SSM manufactured rocket stoves with fires burning in them

Durability Testing at SSM

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SSM manufactured rocket stoves with fires burning in them
Year-long durability testing with real fires

I just returned to the Oregon lab from a two-week visit to Shengzhou Stove Manufacturer. The next few newsletters will be about SSM and progress made. There’s a lot to talk about! SSM has sold over 5 million stoves and the factory is a wonderful place to visit. 

SSM started testing stoves for durability twenty-four hours a day (three eight hour shifts at a nearby farming community) three years ago. The farmers keep fires going in eight SSM stoves and the tests continue for one year of each stove. That’s 8, 860 hours.

It’s great that SSM has been doing long term, real life testing of their stoves. Previously, tests in a kiln with wet, salted pieces of metal resulted in confusing estimates of durability. In 2017, M.P. Brady and T.J. Theiss shocked the stove world by showing that in a wet, salty, hot kiln even very expensive metals were not long lasting. (Energy for Sustainable Development 37 (2017) 20–32, “Alloy Corrosion Considerations in Low-Cost, Clean Biomass Cookstoves for the Developing World”, Michael P. Brady, et al.).

The SSM testing is being written up. It seems to show much longer durability of various combustion chamber metals when real fires are used. Full details to follow.

Lab Tests: Cooking and Heating Stoves

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Unfortunately, although introductions to lab tests warn that results do not predict actual performance, the recent use of lab data to earn carbon credits has made an unfortunate error more commonplace. For decades, introductions to lab tests have warned that only field-testing can determine actual efficiency, emissions, effectiveness, market validity, etc. The World Health Organization based their stove standards aimed at protecting health on field-testing for this reason. 

Lab tests are helpful when comparing performance to understand how fire might be more useful. Starting with the 1985 International Standards, test users were advised not to use lab data to predict actual performance. While improving other carbon methodologies, using field-testing to estimate reductions would dramatically improve the accuracy of offsets.  

Carefully performed lab tests tend to overestimate fuel efficiency and underestimate emissions. This has landed cook stoves and heating stoves in serious controversy. A lab tested Tier 4 cookstove can be Tier 2 in real life – or mistaken for a flowerpot. My first Rocket stoves were often used for this important function in Mexico. 

A lab tested 2 g/hr PM heating stove often emits a lot more smoke when the harmful pollutant is measured from chimneys in houses. In an effort to reduce confounding variables, lab tests show closer to optimal performance. Real life human beings tend to operate stoves with less care, wood is wet, life deserves attention, too.

Maybe the test warnings should have been highlighted in green?

International Standards, 1985

“This is a laboratory test…while it does not necessarily correlate to actual stove performance, when cooking food, it facilitates the comparison of stoves under controlled conditions with relatively few cultural variations.” (Testing the Efficiency of Wood Burning Cookstoves, International Standards 1985).

(ISO 19867-1)

“This document provides a standard test sequence that can be used to compare the performance of various cookstove types, cookstove fuels, and cooking practices under controlled laboratory test conditions, as specified in this document. For evaluation of the performance and predicted outcomes of a cooking system in the field [comprising cookstove(s), fuel(s), cooking vessel(s), kitchen, ventilation, and user(s)], ISO 198691 applies.”

https://www.iso.org/obp/ui/#iso:std:iso:19867:-1:ed-1:v1:en

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

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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

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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!

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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

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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

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“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

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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

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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

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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.