A case study on the performance and economic impacts of natural gas cofiring at American Electric Power's Conesville Unit 3, originally designed to burn coal, was performed using the Coal Quality Impact Model (CQIM). Gas co-firing proved to be economically advantageous over coal firing. The design coal was evaluated at 0, 10, 25,50, and 75% gas co-firing at full-load conditions. Increased gas co-firing decreased the auxiliary power requirements and boiler efficiency. Due to boiler efficiency reduction, the net unit heat rate increased with increased gas co-firing. The worsening of the net unit heat rate was offset by improved availability, auxiliary power, and decreased SO2 emissions with increased gas co-firing. Based on the performance improvements and penalties predicted by CQIM, the total fuel-related costs were reduced with increasing gas co-firing. The annual fuel cost component increased as a result of slightly higher natural gas prices and an increase in the net unit heat rate; however, waste disposal, operation and maintenance costs, and replacement energy costs decreased with increased percentage of gas co-firing. The annual cost benefits associated with gas co-firing would have been much greater if (1) unit derates were experienced when 100% coal was burned, (2) the coal inventory, start-up, and turndown cost elements had been applicable, and
(3) the cost of replacement energy had been higher. Therefore, co-firing natural gas can still offer potential benefits when the full benefits of gas cofiring are not applied.