It takes energy to compress natural gas. Compression increases volumes and flow rates in transmission pipelines. When that gas gets closer to end-use points it must be decompressed. Decompression releases energy that can be recovered. I wrote about Anax Power’s gas letdown generators in my 2021 book: Natural Gas and Decarbonization. That section is reproduced below. Afterwards, I will try to add new developments and projects that have arisen over the past three years or so. One key event is the first deployment in 20 years of a turboexpander on a pipeline.
Turbo expanders, or expansion turbines, have a wide range of
applications in the natural gas liquids, liquefaction, and refinery sectors.
One prominent use is to cryogenically separate natural gas liquids. There are a
wide variety of designs.[1]
While compressing natural gas for long-distance transport requires added
energy, decompressing it releases energy. Lower pressures are required for the
distribution systems. Decompression can occur in different parts of pipelines
but is common at so-called city gates. Traditional methods for decompression
through pressure-regulating valves release the energy. Utilizing an expansion
turbine that turns via the expansion/decompression of the gas can generate
electricity via the gas expansion. This can be used for natural gas and with
other decompressed gases. This is carbon-free electricity that utilizes
recovery of energy from pressure reduction. It is known as a gas letdown
generator (GLG). It can be incorporated where gas is decompressed to be used
for industrial applications and integrated with combined heat and power, using
the CHP waste heat to pre-heat the gas.
There are more than 10,000 Pressure Reduction Stations in the US. US investment
group Anax Holdings and UK engineering company Star Refrigeration designed and
built two turbo expander gas let down generators, the 250kW ASTE250 and the
500kW ATE. These GLGs feature packaged modular, plug-and-play design and can be
used on-grid or off-grid, producing zero emissions power. These are intended
for use at city gates, industrial users, and other connections between high
pressure and lower pressure gas system networks. The electricity can be used to
power local operations or sold to the grid.[2]
Anax touts their turbo expander as “the most efficient natural gas letdown
generator on the planet.” It can even help balance the grid during demand
peaks. Pressure Reduction Stations also require the gas to be heated before pressure
reduction. This can be provided by a variety of sources including low grade
waste heat from CHP. The turbo expander uses no fuel and produces energy around
the clock. These GLGs are expected to have lifespans of 25-30 years with
minimal maintenance requirements. The ASTE250 was performance and safety tested
by the Gas Technology Institute (GTI) beginning in 2017.[3]
Technical feasibility and safe operation were established. GTI’s test brief notes:
“The ASTE’s goal is to deliver renewable energy to public utilities,
pipeline operators, and major industrial users (MIUs). Target customers include
natural gas power plants, MIUs with significant inbound natural gas volume,
city gates, and other pipeline junctions between high-pressure networks and
low-pressure networks. Furthermore, the ASTE qualifies for several state and
federal renewability incentives around clean energy.”[4]
Older turbo expander gas let down generators were based on different
designs. They were developed for liquefaction. Some utilized open-drive
turbines with oil-injected journal bearings. These mechanical drive designs
were bulky, noisy, inefficient, and require maintenance. The ASTE utilizes
closed construction and active magnetic bearings. It offers flexibility,
adapting automatically to a wide range of pressure and flow conditions. Anax
notes in their white paper that “The goal is to provide a package that is
custom-designed for the gas let down application and capable of handling a wide
range of operating conditions automatically, such as would be seen at a typical
primary let down station on the gas distribution back bone. This flexibility
was not designed into previous attempts at gas let down, and so they proved to
be difficult to control in practical applications.”[5]
For an interesting comparison, if the 500kW turbo
expander was running around the clock at its highest capacity factor it would
produce 500kW per day. It takes 2000 250-watt solar panels to have a nameplate
capacity of 500kW. Since the actual solar capacity factor is 25% or less it would
take a minimum of 8000 residential solar panels to match the full power output
of one 500kW turbo expander GLG. Of course, solar would have to be stored and
discharged somehow as well to make the comparison realistic. Putting 100 of
these units in operation would be equivalent in output to the output of 800,000
residential-sized solar panels plus needed storage. Emissions would be
comparable.
Anax
Power’s 2024 First Commissioning of the ATE-500
An article in
the September 2024 issue of Gas Compression Magazine recounts the first deployment
of Anax Power’s ATE-500 GLG. This was installed in July on a natural gas pipeline
in Johnsonburg, PA owned by Pin Oak Midstream, and is thought to be the first
deployment of a turboexpander on a natural gas pipeline in 20 years.
