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 Natural Gas-Fired Power

Natural gas has the lowest carbon content of any fossil fuel. On a Btu basis, burning one million Btu of natural gas will release about 117 lbs. of CO2. Oil has a higher CO2 emissions factor, about 161 lb. CO2/MMBtu for distillate and about 174 lb. CO2/MMBtu for residual fuel. For coal, the CO2 emissions factor is higher still and varies by rank of coal, averaging about 205 lb. CO2/MMBtu for bituminous coal and about 213 lb. CO2/MMBtu for subbituminous coal. Hence, when natural gas is used instead of coal, CO2 emissions per MMBtu are reduced by over 40 percent. CO2 emissions are also reduced when natural gas substitutes for oil, though by a lesser percentage.

In addition, further CO2 reductions can be achieved when the natural gas is used in new high-efficiency applications. In the electric power industry, natural gas combined cycle units have a substantially lower heat rate (Btu consumed per kilowatt-hour of generation) than conventional turbines and steam generators. This, coupled with the inherent lower carbon content, can achieve CO2 reductions of 60 percent or more on a kilowatt-hour basis.

Increased electric utility use of natural gas can take several forms:

Adding gas burning capability to boilers designed for coal and oil allows flexibility to displace those fuels and reduce GHG emissions as opportunities may arise. Additionally, because natural gas has lower fuel nitrogen than other fossil fuels and because of its combustion characteristics, NOx emissions will also be reduced. SO2 and particulate emissions will also be less when burning natural gas.

Although most of the recent expansion of natural gas use for electricity generation has involved large gas turbines (often paired with steam turbines in combined-cycle configurations), future growth will depend in part on the successful introduction of new generating technologies, both large and small. Today's NGCC plants can have outputs of over 500 MW and overall efficiencies surpassing 50% (calculated using the lower heating value of natural gas), compared with 35-44% for today's simple-cycle gas turbine plants — the workhorse for peaking capacity. Further improvements are expected; by 2020, an NGCC plant should have an overall operating efficiency of over 55%. Such a plant would produce about half the CO2 emissions of today's coal plants without carbon capture.

Meanwhile, a variety of smaller, gas-based generation technologies are starting to become more popular. Microturbines, for example, were commercially introduced in 2000 and are now either available or being developed in the 30-350 kW capacity range. They are considered ideal for distributed generation applications because of their flexibility in connection methods, their ability to be stacked in parallel to serve larger loads, and their improved reliability. Typical applications include supplying either stand-alone or backup power for customers ranging from financial services and data processors to hospitals and office buildings. Most micro-turbines feature an internal heat exchanger, called a recuperator, which increases efficiency by preheating inlet air. An additional heat exchanger can be added for combined heat and power applications.

Fuel cells, which substitute an electrochemical process for direct combustion of fuel, potentially offer very high efficiency and low emissions in their use of natural gas. About 200 units of the first commercially available 200 kW phosphoric acid fuel cell are now in service worldwide, often providing on-site premium power for sensitive operations, such as credit card processing. At the same time, research is under way to develop a hybrid generation technology in the 1-20 MW range combining small gas turbines and solid oxide fuel cells (which operate at higher temperatures than phosphoric acid cells); this approach could potentially offer electric service providers their highest-efficiency generating option.

In recent years, the volatility and level of natural gas prices has affected the outlook of natural gas-fired power as a CO2 reduction strategy. Throughout much of the 1990s, U.S. supplies of natural gas were abundant and readily available at prices generally below $3 per million Btu. Coupled with the low capital cost, quick construction, and relatively straightforward permitting of gas-fired powerplants, new gas-fired units came to dominate new capacity additions by the late 1990s. However, in recent years natural gas prices have been far higher and far more volatile, at times rendering much of this recent new capacity uneconomic, and discouraging further additions of new gas-fired capacity.

In recent years, the supply outlook for natural gas in the U.S. has become more favorable, particularly with the expanded reserves and production from hydrocarbon rich shale formations, known as “shale gas.” Shale gas is present across much of the lower 48 States, with the most active shales to date being the Barnett Shale, the Haynesville/Bossier Shale, the Antrim Shale, the Fayetteville Shale, the Marcellus Shale, and the New Albany Shale. Using a technology called hydraulic fracturing to produce natural gas from previously untapped beds of shale has boosted domestic production, lowered prices, and increased potential gas reserves by more than one-third.

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BP has built a significant natural gas-based power portfolio with 12 gigawatts (GW) of total generation. By 2008, BP plans to build two cogeneration plants in the United States, generating nearly 700 MW.

In 2004, Duke Energy Indiana repowered its coal-based Noblesville electricity generating station with natural gas combined-cycle technology. This action reduced the rate at which CO2 is emitted from this facility by more than 50 percent.

