Author:Site Editor Publish Time: 2017-11-06 Origin:Site
Abstract: There are many factors influencing the combustion efficiency of the circulating fluidized bed boiler. This paper mainly analyzes the effect of fuel characteristics on the combustion efficiency.
Effect of Fuel Ash on Combustion Efficiency of Circulating Fluidized Bed Boiler
Fuel characteristics can have a significant impact on the design and operation of CFB boilers. Fuel ash increased, other components unchanged, the boiler efficiency will be reduced. Because with the increase of coal ash, coal calorific value will be reduced, in the same heat transfer under the coal feed rate increases, will produce more bed material, with the increase of bed material, slag discharge and ash discharge Increase proportionally, thus increasing the heat loss of the discharge, reducing boiler efficiency, as shown in Figure 1, Figure 2.
Figure 1 ash on the coal feed rate
Figure 2 coal ash impact on boiler efficiency
Effect of Fuel Moisture on Combustion Efficiency of Circulating Fluidized Bed Boilers
The moisture of the coal has an effect on the calorific value of the coal and on the amount of flue gas generated. With the increase of moisture, the caloric value of coal decreases, and the amount of flue gas increases, which will increase the exhaust loss. And flue gas volume changes, will change the aerodynamic characteristics of the furnace. Figures 3 and 4 show the effect of coal moisture on flue gas volume and boiler efficiency.
Figure3 moisture on the impact of flue gas volume
Figure4 coal-fired moisture on the boiler efficiency
Effect of Fuel Sulfur Content on Combustion Efficiency of Circulating Fluidized Bed Boiler
The sulfur content of the fuel affects the limestone feed system as well as the ash treatment system. For a given molar ratio of calcium and sulfur, with the increase of the sulfur content in the fuel, the feeding amount of limestone and the bed material ash increase correspondingly, resulting in the increase of heat loss in the furnace and the decrease of boiler efficiency. Figure 5 shows the tendency of boiler efficiency and sulfur capture rates to decrease as the sulfur content increases. The change in sulfur content has no significant effect on the coal feed rate because the share of sulfur in the coal accounts for a small fraction of the calorific value of the coal and another significant effect of the limestone is on the particle size distribution in the furnace. Because limestone is added as a desulfurization agent, and limestone is usually finer than coal. Therefore, the limestone must be considered when the particle size distribution within the furnace reasonable to get the appropriate heat transfer and fluidized state.
Figure 5 Effect of coal sulfur content on boiler efficiency and sulfur capture
fuel heat value of the circulating fluidized bed boiler combustion efficiency
Combining the calorific value of coal with operating conditions of the combustion chamber (bed temperature, outlet temperature, excess air ratio, etc.) determines the thermal load distribution between the circulating combustion system and the rear convective heating surface. As can be seen in Figure 6, the flue gas carries approximately 60% of the heat to the rear convective heating surface for poor quality fuels such as stone coal, lignite, gangue, waste wood, etc .; for high quality fuels such as bituminous coal, anthracite Only about 40% of the heat is carried by the flue gas to the rear convection heating surface.
combustion system heat absorption
2 - tail convection heating surface absorbs heat
Fig. 6 Distribution of heat absorption of combustion system and tail heated surface
When a boiler burns coal of much lower calorific value than the design coal, it may cause the fluidized bed dense zone to have a low temperature and affect the combustion. Because when the calorific value of coal is reduced, its conversion to ash and converted water inevitably increase, and the enthalpy of the unit mass of fuel out of the dense phase zone increases, so that the effective heat release in the main circulation loop decreases. The heat released is small, and the absorption of heat, then it may lead to the combustion chamber heat loss and heat loss, the bed temperature is reduced, the combustion instability. While at the same time the end of convection heating surface heat absorption increases, likely to cause overheating superheater. Therefore, the calorific value of coal must be in line with the heating surface arrangement of the boiler combustion chamber so that the heat load is evenly distributed, which is conducive to safety and stable combustion.
According to the above discussion, it can be seen that there are two main effects of the fuel characteristics on the fluidized bed. First, the nature of the fuel determines the optimum operating conditions of the combustion chamber. In the case of the fuel has been set, if the boiler is not adjusted to the appropriate combustion conditions, then inevitably lead to incomplete combustion loss increases, the boiler efficiency. Second, the elemental composition of coal, the level of volatiles combined with the operating conditions of the combustion chamber, determines the heat distribution between the circulating combustion system and the heated surface of the tail, high calorific value of coal, low volatility, less ash, then Unit mass of fuel in the main loop combustion in the effective heat release is large. Conversely, the heat in the main loop is small.