Use of high temperature heat pump for solvent regeneration in post-combustion carbon capture unit with liquefaction facility: Concept and thermodynamic analysis

Cement is the invisible skeleton of the modern world, the material that holds our cities together. Yet, the process of making it is a major contributor to global warming. When limestone is heated to create cement, it releases vast amounts of carbon dioxide, a greenhouse gas that traps heat in the atmosphere. To stop this, engineers have developed ways to capture that gas before it escapes into the sky. One of the most reliable methods involves washing the exhaust fumes with a special liquid that soaks up the carbon dioxide. However, this liquid eventually becomes full and needs to be cleaned so it can be used again. The cleaning process requires a tremendous amount of heat, often more than the factory has available to spare. If the factory cannot find enough heat to clean the liquid, the carbon capture system cannot work efficiently, leaving the pollution problem unsolved.

At the same time, once the carbon dioxide is captured, it must be turned into a liquid to be shipped away for storage. This transformation requires cooling the gas down to extremely low temperatures. In doing so, the machinery that cools the gas generates a large amount of waste heat, which is usually just dumped into the air. This creates a frustrating paradox: the factory needs heat to clean the liquid, but the process of preparing the gas for transport throws away exactly the kind of heat it needs. Researchers at the Norwegian University of Science Technology and SINTEF Energy Research have proposed a way to solve this puzzle. They suggest connecting the cooling system to the heating system in a clever loop, turning a waste product into a useful resource.

The team focused on a specific setup used in cement plants where carbon dioxide is captured and then liquefied. In the standard version of this process, the gas is cooled using a refrigeration cycle that relies on ammonia, a common industrial coolant. As the ammonia absorbs heat from the carbon dioxide to cool it down, the ammonia itself gets warm. In a typical plant, this warm ammonia is cooled back down by releasing its heat into the surrounding air, essentially throwing away the energy it just absorbed. The researchers asked a simple question: what if, instead of dumping that heat, they used it to clean the carbon-capturing liquid?

To make this work, they designed a system that acts like a thermal elevator. They added a second loop to the existing ammonia setup, using a different fluid called pentane. The warm ammonia from the cooling cycle transfers its heat to the pentane. This heat is enough to make the pentane boil and turn into a gas. As the pentane gas is compressed, it becomes even hotter. This superheated pentane is then sent to the part of the factory that needs to clean the carbon-capturing liquid. There, it gives up its heat, warming the liquid just enough to release the captured carbon dioxide so the liquid can be reused. The pentane then cools down, turns back into a liquid, and is ready to pick up more heat from the ammonia, creating a continuous cycle.

The researchers used sophisticated computer simulations to test how well this idea would work in a real-world scenario. They modeled a plant that captures one million tonnes of carbon dioxide every year. Their calculations showed that this new system could successfully recover a significant portion of the heat needed to clean the liquid. Specifically, the system could provide about 22 percent of the total heat required for the cleaning process if the liquid needs a standard amount of energy to be regenerated. If the plant uses a more efficient type of liquid that requires less heat to begin with, the system could cover up to 34 percent of the need. This means the factory would need to burn much less fuel to generate steam, saving money and reducing further emissions.

The study also looked at how much energy the new system would consume to run the pumps and compressors. While adding the extra loop does require some additional electricity, the overall balance is positive. The researchers found that the system is highly efficient at converting the work it does into useful heat. In their simulations, the setup achieved an efficiency rating of over 57 percent, which is a strong performance for a system that does two jobs at once: cooling the gas and heating the liquid. This is notably better than the standard system, which only cools the gas and wastes the heat. The team discovered that the system works best when the carbon dioxide is compressed to a specific pressure before being cooled, a detail that allows the heat recovery to happen at the right temperature.

One of the key advantages of this approach is that it does not depend on the weather or the location of the factory. In many parts of the world, cement plants struggle to find waste heat because the raw materials are wet and need to be dried, which uses up all the available thermal energy. In those wet climates, the usual method of stealing heat from other parts of the factory often fails. This new system, however, generates its own heat source right where it is needed, using the energy from the cooling process itself. It turns the act of freezing the carbon dioxide into a way to warm up the cleaning fluid, effectively recycling energy that would otherwise be lost.

The researchers acknowledged that building this system would require larger and more powerful compressors than a standard plant uses, which is a significant engineering challenge. The machinery needs to handle the flow of the pentane fluid efficiently without wasting energy. They also noted that while their results are based on computer models, the physics behind the design are sound and rely on established principles of thermodynamics. The study suggests that retrofitting existing carbon capture plants with this dual-loop system could be a viable path forward, especially for facilities that lack other sources of waste heat. By linking the cooling and heating needs of the plant, the technology offers a way to make carbon capture less energy-intensive and more practical for the cement industry, helping to turn a major source of pollution into a manageable part of the solution.