This post is based on an article from the October 2007 issue of Biomass Magazine. The information in this article may no longer reflect the current state of the research projects described or current thinking about any scientific issues discussed.
European conquistadors were drawn to the Amazon rain forest by legends of El Dorado, a country where the cities were made of gold. El Dorado was a myth and in their greed for gold, the conquistadors missed a treasure that was right beneath their feet. It was black gold, and that doesn’t mean oil.
“Terra preta” means black earth. It is prized in the Amazon because of its high level of fertility compared with the surrounding red clay soils. The tremendous precipitation in the region washes away any nutrients not taken up by plants, leaving an impoverished soil that is ill-suited for agriculture. In contrast, the terra preta is highly productive year after year. How these soils kept producing good crops was a mystery until a few years ago. Researchers discovered the terra preta was largely man made. Over centuries, the ancient residents of the Amazon incorporated charcoal into the soil. Amazingly, radiocarbon dating showed that some of the terra preta sites were 1,500 to 2,000 years old.
The same properties that enrich the earth could also someday protect the skies. Now that it is known that charcoal can remain in the soil for millennia, some scientists think this may be a possible carbon sink to reduce greenhouse gases in the atmosphere.
Char is the product of partially burned biomass. While charcoal has been produced for almost as long as man has controlled fire, the modern source of char is from a process called fast pyrolysis. In this process, biomass is heated to the point where volatile gases and liquids are driven off and condensed into a product call bio-oil. What remains is almost pure carbon, called char, pyrolysis char or biochar with a varying ash content that depends on the type of biomass used. Interest in the char has grown as more companies explore processing technology and new uses for biomass.
Heartland Bioenergy LLC is proposing to build a biorefinery in central Iowa. The company’s goal is to use corn stalks to produce transportation fuel. “We have been working on biomass issues for five or six years now,” says Lon Crosby, a researcher with Heartland Bioenergy. “If you are going to collect biomass to prove technology, you had better have a way to use the biomass.”
Heartland looked at a number of technologies as a base for its proposed biorefinery. “As we started to look for ways to use biomass, we looked at gasification, fast pyrolysis and a variety of other techniques,” Crosby says. “Probably the best approach we found was to start with fast pyrolysis, because it yields two products: bio-oil and biochar.”
Bio-oil is a relatively easy product to market as a bunker fuel, Crosby says. There is an existing demand for char, but finding new uses for it would make the whole process more economically viable. “Biochar is easy (to market) as long as you are interested in low-value applications,” he adds. “I happen to also farm, so looking at char’s agricultural applications was a natural choice.”
Heartland is testing char in a large-scale project to see how it impacts corn production. Dynamotive Energy Systems Corp. has provided Heartland with 14 tons of biochar. While working with the USDA National Soil Tilth Laboratory, Iowa State University, the Iowa Soybean Association and Prairie Rivers of Iowa Resource Conservation and Development, Crosby created a large-scale test plot to measure the impact of adding char to the soil. The plot consists of three strips that are 30 feet wide and 800 feet long. One strip is untreated while the others were treated with 2.5 and 5 tons per acre of char.
This experiment will overcome some of the deficiencies seen in small-scale char experiments. Crosby says char experiments are greatly affected by edge effects between treated and untreated soils. This skews the results from test plots that are a few meters across. By using large plots, edge effects are reduced producing more reliable data.
In the Black
While the soils of the Amazon have received much of the attention, Crosby says char’s effect on soil is a worldwide phenomenon. “The terra preta soils have gotten all the recent publicity, but agronomists have known for decades that the most productive soils in Europe were char based,” Crosby says. “Up until the research on terra preta soils, no one believed the char was playing an active role in the ecosystem. Research on terra preta showed that char itself is having a significant effect as a direct parameter, instead of just something that happened to be in the soil.”
There are several hypotheses as to why char increases soil productivity. One is that it creates a base for microorganisms that produce substances that hold soil particles and nutrients together. Another hypothesis suggests that the char itself adsorbs nutrients and slowly releases it to plants over time. “We know a lot of reasons why things happen,” Crosby says. “I don’t think anyone understands how all those things interact to create the phenomena we see.”
Laboratory studies suggest biochar can cut down on nonpoint water pollution by holding nutrients like nitrogen and phosphorus in the soil, keeping it out of rivers and lakes. It still isn’t proven that this will work on a larger scale, Crosby says. “No one has done large-scale field studies to show that what happens in the laboratory happens in a production situation,” he says. “That’s one of the reasons we initiated this large-scale field study.”
One of the goals of the project is to learn how to work with biochar as a soil amendment. Most farm chemicals are applied at a rate of a few pounds per acre. Even nitrogen fertilizer is usually applied at no more than a few hundred pounds per acre. In Crosby’s field test, the application rates are 25 to 50 times as great. “We can learn about how you incorporate biochar into the soil profile and how it distributes,” he says. “You can’t just take the top 24 inches of soil and uniformly mix char into it like you see in the terra preta soils.”
Working with the char is unexpectedly complex. Nobody makes equipment to handle char and efficiently incorporate it into the soil. “We are trying a bunch of different pieces of technology,” Crosby says. “We have not yet found the perfect one. The problem is char is very light with a very low bulk density and it’s very granular. That’s very different than your typical potassium or phosphorus fertilizer. Your controllers aren’t set up for it, your chain speeds aren’t set up for it, and your openings aren’t set up for it. You can’t apply it very far above the ground to control wind drift because it is so light and granular.”
Heartland’s biochar study will be a long-term effort. Crosby says char carries a load of plant nutrients, so the first year’s results will be a combination of fertilization as well as the char’s impact. It will take several seasons before he can be sure of the char’s effect on soil productivity. “We have just started a research project that’s going to go on for years,” Crosby says. “Char’s interaction with soil is complicated. In the first year there is a nutrient effect and a char effect and nobody knows how to separate those two factors. So we expect this study will go on for a long time.”