The “Talk series – Porto Towards Carbon Neutrality 2030” is an initiative organized by the Porto City Council, which includes 10 clarification and debate sessions related to sustainability, decarbonization and climate transition in the context of carbon neutrality in cities. The sixth session took place last Thursday, June 22, at the Porto Innovation Hub, with the theme “Carbon sequestration”.

Marta Pinto, Head of the Municipal Environmental Management Division of the Porto City Council, was the moderator of the event, which featured Isabel Sousa Pinto, Professor at the Faculty of Sciences of the University of Porto and Director at CIIMAR; José Paulo Silva, Director of Port’Ambiente (Veolia group); and João Gonçalo Soutinho, President and Co-founder of VERDE – Association for Integrated Nature Conservation.

Each of the speakers had the right to intervene, followed by a space for debate and clarification with the audience present.

At the opening of the session, Marta Pinto began by referring to the “excessive concentration of carbon in the atmosphere,” which she explained is predominantly of human origin. “We know that this concentration of carbon contributes to a global increase in temperature and it is this increase that impacts the delicate structure that regulates the global climate system,” she said.

Therefore, Marta Pinto warned that the main way to remove carbon from the atmosphere is not to put it there, is not to emit it: “However, we know that we are going to emit it and that, mainly, in a transition phase, it will exist. In this sense, the changes being made at the level of mobility, buildings, and food are important.”

In this regard, Marta Pinto pointed out the two available solutions – the natural ones and the technological ones: “This capture of carbon is a challenge because either we go to the big carbon-emitting sources and capture the carbon at the source, or we have to capture it from the air and transform it. These solutions are in their infancy, they are immature and not economically viable. Here, in the city of Porto, we are already working with natural-based solutions, so what Porto has been trying to do is to expand its green areas and requalify them. Among the possible calculations, in the new Alameda de Cartes Park that we are developing, we will have the annual removal of 28 tons of carbon in a 3.8 hectare park,” she said.

The various carbon capture technologies

José Paulo Silva, Director of Port’Ambiente (Veolia group), brought to the discussion the various possibilities for capturing CO2 and presented the embryonic project that Veolia is developing at LIPOR’s Energy Recovery Center.

“We are not yet talking about stopping emissions, we are only talking about slowing down the pace of these emissions. This is not enough. We should make an analogy with waste management, where the first measure is to reduce, then reuse, and only if these first two measures are not effective, does the third measure come in, which is to recycle. If recycling is not possible, other technologies come in. With carbon, it should be the same, the first major measure should be to avoid emissions,” he began.

In this regard, the person in charge of Port’Ambiente presented the various stages of CO2 capture. One of them is the option to capture carbon from the atmosphere, which, according to him, causes negative emissions, since it removes CO2 that is dispersed in the atmosphere. “This is the most virtuous solution at the present time,” he affirmed. Another option is to start by capturing biogenic CO2 (non-fossil origin), which is not a result of human action: “Therefore, as it does not result from human action, if we start by trying to capture biogenic CO2, we create negative emissions”.

In terms of technologies, João Paulo Silva confessed that the first option is not the easiest, “this is because it would be easier to capture CO2 when it is concentrated, which is not the case in the air, where there are other mixed gases”. Even so, being a viable option, he explained that this capture can be done by absorption or adsorption.

The embryonic project that Veolia is developing with Lipor is in line with this last possibility – to produce alternative fuels. “The LIPOR Energy Recovery Center (at LIPOR II) produces 300,000 tons per year of CO2. About 60% are of biogenic origin and the goal is that, with this CO2 capture, synthetic fuel can be produced for aviation,” he concluded.

Trees as allies in CO2 sequestration

In turn, João Gonçalo Soutinho, President and Co-founder of VERDE – Association for Integrated Nature Conservation, explained how trees can be allies in carbon sequestration. He explained that he has been analyzing the trees in Lousada, which he believes to be very similar to those in Porto, and concluded that oaks are the type of tree of greatest importance to biodiversity.

In this regard, they began to discuss ideas to safeguard these trees: “We concluded that we had to involve people, recognize the owners, show them that these trees are valuable while they are alive, and that we should preserve what exists”.

Based on these conclusions, VERDE emerged in 2021, “an environmental association that implemented Biodiverse Carbon, which is a compensation project for companies and citizens, where landowners can receive money to manage them correctly and keep the trees as they are”.

João Gonçalo Soutinho concluded his intervention by warning that, “more important than planting trees, is taking care of those already planted. If you cut one of them, even if you have planted many, we are going to lose, because a year of carbon sequestration from giant trees is equivalent to the first 15 years of trees we just planted”.

The Ocean’s Potential for Balance

Isabel Sousa Pinto, Professor at the Faculty of Sciences of the University of Porto and Director at CIIMAR, also discussed the role of forests in carbon sequestration, in this case, marine forests. “The ocean has already contributed a lot to preventing us from being in a worse situation, because it has already absorbed about 90% of the heat generated by the accumulation of CO2 in the atmosphere and absorbs about 25% to 30% of CO2 emitted annually,” she said.

She therefore assured that the ocean can contribute to the solutions to climate change problems: “21% of the effort that would have to be made to reach a 1.5 degree temperature increase could, in some way, come from the ocean, with ocean renewable energies, changes in maritime transport, nature-based solutions, changes in the way we fish and aquaculture, and carbon storage at the bottom of the sea.”

Algae play a key role in this issue, since, like trees, they can contribute to carbon sequestration. “Algae have a particularity, because they are based on rock, so they cannot be buried there. What happens is that the biomass of these algae is not consumed, it ends up on the beaches and in the sea and, in this case, part of it is going to sink. And it has been determined that any biomass below a thousand meters will take thousands of years to return to the surface, so it is considered that this carbon has been sequestered, even if it is not buried,” she explained.

However, Isabel Sousa Pinto recognized a problem in this process, which is related to the difficulty of knowing the amount of CO2 that is sequestered and the disappearance of marine forests globally: “These algae are receding here in Portugal, in fact, we only have marine forests in the north area and we have been trying to understand why this happened. We know that temperature is a factor, but it is not the only one.”

She concluded her intervention by explaining that the advantage of algae over other ecosystems is that their production can be greatly increased through cultivation. “The idea is to increase carbon sequestration through this cultivation,” she concluded.