Natural gas combined cycles fuel electric plant efficiencies
In 1958, UGI Corp. built a 44-megawatt coal-fired power plant south of Wilkes-Barre, where Hunlock Creek flows into the Susquehanna River.
UGI had morphed five years prior into a public utility licensed to do business in Pennsylvania. Hunlock Power Station was the utility's first electric plant.
For more than 50 years, Hunlock generated electricity using coal. But in 2010, economic forces and environmental concerns throughout the power industry led to UGI Energy Services' decision to convert the plant to natural gas.
In the old Hunlock, coal consumed in a huge burner created steam, which was then used to spin a turbine. The turbine spun a generator, which converted mechanical energy to electrical energy.
Using this method, about 30 percent of the heat energy created from the burning coal could be converted to electricity, said Mike Mara, UGI's vice president of operations and engineering.
"The bulk of the rest of it goes up the stack; it's not usable heat," he said.
In Hunlock after 2010, two General Electric LM600 turbines - derived from jet engines used in aircraft - burn natural gas.
The natural gas turbines also generate excess heat, but instead of allowing it to disperse into the atmosphere, UGI captures this heat and uses it to spin the old turbine. This means UGI added two new 40- to 45-megawatt turbines and continued to run its old 40-megawatt turbine off the gas turbines' waste heat.
Since the change, Hunlock converts 50 percent of its initial heat energy into usable electricity, Mr. Mara said.
The technology is called combined cycle. In the era of cheap natural gas and increased regulatory scrutiny on greenhouse gas emissions, many call it the new paradigm in the electric utility industry.
Before his role as vice president, Mr. Mara worked in power plants and he described how the conversion to natural gas has changed the way they operate.
Per unit of fuel, Hunlock's new combined cycle operation produces a third of the carbon dioxide emissions as the coal-fired version, Mr. Mara said. Switching to natural gas has also reduced emissions of sulphur and nitrogen oxides by many "orders of magnitude," he said. And with no more coal, disposal of toxic coal ash is no longer an issue.
Coal-fired plants require endless truck shipments and large sections of the plant devoted to storing coal, he said.
"They were more of a material-handling enterprise than they were a power plant," he said. "Generating the power was typically the easy part. Now you've got a pipe coming in and you're ready to go."
Hunlock is one of 34 natural-gas-fired power plants in Pennsylvania, according to the U.S. Energy Information Administration. These range from around 1 or 2 megawatts to more than 1,000 megawatts.
"For the big bulk generation, this seems to be the way things are going," Mr. Mara said. For Hunlock, "We didn't do anything cutting edge, we kind of followed the trend of the industry," he said.
Nationally, coal's toll on the environment is one reason for this trend.
Power plants contributed more carbon dioxide to the atmosphere than any other source, including vehicles, according to the latest draft greenhouse gas inventory from the U.S. Environmental Protection Agency.
Energy contributed to 84.4 percent of U.S. greenhouse gas emissions in 2012. For carbon dioxide, the most significant greenhouse gas, energy accounted for 97 percent of emissions that year.
Yet the volume of emissions from power plants has declined over the years. Lower carbon dioxide emissions from natural gas than from coal are driving this reduction.
Coal-fired power plants emit an average of 915 grams of carbon dioxide per kilowatt-hour of power produced, according to findings published Jan. 8 in "Earth's Future," a journal of the American Geophysical Union. Combined cycle plants emit 436 grams.
When looking at electricity generation from all sectors of the economy, coal is still king, though its reign seems to be waning. In 2001, coal generated 1.9 billion megawatt-hours, and natural gas generated 639 million megawatt-hours, according to the U.S. Energy Information Administration.
This held steady until 2008, when power from coal started to fall while gas rose significantly. In 2013, coal generated 1.59 billion megawatt-hours, and gas generated 1.11 billion megawatt-hours.
The EPA attributes the switch to a "significant decrease in the price of natural gas compared to the slight increase in the price of coal."
The drop in the natural gas price occurred after production from tight shale formations, such as the Marcellus, skyrocketed over the past five years. Like many plants, Hunlock benefited from cheap, abundant shale gas in its backyard. Its gas comes from Williams' Transco line, a branch of which runs through Luzerne County.
Any new coal-fired plants will have to make huge strides in efficiency because of new EPA emissions standards. New coal plants must keep emissions below 1,100 pounds of carbon dioxide per megawatt-hour. New combined cycle natural gas plants must emit below 1,000 pounds per megawatt-hour, 1,100 for smaller units.
Most existing combined cycle plants fall within these standards.
Calculating for the factors cited in the "Earth's Future" paper, a coal plant produces, on average, 2,000 pounds of carbon dioxide per megawatt-hour, while a combined cycle natural gas plant emits about 938 pounds of carbon dioxide per megawatt-hour.
Any future coal plants will have to use carbon capture and storage technology to eliminate some of that carbon dioxide.
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