The Navajo Generating Station is configured with tangential fired boilers. In tangential fired combustion, often abbreviated T-fired or corner fired, the boiler consists of jets of air and fuel typically at multiple levels in the furnace. Tangential fired boilers differ significantly from wall fired boilers in that “the furnace is the burner.” Wall fired units have arrangements of multiple burners where each burner controls the mixing of the air and fuel and therefore combustion and flame characteristics of each coal jet. In wall fired units, primary air is used to convey the coal through, most commonly, the center of each jet with secondary and tertiary air providing the necessary air for oxidation of the fuel.
Tangential fired boilers facilitate combustion through a large fireball formed by jets of coal and air blown into the furnace at the corners of the boiler box. This creates significant turbulence and mixing of air and fuel. The advantage of such designs is that it allows for control of coal feed at each level, and can bias air flow at each corner. This creates opportunities for fine tuning the combustion within the furnace to better control emissions such as NOX, as well as maintain proper heat balance and facilitate complete char burnout and such work has a long history of success from the Department of Energy and Southern Company.
For these reasons, tangential fired boilers have been able to operate with extremely low NOX emissions levels such as 0.12-0.15 lbs/MMBtu while still achieving acceptable LOI in the ash of less than 1% for Powder River Basin (PRB) coals. For many plants this can be the difference between having to install expensive selective catalytic reduction (SCR) to manage NOX emissions levels, or being able to achieve compliance solely through the use of combustion controls and boiler tuning. While these results represent some of the best in the industry, this is not achievable for all units. Various factors such as CO emissions limits constraints, furnace/boiler design, fuel type, and operator engagement and skill can all create limits on NOX reduction potential for a given unit.
Furthermore, because these units have the ability to adjust the height of the fireball in the boiler by adjusting corner jet tilts as well as by taking offline lower level jets while maintaining higher level jets, these units can do a very good job of maintaining heat balance within the boiler. This is critical for efficient operation at a variety of firing rates and during load following operation where load must be adjusted up or down based on renewable generation output from wind and solar. Increasingly, coal plants are being required to load follow rather than provide baseload power for which they were designed. Having the ability to control combustion precisely and under a wide operating envelope positions these plants well to adjust to the new role of coal fired power generating stations within the US if they are equipped with knowledgable engineers, skilled operators, advanced combustion controls, and the drive to compete in an ever changing energy landscape.
While this video features an oil fired unit, the tangential fired concept is nicely illustrated with a boiler camera.