By Dawn Santoianni, Managing Director, Tau Technical Communications LLC for Cornerstone
International efforts to mitigate climate impacts have intensely scrutinized carbon emissions from the electricity sector. Coal, in particular, has been targeted as a source of emissions that could be reduced. The International Energy Agency recognizes that “coal is an important source of energy for world…we must find ways to use coal more efficiently and to reduce its environmental footprint.”1 With global coal demand projected to increase 15% through 2040, reducing carbon emissions from coal-fired electricity has become a policy focus in many countries as part of an overall strategy to reduce emissions.2 Although roughly half of new coal-fired power plants constructed during 2011 used high-efficiency low-emissions (HELE) technologies, approximately 75% of operating coal-fired units worldwide are based on less efficient, non-HELE technology.1
Globally, the average efficiency of coal-fired generation is 33% HHV (higher heating value) basis or 35% LHV (lower heating value) basis.3,A In a survey of countries worldwide, the average three-year (2009–2011) efficiency of coal-fired electric generating fleets ranged from a low of 26% in India to a high of 41% in France, normalized to LHV.B Those countries that were among the first to widely deploy HELE technology now have the most efficient coal-fired fleets.
Achieving higher steam temperatures and pressures (see Figure 1), HELE generating units employ advanced steam path design with multiple steam turbine pressure modules to extract the maximum amount of power from the steam produced. As the steam passes through each turbine module, the pressure decreases. These modules are referred to as the high-pressure (HP), intermediate-pressure (IP), and low-pressure (LP) turbine sections. Some turbine designs feature multiple IP or LP modules, while others may have a combined HP/IP cylinder. Steam exiting the HP section is returned to heaters that increase the steam temperature (reheat) to about the primary steam temperature before undergoing further expansion through the IP section. In double-reheat turbines, the steam exiting the IP module is again reheated before passing through the LP turbine module. Reheating is used to keep the steam humidity low, preventing the formation of water droplets that could damage turbine blades. Turbine blades are designed for each module to limit turbulence and efficiently convert steam kinetic energy into torque.
FIGURE 1. The steam cycle is at the heart of coal-fired power plant efficiency.
The upfront cost of ultra-supercritical (USC) HELE technology is 20–30% more expensive than a subcritical unit, but the greater efficiency reduces emissions and fuel costs. Therefore, USC units are being constructed where new coal-fired capacity is integral to maintaining security of energy supply while reducing emissions and also where older, less efficient fossil units are being retired. Although there are numerous examples of highly efficient coal-fired power plants around the world, four generating stations are highlighted in this article because they are particularly notable based on economic, technical, and policy perspectives.
CLAIMING THE WORLD RECORD: NORDJYLLAND POWER STATION UNIT 3, DENMARK
Nordjylland Power Station (Nordjyllandsværket) is touted by its owner Vattenfall as holding the world record for most efficient coal utilization since Unit 3 was commissioned in 1998.4 Nordjyllandsværket is a combined heat and power (CHP) plant located in northern Jutland, Denmark. The decision to build Nordjylland Unit 3 was made in 1992, at a time when European energy markets were being liberalized to create an EU-wide integrated energy market. This market restructuring and competition demanded increased efficiency, improved environmental performance, and cost-effectiveness of heat and power supply. These priorities were used to determine the plant design criteria. In addition to electricity supplied to the Nordic Power Exchange, Unit 3 provides district heating to the city of Aalborg using low-pressure steam extraction.
Nordjyllandsværket Power Station boasts the world’s most efficient coal use.
The 400-MWe USC Unit 3 employs a 70-m-high once-through steam generator and double-reheat steam cycle. To accommodate steam pressures of 29 MPa (4200 psi) and primary and two reheat temperatures of 582°C/580°C/580°C, high-performance superalloys were used for boiler and turbine components. An impulse turbine (in which fast-moving fluid is fired through a narrow nozzle) expands the steam from 29 MPa to 0.7 MPa. The HP and IP steam paths are combined in a common HP/IP module. Steam is passed back to the boiler for reheating before it continues through the IP and LP turbine modules. With the double-reheat cycle and cold seawater for cooling, Unit 3 boasts a net electrical efficiency of 47% (LHV basis). The asymmetric double-flow IP steam path (steam is received in the center of the cylinder and discharges at the ends) is configured to suit district heating requirements. Extracted steam is passed through two heat exchangers where water from the Aalborg city grid is heated to 80–90°C. This dual use allows Unit 3 to utilize up to 91% of the energy content in the bituminous coals it burns.
