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

Abstract

The present regulatory requirements enforce 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 in SO2 and NOx emissions was the primary motivation for these changes. Powder river basin (PRB) coals, classified as subbituminous ranked coals, can lower NOx and SOx emissions from power plants due to their high volatile content and low sulfur content, respectively. On the other hand, PRB coals have also high moisture content, low heating value, and low fusion temperature. Therefore when a power plant switches from the designed coal to a PRB coal, operational challenges were encountered. A major problem that can occur when using these coals is the severe slagging and excess fouling on the heat exchanger surfaces. Not only is there an insulating effect from deposit, but there is also a change in reflectivity of the surface. Excess furnace fouling and high reflectivity ash may cause reduction in heat transfer in the furnace, which results in higher furnace exit gas temperatures (FEGTs), especially with opposite wall burners and with a single backpass. Higher FEGTs usually result in higher stack gas temperature, increasing the reheater spray flow and therefore decreasing the boiler efficiency with a higher heat rate of the unit. A successful modification of an existing unit for firing of PRB coals requires the evaluation of the following parameters: (1) capacities or limitations of the furnace size, (2) the type and arrangement of the firing system, (3) heat transfer surface, (4) pulverizers, (5) sootblowers, (6) fans, and (7) airheaters. In the present study we used a comprehensive methodology to make this evaluation for three PRB coals to be potentially fired in a 575 MW tangential-fired boiler.

Evolution of Fly Ash Aluminosilicates in Slagging Deposition during Oxy-Coal Combustion Investigated by 27Al Magic Angle Spinning Nuclear Magnetic Resonance

“…According to the literature, , the remarkable ash deposition problem for PRB coal is not slagging in furnace but fouling in high-temperature flue, although its ash melting point is much low. The behaviors of aluminosilicates in the PRB fly ash in the ash deposition experiment are consistent with the slagging characteristics of the coal.…”

“…It has a high content of sulfur and iron and tends to readily cause slagging . As main thermal coal in the western United States, Powder River Basin sub-bituminous coal (denoted as PRB coal) has a high content of calcium and a high fouling deposition rate. , From the viewpoint of environmental protection, the combination of oxy-combustion and coal blending is an attractive measure for reducing CO 2 and sulfur emissions. Zhan et al analyzed the element distribution of fine particulates in fly ash of these two coals and their blend in air and oxygen-enriched combustion and discussed the element distribution’s effects on fouling .…”