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: http://aprovecho.org/portfolio-item/project-planning/

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 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

Read more about the Chitetezo Mbaula project at unsustainablemagazine.com. Photo by Deogracias Benjamin Kalima

The Chitetezo Mbaula cookstove is distributed by United Purpose in Malawi with the goal of combating deforestation by replacing the traditional charcoal/firewood cooking stoves. In an effort to assist, ARC worked with stakeholders to see how small changes in the stove might translate into fuel and emissions reductions in lab tests. Of course, this information is only useful to researchers in the field as possible iterations. They determine if the changes might translate into practical conservation. The collaboration continues as possibilities are examined.

In its stock form, the stove achieved an average thermal efficiency of 22.5% during three modified laboratory based IWA 4.2.3 tests at high power. As the stove body got hotter, the thermal efficiency increased from 17.6% to 26.6%. The thermal efficiency Tier rating was 1, and PM2.5 emissions, at 1093.3 mg/MJd, gave a Tier rating of 0.

Simple Adjustments Make Some Performance Improvements

Three one inch in diameter holes were drilled through the back of the clay stove body with the intention of allowing more air into the charcoal bed. The pot gap on top of the stove was also reduced to 6mm. These two changes resulted in an average increase of thermal efficiency with char from 22.5% to 29.6%. 

The CO emissions factor per energy delivered to the cooking pot decreased from 10.45 g/MJd to 5.63 g/MJd, although at the same time the firepower decreased from 8.9 kW to 5.9 kW. Natural draft stoves with lower firepower tend to make less emissions. Since the time to boil (normalized to 75°C temperature rise) also increased from 22.4 minutes to 25.2 minutes, further study is needed to determine if the reduction in CO emissions also occurs at 22.4 min to boil. 

Jet-Flame and Pot Skirt Increase Efficiency, Reduce PM2.5

The Shengzhou Stove Manufacturer Jet-Flame was then inserted into the stove body with a metal Rocket combustion chamber. A 6mm channel gap metal skirt was also used around the 5 liter flat bottomed pot. With these changes, the stove achieved an average thermal efficiency of 47.7% during three laboratory tests at high power. As the stove body got hotter, the thermal efficiency increased from 44.9% to 52.3%. The IWA thermal efficiency Tier rating was 4. Since all of the tests scored within Tier 4, which is the maximum score under the ISO IWA, the 90% confidence interval of the Tier rating was 4 to 4. The PM2.5 emissions of the stove were 69.0 mg/MJd and the Tier rating was 3.

When tested in the field, ARC roughly estimates that emissions will be something like three times higher. This is a “rule of thumb” that is not meant to be an accurate guess but a reminder that many researchers have found that emissions in the field are much higher compared to lab test results! The lab test can point out theoretical “improvements” but only field testing can determine actual performance and practicality. On the other hand, if cooking takes place outdoors, as in the photo above, exposure to harmful smoke can be estimated to be dramatically reduced by the increased air exchange rates.

Last week we wrote about using the LEMS to tune up a stove, so it makes sense to share the actual results of a recent test series with you this week.

When ARC makes an Open Fire, we often use three bricks on end to hold up the pot. The bricks are 16cm high. It has been fascinating to experiment with the SSM Jet-Flame in the open fire to try and determine how fuel-efficient and clean burning the combination can be. Last month, we spent a couple of weeks changing one thing at a time and then completed nine 30-minute ISO high power tests on the close-to-optimized design.

Here are the test results:

test results chart

Description of the changes

  • We kept the pot height at 16cm above the top of the Jet-Flame.
  • Three rebar supports held up the pot replacing the heavier and bulkier bricks.
  • A short 6cm high by 18cm long FeCrAl fence kept the sticks on top of the combustion zone in the Jet-Flame.
  • A lightweight Winiarski 304 stainless steel “0.7 constant cross sectional area” stovetop increased the heat transfer efficiency from the hot flue gases into the pot.
  • Thermal efficiency was also improved with an 11cm high pot skirt creating a 6mm channel gap on the sides of the 26cm in diameter pot.
  • We learned that the sides of the open fire should be partially enclosed for best performance. A 5cm high opening at the lower portion of the sides of the open fire allowed fresh air to enter the combustion zone. 11cm of the upper portion of the sides of the Open Fire were enclosed with aluminum foil.
  • To make sure that there was no backdraft, a 7cm tall, 14cm wide and 7cm deep metal fuel tunnel was added on the outside of the sides of the partially enclosed Open Fire.

