When living organisms like trees, shrubs, grasses and other plants grow, they take up, or sequester, carbon dioxide.
Photo:murphyr, Creative Commons, Flickr
Trees, by virtue of their size, do this best, but all plants do it to some degree, even lowly grasses.
When these organic materials die and decay, they release CO2 back into the atmosphere. When they burn, as in a forest fire, only part of the CO2 is released, and the rest remains contained in the resulting charcoal, or biochar.
As it turns out, biochar is good not only for sequestering CO2, but good for plants as well because it provides the ideal micro-habitat for beneficial soil organisms that promote plant growth. This renewal by fire is the way North America’s prairies evolved and the reason why settlers to the Midwest observed Native Americans regularly burning these prairies. The native inhabitants knew, by observation, that an unburned prairie turns to scrub, eliminating essential habitat for plants like big and little bluestem, switchgrass, and Indian grass.
Biochar has been around for ages. The Amerindians of South America used biochar to create the rich, black soils of the region as far back as 450 B.C. These soils, known variously as terra preta or terra mulata, remain highly fertile compared to surrounding soils, even after generations of tropic-induced leaching. Scientists researching these soils now recognize that biochar makes soil higher in nutrients, better at holding moisture, and lower in pH, or soil acidity – an important factor in the growth of most food crops (except blueberries).
These Amerindians used a process very much like Medieval charcoal making, which was not very efficient either in terms of raw materials or carbon retention – a subject unlikely to have concerned them, though it concerns us very much now, with atmospheric CO2 levels rising 35 percent faster than predicted at the turn of the century.
This concern inevitably led to the modern discovery of pyrolysis, a form of combustion which produces biochar with minimal releases of CO2. Pyrolysis chemically decomposes organic materials by heat, but in the absence of oxygen. Pyrolysis typically occurs under pressure, at temperatures above 800 degrees Fahrenheit, and – while it isn’t possible to achieve a completely oxygen-free atmosphere except in space – the amount of oxygen is so limited that very little oxidation occurs.
Pyrolysis converts organic material into recoverable gases, small amounts of liquid, and the above-mentioned biochar, which can be burned again to create energy but is much more carbon-effective as a soil amendment. Biochar can be made from otherwise useless biomass like straw, grass clippings, wood waste and the like. Biochar doesn’t require anything be cut down, but does mandate that the detritus from forestry, farming and horticulture be recaptured and processed rather than being allowed to decay and release its burden of CO2.
Is there enough detritus to make biochar facilities worthwhile? Very likely. Will it be expensive? Initially yes, but the biochar can be sold to farmers, horticulturalists, home gardeners and others, and the carbon savings can be used to further offset the cost under carbon offset programs. The greatest difficulty I foresee is getting people to deliver their waste for processing.
Biochar is a carbon-negative addition to soils that reduces total fertilizer requirements, reduces runoff, reduces nitrous oxide emissions from crops by up to 80 percent, and enhances crop yields. The gases produced during pyrolysis can be used as fuel to create more biochar, making the process even more carbon negative.
Though experiments by a Swedish university demonstrated biochar was not as good at holding its burden of CO2 as expected – losing more than 20 percent over a 10-year period when mixed with soil – it still retained a remarkable 15 percent. This, coupled with its beneficial effect on plant growth, makes biochar a good prospect for carbon sequestration, and could improve marginal croplands in such places as Africa and Asia. If implemented, improved yields to feed rapidly rising populations would make biochar a life-saving technology from two perspectives – increasing food supplies and saving the planet.
It certainly has my vote.
Disclosure: I don’t own stock in any existing or proposed biochar facility.