Ways of capturing CO2 from diluted sources. Trends and technologies

In recent years, the CO₂ capture processes start to be established as a powerful tool to reduce emissions of contaminant gases in the industry. According to the IEA CCS plans, the evolution must be clear: The capture processes must grow from tens of mega tonnes in 2020, to 1000s of mega tonnes in 2050. All with the objective of achieving agreements in Paris in 2015 in an effort to avoid irreparable harm to our environment. In this article, we will talk about capturing CO2 from diluted sources.

CO₂ capture processes: challenges and limitations

Saying this, huge future challenges are presented related to the efficiency of current capture processes. Perhaps one of the great limitations of the implementation of capture technologies is the price, and the cost of CO2. This is directly related to the energy expenditure of the process in question.

Very diluted CO₂ currents shoot up costs. Source: IEA 

Whenever we design a capture process, we have this aspect in mind, this indicator, like a sword of Damocles always willing to ruin any idea, project or solution. And it is the cost of captured CO2 that depends enormously on the source of emissions from which it comes from. An almost pure stream of CO2, with barely without contaminants or components, is not the same as a very diluted current, where CO2 is markedly minority. And is here where the most of current problems surge.

So we have very cheap processes, in the range of 15-25 €/tonnes of CO2 in industrial processes that produce very strong currents. For example, this is the case of the production of ethanol or the processing of natural gas. But also, we have processes with much more expensive capture costs, in concentrations below 20%, with costs between 35-105 €/tonnes of CO2. For example, this is the case of emissions in ovens of cement or of the production of steel.

Given this, one must take into account, if we attend to the statistics, that more than half of the CO2 emissions to the atmosphere come from very diluted sources, below 2%. In this situation, lots of the processes currently in force or failed promises or are practically unviable. This is due to their high energy cost to reach reasonable efficiency.

Tile companies in the Valencian Community

Likewise, an example, here in the Valencian Community, perhaps a more important focus of emissions comes from tile companies. Their sources contain less than 2% of CO2, as well as lots of contaminants, which the process of capturing is enormously difficult and the use through using more or less technologically established and mature methodologies as it can be the absorption with amines (MEA), membranes, solid adsorbents or different methods of distillation as cryogenic distillation. And if we go further, we have highly dispersed emissions, such as those from transport, calefaction and all those that we put in the sack of what we call direct air capture or atmospherically capture, where we find environment concentrations to 300-400 ppm or 0.03-0.04%. This shoots up the cost of capturing until current levels are unacceptable.

New developments in the capture of CO₂ from diluted sources

However, in recent years it has been a fight to give a solution to these limitations. The new investigations and developing aims, above all, to minimise costs and expenditure in CO2 capture proceeding from very diluted sources. And there is not a single valid line, however, there are variations in the portfolio with very promising results.

On the one hand, we have almost endless fields of solid sorbents. New, very specific adsorbent materials can have great efficiency and selectivity at very low concentrations, below 1-2% are being developed. They are, for example, some types of MOFs (“Metal-Organic Frameworks”). It is certain that not everyone works in that concentration range, but there are some like SIF6-3-Cu that are able to capture CO2 in a very efficient way at lower partial vapour pressure, which corresponds to very diluted sources. Not everything is so perfect and fairly, at present time, these MOFs are expensive and complex of manufacturing on a large scale, while the most affordable are not so efficient at lower pressures. This is a challenge that we hope to overcome in the next years.

SiF6-3-Cu is able to capture CO2 in a very efficient way at lower partial vapour pressure, which corresponds to very diluted sources. Not everything is so perfect and fairly, at present time, these MOFs are expensive and complex of manufacturing on a large scale, while the most affordable are not so efficient at lower pressures. This is a challenge that we hope to overcome in the next years.

A tremendously interesting technology is to apply traditional and cheap liquid sorbents the amino in packaged rotating beds, more known as RPBs (Rotating Packed Beds”). The technology of the “washing machine effect” generates a large centrifuge force by the turn of the rotor. This provokes a very turbulent gas flow that, combined with the “breakdown” of the liquid sorbent into tiny droplets, makes the interaction between sorbent/adsorbent increase exponentially increasing the efficiency of CO2 capture, to higher values of 80% in concentrations below 5%.

Hybrid systems: a viable solution

Perhaps one of the more viable solutions to the short and medium place will be the use of promising hybrid systems, which, from AIMPLAS, we have been working on for some years. It tries to combine different types of traditional capture technologies by connecting them, either in series or in parallel. The aim is to try to take the maximum out of each one of them and to get synergistic effects between them.

In this way, in the first stage they could use, for example, MOFs or zeolites are not so efficient for diluted currents but are much cheaper to produce. This first stage generates a simple concentration of CO2 at levels, let’s say, much more manageable. Leaving this stage of tradition liquid amines can work with this already concentrated flow with total efficiency. This combination of resources in a synergetic way is sensible and cheap, although various iterations and simulation processes are required. This is made to adjust the independent manner to each specific case, either by the composition of the emissions, quantity and flow among other issues.

Currently, we are far from arriving at the end of the road in the development of new CO2 capture technologies. There are several options, deserving of being studied in depth, which require a large investment in developing material and engineering processes. The simulations are very suffering and the scales, are sometimes very complete and unexpected. And, above all, to highlight that there is not a miraculous solution, but adapted, specific and optimised solutions for each specific industrial case.

Adolfo Benedito

Decarbonization researcher