Use of the Brayton cycle for carbon capture and storage

Introduction

The Brayton cycle is a fundamental concept in thermodynamics, used to describe the operation of gas turbines for electric power generation and propulsion in jet engines. Through compression and expansion processes it converts thermal energy into mechanical work in the production of energy globally.

However, in a world where reducing carbon emissions has become critical to mitigating climate change, the Brayton cycle is gaining new relevance. Its integration with Carbon Capture and Storage (CCS) technologies represents a significant advance towards energy sustainability. This cycle, when integrated into CCS systems, contributes to improving the efficiency of energy systems, and plays an important role in reducing the carbon footprint of the energy industry.

The purpose of this article is to analyze how the Brayton cycle, beyond its traditional use in power generation, can contribute significantly to carbon capture and storage. It seeks to highlight the importance of its integration with CCS technologies, underlining the adaptations needed to maximize efficiency and reduce carbon emissions in advanced energy systems.

How does a gas turbine work?

The Brayton cycle describes the ideal behavior of a gas turbine engine and involves a series of processes that convert fuel energy into mechanical work. It is characterized by isentropic compression and expansion, and isobaric addition and rejection of heat. The Brayton cycle can be open or closed. The closed cycle retains the working gas within the system and uses heat exchangers for heat transfer. The process steps are as follows:

1. Intake: Cold air at atmospheric pressure enters through the turbine inlet.
2. Compression: The air is compressed and directed into the combustion chamber by a compressor (driven by the turbine). Since this phase is very fast, it is modeled by an adiabatic compression.
3. Combustion chamber: In the chamber, the air is heated by combustion and expands, so the heating is modeled as an isobaric process.
4. Turbine: The hot air passes through the turbine and moves the blades. In this step the air expands and cools rapidly, which is described by an adiabatic expansion.
5. Exhaust: Finally, the cooled air (but at a higher temperature than the initial one) exits to the outside.

Technically, this is an open cycle since the air that escapes is not the same as the air that enters the turbine mouth, but since it does enter in the same quantity and at the same pressure, the approximation of assuming recirculation is made. In this model the exhaust air simply gives up heat to the environment and is recirculated. This corresponds to constant pressure cooling.

In a closed Brayton cycle with heat recovery, the heat exchanger or heat recovery unit is used to capture and reuse waste heat from the exhaust gases leaving the turbine. This heat is used to preheat the working fluid before it enters the main heat exchanger, where additional energy is supplied to bring the fluid to the temperature required for expansion in the turbine.

Integration with carbon capture technologies

The Brayton cycle, used primarily in gas turbines for electricity generation, has become a key component in improving the efficiency of energy systems and reducing carbon emissions. The integration of the cycle with carbon capture systems represents a significant advance in the quest for cleaner energy. However, this integration requires specific adaptations to ensure effective operation.

In its basic form, the Brayton cycle involves the compression of air, combustion of a fuel, and expansion of the resulting hot gases to generate work in a turbine. This process is inherently compatible with high exhaust gas flows, making it a good candidate for carbon capture.

In order to integrate a carbon sequestration system, it is essential to adjust several stages of the cycle

First, exhaust gas temperature and pressure are crucial factors. Carbon capture by processes such as chemical absorption or oxyfuel capture generally requires exhaust gases at a specific temperature and pressure. To accommodate this, the afterburner stage of the Brayton cycle can be modified by adjusting the operating conditions to optimize capture efficiency.

Second, integrating the cycle with carbon capture technologies may require the incorporation of additional equipment, such as heat exchangers and gas treatment units. This equipment not only recovers waste heat to improve overall system efficiency, but also ensures that exhaust gases are properly treated prior to CO₂ capture.

In addition, modifications to the pressure ratio and turbine efficiency are necessary to compensate for the additional energy required by the carbon capture process. Installing a carbon capture system can increase energy demand, which can reduce the net efficiency of the cycle. Therefore, it is critical to optimize the cycle configuration to minimize this efficiency loss.

Finally, the type of fuel used in the Brayton cycle can also influence integration with carbon capture systems. Fuels that produce less carbon or are easier to process for capture, such as natural gas, can improve the viability of the integrated system.

Technologies and processes in the Brayton cycle for carbon sequestration

The supercritical state is an interesting and often confusing concept because a fluid in this state behaves neither as a liquid nor as a gas in the traditional way. In the case of carbon dioxide (CO₂), when in supercritical conditions (i.e., at a pressure above 73 bar and a temperature above 31 °C, it acquires properties intermediate between those of a gas and a liquid, allowing it to have both a density similar to that of a liquid and a diffusivity comparable to that of a gas.

This happens because, being in the supercritical state, CO₂ does not undergo a traditional phase change (such as from liquid to gas). Instead, the gas densifies as pressure is applied to it, but does not actually condense into a liquid. The result is a fluid that has a much higher density than a conventional gas, which more closely resembles the density of a liquid.

At the same time, this fluid retains a high diffusivity, which is the ability of molecules to move and disperse through a medium. This is typical of gases, where molecules are farther apart and can move relatively freely. The combination of these two properties-high density and high diffusivity-is what makes supercritical CO₂ especially useful in a variety of industrial applications, such as chemical compound extraction and carbon capture.

Current and future trends in the Brayton cycle for carbon sequestration

The Brayton cycle has evolved significantly, especially in its application for carbon capture. As the need to reduce CO₂ emissions becomes more urgent, current and future trends in the Brayton cycle focus on improving energy efficiency, reducing costs and facilitating integration with carbon capture technologies.

