Terra Preta for Carbon Reduction

October 17, 2007

Terra preta (or agrichar, as it is also sometimes called) is not a new concept, but it is probably unfamiliar to most readers. The term terra preta refers to rich black soils found in the Amazon. These soils are not natural, but were human-made, produced by the civilizations living in the region before the arrival of Western settlers. The terra preta has a high level of nutrients, with three times the nitrogen and phosphorus and twenty times the carbon of normal soils. But producing fertilizer is not even the most interesting part of agrichar. The agrichar process also releases gasses which can be used as fuel for electrical generation or even for powering vehicles, and, most interestingly of all, more carbon goes back to the earth than was released in the process.

The process of producing agrichar uses low-temperature burning (called pyrolization) to break down the plant materials and produce two products, syngas and char. Syngas is mostly carbon monoxide and hydrogen, and can be used as a fuel for electrical production. (Wood gas, which is very similar to syngas, has widely used in the past for lighting, heating, and as a fuel for internal combustion engines.) The char turns out to be a good soil amendment that helps fertilize the soil. More importantly, the carbon that has been captured in the char breaks down very slowly so it remains sequestered for a long period of time.

“[B]urn biomass (preferably agricultural waste) in a special way that pyrolisizes it, breaking down long hydrocarbon chains like cellulose into shorter, simpler molecules. These simpler molecules are more easily broken down by microbes and plants as food, and bond more easily with key nutrients like nitrogen and phosphorus. This is what makes terra preta such good fertilizer. Because terra preta locks so much carbon in the soil, it’s also a form of carbon sequestration that doesn’t involve bizarre heroics like pumping CO2 down old mine shafts.”

In addition to providing soil nutrition, the syngas produced from agrichar also has the potential to provide a carbon-negative energy, sequestering more carbon in a state where it will not readily be returned to the atmosphere. Jeremy Faludi’s article at WorldChanging points out that a complete life-cycle analysis has not been completed yet, but the principle seems sound:

“Consider that it takes a certain amount of CO2 to grow a crop, such as corn. You harvest the crop and sell the food part, which leaves you with all the agricultural waste. Instead of burning it in the open air, or landfilling it (which is what’s done today — basically topsoil mining), you gasify it. You then burn the fuel gas you get from gasification, putting some fraction of that CO2 into the air; the agri-char (terra preta) that you’re left with contains the rest of the embodied CO2 which the crops sucked up while growing. There’s more carbon here than there was in the fuel gas. You spread the terra preta on the fields as fertilizer to grow more crops, and repeat the cycle — and with each repeat, you pull more carbon back into the soil than you burn, resulting in a carbon negative fuel as well as crops fertilized with fewer petrochemicals. It’s a double win.”

Karl Schroeder, a science-fiction author as well as a contributor to WorldChanging, first brought agrichar to my attention. In an interview I did with him for EcoGeek.org last month, he mentioned it as one of the projects for environmental improvement he would fund if he had a billion dollars to spend.

Ordinary burning of clear cut forest or jungle does not produce terra preta. Most of the carbon that was sequestered in the organic materials becomes atmospheric and contributes to rising carbon levels. In fact, jungle clear cutting and burning has been identified as a significant contributor to current rising levels of atmospheric carbon. It will take some industrial infrastructure to produce agrichar and fuel on a large-scale basis. But research is underway on exploring these processes. And processing agricultural waste in a manner where it improves the soil and produces energy at the same time holds a lot of promise. It’s a topic we’re likely to hear more about in the future.

via: WorldChanging

Image Source: Kent State University