Sam Bentson and a Winiarski designed down-feed downdraft Rocket stove with added cooling fins.

We used to have problems with the upper portion of the sticks catching fire in Dr. Winiarski’s down-feed downdraft Rocket stoves. The draft had to be strong, and the sticks at the right moisture content and size/weight to burn up completely without any smoke back drafting up into the room. The vertical metal feed tube got too hot and could catch the top part of the sticks on fire, overcoming the draft moving into the stove. (Usually something like 3 MPH.)

When Sam Bentson and Karl Walter were making a 20 watt thermoelectric generator (TEG) for the EPA SBIR supported Integrated Stove project we had the same problem, until Sam added aluminum cooling fins to the top of the vertical feed tube, as seen above. The team had previously designed and built the cooling fins unit that Sam is holding in the photo to cool the cold side of the TEG. I was amazed how well cooling fins work!

But then I remembered how small the radiators are in automobiles with huge horse powers. I saw that Sam and Karl had added a fan to their radiator to make sure it could dissipate the 1.5 kilowatts running through the hot side of the TEG. Adding fins to parts of a stove that could use more dissipation of heat brings to mind several possible applications: the heat exchanger cylinder in the photo, a chimney that has high exit temperatures, or the outside of a metal combustion chamber to preserve the metal. But I would not use fins where they can get dirty! They don’t work, for example, on the bottom of a cooking pot where soot quickly fills the space between the fins. (I didn’t think of this before we tried it.)

Kabanyana Murabukirwa Domina and Jean Marie Vianney Kayonga in Rwanda
Kabanyana Murabukirwa Domina and Jean Marie Vianney Kayonga in Rwanda
Kabanyana Murabukirwa Domina and Jean Marie Vianney Kayonga in Rwanda

One of the roles of the ARC engineer is to give accurate technical information to the in-field decision makers who are directing the stove project. The folks on the ground have to make sure that cooks really like the stove, that the price is market based, that manufacturing is arranged for, etc. ARC engineers and the field team work closely together as the project evolves.

A New Project in Rwanda

In Rwanda, Kabanyana and Jean-Marie and their NGO, ENEDOM, are working with C-Quest Capital and ARC on a carbon credit supported Jet-Flame project. We met Jean-Marie through the internet and realized that he is well known in the sector. In fact, he knows many of our friends in Africa. Dr. Dan Lieberman at Global Health Labs sent Jean-Marie twenty Jet-Flames, and he showed them around to many of organizations, like the World Bank, that have large projects in the country.

Real World Use Guides Product Improvement

Moving the Jet-Flame to the side of the CQC stove
Moving the Jet-Flame to the side of the CQC stove

When we envisioned the Jet-Flame we imagined that it would be inserted into the fuel door of a Rocket stove. Mr. Shen at SSM directed the effort to manufacture the Jet-Flame and it includes a beautiful stainless steel stick support that also protects the fan. However, it only took several weeks of trails for ENEDOM to make a strong recommendation to move the Jet-Flame to the side of the CQC stove. Cooks in their homes were accidentally burning up the cord!

We gratefully thank ENEDOM for helping us make fewer mistakes. It’s another great example of trying to make sure that reality is in the product.

Many years ago, Kirk Smith hired Aprovecho to help Rob Bailis from U. C. Berkeley update and add emissions to the Water Boiling Test in the 1985 International Testing Standards. The Water Boiling Test (WBT) measured in the lab how much wood was used at full power and when simmering water. The writers of the International Testing Standards defined the purpose of the WBT as: “While it does not correlate to actual stove performance when cooking food, it facilitates the comparison of stoves under controlled conditions with relatively few cultural variables.”

