CO2 in Supercritical stage

Carbon dioxide is a gas, that is what is taught in schools, and usually that would be correct. It behaves as a gas, in air at standard temperature and pressure (STP), or if it is pressurized to a certain pressure or temperature is lowered, it becomes solid, and then it is called “Dry Ice”.

However, a midway condition also exists for CO₂, that is if the temperature and pressure are both increased from STP to be at or above the critical point for carbon dioxide, it can adopt properties between a gas and a liquid. Some useful properties of the CO₂ are given below.

Property	Value	Unit	Value	Unit	Value	Unit	Value	Unit
Acidity (pKa1)	6.35
Acidity (pKa2)	10.33
Boiling Point – sublimation point	194.686	K	-78.464	°C	-109.24	°F
Critical density	10.63	mol/dm3	467.6	kg/m3	0.9073	slug/ft3	29.19	lb/ft3
Critical pressure	7.38	MPa=MN/m2	73.8	bar	72.8	atm	1070	psi=lbf/in2
Critical temperature	304.13	K	30.98	°C	87.76	°F
Critical volume	94.12	cm3/mol	0.00214	m3/kg	1.102	ft3/slug	0.03426	ft3/lb
Density	40.8	mol/m3	1.795	kg/m3	0.00348	slug/ft3	0.1121	lb/ft3
Density, gas at 32°F/0°C 1 atm	44.9	mol/m3	1.977	kg/m3	0.00384	slug/ft3	0.1234	lb/ft3
Density, liquid at -34.6 °F/-37°C, saturation pressure	25017	mol/m3	1101	kg/m3	2.136	slug/ft3	68.73	lb/ft3
Density, solid at -109.3 °F/-78.5°C, 1 atm	35492	mol/m3	1562	kg/m3	3.031	slug/ft3	97.51	lb/ft3
Flammability	no
Gas constant – R (individual)	188.92	J/kg K	0.0525	Wh/(kg K)	1130	[ft lbf/slug °R]	35.114	[ft lbf/lb °R]
Gibbs free energy of formation	-394.00	kJ/mol	-8953	kJ/kg	-3849	Btu/lb
Heat (enthalpy) of combustion	0	kJ/mol	0	kJ/kg	0	Btu/lb

Supercritical CO₂ is becoming an important commercial and industrial solvent due to its role in chemical extraction in addition to its relatively low toxicity in the Supercritical state, and resulting reduced environmental impact. The relatively low temperature of the process and the stability of CO₂ also allows most compounds to be extracted with little damage or denaturing. In addition, the solubility of many extracted compounds in CO₂ varies with pressure, permitting selective extractions.

Some uses of the super critical carbon dioxide (sCO₂) are briefly discussed herein.

Transportation

For the commercial use the gas is transported in pressurized cylinders. However, for the purpose of industrial use the bulk transportation is through tanker-trucks, and pipelines. In 1990s and later several pipelines were designed to carry CO₂ gas for processing or for well injection purposes. Except for in 198s the enhanced recovery system injection from the matured oil fields, where pipelines carried supercritical CO₂, all other pipelines carrying CO₂ was being transported in gaseous sate, yet out of abundant caution, and lack of proper knowledge and study by engineering and by the operators several of those pipelines were constructed to meet extreme low temperature requirements. A large majority of pipelines are often designed to carry CO₂ in the temperature and pressure range where the CO₂ is in gaseous state, demanding no specific challenge in selection of material or its properties. A good and clean steel production process, with specified mechanical properties laid out in the pipeline design codes suffice the requirements. However, if the CO₂ is required to be transported in supercritical stage where the CO₂ is fluid state as in sCO₂ state, then specific care should be taken to select proper grade of steel for construction, and other integrity requirements over the pipeline’s life cycle.     

Power generation

The unique properties of sCO₂ present advantages for closed-loop power generation and are applied to various power generation applications. Power generation systems that use traditional air Brayton and steam Rankine cycles are also upgraded to sCO₂ this increases the plant efficiency and power output.

The relatively new Allam power cycle uses sCO₂ as the working fluid in combination with fuel and pure oxygen. In this process the CO₂ produced by combustion mixes with the sCO₂ working fluid and a corresponding amount of pure CO₂ must be removed from the process, for industrial use or sequestration. This process reduces atmospheric emissions to zero, a significant impact on environment.

It presents interesting properties that promise substantial improvements in system efficiency. Due to its high fluid density, sCO₂ enables extremely compact and highly efficient turbomachinery. It can use simpler, single casing body designs, as compared to the steam turbines which requires multiple turbine stages, associated casings, and additional inlet and outlet piping system. The high density of sCO₂ allows for highly compact, microchannel-based heat exchanger technology.

Turbine manufacturers have used sCO2 to redesign their turbines that enabled a 50% efficiency of converting heat energy to electrical energy. In this new design turbines the CO₂ is heated to 700 °C, demanding lower compression and allows heat transfer. The process can reach to full power stage in just couple of minutes, this is significant improvement as compared to the steam turbines that require above 30 minutes to reach full power stage. The improved turbines are more compact requiring about 10% space in the plant.

Reduces Thermal fatigue and corrosion

Due to its superior thermal stability and non-flammability, direct heat exchange from high temperature sources allows higher working fluid temperatures and therefore higher cycle efficiency. The water and steam is a two-phase flow system, however the sCO₂ system is a single-phase this eliminates the requirements of heat input change for phase change, associated with the water to steam conversion. This is a significant improvement on the life and integrity of the turbine system as it eliminates the thermal fatigue and corrosion damages down time for repairs and failures.

The benefits of substantially higher efficiency and lower capital costs, comes with some increased concern with the use of sCO2 in terms design issues, the corrosion engineering, material selection and welding faculties are most critical in this new aspect.

In power generation the materials for components must have some degree of resistance to high-temperature damage, such as oxidation, and creep. Candidate materials that meet these properties and address the performance objectives include nickel-based superalloys for turbomachinery components and austenitic stainless steels for piping. Components within sCO₂ Brayton loops suffer from corrosion and erosion, specifically erosion in turbomachinery and recuperative heat exchanger components and intergranular corrosion and pitting in the piping.

Material selection should be based on the industry experience with material in service in similar service conditions. Testing should be conducted in the simulated conditions. Various Ni-based alloys, austenitic steels, ferritic steels, and ceramics for corrosion resistance in sCO₂ cycles have proven suitable in wide range of conditions. These materials are of interest because of their ability to form protective oxide layer in the presence of carbon dioxide. But since none of the material meet universal usage recommendation, specific service environment must be used to test specific material for use.

Other applications

The development of closed-cycle gas turbine to operate at temperatures near 550°C allows for bulk thermal and nuclear generation of electricity, because the supercritical properties of carbon dioxide at above 500°C and 20 MPa enable thermal efficiencies approaching 45 percent. This increases the electrical power produced per unit of fuel, this would have significant impact on the environmental impact.

Use as refrigerant

Supercritical CO₂ is has emerged as natural refrigerant, that is also a low carbon solution for domestic heat pumps. Supercritical CO₂ heat pumps are commercially marketed in Asia.

Coal Industry

The enhanced recovery method using carbon sequestration is often combined with “Clean coal” technologies. The gasifiers are used, replacing the conventional furnaces, where coal and water are reduced (Broke down) to hydrogen gas, carbon dioxide and ash. This hydrogen gas is then used to produce electrical power in combined-cycle gas-turbines.

The CO₂ is captured and compressed to the supercritical state and injected into geological storage or injected in the aging oil fields to improve yields, the process is called enhanced recovery wells. This way the sCO₂ ensure remains out of the atmosphere, improving environmental quality.