Photo of the experiment

Conclusion

It looks like a Rocket combustion chamber may not be needed to achieve Tier 4/5 results from an “Open Fire” when tested in a lab. A short fence that holds a single layer of sticks on top of the primary air jets seems to be as good.

High temperatures in the combustion chamber seem to have both positive and negative effects on emission rates of biomass. Higher temperatures lower the residence time needed for more complete combustion. At the same time, especially with dry wood, the rate of reactions (how much wood gas is being made per unit of time) is increased. If wood gas is made too quickly, some of it can escape unburned. Our experience has been that in a hotter combustion chamber the fuel must be metered into the fire more slowly to lower emissions. In Rocket and TLUD stoves, the rate of reactions must be controlled to eliminate smoke.

Experiments have shown that elevated temperatures shorten the combustion time for CO and PM 2.5. At 900°C the combustion time required for complete combustion is less than half the time needed at 700°C for biomass particles (Li, 2016). At 900°C, a residence period of 0.5 seconds resulted in close to complete combustion of well mixed CO and PM 2.5 (Grieco and Baldi, 2011; Lu et al., 2008; Yang et al., 2008).

Boman (2005) reports that temperatures above 850°C in a 5kW combustion zone combined with air rich and well mixed conditions for 0.5 seconds resulted in an almost complete depletion of particulate matter. The use of insulation in Rocket stoves can create a combustion zone with temperatures above 1,000°C. Forced draft TLUDs can generate similar temperatures in the secondary air mixing zone above the fuel bed.  Interestingly, when temperatures are around 850°C the near complete combustion of well-mixed carbon monoxide and particulate matter seems to require short residence times. Both forced draft Rocket and TLUD stoves can minimize the emissions of products of incomplete combustion even though the residence time is very limited.

Recently, we ran a series of fifteen experiments trying to optimize performance in a low mass Rocket stove with Jet-Flame. Since we were testing with dry wood we had to be careful not to over insulate the combustion chamber. Insulation made the whole length of the stick catch on fire increasing the rate of reactions and firepower.  When more than 8cm of the stick was burning more mixing was needed to achieve close to complete combustion. When only the tips of the sticks were on fire the metering of woodgas into the fire was slower and less mixing was required.

As seen in the following graph, the emissions of CO were again shown to be reduced at higher temperatures. As a rule of thumb when designing a stove, we try to create temperatures above 700°C about 6cm above the fire, at a minimum.

Testing a Chitetezo Stove with a Jet-Flame

What do I like most about Aprovecho Research Center? We have created a “culture of daily experimentation.”

Scheduling two to three experiments every morning means that we can try less-likely-to-succeed variations. That’s great because experiments that don’t succeed in improving a stove often reveal interesting and possibly unexpected results. And that can lead to new insights. “Accident favors the prepared mind,” Pasteur reminds us.

Nordica MacCarty, our new Executive Director, recently suggested that it would be a good idea to see what happens when the Jet-Flame is used without a Rocket combustion chamber. We have been working with Christa Roth and Christoph Messinger from GIZ, investigating what happens when the Jet-Flame is used in the Chitetezo stove. It was easy to continue the investigation into the effects of primary air jets blowing up into the fuel. We removed the rocket-style combustion chamber that we were testing, and put a short metal fence around the Jet-Flame to keep the sticks on top of the air holes. A SuperPot was used and set directly on top of the stove.

It’s been very interesting to try different sized sticks with the Jet-Flame. Generally, small sticks burn faster and make a lot of flame, but also more smoke. Two 4 cm x 4.5 cm sticks burn more slowly and emit less smoke but the firepower is lower. With no insulation around the combustion zone, we tried burning four sticks that were 2 cm x 2.5 cm. The results were surprisingly good: Tier 4 for thermal efficiency, Tier 4 for PM2.5, and Tier 5 for CO.

