Reducing NOx Emissions by Coal Reburning: Pilot Scale Process Studies

Reburning is a NOx reduction technique which is gaining wide interest for the control of NOx emissions from utility boilers. Unlike alternative control techniques such as low NOx burners and selective non-catalytic reduction, rebuming has been demonstrated to achieve high levels of control with no measurable by-produc t emissions. While natural gas has predominantly been used as the reburning fuel in full-scale demonstrations of reburning technology, oil and coal may also used. Coal is a nitrogen bearing fuel, and the extent to which this impacts the reburning process overall control efficiency depends primarily upon fuel nitrogen content and nitrogen reactivity. In reburning, complete combustion of the reburning fuel is always a concern because of the limited time and temperature for reactions to occur. Because coal does not burn as readily as natural gas, coal reburning has the potential to increase unburned carbon losses. Finally, coal ash can slag under fuel rich conditions leading to increased deposits on the boiler walls in the rebum lone . The use of some coals in specific boiler situations may be unacceptable from a boiler operability point of view. This paper describes the results of a pilot scale study supported by the Canadian Electrical Association of the process parameters influencing the coal reburning process. The goal of the project described in this paper was to identify the characteristics of Canadian utility coals that will result in low NOx emissions, low unburned carbon levels and have minimal impact on boiler performance. To accomplish this, selected Canadian utility coals were evaluated in a small pilot scale furnace over a range of operating conditions simulating full-scale utility boilers. The test results provide signifIcant insight into the impacts of coal type and properties on the reburning process and are provide critical information needed to optimize the application of the coal reburning process to Canadian utility boilers.