BRIDGING THE ENERGY GAP: TRIANEL KOHLEKRAFTWERK LÜNEN, GERMANY
In a country where the transition to renewable energy is being spurred by government investment, building a new coal-fired power plant might seem incongruous. However, the shutdown of Germany’s nuclear plants is presenting challenges to maintaining a reliable and dispatchable power supply. Many of Germany’s existing fossil-fueled power plants are over 25 years old—replacing aging plants with more efficient generation also supports the country’s decarbonization efforts. Construction of the €1.4 billion Lünen plant in North-Rhine Westphalia began in 2008; the plant has been delivering power to the electric grid since December 2013. Lünen is owned by Trianel Kohlekraftwerk Lünen GmbH & Co. KG, a consortium of 31 municipal utilities and energy providers. The plant was built to allow the municipal utilities to be independent and ensure a safe and affordable energy supply for 1.6 million households.5
Trianel Kohlekraftwerk Lünen power plant (photo courtesy of Trianel)
The 750-MW Lünen plant has a USC tower-type once-through boiler that burns low-sulfur hard coal delivered via canal. Main steam is produced at 28 MPa (4060 psi) and 600°C. The Siemens SST5-6000 steam turbine has one HP, one IP, and two LP cylinders. The plant uses Siemens’ advanced 3DV technology (three-dimensional design with variable reaction levels) for the HP and IP blades, which optimizes stage reaction and loading to achieve the highest efficiencies. Using USC technology, the Lünen plant has saved over one million tons of CO2 per year compared to the average German coal-fired power plant.6 In addition to supplying electricity, steam is extracted to heat water for district heating purposes. The plant has an electrical efficiency of nearly 46% (LHV basis) while meeting stringent German environmental requirements, making it the cleanest hard coal-fired power plant in Europe.
While Lünen is one of the most efficient coal-fired power plants in Europe, what makes it particularly notable is the ability of Unit 3 to ramp quickly, making it ideally suited to balance intermittent wind and solar loads.7 To remove the ramping constraint posed by heat transfer into thick-walled HP turbine components, an internal bypass cooling system allows a small amount of cooling steam to pass through radial bores between the HP casings. This system protects the casing surfaces so the wall thickness could be less than without the cooling steam. This design also effectively allows more rapid heat-up (and thus startup) of the turbine.
FIRST USC IN THE U.S.: JOHN W. TURK JR. POWER PLANT
The 600-MW John W. Turk Jr. power plant in Arkansas holds many distinctions. Completed in December 2012, it was the first USC plant built in the U.S. It also reigns as the country’s most efficient coal-fired power plant with an electrical efficiency of 40% HHV basis (~42% LHV basis).8 After the project was announced in 2006, American Electric Power’s (AEP) Southwestern Electric Power Co. (SWEPCO) spent several years trying to secure the necessary permits while fighting legal battles launched as part of national anti-coal campaigns. Under the legal settlement, SWEPCO agreed to retire an older 582-MW coal-fired unit in Texas, secure 400 MW of renewable power, and set aside US$10 million for land conservation and energy efficiency projects. At a final cost of US$1.8 billion to build the plant, the Turk plant also became the most expensive project ever built in Arkansas.
The Turk plant burns low-sulfur subbituminous coal in a spiral-wound universal pressure-type boiler, producing steam at 26.2 MPa (3789 psi) and 600°C. The plant has an Alstom STF60 single-reheat four-casing turbine with a single-flow HP section, double-flow IP section, and two double-flow LP sections.9 Using separate cylinders for the HP and IP turbines allowed the number of stages to be increased by about 25% compared to a subcritical steam turbine. The Turk steam turbine was manufactured such that different superalloys were selected for each section of the rotor to match the exact steam conditions with a specific stage on the rotor, allowing faster startups. The Turk plant is equipped with state-of-the-art emissions control technologies, including a selective catalytic reduction (SCR) system, flue gas desulfurization (FGD), fabric filter baghouse, and activated carbon injection.
