Offsetting carbon emissions with charcoal is a crazy idea that may make sense

According to a new study, as much as 12% of the world’s human-caused greenhouse gas emissions could be sustainably offset by producing biochar – a charcoal-like substance made from plants and other organic materials. That is more than would be offset if the same plants and materials were burnt to generate bioenergy, says the study. Additionally, biochar could improve food production in the world’s poorest regions, as it increases soil fertility.

Biochar is made by decomposing biomass such as plants, wood and other organic materials at high temperature in a process called slow pyrolysis – a form of incineration that decomposes organic materials by heat in the absence of oxygen.

Normally, biomass breaks down and releases its carbon into the atmosphere within a decade or two. But biochar is more stable and can hold onto its carbon for hundreds or even thousands of years, keeping greenhouse gases such as carbon dioxide out of the air for longer.

Other biochar benefits include: improving soils by increasing their ability to retain water and nutrients; decreasing nitrous oxide and methane emissions from the soil into which it is tilled; and, during the slow pyrolysis process, producing some bio-based gas and oil that can offset emissions from fossil fuels.

The carbon-packed substance was first suggested as a way to counteract climate change in 1993.

Scientists and policy-makers have given it increasing attention in the past few years, and this new study – conducted by a collaborative team from the United States Department of Energy’s Pacific Northwest National Laboratory, Swansea University, Cornell University and the University of New South Wales – is the most thorough and comprehensive analysis to date on the global potential of biochar.

The study

For their study, the researchers looked to the world’s sources of biomass that are not already being used by humans as food.

For example, they considered the world’s supply of corn leaves and stalks, rice husks, livestock manure and yard trimmings, to name a few.

The researchers then calculated the carbon content of that biomass and how much of each source could realistically be used for biochar production.

With this information, they developed a mathematical model that could account for three possible scenarios. In one, the maximum possible amount of biochar was made by using all sustainably available biomass. Another scenario involved a minimal amount of biomass being converted into biochar, while the third offered a middle course.

The maximum scenario required significant changes to the way the entire planet manages biomass, while the minimal scenario limited biochar production to using biomass residues and wastes that are readily available with few changes to current practices.

The researchers found that the maximum scenario could offset up to the equivalent of 1.8 petagrams – or 1.8 billion metric tonnes – of carbon emissions annually and a total of 130 billion metric tonnes throughout in the first 100 years. Avoided emissions include the greenhouse gases carbon dioxide, methane and nitrous oxide.

The estimated annual maximum offset is 12% of the 15.4 billion metric tonnes of greenhouse gas emissions that human activity adds to the atmosphere each year.

Researchers further calculated that the minimal scenario could sequester just under one billion metric tonnes annually and 65 billion metric tonnes during the same period.

Making biochar sustainably requires heating mostly residual biomass with modern technologies that recover energy created during biochar’s production, and eliminate the emissions of methane and nitrous oxide, the study noted.

Biochar and bioenergy

Instead of making biochar, biomass can be burnt to produce bioenergy from heat. Researchers found that burning the same amount of biomass used in their maximum biochar scenario would offset 107 billion metric tonnes of carbon emissions during the first century.

The bioenergy offset, while substantial, was 23 metric tonnes less than the offset from biochar. Researchers attributed this difference to a positive feedback from the addition of biochar to soils.

By improving soil conditions, biochar increases plant growth and therefore creates more biomass for biochar productions. Adding biochar to soils can decrease nitrous oxide and methane emissions that are naturally released from soil.

Darren Quick

Article supplied by www.gizmag.com