Solving sorbent use in carbon capture

How to improve carbon capture processes through solids heat exchange technology

It’s easy to capture carbon dioxide (CO₂) from the flue gas at a power plant, right?

Doing so economically? That’s a different story.

Carbon capture dates to the 1930s with the use of solvents in the natural gas industry. Since then, carbon capture and utilization (CCU) has become a growing industry dedicated to re-using captured CO₂ to fight climate change.

Chemical absorption using alkanolamine solvents such as monoethanolamine (MEA) has emerged as one of the more established commercial processes. However, solvents such as MEAs come with significant trade-offs, including high energy consumption requirements for recycling and the solvent degradation (e.g. oxidative degradation) that occurs.

In recent years, the use of solid sorbents has emerged as another solution to removing CO₂ from the atmosphere as well as exhaust and flue gases. The process offers the bonuses of significant energy savings and environmental benefits.

“It’s much easier to use solid sorbents in carbon capture due to the relative simplicity of taking the heat in and out. And easier means more cost-effective,” says Neville Jordison, Chief Executive Officer with Solex Energy Science. The Canadian-headquartered company focuses on solving heat transfer needs from green technology solutions such as solar power, waste heat recovery and carbon capture.

The traditional challenge

Solvents such as MEA that are used in carbon capture processes have a strong chemical reaction with CO₂, and the desorption process can be energy intensive. This results in high costs and unintentional emissions.

Carbon capture 1 Other fluid-based solvents (e.g. ionic liquids) have comparable costs due to high viscosities that restrict large-scale industry applications.

A better way

Solid sorbents used for carbon capture employ either physical (physisorption) or chemical adsorption (chemisorption). In the case of physisorption, target molecules are attracted to a high surface-area sorbent such as silica, activated carbon, graphite, polymers and zeolite. This sorbent has a low heat capacity of adsorption that is greater than heat of the adsorbate. Due to this, they need to be cooled during the adsorption process.

The sorbents regenerate in a desorption process. The desorption needs energy. This can be done by using temperature, pressure or a purge media such as steam. The CO₂ is collected (e.g. separated from the purge media) and either utilized or sequestered. The sorbents can then be re-used.

Because the heat capacity of sorbents is lower than that of liquid solvents, less heat is needed to accomplish the temperature swing. Traditionally, the desorption process is done via distillation, in which all the liquid phase needs to be evaporated and condensed. In solid adsorption, the energy demand for regeneration is lower because the solids are simply heated and cooled.

The role of heat exchangers

As noted earlier, sorbents adsorb CO₂ from a flue gas stream or directly from the gas stream. Those sorbents are then heated to release the CO₂.

The next step involves cooling the sorbents before they return to the adsorption step. This means there are two moving bed heat exchangers (MBHEs) involved:

One to heat up the sorbent to release the CO₂
A second to cool it to the optimum temperature for the adsorption process

A benefit to MBHE technology is its ability to reduce costs by re-using energy from the carbon capture process.

“Bulk solids heat exchange technology is bringing the carbon capture process to a new level by allowing energy recovery between the absorption and desorption processes,” says Gerald Marinitsch, Global Director of Industrials at Solex Thermal Science, whose company collaborates with Solex Energy Science.

“It can efficiently cool the sorbent during the adsorption process. Then, energy rejected in the cooling media can be used to heat the solids during the desorption process.”

More than meets the eye

Jordison notes these types of heat exchangers are as efficient as any other type of heat exchanger. However, they are more difficult to size due the specific knowledge required.

“There are a lot of things going on in these moving bed heat exchangers, meaning not everyone can do it effectively,” says Jordison. “In addition, the thermal modelling is complicated. You’ve got to understand mass flow and the related factors that come into play.

“And there’s a lot of advanced engineering that goes into building out this technology – something Solex Energy Science has focused on for decades. You need a solid understanding of how to evaluate and analyze thermal-mechanical stresses. That's not to forget the construction material properties and the fabrication techniques required for a successful design. There’s no such thing as a cookie-cutter answer to these types of challenges.”

Ready to talk specifics? Contact a Solex team member today.

Carbon capture Heat EX 1