Fir trees and blue sky

Black and White Smoke

Fir trees and blue sky

Biomass: Captured sunlight

Wood burning cookstoves make smoke and many different gases that change climate. Carbon dioxide (CO2) is absorbed when the plant grows and the same amount of carbon dioxide can be released when that biomass is burned. So emissions of CO2 can be zero with no effect on global warming (carbon neutral) if the burned biomass is used at the same rate as it grows.

But other emissions from combustion are also bad for climate change. Generally, wood burning cookstoves do not make a lot of methane and carbon monoxide so these gases do not add a lot to their effect on climate change. 

On the other hand, biomass cookstoves without engineered forced draft can make a lot Black Carbon (the soot in smoke) and BC is very bad for climate change. For this reason, when protecting climate, cookstoves should make as little black smoke as possible. 

White smoke can have a cooling effect on climate. ARC has been learning how to make combustion chambers that emit as little smoke as possible and make 95% white smoke and 5% black smoke. We are working with manufacturers to make stoves with health/climate combustion chambers.

red sign with white letters reading Wood Burning Prohibited

Clean Burning of Biomass

red sign with white letters saying Wood Burning Prohibited

Scotland has banned the use of climate polluting home heating systems such as oil and gas boilers, and wood burning stoves (except in cases of need) in new construction. Heating homes creates one fifth of Scotland’s CO2e. The plan is to switch to electric heat pumps, hydrogen and tighter, better-insulated homes in an effort to achieve carbon neutrality by 2045.

Oil and gas (fossil fuel) burners create too much CO2. Burning renewably harvested biomass can emit close to zero CO2, but old stoves make too much black smoke which is ~2000 times worse for climate change compared to CO2 by weight.

Heating stoves that burn sustainable biomass cleanly enough to protect health and climate are starting to become available. Testing the new generation of stoves in use will show whether biomass can join solar, wind, and hydro as a useful renewable energy resource in the post fossil fuel era.

Maybe those red signs will become green?

Green sign saying Sure, Light Up
bed of charcoal in rocket stove


Boman reports that temperatures above 850°C, in a 5kW combustion zone combined with air rich and well-mixed conditions for 0.5 seconds in the post combustion zone, resulted in an almost complete depletion of particulate matter (Boman et al., 2005).

How can we achieve 850°C in a combustion chamber? 

Yellow flames are around 1,100°C. but: 

  1. Heat flows in one direction, from hot to cold.
  2. Mass, charcoal, and incoming air (primary and secondary) are usually much colder.

Elevating temperatures to 850°C in the combustion chamber is not easy but it is possible!

  1. Use a thermometer.
  2. Create hot fires.
  3. Reduce mass as much as possible.
  4. Replace mass with insulation.
  5. Inject pre-heated air into charcoal to create high temperatures, especially in the top of the pile. Charcoal is a good insulator and when the top of the fuel bed is not red hot the effect can be cooling.
  6. Minimize the pre-heated incoming air, maintaining a minimum 3 to 1 fuel/air ratio.

From The WHO on Lower Emission Solid Fuel Stoves

In 2014, the World Health Organization (WHO) issued the first-ever health-based guidelines on clean fuels and technologies for household cooking, heating and lighting: INDOOR AIR QUALITY GUIDELINES: HOUSEHOLD FUEL COMBUSTION 2014

C:\Users\Owner\AppData\Local\Packages\Microsoft.Windows.Photos_8wekyb3d8bbwe\TempState\ShareServiceTempFolder\image (20).jpeg
Adding forced draft and chimneys to biomass cookstoves helps to meet WHO IAQ guidelines

From section 5.4.1 Roles of clean fuels and lower emission solid fuel stoves

“As recognized in these guidelines, and specifically in Recommendation 2, which addresses policy during transition, improved solid fuel stoves will continue to make an important contribution to the needs of a substantial proportion of lower income and rural homes where primary use of clean fuels is not feasible for some time to come. Work to develop substantially improved solid fuel stoves should continue in parallel with, but not hinder or displace, efforts to encourage transition to clean fuels. The contribution of solid fuel stoves to the mix of devices and fuels promoted will depend on the completeness of combustion that can be achieved when such technologies are in everyday use (as demonstrated through emissions testing), and the consequent reductions in health risks.” (pg.62)

Mixing with Primary and Secondary Jets of Air

Regardless of the velocity of secondary air, flow rate, or the angle at which air is injected into the fire, secondary air tends to lower the temperature of gases. Researchers have found that injecting secondary air into the side of the flame in a Rocket stove results in most effective mixing.*

The Jet-Flame, on the other hand, blows primary air jets up into the bed of made charcoal below the burning sticks of wood, creating a “mini blast furnace.” The jets of primary air increase the temperature in the charcoal, frequently resulting in higher temperatures in the combustion chamber. The mixing function is up into the fire, not into the side as with secondary air jets.

Boman et al., 2005 report that temperatures of 850C or above are needed for close to complete combustion in short residence times, as in a cookstove. Since excess air lowers temperatures, using the minimal volume of air in secondary air jets to achieve thorough mixing seems preferable. Researchers have recommended that the jets should penetrate into the middle of the flame but not enter into each other. (*Lefebvre and Ballal, 2010; Udesen, 2019; Vanormelingen and Van den Bulck, 1999).

