Black gold

Biochar is an idea thousands of years old but one that seemed to have been (foolishly) lost by many along the way.

Like charcoal, biochar is made by baking wood in the absence of oxygen and then quenched. It can then be ground down and worked into the soil to improve fertility and crop yields. It's believed to have been applied thousands of years ago in the Amazon, to generate the Terra Preta.

The biochar locks in much of the carbon captured by the trees and stabilises it. Tom meets Forester Dave Faulkner and his team at Whittlewood to see the productions process in Northamptonshire.

Meanwhile, Josiah Hunt experimented with the process in Hawaii and now supplies across California. As well as capturing carbon and improving the soils, he says they're removing liability wood to reduce forest fires and are helping to produce green electricity.

Can this ancient process help bring new hope?

Listen now on BBC Radio 4

What our experts say

We asked Society Fellows Dr Ondřej Mašek and Professor Stuart Haszeldine from the University of Edinburgh and the UK Biochar Research Centre to offer some observations on the potential of applying biochar to all suitable land worldwide in sequestering carbon. Their points take some of the themes of the programme a step further.

Dr Ondřej Mašek

Among the different Greenhouse Gas Removal (GGR) technologies, biochar and bioenergy with carbon capture and storage (BECCS) rely on sustainably sourced biomass and are often presented as competing for this resource. However, in reality these two technologies would operate mostly on different scales. BECCS is more likely to be in the form of medium to large-scale units (due to the need for access to carbon dioxide (CO2) transport infrastructure). Biochar is more likely to be produced in a distributed form in a large number of small to medium-size units, as this way it can utilise locally available biomass resources and provide biochar for use in the surrounding area. Also, biochar production technology is less sensitive to biomass composition compared to high-efficiency biomass combustion or gasification plants. Due to corrosion and fouling issues that can occur in these high temperature processes when low-quality fuel is used, a wider variety of feedstock can be utilised.

The maturity of biochar technology is sometimes questioned, but there are numerous examples of existing plants in different countries around the world operating at scales from 1,000 tons per annum (t/a) to 100,000 t/a. What is hindering wider deployment of biochar is not the maturity of the technology, but the demand for atmospheric carbon sequestration (most current plants producing biochar generate revenue not from its carbon sequestration value but based on biochar’s other benefits). As the demand for greenhouse gas removal increases, biochar production can be readily scaled-up. This relatively high technology readiness level of biochar is important as it can help in the transition to net-zero economies while other GGR technologies are developed and scaled-up to meet the total GGR capacity required (this cannot be achieved by biochar alone).

It also offers synergies with other GGR technologies, such as rock weathering and BECCS - our work at the University of Edinburgh has shown that ash from biomass combustion, as well as other minerals, can be a useful additives in biochar production.

Professor Stuart Haszeldine

Biochar can and is being used as an animal food additive - this stops methane and CO2 belching and flatulence by cattle and by sheep. That can make a very big decrease in the greenhouse gas (GHG) emissions of upland livestock farms in the UK, enabling high quality meat from marginal land to also be low in carbon production.

Biochar is also being experimented as a seed and fertiliser carrier for re-greening urban brown land faster.

It can be used as an industrial scale filler instead of aggregate crushed rock in low strength concrete, or in foundations of linear infrastructure - as part of the base layers in roads, rail tracks and airports. Therefore, adding a tonne of biochar every 10 metres for each road width can offset the carbon cost of constructing the road.

In the UK, there are no environmental land management scheme (ELMS) payments yet for carbon storage as land management. England may do this, but Scotland, Wales and North Ireland are still quiet. Biochar can be made economic in its own terms by the targeted uses – not paying for livestock emissions, saving money on fertiliser, cheap low carbon heat, and greater success in establishing new and replacement forestry.  

There is uncertainty from UK government regulators and policy advisers as they have been worried about contamination. It is now clear that controlling feedstocks and specifying controlled manufacturing processes can ensure pure biochar. The UK is several years behind European, Chinese, African and Far East countries in targeting the use of biochar within normal agriculture and forestry.

What are the limiting factors?

Dr Ondřej Mašek

• Availability of sustainable biomass (both terrestrial and aquatic, such as microalgae and seaweed).

• Alternative applications for biomass, especially high-value biomass - therefore, focus has been on production of biochar from low-value residues and materials, helping to close loops in circular economy concepts.

• Although the technology for biochar is known and there are numerous historical examples, such as technologies used for dry wood distillation (extensively used until the early 20th century for production of chemicals), the number of currently operating plants is still relatively low, especially at large scale (there are several around the world).

• Low value of carbon, making economics of biochar production challenging (same issue applies to all other GGR technologies).

• Sufficiently established markets and carbon accounting methods are not present yet.

Professor Stuart Haszeldine

The pyrolysis of biomass with insufficient oxygen produces volatile water vapour, gases and oils from about 60% mass (weight) of the biological feedstock, and leaves about 20-40% behind as biochar solid, which can be placed into soil, used as food additive to decrease emissions from cows and sheep burps, used to enhance anaerobic digestion on farms, or used to enhance drinking water filtration mains supply. The amount of biochar and its quality depends on the pyrolysis temperature: hotter than 900ºC makes more gases and less biochar, cooler than 450ºC keeps too many oils in the biochar which in some feedstocks can be contaminants.

• The tonnage of feedstock into the pyrolyser. 

• The purity of feedstock - it is possible to pyrolyse municipal waste or to pyrolyse old tyres. However, these produce seriously contaminated charcoal products which need safe disposal or incineration, not adding them to the ecosystem.

