sticks burning in rocket stove
sticks burning in rocket stove

Here are lessons learned that summarize what we consider to be the ‘state of the art’ in 2021. Please remember that, as scientists, we do not consider these hypotheses to be true. The concepts are evolving and, like all knowledge, will coalesce into a state of greater accuracy as experience, experiment, and analysis shapes understanding.

Optimize the heat transfer efficiency first. 

Then adjust the combustion efficiency of the stove.

  • The fire generates high enough temperatures for a metered amount of wood gas to combust. The stove body reduces temperatures as little as possible.
  • An appropriate amount of wood gas is made. The rate of reactions (how fast the wood is turned into burnable gas) is controlled by adjusting the primary/secondary air (TLUD) or by metering the fuel (Rocket). 
  • The stove creates molecular mixing with sufficient time for combustion to occur. At 850°C, with about one meter per second velocity, the residence time needed for close to complete combustion can be very limited.
  • Even at 900°C, with lots of mixing and residence time, the fuel must be metered at the proper rate. Increasing the rate of reactions too quickly can overwhelm the ability of the combustion chamber to combust all of the wood gas.
  • A zone of mixing of air, gases, smoke, and flame is created using natural draft or forced draft jets of primary and/or secondary air. To date, creating sufficient mixing in a Rocket stove requires forced draft jets of air but natural draft jets work well in a TLUD burning pellets.
  • It is possible that forced draft jets of air are most successful in reducing emissions in a Rocket stove when directed up from the bottom into the fire.
  • In a TLUD stove, jets of secondary seem to be most successful when aimed into the fire, usually just above the batch of fuel.
  • As a rule of thumb, the cooling effect of the primary or secondary air jets decreases thermal efficiency. Primary air tends to have less of a cooling effect than secondary air. 
  • Secondary air jets create a high pressure zone that reduces the draft, limiting the amount of velocity.
  • Secondary air jets are very successful in a cylindrical combustion chamber with a closed bottom, as in a TLUD. However, the cooling effect and lowered velocity of draft in the Rocket stove makes it more difficult to apply this technique with the open fuel door in the side of the stove.
  • Use the emissions hood to tune the stove.
  • The amount of flame, air, and wood gas entering the zone of mixing is adjusted under the emissions hood until close to optimal combustion efficiency is obtained. 
Champion Forge, circa 1920

The blacksmith’s forge is probably the most familiar technology that blows jets of primary air up into charcoal or coal, resulting in the high temperatures needed to shape or melt metals. A forge needs air at high pressure (10” water column) to do its work. A fan (usually a radial pressure blower) capable of moving air against significant resistance is required. You don’t need a big volume of air. What is needed is pressure to keep the air moving and to create molecular mixing of the woodgas and flame.

The Jet-Flame stove accessory
The Jet-Flame Stove Accessory

The SSM Jet-Flame

The Jet-Flame uses the same technology as a forge. It required a year of R&D to create a low cost, 1.5 Watt fan that delivers sufficient pressure into a Rocket stove fire. Thirty 2mm jets of air at 0.4 to 2 inches of water pressure blow up into the charcoal under the burning sticks of wood. When the pressure rises and the vertical jets of air penetrate further into the charcoal and fire, the following effects are seen:

• Temperatures rise in the combustion chamber, resulting in higher thermal efficiency as hotter gases flow past the pot. The higher temperatures also result in lower PM2.5 and CO. However, proper metering of the fuel, mixing caused by turbulence, and sufficient residence time are as important for decreased emissions of PM2.5 and CO. With higher temperatures, the related measures of firepower and CO2 also rise, while the fuel/air ratio decreases – as the fire increases, more oxygen is consumed. Increasing the pressure also increases firepower even when a constant fuel load (usually two 4cm by 4.5cm sticks) is being burned. Larger sticks have a lower surface area to volume ratio and make less smoke. When the outer surface of the sticks are covered with charcoal, emissions decrease as well.

• In a Rocket stove that has a large fuel opening, natural draft pulls room air into the combustion chamber. With forced draft adding more air, the average Lambda in a Rocket stove tends to stay above two times stoichiometric. The jets of air blowing into the charcoal result in higher temperatures as pressure increases resulting in temperatures over 1,000°C even at 2 to 5 Lambda. Secondary air jets blowing into the fire, on the other hand, have a cooling effect. In a Rocket stove with a Jet-Flame, the varying fuel/air ratios are not related to the emission rates of PM2.5 and CO.

