Measuring the smoke from "smokeless" recreational fire pits
Smokeless fire pit emissions testing at the Missoula Fire Sciences Laboratory.
Outdoor wood burning in fire pits, fire rings, and chimineas is a traditional recreational activity enjoyed in campgrounds and backyards across the United States. A by-product of recreational wood burning is smoke, a mixture of particles and gases that can have negative health impacts. In recent years, as the popularity of backyard recreational wood fires has grown, so has interest in fire pits designed to provide high heat output with limited smoke. Purported “smokeless” outdoor recreational fire pits are readily available, and the leading commercial providers of these fire pits are growing rapidly indicating expanding consumer demand. Motivated by the increase in outdoor recreational burning, the consumer desire for low-smoke options, and possible localized smoke impacts from backyard fires, USDA Forest Service scientists based at the Missoula Fire Sciences Laboratory partnered with the Minnesota Pollution Control Agency (MPCA) to determine if self-advertised “smokeless” fire pits produce less smoke than traditional fire pits.
Scientists designed a study to quantify fine particulate matter emission factors (EFPM) for two self-advertised “smokeless” fire pits and a traditional campground fire pit. EFPM quantifies smoke production—the amount of particulate matter (PM) produced per kilogram of firewood burned. EFPM measured in this study can be compared against the EFPM used in the U.S. EPA National Emission Inventory (NEI) for fire pits and fire rings to quantify how much “smokeless” style fire pits could reduce PM emissions. The study was conducted in the Missoula Fire Sciences Laboratory’s large scale combustion chamber. Over the three rounds of testing, EFPM were measured for 31 burns designed to represent a typical recreational fire burning a mix of northern red oak, maple, and birch split firewood from Minnesota.
The study found the three fire pits tested had similar EFPM, probably due to their similar barrel design (only two were specifically advertised as “smokeless” alternatives to traditional fire pits). The study average EFPM was 2.2 g of PM emitted per kg of firewood consumed. This is 80 percent lower than the EFPM used in the NEI for traditional fire pits and fire rings (11.3 g of PM emitted per kg of firewood consumed). The study confirms that the fire pits tested produce significantly less smoke (per mass of firewood consumed) than traditional fire pits and fire rings (assuming the EFPM used in NEI reasonably characterizes traditional fire pits and rings). These results indicate that recreational wood burning using "smokeless" fire pits like those tested in this study can provide robust fires with significantly reduced smoke, if users follow manufacturers’ instructions and use firewood with less than 20% moisture.