High temperatures in the combustion chamber seem to have both positive and negative effects on emission rates of biomass. Higher temperatures lower the residence time needed for more complete combustion. At the same time, especially with dry wood, the rate of reactions (how much wood gas is being made per unit of time) is increased. If wood gas is made too quickly, some of it can escape unburned. Our experience has been that in a hotter combustion chamber the fuel must be metered into the fire more slowly to lower emissions. In Rocket and TLUD stoves, the rate of reactions must be controlled to eliminate smoke.

Experiments have shown that elevated temperatures shorten the combustion time for CO and PM 2.5. At 900°C the combustion time required for complete combustion is less than half the time needed at 700°C for biomass particles (Li, 2016). At 900°C, a residence period of 0.5 seconds resulted in close to complete combustion of well mixed CO and PM 2.5 (Grieco and Baldi, 2011; Lu et al., 2008; Yang et al., 2008).

Boman (2005) reports that temperatures above 850°C in a 5kW combustion zone combined with air rich and well mixed conditions for 0.5 seconds resulted in an almost complete depletion of particulate matter. The use of insulation in Rocket stoves can create a combustion zone with temperatures above 1,000°C. Forced draft TLUDs can generate similar temperatures in the secondary air mixing zone above the fuel bed.  Interestingly, when temperatures are around 850°C the near complete combustion of well-mixed carbon monoxide and particulate matter seems to require short residence times. Both forced draft Rocket and TLUD stoves can minimize the emissions of products of incomplete combustion even though the residence time is very limited.

Recently, we ran a series of fifteen experiments trying to optimize performance in a low mass Rocket stove with Jet-Flame. Since we were testing with dry wood we had to be careful not to over insulate the combustion chamber. Insulation made the whole length of the stick catch on fire increasing the rate of reactions and firepower.  When more than 8cm of the stick was burning more mixing was needed to achieve close to complete combustion. When only the tips of the sticks were on fire the metering of woodgas into the fire was slower and less mixing was required.

As seen in the following graph, the emissions of CO were again shown to be reduced at higher temperatures. As a rule of thumb when designing a stove, we try to create temperatures above 700°C about 6cm above the fire, at a minimum.

The Gates funded Global Health Labs and ARC/SSM invented the Jet-Flame

Global Health Labs, Shengzhou Stove Manufacturer and Aprovecho teams show off the Jet-Flame
Global Health Labs, Shengzhou Stove Manufacturer and Aprovecho teams show off the Jet-Flame

Shengzhou Stove Manufacturer manufactures the Jet-Flame (jet- flame.com) that is being field tested in over 30 locations. Our lab helped to create this accessory that is designed to reduce emissions while increasing thermal efficiency and reducing time to boil. 30 pre-heated primary air jets shoot up into the fire resulting in increased molecular mixing and elevated temperatures. Smoke is reduced by about 90% compared to an open fire.

A forced draft stove can be very clean burning, but start up may create a lot of PM2.5. This is because the cold combustion chamber can allow a higher percentage of the smoke and Carbon Monoxide to escape unburnt. David Evitt, COO of ASAT (the for-profit arm of ARC), invented a method for lighting the Jet-Flame that can be a lot cleaner.

  • Wet 30 grams of left over charcoal with 10 grams of alcohol.
  • Place the small pile of charcoal on top of the holes in the Jet-Flame.
  • Light the charcoal.
  • Turn on the Jet-Flame.
  • Push the tips of the sticks of wood against the pile of burning charcoal.
  • Keep on pushing the sticks into the fire as the tips are consumed.

Here’s a video showing how we light fires in the lab:

Uganda 2-pot rocket style stove

The Uganda 2-pot stove that is described on page 26 in the EPA publication “Test Results of Cook Stove Performance” is a natural draft stove that also uses much less fuel to cook and protects health. The document can be found at: https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P100EKU6.TXT.  Has it been a while since you looked at this book, a comparison of 18 cook stoves? The 2011 book started our surveys with the emissions hood and Test Kitchen, trying to quantify comparisons of fuel use and emissions from available stoves.

