A Control Strategy of the Air Flow Rate of Coal-Fired Utility Boilers Based on the Load Demand

An analysis was conducted on the relationship between the calorific value of different types of coal and the theoretical air requirement. It was found that the theoretical air volume required for generating the same amount of heat during combustion is the same for different types of coal. The concept of the air/coal ratio was improved by proposing the concept of the air/carbon ratio, which refers to the ratio of the mass of air to the mass of carbon during complete combustion; the ratio is approximately 11.5 kg/kg (mass ratio), being roughly constant for different types of coal, unlike the air/coal ratio showing a significant change with coal types. The total air flow rate in a boiler changed with load demand, and the influence of different fuel types can be neglected at the same load level. On this basis, an air flow rate control strategy for coal-fired utility boilers was proposed and implemented in the boiler in which the air flow rate required to the furnace is a function of the unit load. Experiments conducted in a 300 MWe coal-fired utility boiler confirm that the use of the control strategy of the air flow rate for the combustion optimization improves the stability of the main steam pressure and mitigates the fluctuations of the coal flow. These results provide a foundation for the implementation of a new strategy of air/coal decoupling and independent control of boiler combustion.