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Show BACKGROUND Medium sized coal fired boilers continue to be used in the United States where coal supply is nearby and local emissions regulations allow the firing of coal. These boilers are used for both industrial steam generation, local building heating, and small utility electrical power generation. These coal fired boilers are coming under increased regulatory pressure to reduce particulate and sulfur oxide emissions due to Title V of the Clean Air Act. One of the routes a boiler owner has to reduce emissions is to convert completely to natural gas firing. However, the fuel cost associated with the complete conversion to natural gas is prohibitive when considering the marginal improvement needed to bring the emissions of these boilers into compliance. An alternative option is to cofire a minimum amount of natural gas with the coal to achieve the best of both fuels, low fuel cost coal combustion and clean natural gas combustion. The type of boiler targeted in this project is a coal fired stoker boiler where coal is fired on a chain grate type stoker at the bottom of the furnace, see Figure 1. Undergrate air flows through the grate with the use of both forced draft and induced draft fans resulting in a slight negative boiler static pressure. Pulverized coal, nut and pea size, is fed into the spreader stoker boiler by way of a spreader which throws the across the grate from the front of the boiler to the back, see Figure 2. The motion of the grate brings the coal to the front of the boiler with a residence time of 6 hours. During that time, the majority of the coal is reacted and bottom ash is dropped off the front of the grate into hoppers. Stoker boilers are typically fraught with emissions control problems as poor mixing within the furnace precludes complete combustion of smoke, products of incomplete combustion, and fly ash. The problem arises because the distribution of coal onto the stoker grate usually results in uneven piles of coal where complete combustion is hindered and smoke and CO are generated. These columns of smoke and CO rise in the furnace and fail to mix with the rest of the furnace flue gases. This process, called channeling, is responsible for a large amount of the coal stoker boiler emissions. In order to improve the emissions of stoker coal boilers, a small amount of natural gas combustion is added to improve mixing within the furnace and improve the coal combustion process and reduce smoke, CO and fly ash emissions. Stoker boilers also do not respond well to load swings as increases in coal feed rates result in more uneven distribution of coal on the stoker grate and more flue gas channeling. Another problem with coal fired stoker boilers is that excess air within the furnace must remain high to improve the emissions at low boiler loads when the thickness of the coal bed is low. Thus, the benefits of natural gas cofiring over stoker boilers are (1) reduced particulate emissions (2) reduced CO, SOx and NOx emissions, (3) improved boiler efficiency, and (4) improved boiler load following during load swings. Although not a stated goal of the present study, another potential benefit of gas cofiring is NOx reduction through reburning in addition to the reduction from fuel offset. 1 The specific type of boiler considered in this investigation is used to generate electrical power for a utility. This boiler, see Figure 1, is a Babcock & Wilcox spreader stoker boiler originally designed to produce 165,000 lblhr of steam. Due to electro-static precipitator and induced draft fan restrictions, the boiler load has been effectively derated to 145,000 lblhr. The operational benefits of gas cofiring for the boiler owner include (1) improved boiler warm up flexibility, 2 |