Biochar – what is it and why is it generating so much interest?
Fiona Henderson and Evelyn Krull (Australia)
Biochar has the potential to sequester carbon, improve soil health and increase crop yield. Even its production can be classed as beneficial, as it is a by-product of a process that burns waste materials to produce bio-fuel. However, questions remain. Although biochar offers an effective way of offsetting greenhouse gas emissions, while helping to improve soil health and aid in waste management, a number of uncertainties are prompting scientists to dig deeper.
Biochar – what is it?
Biochar is a waste product. When natural organic materials (for example, crop waste, wood chip, municipal waste and manure) are heated to temperatures of up to 700oC in an oxygen-limited environment, a variety of gases are produced. These gases can be used as bioenergy to produce heat and electricity. This process, known as pyrolysis, is gaining interest as a way to produce green energy from a renewable source. The left-over, charred by-product of this process is biochar (Fig. 1).
It is substantially more stable – structurally, chemically and biologically – than the carbon it was made from. Therefore, biochar is one of the hardest known organic materials to breakdown – in some cases, it can remain stable in soil for hundreds to thousands of years. However, Dr Evelyn Krull, Senior Research Scientist from Australia’s CSIRO, warns that not all biochar is the same. The key chemical and physical properties of biochar are greatly affected by the type of material used for its production (also known as the feedstock) and the conditions of the pyrolysis process (that is, temperature and time).
As Dr Krull says:
If we look at biochar made from manure or wood cuttings, for example, the biochar made from manure has a higher nutrient content than the biochar made from wood cuttings. However, the biochar from the wood cuttings is more stable over a longer period of time.
The two different chars will look the same, but will behave quite differently. Similarly, biochar that is produced at 700°C will have a much greater adsorptive capacity and higher degree of micro-porosity than biochar produced at 400°C. Essentially, a higher temperature biochar will have greater potential for adsorption of toxic substances and rehabilitation of contaminated environments.”
Therefore, it is important to understand the characteristics of the biochar being produced so that these can be matched to the requirements of its end use.
Why is it a carbon-negative process?
The production of bioenergy by means of pyrolysis is a carbon neutral process, meaning that it neither adds to the climate change problem nor contributes to its solution. However, if the biochar produced by pyrolysis is used for carbon sequestration, it takes the benefits of the process one step further.
In the natural carbon cycle, plants absorb carbon in the form of CO2 from the atmosphere during photosynthesis. When the plant dies, the carbon-rich plant material decomposes releasing carbon into the soil and back into the atmosphere, completing the cycle. However, if plant matter is used as feedstock for pyrolysis no decomposition occurs.
After pyrolysis, up to 50% of the carbon in the plant matter is converted from being in an unstable and easily broken down form (which will quickly return to the atmosphere), to the highly stable and hard to breakdown form of biochar. This means the carbon cannot re-enter the atmosphere, but is sequestered, with the potential to reside in soil over decades, centuries and even up to millennia. This long-term storage of carbon means that it is essentially removed from the carbon cycle. In this way, biochar production is known as a carbon-negative process (Fig. 2).
What are the environmental benefits of biochar?
The environmental benefits of biochar have the potential to address some of the most urgent environmental issues facing the world today. In an open letter to the International Biochar Initiative, Professor Tim Flannery advocated the great potential of biochar’s multiple benefits. As he wrote:
The biochar approach provides a uniquely powerful solution, for it allows us to address food security, the fuel crisis and the climate problem, and all in an immensely practical manner.
“With the appropriate political and technical recognition, promotion and adoption, it will change our world forever and very much for the better.”
As the highly stable product of a carbon-negative process, biochar has potential as a long-term carbon sink. Its production and application could play an important role in helping to sequester carbon from the atmosphere, partially off-setting greenhouse gas emissions produced by the burning of fossil fuels.
Biochar may also improve soils – an urgent need when more food production is needed from limited soil resources. Early studies in Australia have indicated that, depending on the quality and characteristics of the biochar used, its application may aid in:
- Improving nutrient retention and cation exchange capacity. (A cation is an ion with more protons than electrons.)
- Decreasing soil acidity.
- Decreasing uptake of soil toxins.
