Harnessing Heat For Carbon Control

More and more cement producers have initiated their carbon capture strategy by investigating available technologies and technology pathways. The most common technologies being evaluated by the industry for capturing CO₂ are various oxy-combustion technologies, CO₂-fractionation, and solvent-based post-combustion solutions. Each technology possesses distinct strengths and weaknesses, and there does not seem to be any universally applicable solution. Solvent-based post-combustion technologies are often considered the most mature and most realistic choice for retrofitting CCUS to a cement plant today. Oxy-combustion technologies are considered less mature and can be challenging to retrofit. CO₂-fractionation would be easily retrofittable but less mature than solvent technologies.

Challenges using most solvent-based capture technologies

Although certain post-combustion absorption technologies are commercially available and readily retrofittable (mainly amine-based), their implementation in the cement industry presents several challenges. For instance, the presence of relatively high amounts of contaminants in cement flue gas can lead to increased solvent degradation, resulting in substantial solvent consumption and the potential release of harmful degradation products into the atmosphere. This, in turn, can complicate the emission permitting process. Additionally, these technologies are typically heat-driven and require substantial heat inputs for solvent regeneration.

Carbon capture technologies, such as amines or chilled ammonia, were originally developed to be integrated before a thermal power plant stack to capture CO₂ after the combustion of fossil fuels, hence the designation of ‘post-combustion’ capture technology. The heat extraction to drive the capture process from the thermal power plant will result in a parasitic loss for the power plant, but there is more than enough heat available.

In contrast, the situation is typically quite different for a cement plant. While the cement plant may possess waste heat that the capture plant can utilise, the quantity of waste heat is insufficient to power the capture plant solely. Consequently, the installation of an externally fuelled or electric steam supply becomes necessary to enable the operation of the capture plant. This might be an option for some cement plants, but for others, this rules out the use of such CO₂ capture solutions.

Potassium carbonate as a post-combustion solution

Potassium carbonate is an alternative to amines and has been used commercially to capture CO2 from different gas mixtures in various industries for several decades. This process is often referred to as the hot potassium carbonate process (HPC). Industries where HPC systems are utilised include ammonia and hydrogen plants, natural gas, ethylene oxide recycle gas, vinyl acetate monomer recycle gas, and methanol synthesis plants.

Due to its inorganic chemistry and resistance to thermal and oxygen degradation, potassium carbonate is also well suited for industrial flue gas applications. In addition, the potassium carbonate solvent has a very low vapour pressure, so neither the solvent nor any degradation products will end up in the treated flue gas emitted to the atmosphere or in the CO₂ product. This significantly eases the emission permitting process.

One perceived disadvantage of the HPC process is that it is a slow solvent compared to amine-based solvents used for post-combustion CO₂ capture. Without adding new and significantly improved promoters, there is a need to compress the flue gas to achieve the required CO₂ partial pressure to effectively capture CO₂ from flue gases. Flue gas compression has generally been seen as having too high an energy penalty. Most efforts to make the HPC process work for industrial flue gases have focused on modifying the chemistry by adding new promoters/catalysers to make the process energy efficient. So far, this effort has not been overly successful.

A different approach to post-combustion HPC

Capsol Technologies has attacked the perceived disadvantage of HPC being a slow solvent in a different way. Solvent chemistry is important, but what if the solution to the challenge of making HPC work well in flue gas applications lies not only in manipulating solvent chemistry? Can the focus instead be on adapting the process design and/or manipulating the flue gas?

Instead of viewing flue gas compression as an energy-intensive problem, Capsol Technologies has recognised it as a potential opportunity. Rather than considering the HPC solvent solely as a chemical absorbent, Capsol Technologies also treats it as a working fluid in a thermodynamic cycle.

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