Evaluation of Gas Reburning and Low N0x Burners on a Wall Fired Boiler

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Under the U.S. Department of Energy's Clean Coal Technology Program (Round 3), a project was completed to demonstrate control of boiler emissions that comprise acid rain precursors, especially NOX. The project involved operating gas reburning technology combined with low NO, burner technology (GR-LNB) on a coal-fired utility boiler. Low NOX burners are designed to create less NOX than conventional burners. However, the NO, control achieved is in the range of 30-60-40, and typically 50%. At the higher NO, reduction levels, CO emissions tend to be higher than acceptable standards. Gas Reburning (GR) is designed to reduce the level of NO. in the flue gas by staged fuel combustion. When combined, GR and LNBs work in harmony to both minimize NOX emissions and maintain an acceptable level of CO emissions. The demonstration was performed at Public Service Company of Colorado's (PSCO) Cherokee Unit 3, located in Denver, Colorado. This unit is a 172 MW. wall-fired boiler that uses Colorado bituminous, low-sulfur coal and had a pre GR-LNB baseline NOX emission of 0.73 lb/1 Oe Btu. The target for the project was a reduction of 70 percent in NOX emissions. Project sponsors included the U.S. Department of Energy, the Gas Research Institute, Public Service Company of Colorado, Colorado Interstate Gas, Electric Power Research Institute, and the Energy and Environmental Research Corporation (EER). EER conducted a comprehensive test demonstration program over a wide range of boiler conditions. Over 4,000 hours of operation were achieved. Intensive measurements were taken to quantify the reductions in NOX emissions, the impact on boiler equipment and operability, and all factors influencing costs. The results showed that GR-LNB technology achieved excellent emission reductions. Although the performance of the low NOX burners (supplied by others) was somewhat less than expected, a NOX reduction of 65% was achieved at an average gas heat input of 180A. The performance goal of 70% reduction was met on many test runs, but at higher gas heat inputs. The impact on boiler equipment was determined to be very minimal. Toward the end of the testing, the flue gas recirculation (used to enhance gas penetration into the furnace) system was removed and new high pressure gas injectors were installed. Further, the low NOX burners were modified and gave better NO. reduction performance. These modifications resulted in a similar NO, reduction performance (64%) at a reduced level of gas heat input (-13Yo). In addition, the OFA injectors were re-designed to provide for better control of CO emissions. Although not a part of this project, the use of natural gas as the primary fuel with gas reburning was also tested. The gas/gas reburning tests demonstrated a reduction in NOX emissions of 43% (0.30 lb/1 OG Btu reduced to 0.17 lb/1 OG Btu) using 7% gas heat input. Economics are a key issue affecting technology development. Application of GR-LNB requires modifications to existing power plant equipment and as a result, the capital and operating costs depend largely on site-specific factors such as: gas availability at the site, gas to coal delivered price differential, sulfur dioxide removal requirements, windbox pressure, existing burner throat diameters, and reburn zone residence time available. Based on the results of this CCT project, EER expects that most GR-LNB installations will achieve at least 60% NOX control when firing 10-15% gas. The capital cost estimate for installing a GR-LNB system on a 300 MW, unit is approximately $25/kW. plus the cost of a gas pipeline (if required). Operating costs are almost entirely related to the differential cost of the natural gas compared to coal.