- Providing a suitable habitat for soil microbes.
- Improving soil structure.
- Improving nutrient use efficiency.
- Increasing water-holding capacity.
- Decreasing the release of non-CO2 greenhouse gases (CH4, N2O).
However, before we get too excited, it is important to note that the benefits of biochar are not universal. In fact, some biochars may have adverse effects on plant growth and not all soils respond to biochar in the same way. Studies that have reported positive effects with regard to crop production often involved highly degraded and nutrient-poor soils. On good soils, biochar may not add much at all.
The whole life cycle of biochar, including its feedstocks, production and use, determines its value and role – in terms of both carbon balance and soil benefits. In the same way, its role in the effective management of waste streams may be a significant component of the overall value.
Answering the questions around biochar
Even though there is great potential for biochar production and application to have positive environmental and economic outcomes, there are substantial knowledge gaps that require further research to ensure its safe production and use.
A number of organisations in Australia are exploring these questions further, including the Commonwealth Scientific and Industrial Research Organisation (CSIRO), the University of New South Wales, the University of Western Australia, the New South Wales Department of Primary Industries and the Department of Agriculture and Food Western Australia, as well as biochar producers such as BEST Energies.
Dr Neil McKenzie, Chief of CSIRO’s Division of Land and Water, said CSIRO has an excellent record of research into the dynamics of soil carbon:
Our work on the age, chemistry and abundance of char in soil has been invaluable for new studies into the potential of biochar. This new research is essential for developing one of our more promising mitigation strategies against climate change. Biochar also provides a bonus by improving the fertility of Australia’s naturally depleted and ancient soils.”
A recent report from CSIRO and Rothamsted Research (the largest agricultural research centre in the UK) highlighted the following research questions:
Is all biochar the same? There is the need for a database and rapid screening technique that allows biochar products to be compared and matched to a particular use.
How stable is biochar? At the moment, there is no established method to determine the long-term carbon sequestration potential of biochar. However, most biochar products produced at, or greater than, 4000C are known to be stable over 100 years covering the timeframe of proposed Emission Trading Schemes.
Is biochar safe to use? An environmental risk assessment, which takes into account impacts on ecosystem, needs to be conducted.
What are the agronomic benefits? There is a need to understand and predict the underlying processes associated with the results that have been observed after application of biochar to particular soils (for example, increased nutrient retention and water-holding capacity).
Is it economically viable? The uncertainty around investment in biochar production and the market for carbon offsets needs to be explored further through sound economic and full life-cycle analysis.
What are the environmental and societal benefits in developing countries? Research into the socio-economic constraints associated with biochar’s potential to increase crop yield and restore soil health in developing countries is needed.
Biochar’s potential for carbon sequestration and improving soil health, coupled with the enormity and severity of the issues of climate change and a global food shortage, warrant further research into its application. A complete picture of the properties and potential of biochar is essential to ensure that any benefits from biochar are achieved without inadvertently causing more harm than good.
Further reading
Gaunt, J.L., and Lehmann, J. 2008. Energy balance and emissions associated with biochar sequestration and pyrolysis bioenergy production. Environmental Science and Technology. 42, 4152-4158.
Lehmann, J. 2007. A handful of carbon. Nature. 477, 143-144.
Lehman, J. 2007. Bio-energy in the black. Frontiers in Ecology and the Environment. 5(7), 381-387.
Lehmann, J., Gaunt, J. and Rondon, M. 2006. Bio-char sequestration in terrestrial ecosystems – A review. Mitigation and Adaptation Strategies for Global Change. 11, 403-427.
Lehmann, J., and Joseph, S. 2009. Biochar for Environmental Management. Science and Technology. 384 p.p. (http://www.earthscan.co.uk/?tabid=49381).
Sohi, S., Lopez-Capel E., Krull E., and Bol R. 2009. Biochar, climate change and soil: A review to guide future research. CSIRO Land and Water Science Report 05/09. 64 pp. (http://www.csiro.au/resources/Biochar-climate-change-and-soil.html).
The International Biochar Initiative (www.biochar-international.org/).
Australia and New Zealand Biochar Researchers Network (www.anzbiochar.org/).