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Guidelines for Clean and Safe Biochar

By:Mr Evan Wang
Position:Managing Director
Jun 07, 2022
Biochar is “thermally modified biomass” at high temperatures (∼350 to 1000 °C) in the absence of oxygen through pyrolysis process. As a carbon-rich solid material, when incorporated in soil, biochar can alter the soil’s physical, chemical, and biological properties, and on average, biochar application increases plant growth.

Good biochar has high percentage of stable (non-volatile) carbon, high porosity, extensive microstructure, high cation exchange capacity and adsorption capacity that enable beneficial interactions between microbes, nutrients, and water in the soil. However, some biochar may also contain contaminants that, when such biochar is applied to soil, can adversely affect soil flora and fauna.

From the perspective of SIMEC, pyrolysis unit design is the most important factor to produce clean and safe biochar, neither a specific pyrolysis parameter nor the feedstock. Good biochar shouldn’t be associated with a particular feedstock type.

The pyrolysis of organic materials causes the formation of polycyclic aromatic hydrocarbons (PAH). During biochar production, PAHs are usually released with the pyrolysis gases and are destroyed when these pyrolysis gases are combusted to produce thermal and electric energy. However, depending on the process conditions, a smaller or larger part of the released PAHs can be adsorbed by the simultaneously produced biochar.
biomass-pyrolysis-furnace
 
biochar
 
To allow monitoring and regulation of PAHs, a list of 16 PAHs was compiled by the U.S. Environmental Protection Agency. So it’s also called 16 US EPA PAHs. These 16 compounds were selected from hundreds of PAHs based on environmental relevance, toxicity, and ability to measure them. All 16 individual PAH compounds are listed as follow: Naphthalene, Acenaphthylene, Acenaphthene, Fluorene, Phenanthrene, Anthracene, Fluoranthene, Pyrene, Benz(a)anthracene, Chrysene, Benzo(b)fluoranthene, Benzo(k)fluoranthene, Benzo(a)pyrene, Indeno(1,2,3-cd)pyrene, Dibenz(a,h)anthracene, Benzo(g,h,i)perylene. 

The content of polycyclic aromatic hydrocarbons (PAHs) in biochar is very important factor to define the biochar’s application classes (Feed, Agro Organic, Agro, Urban, Consumer Material, Basic Material).

Limit values for PAH contents according to European Biochar Certification
Certification Class 16 EPA PAHs 8 EFSA PAH
EBC-Feed 4 mg/kg 1.0 g t-1 DM
EBC-AgroOrganic 4 mg/kg
EBC-Agro 6 mg/kg
EBC-Urban declaration
EBC-ConsumerMaterials declaration
EBC-BasicMaterials 30 mg/kg 4 g t-1 DM
 
However, individual PAH differ widely in their toxicity. The type and degree of toxicity (e.g., genotoxicity, carcinogenicity, ecotoxicity) depends on the molecular structure, the concentration, the bioavailability, the exposure route, and the temporal course of the exposure.

According to the latest research, among the 16 US EPA PAHs, naphthalene (NAP) is the compound with the highest content, but the only one with two aromatic rings and the least toxic one. Thus, it is not considered to be carcinogenic nor genotoxic. Furthermore, NAP is also the most volatile PAH (vapor pressure 0.087 mm Hg at 25 °C) and evaporates significantly when present in soil. So, comparing with other parameters, the content of non-NAP PAHs is more important and more actual to define the class of a clean and safe biochar. According to test report issued by a third party lab., biochar made by SIMEC technology presents only 1.4 mg/kg non-NAP PAHs content, and 2.5 mg/kg 16 EPA PAHs content which is much lower than EBC standards.

It’s proved that appropriate production technology with advanced pyrolysis reactors and correct process control can avoid undesired PAH-contamination of biochar. The type of biomass feedstock used for biochar production has a negligible influence on the PAH content. Accordingly, a professionally designed pyrolysis reactor with advanced process control is the most important factor to produce clean and safe biochar.
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