• Polycyclic Aromatic Hydrocarbons (PAH)
    Analysis in Biochar
    At Celignis Biomass Lab

Background to PAH

Polycyclic aromatic hydrocarbons (PAHs) are a group of organic compounds that contain two or more fused aromatic rings. They are formed by the incomplete combustion of organic matter and can be found in many different environments, including air, water, soil, and sediment.

PAHs are considered a carcinogen, meaning they have the potential to cause cancer in humans. They are also mutagenic, meaning that they can cause mutations in DNA, which can lead to genetic disorders.

In the environment, PAHs can be toxic to aquatic organisms, including fish and amphibians, and can also have negative effects on soil microbial communities. They are also persistent in the environment, which means they can remain in soil, water, and air for long periods of time.

Polycyclic Aromatic Hydrocarbons in Biochar

The production and use of biochar can result in the release of PAHs into the environment. The amount and composition of PAHs produced during the pyrolysis of biomass depend on several factors, including the type of feedstock used, pyrolysis temperature, and residence time.

PAHs can be released from biochar into the environment through several pathways, including leaching, volatilization, and erosion. Leaching occurs when water percolates through the soil and carries PAHs with it. Volatilization occurs when PAHs evaporate from the surface of biochar and are released into the air. Erosion occurs when biochar is transported by wind or water and is deposited in other areas.

To minimize the release of PAHs from biochar, several strategies have been proposed. One strategy is to use low-temperature pyrolysis, which has been shown to reduce the formation of PAHs. Another strategy is to use feedstocks that result in lower levels of PAHs production, such as hardwoods. In addition, post-processing techniques, such as washing and air-drying, can be used to remove PAHs from biochar before it is applied to the soil.

Effect of Feedstock on PAHs in Biochar

With regards to feedstock, their differences in chemical compositions and structures can influence the formation and distribution of PAHs during the pyrolysis process. For example, biochar produced from wood feedstocks is often found to contain high levels of Benzo[a]pyrene, a highly toxic and carcinogenic PAH. This is because wood contains high levels of lignin, a complex polymer that can break down during pyrolysis to form PAHs such as Benzo[a]pyrene. Additionally, wood feedstocks tend to have a higher carbon content than other feedstocks, which can result in higher temperatures during pyrolysis and greater production of PAHs.

In addition to the formation of PAHs during pyrolysis, the presence of PAHs in the original feedstock can also influence their presence in the resulting biochar. For example, feedstocks that have been contaminated with PAHs from sources such as air pollution, waste disposal, or industrial activities may contain higher levels of these compounds, which can be transferred to the biochar during pyrolysis.

Effect of Pyrolysis Conditions on PAH Levels

Temperature is one of the most important factors in determining the types and amount of PAHs in biochar. Generally, higher pyrolysis temperatures lead to greater production of PAHs, including more toxic and carcinogenic forms such as benzo[a]pyrene and indeno[1,2,3-cd]pyrene. This is because high temperatures can break down complex organic molecules into simpler ones, including PAHs, and can promote reactions that favour their formation.

However, there is a limit to the amount of PAHs that can be formed at high temperatures, as excessive heating can cause the PAHs to decompose into simpler compounds.

With regards to residence time, longer residence times typically lead to increased formation of PAHs, as they provide more time for reactions to occur.

Main PAHs in Biochar

Some of the main PAHs found in biochar include:
  • Benzo[a]pyrene - a PAH consisting of five fused benzene rings. It is a highly toxic and carcinogenic polycyclic aromatic hydrocarbon that is commonly found in biochar produced from wood feedstocks. It is formed during high-temperature pyrolysis and can remain in the environment for long periods of time.
  • Indeno[1,2,3-cd]pyrene - a polycyclic aromatic hydrocarbon that consists of six fused benzene rings and a five-membered ring. It is another highly toxic and carcinogenic PAH that is commonly found in biochar.
  • Phenanthrene - a PAH that consists of three fused benzene rings. It is commonly found in biochar produced from agricultural waste feedstocks.
  • Fluoranthene - consists of four fused benzene rings and is commonly found in biochar produced from wood feedstocks.
  • Pyrene - also consists of four fused benzene rings and is also commonly found in biochar produced from wood feedstocks.

