Chart comparing energy output of 1 acre of grain vs 1 acre woodlot

The direct burning of biomass seems to be dramatically more efficient compared to ethanol for applications such as home heating, cooking, heating water, or drying clothes. It makes sense that not having to create alcohol from biomass would save energy. When the use of natural gas is decreased (due to climate change), burning biomass for heating seems like a fuel-efficient option that could reduce the extra burdens on electricity. 

One of my favorite reference books is “The Energy Primer” published in 1974. It has comprehensive review articles on solar, wind, water, and biomass energy. The following chart comes from a great article on biomass written by Richard Merrill. It shows that when renewable biomass is combusted, the efficiencies are much higher compared to making alcohol from biomass and then burning it.

The very clean burning of biomass allows efficient heating applications.

Chart comparing energy output of 1 acre of grain vs 1 acre woodlot

Cooking outside

When the air exchange rate is doubled, the concentration of smoke is reduced by half. The average house in low middle income countries is estimated to have around 12 air exchanges per hour (ISO 19867-3). When ARC measured the air exchange rate outdoors in a gentle breeze it was found to be about 120 per hour. Exposure to smoke could theoretically be dramatically reduced by moving outside.

Changing stoves costs money and the transition to an improved stove is often a time consuming process. ISO reports that even stoves with chimneys (that often leak) only reduce concentrations of smoke in houses by around 75%. Experimenting with increasing the air exchange rate is probably the most cost-effective intervention to protect health. 

When we went to villages where women cooked outside, Dr. Winiarski would frequently ask the women if digging a well, etc. might be more important than new Rocket stoves. That is one of the things that we loved about Larry!
Dr. Mouhsine Serrar and the Rocket institutional stove designed by Dr. Larry Winiarski

There are at least three ways to make institutional stoves with chimneys, all of which work well and save fuel and decrease emissions. Here they are:

  1. Shell Foundation supported the making of an eight-part video, a step by step guide to making a 50 to 100 liter institutional Rocket stove, with a heat resistant metal Rocket combustion chamber. It is a great stove with lots of successful field testing but it costs the most to construct because heat resistant metals like 410 stainless or FeCrAl are increasingly expensive. The super insulated combustion chamber requires these types of metal. 304 stainless will not last.
  2. Cooking With Less Fuel: Breathing Less Smoke shows how to make the same institutional stove using bricks for the Rocket combustion chamber. Construction details can be found at in the publications section. The book was written with the World Food Program in Rome. This is a less expensive stove that is slightly less fuel efficient at cold start but lasts longer and is easier to make in places where 410 stainless and FeCrAl are not available.
  3. Making a VITA style institutional stove without a Rocket combustion chamber is the least expensive way to create an institutional stove. The open fire under the pot is supported on a grate and the hot gases flow up the inside of the skirt, down the outside of the skirt and exit out the chimney placed below the bottom of the pot as in the Rocket stoves shown above. You can find a video we made about constructing the VITA stove at:

Lots of manufacturers do not use the chimney but we think that protecting health is very important. We try to follow Don O’Neals advice (HELPS International) to always include chimneys whenever possible, imagining our mothers cooking and getting ill from exposure to the harmful emissions without the protection of the chimney.

by Kirk Harris

Blade cross sectional shapes:

There are several main physical principles that can be used to make a functional cross-sectional shape.

  1. The gas enters the stationary fan vertically from below, and should leave the fan at as flat an angle as possible.
  2. Deflection:  The side of the blade that faces the oncoming gas deflects the gas to change its direction.
  3. A space between the blades can allow gas to pass upward without deflecting it, reducing the stationary fans efficiency.
  4. The Coanda effect can be used on the side of the blade facing away from the oncoming gas to deflect the gas.  The Coanda effect states that a stream of gas moving parallel and close to a surface will experience a force holding it close to the surface.  This force results from Bernoulli’s principle.  The moving gas is at a lower pressure then the atmospheric gas next to it, and so is pushed toward the surface.  The surface blocks atmospheric pressure from pushing back.

 Three shapes of blade cross-section to consider:

Straight angled

The gas on the bottom side of the blade is moderately deflected and leaves the blade at a bad angle, reducing swirl.  The gas on the top side of the blade cannot form a Coanda type connection to the blade because it is not flowing parallel to the surface.

