Calibrated Operating Presets
Cycle & Financial Parameters
Thermodynamic State Trajectory: sCO₂ Temperature-Entropy (T-s)
Supercritical Working FluidThe cycle operates entirely above the critical pressure of CO₂ (73.8 bar). At point 1, sCO₂ behaves like an incompressible liquid, minimizing pump power before being recuperated to 720°C and combusted to 1150°C.
Levelized Cost of Electricity (LCOE) Benchmark Comparison
$/MWh Baseload Generation25-Year Cumulative Project Cash Flow & Payback
Two-Dimensional Sensitivity Matrix: Gas Price vs Section 45Q Credit
Displays Net Levelized Cost of Electricity ($/MWh net generation).
| 45Q Credit \ Gas Delivered | $2.00/MMBtu | $3.00/MMBtu | $4.00/MMBtu | $5.50/MMBtu | $7.00/MMBtu |
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Formative Assessment: Allam Cycle Thermodynamics & Project Finance
Evaluate your understanding of sCO2 fluid physics, oxy-combustion chemistry, ASU parasitic loads, and Section 45Q economics.
Engineering Architecture & Thermodynamics of the Allam-Fetvedt Cycle
1. Supercritical CO₂ Fluid Mechanics & Turbomachinery
Above its critical point (Tcrit = 31.0°C, Pcrit = 73.8 bar), carbon dioxide transitions into a supercritical fluid (sCO₂). Supercritical fluids do not possess distinct liquid or vapor phases; rather, they combine the high density of a liquid (~800 kg/m³ at 300 bar) with the low viscosity and rapid diffusivity of a gas.
In a conventional steam Rankine cycle, low-pressure steam expands to enormous volumetric volumes (~100 m³/kg at 0.05 bar), requiring massive low-pressure turbine casings and 40-inch titanium blades. In the Allam cycle, because sCO₂ remains highly compressed even at the 30-bar turbine exhaust, the specific volume is under 0.04 m³/kg. This produces an astounding 90% reduction in turbomachinery physical volume, dramatically cutting forged rotor costs and civil foundation footprints.
2. Oxy-Combustion & Inherent Carbon Capture
Conventional combined cycle gas turbines (CCGT) burn fuel in atmospheric air, which is 78% nitrogen. The resulting exhaust contains only 4%–5% CO₂ diluted in vast volumes of nitrogen and excess air. Separating CO₂ from dilute exhaust requires massive post-combustion amine absorption towers and enormous thermal energy (~3.0 GJ/tonne CO₂) to boil and regenerate the chemical solvents.
The Allam cycle completely eliminates nitrogen by burning pure methane in pure oxygen inside an atmosphere of 95%+ recycle supercritical CO₂:
The exhaust contains only CO₂ and water vapor. When cooled in the printed circuit recuperator heat exchanger (PCHE), liquid water condenses and is drained. The remaining gas is pure supercritical CO₂ at 30 to 100 bar, ready for geological saline sequestration with zero post-combustion chemical scrubbers.
3. Cryogenic Air Separation Unit (ASU) Parasitic Loads
The primary efficiency penalty of the Allam cycle stems from generating the pure oxygen required for oxy-combustion. Modern cryogenic Air Separation Units (ASU) cool atmospheric air to -196°C to fractionally distill pure oxygen (99.5% purity).
Producing 1 tonne of pure O₂ requires approximately 180 to 210 kWh of electrical compression energy. Because burning 1 kg of methane consumes 4.0 kg of oxygen, the ASU requires ~0.8 kWh of electricity per kg of natural gas combusted. This parasitic load consumes 12%–15% of the gross turbine generator output. Modern designs integrate ASU thermal waste heat back into the sCO₂ recuperation loop, boosting net electrical efficiency up to 54%–58% LHV.
4. Levelized Economics & Section 45Q Project Finance
Under the US Inflation Reduction Act, Section 45Q provides a 12-year guaranteed tax credit of $85 per metric tonne of geologically sequestered CO₂ in saline aquifers ($60/t for EOR).
Because an Allam cycle plant generates approximately 0.38 tonnes of pure CO₂ per net MWh of electricity delivered, the Section 45Q credit produces approximately $32.30/MWh in direct non-dilutive credit revenue. When combined with wholesale electricity sales ($55–$85/MWh) or bilateral clean firm PPAs with hyperscale AI data centers, the plant delivers robust 25-year equity IRRs (>15%) and rapid capital payback (6 to 9 years).
Frequently Asked Questions
What is the Allam-Fetvedt cycle and how does it achieve 100% carbon capture?
The Allam-Fetvedt cycle is a modern oxy-combustion thermal power cycle that utilizes high-pressure supercritical carbon dioxide (sCO2 at 300 bar) as the primary working fluid instead of water or steam. Pure natural gas and pure oxygen (from an air separation unit) combust within the sCO2 stream, producing an exhaust of exclusively CO2 and water. As the exhaust cools in a recuperator, water condenses and is removed, leaving pure pipeline-ready supercritical CO2 at 30 to 100 bar. This eliminates the need for post-combustion amine chemical capture plants and their heavy efficiency penalties.
How does the Allam cycle thermal efficiency compare to conventional CCGT power plants?
The Allam cycle delivers a net plant thermal efficiency of 48% to 58% LHV, which is directly competitive with modern combined cycle gas turbines (CCGT). While an unabated CCGT achieves ~60% efficiency, retrofitting it with post-combustion amine capture incurs an 8 to 10 percentage point parasitic thermal penalty (dropping to ~50% net efficiency). In contrast, the Allam cycle includes 100% inherent carbon capture within its baseline 48%–58% efficiency rating.
Why is an sCO2 power turbine so much smaller than a conventional steam turbine?
Supercritical CO2 at 300 bar exhibits a density near 800 kg/m³—comparable to liquid water—while retaining the low viscosity and rapid transport properties of a gas. Because the working fluid is orders of magnitude denser than low-pressure steam, the volumetric flow rate required to produce equivalent megawatts is drastically lower. This allows an sCO2 turbine rotor and casing to be approximately one-tenth the physical size and footprint of an equivalent steam turbine.
What is the primary parasitic energy penalty in an Allam cycle plant?
The largest auxiliary parasitic energy consumer is the cryogenic Air Separation Unit (ASU), which separates atmospheric nitrogen from oxygen to provide 99.5% pure O2 for combustion. The ASU consumes approximately 180 to 210 kWh of electricity per metric tonne of oxygen produced, representing roughly 12% to 15% of the gross turbine output. In contrast, the sCO2 recycling pump consumes modest power because CO2 near its critical point behaves like an incompressible liquid.
How does US IRA Section 45Q monetize carbon capture from an Allam cycle facility?
The US Inflation Reduction Act Section 45Q provides a tax credit of $85 per metric tonne of CO2 permanently stored in secure geologic saline formations (or $60/tonne for enhanced oil recovery). An Allam cycle plant burning natural gas captures roughly 0.36 to 0.42 tonnes of CO2 per net MWh generated, translating into $30 to $35/MWh in tax credit value. This direct subsidy offsets a significant portion of capital and fuel expenses, lowering net LCOE to $40–$55/MWh.