Dr. Nordica MacCarty’s paper with comparisons of 50 stoves is a much more complete survey. See: “Fuel use and emissions performance of fifty cooking stoves in the laboratory and related benchmarks of performance” (MacCarty, et al, 2010) https://www.sciencedirect.com/science/article/pii/S0973082610000311

The Uganda 2-pot stove has a Rocket combustion chamber. The hot gases made by the fire pass through narrow, insulated channels around the first pot, which is sunk into the stove. The gases then flow through an insulated tunnel and are forced into narrow channels around the second pot before exiting the chimney. The pots fit tightly into holes in the sheet metal top, preventing smoke from escaping into the kitchen. This stove is fast to boil and, because of the sunken pots, uses less wood than most stoves with chimneys. 

When we build and test stoves we often reflect on Larry Winiarski’s advice that helped to improve the Ugandan stove. Larry advised us that in a 2-pot horizontal stove, channel gaps around the pots that are 0.75 constant cross sectional area are a good compromise between maintaining needed draft and increasing heat transfer efficiency. The cross sectional area of the Ugandan fuel entrance in the Rocket combustion chamber was about 16 square inches so we made the channel gaps all the way to the chimney at 0.75 times 16 square inches. We use Larry’s “rule of thumb” and tests remind us how well Larry knew stoves. He had a good touch.

Constant cross sectional area through a cookstove.


A 2014 survey of biomass stoves for DOE showed that tight pot skirts are great!

We had a couple of days between jobs at the lab and decided to see if a simple Rocket stove manufactured in India, patterned after the BURN stove, could get better thermal efficiency. Low grade stainless steels, like 304, can’t withstand the hotter combustion chamber temperatures generated when insulated, so in the BURN stove room air is used to keep the steel cool enough to increase durability.

One of the key properties of any stainless steel alloy is its resistance to oxidation. High temperatures can compromise the oxidation resistance of steel alloys, leading them to become rusted and weakening their structural integrity.

As stated by AZO Materials, grade 304 stainless steel possesses “good oxidation resistance in intermittent service to 870°C and in continuous service to 925°C.” However, they warn that “continuous use of 304 in the 425-860°C range is not recommended if subsequent aqueous corrosion resistance is important.” In other words, you can expose grade 304 alloy steel to temperatures of up to 870°C for short periods of time without ill effect, and for extended periods of time in temperatures of up to 925°C. However, this can compromise the corrosion resistance of the metal, making it more susceptible to damage from exposure to moisture. (https://www.marlinwire.com/blog/what-is-the-temperature-range-for-304-stainless-steel-vs-316-vs-330)

When the low mass, uninsulated BURN Rocket stove has (1) 6mm high pot supports, (2) a pot skirt that creates a 6mm channel gap around a family sized pot, and (3) a fire that creates hot, tall flames that transport 800°C to 1,000°C gases to the pot, the thermal efficiency has been measured at around 52%. 

We lowered the pot supports in the simple Indian Rocket stove to (1) 6mm high and used a (2) 12cm high, 6mm channel gap pot skirt around a 25cm in diameter steel pot filled with 5 liters of water. Thinking that the simple Indian Rocket stove could use a 1,200°C thin walled refractory ceramic combustion chamber, (less than $1 from Shengzhou Stove Manufacturer), we (3) surrounded the combustion chamber with ceramic fiber insulation. (4) The fire was made from tiny sticks. Tiny sticks make hot, tall, dirty flames and use up the least amount of wood while making really hot gases. When burning tiny sticks, gas temperatures under the pot can be over 1,000°C. The 1,000°C gases heat water quickly and efficiently when 6mm channel gaps are used below and on the sides of the pot.

With these changes, the simple Indian Rocket stove scored an average of 56% thermal efficiency (3 tests to boil). 

If (1) 6mm pot supports, (2) 6mm pot skirts, (3) insulation, and (4) tiny sticks making 1,000°C gases had been used in the 2014 DOE stove survey the average scores would have been a bit higher. One lesson is that channel gaps and types of fires can have a big effect on heat transfer efficiency.

Go for those 1,000°C gases flowing right next to surfaces for high thermal efficiency. 

Add metering and mixing to 1,000°C gases with sufficient residence time and combustion efficiency is also improved.