Unfortunately, raising the temperature of pre-heated secondary air by a lot more than ~ 100C seems to be difficult. Cookstove combustion chambers are usually small, limiting the area exposed to high temperatures. The heat transfer efficiency is much lower from degraded temperatures further from flame.

 Residence time and temperature are easily measured. However, “thorough mixing” has not been defined and is not yet measured in our experiments. We infer that the woodgas/air/flame was thoroughly mixed when the emissions of PM2.5 and CO are close to zero as measured with the LEMS emissions hood. 

Cleaner Burning Biomass Stoves: In Homes!
The British Petroleum clean burning Oorja FD-TLUD stove from India

If protecting health and climate are important in stove projects, why not monetize the reductions of health/climate pollutants in carbon-offset projects?

Only the reduction in fuel use earns carbon income now!

With equal heat transfer efficiency, dirty burning stoves earn as much as clean burning stoves.

Dirty burning stoves are less expensive. “Market demand” reinforces the use of biomass stoves with low combustion efficiency.

Why not add income from reductions in CO, PM2.5 and Black Carbon, etc. to carbon projects to get cleaner burning stoves into use?

The approved 2017 Gold Standard Methodology already exists to do this! See:

Clean Burning with Metering and Mixing

Primary air usually controls the rate of reactions: How fast the solid biomass changes into wood gas.

Jets of primary air into the charcoal beneath the biomass add mixing while raising temperatures.

Secondary air jets into the side of the flame can also supply needed mixing but tend to lower temperatures.

Metering the gas supply into the tuned combustion chamber is important!

Too much wood gas can easily overcome the ability of even a well-tuned combustion chamber to achieve close-to-complete combustion. 

You can’t pour too much gas into a carburetor!

Updating a 9-year-old “Clean Combustion” YouTube Video

The great and upsetting thing about YouTube videos is that they don’t go away! A lot of this old video now seems incomplete:

Our understanding of how to come closer to complete combustion has changed and includes details missing before. The following describes a hopefully less wrong set of design principles. David Evitt is currently deep diving into clean combustion and who knows what details will be added in the next few years?

Nine years ago, we thought that close to complete combustion could be achieved by forcing all gases and smoke into flame for a sufficiently long amount of time.

  1. Today we add that the air/fuel ratio in the combustion zone needs to be air rich.
  2. We add that the flame/gas/smoke/air mixture needs to be thoroughly mixed. 
  3. We add that the temperature in the combustion zone needs to be above 850°C to allow short residence times to become effective.
  4. We add that at a minimum of 850°C the residence time needs to be more than 0.2 seconds. Longer is better.
  5. Wet wood, un-preheated air jets, and mass tend to reduce temperatures to below 850°C.
  6. Optimized heat transfer efficiency helps to reduce harmful emissions since less fuel accomplishes tasks.

Aprovecho Announced as a Winner of the Wood Heater Design Challenge

The U.S. Department of Energy (DOE) Bioenergy Technologies Office (BETO), in collaboration with Brookhaven National Laboratory (BNL), Lawrence Berkeley National Laboratory (LBNL), and the Alliance for Green Heat, today announced the winning teams for the Wood Heater Design Challenge (WHDC). 

Aprovecho Research Center, from Cottage Grove, Oregon, came in second place and won $25,000 with a novel burn pot, airflow configuration, and sensor package for pellet heaters. Davidon Industries from Warwick, Rhode Island was awarded first place for their mechanically automated, combustion-air control technology for cordwood heaters. Kleiss Engineering from Cloverdale, Indiana, won the third-place prize with a smart wood stove heater.

“Embracing innovation allows us to challenge existing norms, push boundaries, and discover new solutions that can reshape the entire industry,” said Dr. Valerie Sarisky-Reed, Director of BETO. “Wood stove research is part of DOE’s overall strategy to develop affordable bioenergy technologies and convert our nation’s renewable resources into fuels, power, products and in this case, more efficient wood stoves for homeowners.”

Aprovecho, Davidon, and Kleiss were selected from nine teams competing at the Wood Heater Technology Slam in September 2022. Teams pitched new wood stove ideas to retailers, the public, and experts, who assessed which stoves were the most innovative, efficient, and offered the greatest market potential. The three finalist teams moved forward to the testing phase of the competition, which was held this past spring at BNL in Upton, New York.

Read the full press release on the DOE Website. Learn more about the BETO-funded Wood Heater Design Challenge.


Watching a Rocket stove or a pellet stove (as above), it becomes obvious that metering the fuel is a primary factor in achieving close to complete combustion. When too much fuel is introduced into the combustion chamber, the emissions of smoke increase almost immediately.

For the clean burning of biomass, the controlled metering of fuel seems to be as necessary as it is in the engine of an automobile. The rate of reactions (how fast the solid biomass is being converted into wood gas) is then matched with the corresponding amounts of Time, Temperature, and Turbulence required to minimize CO and PM2.5.

ARC has added Metering to Time, Temperature, and Turbulence while unsuccessfully searching the thesaurus for a synonym that starts with the letter T. Maybe someone can succeed where we have failed?