From: EPA’s Lab Test Results for Household Cookstoves, Jim Jetter, 2012 

Since 2012, optimized biomass cook stoves have been tested at ~50% thermal efficiency

The temperature of the hot gases flowing past the surface of the pot is increased by

  1. Creating as much flame (1,100C) as possible in a low mass, insulated combustion chamber.
  2. Decreasing the distance between the fire and the pot without making excess smoke.
  3. Not allowing external air to cool the combustion gasses.

In convective heat transfer, the primary resistance is the surface boundary layer of still air immediately adjacent to a wall. 

Increasing Temperatures, increasing exposed Area, increasing Radiation, increasing Velocity in a 6mm to 7mm channel gap (10cm or higher) pot skirt has been shown (up to 5kW firepower) in a 24cm or larger diameter pot to result in ~50% thermal efficiency. Reducing losses from the exterior of the pot skirt with refractory ceramic fiber insulation also increases thermal efficiency. 

60% thermal efficiency has been demonstrated in the lab.

Helpful links:

From: EPA’s Lab Test Results for Household Cookstoves, Jim Jetter, 2012

Key findings compared with the 3-stone fire:

  • Most stoves that were tested had better thermal efficiency, but some did not.
  • Compared with the 3-stone fire, many stoves that were tested had better combustion efficiency, but many did not.
  • A natural-draft TLUD stove (ARC) had very high efficiency with processed, wood-pellet fuel with low-moisture content.
  • Some forced-draft (fan) stoves had very low emissions – but not all fan stoves did.
  • Most natural-draft stoves that were tested showed a bigger improvement (lower emissions) over the 3-stone fire with high moisture fuel than with low-moisture fuel.
  • A natural-draft TLUD stove (ARC) had very low emissions – but required processed, wood pellet fuel with low-moisture content.
  • Two rocket stoves were tested at a “medium power” level – and had lower emissions (per energy delivered to cooking pot) than at maximum power.
  • Charcoal stoves had high emissions of CO and high emissions of PM during start-up.

Moving Forward: Thanks to Jim Jetter’s EPA Lab!

Champion (2021) average energy emission factors (g/MJ) from ISO high, medium, and low tests. 

Champion, Wyatt M., et al. “Cookstove Emissions and Performance Evaluation Using a New ISO Protocol and Comparison of Results with Previous Test Protocols.” Environmental Science & Technology, 2021, 55, (22), 15333-15342.
DOI: 10.1021/acs.est.1c03390

Lab testing can quickly compare emissions from stoves. The EPA and ARC labs now measure the climate emission factors, not just PM2.5 and CO. It has been proven that only field tests show real world performance. Together, lab and field tests help to move stoves forward as we get closer to market driven stoves that please cooks, successfully cook food, use a lot less fuel, and protect health/climate.

The above chart contains a lot of information. Some takeaways are:

  1. Wow! The Three Stone Fire (TSF) was pretty bad! 943g/MJ for PM2.5, 15.5 g/MJ for CO.
  2. Charcoal made ~90% less PM2.5.
  3. The Carbon Monoxide (CO) from charcoal was only a bit higher than the Three Stone Fire (19.2g/MJ).
  4. LPG did so well! (Too bad that we are entering the end of the fossil fuel era).
  5. The forced draft pellet stove looked great, as well. (PM2.5: 30g/MJ, 2.2g/MJ CO)
  6. Black Carbon (EC) is much worse than CO2 for climate change. Many of the stoves, except the Rocket stove, successfully reduced Black Carbon. 
  7.  In this recent lab test, as in the previous MacCarthy study (2008), the Rocket stove emitted a lot of Black Carbon.  www.sciencedirect.com/science/article/abs/pii/S0973082608604299
  8. R&D has shown that the Rocket stove requires successful forced draft mixing at high temperatures to decrease emissions of Black Carbon and potentially address climate. 

When the emissions factors are summed and converted to global warming potential the forced draft stoves have the potential to generate large amounts of carbon offsets. 

Pot Skirts – basic theory

Dr. Sam Baldwin describes the use of a pot skirt in his book “Biomass Stoves: Engineering Design, Development, and Dissemination (1987).” Changes in the length and diameter of the channel gap (between the pot and the interior of the skirt) result in dramatic changes in heat transfer efficiency.

“In fact, the channel efficiency, defined as the fraction of the energy in the hot gas entering the channel that is transferred to the pot, is extremely sensitive to changes in the channel gap. For a 10cm long channel, the channel efficiency drops from 46% for an 8mm gap to 26% for a 10cm gap. Thus the stove and pot dimensions must be very precisely controlled.” (pg. 45)

If stoves are to be compared, these types of variables must be controlled. The use of a standard pot, or pots, without pot skirts will result in performance scores that are significantly reduced. If a pot skirt is used on testing pots it should be identical in all aspects. Again, the use of a standard pot(s) seems to be required.


Iterative Development of Stoves and Black Box Theory

https://tse3.mm.bing.net/th?id=OIP.wXqUUCsn5xHBA6meZOienQHaBg&pid=Api&P=0&h=220

“The concept of black boxes has been around since the early days of systems theory though some attribute the first use to the field of electrical engineering.

It is a simple concept and has a straightforward definition: we know the inputs and subsequent outputs to a system but the internal workings of the system are not visible to us.

Black boxes approaches focus on input and output rather than the details of how inputs are transformed into outputs”.

