Supercritical CO2: Powering the Future with Efficient Waste Heat Recovery

With most industries working to reduce carbon emissions before 2030 and a goal of net zero by 2050 fast approaching, more action is needed to decarbonise operations. The International Energy Agency (IEA) points out that energy-related CO₂ emissions grew by 1.1% in 2023 to reach a new record high of 37.4 billion tons — an increase incongruent with limiting global warming to 1.5°C. Innovative solutions are required to accelerate decarbonisation efforts while also maintaining energy security.

One promising approach to low-carbon power generation involves harnessing excess industrial heat to produce electricity. This emerging class includes waste heat strategies that power technologies using supercritical CO₂ (sCO₂). These sophisticated systems enable new emissions-reduction possibilities for various industries, including those traditionally considered difficult to abate.

Reducing Emissions with Waste Heat to Power

Industrial processes can release significant amounts of heat into the atmosphere. Waste heat to power (WHP) captures and uses this heat to generate electricity.

Besides strengthening energy reliability and security by converting waste into a valuable resource, WHP systems reduce greenhouse gas (GHG) emissions by limiting the demand for fossil fuel-based electricity production. The steps in a conventional closed-loop waste heat-to-power process include:

1. Capture the hot exhaust gasses or high-pressure steam from operations and use it to heat a working fluid, boiling it to steam.
2. Use the working-fluid steam to power a generator or turbine, producing electricity.
3. Cool the steam back to a liquid and repeat the process.

Because WHP occurs in a closed loop or semi-closed loop, the working fluid (traditionally water) never encounters the industrial equipment or chemicals that create the waste heat. WHP systems are process-agnostic and modular, making the technology feasible for many applications.

Waste Heat to Power Applications

In the power generation industry, a “combined cycle” plant is a facility that uses waste heat from its initial cycle to produce additional electricity. Combined cycle technology is currently used with fossil fuel-based — as in, gas-fired or coal-fired — nuclear and geothermal power plants.

The energy transition is driving interest in operating plants with low-carbon fuels or renewables; these facilities also have the potential to benefit from WHP. Likely candidates for WHP include using the waste heat from concentrated solar energy as well as from hydrogen-fueled power plants; and decarbonising high-temperature operations, such as cement, steel and glass manufacturing, which are hard to abate.

Working Fluid Variations

There are several ways to generate power from waste heat, with the major differentiator being the type of “working fluid” within the system. Water is the most prevalent working fluid; using steam to power turbines is a well-understood, centuries-old technology. However, transforming water into steam requires very high waste-heat temperatures for energy production.

Since the temperature level of waste heat from industrial processes varies greatly, some designers adjust the working fluid to enable electricity production at a broader temperature range. Some WHP systems, for example, use a water-ammonia mix or a liquid hydrocarbon.

The use of sCO₂ as a working fluid has recently emerged. This exciting innovation will increase the efficiency and operating envelope of WHP systems.

Waste Heat to Power Using sCO₂

Using sCO₂ as a working fluid helps overcome inherent waste heat challenges such as low temperatures and temperature fluctuation. When CO₂ is supercritical, it is neither a gas nor a liquid — it exhibits properties of both, with low viscosity and high density. These physical characteristics boost power cycle efficiency, making sCO₂ an excellent choice for converting waste heat to electricity.

Carbon dioxide attains a supercritical state at temperatures and pressures above its critical point. Relative to the critical temperature for water, which is over 700°F (371°C), the critical temperature for CO₂ is low, below 90°F (32°C).

Although sCO₂ is an advantageous working fluid, maintaining a constant supercritical state is challenging. Carbon dioxide's critical temperature falls within the parameters of many industrial operations. If temperatures swing one way or the other, sCO₂ could transition to a gas or liquid, causing issues.

Fortunately, cutting-edge technologies address this issue and enable safe, effective handling of carbon dioxide. John Crane's on-demand webinar "Sealing the Future: Using Innovation to Accelerate CCUS" explains the latest advancements and technology used for sealing sCO₂ to support CCUS and new energy applications.

Advantages of sCO₂ for Power Generation

Converting waste heat reduces environmental impact and improves energy efficiency, and advanced WHP using sCO₂ as the working fluid offers further prospective benefits.

