Biochar is a supercharcoal made by heating any residual biomass – for example, corncobs, husks or stalks, potato or soy residues, grass hay, rice or wheat straw, woody forestry residues, etc. – with little or no oxygen. All the cellulose, hemicellulose and lignin in biomass is converted to other forms of carbon including gases such as carbon dioxide (CO2) and charcoal, which is similar to graphite, a very stable form of carbon. Most of the minerals in biomass are converted to ash, which can provide nutrients such as potash and phosphate to crops when the biochar is added to soil.
Jim Amonette, a soil chemist at the Pacific Northwest National Laboratory and Washington State University, says this heating, known as pyrolysis, is done under controlled conditions to have a consistent product.
“I have been looking for ways to store more carbon in the soil. The advantage of producing charcoal is that charcoal carbon is 20 to 500 times more stable than the original biomass carbon. Less carbon is released into the atmosphere over decades than if the biomass had simply decayed in place,” he says.
Another aspect of interest is how it can improve soil health. “Not only are you retaining and storing biochar carbon but increasing soil organic matter content, especially long term. In some cases, it increases crop yields and, in some cases, increases moisture-holding capacity of the soil,” says Amonette. This is variable, however, and we don’t yet have enough information to fully understand this.
The concept of biochar is new, in North America. “We just started using this term in about 2005. There have been some studies looking at crop responses, but we don’t know how long these responses last. We do know that crops respond well to addition of biochar to acid soils. For soils like wheat growers in eastern Washington are dealing with, that have had nitrogen fertilizer added for years but no lime, and are too acidic, biochar can be a good substitute for lime,” he explains.
It could benefit farmers in other regions with highly leached acidic soils, such as in the eastern part of the country and particularly the Southeast. “Whether it pencils out in terms of cost, versus liming, depends on how the biochar is made, how far it is to a limestone deposit, etc. The economics are not necessarily there, but from a technical perspective, it should help,” he says.
“People tend to think about biochar as a single entity, but there are many different kinds, just as there are many different kinds of soil. You must match the biochar to your soil. You don’t want to apply a high-pH biochar to a high-pH soil, so you need to know what your soil is,” says Amonette.
He currently lives in Montana where there is lots of hay ground, and on some ranches there are old hay bales that are weathered and moldy – not good enough quality to feed. Burning them is not a good option, so using them to make biochar might be an opportunity to create something useful. “The trick is to get them into the right form to put through a pyrolyzer. There are different types of pyrolyzers available. Typically, for hay you’d need to chop it into smaller pieces (2 or 3 inches long) to go through it. Some methods also capture the energy released during pyrolysis and use it for space heating or to create steam to make electricity. If you have a greenhouse, you could use this to heat and light the greenhouse,” he says. A greenhouse operation could set up a small biochar unit next to it and get some CO2 as well as warmth off the pyrolyzer – and keep the greenhouse going through winter.
“Some folks are looking into water-retention properties. In the lab, most of the experiments I’ve done show that the water-holding capacity is simply due to spaces between the biochar particles and other soil particles. As they weather over time, the biochar particles become smaller and likely lose that storage capacity,” he explains. On the other hand, biochar helps generate new soil organic matter, which can also increase water-holding capacity of the soil. We don’t know yet how it balances out.
“We need more funding to get long-term research so we can understand what the benefits are and in what situations. We might want 20 sites around the country, with different biochar production methods and different applications – looking for what works best in certain regions versus what works in other regions. Currently most of the research only looks at it for a year or two. In our situation, we put it in the ground, grow a crop on it for a year or maybe two, and then leave it because our funding runs out. It’s still there; someone could come back in five to 10 years and measure it, but there’s not enough long-term integrated research – which would be the most efficient way to answer the questions,” Amonette says.
Another area of interest would be production of biochar when thinning forests (to improve forest health and reduce risk of catastrophic wildfires). Most of this material is going to waste, burning slash piles, etc., and creating air quality issues and respiratory problems in humans and animals. Creating biochar would be a better solution.
“There are different ways to do that. One is a so-called conservation burn pile where wood is stacked a certain way and lighted from the top. The flame is on top and smoke comes up through the flame – diminished significantly by the flame before it leaves. The top-lit method may double the amount of charcoal at the end because you have a larger bed of coals at the bottom,” he explains. Small portable “flame-cap kilns” expand on this concept and can yield larger amounts of charcoal with little smoke. These are suitable for small landholders but require constant supervision during operation.
The next step is air-curtain incinerators which burn very cleanly. “These are designed like a garbage bin in the city, but they have a big fan that blows air over the top and back into the flames. This captures the smoke as it comes up and recycles it through the flames before it gets out.” This creates a more complete burn.
“This gets rid of a large fraction of most of the particulates and all the methane is taken care of, but it doesn’t produce much biochar – maybe 5 to 15 percent of the carbon you started with. This is, however, a step in the right direction,” says Amonette.
“Then there are gasifiers, which are even more efficient. They can be as clean, or even cleaner, than an incinerator. Finally, we go to a slow-pyrolysis kiln in which there is an afterburner to capture and take care of whatever methane and particulates are coming off. This is by far the most efficient process. Of the original carbon in the biomass, you might end up with as much as 50 percent of it in the biochar,” he explains. .png){kind=icon}
To learn more about biochar, visit websites for the U.S. Biochar Initiative or the International Biochar Initiative. The report from a recent workshop hosted by Washington State University also provides a great introduction (Biomass to Biochar).
