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.

Short-lived Climate Forcers and Climate Change

It is not difficult to estimate the emissions of Black Carbon from cook stoves.

The emissions that change climate include various gases and the colors of smoke. If the wood used for cooking is 100% renewably harvested, the emissions of CO2 can be carbon neutral. Why? CO2 is absorbed when the plant grows and the same amount of CO2 can be released when biomass is burned. 

100% renewability can help CO2 to become climate neutral. However, the fraction of non-renewability (fNRB) does not change the amounts of other climate forcing emissions. Smoke is smoke. Etc.

What are the most powerful cook stove emissions affecting climate?

In general, adding methane and carbon monoxide to Carbon dioxide (CO2) adds a bit to the total warming influences (CO2e). 

However, adding short-lived climate forcers such as NOx, SOx and Black Carbon to the above has been estimated to more than double the warming potential. 

For this reason, it seems to be important to add the short-lived climate forcers when calculating how to address climate change with cook stoves. 

A chef cooks at a stove with a flaming wok

Turn Down Ratio: Cooking and Heating Stoves

The successful stove delivers the needed amount of heat to perform a task. In Haiti, our little charcoal stove could not bring the big pots of rice and beans to boil although the thermal efficiency at low power (simmering) was above 40%.

In the same way, a tight, well-insulated house requires a low amount of heat to stay warm. A “leaky” house needs higher firepowers to replace the constant flow of hot air lost through cracks under doors, etc.

Generally, a good stove has a minimum three to one turn down ratio. Heating stove experts have suggested a high power of 5 pounds burned per hour and a low power of 1.5 pounds. If a heating stove cannot turn down sufficiently, the tight, well-insulated house gets too warm. On the other hand, a “leaky’ house needs a big fire.  To save fuel, a tight house is more important than a new stove.

To boil 5 liters of water in less than 25 minutes in an uncovered pot, low mass cook stoves with tight pot skirts typically need a high power between 3kW and 2.5kW. However, firepower is often a lot higher when cooks are trying to get food on the table. In our experience, many cooks prefer ~ 5kW. 

Experiments at ARC have shown that with a low mass stove, lid, tight skirt on a 5 liter pot, it takes only ~0.4 kW to maintain a 97°C simmering temperature. But, cooking requirements vary a lot from country to country. Chinese cookstoves tend to use 10-15kW and may not need low power. 

Village cooks in Southern India cooked with many small pots and often did not bring the water to a full boil. For India, Dr. K. K. Prasad proposed, “…an ideal burner design with the power output ranging from 2.64 kW to 0.44 kW… (Prasad and Sangen, 1983, pp. 108-109). Dr. Baldwin adds, “One of the most important factors determining field performance of a stove is the firepower it is run at during the simmering phase. Because simmering times tend to be long, quite modest increases in firepower above the minimum needed can greatly increase fuel consumption.” (Baldwin, 1987)

With careful operation, the heat exchanger efficiency of houses and pots combined with delivering the appropriate firepower largely determines the fuel used per task. 

thermal image of house juxtaposed with daylight image of same house

COP 28: Near-zero emissions in global building sectors

thermal image of a house juxtaposed with daylight image of same house
Heat can constantly leak out of older homes. Photo: Gina Sanders

Aprovecho is investigating how to design and manufacture biomass-heating stoves that protect health and climate when burning renewably harvested biomass. Of course, staying warm depends on many factors including how much energy is being leaked from the building.

Net-zero buildings are usually tight and well insulated. A net-zero home can have a heating load of 10,000 to 15,000 Btuh (or ~3 to 4 kW) in a cold, northern climate. At COP 28, a minority of nations agreed to move towards net zero homes to reduce climate change by heating the better buildings with renewables. Green Building Advisor: 28 Countries Sign Buildings Breakthrough Agreement at COP28

Since the 1970’s, architects and engineers have learned how to dramatically reduce energy losses in buildings. Many net-zero homes take advantage of solar power to assist heating and create electricity. Solar gain helps a tight, well-insulated home to stay warm.

The United Nations found that buildings and construction account for 39% of total carbon emissions annually. Net Zero Homes: Your Guide to the Greenest Housing Option  If a new generation of very clean burning biomass heating stoves can protect health and climate, might they assist COP* countries to move towards near-zero emissions in global building sectors? *COP is the decision-making body of the UN Framework Convention on Climate Change.

From: EPA’s Lab Test Results for Household Cookstoves, Jim Jetter, 2012 

Since 2012, optimized biomass cook stoves have been tested at ~50% thermal efficiency

The temperature of the hot gases flowing past the surface of the pot is increased by

  1. Creating as much flame (1,100C) as possible in a low mass, insulated combustion chamber.
  2. Decreasing the distance between the fire and the pot without making excess smoke.
  3. Not allowing external air to cool the combustion gasses.

In convective heat transfer, the primary resistance is the surface boundary layer of still air immediately adjacent to a wall. 

Increasing Temperatures, increasing exposed Area, increasing Radiation, increasing Velocity in a 6mm to 7mm channel gap (10cm or higher) pot skirt has been shown (up to 5kW firepower) in a 24cm or larger diameter pot to result in ~50% thermal efficiency. Reducing losses from the exterior of the pot skirt with refractory ceramic fiber insulation also increases thermal efficiency. 

60% thermal efficiency has been demonstrated in the lab.

Helpful links:

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:

Moving Forward: Thanks to Jim Jetter’s EPA Lab!

