Combustion stability assessment for utility pulverized coal-fired boilers under low loads

Based on the influence of chemical equivalence ratio on the combustion stab ility of utility pulverized coal-fired boilers and the control theory about system stability, a combustion stability index, CSI, which refers to the m aximum reduction ratio of the fuel mass flow rate that can be overcome by t he stable combustion process under a constant air mass flow rate, was propo sed to assess quantitatively the combustion stability in the boilers. MLO, the Minimum Load of Operation with stable combustion not supported by firin g oil, and MCQ, the Minimum Coal Quality, which gives the lowest heat value s of coals with different volatile matter contents for stable operation of boilers, are defined on the basis of CSI. In order to predict MLO and MCQ, a simple chemical reaction system model has been modified by means of the c oncept of lean flammability of gaseous fuels. A three-dimensional combustio n simulation code integrated with the modified model was used to study the stability of combustion process in a 200MWe pulverized coal fired utility b oiler. The predictions of MLO and MCQ agreed confidently with operational e xperiences.