Cold Cyclone Design Problems
The cold cyclone design of Units 2, 3, and 4 may have less high-velocity gas that can cause accelerated erosion of tubes exposed in the gas path, but that doesn’t mean the design is immune to other operational problems.
In the cold cyclone design, ash grain size of approximately 0.8 mm is separated from the flue gas and drops into the siphons as the ash travels back to the combustion section of the CFB. In the cold cyclone CFB boiler, the heat of the flue gas is taken up by the screen, superheaters, evaporator, and one bank of economizer placed in the first pass of the boiler. This way the temperature of the flue gas, along with the fine ash, is considerably reduced. Circulating the cyclone ash not only enhances heat transfer to pressure parts, but it also substantially controls the bed temperature. In this design there is no loop seal arrangement; the cold ash (about 400C) is circulated through a siphon, back to the bed.
However, the boilers of three new units did experience excessive erosion on the wear resistance plates at the cyclone inlet area, damaged refractory in the area of the vortex plates and down to conical pipes, choking the downcomer pipe after the conical portion at the strainer area, and damage to the siphon fluidizing nozzles. Wear resistance plates were hard faced as in the hot cyclones, and the refractory was recast with improved anchors to solve the erosion problems. The fluidizing nozzles also were welded down to the windbox bed plates to permanently secure them in place.
Compared with the problems associated with the hot cyclone CFB boiler, the cold cyclone design problems were very manageable.
Reducing Auxiliary Power
Responsibility for controlling energy consumption at Hirakud Power begins with top management and moves down through the entire organization to the shop floor. Every kilowatt-hour consumed by inefficient equipment or work processes is one more kilowatt-hour that must be generated by the power plant, reducing overall plant efficiency and increasing the cost of aluminum products sold on the global market.
The entire plant workforce is grouped into 18 separate cross-functional teams that look for ways to reduce waste, improve maintenance processes, and improve the plant operating efficiency (table). Several significant improvement projects were recently completed that continue to push auxiliary power usage down at Hirakud Power. A sampling of these projects follows.
Cut your losses. Hirakud Power has almost quintupled production in the past five years while also significantly reducing plant auxiliary power losses. Source: Hirakud Power
Add Fluid Couplings to ID Fans. Units 2, 3, and 4 have six induced draft (ID) fans each (two per boiler for a total of 18), each rated at 480 kW. By regulating the speed of the fans with a fluid coupling, the power consumed by each motor was reduced by approximately 60 kW, thereby reducing annual energy usage by 9,460 MWh. The simple payback on this project was less than two years.
Add VFD to SA Fans. The nine newest CFB boilers also have a total of 18 secondary air (SA) fans, each rated at 180 kW and sized at 60% of the SA flow. The CFB boilers typically operate at 85% of maximum continuous rating. Instead of using inlet guide vanes to control flow at reduced load operations, variable frequency drives (VFD) were added to each 415-V motor. Each 180-kW motor saves 32 kW when operating, for a total annual energy savings of 5,050 MWh. The simple payback on this project was less than three years.
Add Fluid Couplings to Boiler Feed Pumps. Units 2, 3, and 4 have three boiler feedwater pumps each, each rated at 1,300 kW. Fluid couplings were added to each pump, which reduced auxiliary power by 268 kW per pump, resulting in an average annual energy savings of 14,090 MWh. The simple payback on this retrofit was less than one year.
Above Average
Hirakud Power continues to go beyond what is legally required by its plant operating permits to maintain a small environmental footprint. For example, the plant’s stack particulate emissions remain at about 25% below the permit limit of 80 mg/Nm3, and the staff have reduced fugitive dust emissions in the entire coal and ash lifecycle far below the local residential standard of 200 µg/m3 and close to the background air quality.
In addition, Hirakud Power was able to recycle more than 40% of the ash generated by the plant last year into beneficial uses, such as construction materials, including bricks and cement for the construction of roads and stormwater holding and protection dikes.
—Dr. Robert Peltier, PE is POWER's editor-in-chief.
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