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 

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.

It can be challenging to find the time to read a book so here’s a ten-minute video introduction to “Clean Burning Biomass Cookstoves” (2020), our recent updating of the 2015 edition. Download the book for free on the publications page. We spent a significant part of last year rewriting most of the book trying to put in one place pretty much everything that ARC has learned in the last five years. The plan is to do the same in 2025.

The summary includes:

  • The energy ladder and renewable biomass
  • Fire is investigated in the lab
  • Stoves are designed by cooks in the field
  • Factories tell us how to help them
  • Chimneys and air exchange rates protect health
  • A new outside air model helps to predict PM2.5
  • Increasing heat transfer efficiency! No problem
  • Residence time is shorter than previously imagined
  • Metering and mixing get us most of the way to cleaner combustion
  • There are great new stoves!
  • Let’s move faster

Aprovecho Research Center is pleased to announce that it has just been awarded a $50,000 grant from The Osprey Foundation in support of expanding research into the connection between biomass combustion and climate change.

To date, a handful of lab and field studies have resulted in relatively little information on how stove/fuel interventions could impact emissions. The Gates funded GH Labs has partnered with us in this project because information is vitally needed to make sure that stove interventions are most productive.

Research has identified renewable biomass as carbon neutral but only when burned without making climate forcing emissions.  Aprovecho manufactures and sells the Laboratory Emissions Monitoring System (LEMS) as a tool to characterize cook stove performance. The LEMS enables ARC to develop stoves addressing climate, health, and effectiveness. It has become the centerpiece of more than 60 cookstove laboratories worldwide.

The LEMS measures thermal efficiency and the emissions rates of PM2.5, CO, CO2, and Black Carbon. Minimizing those emissions is important for addressing climate change and protecting human health. The effectiveness of the stoves is assured when cooks are deeply involved in designing them.

Non-methane hydrocarbons (NMHC) and methane contribute to a significant fraction of the global warming potential, especially from charcoal burning stoves, but to date have not been measurable with the LEMS.

The Osprey Foundation sponsored project goals are:

  • Adding the measurement of methane and NMHC to the LEMS.
  • Surveying the global warming potential of wood burning stoves and charcoal stoves and creating market driven designs that address climate/health/effectiveness.
  • Widely distributing open source information (including CAD drawings) describing how to minimize climate forcers in biomass stoves.

At the Leaders Summit on Climate hosted by President Biden, the U.S. government pledged to help countries achieve their climate ambitions through expanding access to clean cooking. We feel very lucky to be investigating how to manufacture improved biomass stoves that cook well, are affordable, and protect personal and planetary health. Thank you, Osprey Foundation!

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.

Prices of carbon credits used by companies to offset their emissions are currently low, due to an excess of supply built up over several years.

According to recent research at the University of London, without the excess supply, prices would be around $15/tCO2e higher, compared to $3-5/tCO2e today. (tCO2e means tonnes of carbon dioxide equivalent, “carbon dioxide equivalent” being a standard unit for counting greenhouse gas (GHG) emissions.)

The paper predicts that the surplus will not last forever, with demand for carbon credits expected to increase five to ten-fold over the next decade as more companies adopt Net Zero climate commitments. This growth in demand should see carbon credit prices rise to $20-50/tCO2e by 2030, as more investment is required in projects that take carbon out of the atmosphere in the long-term. With a further increase in demand expected by 2040, carbon credit prices could rise in excess of $50/tCO2e.

Guy Turner, CEO of Trove Research and lead author of the study said “It is encouraging to see so many companies setting Net Zero and Carbon Neutral climate targets. What this new analysis shows is that these companies need to plan for substantially higher carbon credit prices and make informed trade-offs between reducing emissions internally and buying credits from outside the company’s value chain.”

Imagine the stoves that could be supported by a $20 price for an avoided tonne of carbon dioxide. Stoves with good thermal efficiency can save three tonnes or more per year making long lasting, super clean stoves with chimneys a great deal for carbon developers and investors.

And a great deal for consumers.

Report: https://trove-research.com/wp-content/uploads/2021/06/Trove-Research-Carbon-Credit-Demand-Supply-and-Prices-1-June-2021.pdf

When this sort of fire is maintained, a high mass Rocket stove can get close to Tier 3 for CO (less than 7.2g/MJd) and PM2.5 (less than 218mg/MJd).

Testing stoves means that many, many hours are spent watching the flames and pushing sticks of wood into the fire. We watch the real time emissions, water temperature, excess air, and temperatures in the combustion chamber on a computer screen as the testing continues. After hundreds of hours it becomes obvious that clean combustion has a certain “look.” For instance, when there is a lot of flame above the sticks the real time CO goes down on the computer screen. Without flame above the fuel, the gas rises up and is not combusted. That’s why charcoal can be dangerous, because burning charcoal does not create a lot of flame above the fuel.

When the wood sticks are changed into charcoal, the PM2.5 is dramatically reduced, as well. Pushing the unburned sticks into the fire creates smoke. Pushing the sticks in quickly makes a lot of smoke and pushing the sticks in slowly makes less smoke. Charcoal does not make much smoke and that’s one of the reasons that people like cooking with it. Feeding a fire is a compromise, as firepower and PM2.5 tend to rise together. A rocket stove can look OK as long as the sticks are fed slowly into the fire at less than about 2.5 kW. But at high power, Rockets start to smoke like crazy.

How can these two experiences be factored into a mathematical model of combustion? By using a video camera hooked up to a computer program? Residence time and temperature are easily measured, but the extent that turbulence occurs is not easily quantified. Engineers get past these sorts of problems by figuring out how to optimize mixing (using jets of air, for example), but a mathematical model is more easily filled in with numbers for other sorts of phenomena, such as the excess air ratio.

Both experiments and mathematical modeling shed light on how to make better stoves and hopefully complement each other. Arguments have been known to happen. I have been trying to figure out how to make clean burning biomass fires since 1989, and one thing has not changed since Dr. Winiarski started me on this path. I continue to be happier trying to answer questions that begin with the word “What” instead of “Why.”