New technologies help wood-burning stoves burn more efficiently, produce less smoke

Oregon State University researchers are gaining a more detailed understanding of emissions from wood-burning stoves and developing technologies that allow stoves to operate much more cleanly and safely, potentially limiting particulate matter pollution by 95%.

The work has key implications for human health as wood-burning stoves are a leading source of PM_2.5 emissions in the United States. PM_2.5 refers to fine particulate matter with a diameter of 2.5 micrometers or smaller that can be inhaled deeply into the lungs and even enter the bloodstream. Exposure to PM_2.5 is a known cause of cardiovascular disease and is linked to the onset and worsening of respiratory illness.

Even though a relatively small number of households use wood stoves, they are the U.S.’s third-largest source of particulate matter pollution, after wildfire smoke and agricultural dust, said Nordica MacCarty of the OSU College of Engineering.

Residential wood combustion, especially the use of inefficient stoves, is also a significant source of other harmful emissions including polycyclic aromatic hydrocarbons, carbon monoxide, nitrogen oxides, methane, benzene and formaldehyde.

“Wood is an affordable, local, renewable, low-carbon fuel that should be an important part of the U.S. energy mix, but it must be burned cleanly to effectively protect health,” MacCarty said.

“Folks typically think of pollution as coming from vehicles and industry, but household wood stoves are a larger source—just a few smoky stoves can create a harmful effect on air quality in an entire community.”

MacCarty published a paper in the Journal of the Air & Waste Management Association showing that 70% of the pollution emitted from wood stove flues happens at two points in time: when a stove is first lit, and when it’s reloaded. MacCarty’s team gained that knowledge by developing a new monitoring technique and deploying equipment at a collection of wood stove users’ homes in rural Oregon.

According to the Environmental Protection Agency, there are an estimated 6.5 million inefficient stoves in the U.S., most of them models that predate EPA clean-burning standards. In all, there are roughly 10 million wood-burning stoves in the country, or one for every 35 people.

“A lot of the older stoves are essentially just metal boxes with chimneys and they don’t incorporate modern engineering principles to optimize heat transfer and combustion,” said MacCarty, the Richard & Gretchen Evans Professor of Humanitarian Engineering and an associate professor of mechanical engineering.

“They have no catalysts or secondary combustion to reduce emissions and lower the risk of creosote buildup that can cause chimney fires.”

MacCarty’s group is developing automated technologies that inject jets of primary and secondary air into the fire to provide just the right amount of air and mixing at the right time and place in the fire. Prototypes are showing about a 95% reduction in particulate matter emissions compared to older models, she said.

The EPA has been reducing the allowable PM_2.5 emissions rate regularly since the 1980s. In 2015 it was 4 grams per hour for cordwood stoves, and five years later it was reduced to 2.5 grams per hour. Regulation is driving innovation as stove makers improve their designs to meet certification requirements, MacCarty said.

But wood stoves perform differently in the lab than they do in real life, she noted, and stoves are certified based on laboratory tests—and often designed to pass the tests, rather than to operate well in someone’s home.

“It’s difficult to measure wood stove emissions in the field, so there has been relatively little in-use performance data available in the past to guide designs,” MacCarty said. “Our study introduces a new system that makes collecting real-world emissions data more practical.”

The project included Oregon State undergraduate student Jonah Wald and was a collaboration between OSU and the nonprofit Aprovecho Research Center based in Cottage Grove, Oregon. It builds on OSU and Aprovecho’s ongoing work on efficient combustion for cooking with wood in the developing world.

Roughly 2.7 billion people rely on open fires for cooking, MacCarty said, and her team has been designing efficient cook stoves for them to use instead.