Prediction of Performance From PRB Coal Fired in Utility Boilers With Various Furnace and Firing System Arrangements

Abstract

Present regulatory requirements enforces the modification of the firing modes of existing coal-fired utility boilers and the use of coals different from those originally designed for these boilers. The reduction of SO2 and NOx emissions were the primary motivation for these changes. However, economic considerations played a major role too. Using sub-bituminous coals has become an important solution for emissions compliance due to their unique constituents and combustion characteristics; these coals are often referred to as enviro coals. Powder River Basin (PRB) Coals are classified as sub-bituminous ranked coals. Unlike higher ranked bituminous coals, which have tight pore structures that limit the amount of moisture they could hold, low rank coals, such as PRB coals, have looser pore structure and additional moisture retention capacity. PRB coals differ in many properties from those of the commonly burned bituminous coals, including low heating value, low fusion temperature, and high moisture content. However, PRB coals have low sulfur content and are relatively low cost. They can also lower NOx emission from power plants due to higher volatile content. When power plants switch from the designed coal to a PRB coal, operational challenges, including transportation, handling, storage, and combustion, were encountered. A major problem faced when using PRB coals is severe slagging and excess fouling on the heating surface. Not only is there an insulating effect from deposit, but there is a change in reflectivity of the surface. Excess furnace fouling and high reflectivity ash may cause reduction of heat transfer in the furnace, which results in higher furnace exit gas temperatures (FEGT), especially with opposite wall burners and with single backpass. Higher FEGT usually result in higher stack gas temperature and increasing in the reheater spray flow and therefore decreasing the boiler efficiency with higher heat rate of the unit. The modification of an existing unit for firing of PRB coals is confined to — and constrained by — existing equipment. All successful conversions happen when in the design phase of a project the following parameters are evaluated: (1) capacities or limitations of furnace size, (2) firing system type and arrangement, (3) heat transfer surface, (4) pulverizers, (5) sootblowers, (6) fans, and (7) airheaters. In the present study we used a comprehensive methodology to predict the behavior of three PRB coals fired in a 575MW T-fired boiler.