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Wednesday, November 07, 2007

A Carbon-Negative Fuel

A Carbon-Negative Fuel
Jeremy Faludi



"Impossible!" you say. "Even wind and solar have carbon emissions from their manufacturing, and biofuels are carbon neutral at best. How can a fuel be carbon negative?" But listen to people working on gasification and terra preta, and you'll have something new to think about.

We've mentioned terra preta before: it's a human-made soil or fertilizer. "Three times richer in nitrogen and phosphorous, and twenty times the carbon of normal soils, terra preta is the legacy of ancient Amazonians who predate Western civilization." Although we don't know how it was made back then, we do know how to make it now: burn 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. What's more, it may reduce other greenhouse gases as well as water pollution: according to Biopact, a network that promotes biofuels and biomass energy,


Not all gasification is green. The coal industry routinely uses gasification all around the world to create syngas (synthetic gas) as a petroleum substitute of chemical feedstock. However, gasification plus terra preta has potential to be revolutionary.

I can't promise that using gasification for energy and using the resulting char as terra preta fertilizer will be a carbon negative fuel, because I haven't seen a credible lifecycle analysis of it. (If anyone has, please post it to the comments.) But it's quite plausible. 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.



Gasification and terra preta as a means of sequestering carbon is far cheaper than injecting CO2 into mine shafts, but it's still not cheap. Biopact calculated that "under a basic scenario sequestering biochar from biofuels produced by pyrolysis would be competitive when carbon prices reach US$37 (carbon currently fetches €21.55 on the European market, that is $30.5, and prices are expected to increase strongly in the near future)."

However, "[T]he great advantage of biochar is the fact that the technique can be applied world-wide on agricultual soils, and even by rural communities in the developing world because it is relatively low tech." In fact, the guts of Jim Mason's Mechabolic was mostly built with scrap steel tanks and whatever miscellaneous piping was handy, with nothing but a couple welders and some power tools -- nothing a well-equipped farm mechanic wouldn't have.

One of the 2007 Ashden Awards went to a company in India making gasification / char systems in Kerala:

BIOTECH has succeeded in tackling the problem of the dumping of food waste in the streets of Kerala through the installation of biogas plants that use the food waste to produce gas for cooking and, in some cases, electricity for lighting; the residue serves as a fertiliser.
To date BIOTECH has built and installed an impressive 12,000 domestic plants (160 of which also use human waste from latrines to avoid contamination of ground water), 220 institutional plants and 17 municipal plants that use waste from markets to power generators. The disposal of food waste and the production of clean energy are not the only benefits of BIOTECH's scheme. The plants also replace the equivalent of about 3.7 tonnes/day of LPG and diesel which in turn results in the saving of about 3,700 tonnes/year of CO2, with further savings from the reduction in methane production as a result of the uncontrolled decomposition of waste, and from the transport of LPG.
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