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

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.

A major accomplishment of the past few years has been the creation of thirty Regional Testing and Knowledge Centers (RTKCs). Many of these facilities rely on emissions equipment and training from Aprovecho Research Center. They are usually created as an addition to a university in a developing country, and were initially funded by large development organizations such as the Global Alliance for Clean Cookstoves.

Once a month we’re turning our newsletter over to Sam Bentson, to tell you more about their activities:

Hello to everyone at the Regional Knowledge and Testing Centers (RTKCs), and to our newsletter readers, from Sam Bentson, General Manager at Aprovecho!

Sam was recently in Ghana and Senegal and then visited the Instituto de Investigación y Desarrollo de Procesos Químicos (CPC) in La Paz, Bolivia helping with stove testing and their LEMS emission hood. La Paz has the highest elevation of any government city in the world at an altitude of 3,650m!

The atmospheric pressure at CPC in La Paz is 20Hg. Our lab in Oregon is 241 meters above sea level where the atmospheric pressure is 30Hg. Sam and the CPC staff determined that at their high elevation, and with the voltage applied to the Jet-Flame motor increased to 8V, the mass flow in the Jet-Flame was 82% of the mass flow measured at the ARC lab.

Altitude had a big effect on boiling water and on the Jet-Flame!

CPC in La Paz, Bolivia from left to right: Libertad Mariana Casanova Velasquez, Dalia A. Borja, Sam Bentson, Jazmin Gidari Ruiz Mayta, and Karen Fabiana Paz Quispe

When Sam returned home, he started thinking about keeping in touch with all of his friends at the RTKCs and to share reports of activities. We are starting with CPC and highly recommend that anyone interested in doing research or a stove project make use of this wonderful resource in Bolivia!

Contact:

Marcelo Gorritty
Email: mgorritty@gmail.com
Calle Campos, Esq. Pasaje Villegas.
Edificio Artemis 367. PB Of. 7
La Paz, Boliviawww.cpc-bolivia.org

Testing the SuperPot on a three-stone fire, Batil Camp, South Sudan

ARC engineers rely on feedback from field testing to improve the real-world function of biomass cooking systems. Sometimes the news is challenging, but in this instance the news was very encouraging!

In 2014 the UNHCR (The UN’s Refugee Agency) conducted pilot testing of the SSM SuperPot in seven refugee camps in four countries in East Africa: Kenya (Kakuma, Dadaab), South Sudan (Yida, Maban), East Sudan (Kilo 26), and Ethiopia (Dollo Ado; Bambasi). 

Kakuma: “Tests conducted in Kakuma overall yielded very positive results. The participants confirmed that cooking time is faster, fuel is saved, and water is conserved even if only by a scant amount. Participants agreed that SuperPot is a much better option than the regular cooking pots not only because of the efficiency but they are apparently also easier to clean, saving more energy and water.”

Dadaab: “Smoke expelled from the sides of the pan and does not enter the pot thus no change in the smell and taste of food. SuperPot cooks food faster and thus less firewood used. Less usage of firewood and faster cooking would mean less protection incidents, more time for infant/child care. With the SuperPot there was less heat loss and firewood consumption by wind as most of the surface was covered with the pan unlike the traditional pot.”

Batil: “Significant differences in cooking time were noted: for CSB++ (corn-soy blend flour) the Stovetec SuperPot cooked 8 minutes faster than the local pot; for cereal, there was a difference of 4 minutes. With pulses, super pot cooked faster by 5 minutes. Overall, Stovetec is time efficient. The fuel savings are particularly impressive.”

Yida: “Together, both tests saved women 20 minutes in overall cooking time. According to the participants, this time saved ‘can be used for other productive household economic activities or be dedicated to childcare which will effectively improve the nutrition and health status of the children and the entire household members.'” 

East Sudan: “Testing was conducted at hospital kitchen inside Kilo 26 hospital complex by four people including two cooks and the HAI nutrition coordinator. 500g of lentils were cooked in 750ml of water in both pots on improved stoves. The super pot cooked the lentils in 27 minutes, as opposed to aluminum pot, which took 34 minutes, for a difference of 7 minutes.”

Assossa: “Results indicate that community perspectives are positive for the StoveTec super pot. The water boiled faster in the super pot by 3 minutes and the lentils were cooked 15 minutes earlier on kerosene stove, while also being 9% more fuel efficient than the regular pot. When testing CSB on kerosene stove, super pot was 4% more fuel efficient and saved 7 minutes of cooking time.”

Hilaweyn: “Tests were ran in Buramino Block 13 and Buramino Block 24 Line A with woman groups. In Block 13, the women tested cooking time for 500g of rice over an improved stove (with windshield). The Stove Tec pot cooked the rice faster by 8 minutes. In Block 24, women cooked 500g of lentils over firewood. Stove Tec pot out performed local pot only by 2 minutes. Neither water used nor fuel consumption were measured.”

Summary:

“Results indicate that the super pot is fuel efficient, effective in saving time, safe and well accepted by the community.”

Recommendation:

In their summary report, the UNHCR Food Security and Nutrition Unit advised “Procurement and distribution of SuperPot in select humanitarian contexts within priority countries according to needs of the most vulnerable households.”

