Biochar can sequester carbon cheaply

Amazonian topsoil enriched with charcoalOrganic matter, such as agricultural waste, heated in the absence of oxygen splits into two types of material: a charcoal (biochar), and hydrocarbon gases and liquids. When added to soils, the charcoal can provide a powerful fertiliser. The hydrocarbons can be burnt, either to generate electricity or to power an internal combustion engine. Biochar is exciting growing attention around the world. Charcoal’s ability to improve soils can sometimes be spectacular. But more importantly from a climate change perspective, charcoal is almost pure carbon and is strangely stable in soils. It seems to persist for centuries. Charcoal can therefore offer substantial opportunities for long-term sequestration of carbon. The valuable fuels from the biogases and liquids are also carbon-neutral since they contain CO2 previously captured during photosynthesis. As a third major benefit, soils fertilised with charcoal seem to need less artificial fertiliser, thus saving fossil fuels. Fewer applications of fertiliser would reduce the level of emissions of nitrous oxide, a particularly dangerous greenhouse gas.

Biochar manufacture represents a way of productively storing large amounts of carbon. But the carbon in the charcoal could be burnt to generate electricity instead of being stored in soil. Current emissions trading schemes, such as the European ETS, do not allow sequestered carbon to be considered as equivalent to a reduction in greenhouse warming emissions. This is a mistake that will need to be rectified. It make more sense to use agricultural land to make biochar and biogases/bioliquids than to burn the biomass in power stations. Power stations burning wood benefit from buying fewer emissions certificates and from the renewable energy subsidy, but there is no comparable benefit from storing carbon in the soil. This is an anomaly that should be removed.


Some Amazonian soils are extremely rich and fertile. Thick and almost black, these soils contain a high percentage of very stable carbon. The pre-Columbian population appears to have fed their naturally thin soils with large amounts of charcoal which have remained until today. Known as ‘terra preta’ soils, they have remained fertile for hundreds of years after the application of carbon in the form of charcoal.

Researchers have noted that charcoals can improve soil productivity in other soils around the world. No one quite understands the process, and a great deal more scientific work needs to be done, but the evidence is strong that the highly porous structure of biochar helps retain other nutrients and provides a protective structure that encourages the growth of beneficial micro-fungi. Its sponge-like porosity gives it a huge surface area on to which nutrients and useful organisms can cling. Biochar that has been laced with applications of potassium and phosphorus appears to achieve even better results than the simple charcoal. (See

Established terra preta soils have high productivity. Yields can be more than twice those of adjacent areas to which charcoal has not been applied. From the point of view of improving soil fertility, it may make good sense to apply biochar to a large percentage of the world’s soils.

How to make biochar Wood and other organic residues such as crop wastes can be burnt in air, and will leave a small amount of black charcoal. Most of the carbon in the organic matter is burnt, generating heat, carbon dioxide, and other gases. Unlike biochar, the carbon that is left unburnt tends to be quite quickly digested by the soil.

Heating in the absence of air is usually called ‘pyrolysis’. (Much oven cooking occurs as pyrolysis. The external layer of the food seals the inside from oxygen.) During pyrolysis, the heat drives off hydrocarbon gases and liquids (as well as the remaining water). Eventually, all that is left is biochar, which can be almost pure carbon. The carbon at the end of the pyrolysis process can represent as much as 50% of the original weight of the organic waste. The percentage depends on the original material and on the temperature to which it was heated.

Pyrolysis process

Pyrolysis can be carried out on a very small scale, producing charcoal and gases for cooking use. In the past, the charcoal would have been employed as an efficient source of heat for other uses, whether metal smelting or the cooking of food. The challenge now is to produce equipment that can handle hundreds of tonnes of bio-waste or wood residues every day. Some businesses are well on the way.

In today’s pilot plants, the original organic material passes through sealed vessels to which heat is applied. This heat may be derived from burning the gases driven off in previous pyrolysis. The hydrocarbons are collected, either as gas or as liquid and the solid material is then cooled and crushed into very fine carbon granules.

Dynamotive in Canada and BEST Pyrolysis (Australia/US) are making good progress towards commercial-scale plants processing hundreds of tonnes of material a day. I don’t doubt that large industrial-scale biochar manufacturing facilities will be successfully developed within three or four years. This area is already attracting substantial sums of private capital, and the technological challenges are not of the greatest difficulty. The crucial determinant of whether the sequestration of biochar becomes a large-scale worldwide activity is financial. Does it make sense to store charcoal in soil rather than burn it?

The financial value of biochar The short section that follows contains dense financial calculations. Its purpose is simply to say that:

  • Emissions trading schemes should incorporate carbon sequestration in the soil as a valid reduction in CO2.
  • The current UK system for subsidising renewable electricity generation is extremely generous to the burning of renewable fuels. The system offers a misplaced incentive to use charcoal to burn in power stations rather than store it in the soil, although the net effect on carbon emissions will be approximately equal.