As noted, gas cools
as it is decompressed, which can require, depending on the site, subsequent
heating of the gas with relatively inefficient line heaters. Anax’s solution is
to pair the GLG with a nearby data center and use the waste heat from the data
center to heat the gas much more efficiently. The ATE-500 utilizes oil-free
magnetic bearings like the Baker Hughes turboexpanders explored later in this
post. It also utilizes a SCADA system for monitoring and diagnostics. Anax
chose a 670 hp system that would fit most industry needs. Where there are
higher flows, the GLGs can be stacked. The ATE-500 was designed for gas
transmission companies. A smaller 335 hp design is in the works for gas distribution
companies.
The use of
turboexpanders in cryogenic separation plants, air separation plants, and LNG facilities
is commonplace. These applications do not experience changes in flow rates or
pressures. Pipelines experience flow rate and pressure changes. Thus, designing
turboexpanders for them is a bit more challenging. The Anax ASTE-250 turboexpander
was originally tested by the Gas Technology Institute (GTI) in 2017 in the UK.
Since then, both models have been tweaked.
In 2022 a
study funded by Emissions Reduction Alberta explored the emissions reduction potential
of the ATE-500 based on the 2022 Grid Emissions Factor for the Province of
Alberta. They concluded that 3723 MWh of carbon-free electricity, 0.57 tonnes
of CO2eq/MWh, and 2124 tonnes of annual CO2 emissions reductions can be achieved
per unit deployed. That means that just under 500 units deployed can mitigate a
Gigaton of CO2 emissions. Anax notes that turboexpander power can be utilized
in a number of ways in addition to providing grid power: utilizing waste heat from data centers for further efficiency improvements, used to power electrolyzers
for H2 production, or powering resistive heaters. They can be used to
retire renewable energy credits and they are eligible for Investment Tax
Credits (ITC) tax credits of 30-50%, similar to wind or solar. Pin Oak has
partnered with a Bitcoin mining operation data center to use the energy generated
from the GLG.
Langson Energy’s Total Flow Gas Letdown Generator
Utilizing Helical Screw Technology: Applicable for Data Centers
Langson Energy
developed a GLG that instead of being a turboexpander, utilizes helical screw
technology. Their 2021 white paper notes the scale of the U.S. natural gas
pipeline delivery system:
“The U.S. natural gas pipeline network is a highly
integrated transmission grid that delivers natural gas to and from nearly any
location in the lower 48 States. It is comprised of more than 210 natural gas pipeline
systems; 305,000 miles of interstate and intrastate transmission pipeline; more
than 1,400 compressor stations that maintain pressure on the network and ensure
continuous forward movement of supplies; more than 11,000 delivery points,
5,000 receipt points, and 1,400 interconnection points that provide for the
transfer of natural gas throughout the United States.1 The pipeline grid
efficiently and safely moves more than 20 trillion cubic feet (Tcf) of natural
gas annually to residential, commercial and industrial consumers.”
“Pressure letdown is accomplished in stages and at
multiple locations throughout the system (e.g. 1000 psi to 600 psi, 600 psi to
250 psi, 250 psi to 100 psi, etc.) The high pressure is reduced through
mechanical regulating devices known as pressure reducing valves (PRVs). These devices
reduce the pressure of the gas to the desired levels but wastefully dissipate
the kinetic energy contained in the gas.”
Helical screw
technology has been used to compress gas for over 100 years. Langson applied it
to decompression which they say results in lower capital and operating costs
compared to turboexpanders for gas letdown generation. They also note that no
heat exchangers are required, no organic medium to transfer energy is required,
and operating efficiencies are in the 60-75% range. They claim that the only maintenance
requirements are a bearing inspection and replacement every 100,000 hours (11.4
years). They also tout “significant advantages in durability and for
handling contaminants.” They also claim that their GLG handles fluctuations
in pressure, temperature, and flow much better than turboexpanders. These, they
say, can operate 24 hours a day and seven days a week, providing baseload
power. They can also provide distributed power and redundancy capability.
In September
2021 Langson Energy put out a white paper about GLGs as a green energy solution
for data centers that are located near a natural gas pressure reduction valve
(PRV). One issue with decompression is that the gas is cooled in the process
which usually requires some heating. The cooled gas can instead be used for
cooling. In the case of data centers, one of the most energy intensive
processes is cooling for liquids or air, that makes up about 43% of data center
energy use.
