velocity changes and affect the particle impact frequency.

　　In general, an increase in air velocity on the one hand enhances the convective heat transfer of the gas, and on the other hand reduces the thermal conductivity of the particle population due to a decrease in particle concentration. For the upper heating surface of the circulating fluidized bed boiler chamber, the heat transfer coefficient increases after the fluidization air velocity is increased due to the influence of factors such as increased ash concentration.

　　What are the effects of particle circulation, solid particle size and bed temperature on the heat transfer coefficient?

　　(1) With the increase of the return volume, the amount of suspended fine particles inside the furnace also increases, and the concentration of particles inside the furnace increases, which increases the impact frequency between the particle population and the heating surface, causing the heat transfer coefficient to increase.

　　(2) Under the same solid particle concentration condition, the smaller the solid particle size is, the larger the heat transfer coefficient is.

　　(3) When the bed temperature increases, the share of radiation heat transfer inside the furnace chamber increases. In addition, the higher temperature increases the thermal conductivity of the gas, reducing the thermal resistance between the tube wall and the particle mass, while the high activity of the particles at high temperatures increases the impact frequency, thus strengthening the heat transfer and making the heat transfer coefficient of the combustion chamber area heat surface increase. Conversely, the bed temperature decreases, the heat transfer coefficient decreases.