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Blowing steam

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Although fuel prices, climate change and other issues have created a push to increasingly use natural gas for electricity generation, over the past several years there has been a shift in how power producers actually derive electricity from natural gas.

Many electricity producers have moved from primarily using traditional steam turbines to convert natural gas into electricity, to more efficient systems, called combined cycle power generators, which use heat that would otherwise be wasted.

Traditional steam turbines use steam from boiling water heated by the burning fuel source (such as coal, natural gas, nuclear fission and others) to turn the blades of the turbine, which in turn produces electricity. Combined cycle power plants use the hot air created from burning fuel, usually natural gas, to spin the gas turbine at a higher pressure and temperature, while the exhaust heat not used to power the first generator is captured and sent into a heat-recovery generator where it is used to make steam and turn a second turbine.

There is a big difference in the efficiency of regular steam units and combined cycle units. In steam-only generation units, only about 33 to 35 percent of the thermal energy is converted to electricity; combined cycle generators can have a thermal efficiency of up to 50 to 60 percent, according to the Natural Gas Supply Association.


Generations Of Generators

A review of the
most recent data from the U.S. Energy Information Administration shows
the median beginning operating year of natural gas-powered steam
generators in operation in Oklahoma is 1959, with a range of 1950 to
1977.

Meanwhile,
the median initial operating year of currently operating combined cycle
generators in the state is 2002, though such generators began operating
as early as 1961 and are still in use, according to the EIA data.

“For people that are going to burn gas, you see more people building (combined) cycle than traditional steam cycle ones. That is a trend,” said Gary Knight, vice president of generation for Public Service Company of Oklahoma.

PSO’s newer combined cycle generators are about 35 percent more efficient than the simple steam turbines, Knight said, and newer models being manufactured by General Electric have shown even greater efficiencies.

OGE Energy Corp. was the first utility company in the country to implement a combined cycle generator, installing one at the old Belle Isle power plant in 1949, said Brian Alford, spokesman for the company.

OGE is hoping to delay building any new fossil fuel generators until 2020; improving
efficiency, implementing renewable energy generation and encouraging
consumer conservation are the keys to achieving this goal, Alford said.

“To
do that, we have to help our customers be better managers of their
energy consumption by doing things like deploying smart grid and working
to improve efficiency and adding in renewable resources like wind,”
Alford said.

You see more people building combined cycle than traditional steam cycle ones. That is a trend.

—Gary Knight

However,
there is also a downside to the more efficient generators, Knight said,
because of the more exotic metals involved to withstand the higher
temperatures.

“You balance those and you’re looking for the design that’s going to minimize the cost to the customer,” Knight said.

On
the natural gas-fired electricity generation front, simple steam
turbines still play a role as well, Knight said, allowing utility
companies to use them for peak demand and allowing the company the
option to not purchase additional energy from the market if prices are
too high.

Coal-fired
electricity generation, which still comprises the majority of
electricity produced in the state, relies on steam turbines, Knight
said. Though advances have been made in improving efficiency of burning
coal, attempts in the 1980s to make combined cycle generators for coal
failed, Knight said.

“They
just couldn’t make it work. Coal and the ash leftover are so abrasive,
it was deemed at that time just not to be feasible.”

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