Low-temperature Operation

One of the primary benefits of using sCO₂ for waste heat to power lies in its ability to use heat at lower temperatures than conventional working fluids, particularly water. Waste heat below 100-230°C (200-450°F) cannot sustain effective power generation using traditional water-based WHP solutions. Alternatively, sCO₂, with its low critical temperature, can harness lower-temperature heat, expanding the viability of WHP to a wide range of energy and process applications.

Increased Efficiency

Energy efficiency is often referred to as the "first fuel", and it's a fundamental strategy for achieving net zero goals. Using sCO₂ as a working fluid avoids undergoing an energy-wasting phase change (i.e., from liquid to vapor) by maintaining a supercritical phase throughout the WHP process. The stability translates into greater efficiencies and more power generation, lowering the carbon intensity of electricity produced.

Compact Footprint

Another advantage of using sCO₂ for WHP is the compact nature of the equipment. Smaller turbines can be used with sCO₂ than with other working fluids, minimising system footprint. This is favorable for applications where space is a premium, such as retrofitting WHP into existing facilities.

Few Hazards

Working fluids, such as liquid hydrocarbons and ammonia, require special handling procedures due to the potential hazards to workers, equipment and the environment. sCO₂ is non-flammable and non-toxic; it is relatively safe for use in an industrial setting.

Applications for Waste Heat to Power Using sCO₂

Hard-to-abate sectors face unique challenges in reducing GHG emissions due to various barriers, one being operational conditions and quality of waste heat. WHP using sCO₂ as a working fluid is suitable for lower-temperature applications, making it a possible pathway to decarbonisation for these industries.

Cement Manufacturing

Heating limestone with other raw materials to manufacture cement generates significant heat, often vented to the atmosphere. sCO₂ power systems can harness this waste heat, turning it into electricity that can be used in the plant itself or sold back to the grid.

Steel Manufacturing

Steel mills and metal processing facilities create vast amounts of waste heat during operations. Equipment using sCO₂ can recover the heat from coke ovens and blast furnaces, converting it into electricity and reducing energy costs and environmental impact.

Glass Production

Another target industry is glass production. The concentrated heat created by the melting and annealing processes used to manufacture glass is often discarded as waste. Glass producers can convert waste heat into electricity using WHP.

Chemical Processing

In the chemical processing industry, high-temperature heat is generated to create petrochemicals, industrial gasses, plastics, fertilizers and more. Instead of letting this heat go to the atmosphere, a compact sCO₂ WHP setup can be retrofitted into the process to turn this waste heat into valuable electricity.

Aluminum Manufacturing

Aluminum manufacturing is an energy-intensive process that creates large amounts of heat. By implementing a sCO₂ WHP system, these facilities can transform waste heat into power, thereby reducing costs and their carbon footprint.

John Crane's sCO₂ Solutions

Carbon dioxide in any phase can pose challenges. John Crane's proven experience with CO₂ and sCO₂ stems from nearly three decades of supporting carbon capture initiatives.

Safely and reliably sealing sCO₂ in rotating equipment requires a design that addresses application-specific performance requirements such as high pressure, high speed and high temperatures. John Crane has experience overcoming these challenges.

Our sealing solutions were selected for the turbomachinery in the world's first Allam Cycle power plant, using sCO₂ as the working fluid. The success of this pilot project led to the construction of additional facilities employing this advanced power generation cycle.

Numerous John Crane sealing technologies, seal gas systems and filtration solutions are applicable for sCO₂:

• Aura® 220
• Type 28XP
• Type 28EXP
• Separation seals
• Gas seal systems
• John Crane's Metastream® power transmission couplings
• Filtration solutions

Powering sCO₂ Innovation

Innovation is helping drive the energy transition forward, presenting new ways to lower emissions and improve process efficiency. Waste heat to power systems contribute to achieving net zero goals by maximising resources and sustainability. State-of-the-art WHP technology using sCO₂ as a working fluid offers advantages over traditional WHP solutions, enabling electricity generation at a wider temperature range, posing few hazards and taking up less space.

John Crane is proud to put our sCO₂ expertise to work by providing technologies, solutions and services that support customers on their net zero journey. Contact us today to accelerate your energy transition.