The 1985 Kitchen Performance Test (KPT) measured fuel use in actual households, and the Controlled Cooking Test (CCT) was a bridge between the WBT and the KPT. ARC uses the Controlled (or Uncontrolled) Cooking Test to develop stoves with local committees of all stakeholders, as recommended by Sam Baldwin. In this test, locals cook with their own fuel, pots, and cooking practices, hopefully at Regional Testing and Knowledge Centers under the total capture emissions hood. Using the WBT in the lab has been a good tool for ARC to improve heat transfer and combustion efficiency. The cooks, marketers, manufacturers and funders in the project have to make the stove. It must work for users. They are experts.

We now use the new, updated Water Heating Test (ISO 19867) to improve heat transfer and combustion efficiency in the lab and it’s great. We are directed to try to use the type of wood, pot, and cooking practices from the intended project location. ISO 19867 also has us test the prototypes at high, medium, and low power to learn more about performance. As said, there are many other variables that can only be learned from the local cooks and everyone involved in the project. How much the can stove cost, that chapatis have to be toasted in the fuel door, that cooks in southern India sit cross legged so the stove must be pretty short, etc. is information that is obviously necessary and field based. The idea is that lab tests inform the prepared mind of the engineer who then works hand in glove with the project stakeholders in their location to make an effective product.

Kelsey Bilsback from Colorado State University advised that lots of times stoves in actual use are operated at exceedingly high fire powers. We agree! When applicable we use very high power (and relatively untended fires with sticks gathered from the forest). We are trying to find out whether a biomass stove burning found fuels can be clean burning at the equivalent of 85 MPH.

Thanks, Kelsey! Good idea!

Intro image for YouTube Video

Watch what happens with PM2.5, CO2, Oxygen and more during a wood burning stove test in this real-time video from Apro’s Laboratory Emissions Monitoring System. The LEMS provides a display of what’s being recorded by the various sensors in the stove being tested, and in the emissions hood. In this video, Dean Still gives an overview of what the five lines on screen represent, and how they relate to each other as the fire progresses.

For more info about Aprovecho’s emissions monitoring systems, see aprovecho.org/portfolio-item/emissions-equipment.

Rocket Stove 2021 - Pot Skirts

In this video, Dean Still explains why a pot skirt – a sheet of metal wrapped around the cooking pot – is a simple yet important way to improve the fuel efficiency of a rocket stove. He also explains how to calculate the appropriate distance between the skirt and the pot. Stay tuned to the end of the video to find out who is causing all the ruckus in the background…

Helpful references:

simplified diagram of constant cross sectional area
Simplified drawing of the concept of constant cross sectional area.

This is a very simplified illustration of what “constant cross-sectional area” means. The top circle represents the cross-sectional area of a stove riser. The bottom ring shows the same area translated into the space around a pot. It’s important to keep the cross-sectional area that the hot gasses flow through consistent, so they don’t slow down. Hot, fast flowing gasses transfer heat most efficiently. 

graph helps calculate proper skirt gap for best heat transfer efficiency
Chart for calculating channel gaps, from Dr. Samuel Baldwin’s “Biomass Stoves: Engineering Design, Development, and Dissemination.” 1987, Volunteers in Technical Assistance.

This is the chart for determining efficient channel gaps, explained towards the end of the video. It was developed by Dr. Samuel Baldwin in 1987.

Here is the Ten Stove Design Principles poster referred to in the video. Many more helpful documents are also linked on the Publications page.

sticks and charcoal start to combust in a rocket stove

The Jet-Flame was developed from combustion concepts used in fluidized beds and TLUDs.

Fluidized Bed

fluidized bed combustion diagrams

“In its most basic form, fuel particles are suspended in a hot, bubbling fluidity bed of ash and other particulate materials (sand, limestone etc.) through which (under air) jets of air are blown to provide the oxygen required for combustion or gasification. The resultant fast and intimate mixing of gas and solids promotes rapid heat transfer and chemical reactions within the bed.”   https://en.wikipedia.org/wiki/Fluidized_bed_combustion

Top Lit Up Draft

diagram explaining how a top loaded up draft stove works

The TLUD uses under air flowing up through the fuel to transport wood gas into the hot layer of charcoal and flame above the fuel assisting more complete combustion efficiency.

