This paper deals with an experimental study on the influence of coal quality on NOx emission and unburned carbon in flying ash. Chinese lean coals (fixed carbon content varied from 50%-70%) have been fired in a tangentially full scale boiler (300Mwe power unit, 1024t/h boiler). In furnace NOx emission and unburned carbon in flying ash concentrations have been measured synchronously with different combustion condition. Identical trends in NOx emission as a function of coal analysis content and excessive oxygen content have been obtained. NOx emission was found to be proportional to the fuel nitrogen content of the coals. Coal quality characteristic, especially fix carbon content and ash content have been studied on the influence of boiler burnout performance. A relationship has been found between operation condition and unburned carbon in flying ash content. Additionally, a three-dimensional mathematical model for pulverized coal combustion has been developed which describes coal combustion, including volatile devolatilization and char burnout, and radiation heat transfer in full scale furnace. The model can predict the levels of velocity, temperature, oxygen, and carbon in ash. It can predict accurately the influence of coal quality on combustion status in the furnace, and numerical simulation is more useful as a tool to investigate the influence of coal type on furnace performance related to differences in furnace design.