NET Power, Supercritical CO2 Technology, and the Development of the
First Allam Cycle Natural Gas Power Plants: Featuring Carbon, No Pollution, and Comparable Efficiencies and Costs to Existing Combined -Cycle Gas
Plants
Durham, North Carolina company NET Power announced a few weeks
ago that they fired up their new $140 million 50MW test plant in LaPorte, Texas
that runs on natural gas and captures the CO2 at no additional cost to other
modern gas plants when done at scale. This is a big deal, possibly a very big
deal. It is good for natural gas. It is good for decarbonization. It is good
for pollution abatement. It is good for electricity consumers. It is good for people
who live nearby who otherwise might be close to a major pollution source. This
is likely to be a major victory for carbon capture technology. The main
limiting factor to scaling this technology is the need for sequestration and/or
to sell the by-products – mainly CO2, nitrogen, and argon. Oil companies
purchase CO2 for CO2 flooding to recover oil and related plastics companies use
it in making ethylene. Another advantage of these types of plants is that they
have a smaller physical footprint than current natural gas combined cycle
plants. NET Power’s parent company 8 Rivers Capitol is funding ongoing tech
development, Exelon Generation is operating the plant, and Toshiba is working
on turbine development. The plants also use far less water for cooling and
could even be air cooled if necessary. They think that they will surpass the
economics of a conventional modern combined cycle gas plant when they scale up,
with the 30th plant (presumably 300MW).
Supercritical CO2
The key to the plant’s function is supercritical CO2, which
is CO2 that is heated to a certain temperature (31.1 deg C – a hot day in
Phoenix) and pressurized to 7.39 megapascals. This makes the CO2 expand like a
gas but flow like a liquid. Liquids can be pumped. The CO2 is compressed,
pumped, and guided to spin a turbine. It is compressed to the needed pressure
then pumped since pumping requires far less energy than compression. That is
one key to its lower costs. It is the pressurized supercritical CO2 (SCO2)
itself that runs the turbine.
Oxyfuel Combustion
Like combined-cycle gas plants the exhaust from combusting
the gas in near-pure oxygen runs a turbine (here by heating CO2 to run the
turbine), but instead of the second cycle being still-hot exhaust heating steam
to run a second turbine the second cycle uses the waste heat to reheat the next
batch of CO2 so that the heat is basically recycled. The plant also relies on oxyfuel
combustion, or oxy-firing. This involves burning in pure oxygen. It was tried
in the past in coal projects with limited results but works better in natural
gas combustion, presumably due to less impurities in gas relative to coal. Using
it for coal also requires a desulfurization system and generates waste in the
form of sulfur and heavy metals. Oxyfuel combustion has been in common use for
some time in other industries such as aluminum, steel, and glass. Nitrogen,
which makes up over 70% of air, is removed. Another problem with coal is the
ash it generates which gets sticky and is hard to handle. It is oxyfuel
combustion that causes the waste stream from the combustion to be basically
pure CO2, with magnitudes less impurities than a traditional combustion system.
This makes carbon capture vastly easier and cheaper.
The Allam Cycle
UK engineer Rodney Allam is credited with the invention of
the Allam Cycle. Allam has defined it as “a high-pressure, highly recuperative,
oxyfuel, supercritical CO2 cycle.” The combustible mix by mass is 94% CO2, 4.75%
oxygen, and 1.25% natural gas. The pressurized CO2 runs the fluid turbine
(different from a steam turbine). Apparently, much of the CO2 can be reused
(thus the term “highly recuperative”), some water is condensed out, and some
90+% pure CO2 (considered a pipeline-quality CO2 product) is ready for
utilization or sequestration. It is unclear just how much CO2 is used up in the
process and how much is produced as a by-product that must be utilized and/or
sequestered. Allam notes that the process is not “parasitic, “or added on, as
are all other conventional carbon capture technologies. He describes it this
way in the 2013 Modern Power Systems
article referenced below:
“NET
Power turns the CO2 problem into the solution by exploiting the special
thermodynamic properties of carbon dioxide as a working fluid. This avoids the
energy losses that steam-based cycles encounter as a result of heat loss
inherent in the unavoidable vaporization and condensation of water. In the
process, NET Power generates - at no additional cost - a high-pressure,
high-quality CO2 byproduct that is ready for pipeline removal.”
The process is termed “highly recuperative for a number of
reasons: 1) much of the CO2 is recycled, 2) waste-heat from the air compressors
of the cryogenic air separation plant associated with the oxy-combustion system
(oxy-combustion is normally much more parasitic) is recycled to reheat the recycled
CO2 – thus the heat-exchange process is highly efficient , 3) the energy
savings from running compression to running pumps as the CO2 gets into a supercritical
state, and 4) the CO2 is removed from the recycle flow at high purity and at
pressures which can flow in a CO2 pipeline – it is both pipeline quality and
pipeline ready.
Of course, the projects will be initially confined to places
where the extra CO2 can be used which requires both a market for the CO2 and some
CO2 pipeline infrastructure. Thus, it is likely to be confined to projects near
major secondary oil recovery operations. He also notes that Toshiba is well-positioned
to build the turbines required as they have the expertise in high pressure
turbines, materials, and manufacture. The NET Power cycle systems are cheaper
due to smaller footprint (in part due to higher pressures) and the lack of a
need for smokestacks and emissions control systems. This saved cost is partially
offset by NET Power’s requirements for a cryogenic air separation unit and a
heat exchanger block.
Allam also mentions the NET Power cycle as a ‘platform’ that
can be used for other processes such as a coal plant, LNG regasification facilities
where it could increase efficiencies, hybrid concentrated solar-natural gas
plants where it could increase efficiencies, by superheating steam at greater
efficiencies than current in steam cycle turbines, and direct link-up to secondary
recovery of oil where associated gas could be the combustible source. He also
notes that the NET Power cycle can be utilized for coal just as readily and
this could be useful for countries reliant on coal like India and China,
although it is unclear what they could do with the excess CO2.