The size of the sticks made a significant difference on all three measures, which makes sense and has been seen in natural draft stoves. But it’s noteworthy that the size of the sticks results in big differences with the Jet-Flame. Following up on Nordica’s suggestion made for a more interesting week here at the lab. Completing ten to fifteen half-hour experiments weekly makes it more likely that progress is being made.

Reading/learning in front of the computer is important, and OK for a while, but we also like to get into the lab to try and create/learn something new.

The Gates funded Global Health Labs and ARC/SSM invented the Jet-Flame

Global Health Labs, Shengzhou Stove Manufacturer and Aprovecho teams show off the Jet-Flame
Global Health Labs, Shengzhou Stove Manufacturer and Aprovecho teams show off the Jet-Flame

Shengzhou Stove Manufacturer manufactures the Jet-Flame (jet- flame.com) that is being field tested in over 30 locations. Our lab helped to create this accessory that is designed to reduce emissions while increasing thermal efficiency and reducing time to boil. 30 pre-heated primary air jets shoot up into the fire resulting in increased molecular mixing and elevated temperatures. Smoke is reduced by about 90% compared to an open fire.

A forced draft stove can be very clean burning, but start up may create a lot of PM2.5. This is because the cold combustion chamber can allow a higher percentage of the smoke and Carbon Monoxide to escape unburnt. David Evitt, COO of ASAT (the for-profit arm of ARC), invented a method for lighting the Jet-Flame that can be a lot cleaner.

  • Wet 30 grams of left over charcoal with 10 grams of alcohol.
  • Place the small pile of charcoal on top of the holes in the Jet-Flame.
  • Light the charcoal.
  • Turn on the Jet-Flame.
  • Push the tips of the sticks of wood against the pile of burning charcoal.
  • Keep on pushing the sticks into the fire as the tips are consumed.

Here’s a video showing how we light fires in the lab:

Years of experimenting with stoves at ARC taught us that a high mass combustion chamber absorbs a lot of heat that could be going into the cooking pot. That led us to presume that efficient combustion chambers had to be lightweight, which are harder to make. Once we started experimenting with forced draft, we were surprised to learn that adding forced draft (FD) to a TLUD or a Rocket stove increases the temperature of the gases and largely overcomes the negative effect of a high mass combustion chamber.

The Oorja FD-TLUD and the FD-CQC mud brick stoves generate temperatures of around 1,000°C in the combustion chamber. The option to use a high mass combustion chamber lowers cost and dramatically increases durability when designing forced draft, health protecting, affordable, clean burning, carbon neutral stoves.

The combustion chamber in the pellet burning FD-Oorja that we have in the lab is made from castable refractory ceramic and is over 20 years old. The retail price of the 400,000 British Petroleum Oorja stoves sold in India was around $18. The CQC Rocket combustion chamber is less expensive and is manufactured from sand, clay, and cement. With the addition of forced draft via a Jet-Flame, it reaches Tier 4 for  thermal efficiency and PM 2.5.

To replace health protecting stoves that use natural gas (a fossil fuel) it seems likely that FD-TLUDs and FD-Rockets can be built with lower cost and 10 year durability combustion chambers. The renewably-harvested-biomass fueled stoves need to be manufactured and field tested, and there is a lot of ground to be covered (an understatement), but it’s great that harder-to-make low mass combustion chambers may not be necessary.

Champion Forge, circa 1920

The blacksmith’s forge is probably the most familiar technology that blows jets of primary air up into charcoal or coal, resulting in the high temperatures needed to shape or melt metals. A forge needs air at high pressure (10” water column) to do its work. A fan (usually a radial pressure blower) capable of moving air against significant resistance is required. You don’t need a big volume of air. What is needed is pressure to keep the air moving and to create molecular mixing of the woodgas and flame.