For more SuperPot info:
 https://www.ssmstoves.com/Product/Accessories/49.html

To read the summary report: http://aprovecho.org/publications-3/, scroll down to “Pots” section.

The Field Informs the Lab

In Part 1, we gave examples of how field studies can provide unpleasantly surprising results. Rocket stoves were designed to make a little less smoke and use substantially less fuel. So when the rocket stove was field tested by USAID the inventor, Dr. Larry Winiarski, was not surprised that the stove still made smoke. But the ARC team was surprised that it was not a real improvement over the open fire.

In 2011 the goals for cookstoves published by the Department of Energy asked that a stove use 50% less fuel and make 90% less PM2.5 to protect health when used indoors. Now in 2022 stoves are also supposed to address climate change, which means emitting less PM2.5 and hopefully making less than 8% black carbon. Field tests show that we need to make more improvements to meet these specific goals.

How are these reductions achieved in the lab?

  1. Use a chimney to reduce in-home concentrations of CO and PM2.5.
  2. In lab tests, approximately 850°C gases need to flow in tight channel gaps around the pot(s) to reduce the fuel used to cook by about 50%.
  3. Molecular mixing at 850°C (0.2 second residence time) can achieve something like a 90% reduction of PM2.5 (requires forced draft in a Rocket stove).
  4. This mixing reduces greenhouse gas emissions by about the same amount.

Natural-draft and forced-draft TLUD stoves burning pellets and forced draft Jet-Flame stoves burning dry sticks without bark get close to these reductions in the lab. Unfortunately, they frequently do not yet meet these goals in the field.

The lab has to move into the field to learn if current technology can accomplish modern goals. Let’s go!

Next week in Part 3: sometimes field tests show success.

Chart showing turn down ratios and firepower of 18 stoves
Firepower and turn-down ratio of 18 stoves, from “Test Results of Cook Stove Performance”

The ARC/EPA 2011 book “Test Results of Cook Stove Performance” compares performance and emissions, including turn down ratio and firepower, from survey of 18 stoves. Firepower is a measure of how much energy is released per unit of time. More energy is required to quickly boil water. Less energy is needed to simmer food.

The most effective cooking stove should be fuel efficient at both high and low power. The ratio between high and low firepower is called the turn-down ratio (TDR). It is a measure of how well the stove can be “turned down” from high to low power. 

A TDR of 2 means that half as much fuel was consumed while maintaining a simmer, compared to bringing water to boiling. Cooks usually appreciate a stove that is capable of both high-and low-power operation. Many foods will burn if the heat can’t be reduced enough.

It is interesting that the liquid-fueled stoves were generally low powered, at less than 2kW. Most of the wood burning stoves ranged from 8kW to around 6kW. In Mexico, gas stoves can have a hard time cooking tortillas.

The Mud/ Sawdust (TDR 3.9) and VITA (TDR 3.8) stoves had the highest Turn Down Ratio. The average for the other wood-burning stoves without chimneys was 2.4. The average for stoves with chimneys was 2.2. The Gyapa charcoal stove (TDR 2.8) scored slightly higher. 

The chart shows the average high firepower and the low firepower for each stove. It should be noted that in these tests the pot was uncovered, which increases the energy input needed to maintain the water at simmering temperatures.

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.

PM2.5, CO2, CO and other metrics measured by the LEMS are displayed in a real-time graph during testing.

When enough data is available in real time, it is not hard to make progress improving a stove. The LEMS emission hood provides real time, holistic feedback as the experimenter makes changes. Watching all of the measures simultaneously makes it possible to tune a stove like a car. The goal is to burn biomass at high, medium, and low power with Tier 4/5 levels for thermal efficiency and emissions of CO and PM2.5.

It is easier in a forced draft stove.

Watching the real time PM2.5 data on a screen while turning two knobs that control the velocity/volume of primary and secondary air jets going into a combustion chamber quickly establishes a close to optimal compromise resulting in the lowest emissions. The stove “tune-up” process includes making sure that the temperature of the gases touching the pot stay as hot as possible, keeping the thermal efficiency high. Maintaining high temperatures in the combustion chamber is important as well. At the same time, the CO2 (a proxy for firepower) needs to stay around 5kW (high power), 4kW (medium power) and 3kW (low power). Since burning wood does not make much CO, the CO should stay low. The Oxygen (O) sensors (air/fuel ratio) warn us if the available Oxygen is too low.

Establishing a close to optimal balance of PM2.5, temperatures, CO, CO2, O is easier when working on a stove with metered fuel like a TLUD or a pellet burning heating stove. Consistent metering reduces disturbances in the rate of reactions (how fast the solid wood turns into woodgas). A standardized test, such as ISO 19867, provides data on fuel use, time to boil, thermal efficiency, firepower, emissions rates, etc.

After the real time balances look good, we use the more accurate pump and filter PM2.5 system with enough repetitions to establish statistical confidence. It usually takes two to four weeks to tune up a stove. After tuning up several similar stoves, the data can coalesce into time saving design principles. Data derived design principles are more likely to be predictive.

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.