Industrial residues could help capture and permanently store carbon dioxide

Sweden's pulp and paper industry generates large quantities of residual materials every year, much of which has to be managed as waste. Researchers at Luleå University of Technology have shown how some of these residues could instead be used to capture carbon dioxide – and how naturally occurring enzymes can significantly accelerate the process.

“We are bringing together two challenges: the need to reduce carbon dioxide emissions and the need to find better uses for industrial residues,” says Ayanne de Oliveira Maciel, who recently defended her doctoral thesis in Biochemical Process Engineering at Luleå University of Technology.

Using enzymes to capture carbon dioxide

In the thesis Enzyme-Assisted Accelerated Weathering of Industrial Residues for Carbon Capture and Storage, she investigates residues from the pulp and paper industry, including lime mud and other mineral-rich materials.

The method is based on a process similar to the natural weathering that occurs when carbon dioxide reacts with minerals. The problem is that these natural processes are slow. Ayanne de Oliveira Maciel therefore investigated whether an enzyme could be used to accelerate the reaction.

The enzyme helps carbon dioxide react with water while also promoting the release of elements such as calcium and magnesium from the residual materials. Under certain conditions, the experiments showed that the enzyme could increase carbon dioxide capture by up to four times compared with processes without the enzyme.

“Nature can bind carbon dioxide in minerals, but it takes a long time. We are trying to accelerate these processes while using materials that are already available within industry,” says Ayanne de Oliveira Maciel.

Reusing the enzyme

One challenge is that enzymes can be expensive and sensitive, making their use in large industrial processes difficult. The researchers therefore tested attaching the enzyme to extremely small magnetic particles.

This allowed the enzyme to be separated from the mixture after the process and used again. In the experiments, it remained active for ten consecutive cycles and achieved higher productivity than the free enzyme.

“If an enzyme is to be interesting on an industrial scale, we need to be able to reuse it. The possibility of separating and recovering the enzyme is therefore an important part of the work,” she says.

The researchers also investigated how factors such as the amount of residual material, temperature, pressure and carbon dioxide flow affect the process. Under certain conditions, the rate of carbon dioxide uptake doubled.

Making sure the carbon stays stored

Capturing carbon dioxide is only one part of the challenge. To reduce the amount of carbon dioxide in the atmosphere, it also needs to remain securely stored for a very long time.

The research therefore goes beyond capture to investigate how carbon dioxide can react with rocks and become locked into solid carbonate minerals. In cooperation with the Ore Geology group, tests using Swedish rock samples confirmed that carbon dioxide could be stored in stable mineral forms.

This opens the possibility of a process in which carbon dioxide is first captured using enzymes and industrial residues and then permanently stored through natural reactions with suitable rocks.

“The important thing is to consider the entire chain. Carbon dioxide should not only be captured, but converted into a form in which it can remain stable for a very long time,” says Ayanne de Oliveira Maciel.

Reducing both waste and transport

One potential advantage is that residues from pulp and paper production could be used close to where carbon dioxide emissions occur. If suitable rocks for storage are also available nearby, this could reduce the need for energy-intensive compression and long-distance transportation of captured carbon dioxide.

This is particularly relevant for industries using biomass. If carbon dioxide originally absorbed from the atmosphere by growing trees can be captured and permanently stored, the process could ultimately contribute to negative emissions.

At the same time, materials that would otherwise require disposal and generate costs could be given a new use.

“We want to show how biological tools, industrial residual streams and minerals can be connected within the same system. This could create new opportunities for managing carbon dioxide while making better use of resources that industry already has,” says Ayanne de Oliveira Maciel.

Carbon dioxide must remain

Capturing carbon dioxide is only part of the problem. To truly reduce the amount of carbon dioxide in the atmosphere, it must also be stored in a stable manner for a very long time.

That is why research has moved beyond the capture process itself to investigate how carbon dioxide can react with rocks and be converted into solid carbonate minerals. In collaboration with the university’s ore geology group, tests using Swedish rock samples showed that carbon dioxide could be bound in stable mineral forms.

This paves the way for a process in which carbon dioxide is first captured using enzymes and industrial byproducts and then permanently stored through natural reactions with suitable rock.

“The important thing is to look at the entire chain. Carbon dioxide should not only be captured, but also converted into a form where it can be stored stably over a long period of time,” says Ayanne de Oliveira Maciel.

Can Reduce Both Waste and Transportation

One potential benefit is that byproducts from the pulp and paper industry could be used close to where the carbon dioxide is generated. If suitable rock formations for storage are also nearby, the need to compress and transport the carbon dioxide over long distances could be reduced.

This is particularly interesting for industries that use biomass. If carbon dioxide that was originally absorbed from the atmosphere by growing trees is captured and stored permanently, the process could, in the long term, contribute to negative emissions.

At the same time, materials that would otherwise incur costs for handling and disposal gain a new use.

“We want to show how biological tools, industrial waste streams, and minerals can be integrated into the same system. This can create new opportunities for managing carbon dioxide while making use of resources that already exist within the industry,” says Ayanne de Oliveira Maciel.