February 24, 2024

Processing Biochar into Pellets for Emission Offset in Concrete Production

Switzerland aims to be climate neutral by 2050, and to achieve this, strategies and processes with a negative CO2 balance are necessary. Negative emission technologies (NET) are being developed to counterbalance remaining hard-to-avoid emissions in 2050 and ultimately achieve net zero emissions.

The construction sector, as one of the main emitters, has a significant role to play in meeting these goals. Cement production alone is responsible for approximately 8% of global greenhouse gas emissions. At the same time, there are emerging efforts to utilize the construction sector, known for its resource-intensive nature, as a possible carbon sink.

The idea of building with CO2, or using carbon to produce building materials and effectively removing it from the atmosphere in the long term, may seem paradoxical but has the potential to be successful. To make such visions a reality, extensive research is required. At Empa’s Concrete & Asphalt lab, a team led by Pietro Lura is currently developing a process for integrating biochar into concrete.

Biochar is produced through a pyrolytic carbonization process of biomass in the absence of oxygen. It consists primarily of pure carbon derived from the CO2 that plants extract from the atmosphere as they grow. While burning plants releases CO2, biochar retains carbon over the long term.

Although the first concrete products with integrated biochar are already available, the addition of untreated biochar often poses challenges. Biochar is highly porous, leading to excessive water absorption as well as the absorption of expensive admixtures used in concrete production, according to Empa researcher Mateusz Wyrzykowski. Furthermore, it is difficult to handle and potentially hazardous due to the fine coal dust, which can pose respiratory and explosion risks.

To address these issues, the researchers propose processing biochar into pellets. Similar lightweight aggregates already exist from materials like expanded clay or fly ash, and the industry possesses the necessary knowledge to handle such materials. This increases the likelihood of implementing this concept, says Wyzrykowski.

To produce the pellets, the team used a concrete mixer with a rotating pan. Biochar was mixed with water and cement, and as a result of the rotation, small pellets with a diameter ranging from 4 to 32 millimeters were obtained. These pellets were then used to produce normal concrete of various strength classes commonly used in civil engineering, from C20/25 to C30/37.

Wyrzykowski states that with a proportion of 20% by volume of carbon pellets in the concrete, net zero emissions can be achieved. This means that the amount of carbon stored offsets all emissions produced during the production of both the pellets and the concrete. While the limit has likely not been reached yet for normal concrete with 20% volume, the negative emission potential is particularly significant for lightweight concrete, where adding 45% by volume of carbon pellets results in total negative emissions of -290 kg CO2/m3. In comparison, conventional concrete emits around 200 kg CO2/m3.

Head of the laboratory, Pietro Lura, views research in his lab as a crucial contribution to meeting climate targets. He does not consider biochar, which has been used as a model material in current research, as the primary carbon source. Instead, Lura highlights the broader concept of “Mining the Atmosphere,” which several Empa labs are pursuing. This concept involves producing synthetic methane using solar energy, water, and atmospheric CO2 in sunny regions of the world, followed by the pyrolysis of the synthetic gas. The process yields hydrogen, which can be used as an energy carrier in industry or mobility, and solid carbon, which can be processed into pellets like biochar and incorporated into concrete, explains Lura.

In conclusion, the development and utilization of negative emission technologies, such as processing biochar into pellets for use in concrete production, offer promising potential for reducing greenhouse gas emissions. Further research and implementation of these strategies are necessary to achieve climate targets and ensure a sustainable future.

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1. Source: Coherent Market Insights, Public sources, Desk research
2. We have leveraged AI tools to mine information and compile it