This is an excerpt from my 2022 book, Natural Gas and
Decarbonization: Key Component and Enabler of the Lower Carbon, Reasonable Cost
Energy Systems of the Future: Strategies for the 2020’s and Beyond
A combined-cycle natural gas plant has two power cycles:
a combustion cycle where the gas is burned to run a gas turbine and a steam
cycle where waste heat in the form of steam runs a steam turbine. Combustion
turbines can go from cold shut down to full load in an avg. of 10 min to an
hour, but natural gas steam turbines take more than 12 hours on avg. A combined
cycle natural gas plant takes on avg. 1-12 hours to get from cold shut down to
full load.[1]
Other types of plants may have just a single combustion cycle or the much less
common internal combustion engine that runs on natural gas. However, a type of ICE
engine known as a gas reciprocating engine can have some advantages applicable
to peak demand response. Combined-cycle plants are much more efficient,
economic in the long run, but cost more upfront and take longer to construct.
They are best for what has become known as baseload generation. Units with a
single combustion cycle may be applicable where gas is needed sooner or where
gas use is deemed more temporary. These units are often built for peak demand and
those have become known as gas peakers or peaking plants. They provide reserve
capacity for peak demand times often brought on by daily and/or seasonal
declines of wind and solar generation. Their ability for quick start and ramp
up makes them the preferred choice for demand response. In the future they will
face more and more competition from energy storage. If better and cheaper
batteries are developed that competition will increase. At some point it seems plausible
that battery storage and perhaps other forms of energy storage will largely replace
the need for natural gas peaking plants. But for now, and the near future, gas
peakers are most often the best choice to back up variable generation. Energy
storage can also be hybridized into gas peakers, making them more flexible,
more economic to run, and decreasing emissions. These peaking plants are often
quite underutilized with many running much less than 10% of the time and some
with capacity factors as low as 1 or 2 %. That makes them less profitable to
build and maintain. Operating costs are higher. The frequent starts and stops
also increase maintenance costs. These units are often used specifically to
back-up renewables generation fluctuation events like the evening solar “duck
curve.” As I have argued before, these peakers should be included as a
necessary renewables integration cost, one that should be accounted as part of
the renewable energy system. In this sense, one might even argue that they
could even be eligible for subsidization.[2]
The US Energy
Information Administration classifies modern Combined Cycle Gas Turbines into
two types: Natural Gas Combined Cycle and Advanced Natural Gas Combined Cycle. EIA
notes in a June 2019 report that: “The latest generation of larger Frame H
natural gas-fired turbines was first installed in 2015 and was incorporated
into the design for 45% of the combined-cycle units installed in 2017. Based on
EIA’s survey of announced capacity additions, Frame H turbines are expected to
be incorporated in 33% of future natural gas combined-cycle power plants
through 2020 and in close to 40% of those installed in 2021 and 2022.”[3]
The graph below adapted from EIA shows that avg. construction costs for
advanced combined cycle units built from 2013-2016 were 26.3% cheaper than
combined cycle units and just 20% higher than single simple-cycle combustion turbines.