Champion (2021) average energy emission factors (g/MJ) from ISO high, medium, and low tests. 

Champion, Wyatt M., et al. “Cookstove Emissions and Performance Evaluation Using a New ISO Protocol and Comparison of Results with Previous Test Protocols.” Environmental Science & Technology, 2021, 55, (22), 15333-15342.
DOI: 10.1021/acs.est.1c03390

Lab testing can quickly compare emissions from stoves. The EPA and ARC labs now measure the climate emission factors, not just PM2.5 and CO. It has been proven that only field tests show real world performance. Together, lab and field tests help to move stoves forward as we get closer to market driven stoves that please cooks, successfully cook food, use a lot less fuel, and protect health/climate.

The above chart contains a lot of information. Some takeaways are:

  1. Wow! The Three Stone Fire (TSF) was pretty bad! 943g/MJ for PM2.5, 15.5 g/MJ for CO.
  2. Charcoal made ~90% less PM2.5.
  3. The Carbon Monoxide (CO) from charcoal was only a bit higher than the Three Stone Fire (19.2g/MJ).
  4. LPG did so well! (Too bad that we are entering the end of the fossil fuel era).
  5. The forced draft pellet stove looked great, as well. (PM2.5: 30g/MJ, 2.2g/MJ CO)
  6. Black Carbon (EC) is much worse than CO2 for climate change. Many of the stoves, except the Rocket stove, successfully reduced Black Carbon. 
  7.  In this recent lab test, as in the previous MacCarthy study (2008), the Rocket stove emitted a lot of Black Carbon.
  8. R&D has shown that the Rocket stove requires successful forced draft mixing at high temperatures to decrease emissions of Black Carbon and potentially address climate. 

When the emissions factors are summed and converted to global warming potential the forced draft stoves have the potential to generate large amounts of carbon offsets. 

Happy Holidays, 2023!

That the days are shorter is easy to attest to here at Blue Mountain. Our campus is wedged between parallel rows of 100-foot tall Fir trees. Yesterday, the rare sun fell down below the celestial horizon at 2PM.  We envy the valley farmers whose day lasts until around five. On the other hand, being surrounded by the forest up here makes the air sweet and clean. But only when the wood burning heating and cooking stoves that we are developing and testing are protecting health and climate.

It can be so terrible when traditional stoves are belching smoke! I have had pneumonia three times and get nervous when my throat gets sore. Clean burning makes me a lot happier. 

HEALTH: Can biomass be burned cleanly enough to protect air quality when warming houses and cooking food? 

CLIMATE: Can the Global Warming Potential of burned biomass meet the Paris Agreements and join solar, hydro and wind as a renewable energy source? 

Sure, we do both every day.  

Celebrating life and scientific endeavor in the forest becomes pleasant and comforting when we are toasty warm, and the smoke disappears. 

Feels downright civilized.

Iterative Development of Stoves and Black Box Theory

“The concept of black boxes has been around since the early days of systems theory though some attribute the first use to the field of electrical engineering.

It is a simple concept and has a straightforward definition: we know the inputs and subsequent outputs to a system but the internal workings of the system are not visible to us.

Black boxes approaches focus on input and output rather than the details of how inputs are transformed into outputs”.

-John M. Green, The Application of Black Box Theory to System Development

Data Driven Hypothesis Generation

The results of experiments guide the subsequent experiments. For instance, adding secondary air jets into flame is shown to decrease PM2.5. Guided by the result, further investigation in a prototype under the emission hood determines the most successful application by varying parameters.

Random Experimental Design

In a Black Box (where the situation is complex and not understood) randomized approaches to experimental design can be effective and efficient.

H.R.6316 – Clean Cooking Support Act

Following the COP26 international climate conference, Senator Susan Collins (R-ME) and Senator Dick Durbin (D-IL), have introduced a bill to accelerate access to clean cooking. Here is the opening text of the section defining the activities directed in the bill. H.R.6316 has been referred to the Subcommittee on Energy:


(a) Department Of State; United States Agency For International Development.—The Secretary of State and the Administrator of the United States Agency for International Development shall work with the Clean Cooking Alliance, founded in 2010—

(1) to engage in a wide range of diplomatic activities, including with countries across the globe and with United States embassies abroad, to support activities of the Clean Cooking Alliance and the clean cookstoves and fuels sector;

(2) to continue the clean cooking initiatives supported by the Climate and Clean Air Coalition, an intergovernmental organization formed in 2012, to reduce emissions of climate pollutants;

(3) to advance programs that support the adoption of affordable cookstoves that require less fuel to meet household energy needs and release fewer pollutants, as a means to improve health, reduce environmental degradation, mitigate climate change, foster economic growth, and empower women; and

(4) to carry out other activities authorized under this Act.

(b) Department Of Energy.—The Secretary of Energy shall work with the Clean Cooking Alliance—

(1) to conduct research to spur development of low-cost, low-emission, high-efficiency cookstoves through research in areas such as combustion, heat transfer, and materials development;

(2) to conduct research to spur development of low-emission, high-efficiency energy sources;

(3) to support innovative small businesses in the United States that are developing advanced cookstoves and improved cookstove assessment devices; and

(4) to carry out other activities authorized under this Act.

The bill continues on in sections (c) through (f) to direct the National Institutes of Health, Centers for Disease Control and Prevention, Environmental Protection Agency and other federal agencies to engage in supportive activities. You can read the full text of the bill at