Improved NOx Emissions and Combustion Characteristics for a Retrofitted Down-fired 300-MWe Utility Boiler

A new technique combining high boiler efficiency and low-NOx emissions was employed in a 300MWe down-fired boiler as an economical means to reduce NOx emissions in down-fired boilers burning low-volatile coals. Experiments were conducted on this boiler after the retrofit with measurements taken of gas temperature distributions along the primary air and coal mixture flows and in the furnace, furnace temperatures along the main axis and gas concentrations such as O-2, CO and NOx in the near-wall region. Data were compared with those obtained before the retrofit and verified that by applying the combined technique, gas temperature distributions in the furnace become more reasonable. Peak temperatures were lowered from the upper furnace to the lower furnace and flame stability was improved. Despite burning low-volatile coals, NOx emissions can be lowered by as much as 50% without increasing the levels of unburnt carbon in fly ash and reducing boiler thermal efficiency.