Analysis of PAHs at Celignis

Types of PAHs Determined

At Celignis we can determine the content of 23 different types of PAH in biochar. These cover the 5 main types listed above as well as:
  • Napthalene - a polycyclic aromatic hydrocarbon that consists of two fused benzene rings.
  • 2-Methylnaphthalene - consists of a naphthalene ring with a methyl group attached at the second position.
  • 1-Methylnaphthalene - consists of a naphthalene ring with a methyl group attached at the first position.
  • Acenaphthylene - consists of two fused benzene rings with a triple bond between them.
  • Benzo[j]fluoranthene - a polycyclic aromatic hydrocarbon that contains five fused benzene rings. It is a member of the fluorene family of PAHs
  • Anthracene - consists of three fused benzene rings. It has a blue fluorescence and is primarily used in the production of dyes, plastics, and other industrial chemicals
  • Benzo[a]anthracene - a polycyclic aromatic hydrocarbon that consists of four fused benzene rings and a five-membered ring.
  • Dibenzo[a,h]anthracene - a PAH that consists of five fused benzene rings.
  • Chrysene - a PAH consisting of four fused benzene rings.
  • Benzo[b]fluoranthene - consists of five fused benzene rings.
  • Benzo[k]fluoranthene – a PAH that also consists of five fused benzene rings.
  • Benzo[ghi]perylene - a polycyclic aromatic hydrocarbon consisting of six fused benzene rings.
  • Benzo[e]pyrene- a PAH consisting of five fused benzene rings.

Our packages that quantify PAHs in biochar are listed below:

Checking PAH Levels Against Threshold Values

Polycyclic aromatic hydrocarbons are regulated by various governmental agencies worldwide due to their potential toxicity and carcinogenicity. For example, in the United States, the Environmental Protection Agency (EPA) regulates PAHs under the Clean Air Act, Clean Water Act, and the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA).

In the European Union, PAHs are regulated under the Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) regulation. The regulation sets limits on the amount of PAHs allowed in consumer products and the EU also sets limits on PAHs in ambient air, surface water, and sediments.

The European Biochar Certificate (EBC) is an organisation set up to ensure the quality and safety of biochar produced in Europe. It was established in 2012 by a group of experts from different European countries and is now supported by various research institutions, companies, and associations.

The EBC is based on a set of criteria that cover the production, properties, and use of biochar. The EBC includes guidelines for polycyclic aromatic hydrocarbons in biochar to ensure that the biochar produced meets strict safety and quality standards. Two different threshold values (16 EPA PAH and 8 EFSA PAH) are provided, as described below:
16 EPA PAH - The US EPA has identified 16 priority PAHs, chosen due to their persistence in the environment, toxicity to human health, and potential for bioaccumulation. The EBC sets a threshold value for the total of the 16 PAHs at 4 grams per tonne of biochar if the biochar is to be use in the Agro-Organic sector and 6 grams per tonne of biochar if the planned application is in the Agro sector. The 16 PAHs covered by the EPA are: Acenaphthene, Acenaphthylene, Anthracene, Benzo[a]anthracene, Benzo[a]pyrene, Benzo[b]fluoranthene, Benzo[g,h,i]perylene, Benzo[k]fluoranthene, Chrysene, Dibenzo[a,h]anthracene, Fluoranthene, Fluorene, Indeno[1,2,3-cd]pyrene, Naphthalene, Phenanthrene, and Pyrene.

8 EFSA PAH - These are eight Polycyclic Aromatic Hydrocarbons that the European Food Safety Authority (EFSA) identified as being of priority concern for human health. These eight PAHs are classified as priority pollutants due to their persistence in the environment, bioaccumulation potential, and toxicity to human health. The EFSA has set maximum limits for these PAHs in food and feed products, and they are also included in the European Union's REACH regulation. The EBC guidelines provided different threshold values for the total of these 8 PAHs according to the planned application market for the biochar. For example, a threshold value of 1 g per tonne of biochar is set for most applications with the exception of biochar used in basic materials, where the threshold value is 6 grams per tonne. The 8 PAHs covered by the EFSA are: Benzo[a]anthracene, Benzo[a]pyrene, Benzo[b]fluoranthene, Benzo[k]fluoranthene, Chrysene, Dibenzo[a,h]anthracene, Indeno[1,2,3-cd]pyrene, Phenanthrene.

At Celignis we check as to whether the total PAH values are above or below the threshold values for each of the EBC application sectors and provide the results in a PASS/FAIL table for each feedstock according to the specific sector and test.

These tables are provided in our pdf reports. An example table is provided here.

Understanding Links Between Process, Feedstock and PAH Levels

If you are currently producing a range of different from biochars covering different feedstocks and/or process conditions then we can help in screening these biochar samples for their PAH contents and compositions. We can then use data analysis tools to look for trends and correlations across the dataset. This can lead to greater understanding on what factors are most important, according to your particular conditions, with regards to PAH levels in biochar. From this we can provide guidance on how conditions can be optimised for reduced PAH concentrations, whilst also considering other factors such as target biochar yields and composition. Such data can feed into our technoecomomic analyses of biochar production scenarios.

Additionally, at Celignis we can also produce biochar samples from your feedstock(s), with factors such as pyrolysis temperature, residence time, and heating rate being varied in order to see the effects on the yield and quality of biochar. We can formulate a design of experiments (DoE) for exploring the relationships between these factors and responses, also taking into account the production and distribution of PAHs. We can also potentially explore the use of certain additives during the pyrolysis process (e.g. potassium hydroxide) that may help to can reduce the formation of PAHs during pyrolysis. These additives can help break down PAHs into simpler compounds, which are less harmful to human and environmental health.