Check out the heat transfer and combustion chapters in “Clean Burning Biomass Cookstoves, 2021” at www.aprovecho.org

Dr. Larry Winiarski
Dr. Larry Winiarski
Dr. Larry Winiarski, 1940-2021

Dr. Larry Winiarski, the Technical Director of Aprovecho Research Center (ARC), died this past week at the age of 81. In the 1980’s and 90’s, Dr. Sam Baldwin defined how to improve heat transfer efficiency in biomass cook stoves (pot skirts, etc.), Dr. Tom Reed created the TLUD, and Dr. Winiarski invented the Rocket stove. The saying “We stand on the shoulders of giants” certainly applies to the stove community.

Larry led teams from ARC around the world starting in Central America, where the plancha stove was evolved, after he found that a floor tile called a baldosa made a long lasting and relatively low mass combustion chamber that was surrounded by wood ash, a great natural source of refractory insulation. Larry discovered that Rocket type stoves, like plancha stoves, can be described by ten design principles and that these simple engineering principles could be taught to indigenous people, mostly women, who were the experts in using the stoves. My memories of Dr. Winiarski, who was born in Nicaragua, are often about him having a wonderful time speaking Spanish as stoves were constructed and flavorful food prepared.

It is not an exaggeration to say that Larry had a heart of gold. He picked up sick kids and walked from the city dump in Managua to a distant hospital. He slept on cement floors for months at a time in Haiti. Larry lived as others lived in Africa for years and because of his character was loved and respected in villages worldwide. His Rocket stove found a place in people’s homes in the same way that Larry was cared for, accepted, and loved by strangers. Larry is missed by thousands of friends and he was blessed with a life well lived.

There will be a Celebration of Larry’s Life on Saturday, August 28, 2021 at 1:30 PM, at Colgan’s Island, 79099 Hwy 99 N in Cottage Grove, Oregon.

Fred & Lise Colgan, founders of InStove
Fred & Lise Colgan, from InStove

Fred and Lise Colgan created InStove, manufacturing and distributing institutional stoves initially designed by Dr. Larry Winiarski. They developed and sold large Rocket stoves that cooked food, sterilized medical equipment, and pasteurized water.

The autoclave, sold by Wisconsin Aluminum Foundry, works like a big pressure cooker and sterilizes quickly using steam and pressure. The unit fits into a Rocket stove that delivers the heat using less wood compared to traditional stoves. A chimney removes smoke from the room. The system can sterilize about 7 gallons of surgical instruments, dressings, and other medical supplies at a time, making them safe for either reuse or hygienic disposal.

These larger Rocket stoves combine the same strategies that are used in smaller versions. A pot skirt cylinder surrounds the sterilizer creating a narrow channel gap that is especially effective in transferring heat, in part because the pot is larger. Big pots have more surface area so increased percentages of heat pass into the water. When a chimney is attached to the stove, the hot gases are forced to flow down another channel on the outside of the pot skirt. In this way, adding a chimney to the stove does not diminish the fuel efficiency. A lot of the heat has already scraped against the pot and been absorbed before it exits out of the chimney. The light weight bricks used in a larger Rocket stove combustion chamber can be thicker and larger than bricks used in a smaller stove. The Institutional Stove described in the Institutional Rocket Stove pdf on the Publications page can handle pots from 50 to 300 liters. The downloadable Excel worksheet Institutional Stove Gap Calculator can help you determine the measurements of an institutional stove designed to fit the large pot you have available for use.

Photo from the BURN Newsletter, May 2021
Photo from the BURN Newsletter, May 2021
Photo from the BURN Newsletter, May 2021

In a recent BURN newsletter it was announced that the natural draft Kuniokoa stove with a new pot skirt achieved 51.3 % thermal efficiency. That’s Tier 5, the highest score on the voluntary tiers of performance. Achieving great thermal efficiency involves improving heat transfer efficiency which is summarized in the acronym TARP-V: Increase Temperature, Area, and Radiation, use narrow channel gaps to achieve Proximity, and increase the Velocity of the gases flowing past the pot without decreasing the temperature.

Making a clean burning fire does not help very much to increase thermal efficiency. Even 97% combustion efficiency is very smoky.

How can your stove get around 50% thermal efficiency?