Cleanly Starting the Jet-Flame

High velocity under air jets blow up into the lit charcoal placed on top of small sticks of wood. When the charcoal and wood are on fire, long pieces of wood are pushed into the made charcoal to start a Rocket Jet-Flame without making visible smoke. The sticks of wood are burned at the same rate as the continual production of charcoal creating a cleaner combustion process related to a fluidized bed and the TLUD.

sticks and charcoal start to combust in a rocket stove

Charcoal over wood is lit.

bed of charcoal in rocket stove

The charcoal becomes superheated with jets blowing up into the pile.

sticks burning in rocket stove

After 30 seconds, long sticks of wood are pushed against the burning charcoal creating flame.

Chart showing how more air exchanges reduces indoor air pollution from cooking
Chart describing the influence of air exchange per hour rates on the concentration of PM2.5 in a 30 cubic meter room. Higher air exchanges equal lower PM2.5 concentrations.
Using the ISO box model, Sam Bentson has calculated how increased ventilation helps a classic Rocket stove (around 30 mg/minute of PM2.5) and a modern TLUD burning pellets (about 5mg/minute PM2.5) to protect health.

In the lab, we are used to thinking of the ISO Tiers as static, based on how much pollution enters a 30 cubic foot kitchen during four hours of cooking with 15 air exchanges per hour. However, in 2018 ISO published 19867-3 that further explains how, for example, increasing the air exchange rate (ACH) changes the Tier rating. Generally, doubling the air exchange rate cuts pollution (PM2.5 and CO) in half.

In a low ventilation situation (10 ACH), Tier 4 requires that the emissions of CO are lower than 2.2 grams per megajoule delivered to the pot (g/MJd). But in a higher ventilation condition (30 ACH) the stove can be three times dirtier, emitting up to 7 g/MJd, and still be in Tier 4. Cooking outside is often employed by the cooks we work with because smoke is bothersome and unhealthy.

ISO 19867-3 reports that studies of air exchange rates have found a lot of variation in ventilation, from 4 ACH in very tight buildings to 100 ACH outside in the fresh air. When I lived on a ranch in Mexico, most of the cooking took place outside under a veranda (which also made it easier to smell the wonderful homemade coffee brewing in the early mornings). When Sam Bentson carefully measured the ventilation rate under our veranda in Oregon he also found that when a gentle breeze was blowing (2 MPH) the air exchange rate per hour was around 100.

At 100 ACH, with so much dilution occurring outside, achieving Tier 4 for PM2.5 and CO is easier. In our experience, the most successful and cost effective interventions are situation dependent. We find that a combination of approaches to protecting health enables a welcome adaptability to the actual and interwoven circumstances.

Thumbnail from Rocket Stove 2020 video about height and weight

Why is a heavy stove an inefficient stove? A tall combustion chamber makes a lot of draft to keep a fire roaring, how can that be a bad thing? What is TARP-V and how will it improve your stove? Dean Still has the answers for you in the latest Rocket Stove 2020 Video.

Here is the Ten Stove Design Principles poster. Many more helpful documents are also linked on the Publications page.

link to Rocket Stove 2020 YouTube video

How can burning wood, agricultural waste or even cow dung be a carbon neutral energy source? How do you start a fire without making a lot of smoke? How can a metal skirt around a cooking pot help with fuel efficiency? Dean Still has the answers for you in this new video.

Find out more about the Jet-Flame combustion accessory used in this video at www.jet-flame.com.

YouTube Video explains the importance of mixing for clean combustion

In this video, Dean Still explains why mixing air into flame is important for cleaner combustion. He uses several Rocket Stoves to demonstrate the effects of both natural draft and forced draft secondary air jets. Which style is more effective? Watch to find out!

For a simple way to add mixing to a Rocket Stove, check out the Jet-Flame.