Entergy recently acquired 1,198 MW of combined-cycle gas turbine capacity to help meet projected demand while improving generation efficiency and improving overall emissions.

In the early 1990s, Exelon converted Cromby Generating Station Unit 2 and Eddystone Generating Station Units 3 and 4 to burn natural gas in addition to residual fuel oil. The summer of 2006 was the first time in nearly five years that it was economic to run the units on natural gas rather than residual fuel oil. On May 31, 2006, Eddystone Unit 4 and Cromby Unit 2 came online firing natural gas. Natural-gas firing of the Cromby and Eddystone units continued over the balance of the summer generation period.

Austin Energy in Texas recently completed construction of the Mueller Energy Center - the most efficient and comprehensive on-site generation plant available today. CO2 emissions from the plant will be less than half of those produced from traditional power plants and gas boilers. The lower emissions are equivalent to removing 1,800 automobiles from the road or planting 2,700 acres of trees.

MEAG Power responded to new generation needs with the construction of a new combined-cycle natural gas unit at Plant Wansley in Lowell, Georgia. The 503-MW unit became operational in 2004.

Wisconsin Public Power added a 54-MW natural gas combustion turbine to its generation portfolio in 2004.

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Electric Power Research Institute, “Generation Technologies For a Carbon-Constrained World”, EPRI Journal (Summer 2006)
http://my.epri.com/portal/server.pt?space=CommunityPage&cached=true&parentname=
CommunityPage&parentid=0&in_hi_userid=234&control=SetCommunity&CommunityID=260&PageID=486

Planning future generation investments can be difficult in the context of today’s high fuel costs and regulatory uncertainties. Of particular concern are sharp changes in the price of natural gas and the possibility of future mandatory limits on the atmospheric release of CO2. Research on advanced coal, nuclear, natural gas, and renewable energy technologies promises to substantially increase the deployment of low- and non-carbon-emitting generation options over the next two decades. Prudent power providers are likely to invest in a number of these advanced technologies, weighing the advantages and risks of each option to build a strategically balanced generation portfolio.

Electric Power Research Institute, “2007 Portfolio: 80 New Combustion Turbine/Combined-Cycle Design, Repowering & Risk Mitigation”
http://mydocs.epri.com/docs/Portfolio/PDF/2007_P080.pdf

This program provides up-to-date information and evaluation software that enables project developers to make better procurement decisions, thereby minimizing capital and O&M costs while ensuring high plant performance, reliability, and operational flexibility for simple-cycle and combined-cycle combustion turbine (CT) plants. This program also provides benefits to those seeking to acquire equipment or entire plants from other owners. Additional supplemental projects associated with this program offer application software and services that support plant design performance and economic evaluations and the development of CT maintenance strategies on a life-cycle basis.

Energy Solutions Center, “Natural Gas Co-Firing & Reburning”
http://www.energysolutionscenter.org/tech/tech_cofiring.asp

Natural Gas and Coal Cofiring is a technology that uses gas burners inserted into the boiler in a way that allows both coal and natural gas to be burned at the same time. The Energy Solutions Center is a technology commercialization and market development organization representing energy utilities, municipal energy authorities, and equipment manufacturers and vendors. The mission of the Center is to accelerate the acceptance of and deployment of new energy-efficient, gas-fueled technologies.

Environmental Protection Agency, “Electricity from Natural Gas”
http://epa.gov/cleanenergy/energy-and-you/affect/natural-gas.html

An overview of natural gas use for power, technologies, and environmental impacts.

U.S. Department of Energy, Office of Fossil Energy, “The Turbines of Tomorrow”
http://www.fe.doe.gov/programs/powersystems/turbines/index.html

In 1992 the U.S. Department of Energy's Fossil Energy program began an intensive effort to break through technical barriers that had essentially capped gas turbine efficiencies. This program produced turbine systems with efficiencies above 60 percent, a mark once thought unachievable. At the same time, new combustion techniques were developed to limit the formation of nitrogen oxide (NOx) emissions. 

U.S. Department of Energy, Office of Fossil Energy and National Energy Technology Laboratory, "Modern Shale Gas Development in the United States: A Primer" (April 2009 report prepared by Ground Water Protection Council and ALL Consulting)
http://fossil.energy.gov/programs/oilgas/publications/naturalgas_general/Shale_Gas_Primer_2009.pdf

Because shale gas development in the United States is occurring in areas that have not previously experienced oil and gas production, the GWPC has recognized a need for credible, factual information on shale gas resources, technologies for developing these resources, the regulatory framework under which development takes place, and the practices used to mitigate potential impacts on the environment and nearby communities. While the GWPC’s mission primarily concerns water resources, this Primer also addresses nonwater issues that may be of interest to citizens, government officials, water supply and use professionals, and other interested parties.
 

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