|The most common problem with aging coal-fired units involves erosion of the steam turbine’s blade path. More efficient blading will increase the steam output and improve the unit’s overall heat rate.|
By Russell Ray, Chief Editor
The Environmental Protection Agency’s (EPA) Clean Power Plan calls for a 6 percent improvement in heat rates at coal-fired power plants. A significant improvement in heat rate is one of several benefits of steam turbine rehabilitations.
Under the proposed Clean Power Plan, each state’s reduction target for carbon dioxide is based on four “building blocks.” One of those building blocks centers on heat rate improvement, or the fuel efficiency, of coal-fired power plants.
The Clean Power Plan would require existing power plants to reduce carbon dioxide (CO2) emissions 30 percent below 2005 levels by 2030. Although the national average would be 30 percent below 2005 levels by 2030, the specific reduction target for each state varies. Under the proposed rule, the benchmark year would be 2012, which means any improvements completed during that year or before will not be recognized.
A plant’s heat rate depends on its design, its operating conditions and its level of output. For existing coal-fired power plants, heat rates range from 9,000 to 11,000 Btu/kWh. A plant with the industry average heat rate of 10,300 Btu/kwh will operate with an overall plant efficiency of about 33 percent, according to the Electric Power Research Institute.
The first step in reducing CO2 is improving the heat rate. Heat rate reduction is the most cost-effective tool for controlling CO2. According to EPRI, a 1 percent heat rate reduction effects a 1 percent reduction in CO2 emissions, or about 40,000 tons a year.
The quest for greater efficiency is often the catalyst for a steam turbine upgrade.
A number of coal-fired generating units have been modified to improve heat rates. Many of these heat rate improvements stem from physical upgrades to steam turbine generators. According to EPRI, such upgrades have led to heat rate improvements in the range of 2 to 4 percent.
Although a large chunk of U.S. coal-fired generation will be retired amid stricter emission standards for air and water, the majority of America’s coal-fired generation will survive as power producers spend billions to bring these aging coal-fired units into compliance with new and pending emission limits on a wide range of pollutants.
“With the modern design tools, manufacturing practices and sealing practices, you can redesign the steam path and optimize the HP or IP steam path to gain an additional 3 to 5 percent in design efficiency,” said William Ray, vice president of Sales Service and Marketing for Mitsubishi Hitachi Power Systems Americas. “If you have additional available steam flow, you can add that on top of the efficiency gain.”
The incentives for upgrading most steam turbines are longer life, increased output, greater efficiency, and improved reliability, including improved solid-particle erosion capability.”
“When you do these steam path retrofits, you’re really making things new again,” Ray said.
|A 500-MW class low pressure rotor with 40″ – steel Hitachi last stage continuous cover blades. Photo courtesy: MD&A Hitachi|
To justify the expense, an improvement in performance must be realized. Any upgrade should focus on optimizing the entire design system, including balance of plant.
“Most of the efficiency gains are derived from managing the flow in the turbine,” Ray said. “The larger the unit, the greater those advantages are. A 900-MW unit will see a greater efficiency change than a 250-MW unit.”
The most common problem with aging coal-fired units involves erosion of the steam turbine’s blade path. More efficient blading will increase the steam output and improve the unit’s overall heat rate.
“The heat rate trails the output,” Ray said. “When you make a more efficient HP section, that steam has a lower energy content. It goes back to the reheater and is going to draw more heat to get it back up to design reheat temperature. That extra energy absorbed in the reheat section makes the heat rate go up.”
Steam turbines have many stages and components. Degradation of each stage or component affects subsequent stages and other components. Several mechanisms cause the degradation of rotating machines. Fouling is caused by the adherence of particles to flow path surfaces. Corrosion is the loss or deterioration of machine component material exposed to fluids. Erosion is the abrasive removal of material from the flow path by hard or incompressible particles impinging on flow surfaces. Abrasion is caused when a rotating surface rubs on a stationary surface.
“One of the things in designing the new steam paths, especially for an impulse machine, is that you’re able to manage the solid particle erosion much better,” Ray said. “There are some inherent design changes that go into it that make it more tolerant to erosion and therefore makes the steam path sustain its efficiency over a greater period of time.”
The aerodynamic performance of the steam turbine blade is crucial to achieving acceptable performance. Turbine manufacturers now provide bowed blade designs in the name of greater efficiency and reduced erosion, Ray said.
These designs “manage the steam flow to minimize sidewall losses,” he said. “You’re directing more steam flow into the body of the blade, which minimizes those losses.”
American Electric Power’s ultra-super critical plant, John W. Turk, is the most efficient coal-fired plant in the U.S with a heat rate of 8,858 Btu/kWh, according to Peabody Energy. That’s 16 percent lower than the average (10,424 Btu/kWh) for the entire coal fleet in the U.S.
The next four coal plants with the best heat rates in the U.S. are: Longview Power, 9,115 Btu/kWh; Sandy Creek, 9,151 Btu/kWh; Belews Creek, 9,167 Btu/kWh; and James E. Rogers Energy Complex, 9,174 Btu/kWh.
The top 5 most efficient coal plants had an average heat rate of 9,120 Btu/kWh, which is 13 percent better than the U.S. average.
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