On this page we are looking at biochar as a way of fixing carbon underground or in the soil.
Biochar has the advantage of letting the trees collect the carbon, then giving us energy in the biochar production process, then once added to the soil it increases its' fertility.
If we were prepared to convert all trees in Australia to biochar, then we could balance out the emissions from Australian lifestyle and coal exports. However converting biomass waste to biochar will still help. It is a matter of whether biochar or coalification is the best answer.
There is still a lot of research needed on biochar so we cannot yet say it is a solution.
For the production of biochar see the page on pyrolysis of biomass.
Bio-char is the charcoal left by a wood fire.
Biochar seems to be win-win. It increases the fertility of the soil, and it fixes carbon in the soil.
The questions are: how long will it last, and what will it cost?
Industrially biochar is made by heating biomass in a sealed container with no air in a process known as pyrolysis. The gases and liquids are driven off and the char remains. The smoke (wood gas) can be burnt to provide the heating for the process and the liquids can be made into oil. Traditional methods allowed some of the char to burn.
Coal is similar, if it is heated without air, it produces coke, liquids, and gases.
History of biochar
Terra Preta, or black soil, is an incredibly fertile soil produced by ancient slash and burn farmers in the Amazon rainforest. They discarded charcoal, bones and pottery which became part of the the soil and it has stayed fertile for thousands of years.
How long will biochar last in the soil?
Researchers are looking at the ages of biochar in different soils and producing quite different and conflicting results.
Amazon: 500-7000 yrs Tropical
Australia: 1300-2600 yrs - dry land conditions
Vancouver: 6,600 years - cold moist
Russia: 300 yrs - May be destroyed by freeze/thaw.
Zimbabwe: 8 yrs - may be erosion.
Ocean sediments: 13,900, & several millions of years.
Traditional charcoal production is to burn wood slowly under a covering of earth to exclude air.
Traditional methods have always been wasteful destroying forests in the process. Char is in demand for steel working and even steel making. It burns hotter than normal wood.
Sequestering CO2 in the soil using biochar
There is a lot of hope based on the ability of biochar to remain stable in the soil for thousands of years. It locks up carbon and increases soil fertility.
However, there are very few academic studies into the effect of biochar on soil. It seems that it works in warm climates, but not in cold. In the forests near the Arctic, adding biochar seems to reduce the total carbon in the soil. Could this be because there are no earthworms in the cold?
Biochar is eventually oxidised by microorganisms. The question is how long does it take? If it lasts for a few thousand years, then surely this gives farmers time to renew the biochar making up for losses? So it could be argued that bio char is a valid way of fixing carbon for ever.
Cultivation of soil destroys organic carbon, but seems to have no effect on biochar.
Three main factors make biochar even more stable:
1) The surface slightly oxidises forming a protective coating. This takes time.
2) biochar adsorbs onto mineral and metals. Especially after it has passed through earthworms.
3) humous often coats the surface giving it some of it's properties.
Putting it all back where it belongs
There is a well known bushwalking saying:
"The further you walk in a day, the further it is to get home."
We are burning carbon at a furious rate. At some time we are going to have to put it all back in the ground so we can return the Earth to normal.
It is easy to imagine the post apocalyptic landscape covered in wind turbines and PV cells powering equipment collecting and pumping CO2 underground. The survivors maintain these pumps hoping that one day their descendants will be able to enjoy a once again beautiful planet. But I digress....
One way is to bury biochar. It does not need to be held under pressure as CO2 does, and it will take up one sixth of the volume of CO2. Perhaps it could be paid for, by the energy released, and the improvement in soil fertility.
Hansen calculated that producing biochar by current methods of burning waste organic materials could reduce global carbon dioxide levels in the atmosphere by 8 ppm (parts per million) over the next 50 years. That is the equivalent of three years of emissions at current levels.
Tim Lenton, a climate scientist at the University of East Anglia, calculates that by 2100, a quarter of the effect of human-induced emissions of CO2 could be sequestered with biochar production from waste organic matter, giving a net reduction of 40ppm in CO2 concentration.
Johannes Lehmann of Cornell university has calculated that it is realistically possible to fix 9.5bn tonnes of carbon per year using biochar. He estimates that pyrolysis can be cost-effective for a combination of sequestration and energy production when the price of CO2 reaches US $37 per ton.
How many trees would we need?
Hopefully, biochar is stable, and could be used to fix carbon underground.
Let's look at how much we would need to balance our coal mining.
If we count our yearly emissions plus our exported coal, then we total 507 MT for 2013.
To counter this we would need to put about 500 MT of biochar into the soil every year.
To produce this much biochar, we'd need to cut down every tree in Australia when it reached maximum size, and convert it to biochar.
Or we could stop coal mining.
12 T C + 32 T O2 ---> 44 T of CO2
1 T of C ---> 3.67 T CO2
Coal is aprox. 80% Carbon
1 T coal ---> 2.9 T CO2
Australian emissions = 339 MT Co2 e Source
Aust. emission equivalent to 170 MT of coal
Export of coal is 337 MT Source
Total 507 MT coal
We would need to bury 500 MT of biochar per year
(Assuming biochar has the same carbon content of coal)
Trees in a plantation:
I have to declare a bias here- I don't want this to happen.
I like trees.
Soil properties of biochar
Biochar is reported as having many benefits:
In boreal forests (near the arctic circle) it reduces the carbon content of soils. Source: Wardle
Fertility increase varies and there has been very little research on the subject..
Biochar varies depending on the source and production process.
Different soil types and climates also cause the effect to vary.
Aged biochar better than new
To date, scientists have been unable to completely reproduce the beneficial growth properties of terra preta. It is hypothesized that part of the alleged benefits of terra preta (Amazonian black soil) require the biochar to be aged so that it increases the cation exchange capacity of the soil, among other possible effects.
There is no evidence natives made biochar for soil treatment, but really for transportable fuel charcoal. Abandoned or forgotten charcoal pits left for centuries were eventually reclaimed by the forest.
In that time the harsh negative effects of the char (high pH, extreme ash content, salinity) had worn off and turned to positive as the forest soil ecosystem saturated the charcoals with nutrients. At high temperatures (30–70°C), cation retention occurs within a few months.
Lehmann 2007 pp. 381–387
Production of biochar
The process conditions can be optimized to produce either energy or biochar. The carbon price and the energy price will determine the economics. The process will be economic at a price of $37 per tonne of CO2.
At 400-500oC -50% char is produced. Takes several hours.
Above 700oC , 20% char, 20% gas, 60% bio oil is produced in only a few minutes. Gas is used to heat the process and generates 3-9 times the energy needed. The oil has several options.
The process can be carried out in a central plant, the excess energy can be used to generate electricity. It could be carried out in a mobile plant, to save transporting the biomass or the char. However excess energy would probably be wasted.
Microwaves can be used and produce about 50% char.
Temperature, and biomass composition, affect the properties of the biochar.