Rocket Stoves Archives | Page 3 of 4

Detailed Observations At Work

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.

April 7, 2021/0 Comments/by Kim Still

Cleaner Burning Technologies, Jet-Flame, Rocket Stoves

Looking at High Mass and Insulation

*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.*

March 24, 2021/0 Comments/by Kim Still

Rocket Stoves

Cooling Fins on a Downdraft Rocket Stove

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.)

March 10, 2021/0 Comments/by Kim Still

International Work, Jet-Flame, Rocket Stoves

Making it real!

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

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.

February 17, 2021/0 Comments/by Kim Still

ISO Standards, Rocket Stoves

“Going Faster” on ISO 19867

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!

February 3, 2021/0 Comments/by Kim Still

LEMS, Rocket Stoves, Videos

New Video: Rocket Stove 2021 – LEMS+ Realtime Combustion Analysis

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.

January 20, 2021/0 Comments/by Kim Still

Cleaner Burning Technologies, Rocket Stoves, Videos

New Video: Rocket Stove 2021 – Pot Skirts to Increase Heat Transfer

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.

January 6, 2021/0 Comments/by Kim Still

Cleaner Burning Technologies, Jet-Flame, Rocket Stoves, TLUDs

Fluidized Bed Combustion, Top Lit up Draft, and the Jet-Flame

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

Fluidized Bed

“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.

Charcoal over wood is lit.

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

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

December 30, 2020/0 Comments/by Kim Still

Indoor Air Quality, News, Rocket Stoves, TLUDs

Air Exchange Rates and the ISO Tiers of Performance

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 (PM_2.5 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 PM_2.5 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.

December 16, 2020/1 Comment/by Kim Still

Thumbnail from Rocket Stove 2020 video about height and weight

Materials, Rocket Stoves, Videos

New Video: Rocket Stove 2020 – Height & 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.

December 9, 2020/1 Comment/by Kim Still