Straight bent

The gas on the bottom side of the blade is moderately deflected, leaving the blade at a bad angle.  The sharp bend in the blade slows and disrupts the gas flow.  The gas on the top side begins by moving parallel to the blade surface, and so can attach to the bottom half, but cannot stay connected because of the sharp bend.  Sharp bends destroy the Coanda type connection.

Smooth curve

The gas on the bottom side of the blade is fully and smoothly deflected to the horizontal direction.  The gas on the top side of the blade begins by moving parallel to the surface, and so can form a Coanda attachment to the blade.  Because of the Coanda effect, the gas can follow the smooth curve of the blade to a flat exit path.  Note that the leading edge of the blade is at a right angle to the trailing edge.

Draft (buoyancy) vs. the Coanda effect

The gas moving horizontally on top of the blade is subject to two competing forces, the draft and the Coanda effect.  The draft tries to direct the gas upward while the Coanda effect tries to keep the gas attached to the blade going horizontally.  If the gas speed is slow and/or the connection of the gas to the blade is weak, the draft wins and the gas breaks away from the blade.  The gas may be split between the two, some gas going each direction.

The gas deflected by the previous blade can join the Coanda effected gas on the current blade, countering the draft induced upward motion and improving the swirl.

Blade Construction

The simplest stationary fan for creating swirl is made from a single round disk of sheet metal. 

There may be 4 or more blades, but I will describe a 6 bladed fan, which works best for me.  Draw a small circle in the center which will hold the blades together.  Draw 6 radial lines from the center, outside the small circle evenly spaced at 60o each.  Cut the radial lines to the small circle.  If the fan is to be mounted on a shaft, drill a center hole of the appropriate size.

Bend the 6 blades as shown with red lines around the point of intersection of the radial lines with the small circle.  Make the curve smooth and round so the gas can stay connected using the Coanda effect.  The lower edge of the blade should point straight down, the upper edge horizontal.

The stationary fan will cause flow resistance for the gasses.  This will slightly reduce the highest power level of the stove.  Lower power levels are more sensitive to flow resistance and the flame may go out.  A small slit, as shown in red, can reduce the flow resistance and allow higher and lower power flames.  The blade must be bent as shown, from the end of the slit.

There will be an open space between the blades which will allow gasses to pass upward without being effectively deflected, reducing swirl.  This type of fan can work quite well, but can be improved.

An improved version which eliminates the gap can be made from two circles.  Mark three evenly spaced blades at 70o on both circles.  Cut out the 50o space between.  Make a short cut on the red lines.  This will make room to place one circle on top of the other to form one fan with 70o blades.  Spot weld the two circles together in the small center circle.

Bend the blades as before around the red lines, but with the center at what would be the 60o blade point.

The finished fan will not have the gaps and will be more efficient, especially for slow moving flame gas. 

Flame gasses may escape deflection around the ends of the blade.  This can cause incomplete combustion and increased particulates.  This problem may be fixed by extending the blade beyond the flame, placing something to block the gasses from escaping, or by raising the center of the fan and bending the blades slightly downward as shown along the dotted lines (thus directing the flame inward).

swirl and tumble in ic engine

Turbulence is very important for close to complete combustion. Swirl, Squish, and Tumble are used to create turbulence in internal combustion engines.

Kirk Harris discovered that a static fan shape with overlapping 70 degree blades creates lots of fast moving swirl at the approximately one to two meter per second velocities found in TLUDs.

1. Swirl:

The rotational motion of air within the cylinder is called Swirl. Swirl enhances mixing and makes the flue air mixtures homogeneous. Swirl is the main mechanism to spread the flame within the combustion zone.

2. Squish:

The radial inward movement of air is called Squish. Squish can be defined as an inward flow of air towards the combustion recess. 

3. Tumble:

Squish generates secondary motion about the circumferential axis near the outer edges. This motion is called ‘tumble’. To achieve this either the fuel is directed towards air or air is directed towards the fuel.

In the last Newsletter, we announced the publication of Aprovecho’s recent research on the Jet-Flame, “Retrofitting stoves with forced jets of primary air improves speed, emissions, and efficiency: Evidence from six types of biomass cookstoves” Here is a simplified summary of the findings:

When the goals for biomass cook stove interventions were raised to include protecting health, it was obvious that adding a chimney or cooking outdoors continued to be the historically proven solutions. USA heating stoves create more smoke than cook stoves but the smoke is transported outdoors in the chimney and diluted by clean air to meet EPA outdoor air standards for PM2.5. Cooking outdoors, especially in a bit of wind, directly dilutes the PM2.5.

