27 July 2017
By Madeleine Jepsen
Cookstoves meant to curb carbon emissions and reduce pollutants may not be as climate-friendly as previously thought, a new study finds.
Traditional bio-fueled cookstoves, used by one-third of the global population, burn wood and other organic material to cook food. A new study measuring the types of carbon in fine particulate matter emitted from these types of cookstoves in India found newer stoves designed to reduce emissions were not much of an improvement over traditional chulha stoves.
The ‘rocket-style’ stoves, which insulate the cooking fire inside a vertical clay tube and metal casing, released only marginally fewer emissions per the same amount of fuel burned than the traditional stoves, which rely on a more open fire, according to the new study.
Burning solid fuel in households for cooking contributes about 25 percent of global emissions of black carbon or soot, which has been shown to contribute to climate warming. Fine particulate matter emitted from cooking fires also contributes to household air pollution, which was estimated to cause 2.9 million deaths in 2015.The newer style of cookstoves were expected to emit less particulate matter and use less fuel, making the new stoves better for the environment and human health.
In the new study, researchers were testing whether the new stoves would emit less particulate matter per amount of fuel burned, and the properties of those emissions. They also wanted to see if the stoves reduced fuel use, since more efficient combustion reduces the amount of fuel needed to complete the same tasks and more completely burns the fuel used.
“The idea is that you have more insulation and you have better control over the air coming into the fire, so you typically get more efficient combustion,” said Andy Grieshop, an assistant professor of environmental engineering at North Carolina State University in Raleigh and co-author of the new study in GeoHealth, a journal of the American Geophysical Union.
The new study was built around an effort by an Indian NGO to change out stoves in the country to reduce carbon dioxide emissions.
Grieshop and his colleagues measured particulate emissions from 31 households in India, 11 of which completely transitioned to the new stoves for cooking. For households with the newer stoves, they measured emissions before and after the newer stoves were introduced.
The researchers measured amounts of organic carbon and black carbon emitted from the stoves. Black carbon has been shown to contribute to climate warming by absorbing solar energy and disrupting cloud reflectivity in the atmosphere. It also darkens the snow or ice it lands on, speeding up the melting process. Organic carbon, on the other hand, is thought to have a net cooling effect on the climate by scattering short-wave solar radiation. Both types of carbon are present in the fine particulate matter from the fire, which is hazardous for human health.
Their measurements showed particulate carbon emissions — a combination of organic and black carbon found in particulate matter — were only marginally reduced in households with new stoves and there was not a significant reduction in fuel use. These particulate emissions also contained a higher proportion of black carbon than traditional stoves. Emissions from the new stoves contained almost 70 percent more black carbon than the traditional stoves’ emissions, according to the new study.
“When you change the balance of the two — organic versus black carbon — you can basically shift the overall particle climate effect from cooling to warming, or vice versa,” Grieshop said. “This study is a bit of a bummer, because we hope for benefits, and what we saw was a pretty minimal benefit in terms of indoor air pollution, a pretty minimal benefit in terms of particulate matter emissions and then in the particles that are made, the balance between organic and black carbon is shifted in the wrong direction.”
He said more research was needed in order to pin down the exact causes for these results.
In addition, the quantity and quality of carbon emissions from cookstoves depended on the season, according to study. After taking follow-up measurements in households still using traditional cooking stoves, the researchers found significantly higher particulate matter emissions in measurements taken during the monsoon season than during the dry season.
Part of the seasonal difference may be due to differences in fuel composition or its moisture content, but Grieshop said more research is needed to confirm these causes.
“It has pretty big implications for doing these kinds of studies because, for example, if you do a study where you say, ‘We’re going to do a bunch of measurements now, then have people switch [stoves] and do the same kinds of measurements later,’ and you don’t have any kind of control, you might see a change and attribute it to the intervention, but it’s actually just because seasonal conditions have varied,” he said.
Although the study suggests the new stoves may not significantly lower particulate matter emissions, the link researchers uncovered between seasons and emission levels could help future studies better estimate the environmental impact of other stove designs, according to Grieshop. He said studies like this can save organizations money by testing the technology before it is used in a widespread, more expensive trial.
“In this case, despite very good efforts, it didn’t really have much of an effect on the things we were hoping to see an effect on,” he said. “That’s one of the messages here: doing this type of assessment early on can be very helpful.”
— Madeleine Jepsen is a science writing intern at AGU.