Biochar Goals

A few years ago, on another series of hot summer days, Aprovecho hosted a seminar on biochar. About twenty enthusiasts from all over the US joined us for a biochar summer camp, lighting a lot of stoves and scaring the safety advisors.

We needed more spark screens.

At the closing event, we created a list of three goals for biochar production:

  1. The biomass should be renewable
  2. The generated heat should be used for a purpose
  3. The total package of emissions, including buried biochar, should be carbon negative (-CO2e). By weight, smoke is ~ 700 times worse for climate than CO2. Methane is ~ 25 times worse than CO2, etc. 

Since most of the biochar production at the seminar resulted in a lot of smoke and the heat was lost into the sky, it was appreciated that R&D was needed to perfect our efforts. 

Having goals is a first step.

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

A Short History of Cookstove Durability

Tin can stove by Katska on Flickr

The Problem with Metal

It’s great to start making stoves and testing ideas with tincanium (cut up tin cans). Making new prototypes from tin cans is a quick, inexpensive way to start the design process but tin cans only last for a limited amount of time, depending on the temperature of the combustion chamber. At 1000C in fire, a tin can burns out in an hour or so!

One of the most challenging components in a cookstove is the combustion chamber, which can operate at high temperatures (often ≥600 °C) in wet and salty conditions. Wood can be salty and water vapor is produced when wood is burned.

In 2017, M.P. Brady and T.J. Theiss shocked the stove world by showing that in their tests even expensive metals could not be estimated to be long lasting. (Energy for Sustainable Development 37 (2017) 20–32, Alloy Corrosion Considerations in Low-Cost, Clean Biomass Cookstoves for the Developing World Michael P. Brady, et al.).

“Corrosion evaluation under cookstove-relevant conditions was studied by two methods: 1) lab furnace testing and 2) in-situ exposure in an operating cookstove. The lab furnace testing was conducted in air with 10 volume percent of H2O to simulate water vapor release from burning biomass, and direct deposition of salt onto the test samples to simulate the burning of highly corrosive biomass feedstocks. In particular, relatively high levels of salt species are encountered in many types of biomass and can lead to significantly accelerated alloy corrosion rates (Antunes and de Oliveira, 2013; Baxter et al., 1998; Saidur et al., 2011; Okoro et al., 2015). The in-situ cookstove testing was conducted using wood fuel that was pre-soaked in a salt water solution to yield accelerated, highly corrosive conditions.”

“Each day of testing, cookstoves were burned continuously for an average of ~6 h. The average fuel consumption rate was 570 g/h. To determine the range of temperatures that the alloy test samples would experience, a thermocouple was placed inside the chimney of each stove at the same height as the coupon fixture. Typical combustion chamber temperature profiles for the cookstoves, where test coupons were placed, are shown in Fig. 2. The average gas temperature range during steady state in-situ testing was 663 °C ± 85 °C”

“Much faster corrosion rates were observed in the 800 °C lab furnace testing where evaluation of most alloys stopped after 500 h of exposure due to excessive corrosion. Of the alloys tested to 1000 h, only the FeCrSi and pure Ni samples exhibited good corrosion resistance. The FeCrAlY and 310S alloy samples were consumed through-thickness in some crosssection locations.”

“Type 201 stainless steel, type 316 L stainless steel, and the 12 and 20Ni AFA alloys all exhibited relatively poor corrosion resistance in the in-situ cookstove testing, with metal losses in excess of −200 μm after only 500 h of exposure, consistent with the lab furnace trends. The types 310S and 446 stainless steels exhibited moderately worse corrosion resistance, with metal loss values of −190 μm and -230 μm after 1000 h. Despite exhibiting the best corrosion resistance in the lab furnace testing, the pure Ni suffered from −300 μm metal loss after only 500 h in the in-situ cookstove testing.”

What could stove companies do? Attempts were made to reduce temperatures in combustion chambers. Insulation was removed and external air was directed to cool the external surfaces of the metal.

Refractory Ceramic, A Viable Alternative

When Dr. Winiarski insulated the combustion chamber in his Rocket stoves, it became all too obvious that available metals were short lived. Unfortunately the alternative – heavy ceramic materials that were free and available – absorbed heat which lowers temperatures, resulting in reduced thermal efficiency and higher emissions. 