• Higher temperatures caused by greater pressure also have detrimental effects. The percentage of black carbon is higher when temperatures/firepower/CO2 become elevated – hot yellow flames cause the formation of black carbon. The lifespan of affordable refractory metals is greatly decreased by very high temperatures such as 1,000°C. A durable refractory ceramic material is better suited to higher operating temperatures. Replacing metal combustion chambers with low mass, refractory ceramic was recommended by the 2011 DOE biomass stove conference.

• Lowering the firepower by adjusting the pressure in the air jets assists the metering of fuel to achieve a 3 to 1 turn down ratio. When the pressure is too high, the fire can be blown out when the charcoal has disappeared.  A layer of charcoal under the fire (or charcoal coating the sticks) helps to maintain the fire and lower emissions as charcoal emits much less PM2.5 compared to wood. The levels of CO tend to rise when the flame above the sticks decreases. The amount of flame above the sticks seems to be related to lower emissions of CO and PM2.5. The amount of flame above the burning sticks, the metering of the fuel, and the mixing of the wood gas are not as easily quantified as temperature and residence time but are as important for more complete combustion.

pdf available at:

I hope that you are living in a smoke free environment! There’s a forest fire about 40 miles east of the lab that floods our valley with smoke when the wind slows down. I just looked up and noticed that it was getting hard to see Blue Mountain, a sure sign that the northwesterly wind wasn’t pushing hard enough to clear the skies. It reminded me of living on the Coromandel Coast in India where a blue sky was unlikely even at the beach.

Burning up smoke is not all that difficult to do: just thoroughly mix the smoke into the flame. But that doesn’t happen in a forest fire (or in a three stone fire). The smoke and flame go in different directions. The industrial reduction of PM2.5 often depends on both improved combustion efficiency and the post-combustion filtration/scrubbing of emissions. When I can’t see Blue Mountain anymore, I switch on a box fan that has a 20” by 20” by 1” furnace filter taped onto the inlet side of the fan. The fan pulls the dirty air in my office through the filter and the PM2.5 is removed from the air that I’m breathing.

Simple cooking enclosure

We wrote a paper describing how the same fan and filter reduced PM2.5 when installed in a hood over the stove. We used a washable filter and hoped that the combination of a clean burning stove with post combustion filtration of smoke might help to protect inside and outside air quality. Check out the paper: Still, D. K., Bentson, S., Murray, N., Andres, J., Yue, Z., & MacCarty, N. A. (2018). Laboratory experiments regarding the use of filtration and retained heat to reduce particulate matter emissions from biomass cooking. Energy for Sustainable Development, 42, 129–135.

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. (

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

Ryan Thompson and Sam Bentson spent a year exploring charcoal burning

Ken Newcombe and C-Quest Capital have seen firsthand how charcoal production has wiped out forests in Africa. Making charcoal is a very energy inefficient process (Aprovecho Institute, 1984a). Burning off the volatile compounds in wood consumes and wastes between 50% and 80% of the energy!  Ken recently observed that saving charcoal was not easy, and  that field tests indicated that traditional stoves were as fuel efficient as more modern stoves. So I asked Ken if he had read Sam’s paper* on how to save fuel in charcoal stoves, and Sam sent it to him.

Busy people are out in the world accomplishing things. There’s a ton of information on the Internet and lots of it is conflicting. Publishing information in journals and books sometimes seems to be a fairly ineffective way to reach actively engaged stovers. That’s why we publish this weekly newsletter, trying to disseminate data driven information. Maybe you can pass it to on to your colleagues?

After testing many charcoal stoves, Sam concluded that:

“Using the minimum load to complete the WBT [water boiling test] results in similar performance between most charcoal stoves, while filling the combustion chamber with fuel results in large differences on the same measures. Based on these laboratory results, limiting the size of the combustion chamber so as to prevent excess fuel loading may be an effective technique for decreasing fuel consumption while cooking.” *

The loaded charcoal is used up, so loading only the amount needed for the cooking process is very important for saving fuel.  At the same time, Sam and Ryan Thompson spent a year exploring charcoal burning and came up with an “all Tier 4” charcoal stove that was higher scoring than the other optimized wood burning stoves that ARC made for the US DOE.

CAD drawings and a chapter on charcoal describe what Sam and Ryan did and how to build the stove they invented. (Clean Burning Biomass Cookstoves, 2021,

*(Energy for Sustainable Development, 2013, Bentson, Still, Thompson, Grabow)

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.