When the outdoor air is cleaner, the emissions from the stove can be higher. When the outdoor air is dirtier, the emissions need to be cleaner. Simple! Aprovecho published a model that estimates emissions based on the quality of the outdoor air. See:

ISO Tier Mapping for CO and PM2.5 per MJdelivered for the natural (blue triangle) and forced draft (orange dot) cases. Note the log scale on both axes.

As seen on the upper right side of the graph above, stick burning stoves (even in the lab) emit very high levels of PM2.5. That can be OK when used with a functional chimney or outdoors in rural locations with limited numbers of cooks per hectare. But in many more crowded situations the emission rates need to be much lower to protect health.

Adding forced draft mixing to many types of stoves, including the open fire, can be very effective in reducing the emission rates of PM2.5. The Jet-Flame shoots primary-air-only jets into the bottom of the fire and this simple technique reduces emissions of PM2.5 and CO, while reducing fuel use and time to boil. We hope that technologies like the Jet-Flame can assist stove projects to protect health especially when combined with chimneys and/or outdoor cooking.

The Jet-Flame in a home made CQC Rocket Stove
The high mass CQC stove with Jet-Flame inserted from the side.
The SSM Jet-Flame in the C Quest Capital 15 brick stove

The Journal “Energy for Sustainable Development” has just published Aprovecho’s most recent research paper, “Retrofitting stoves with forced jets of primary air improves speed, emissions, and efficiency: Evidence from six types of biomass cook stoves.” It was authored by Samuel Bentson, David Evitt, Dean Still, Dr. Daniel Lieberman and Dr. Nordica MacCarty (Energy for Sustainable Development 71 (2022) 104–117)

Read the full research paper at, available to all as an open access document thanks to Dr. Dan Lieberman of GH Labs.

Quoting from the Abstract:

Incorporating jets of forced air into biomass cook stove combustion has been shown to potentially decrease harmful emissions, leading to a variety of designs in recent years. However, forced draft stoves have shown mixed success in terms of real world performance, usability, and durability. The Shengzhou Stove Manufacturer Jet-Flame forced draft retrofit accessory was developed by the Gates funded Global Health Labs and ARC, to implement forced jets of primary air at a low cost into a wide range of types of cook stoves using a small 1.5-W fan housed in a low-cost cast iron body to be inserted beneath the fuel bed of a biomass cooking fire.

This research sought to quantify the potential efficiency and emissions performance impacts of the Jet-Flame when installed in six different types of biomass cook stoves (three open or shielded fires and three rocket stoves) versus the natural draft performance of each. The effect of the operating fan voltage was also measured. A series of tests following a modified ISO 19867-1:2018 protocol were performed in the laboratory using the Aprovecho Laboratory Emissions Measurement System (LEMS) equipped with additional oxygen and temperature sensors. 

Results for each stove, carefully tended with a single layer of sticks, showed that the global average PM2.5 reduction with the Jet-Flame was 89% relative to the natural draft cases, with larger relative improvements seen in the most rudimentary stoves. CO was reduced by a global average of 74%, reaching Tier 4 or 5 for all stoves. Thermal efficiency was also improved by 34% when calculated without taking into account the energy content of the remaining char (or 21% with char), illustrating the value of burning char to provide cooking energy rather than leaving it unburned in the combustion chamber as is common in many natural draft stoves. Time to boil was also reduced by 8%.

In addition, adjusting the voltage of the jet-flame assisted in modulating firepower, possibly improving the usability of the stove.

For more about the Jet-Flame, see

Illustration of a Reaction Vessel and a Burner for a TLUD stove
Illustration of a Reaction Vessel and a Burner for a TLUD stove

The Rocket stove requires forced draft to create sufficient mixing in the flame to achieve Tier 4 for PM2.5. In addition, when sticks are pushed too fast into the Rocket, too much wood gas enters the combustion zone and a lot of smoke is emitted. Metering of the fuel is necessary to keep the Rocket stove even moderately clean burning. Unfortunately, that is unlikely to happen in real life.

On the other hand, the TLUD supplies a constant amount of wood gas into the combustion zone. A five inch in diameter reaction vessel produces around 4kW of energy (5 liters of water boils in about 20 minutes using a pot skirt with a 6mm gap).

The emissions of CO and PM2.5 can be very low in a natural draft TLUD when:

  1. The primary air controls the rate of reactions (how fast the wood pellets are turned into wood gas).
  2. The jets of pre-heated secondary air provide sufficient mixing of wood gas, flame, and air by completely covering the top of the fuel bed.
  3. The burner section allows sufficient Temperature, Time, and Turbulence to complete the combustion process.