-John M. Green, The Application of Black Box Theory to System Development

Data Driven Hypothesis Generation

The results of experiments guide the subsequent experiments. For instance, adding secondary air jets into flame is shown to decrease PM2.5. Guided by the result, further investigation in a prototype under the emission hood determines the most successful application by varying parameters.

Random Experimental Design

In a Black Box (where the situation is complex and not understood) randomized approaches to experimental design can be effective and efficient.

Predictive Testing of Cars and Wood Stoves

Photo: Car on dynamometer
Testing a car on a dynamometer

Written decades ago, the lab based tests for both biomass heating and cooking stoves were designed to achieve statistical validity by controlling variables. Because many real world variables were removed from the heating and cooking stove protocols, the results were known not to predict real world performance.

Automobiles are currently tested on a dynamometer instead of being driven around town. EPA estimates are based on dyno tests designed to reflect “typical” driving conditions and driver behavior. Even so, The EPA warns customers that actual mileage will probably be significantly different.

To predict real world performance, each car could be driven around town by enough people until a meaningful average was mathematically determined. Cook stove tests could also rely on field tests generating complicated data resulting in accurate predictions. However, doing real life testing for every manufactured car or stove has been thought of as rather cumbersome.

Another approach might be to have regional survey data inform a predictive model. The model teaches the dynamometer how to test the car. Stove use could be modeled in the same way, so lab tests get closer to predicting what actually happens when cooks use wood to make meals.

Making a model probably didn’t seem to be worth the trouble in the past. However, things have changed. The harmful emissions from cars and biomass stoves damage health and contribute to climate change. Actually knowing what a new car or stove will do when used should help to create better technologies and reduce pollution. Any kind of predictive testing seems like a great idea!

Emission Testing: Tuning a Stove Like a Race Car

https://i.ytimg.com/vi/FLSI-62aIYc/maxresdefault.jpg

Yesterday, two fine fellows who manufacture stoves in southern Oregon visited our lab. One of the very competent guys had just installed a Corvette engine in a Jaguar, for fun. 

We quickly got on the same page when the ARC staff showed them how an emission hood (with both real time and gravimetric measurements) enables quick experiments to improve performance and achieve clean burning.

Anyone involved with racing (or fixing cars) knows how a computer helps to tune a modern car. The biomass emission hood allows folks to tune stoves like race cars.

Testing for development means that the testing of a manufactured product will have known results. 

You win the race.

sticks burning in rocket stove

How To Achieve Close To Complete Combustion of Biomass

https://www.jet-flame.com/wp-content/uploads/2019/10/Bricks.jpg
The Jet-Flame pushes jets of primary air into the fire to aid combustion.
  1. When a wooden stick is burned a lot of smoke is produced but the made charcoal at the tip of the wooden stick does not make much smoke.
    Rocket Stove: Push the sticks in slowly so the charcoal at the tip is burning.
    TLUD: Charcoal covers the slowly burning fresh wood.
  2. If the stove begins smoking, the solid wood is being turned into gas too quickly, too much wood gas is being produced and un-combusted fuel is escaping.
    Rocket Stove: Pull the sticks back until just the tips are burning.
    TLUD: Reduce the primary air.
  3. Mixing the smoke, gases, flame, and air reduces emissions.
    Rocket Stove and TLUD: Cut up the laminar flames with static mixing devices or jets of primary or secondary air. Aim the jets of secondary air into the flame and adjust the velocity of the jets to completely cover the burning fuel. Primary air jets can also achieve close to complete combustion. Excess velocity in primary or secondary jets is detrimental when it reduces the combustion temperature.
  4. For close to complete combustion the temperature in the combustion zone needs to be 850C or above. The woodgas and air and flame have to be thoroughly mixed. The residence time needs to be 0.2 seconds or more. Reduce the amount of woodgas entering the combustion zone until close to complete combustion is achieved. Biomass fuels with 15% or lower moisture content are easier to burn.
  5. It is necessary to tune the stove under an emissions hood to achieve close to complete combustion. Change one variable at a time and test until significance is achieved.
The Jet-Flame in a home made CQC Rocket Stove

Jet-Flame Research Results From Six Types of Biomass Cookstoves

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 https://doi.org/10.1016/j.esd.2022.09.013, 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 www.jet-flame.com

A Recent History of the Rocket Stove: 2009

In 2009, The New Yorker published an article about the Rocket stove entitled Hearth Surgery: The quest for a stove that can save the world. One year later, USAID funded field tests in Africa showed that the insulated Rocket stove was not cleaner burning than the open fire. The Rocket with skirt saved 40% of the fuel to cook and emissions were only reduced by that amount.

Not a Planet Saver, yet!

The insulated Rocket combustion chamber raised temperatures but as Dr. Winiarski realized at the time, flame, air, and gases were not adequately mixed to achieve sufficient combustion efficiency. Larry knew that the Rocket was smoky but it was simple to make and with a pot skirt saved fuel. He wanted to provide folks with a stove that was helpful and he realized that it wasn’t perfect.

Larry’s idea went viral worldwide and continues to be a favorite on the internet and in many low- and middle-income countries. Millions of Rocket stoves are manufactured and sold yearly by factories large and small.

Going viral is great but can have a downside especially when the initial products are not technically mature. It’s normal for first generation products to be improved as time goes by. The process of development continues in 2022.