How is biochar made?

Biochar is produced by heating biomass in a reduced oxygen environment. The process is referred to as pyrolysis. The volatile substances in the original material are gasified by the high temperature and driven off, and can be captured to be used as fuels, or fully combusted to yield process heat.

What materials can be used to make biochar?

Any organic material with a significant carbon content can be a feedstock for biochar. Woody material, bamboo, crop residues, municipal green waste, manure, sewage solids, and seaweed are examples of feedstocks. Feedstocks with toxic contaminants, such as tanalised timber, should be avoided.

Does biochar production require energy inputs? What about the carbon footprint of the process?

Some form of heat source is needed to begin pyrolysis. This can be supplied externally or by burning some of the feedstock itself in the outset. As soon as the outgassing of volatile fractions is underway, these products can be fed back into the combustion environment and used to drive an exothermic process. As long as the heat is not supplied by fossil fuels, the process footprint is carbon neutral.

Does biochar production require specialised equipment and facilities?

The size, complexity, and capital requirements for pyrolysis technology cover a wide range of use cases. All that is needed to produce biochar is heat and a means of excluding oxygen. This can be as simple as a positive-pressure retort inside an oven, or a pit or container which is lit so as to burn with an active flame cap across the top which prevents air from penetrating the material underneath. More advanced designs permit the capture of products such as syngas, bio-oil, and wood vinegar, and can exploit the exothermic nature of pyrolysis to provide cogeneration or industrial process heat.

What are the implications of transporting the materials? Does this negate the sequestration benefits?

In settings where large amount of feedstock are produced or received already (e.g. sawmills, municipal transfer stations) there would be an incentive to have stationary pyrolysis facilities. Mobile technology would be better suited to distributed source material, and because conversion of feedstocks to biochar involves considerable volume and mass reduction (up to 75%) it makes far more sense to transport the finished product.

Charcoal production in third world countries is dirty and pollutes the air. What about biochar?

Several traditional methods of making charcoal involve partial combustion in smoldering fires which burn at lower temperatures than we use for making biochar. Any well-managed pyrolysis process will release little aside from carbon dioxide and water vapour. Particulate and gaseous fractions such as syngas are either completely combusted or harvested as biofuel yields.

How much carbon is sequestered in biochar?

Biochar is mostly carbon, with varying percentages of minerals and organic substances depending on what it was made from and the process utilised. Typically, up to half of the carbon originally present in woody feedstock will be retained in durable form as biochar.

Does the carbon in biochar break down eventually?

Pyrolysis at the temperatures used for biochar production promotes the formation of aromatic bonds between the carbon atoms in the feedstock. The resulting graphene complexes, a stable chemical form, means that the carbon in biochar tends not to be greatly affected by processes such as oxidation or microbial activity.

Laboratory simulations of accelerated weathering on samples of biochar show losses at longer timescales, but the majority of the bound carbon can be expected to remain stable for decades at the very least, and centuries in the median prospect. Radiocarbon dating of the pyrogenic carbon fraction in soils has yielded ages of over 2,500 years in the terra preta soils of the Amazon, 7,000 in the US Midwest, and 12,000 in Russia.

What does biochar do to soluble nutrients or contaminants?

Biochar has a high surface area, a result of the pore structure of the feedstock, and on this surface are multiple charge sites at edges of the graphene complexes which provide a high cation exchange capacity (CEC). This property gives biochar the ability to “grab” positively-charged ions from solution and retain them. The porous structure of biochar also promotes a rich microbiota of bacteria and fungi which can in turn attract and metabolise a range of nutrients and pollutants. In both of these cases the nutrients become highly available to plants, and the biochar performs the simultaneous functions of mitigating nutrient leaching and reducing the need for fertiliser inputs.

What does biochar do to soil life, such as bacteria, fungi, insects and earthworms?

Biochar provides an attractive lodging for soil microbial life. A community of bacteria and fungi will rapidy populate biochar in soil, and this provides a reservoir of activity and resources to the surroundings. Invertebrate populations have been observed to increase in the presence of biochar, and earthworms transport it from the soil surface to depth via their feeding and burrowing.

What effect does biochar have on soil structure and fertility?

Biochar has beneficial attributes owing to its porosity and high surface area. The structural effects of these attributes are to increase aeration and water retention in soils. The effects on fertility come from several factors, including CEC and establishment of surface microbiota, which can provide plant roots with more bioavailable nutrients than unamended soils. Biochar may also contain significant quantities of mineral nutrients depending on the feedstock and method of pyrolysis used.

New Zealand agricultural soils are already high in carbon. What is the point of adding more?

