The Magazine of The Sierra Club
Stockholm Explores Biochar Solution to Carbon Reductions
Leaves and lawn trimmings are key to a process for generating thermal energy
Combining wood and leaves with blazing-hot temperatures can mean bad news for the climate. When those materials burn, the carbon stored inside them is turned into carbon dioxide, which is released into the atmosphere. But the city of Stockholm, Sweden, is experimenting with a system that heats garden waste to produce modest amounts of power while actually locking away carbon. This "carbon negative" process relies on the production of biochar, a type of charcoal that can be added to soils to help gardens grow.
City residents collect twigs and lawn trimmings from their gardens and bring them to the biochar facility.
The garden waste is heated to more than 1000°F in a reactor that contains no oxygen, which prevents combustion. Instead, the chemical process of pyrolysis transforms the plant material into glossy lumps of biochar and gas, both of which are captured by the reactor.
The biochar is tilled into the soil in Stockholm's parks. These porous chunks of charcoal improve soil health by retaining water, nutrients, and beneficial microorganisms. Because the biochar doesn't degrade, it also locks away the carbon that was previously stored in the plants. Stockholm's program aims to create a carbon-sink equivalent to the yearly CO2 emissions from 3,500 cars.
The gas produced by the pyrolysis process is burned in a combustion chamber, and the resulting thermal energy is used for the city's heating and hot water systems. The goal of the project is to generate enough heat for 400 apartments.
This article appeared in the July/August 2017 edition with the headline "The Biochar Solution."
Infographic by Brown Bird Design
