The Why Files The Why Files --

Buried charcoal: global warming star?

Charcoal's hidden benefit: biofuel

Making charcoal is one of the uglier traditional technologies: wreathed in blue smoke, charcoal makers releases polluting gases, leaving relatively pure carbon in the charcoal and a cloudy mess in the surrounding air.

But it doesn't have to be that way: That nasty smoke becomes an asset in a modern pyrolyzer.

 Large metal machine in a laboratory
Photo: Agricultural Research Service
This experimental fast-pyrolysis machine is being used to explore how to obtain biofuel while making biochar from biomass.

Charcoal is made by heating biomass in the absence of oxygen, a process called pyrolysis. Varying the design and temperature of the pyrolysis reactor alters the proportions and chemistry of the three major outputs: charcoal, the condensed smoke called bio-oil, and the gases hydrogen and carbon monoxide, which can be collected as a low-energy fuel called syn-gas.

In fast pyrolysis, pulverized biomass is dried, ground, and flash heated to between 450 and 500 C. as it is pumped through hot sand. Within one second, a mix of charcoal, smoke and syn-gas is created. The charcoal is separated, the smoke condensed and cooled into bio-oil, and the syn-gas is collected and stored.

Fast pyrolyzer for making biochar and bio-oil diagram of fast pyrolysis

Diagram: David Laird, USDA ("Review of the pyrolysis platform..." in bibliography).
In a fast pyrolyzer, shredded biomass enters a bed of hot sand and is heated to between 450 and 500 degrees C within a second. The cyclone removes biochar, and the bio-oil is captured and transformed into biofuel.

The Why Files talked to Akwasi Boateng of the U.S. Department of Agriculture's Agricultural Research Service, who is investigating the production of liquid fuel during pyrolysis. Boateng said that bio-oil, even before being processed into bio-fuel, already contains about half as much energy per kilogram as diesel fuel.

Bio-oil is strongly acidic and contains excess oxygen, so it must be "upgraded" before being used as fuel. Boateng says this process is attracting some attention from industry, which is trying to concoct a liquid that could be added to diesel fuel or gasoline, much as ethanol is added today.

Pile of charred, blackened sticks broken in small pieces
Photo: Biochar Fund
This charcoal was made in Congo from waste stems of cassava, a staple root crop. Some biochar advocates say the bio-oil could be captured and used to fuel village electric generators.

No fuel fool

Whether the process is optimized for biochar or bio-oil, small-scale pyrolysis units can eliminate the need to haul biomass from fields or forests to processing plants. "The process could be economic if you could make liquid at the farm," says Boateng, "leave some charcoal behind, and take the liquid, which is more easily transported than the bulky biomass, to a central place where it can be upgraded."

Bio-oil could also be used to run pumps or electric generators, or heat homes, in remote villages.

Because the carbon in the fuel comes from plant material, the fuel is carbon neutral, and so bio-oil can add to the carbon-storages benefits obtained from burying biochar.

Boateng cautions that lab-scale pyrolysis still requires more energy input than is contained in its three products, but adds that such reactors usually become more efficient with the economies of scale: waste heat extracted from the hot bio-oil, for example, could be recycled to heat the pyrolyzer. "If we can do all this efficiently, there is a possibility that we can do this sustainably, and make fuel and food at the same time."

And still leave some charcoal to bury!

Nothing charred in our bibliography.

Terry Devitt, editor; Steve Furay, project assistant; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive

©2021, University of Wisconsin, Board of Regents.