Building on CO2

The construction industry as a CO2 sink? Researchers at Empa's Concrete & Asphalt lab are working on this. By incorporating biochar into concrete, they are exploring the potential of CO2-neutral or even CO2-negative concrete. For optimal applicability, they process the biochar into pellets and use them to replace conventional aggregates.

To achieve the goal of a climate-neutral Switzerland by 2050, strategies and processes with a negative CO2 balance are necessary.

These so-called negative emission technologies (NET) are intended to counterbalane the remaining "hard-to-avoid" emissions in 2050 and should help ensure that we eventually achieve net zero.

As one of the main emitters, the construction sector has a particular obligation.

Around eight percent of global greenhouse gas emissions are caused by cement production.

At the same time, initial efforts are emerging to use the construction sector, with its massive consumption of resources, as a possible carbon sink.

What sounds paradoxical will succeed if we start "building with CO2" -- or rather, using carbon to produce building materials and thus removing it from the atmosphere in the long term.

For such visions to become reality, a great deal of research is needed -- such as is currently being done at Empa's Concrete & Asphalt lab.

A team led by Pietro Lura is developing a process for integrating biochar into concrete.

Difficulties due to porosity

Biochar is produced by a pyrolytic carbonization process of biomass in the absence of oxygen and consists to a high extent of pure carbon -- the carbon that the plants have extracted from the atmosphere in the form of CO2 as they grow.

While CO2 is emitted when plants are burned, it remains bound in the biochar over the long term.

The first concrete products with integrated biochar are already on the market.

However, biochar is often introduced into the concrete untreated, which can lead to difficulties.

"Biochar is very porous and therefore not only absorbs a lot of water, but also expensive admixtures used in concrete production," explains Empa researcher Mateusz Wyrzykowski.

"Moreover, it is difficult to handle and not completely harmless either." The fine coal dust is problematic for the respiratory tract and carries a certain risk of explosion.

For these reasons, the researchers propose in a paper that has just published in the Journal of Cleaner Production processing the biochar into pellets.

"Such lightweight aggregates already exist from other materials such as expanded clay or fly ash. The knowhow in handling these materials is available in industry, and this increases the chances that the concept will be put into practice," says Wyrzykowski.

Net zero at 20 percent share

To produce the pellets, the team used a concrete mixer with a rotating pan in which they mixed the biochar with water and cement and, as a result of the rotation, obtained small pellets with a diameter of between 4 and 32 millimeters.

In turn, they used these pellets to produce normal concrete of strength classes C20/25 to C30/37 -- the classes that are most widely used in civil engineering today.

"With a proportion of 20 percent by volume of carbon pellets in the concrete, we achieve net zero emissions," says Mateusz Wyrzykowski.

That is, the amount of carbon stored offsets all the emissions produced in the production of both the pellets and the concrete.

While the limit has probably not yet been reached for normal concrete (density between 2,000 and 2,600 kg/m3) with 20 percent by volume, the negative emission potential is particularly striking for lightweight concrete (density approx.

1,800 kg/m3): An admixture of 45 percent by volume of carbon pellets in the concrete leads to total negative emissions of minus 290 kg CO2/m3. By comparison, conventional concrete emits around 200 kg CO2/m3.

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Researchers want to use 'biochar' to combat climate change

A new review of research suggests that the nature-based technology biochar -- a carbon-rich material -- could be an important tool to use in agriculture to help mitigate climate change.

Made by pyrolysis, a process that involves heating organic material in a low-oxygen environment, biochar -- a charcoal-like, porous substance -- has long been utilized for crop production as a soil amendment or carbon sequestration agent. In recent years, researchers have seen a resurgence of heightened interest in the technology due to its unique physical structure and its various agricultural and environmental benefits.

It's for these reasons that biochar's potential to remove large amounts of greenhouse gases from the atmosphere deserves to be re-evaluated, said Raj Shrestha, lead author of the study and a research associate in horticulture and crop science at The Ohio State University.

"When farmers grow their crops, they apply fertilizer and/or manure and use different kinds of machinery to till the soil," said Shrestha. "In the process, greenhouse gases are produced and released into the atmosphere."

But farmers could lessen this impact by applying biochar to their fields, according to the paper, recently published in the Journal of Environmental Quality.

"If we can convince farmers that converting biomass to biochar is good for the long-term sustainability of soils, the economy, and good for the environment, then we'll be able to see wide adoption of this technology," said Shrestha.

The researchers reviewed more than 200 field studies conducted across the globe that examined the impact of biochar application in agriculture on emissions of nitrous oxide, methane and carbon dioxide -- heat-trapping gases that cause Earth's atmosphere to warm.

The team found that the amount of biochar in the soil does have variable effects on local greenhouse gas emissions, which range from a decrease to an increase, and, in some cases, no change. But in general, the team discovered that the use of biochar in field settings lowered the amount of nitrous oxide in the air by about 18% and methane by 3%.

Biochar alone was also not effective at reducing carbon dioxide emissions, but did help when combined with commercial nitrogen fertilizer or other organic materials, like manure or compost.

"We can achieve negative emission in our agroecosystems by reducing the carbon source and enhancing carbon sink," said Shrestha. Reducing Earth's carbon source can be achieved by reducing greenhouse gas emissions from our activities, and enhancing carbon sink -- increasing the technology's ability to absorb more carbon than it releases into the atmosphere -- can be done by increasing the long-term soil carbon pool through conversion of organic waste into biochar, he said.

"What's good about biochar is that it contributes to both these aspects to create net negative agriculture," said Shrestha.

Right now, when farmers leave crop residue on the field, only about 10% to 20% of the residue carbon is recycled into soil during the decomposition process, but by converting the same amount of residue to biochar and then applying it to the field, we can store about 50% of that carbon into stable carbon forms."

As biochar-carbon placed in the soil can also last anywhere from a few hundred to thousands of years, it's currently one of the proposed best management practices for achieving negative emissions and preventing Earth's average temperature from increasing to 1.5 degrees Celsius above pre-industrial levels.

According to the study, between 2011 and 2020, global greenhouse gas emissions rose: carbon dioxide by about 5.6%, methane by 4.2%, and nitrous oxide by 2.7% -- and agriculture accounts for about 16% of these emissions. While such levels have already led to irreversible changes to the global climate system, Shrestha said that future damages could be slowed by helping to curb the extent of emissions from the farming and forestry sectors.

Yet despite biochar's potential as a negative emission technology and the recent increase in biochar-related research, it's difficult to get farmers to apply it, partly because it hasn't been commercialized for widespread use or promoted well, said Shrestha.

To better deliver more science-based, practical information about the technology and its benefits to farmers and agriculture-related businesses, many lawmakers have enacted policies meant to investigate its effectiveness across many different soil types and environmental conditions. It's an objective that Shrestha shares, as the main goal of his team's review paper is to improve farmers' confidence in biochar so that more of them choose to adopt it sooner.

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