Exploring Strategies to Reduce PAH Levels in Biochar

There are a number of different strategies that we at Celignis can test to reduce the levels of PAHs in your biochar samples. For example, we can wash or leach the biochar (using water or other solvents). This treatment can potentially remove any remaining ash or soluble organic compounds that may contain PAHs.

If we are also involved in the production of biochar samples from your feedstock(s) then we can also undertake feedstock pre-treatment experiments. These may include washing or soaking the biochar samples in water, which can help remove any contaminants or impurities that may contribute to the formation of PAHs during pyrolysis.

Additional Information on PAH in Biochar

Feel free to get in touch with us if you have any questions about our analytical services, if you are looking to screen biochar for PAHs, or if you need to explore strategies to reduce the PAH levels in biochar. Relevant members of the Celignis biochar team will be happy to assist. Those team members with the most experience with undertaking these tests and interpreting the resulting data are listed below.

Sajna KV

Bioanalysis Developer


Our Biomass Detective! Designs, tests, optimizes and validates robust analytical methods for properties of relevance to the various biochar market applications.

Dan Hayes

Celignis CEO And Founder

PhD (Analytical Chemistry)

Dreamer and achiever. Took Celignis from a concept in a research project to being the bioeconomy's premier provider of analytical and bioprocessing expertise.

Other Celignis Tests and Services for Biochar

Global Recognition as Biomass and Biochar Experts

Celignis provides valued services to over 1000 clients. We understand how the focus of biochar projects can differ between countries and have advised a global network of clients. We also have customs-exemptions for samples sent to us allowing us to quickly get to work no matter where our clients are based.

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Feedstock Evaluation

Our analysis packages can screen biochar feedstocks. We can estimate biochar yield and quality using feedstock chemical composition and can estimate biochar composition using the ultimate and major/minor elements analyses of the feedstock. With TGA analysis we can also monitor feedstock behaviour under pyrolysis conditions.

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Biochar Production

We can produce biochar samples from your feedstocks using a wide range of temperatures, heating rates, and residence times. We can formulate a Design of Experiments (DoE) to study the effects of varying process parameters on biochar yield and quality and can optimise these outputs according to your desired biochar market applications.

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Biochar Analysis

We have an extensive array of analysis packages to evaluate the suitability of biochar for a range of applications. These analyses cover properties relevant to combustion, soil amendment, feed, and biomaterials. Our reports compare the results against internationally-recognised limits for using the biochar in specific end-products.

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Biochar Combustion Properties

Biochar can be a superior fuel versus virgin biomass due to its greater carbon content and energy density. We offer a wide array of analysis packages to fully evaluate biochar as a fuel. For example, we can determine both organic and inorganic carbon and can monitor the behaviour of the biochar ash over wide temperature ranges.

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Soil Amendment & Plant Growth Trials

We can test biochar for several properties (e.g. water holding capacity, electrical conductivity etc.) relevant to its use in soil amendment. We can also grow plants in biochar-amended soils and assess the impacts of this approach on germination, plant growth, plant health, and soil biology.

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Surface Area and Porosity of Biochar

The suitable markets for a biochar are often greatly dependent on its surface area and pore size-distrubtion. We provide detailed reports on biochar surface area and porosity and can provide guidance on the implications of the results. We can also work on strategies to increase the surface area and modify the pore-size distribution of biochar.

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Thermogravimetric Analysis of Biochar

TGA is a powerful analytical technique for the study of biochars because it allows us to examine the thermal stability of the material as a function of temperature. The thermal stability of biochars is an important factor to consider when evaluating their potential use as a soil amendment or for carbon sequestration.

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Biochar Upgrading

There are several different methods (covering physical, chemical and biologial routes) by which we can upgrade your biochar in order to increase its value and make it more suitable for the desired market applications. We are able to fully characterise the changes in physicochemical properties associated with upgrading.

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Biochar for Carbon Sequestration

Biochar's efficacy as a means for sequestering carbon depends on a range of factors (e.g. feedstock and pyrolysis conditions). We can undertake a range of analytical tests to help you determine the stability of your biochar's carbon. We can also suggest alternative approaches to improve carbon sequestration potential.

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Technoeconomic Analyses of Biochar Projects

Our TEA experts work with you to evaluate the economic prospects of your biochar facility, considering various scale, technology, and feedstock options. We apply accurate costing models to determine CAPEX/OPEX of simulated and pilot scale processes which are then used to determine key economic indicators (e.g. IRR, NPV).

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Research Project Collaborations

Celignis is active in a number of important research projects focused on biomass valorisation. Biochar is a key component in some of these ongoing projects as well as in several prior projects. We are open to participating in future collaborative research projects where our extensive infrastructure and expertise in biochar can be leveraged.

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