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  • Guenther J. Schulz

4CleanTech's Bioenergy Insight article: Improving the performance of AD plants

Updated: Mar 7, 2022

Could Biochar be the solution for AD plants with minor gas yields? 4CleanTech, a joint venture between Biogas Clapton and CleanTech Leasing, shares its findings.

After investing in an AD plant, the majority of operators are confronted with the fact that the gas yield is not nearly as high as previously calculated. Kent, UK-based 4CleanTech sees biochar as a way to significantly increase the gas yield from a given feedstock.

Asked about the reasons for the often-disappointing performances, Guenther J. Schulz, one of the directors of 4CleanTech, addresses as a major problem the design of the plant’s digester(s): “In many AD plants, especially the bigger ones, the retention times for feedstock such as grass or maize are far too short, which is due to the digesters being too small,” he said. “The result of this is that the substrates are not broken down properly and energy is wasted.”

To support this view, 4CleanTech refers to the KTBL calculator, a widely recognised data evaluation tool used by major biogas research institutes in Germany. According to this calculator, for example, a maize silage AD plant with 1,400 cubic metres (m3) of biogas output per hour (2,800 kWh combined heat and power) requires a digester capacity of net 18,000 m3.

“After conducting a representative survey, we found that in reality these bigger AD plants usually have only a digester capacity of between 9,000 m3 and 13,000 m3.”

The problems created by these reduced digester capacities are often twofold, with not only a low gas yield but also a dry matter content that is too high. In many conversations with biogas plant operators, it became clear that the operators were doing everything right, but they ultimately had to accept lower gas yields from the feedstock, because the substrate leaves the digesters before it has had time to reach its full potential.

At the same time, the higher dry matter content leads to earlier wear of pumps and mixers and sometimes expensive separators have to be purchased and operated to be able to handle the thicker substrate.

Upgrading the digester capacity of existing AD plants is generally not an option for most, so while researching these performance issues, 4CleanTech has identified biochar as an additive that can significantly increase the gas yield and reduce the dry matter content from a given feedstock.


What is biochar and how does it work?

Biochar is made from renewable raw materials such as wood or vegetable residues (e.g. trimmings, leaves or green waste) and significantly reduces the carbon footprint. To produce biochar, the biomass is burned within a modern pyrolysis plant with very little oxygen.

This pyrolysis method allows all gases and vapours to be filtered out completely, making it 100% pollutant-free. This production process is often referred to as the carbonation of biomass. Biochar is a porous structure, and this significantly increases the surface in the digester for micro-organisms to settle on. Biochar has a surface area of up to 300 m2 per gram, which allows it to act like a sponge that can absorb up to five times its own weight.

Testing phase

Over a period of two-and-a-half years, 4CleanTech conducted trials with biochar at a 500kWh plant in Somerset, UK, which uses as feedstock 90% maize and 10% grass. The first year was spent optimising the biochar’s feeding schedule to allow the fermenter biology to adapt successfully, while at the same time ensuring that the cost and benefit ratio was secured. Over 12 months, the result was a 10.5% increase in the biogas yield and the organic dry matter (oDM) reduced 9%. Additionally, the ammonium also declined by 5%. The biological process of the plant stabilised at a new operating point of 6-6.2% oDM.


Biochar cannot fully compensate for the lack of capacity in a plant’s digester, but without question biochar can offer the microorganisms in the digester a considerable increase in surface area and thus achieve a significant increase in gas production. At the same time the dry matter reduces and the digestate is more fluid. Furthermore, the biological process stabilises and the ammonium, which can block the biological process, is reduced. All these factors lead on different levels to an increase in the profitability of the plant. Based on these results 4CleanTech has developed its own biochar product – Advance Biochar – a premium-quality product imported from Germany.

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4CleanTech's Bioenergy Insight article
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