Nothing is perfect. The TLUD, whether forced draft or natural draft, has trouble achieving enough Turn Down Ratio to simmer food efficiently. It’s also generally necessary to burn pellets to lower emissions to Tier 4 levels. At the same time, we love to cook on TLUDs and it is great not having to constantly adjust the fuel!

As Dr. Tom Reed said, “Now you’re cooking with the real natural gas.”

Jets of air at the top of a SupaMoto TLUD chamber cause flames to cover a bed of burning biomass pellets.
Jets of air at the top of a SupaMoto TLUD chamber cause flames to cover a bed of burning biomass pellets.
The SupaMoto Forced Draft TLUD

Before starting to develop cleaner burning cook stoves in the 2013-2015 DOE project, ARC researchers completed a survey of best performing existing stoves. Improving combustion efficiency to protect health (and climate) has continued as various cook stove organizations have worked tirelessly to meet the WHO 2015 PM2.5 Intermediate Emission Rate Target of 1.75mg/minute, calculated to protect health in homes using biomass to cook.

The results of the survey are described in Clean Burning Biomass Cookstoves, 2nd edition, 2021.

  • With a 6mm channel gap pot skirt, many stoves scored close to 50% thermal efficiency.
  • TLUDs were not able to achieve enough Turn Down Ratio (TDR) to simmer water.
  • Burning wood does not emit much CO so meeting the WHO CO Target (.35g/min) was easy.
  • Forced draft TLUDs scored between 2mg/min PM2.5 to around 5mg/min.
  • Stoves with chimneys met the aspirational WHO PM2.5 Emission Target of 0.23mg/min since the smoke was transported outside.

SupaMoto Forced Draft TLUD Bests WHO Goals

The new SupaMoto stove from Emerging Cooking Solutions with combustion technology from partner company Zemission has made great progress! For information contact Mattias Ohlson at: As seen below, in Water Boiling Tests at ARC, the SupaMoto Forced Draft TLUD achieved:

  • 51% to 56% thermal efficiency (without pot skirt)
  • 0.1g/min to 0.6g/min for CO
  • 0.19mg/min (simmer) to 1.11mg/min (high power) for PM2.5
  • The temperature corrected time to boil the 5L of water was fast, about 18 minutes

As in the FD TLUD Mimi-Moto stove, turn down in the Supa-Moto is achieved by inserting an accessory into the combustion chamber. The Supa-Moto Turn Down Ratio (TDR) varied between 1.91 to 2.11.  When a lid is used on a pot, a TDR of around 3 saves more fuel when a lower firepower is needed to simmer food to completion. Reducing the forced air jets in a TLUD does not create sufficient TDR.

It is so gratifying to witness progress! I never thought that we would see a biomass stove come so close to meeting the aspirational PM2.5 WHO Emission Rate Target.  To test a stove that easily meets the Intermediate PM2.5 Target is amazing. Mattias and the Zemission team have moved TLUD technology forward and it is a very valuable achievement!

Thank you for your work! Learning how to cleanly combust biomass has important ramifications in all parts of the world now that climate change reinforces the importance of renewable biomass as a health and climate friendly energy source.

Smokestacks belch out smoke, spelling out CO2 in a blue sky. A Euro symbol floats to the right.
Smokestacks belch out smoke, spelling out CO2 in a blue sky. A Euro symbol floats to the right.
Image by Petra Wessman via Flickr

How can smoke, extremely dangerous for health and climate change, be ignored in carbon credit equations? Carbon dioxide and methane are counted but not smoke. Carbon dioxide is reduced when heat transfer is improved resulting in less wood being burned. Wood doesn’t make appreciable amounts of methane. 

Because smoke is not counted to earn carbon credits, smoky stoves with good heat transfer efficiency make as much money as clean burning stoves even though the Black Carbon in smoke is something like 680 times worse than CO2 by weight for warming. Because smoke is not included in climate credit math, adding clean burning to biomass cook stoves usually has to be as inexpensive as possible.

We know that adding high pressure mixing to Rocket stoves dramatically reduces smoke. As of 2022, forced draft is required to achieve adequate amounts of mixing. Mixing requires high pressures that (so far) cannot be made with natural draft. We know how to improve the Rocket but are in the process of completing the transformation to clean burning.

Nice to know the solution!