More than 20 years ago, the quest began for an inexpensive, refractory metal and/or a durable, low mass, abrasion resistant refractory ceramic material. In Central America, Don O’Neal and Dr. Winiarski found a locally manufactured, inexpensive, thin walled refractory tile called a baldosa that can last for about seven years in a plancha stove and is now in use in hundreds of thousands of stoves.

The wisdom of using refractory ceramic was confirmed in 2011. Metallurgy experts at a DOE Biomass Cookstoves Technical Meeting pointed out that only refractory ceramic seemed to meet the requirements of being affordable with a prolonged longevity. Unfortunately, making lightweight, abrasion resistant refractory ceramic has proven to be difficult.

Shengzhou Stove Manufacturer has for years manufactured low mass, abrasion resistant refractory ceramic combustion materials in China. Since 1407AD, potters in eastern China have used rare local clays to make and sell these combustion chambers to East Africa. In 2023, SSM sells ceramic combustion chambers in Rocket stoves globally.

Though not necessarily refractory, simple earthen ceramic stoves continue to to be the most popular models in many countries. These stoves are locally produced, inexpensive and are replaced relatively frequently as needed. These heavy bucket shaped stoves can save fuel when used with a pot skirt.

The Importance of Durability

Durability has became more important as carbon credits, which currently support most large-scale stove projects, are generated only when the stove is in use. Carbon credits are based on improvements in fuel use while emissions of CO and PM2.5 are not counted. The ‘perfect’ carbon credit stove is least cost, long lived, and as fuel efficient as possible. When ARC is asked to develop a cookstove for a carbon project we usually aim for cost under $20, over 40% thermal efficiency, with a minimum 5 year durability.

The ARC carbon credit stove is dependent on a tight fitting pot skirt (close to optimal heat transfer efficiency) coupled with as cool as possible temperatures in metal parts, resulting in improved lifespan. Shengzhou Stove Manufacturer sells millions of carbon credit stoves with their light weight, abrasion resistant combustion chambers. 

ARC tries to add mixing in the combustion chamber and a chimney whenever possible. Cooking outside/increasing the air exchange rate in houses is also effective in reducing exposure. The Jet-Flame is moving into greater use and has been field tested in Africa. Carbon revenue is moving better stoves into homes as humanitarian oriented partners like C-Quest Capital replace older stoves with better stoves. Progress has picked up in the last five years.

Predictive Testing of Cars and Wood Stoves

Photo: Car on dynamometer
Testing a car on a dynamometer

Written decades ago, the lab based tests for both biomass heating and cooking stoves were designed to achieve statistical validity by controlling variables. Because many real world variables were removed from the heating and cooking stove protocols, the results were known not to predict real world performance.

Automobiles are currently tested on a dynamometer instead of being driven around town. EPA estimates are based on dyno tests designed to reflect “typical” driving conditions and driver behavior. Even so, The EPA warns customers that actual mileage will probably be significantly different.

To predict real world performance, each car could be driven around town by enough people until a meaningful average was mathematically determined. Cook stove tests could also rely on field tests generating complicated data resulting in accurate predictions. However, doing real life testing for every manufactured car or stove has been thought of as rather cumbersome.

Another approach might be to have regional survey data inform a predictive model. The model teaches the dynamometer how to test the car. Stove use could be modeled in the same way, so lab tests get closer to predicting what actually happens when cooks use wood to make meals.

Making a model probably didn’t seem to be worth the trouble in the past. However, things have changed. The harmful emissions from cars and biomass stoves damage health and contribute to climate change. Actually knowing what a new car or stove will do when used should help to create better technologies and reduce pollution. Any kind of predictive testing seems like a great idea!

Reaching 50% Thermal Efficiency

Adjustable pot skirt,

For good thermal efficiency, be sure that as much heat as possible is being transferred to the outside of the cooking pot. The temperature of the hot gas 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 in the surface boundary layer of very slowly moving gas immediately adjacent to a wall. Increasing the velocity of the hot gas as it flows past the pot without reducing the temperature is aided by a pot skirt. Reduce the thermal resistance with appropriately sized channel gaps under and at the sides of the pot. ( see “Biomass Stoves:” Sam Baldwin).