Smoky kitchen with no chimney vs. kitchen with chimney and clear air
Reducing/removing smoke from the kitchen improves health.
Adding a chimney is one recommended solution.

In 2014, the World Health Organization (WHO) published intermediate and final indoor air guidelines for vented and unvented biomass cooking stoves. Their strong recommendation directed governments and implementers to advocate technologies and fuels that are proven to protect health. The WHO advises implementers that to protect health the cook stove intervention should not exceed the following air pollutant emission rates in actual use:

WHO Intermediate Emission Rate Targets:

Unvented stove Vented stove
PM 2.5:  1.75 mg/minPM 2.5 7.15 mg/min
CO:   0.35 g/minCO:  1.45 g/min

Many newer biomass cookstoves with chimneys meet the WHO Targets of 7mg/minute for PM2.5 and 1.45 g/minute for CO when tested in the laboratory. Adding chimneys to cook stoves makes them more costly, but ARC designers are relieved to have a “line drawn in the sand.” As seen in Sam and Shikhar’s video last week, experiments in the ARC Test Kitchen showed that a natural draft hood can also be a big help in protecting health.

Protecting outdoor air quality is equally important. Dr. Nordica MacCarty (Oregon State University) and Ken Newcombe (C-Quest Capital) will be investigating effects on indoor and outdoor air when the combination of a natural draft earthen hood and a CQC earthen stove with Jet-Flame is used in houses in Malawi. Ken and CQC are invested in protecting health and have funded the study.

Industrial technologies routinely achieve strict standards for combustion efficiency and further reduce emissions with post combustion techniques. Introducing these well-known applications into cook stoves seems a logical progression. Clean up combustion and, at the same time, clean up the indoor and outdoor air. We have been very successful doing this in the US and Europe, and China is now on the same path.

The WHO vented stove Emission Rate Targets are based on 75% of the smoke and gases being removed up the chimney and out of the house. In their review of field studies, an average of 25% of the smoke and gas remained in the kitchen. Almost none of the residential biomass heating stoves in the United States meet the WHO Targets for PM 2.5 but the chimney transports the smoke outside where it is diluted by clean outdoor air to safe levels of concentration.

Meeting emission targets is a necessary and ever present goal. At the same time, wood burning stoves can be improved in many other ways. Improving the smoky mud stove to use less fuel is not a complete cure but is very helpful, benefitting the user who either pays for the fuel or has to collect it. The functional chimney makes a tremendous difference by sending smoke and gas out of the kitchen, making it a more pleasant and healthy environment. Making the high mass stove safer results in fewer burns. The list of improvements goes on and on – making the stove better at cooking local foods, increasing the number of air exchanges per hour in the kitchen, moving the kitchen outdoors, etc.

In the real world, positive changes are hard to accomplish but are always great.

The old saying, “if you can’t handle the heat, get out of the kitchen,” is not lost on us when we are working in the test kitchen. It takes us a while to get the vertical and horizontal grid of Climate Solution Consulting’s HAPEx particulate matter monitors started and positioned. Then ARC’s PEMS-PC partial-capture based emissions sampler has to collect a zero point of the gasses in the atmosphere. Don’t forget the temperature sensors (where is that data?) and the wood. This is after reviewing yesterday’s work, discussing a plan for the day, and watering the garden. So, by the time the young scientist rolls into the test kitchen in the Oregon summer, currently home of America’s largest wildfire, it’s about 100°F and rising. But science must go on.

ARC’s four year old test kitchen is currently being used to test a natural draft hood of our design. Our experiment allows us to operate the fire without being exposed to the emissions from the fire. We used to use a vacuum cleaner as a positive pressure ventilator, but now we sit outside of the room and feed the fire through a glove box.  After seeing that the hood was effective enough to reduce the concentration of PM2.5 in the test kitchen to below 35 ug (averaged over 24 hr), we turned the hood around and made a video for you of us doing the water boiling test while enjoying a smoke free kitchen.

Enjoy the video (and know that those loud pumps and fans go with the bit about it being hot in the kitchen).

Please send your photos and stories of natural draft hoods! We don’t want to lose this beautiful technology.

-Sam Bentsen, Aprovecho Research Center General Manager

General Manager Sam Bentsen is happy about some LEMS test results.

Why test a stove?