Natural gas
reciprocating engines can also be powered by other fuels and blends of fuels –
diesel, biogas, syngas, propane, and hydrogen. They have many applications,
often as generators for offsite power where there is access to natural gas. They
are used to run gas pipeline compressors. They are a key player as fuel
blending engines in fracking and other oilfield applications, with the
Caterpillar models running on 70-75% natural gas blended with diesel. Companies
Jenbacher and Waukesha make them up to 10MW in output. Those larger ones could
be utilized for niche peak demand applications for stabilizing the power grid. The
Jenbacher J920 engine has also been tested for use in large combined-heat-and-power/cogeneration
plants with very high electronic and thermal efficiencies.[4]
Siemens touts their reciprocating gas engines for power generation, combined-heat-and-power
(CHP), waste-to-power, marine applications, other distributed energy apps, and
to drive pumps. They have many models, some tailored to run on minimally processed
biogas. They offer a 2MW electronically carbureted model with very high
electrical efficiency and high combustion efficiency for power generation
applications. The supply of many sized models and types for different applications
allows for optimum designs for efficiency.[5]
Natural gas reciprocating internal combustion engines have been growing in use
over the last several years as a preferred source to backup intermittent renewables
on power grids. They start-up quickly (black start capable) and provide
flexibility for changing loads as they can operate at smaller loads. There have
been advances in efficiency and emissions reduction, particularly of nitrogen
oxides, or NOx. They are a good choice for use as backup power during weather
outages, including from floods. They helped stores and businesses remain open
during Hurricane Harvey in Houston. They also require less water. They are typically
usually used for smaller peaking applications than combustion turbines. Larger
ones of 16-19 MW have been employed for the last several years in the US,
beginning around 2011. In 2019 they accounted for 1% of the US natural gas-fired
fleet. Petroleum reciprocating engines are used as well in some places, but
their use is limited. According to the EIA in 2019: “The recent increase in
natural gas or dual-fuel capable reciprocating internal combustion engine units
has been driven in part by advancements in engine technology that increase operational
flexibility and by changes in natural gas markets that have generally provided
ample supply and relatively stable fuel prices.” They also note that these
engines are often banked together at power plants with the largest one in
Denton, Texas coming online in July 2018 with 12 18.8 MW engines for a total
output of 225 MW. Wind states like Texas and Kansas and solar states like
California use them specifically for backing up renewables.[6]
However, reciprocating engines are not as efficient as aeroderivative gas
turbines. GE gives a comparison of reciprocating engines and aeroderivative gas
turbines with the bottom line that aeroderivative turbines provide cheaper, cleaner,
and faster power. They note that an aeroderivative turbine is more efficient,
cheaper to run, uses 200 times less lube oil, and has 50 times less maintenance
events, so O&M costs are much lower than for reciprocating engines. They
are also fuel flexible, able to run on “natural gas, LPG (propane and
butane), isopentane, ethanol, diesel, and Coke Oven gas.” GE touts that
their aeroderivative turbines offer up to a 17 times better carbon footprint
and up to 4 times smaller physical footprint than reciprocating engines.[7]
I’ll get back to reciprocating engines later when talking about CHP,
microgrids, and oilfield power, and to more about aeroderivative gas turbines
in new power plant designs.
Fig. 1.17. Average
Construction Cost of Selected US Natural Gas-Fired Generators. Source: adapted
from Energy Information Administration.
[1]
Comstock, Owen, November 19, 2020. About 25% of US power plants can start up
within an hour. Energy Information Administration. About 25% of
U.S. power plants can start up within an hour - Today in Energy - U.S. Energy
Information Administration (EIA)
[2]
Gerardi, Jeff, Feb. 22, 2021. Power Plant Construction: How Much Does It Cost?
ProEst. Power
Plant Construction: How Much Does It Cost? | ProEst
[3]
Jell, Scott, principal contributor, June 19, 2019. More new natural gas
combined-cycle power plants are using advanced designs. Energy Information Administration.
More
new natural gas combined-cycle power plants are using advanced designs - Today
in Energy - U.S. Energy Information Administration (EIA)
[4]
Jenbacher J920 Engine. Case Studies. Clarke Energy. J920
Gas Engine | INNIO Jenbacher (clarke-energy.com)
[5]
Best-in-class solutions for efficient power generation. Siemens Energy. Gas
Engines | Power Generation | Siemens Energy Global (siemens-energy.com)
[6]
Natural gas-fired reciprocating engines are being deployed more to balance
renewables. Energy Information Administration. February 19, 2019. Natural
gas-fired reciprocating engines are being deployed more to balance renewables -
Today in Energy - U.S. Energy Information Administration (EIA)
[7]
The aero advantage: cheaper, cleaner, faster power. General Electric. GE Gas
Power. Aeroderivative
Gas Turbines vs. Reciprocating Engines | GE Gas Power
No comments:
Post a Comment