For those interested, here are the numbers:

A tonne of good quality biochar has an energy value of about 28 gigajoules (GJ), slightly less than the best quality coal. (Pure black carbon is about 32 GJ/tonne.) Standard coal costs about £1.50 per GJ. If a power station operator is prepared to pay the coal-equivalent price, biochar is worth about £42 per tonne in the UK.

Burning a tonne of biochar will produce about three and a half tonnes of CO2. (Pure carbon would generate 3.667 tonnes.) The current price of CO2 in the European Emissions Trading Scheme (ETS) is about £16, meaning that sequestering 3.5 tonnes ought to be worth approximately £56. Since £56 is greater than £42, the economic logic suggests that we should hold the carbon in the soil rather than burning it. This is before considering the secondary climate change benefits of reduced fertiliser use and lowered nitrous oxide emissions.

Of course, today it isn’t yet possible to make the rational choice and plough biochar into the soil. The farmer cannot gain credit for sequestering organic matter in this way. The ETS doesn’t recognise the storing of inert carbon as a valid way of reducing emissions. So charcoal gets burnt in the UK, rather than being stored. This needs to change: sequestering biochar in the soil is a reasonably inexpensive way of reducing net CO2 emissions.

In fact, the position is far more illogical even than this. If a power station burns an ‘energy crop’, such as willow or miscanthus grass, it gets two Renewable Obligation Certificates per MWh generated, worth over £90. Per tonne of biochar used in a specialist biomass power station, such as E.ON’s at Lockerbie, the power station operator will generate about 2.3 MWh, and therefore get ROCs to the value of over £200. Carbon Commentary has written before about the excessive generosity of this arrangement – please see issue 1 of this newsletter.

The net effect of the subsidy regime in the UK is that charcoal sequestration is financially unattractive before considering the benefits in terms of reduced fertiliser use. But in terms of the underlying carbon saving, it would be better to plough the char into the soil.

Weighing the climate change benefit of smaller amounts of fertiliser Artificial fertilisers are bad for global warming because they take substantial amounts of natural gas to make, and because they seem to cause greater amounts of nitrous oxide to be emitted from fields and local watercourses as the fertiliser breaks down. Nitrous oxide has over 300 times the global warming effect of CO2. It may be that at least 2% of the nitrogen in fertiliser ends up in the form of nitrous oxide after being applied on a damp British field.

I have seen no complete studies of how much it might be possible to reduce fertiliser use by adding biochar to a field. But a few illustrative numbers might be helpful. A hectare of wheat gets about 200kg of fertiliser a year. The CO2 cost of making this is something over 1 tonne. The nitrous oxide effect might triple this. In the ETS, four tonnes of CO2 costs perhaps £64. So even if biochar meant that the farmer could completely get rid of fertilisers, it would still only have a small CO2 benefit if all the emissions were valued at current prices. All available biochar would still probably be burnt for its value in ROCs.

The more interesting question is whether agricultural yields would go up. Terra preta soils succeed partly because they have much higher carbon content than other land in the same area. UK soils already have reasonable carbon content, so the impact of biochar is likely to be much less. We need further research on this urgently.

In Australia soil carbon levels are generally very low. There the value of adding biochar may be very much greater. Unsurprisingly, a good portion of the research into adding charcoal to soils is being carried out in Australian universities and companies.

Could sequestering biochar make a big difference to UK emissions? The answer to this question is quite complicated, but broadly speaking the answer is yes. Soil used to grow miscanthus grass can produce about 20 tonnes of organic matter per year. Burnt in a low temperature pyrolysis furnace, this grass will give off substantial amount of gas and liquids that can be burnt as fuel, reducing fossil fuel use. This has value, and replaces fossil fuel. About 10 tonnes of char might be left, producing 35 tonnes of CO2 if it had been burnt, per hectare. To sequester 70 million tonnes of CO2, or somewhat over 10% of UK emissions per year, we would need to devote about 2m hectares to growing grasses or fast-growing willow for turning into charcoal. The UK has about 19m hectares of land in agricultural use of which over 7m is used as grassland. In other words, we probably could sequester the equivalent of 10% of our emissions every year, but it would need us to convert a lot of low-value grazing land to miscanthus. We’d get additional benefits from improved soil fertility, lowered fertiliser use, and the benefit of burning the biogas and bioliquid.

It is probably more important to get people in the tropics using biochar for soil improvement. Tropical agriculture often uses ‘slash and burn’ for clearing an area prior to cultivation. Changing this to ‘slash and char’ would sequester most of the carbon, and increase the number of years the soil could be used before the farmer moved to other land. The potential in the tropics for carbon sequestration is far greater than in high latitudes.

Biochar appears to have real significance as a technique for reducing emissions around the globe. The case for substantial investment in R+D, as well as changing the regulatory incentives to sequester carbon, is overwhelming.

(Readers interested in learning more about the worldwide research into biochar may like to go first to the web site of Cornell professor Johannes Lehmann, a leading figure in the scientific investigation: