Burning wood
Fuel delivered to the CFB is stored in one of two silos, for a combined 60 minutes of operation at maximum continuous rating (MCR). Each silo has a volumetric feeder, two feeder discharge chutes and isolation valves, and pneumatic spreaders that route wood fuel to the boiler.
A dedicated silo sized for 10 days' operation at full load receives truck deliveries of limestone, which is used for SO2 removal when coal is burned. From the silo, the limestone is metered through a rotary valve into the injection piping. Two blowers supply the transport and injection air needed.
Inside the CFB, a bed material transfer system adjusts the height of the bed to tune the steaming rate during load changes. Bed material is pneumatically transferred to and from a refractory-lined bed material silo.
A gas-fired duct burner sized at 20% MCR preheats the fluidized-bed material in the furnace prior to solid fuel firing. Separate, 100%-capacity forced-draft and induced-draft fans move the air and combustion products.
A water-cooled screw removes bottom ash, whose temperature is reduced from about 1,590F to 400F by a cooler. A dense-phase, pressurized ash transport system handles flyash, bottom ash, and economizer ash. All types are transported to a single storage silo that straddles the roadway between the boiler building and the baghouse.
Air pollution controls on the Alstom CFB include a selective noncatalytic reduction (SNCR) system that uses urea supplied by Fuel Tech Inc. (www.fueltechnv.com) for NOx control, a limestone injection system to control SO2 and acid gases when burning coal, and a baghouse fabric filter from Wheelabrator Air Pollution Control Inc. (www.wapc.com) for limiting particulate emissions. The repowering of the Unit 5 boiler has produced net decreases in emissions of the criteria pollutants PM, PM10, NOx, and SO2. However, the project will result in increased net emissions of CO and volatile organic compounds. The table summarizes the net emissions increases/decreases permitted for the repowered Unit 5.
The Northern Wood Power Project's emission limits reflect lower wood emission rates. Source: PSNH
Overcoming obstacles
One of the biggest challenges that the NWPP project team (Figure 7) faced was coordinating all the demolition and construction activities between the two operating units—with Unit 5 continuing to operate on coal for the first year of construction. Another unique challenge was the railroad spur that bisects the plant and afforded little room for the CFB boiler, which had to be built in close proximity to an active railroad line. Yet another was integrating the digital controls for the repowered Unit 5 in Schiller Station's control room, where Units 4 and 6 are controlled pneumatically. Operators rose to the challenge and upgraded their skill sets so they can successfully operate both types of units.
7. Teamwork pays. Schiller Station staffers assumed leadership positions on the NWPP team. From left to right are: Richard Despins (station manager), John Cloutier (I&C/electrical construction supervisor), Mike Hitchko (project manager), Henry Patrick (mechanical/civil construction supervisor), and Jim Granger (project engineer). Missing from the photo is David Packard (senior contract administrator). Courtesy: PSNH
With Northern Wood Power in service for its customers, PSNH is now looking for opportunities to build on what it learned from this project to help New Hampshire meet its goals for significant increases in the state's green power production.
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