Here are some of the most relevant trends:

Use of Supercritical CO₂ (sCO₂) as working fluid.

• Current trend: s-CO₂ due to its high thermal efficiency and ability to operate at high temperatures allows for more compact and efficient cycles, which is ideal for integration into power generation plants and industrial processes seeking to reduce carbon emissions.
• Future Trend: Continued development of materials and technologies that withstand the extreme temperature and pressure conditions of s-CO₂ is expected, thereby improving the durability and efficiency of Brayton cycle components.

Integration with combined cycles and heat recovery cycles

• Current trend: Brayton cycles are increasingly being integrated with Rankine cycles and other heat recovery systems to maximize energy efficiency and minimize emissions. This integration allows more energy to be captured from the process, reducing the need for fossil fuels and associated CO₂ emissions.
• Future trend: Future research could focus on optimizing these combined cycles, looking for configurations that maximize heat recovery and overall cycle efficiency, with a particular focus on CO₂ emissions reduction.

Development of oxy-combustion technologies

• Current trend: Oxyfuel combustion, which uses pure oxygen instead of air for combustion, is a technique that is being integrated with Brayton cycles to facilitate CO₂ capture. This technology produces an exhaust gas stream with a high concentration of CO₂, which simplifies its capture and storage.
• Future trend: Oxyfuel combustion is expected to be refined, with improvements in process efficiency and CO₂ capture and compression technology. Integration of oxy-combustion with advanced Brayton cycles could become more common in power generation plants and industrial applications.

Renewable energy and energy storage application

• Current trend: The use of the Brayton cycle with s-CO₂ in energy storage systems and integration with renewable energy sources, such as concentrated solar, is being explored. These systems allow thermal energy to be stored and converted to electricity when needed, with high efficiency.
• Future trend: Combining cycling with energy storage technologies will be key to stabilizing renewable energy generation and reducing dependence on fossil fuels. This could also include capturing CO₂ produced during peak demand, thus optimizing emissions management.

Examples: Industrial facilities using the Brayton cycle for carbon sequestration.

The Brayton cycle has been adopted in various industrial applications, especially in projects that seek to capture and reduce carbon dioxide (CO2) emissions. Here are some examples of facilities using the Brayton cycle in the context of carbon capture:

Power generation plants with gas turbines

A combined cycle power plant, which uses gas turbines to generate electricity, can integrate a modified Brayton cycle with carbon capture. These plants combine the Brayton cycle with a Rankine cycle to take advantage of waste heat.

_CO2 capture is achieved through oxy-combustion, where fuel is burned in the presence of pure oxygen, generating an exhaust gas stream with a high concentration of CO2, which is then captured before the gases are released into the atmosphere.

Carbon capture projects in industrial processes

In the steel industry, some facilities use the Brayton cycle to generate power from the hot exhaust gases produced during steelmaking. These gases contain CO2, which is captured after the heat has been harnessed to generate power.

Contribution to carbon capture and storage

The Brayton cycle can contribute to carbon capture and storage in several ways:

1. Integration with CCS technologies: Post-combustion: Post-combustion carbon capture technology can be applied after the turbine, where exhaust gases are treated to remove CO₂ before being released into the atmosphere. Pre-combustion: In this approach, the fuel is gasified to produce a mixture rich in hydrogen and CO₂. The CO₂ is captured prior to combustion and the turbine uses hydrogen as fuel.
2. Brayton cycle with supercritical CO₂: Instead of using air, some advanced cycles use supercritical carbon dioxide as the working fluid. This approach not only improves thermal efficiency, but also facilitates CO₂ capture and storage, since CO₂ is the primary working fluid.
3. Improved efficiency: Improvements in cycle efficiency reduce the amount of fuel required to generate the same amount of energy, which in turn reduces CO₂ emissions. A more efficient cycle means less carbon produced per unit of energy.
4. Implementation example: A practical example is the use of combined cycles where a gas turbine (Brayton cycle) works together with a steam turbine (Rankine cycle). In this system, the waste heat from the gas turbine is used to generate steam, increasing the overall efficiency of the plant. The implementation of CCS technologies in these combined cycle systems can capture a larger fraction of the CO₂ emitted.

Conclusions

The supercritical_CO2-based recompression Brayton cycle offers the opportunity to increase cycle efficiency compared to Rankine and some other Brayton cycles and result in power plants with higher process efficiency.

_CO2 appears to be an ideal working fluid because it has a critical point well suited for land-based applications with a moderate critical pressure and a critical temperature that is low enough to be achieved with ambient cooling when the wet cooling option is available.

The supercritical_CO2 Brayton cycle can be adapted to direct heating applications, with potential for high efficiency and process water capture; such a cycle also facilitates_CO2 capture from the combustion process. When its availability and low cost are taken into account, no other substance is a more attractive working fluid for the supercritical Brayton cycle.

It is possible to minimize pollutant emissions from a coal-fired power plant by converting coal to fuel gas (synthesis gas or syngas) by partial oxidation (or gasification), cleaning the gas and then burning it in gas turbines. The_CO2 generated from a given fuel source per unit of energy produced is inversely proportional to the thermal efficiency of the plant for a given coal conversion.

References

1. https://oa.upm.es/61515/
2. https://www.energy.gov/sites/default/files/2016/06/f32/QTR2015-4R-Supercritical-Carbon-Dioxide-Brayton%20Cycle.pdf
3. https://scienceinfo.com/brayton-cycle/
4. https://en.wikipedia.org/wiki/Brayton_cycle