  1. The BURN stove is very light weight, weighing in at around 3 kilos. Thermal mass in the stove body absorbs heat from the fire lowering the temperature of the gases trying to heat the water in the pot. MAKE THE GASES AS HOT AS POSSIBLE!  Hotter gases in narrow channels flowing past the bottom and sides of the pot thin the boundary layer of still air next to the pot and result in better heat transfer efficiency. The insulation in the BURN stove is 15mm of trapped air – a cylinder surrounds the riser in the Rocket combustion chamber.
  2. The channel gaps on the bottom and sides of the pot can be 6mm. 10cm or higher pot skirts are better. It’s great if the pot skirt is as high as the water level in the pot.
  3. Small, kiln dried sticks make a lot of flame (and smoke). Small sticks (we used 1cm by 2cm in a recent test) create hotter fires and gases, using less fuel compared to burning larger sticks. The hotter gas temperatures get a higher percentage of the heat into the pot.
  4. A big pot has more surface area and can be a better heat exchanger. Dr. H. S. Makunda found that larger pots (32cm in diameter) could score in the 50% range, while smaller pots (25cm) tended to get around 40% thermal efficiency. (H. S. Mukunda, CURRENT SCIENCE, VOL. 98, NO. 5, 10 MARCH 2010). 
  5. Don’t make a big fire. A moderate fire (3 to 5Kw) is better matched to family sized pots. (Prasad, Some studies on open fires, shielded fires and heavy stove, Eindhoven, 1981
  6. A hot start test usually adds something like 5% to the thermal efficiency. A cold start test transfers more of the heat from the fire into the stove body.

We built a Rocket stove that combined these characteristics. The stove top had 6mm high pot supports and the 6mm channel gap pot skirt was 10cm high. The pot had a diameter of 30cm. We used very light weight ceramic fiber insulation around the combustion chamber. The stove weighed 2.9 kilos and was 24cm high and 32cm wide. We tested it by burning five kiln dried 1cm by 2cm sticks in a hot, small fire that started quickly. The Rocket stove smoked like crazy at a firepower of around 4.5Kw, but the thermal efficiency from one high power, hot start test was 52.7%.

This week we will see what happens when we use the same Rocket stove/big pot with a Jet-Flame that should increase the Temperature of the gases and their Velocity.

In our newsletter “Making It Real,” we described how feedback from the field in Rwanda suggested that the Jet-Flame’s power cord would last longer if the whole device was inserted from the side of the combustion chamber. (It was originally designed to go through the door, with the sticks placed on top.) So of course we ran some tests, and discovered more benefits.

Is the Jet-Flame, when inserted into the combustion chamber from the side of the CQC stove, as effective in reducing emissions as when it enters through the fuel door?  

Yes, performance seems to have even improved a bit. After testing the Jet-Flame with side entry, it seems that it’s better to get the hot metal out from under the parts of the fuel that you don’t want to heat up. To burn cleanly, natural draft Rockets like to burn something like 8cm of the end of the sticks. Instead of laying the entire length of the sticks on the heated metal of the Jet-Flame, the side entry only exposes a limited amount of the sticks to high temperatures.

As seen in the photo, the sticks are now supported by a white homemade high mass brick and only the tips are exposed to Jet-Flame heat well inside the stove. It’s nice how a suggested change from Jean Marie Kayonga in Rwanda ends up having some unexpected benefit, not just better protecting the cord. Thanks again, Jean Marie! www.Jet-Flame.com

The time to boil, thermal efficiency, temperature in the combustion chamber, CO, and PM were improved with side entry while firepower rose. Excess air fell from 3.38 times stoichiometric to 2.57. I liked operating the stove because the sticks seemed to burn more at their tips as Dr. Winiarski described in the Rocket Design Principles. See: http://bioenergylists.org/stovesdoc/Still/Rocket%20Stove/Principles.html

CQC stove with swinging door
CQC stove with swinging door
A door was added to the CQC stove to block some of the air entering the combustion chamber.

Last week we shared some thoughts about the importance of gathering detailed data and making direct observations when testing stoves. Here’s a question we recently considered.

Did a partial cover over the sticks entering the combustion chamber help reduce emissions in the CQC stove with Jet-Flame?