The Jet-Flame stove accessory
The Jet-Flame Stove Accessory

The SSM Jet-Flame

The Jet-Flame uses the same technology as a forge. It required a year of R&D to create a low cost, 1.5 Watt fan that delivers sufficient pressure into a Rocket stove fire. Thirty 2mm jets of air at 0.4 to 2 inches of water pressure blow up into the charcoal under the burning sticks of wood. When the pressure rises and the vertical jets of air penetrate further into the charcoal and fire, the following effects are seen:

• Temperatures rise in the combustion chamber, resulting in higher thermal efficiency as hotter gases flow past the pot. The higher temperatures also result in lower PM2.5 and CO. However, proper metering of the fuel, mixing caused by turbulence, and sufficient residence time are as important for decreased emissions of PM2.5 and CO. With higher temperatures, the related measures of firepower and CO2 also rise, while the fuel/air ratio decreases – as the fire increases, more oxygen is consumed. Increasing the pressure also increases firepower even when a constant fuel load (usually two 4cm by 4.5cm sticks) is being burned. Larger sticks have a lower surface area to volume ratio and make less smoke. When the outer surface of the sticks are covered with charcoal, emissions decrease as well.

• In a Rocket stove that has a large fuel opening, natural draft pulls room air into the combustion chamber. With forced draft adding more air, the average Lambda in a Rocket stove tends to stay above two times stoichiometric. The jets of air blowing into the charcoal result in higher temperatures as pressure increases resulting in temperatures over 1,000°C even at 2 to 5 Lambda. Secondary air jets blowing into the fire, on the other hand, have a cooling effect. In a Rocket stove with a Jet-Flame, the varying fuel/air ratios are not related to the emission rates of PM2.5 and CO.

• Higher temperatures caused by greater pressure also have detrimental effects. The percentage of black carbon is higher when temperatures/firepower/CO2 become elevated – hot yellow flames cause the formation of black carbon. The lifespan of affordable refractory metals is greatly decreased by very high temperatures such as 1,000°C. A durable refractory ceramic material is better suited to higher operating temperatures. Replacing metal combustion chambers with low mass, refractory ceramic was recommended by the 2011 DOE biomass stove conference. https://www1.eere.energy.gov/bioenergy/pdfs/cookstove_meeting_summary.pdf

• Lowering the firepower by adjusting the pressure in the air jets assists the metering of fuel to achieve a 3 to 1 turn down ratio. When the pressure is too high, the fire can be blown out when the charcoal has disappeared.  A layer of charcoal under the fire (or charcoal coating the sticks) helps to maintain the fire and lower emissions as charcoal emits much less PM2.5 compared to wood. The levels of CO tend to rise when the flame above the sticks decreases. The amount of flame above the sticks seems to be related to lower emissions of CO and PM2.5. The amount of flame above the burning sticks, the metering of the fuel, and the mixing of the wood gas are not as easily quantified as temperature and residence time but are as important for more complete combustion.

In our newsletter “Making It Real,” we described how feedback from the field in Rwanda suggested that the Jet-Flame’s power cord would last longer if the whole device was inserted from the side of the combustion chamber. (It was originally designed to go through the door, with the sticks placed on top.) So of course we ran some tests, and discovered more benefits.

Is the Jet-Flame, when inserted into the combustion chamber from the side of the CQC stove, as effective in reducing emissions as when it enters through the fuel door?  

Yes, performance seems to have even improved a bit. After testing the Jet-Flame with side entry, it seems that it’s better to get the hot metal out from under the parts of the fuel that you don’t want to heat up. To burn cleanly, natural draft Rockets like to burn something like 8cm of the end of the sticks. Instead of laying the entire length of the sticks on the heated metal of the Jet-Flame, the side entry only exposes a limited amount of the sticks to high temperatures.

As seen in the photo, the sticks are now supported by a white homemade high mass brick and only the tips are exposed to Jet-Flame heat well inside the stove. It’s nice how a suggested change from Jean Marie Kayonga in Rwanda ends up having some unexpected benefit, not just better protecting the cord. Thanks again, Jean Marie! www.Jet-Flame.com

The time to boil, thermal efficiency, temperature in the combustion chamber, CO, and PM were improved with side entry while firepower rose. Excess air fell from 3.38 times stoichiometric to 2.57. I liked operating the stove because the sticks seemed to burn more at their tips as Dr. Winiarski described in the Rocket Design Principles. See: http://bioenergylists.org/stovesdoc/Still/Rocket%20Stove/Principles.html