Most of the time, our lab uses testing for product development. If we did not test a stove prototype we would be guessing whether it met expectations. Testing in the lab gets us ready for the field testing of prototypes. Then, customers take the prototype and make it work. The factory and distributors frequently ask for design changes as the product gets closer to shelves. From initial design to market usually takes about a year of testing/iteration/development.

Recently, a factory in Africa asked us to design a $10 wholesale, pellet burning forced draft TLUD prototype that achieves Tier 4 for thermal efficiency, CO, and PM2.5. The stove has to last ten years with scheduled maintenance and require as low wattage as possible.

We had tested the Oorja several times during surveys of commercially available stoves. ARC published the results in books and papers trying to inform the public how stoves compare on various measures of performance. We were trying to make available a Consumer’s Report on stoves (see list below). We knew that the Oorja stove met the Tier rankings and that it used a high mass, low cost, durable combustion chamber. We tried a castable refractory in the lab and we also found several manufacturers that make inexpensive ceramic combustion chambers.

The factory wants a high-powered stove to meet the needs of cooks in their region. Protecting health is also a major concern. Delivering a design that can be made for $10 is also very important. All the interconnected partners in the business plan have to make a healthy profit to bring a “Tier 4” technology to the public. The designer is only the first step in a web of stakeholders.

After all of the necessary parts were combined in the lab, testing with the LEMS (Laboratory Emissions Monitoring System) started. Many iterations were needed to get close to optimal performance. Adjusting the primary and secondary air at high power took experimentation. In several weeks of daily testing, the prototype was repeatedly achieving best scores. A CAD drawing was made and the design was sent to the factory. The factory is making their version of the stove, we will test it here and make adjustments if needed, and then field testing of the prototypes will begin, including home trails and test sales in stores.

Does it sound like a lot of work? The payback to know, rather than guess, that the product can be successfully sold. It’s great to make data based decisions, and a careful approach attracts investment. Failing miserably with products we loved (and lost money on) has made ARC consider external input carefully, especially from field testing.

Cook Stove Performance Reports:

Testing the Oorja Stove under the LEMS hood.

There are many forced draft TLUDs that are quite similar to Dr. Tom Reed’s 2001 version, the WoodGas stove. The Oorja stove can be about as clean burning but has several obvious differences: a high mass refractory ceramic combustion chamber, much bigger secondary air holes, and high firepower. Like other forced draft TLUDs the turn down ratio, created by limiting the combustion air, is narrow. The Mimi-Moto had to turn to a smaller combustion chamber for simmering to achieve Tier 4 for low power metrics. It’s a problem for Forced Draft TLUDS.

I have been a fan of the Oorja stove since 1999. In 2003, when I was living in India, hundreds of thousands of British Petroleum Oorja stoves were in use, burning pellets made from field residue. It’s been fascinating recently to read Dr. H. S. Mukunda’s 2010 paper describing the development of the Oorja.* When his team tested the lifespan of a metal combustion chamber it was only about 12 months and cast iron was expected to last about twice as long. The team developed a ceramic combustion chamber to create a better, longer lasting stove. I’m testing an Oorja stove with ceramic combustion chamber that is 20 years old!

Mr. Prasad Kokil from the San Jay Group writes: “We had developed this Oorja stove for BP in our company. We developed the ceramic refractory for the Oorja at that time. Our Elegant Model (now for sale) has a ceramic refractory combustion and is a forced draft TLUD”.

Large secondary air holes near the top of the combustion chamber.

Dr. Mukunda and team decided that at a burn rate of 12 grams per minute the primary air should be 18 g/min, and the secondary air was set at 54 g/min. The 18 secondary air holes, just below the top of the combustion chamber, are larger than in other FD-TLUDs at 6.5 mm in diameter creating a velocity of 1.8 meters per second. Using larger holes means that a low wattage computer fan supplies air jets with sufficient volume and velocity. Emission measurements made by the development team, carried out at fuel consumption rates of 12 and 9 g/min, showed that the CO emissions were 1 and 1.3 g/MJ whereas particulate emissions were 10 and 6 mg/MJ for the high and low power levels. When burning the made charcoal, CO rose but did not exceed the Indian standards.  

The Oorja stove has been tested at various times in our lab with impressive results. Learning from Dr. Mukunda and team how to make stoves that are super clean burning and last a really long time is an important development. Thanks for such a great stove!

* Gasifier stoves: Science, technology and field outreach H. S. Mukunda, et al., CURRENT SCIENCE, VOL. 98, NO. 5, 10 MARCH 2010