By Steve Blankinship, Associate Editor
NRG Energy’s Dunkirk Generating Station in Dunkirk, N.Y. is a four-unit, 530 MW facility consisting of four coal-fueled units, including two 100 MW units running since 1950 and two 200 MW units that went into service in 1959 and 1960.
In nearby Tonawanda, N.Y., NRG’s Huntley Generating Station consists of two 200 MW units that began service in 1957 and 1958. Both Huntley units are the same design as Dunkirk units 3 and 4. Net output of the two facilities is slightly less than the original due to a switch from high sulfur Eastern bituminous coal to lower sulfur Powder River Basin (PRB) coal. All six units are tangentially-fueled with boilers and GE steam turbines.
Between 2003 and 2006 NRG converted all units to PRB coal with a heating value of 8,800 Btu/lb. Pictured is the Dunkirk plant near Buffalo, NY. Courtesy NRG Energy.
NRG bought the Dunkirk and Huntley stations in 1999 from Niagara Mohawk Power Corp. and has since invested millions of dollars in low sulfur fuel conversion, combustion tuning and routine maintenance to help provide reliable, stable-priced electricity to New York’s power market.
For most of the plants’ 50 years in operation, Dunkirk and Huntley have used Eastern bituminous fuel with a heating value of about 13,200 Btu/lb. Between 2003 and 2006, NRG converted all units at both plants to PRB with a heating value of 8,800 Btu/lb. The conversion was gradual, involving blending PRB with Eastern coal during the transition. Since late 2006, the plants have burned PRB coal exclusively.
The coal switch, combined with other NRG’s investments, has reduced the plants’ sulfur dioxide (SO2) and nitrogen oxides (NOX) emissions. Particulate emissions also were reduced by improving electrostatic precipitator (ESP) ash removal system efficiency along with the lower ash content of PRB. In the mid-1990s, the plants installed burners with over-fueled air, which further reduces NOX emissions. Moreover, the plants have reduced NOX emissions using advanced staged coal/air combustion and continued combustion efficiency tuning. More environmental projects are in the works. Baghouses came on-line at Huntley in 2008 with Dunkirk following in 2009.
The planned use of activated carbon along with the baghouses also will reduce mercury emissions. The collective goal for NRG’s New York State coal power plants is to reduce SO2 emissions by 87 percent and NOX emissions by 81 percent. Additionally, NRG plans to reduce mercury by more than 50 percent by 2010 and by greater than 90 percent by 2015.
Dunkirk Plant Manager Tom Kilburn said PRB coal, while not incompatible with the existing ESPs, requires more frequent plant maintenance. Dunkirk currently schedules routine outages twice a year on each unit, primarily to clean the precipitators. “That should improve once we tie in the baghouses that are currently under construction,” he says. The baghouses will replace the precipitators and address not only particulate matter and opacity but reduce SO2 and mercury emissions even further. Even before the baghouses were added, the facilities were below their particulate limits. Plans call for eventually moving to 18-month maintenance intervals.
PRB coal is different to handle than the design coal and can pose some challenges. NRG made changes to operating procedures and equipment design throughout the coal handling system.
“PRB is much dustier so we needed dust suppression and containment installed throughout the conveying system,” says Kilburn. Because of its lower heating value, throughput increased 50 percent, requiring changes to the coal handling system.
“We installed blending capability for the period we were blending both coals to safely make the conversion,” he says. The goal was to reach 100 percent PRB, so the plant made changes during this process and learned how PRB was going to react in boilers designed for Eastern coal.
Soot blowing changed, too. The plant converted from compressed-air soot blowing to steam and installed additional blowers to handle PRB ash. Tighter controls also were needed on furnace outlet temperature because of the PRB combustion characteristics. “We had to be careful of overheating superheater elements and the electrostatic precipitators had to be tuned to efficiently handle PRB ash,” Kilburn says.
PRB coal’s higher moisture content results in its lower heating value. That, in turn, reduces boiler efficiency, requiring higher pulverizer system throughput to maintain existing boiler output capacity. NRG adjusted the pulverizer system to maintain as closely as possible the original gross heat input capability and boiler output. Coal bunkers were modified to eliminate flat laydown areas due to PRB coal’s ability to self combust. Sloped bunker bottoms were installed so the fuel could not sit for long periods of time. Pulverizer fire detection and fire protection systems also were installed.
Dunkirk also has a small capacity to use biomass as a fuel source. While still owned by Niagara Mohawk, the utility and Department of Energy built a forestry wood product biomass pilot project on Unit 1 to produce 8 to 10 MW. That project is now restarted to evaluate its performance using PRB coal.
Although it’s one of NRG’s older plants, Dunkirk remains a baseload facility. “We may drop load a bit during weak market periods, but the plant is mainly baseload,” says Kilburn. Although the plant is on Lake Erie, fuel currently is delivered by rail, although it canand hasarrived by barge.
Dunkirk maintains a workforce of around 160. “Given the challenges we had to overcome with the fuel conversion, we are striving for and approaching top decile performance in our industry,” says Kilburn. “We have accomplished this with attrition due to retirement of about 30 percent over the past five years.” He says the right team is in place to operate the plant safely, efficiently, reliably and in an environmentally responsible manner for many years to come.
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