Soil carbon takes more than one form and is often characterised in terms of how quickly it is moving through the biological cycle. Labile carbon is that which is embodied in the microbial life and the decomposition products of organic matter such as plant litter and animal manure, and is responsible for a great deal of soil fertility. Humic carbon is the collection of end products of this decomposition: a set of complex, stable forms which are no longer available as an energy source for most microbial activity but still act to promote CEC, pH buffering and other indicators of soil fertility. Most carbon in New Zealand agricultural soils is in the labile fraction, with mean residence times in the range of months to years. Pyrogenic carbon, including biochar, can have residence times of centuries to millenia and is comparatively low or even absent in most of New Zealand because of the lack of fire as a recurring presence in our native ecosystems prior to the arrival of humans.

The labile fraction of soil carbon is most susceptible to land use change, poor management, and adverse climate driven events, and can be lost in quantity as a result of such simple activities as tilling, overgrazing, and fertiliser over-application, at timescales as short as days or weeks. The humic fraction is durable, degrading only slowly in the presence of specific microbial activity. Biochar is solely at risk from rapid destruction through burning, and once emplaced in soil (or water) it is unlikely to be present in sufficient ratios to support combustion except in the most extreme scenarios.

What happens if soil with biochar added erodes?

Even if soil containing biochar is washed out to sea, the durable carbon is still sequestered and will make its way into deep marine sediments to become a part of some future epoch’s stocks of fossil fuels millions of years in the future.

What is the process of incorporating biochar into soils under pasture? Will this require tilling, machinery and fossil fuels?

Biochar can be emplaced in soil by a number of methods, including various forms of tillage, direct drilling/chisel ploughing, surface spreading and mixing with water or effluent and spraying. Surface application can be effective provided it is not done with small particle sizes under windy conditions, and treading by grazing livestock will move it quickly and effectively into the topsoil. Earthworms and dung beetles can also work significant amounts of biochar below the surface.

Can we make enough biochar to have a meaningful impact on New Zealand’s net emissions?

Let us take three sectors with abundant feedstocks for production and consider some potential yields:

Forestry creates 4 million tonnes of waste products annually in the form of prunings, thinnings, and post-harvest slash. If we turned half of this into biochar we would sequester 2 MT of carbon, for a net emissions reduction of 7.3 MT CO₂e.

If all New Zealand dairy farms were to devote five percent of their land to fast-growing carbon crops harvested once per year, the resulting biochar would be responsible for another 2.5 MT CO₂e.

And if we diverted one third of the nearly 5 MT of green waste currently going to municipal landfills each year, we could gain another 4.6 MT CO₂e.

The sum of these three streams would account for 14.4 MT CO 2 e annually, nearly one fifth of our gross GHG emissions of 80.9 MT CO₂e.

Also, instead of planting ever increasing swathes of Pinus radiata on sheep and beef country and crossing fingers against the changing climate and the increasing risk of wildfire, we could instead be growng low-input carbon crops such as coppicing forestry, switchgrass, miscanthus, or hemp. A commitment of just 50,000 ha to carbon crops yielding 20T/ha pa would translate to 1.9 MT CO 2 e each year as well as provide jobs for the regions currently facing the prospect of being turned into “pine deserts” by wholesale forestry conversion.

What can biochar be used for in animal husbandry?

Some applications of biochar are obvious: it makes a valuable additive to bedding, where its absorbency and CEC can reduce ammonification in calf rearing, standoff pads, and poultry sheds. Biochar is widely used as a feed additive in Europe, where it demonstrates multiple benefits to animal health and in some settings has shown potential to reduce ruminant methane emissions. Precision feeding equipment could allow New Zealand pastoral farmers to offer supplements based on biochar to their livestock.

Will farmers have to radically alter their management in order to produce biochar to use on farm?

Integrating carbon crops can work well in an existing farm setting, as many of the plants which are suitable can be grown in marginal areas which are the least fit for grazing or cultivation. Annual rotations of plants such as hemp can provide benefits through the root mass left in the soil, and a crop such as maize could provide a dual yield if the grain were harvested and the stover used for biochar. Marginal plantings of coppice forestry along drains and raceways would help mitigate some nutrient loss during the growing season, and offer a recurring harvest of high-carbon feedstock.

Isn’t biochar still unproven on a landscape level and at longer time scales?

We have evidence in the form of well-developed, fertile soils in regions such as the US midwest, the Russian and Ukrainian grain belt, and the Argentine pampas to show us how the presence of pyrogenic carbon has helped create some of the most productive growing conditions on the planet. We have the record of deliberate addition of charred matter which turned parts of the highly leached rainforest soils of the Amazon into terrapreta, whose abundance allowed settled agriculture to develop and sustain a thriving culture. These case studies extend thousands of years into the past and provide a sound basis for a call to produce and incorporate biochar into productive landscapes over the long term. Even if the secondary benefits were deemed unimpressive, the carbon sequestration potential of a large-scale biochar application programme merits priority treatment as part of New Zealand’s climate mitigation strategy.