Airflow and combustion characteristics and NO_x formation of the low-volatile coal-fired utility boiler at different loads

In order to solve the problems of lower heating rate of the primary air/fuel mixture and higher NO_x emission of the boiler, both the small-scale cold air experiment of a single burner and industrial-scale experiments of the centrally fuel-rich (CFR) swirl burner on a retrofitted 300 MW wall-fired boiler fed with low-volatile coal under deep air staging were performed. The aerodynamic characteristics, flue gas temperature, and temperature distribution of the furnace and carbon content in the fly ash were measured at loads of 300, 230, and 150 MW. Results illustrate that a central recirculation zone appeared close to the outlet the CFR burner under deep air staging. Compared with the enhanced ignition-dual register (EI-DR) burner, the gas temperature and the heating rate of the CFR burner are much higher, so it can guarantee the timely ignition and stable combustion of the pulverized coal. As for the CFR burner, with a decreasing load, the heating rate of the gas temperature decreases and the ignition position of the primary coal/air mixture becomes backward. The overall temperature of the furnace also decreases with decreasing load, as does the difference between the temperatures in the burning region and the lower position of the burnout region. After the retrofitting of the combustion system, the exhaust gas temperature decreases from 149 to 146 °C, the NO_x emissions at the air preheater exits decrease from 1354 to 778 mg/m³ (6 % O₂), and the boiler thermal efficiency increases from 89.75 to 90.57 % at the rated load.