Effect of the anthracite ratio of blended coals on the combustion and NOx emission characteristics of a retrofitted down-fired 660-MWe utility boiler

Industrial experiments were performed for a retrofitted 660-MWe full-scale arch-fired boiler. The ignition position of the primary air/fuel mixture flow, furnace temperature and flue gas component after the air heater were measured with different anthracite ratios at loads of 660- and 380-MWe. As the ratio of anthracite increased, the gas temperature decreased in the fuel-burning zones of the C4 and C6 burners, indicating a farther ignition position of the primary fuel/air flow under the arches, the gas temperature of the lower furnace decreased while that of the upper furnace increased, NOx emissions decreased while both the exhaust gas temperature and carbon content in fly ash increased, and the boiler efficiency decreased. At the rated load, an anthracite ratio of 35% was optimal for economical efficiency. At low load, the negative gas pressure fluctuated remarkably, which led to unstable combustion in the furnace. To ensure safe and stable operation of the boiler, a mass ratio of anthracite of 25% is advisable.

► Tests were performed for a 660 MWe unit to optimize the blended ratio of coals.
► As the ratio of anthracite increased, NOx and boiler efficiency both decreased.
► At the rated load, an anthracite ratio of 35% was optimal for economical efficiency.
► At low load, a mass ratio of anthracite of 25% is advisable.