With inexpensive natural gas and proposed carbon standards for new power plants that would require carbon capture for coal-fired units, permitting another HELE plant in the U.S. could be extremely difficult for economic reasons.10 Thus, despite its efficiency and excellent environmental performance, the Turk plant may be the last HELE plant built in the U.S. for the foreseeable future.
Turbine blading design plays a role in achieving high-efficiency operation.
SETTING THE STANDARD FOR CLEAN COAL: ISOGO NEW UNITS 1 & 2, JAPAN
The Isogo Thermal Power Station is located only six kilometers from Yokohama, the second largest city in Japan. The power station originally consisted of two 1960s-vintage 265-MW subcritical units. During the late 1990s, Yokohama’s environmental improvement plans aimed to enhance the stability of electric power supply while retiring older facilities. Electric Power Development Co., Ltd. (J-POWER), which owns and operates Isogo, entered into a pollution prevention agreement with the city. The new USC Unit 1 (600 MW) was built while the original facility remained in operation, becoming operational itself in 2002. The two older units were then shut down and demolished. The new USC Unit 2 (also 600 MW) was constructed on the site of the old plant and started commercial operation in 2009. Isogo Unit 2 operates at 25 MPa (3626 psi) and 600°C/620°C reheat achieving 45% efficiency, while Unit 1 operates at a slightly lower 600°C/610°C. Completion of both units more than doubled the power generated at the small peninsula site while lowering emissions levels to that of a natural gas-fired combined-cycle plant.
Combined, the two larger new units emit 50% less SOx, 80% less NOx, 70% less particulate, and 17% less CO2 than the older subcritical units that were replaced.11 The reduction in criteria emissions has been accomplished using a multipollutant regenerative activated coke dry-type control technology (ReACTTM) that captures SOx, mercury, and NOx while only using 1% of the water required by conventional wet FGD systems.12 ReACTTM technology consists of a moving bed adsorber with activated coke pellets downstream of the electrostatic precipitator. Mercury, SOx, and NOx are adsorbed onto the carbon pellets with ammonia injected to promote the nitrogen and sulfur reactions. In addition, the ReACTTM system offers a secondary method of particulate control as the flue gas impinges on the coke pellets. Activated coke from the adsorber is regenerated to reduce NOx to N2 and drive off SOx. In the process, the concentrated sulfur-rich gas stream created is used to produce sulfuric acid as a byproduct for commercial sale. Isogo’s Unit 2 has permit levels of 10 ppm and 13 ppm for SO2 and NOx, respectively, and usually achieves single-digit ppm concentration emissions. The system provides such exceptional pollution control that Isogo is ranked the cleanest coal-fired power plant in the world in terms of emissions intensity.
THE FUTURE OF HELE TECHNOLOGY
With USC well established, R&D is underway to increase steam temperatures to 700°C and beyond, which could achieve coal-fired efficiencies as high as 50%. Known as advanced ultra-supercritical technology (AUSC), such high pressures and temperatures will require more advanced (nickel or nickel-iron) superalloys that are expensive and currently present fabrication and welding challenges. In early 2014, Alstom and Southern Company (U.S.) announced a milestone in the development of AUSC, with steam loop temperatures maintained at 760°C for 17,000 hours during a trial at Plant Barry Unit 4 in Alabama. The loop contained an array of different superalloys and surface coatings that enabled it to withstand the exceedingly high temperatures within the boiler.13 Further advances in HELE technology, material science, and emissions control will enable coal-fired power to retain a primary role in future power systems.
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A. Although HHV is the efficiency convention most widely used in the U.S. for coal-based systems, in parts of Europe and elsewhere LHV is commonly used to report efficiencies.
B. Efficiencies normalized based on lower heating value (LHV), which refers to the quantity of heat liberated by the complete combustion of a unit of fuel when the water produced is assumed to remain as vapor and the heat not recovered.
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