No, not in this preliminary test series, although lots of interesting things happened! Placing a swinging door over the fuel opening into the combustion chamber has often occurred to Rocket stove designers and it appears now and then in stoves. Dr. Winiarski liked the swinging door.  The thought is to get the excess air down by reducing the amount of air coming into the combustion chamber through the door. With sufficient but less excess air, the temperatures needed for cleaner combustion should rise.

Excess air did go down when a metal cover was near to touching the tops of the sticks in our experiments. It fell from an average of 2.91 times stoichiometric to 2.54. The average temperature in the combustion chamber did rise as a result, from 557°C to 596°C. The partial cover was doing its job. Firepower went up (4359 watts up from 4012 watts) and that might have helped with heat transfer efficiency. However, in this particular case, the thermal efficiency was unchanged (36% covered and 37% uncovered).

The positive changes in excess air and temperature also did not affect the emissions. PM2.5 (56mg/MJ-d covered and 55mg/MJ-d uncovered) and CO (2.63g/MJ-d covered and 2.72 g/MJ-d uncovered) were not changed enough to show a difference. Of course, using a Jet-Flame that is introducing lots of changes into the combustion chamber, including more excess air, probably makes this an unusual set of circumstances.

The swinging door may be great in a natural draft Rocket stove and as usual, Dr. Winiarski was right. It didn’t seem to be needed in this scenario. That’s OK with me, because the cover obscured the visual clues that help to tend a fire, and make tending more fun. I felt like I was flying a plane through the fog and was glad to land.

The high mass CQC stove with Jet-Flame inserted from the side.
The Jet-Flame in the CQC high mass brick Rocket stove

I ask for help when moving the CQC stove. We built it on a piece of plywood and two folks can, with care, move it around the lab but it is heavy. The sand/clay/cement bricks are dense at 1.4 grams per cubic centimeter after being baked many times in the stove. Dr. Winiarski advised that, when possible, Rocket stoves should float in water at less than 1 gram per cubic centimeter.

For a long time, people have added sawdust and other lightweight materials into earthen mixtures to try to lighten up stoves. I ended up at Shengzhou Stove Manufacturer (SSM) in China because for hundreds of years ceramicists had manufactured (and sold in Africa since 1407) durable earthen stoves that weighed around 0.7 grams per cubic centimeter. Their amazing clay floats when dug out of the ground! It is full of diatomaceous earth. The Shens own a 100 year supply of clay in two mines next to the factory.

Why go to all of this trouble to lighten stoves?

The heat from the fire is diverted into the mass of the stove body and less heat is available to cook food. It is harder to start a hot, intense fire in a high mass combustion chamber. In a natural draft stove, this can be disadvantageous. The open fire has other problems but, out of the wind, the hot gases from the flames directly contact the pot and it’s common for open fires to have higher thermal efficiencies compared to high mass stoves, including Rocket stoves. Lightening the bricks helps to address this difficulty. Heat is still diverted into the stove body, but less. Well insulated, mostly metal, Rocket stoves successfully avoid most of these losses.

Indigenous cooks, experts at using fire, often use grasses and twigs to start a hot, fast fire in a high mass stove. You need to pour the BTUs into the stove to quickly prepare food. Speed to cook is almost always the first priority when talking to cooks around the world. When the SSM Jet-Flame is added to the high mass stove, the mini blast furnace immediately starts a hot, over 1,000°C fire that delivers relatively hot gases into the channel gap around the pot created by the pot skirt. (The CQC skirt creates a 5mm channel gap that is 7cm high.) 

The Jet-Flame creates a surprising result

The thermal efficiency in the first CQC/Jet-Flame test (see below) was 33%. The 5 liters of water boiled in 12.5 minutes. After the first 12.5 minutes of heating, the over 1,000°C fire started to heat up the mass and the water boiled more quickly in 10.2 minutes at 38% thermal efficiency. Three more short, but intense, heating phases resulted in the thermal efficiency incrementally rising to 41%, 42%, and 45%. The progressively hotter gases scraping against the sides and bottom of the pot in the small channel gap were more and more successful at transferring heat through the metal walls of the pot into the water.

When thermal efficiencies are in the 40% to 45% range, the performance of the high mass stove is similar to low mass, insulated Rocket stoves. This similarity was completely unexpected at ARC.

Results of five tests of the CQC Stove with Jet-Flame.