• Critical is the use of the heat, oils and gases generated by the pyrolysis process. If these are simply vented to the atmosphere, that is waste for no benefit. If the gases and oils are used to displace heat from mains North Sea gas, or heat from gas fuelled electricity then there is a benefit. Thus, efficient biochar pyrolysers should be 'plumbed in' to use the heat, or to collect oils as biochemical feedstocks. Examples may include heating a commercially large greenhouse, heating a timber drying kiln, heating a village or town district scheme for buildings - which is fed by locally grown biomass providing heat and biochar.

What are the co-benefits?

Dr Ondřej Mašek

Appropriately selected and applied biochar can have a range of beneficial effects on soils and crop production, related to nutrient management, water management (water holding capacity, drainage), and physical and chemical properties of soil. The vast variety of biochar that can be produced allows for the right type of biochar to be used for each type of soil, climate and crops.

When all co-products of the biochar production process (pyrolysis liquids and pyrolysis gas) are used (for heat or electricity, and for chemicals), emissions associated with the production of fossil-based alternatives can be avoided. Examples of chemical products include organic acids (especially acetic and formic acids), bio-based herbicides and pesticides, phenolic compounds (production of resins) and many more.

Biochar is suitable also for a range of environmental applications (e.g. treatment of water and effluents, contaminated soil remediation) as well as in materials (additives, catalysts, fillers) and engineering applications, especially in construction.

Professor Stuart Haszeldine

Biochar enables using waste biomass products in a way which benefits the climate by storing carbon for medium timescales of decades to centuries. That compares well against nature letting biomass rot in soil, which returns methane and carbon dioxide to atmosphere within only years. Alternatively, combusting biomass with carbon capture and storage (BECCS) stores CO2 for more than 10,000 years.

Biomass from trees can create durable long-lasting value into local communities for a sustainably harvested forest or woodland. That is by employment in forestry, in harvesting forest, and in making the biochar and heat.

Biochar can be deployed anywhere globally, but planning is needed to avoid competition for land and water against long duration ecosystems, or against land and water use for food. Sustainable biomass feedstocks can be supplied from limited diversity plantations, authenticated forest wastes, or sustainable crop wastes

Biochar from harvests of wood or straw can capture and store the carbon on a much more rapid timescale (yearly to decade) than full-duration forests of 50–200-year cycles. That decreases the risk of losing a forest to wildfire - which will become increasingly common.

Biochar can also be added into peatland for even longer duration storage (UK peats are often thousands of years old) but the peat will need to remain continually wet as climate changes in the future.

The best benefits in the UK are co-deployment with integrated quantities of one or two tonnes biochar per hectare annually, e.g. sowing seeds or planting seedlings. The biochar can be 'tailored' as a manufactured substance and mixed with crushed rock dust to provide highly targeted slow-release fertiliser. This can greatly reduce wasted run-off of nitrogen oxides which contaminate aquifers, river water and lakes, as well as being a greenhouse gas and can save a large arable farm tens of thousands per year in chemical fertiliser. Our work at the University of Edinburgh with tree seedlings shows that death rates can be reduced from 15-20% to less than 5%, and that the first five years of tree growth are much faster - so the carbon payback time of a forest is greatly accelerated.

Are there any potential negative impacts of this idea?

Dr Ondřej Mašek

Negative environmental effects can result from the use of unsustainably sourced biomass for biochar production (same issue as for other technologies dependent on biomass).

Use of low-quality biochar (biochar with high content of contaminants) resulting from use of inappropriate (contaminated) feedstock or poorly designed/operated process can have short-term or long-term negative environmental effects. It is important to have clear quality standards in place (several voluntary standards are in place already, such as those by the EBC). The potential to produce safe biochar has been extensively demonstrated and issues affecting biochar quality are known.

The incorrect choice of biochar for a given application could result in a lack of desired effect or even a detrimental impact, therefore informed selection of biochar for a specific application is key.

Professor Stuart Haszeldine

In the UK, it is not useful to apply large quantities (tens of tonnes per hectare) onto grassland, forests or ploughed soils. The biochar is expensive and has very little or no effect in stimulating crop growth. However, in depleted tropical soils, biochar does stimulate extra crop growth.

A big problem is that anybody can make charcoal. That is not biochar. Firstly, the feedstock is not known so security of use on agricultural soil is not known. Second, the heat from gases and oils is wasted, third an amateur kiln or partial burn can create a lot of 'smoke', i.e. particulate carbon air contamination. So just like wood stoves, you need a good kiln. Uncontrolled charcoal making is one of the problems in the UK leading to cases of soil contamination.

Further reading

• Paustian, K. (2016) Climate-smart soils, Nature, 532, 49-57

• Smith, P. (2016) Soil Carbon sequestration and biochar as negative emission technologies, Global Change Biology, 22, 3, pp. 1315 - 1324

• Sohi, S. (2012) Carbon Storage with Benefits, Science, 338, 6110, pp. 1034-1035

• UK Biochar Research Centre

• Yang, Q. et al. (2021) Prospective contributions of biomass pyrolysis to China’s 2050 carbon reduction and renewable energy goals, Nature Communications, 21, 1698

About the series

39 Ways to Save the Planet is a new radio series by BBC Radio 4 developed in partnership with the Society and broadcast in 2021. It showcases 39 ideas to relieve the stress that climate change is placing on the Earth. In each 15 minute episode Tom Heap and Dr Tamsin Edwards meet the people behind a fresh and fascinating idea to cut the carbon.

Programme website

Over the course of 2021, the Society will be producing events and digital content to accompany the series.