The Danish government's ambitions to apply biochar to agricultural land on a large scale have created a need for closer scrutiny of hazardous substances that may be present in biochar.
A research project led by DTU has investigated the presence of environmentally hazardous substances in feedstocks and biochar from Danish pyrolysis plants and assessed the effect of the pyrolysis process on selected hazardous substances through laboratory experiments.
The studies form part of the research project 'Undersøgelse af miljømæssige problemstillinger ved produktion og anvendelse af biokul' (Investigation of environmental issues related to the production and use of biochar), which is being carried out for the Danish Environmental Protection Agency over a four-year period. The project’s findings are continuously incorporated into the scientific basis for authorities' work on future biochar regulation in Denmark.
Pyrolysis can reduce hazardous substances in biomass
In Denmark, biochar is produced from straw, residual fibres from biogas production, and sewage sludge. This takes place in pyrolysis plants, where biomass is heated without oxygen. Instead of burning, the material is broken down into gas, oil, and char. The solid carbon-rich residue is known as biochar.
“Our results indicate that pyrolysis can be an effective technology for reducing the content of a range of organic pollutants found in biomass. At the same time, they show that biochar quality depends on both the feedstock and the production conditions. Therefore, monitoring and clear regulatory frameworks are needed if biochar is to be used on a larger scale,” says Senior Researcher Wolfgang Stelte of DTU Chemical Engineering.
The studies show that the pyrolysis process often reduces the concentrations of several organic pollutants relative to the original feedstocks. This includes PFAS and several other industrial chemicals. The experiments also show that biochar generally releases fewer of these substances into the environment than the materials from which it is produced.
This is significant because the feedstocks used today are commonly applied to agricultural land without any real pre-treatment. Sewage sludge, in particular, may contain organic contaminants such as PFAS, pharmaceutical residues, and other hazardous substances, as well as heavy metals that can be released into the environment and potentially leach into groundwater.
Feedstock and process are crucial
The studies also reveal clear differences between feedstocks. Straw generally produces the cleanest biochar, with low levels of both heavy metals and organic contaminants. Residual fibres from biogas production fall into a middle category, while sewage sludge presents the greatest challenge due to higher concentrations of heavy metals and many organic pollutants.
The results further show that biochar from Danish pyrolysis plants contains only very low concentrations of PAHs (polycyclic aromatic hydrocarbons), commonly known as tar compounds, which can form during pyrolysis. However, a small number of samples from industrial facilities showed elevated levels of these substances.
Although many organic pollutants are reduced during pyrolysis, not all substances are. Heavy metals and metalloids such as cadmium, nickel, copper, and arsenic are not broken down during pyrolysis and may therefore become concentrated in the resulting biochar.
Many of these metals are strongly bound within the biochar structure and are released only to a limited extent. However, their long-term environmental effects have not yet been fully clarified.
Need for standards and further research
Overall, the findings show that biochar from Danish pyrolysis plants often results in lower emissions of hazardous substances than the feedstocks from which it is made. At the same time, not all biochar is suitable for application on agricultural land. Biochar produced from feedstocks with high contaminant levels may itself contain excessive concentrations of heavy metals or other undesirable substances, and there is considerable variation in both feedstock and production conditions.
According to Wolfgang Stelte, this underscores the need for quality assurance and strict regulation if biochar is to be widely used in agriculture. This is particularly important given the possibility that imported biochar with an unknown risk profile could enter the Danish market.
“There are already certification schemes, such as the European Biochar Certificate, which set requirements for documentation and analysis of biochar. Our results support the need for clear standards, continuous monitoring, and further research,” he says.
The ongoing project will continue to investigate other key environmental aspects of pyrolysis, including process emissions and potential risks associated with large-scale handling and storage of biochar.
