The study was published in 2011 and provides an overview of the various processes used to produce biochar (pyrolysis process) and biochar (hydrothermal carbonisation) for soil application. The focus is on technological, economic and climate-relevant aspects of biochar production. The authors evaluated almost 30 scientific studies.
The following production processes for biochar and biochar were used by the authors for the analysis:
- Torrefication (pyrolysis at approx. 250–300 °C, main product is coal)
- Slow pyrolysis (approx. 400 °C, main product is coal)
- Medium speed pyrolysis / intermediate pyrolysis (approx. 450–600 °C, main product is a pyrolysis oil)
- Fast pyrolysis /flash pyrolysis (approx. 450–600 °C, main product is a pyrolysis oil)
- Gasification (approx. 800°C, partial oxidation, main product is a gas)
- Hydrothermal carbonisation/ HTC (approx. 180–220 °C, main product is a liquid)
Pyrolysis and gasification differ in the almost complete absence of oxygen in the pyrolysis conversion process. Pyrolysis techniques can be distinguished by the following parameters
- Reaction time (slow, fast process)
- Heating processes (combustion, electrical, microwaves)
- Dwell time of the input material
In hydrothermal carbonisation, the biomass is treated at around 180–220°C together with water and under pressure. Output are liquid and gaseous products.
The authors could not make any statement about the technical maturity/marketability of the individual systems. The data situation was not sufficient for this.
The authors faced the following challenge in evaluating the economic data: Biochar is often produced together with biogenic energy sources (oil, synthesis gas). An essential part aims primarily at the provision of energy (electricity or heat) and not for the production of biochar. Biochar is often only the by-product.
The authors also point out that the markets for renewable energies (biogas plants) and biochar compete for the same raw material. The market for biochar as a soil improver is similar, competing with products such as peat or compost. This factor will also determine the economic efficiency of a manufacturing process in the future.
The production costs vary strongly in the studies between 51 USD/t (pyrolysis biochar from garden waste) and 386 USD/t (charcoal from retort process). However, according to the authors, a meaningful economic comparison of the methods is not possible due to the low data situation.
Greenhouse gas balance
According to the authors, the following factors must be considered for a complete CO2 balance or greenhouse gas balance (GHG) of the plant coal production process:
- Information on the supply of raw materials (including direct and indirect land-use changes)
- Transformation process itself
- Use of by-products
- Biochar application
- Biochar stability in the soil
- Influence of biochar application on soil-related N2O and CH4
The authors could only find reliable data for pyrolysis processes. A positive influence on the GHG balance of the manufacturing processes is mainly due to the CO2 storage
effect of biochar in the soil. A study by Woolf et. al. (2010: Sustainable biochar to mitigate global climate change), this results in avoided GHG emissions of ‑1054 kg/CO2 per ton of dry biomass (with an average carbon content of 50%) per year.
However, even here the values vary greatly between the studies. Using a reference scenario, the authors identify an opposite effect, namely increased greenhouse gas emissions of 123 kg CO2/t dry biomass feedstock in the production of biochar. Indirect land use changes can have a negative impact on the balance. This means the effect caused by the displacement of existing agricultural production by the cultivation of energy crops on the same land, meaning new areas must be exploited for agricultural production.