Marketing CO2 and Other Gases
Finding a market for the captured CO2, nitrogen, argon, and
a few other gases is a current focus. Power plants near oil fields could
provide CO2 for enhanced oil recovery. The pure CO2 could even be used as a
source to make gasoline or ethanol as analyses show these new processes to be
economically viable at scale, especially CO2-to-ethanol. Of course, the extra
CO2 could also be sequestered which would render the projects less economic and
perhaps uneconomic. Perhaps they could even send some to Europe who has a
current shortage of food-grade CO2 for things like carbonated beverages –
although it is a temporary shortage due to plant maintenance! I wonder if CO2
in whatever state could also be utilized for energy storage – of the compressed
air type. Indications are that the CO2-to-ethanol chemical reaction has very
low energy input requirements and could very well be developed for energy
storage, especially of intermittent renewables in times of overgeneration. Carbon
capture, utilization, and sequestration (CCUS) has been at an economic
standstill for some time since critics have argued that renewables, especially
wind, are cheaper to deploy without CCUS than fitting fossil fuel plants with
CCUS. The Vox article explores the problem of what to do with the CO2 and who
pays for its sequestration. If the plant pays then the economics go way down.
If the public pays then it becomes a fossil fuel subsidy. Of course, if there
is a taker or buyer then the economics stay the same or get better.
The Current State of Carbon Capture, Utilization, and Sequestration
(CCUS)
According to the article referenced below about the new
clean energy incentives bill signed by Trump:
“Currently, there are
17 large-scale carbon capture plants in the world, sequestering 40 million
metric tons of carbon dioxide in total—about 0.1% of total global emissions.”
That is a dismally small amount and critics probably
correctly point out that CCUS seems unlikely to make a major impact on carbon
emissions, especially in the near-term, as the costs have been consistently too
high. The International Energy Agency suggests that by 2050 the global need for
CCUS will be for 6 billion metric tons to be diverted from the atmosphere – or about
15% of emissions. This will be unlikely to happen without efficiency, cost, and
technological improvements.
Public perception of CCUS has waned over the years as the
technology has stagnated. With wind and solar an added advantage is companies
and residents that want zero or low emissions technologies. It is unclear if and
probably unlikely that low-emitting tech that still uses fossil fuels will be
as warmly welcomed and pursued in things like power purchase agreements. Thus,
I doubt companies like Google, Amazon, or Facebook, would embrace the tech over
wind and solar for their data centers, for example.
US DOE Energy Research Contributes to Technology Solutions
The National Energy Technology Laboratory (NETL) is involved
with several research projects utilizing supercritical CO2 as well as turbine
development, carbon capture, and sequestration. They are involved with R&D
in several SCO2 cycles and turbomachinery in their Advanced Turbines Program. Their
current projects include working with what’s called the Brayton Cycle which is
a closed-loop system that is also non-condensing. The Alam Cycle involves
condensing out water but could run without doing so at additional loss of
efficiency – so added cost. The Brayton Cycle and other SCO2 cycles may work
better with oxy-combusted coal. A 10MW natural gas test plant of the Brayton
Cycle will open in Texas in 2019. Other SCO2 projects include ‘indirect firing’
which utilizes turbine waste heat to heat SCO2 in a conventional Combined Cycle
Gas Plant steam cycle for a 2-4% increase in efficiency for little cost. Other
projects involve converting waste heat from turbines and/or engines to heat SCO2
for a 20% increase in efficiency at a small scale. In the Netherlands Statoil,
Mitsubishi, and other companies are working on a combined cycle gas plant that
burns 30% hydrogen for a 10% reduction in emissions, although that project does
not use SCO2 or oxyfuel combustion. Sequestration and utilization of CO2 also
require transportation to point of use or sequestration, either via truck or
pipeline so that is another area of expense and research. Obviously, being
close to area of utilization and/or sequestration is an economic advantage.
There are also other incentives including a recently passed
federal law that gives tax credits of $50 per metric ton of CO2 buried or $30
per metric ton of CO2 captured and used for oil production. These are the 45Q
tax credits and plants beginning construction before 2024 are eligible and
credits can be taken for 12 years.
References:
That Natural Gas Power Plant with No Carbon Emissions or Air Pollution?
It Works: The Carbon-Capture Game is About to Change – by David Roberts, in
Vox, June 1, 2018
This Natural Gas Plant Could Be A Big Breakthrough – by Nathanael
Johnson, in Grist, May 31, 2018
NET Power Achieves Major Milestone for Carbon Capture with
Demonstration Plant First Fire – Cision PR Newswire, May 30, 2018
This Power Plant Runs on CO2:
Carbon Capture Costs Nothing in NET Power’s New Plant, Which Uses
Supercritical Carbon Dioxide to Drive a Turbine – by David Wagman, in IEEE
Spectrum, May 30, 2018
Winner: Restoring Coal’s Sheen: Swedish Energy Company Takes a Novel
Approach to Carbon Capture – by William Sweet, in IEEE Spectrum, Jan. 1, 2008
Supercritical CO2 Turbomachinery: Technology Development for
Supercritical Carbon Dioxide (SCO2) Based Power Cycles – by National Energy
Technology Laboratory (NETL) – website
NET Power’s CO2 Cycle: The Breakthrough that CCS Needs – by Rodney
Allam, in Modern Power Systems (modernpowersystems.com), July 2013
Trump signed a landmark bill that could create the next big technologies
to fight climate change – by Akshat Rathi, in Quartz (qtz.com), Feb 9, 2018
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