“Langson’s GLG utilizes 2 forms of often wasted
energy:
1. The wasted kinetic energy at the PRV, and
2. The wasted cooling, often described as
Joule–Thomson effect (or Joule–Kelvin effect), which decreases the temperature
of a real gas that is allowed to expand freely at constant enthalpy (or
adiabatic free expansion - which means that no heat is transferred to or from
the gas, and no external work is extracted).”
Data centers
that utilize liquid immersion cooling can further benefit from GLGs that cool
more efficiently than cooling towers which use massive amounts of water. They
can also improve the efficiency of dry cooling systems.
Langson Energy
also offers CHP, or cogeneration solutions, and can be ideal for on-site power
needs for uninterrupted power for industry. They tout their products’
capabilities for baseload power, distributed power, black start, standalone,
off-grid, and redundancy
Baker Hughes Oil-Free Turboexpander Generators with
Active Magnetic Bearing Technology
Turboexpander
GLGs are also used in natural gas and LNG processing. Baker Hughes turboexpanders
are used in “cryogenic gas treatment and dew point control applications, petrochemical
(syngas, H₂ rich process), and waste-to-value applications (LAES, pressure
let-down, geothermal, waste-heat recovery.” Their fleet outputs range from
0.3 to 20 MW. In 2019 Baker Hughes introduced their Heat Injection Pressure
Energy Recovery (HIPER) system to optimize natural gas waste pressure recovery
at pressure reduction stations. That system offers oil-free magnetic bearings
in a sealed case, and no leakage, flares, or venting.
Baker Hughes has
three 17MW GLGs operating, two in Canada, and one in Brunei at waste-heat
recovery plants. They utilize an Organic Rankine Cycle (ORC) to efficiently
recover gas turbine exhaust waste heat.
Baker Hughes
touts the advantages of Active Magnetic Bearing (AMB) Technology over bearings
that require oil:
• No contact friction or components wear
• No oil lubricant or Lube-oil system required, no
flushing
• Compact skids and footprint, weight reduction
• Lower mechanical losses
• Environmentally friendly solution
• Proven solution with high reliability and availability
• Longer asset life and light maintenance
• Enhanced built-in machine monitoring capabilities
• Ready for unmanned operations, remote analysis, big
data
Baker Hughes has
been involved in AMB technology development and collaboration for a long time
as the following timeline shows. They utilize AMB tech for both compressors and
turboexpanders.
Baker Hughes
utilizes their SKF MBScope software suite as a monitoring, diagnostic, and
servicing interface which can be used for AMB tuning, troubleshooting, and
high-sampling data acquisition and diagnostics.
References:
Langson
Energy. Gas Pressure to Green Energy. Energy Recovery from Natural Gas Letdown
Stations. Langson Energy. September 2021. White
Paper LEI 2011-01 (langsonenergy.com)
Anax
Power. Anax-Star Turboexpander: Generating Electricity from Natural Gas
Pressure Reduction. May 2018. Microsoft
Word - ANAX WHITE PAPER.docx (anaxpower.com)
Turboexpander
generators. Baker Hughes. Turboexpander
Generators | Baker Hughes
Case
study: Turboexpander generator in ORC package for high‑enthalpy energy recovery. Baker
Hughes. July 2020. BakerHughes_Case_TEXGen_ORC_A4-062520-2.pdf
Gas
Letdown Generator for Data Centers. Langson Energy. White
Paper LEI 2011-01 (langsonenergy.com)
Oil-free
Turbomachinery with active magnetic bearing technology. Baker Hughes. April 13,
2023. bakerhughes_oilfreeturbomachinery_amb-041323.pdf
Anax
Power Turns Compressed Gas into Clean Energy. Drew Robb. Gas Compression
Magazine. September 2024. September
2024 | Gas Compression Magazine
References from Natural Gas and Decarbonization
[1] Turbo-expanders. IPIECA. Turbo-expanders | IPIECA
[2] Turbo Expansion. Longo. Longo - Turbo Expansion (elongo.com)
[3]
Anax Turboexpander 500 kW ATE Technical Data. Anax Power. ATE-Technical-Spec-Sheet.pdf
(anaxpower.com)
[4]
Anax-Star Turboexpander ASTE Safety & Performance Test Brief. Gas
Technology Institute. March 2020. GTI
Test Brief.pdf (anaxpower.com)
5] Anax-Star Turboexpander: Generating Electricity from Natural Gas Pressure Reduction. Anax Power. Microsoft Word - ANAX WHITE PAPER.docx (anaxpower.com)
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