A 6mm channel gap in a 10cm or higher pot skirt has been shown to work well with up to 6kW firepower with a 24cm or larger diameter pot. 

Reducing thermal losses from the exterior of the pot skirt with 1cm of refractory ceramic fiber insulation increases thermal efficiency by approximately 8%.

Combustion Chamber Heat Loss

Illustration from Biomass Stoves: Engineering Design, Development, and Dissemination

“Lightweight walls have the intrinsic potential for much higher performance than massive walls due to their lower thermal inertia.” –Baldwin, Biomass Stoves: Engineering Design, Development, and Dissemination, 1987

After about 80 minutes, the earthen mass wall in the illustration above gets hot enough to equal the heat loss in a single metal wall.

After about 20 minutes, the fired thin walled fired clay wall gets hot enough to equal the heat loss in a single metal wall.

After 80 minutes, the earthen high mass wall loses less heat compared to the bare metal wall resulting in better performance when used in long-term applications.

After heating up, fired clay walls and high mass earthen walls lose around 300 watts compared to 500 watts from the bare metal wall.

Insulated metal walls with 1cm insulation lose around 75 watts and food is cooked more quickly while using less fuel. The problem is that insulated metal walls get too hot and do not last very long.

For this reason, stove companies started making double walled stoves with cold air moving between the walls to increase longevity.

Thanks to Dr. Sam Baldwin for quantifying the effect of design choices!

Emission Testing: Tuning a Stove Like a Race Car

Yesterday, two fine fellows who manufacture stoves in southern Oregon visited our lab. One of the very competent guys had just installed a Corvette engine in a Jaguar, for fun. 

We quickly got on the same page when the ARC staff showed them how an emission hood (with both real time and gravimetric measurements) enables quick experiments to improve performance and achieve clean burning.

Anyone involved with racing (or fixing cars) knows how a computer helps to tune a modern car. The biomass emission hood allows folks to tune stoves like race cars.

Testing for development means that the testing of a manufactured product will have known results. 

You win the race.


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?

Clean Combustion Needs More Than Just Heat

Dr. Larry Winiarski wrote the ten rocket design principles

We have been having a lot of fun doing a modern literature search: Surfing YouTube. YouTube is often years ahead of the slower, but probably more accurate, information in peer reviewed journal articles. I suppose that many people are looking at both.

A shared misunderstanding seems to be that making the combustion zone hotter cleans up combustion. Yes, it is great to keep the temperatures around 900°C, which shortens the residence time needed to burn up the wood gas. However, just raising the temperature misses other necessary components that also move stoves closer to complete combustion. They are:

1. FUEL/AIR RATIOS: Fuel and air are needed for complete combustion.

2. MOISTURE: Biomass has to be relatively dry to burn.

3. MIXING: Turbulence needs to completely mix the fire, air and wood gas.

4. RESIDENCE TIME: Less time is required at higher temperatures to burn up the wood gas.

5. TEMPERATURE: Good – higher temperatures decrease the residence time. Bad –higher temperatures increase the rate of reactions possibly producing more wood gas than can be cleanly combusted.

6. METERING: As Dr. Winiarski wrote in his Rocket Design Principles: “Only make the amount of wood gas that can be combusted.”

Electricity: Planning for Net Zero by 2040
  • Transitioning to carbon neutral electric generation would replace a big climate problem in the U.S., since about 60% of its electricity comes from burning natural gas. 
  • The World Energy Forum forecasts that around 40% of electricity could be from wind and solar doing most of the heavy lifting by 2040, enabling a net zero global future. 
  • Today hydropower provides about 16% of the world’s electricity, generating power in all but two U.S. states. 80% to 90% of our electricity at the lab comes from the wonderful Columbia River.
  • ARC is working to clean up combustion so renewable biomass (domestic switch grass, for example) could cook food and heat homes when fossil fuels are no longer available.
  • Reading a book at night in a warm house is a wonderful thing. Somebody is playing the piano… Dinner was great.