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: mattias@emerging.se. 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!

In 2021, ASAT (the for profit arm of ARC) won the Small Business Administration’s Tibbetts Award for work funded by their Small Business Innovation Research (SBIR) program, awarded through the U.S. Environmental Protection Agency (EPA). ASAT Inc. staff pose with their Tibbetts Award: Sam Bentson, David Evitt, Jill Allen, Dean Still, Kim Still, and Dr. Nordica MacCarty.

The investigation of how to reduce emissions and fuel use in biomass stoves continued with support from an EPA SBIR award. Two products were manufactured by our Chinese partner SSM, a heating/cooking stove and the Jet-Flame, a $12 insert that has made stoves 67% cleaner burning in field tests. https://www.jet-flame.com/

The Gates funded Global Health Labs (Dr. Daniel Lieberman) also worked with ARC/ASAT and BURN (Peter Scott) to improve the Rocket stove. BURN and ARC/ASAT added fan driven mixing to the Rocket stove.

Learning how to optimize the use of high pressure jets of air at high, medium, and low power required hundreds of experiments. Different pressures are needed as firepower is adjusted. The size of the fuel also affects emission rates. Experiments under the LEMS hood determine the location of jets, pressure, and volume of air for varying applications.

Dr. Samuel Baldwin

In 2011, Dr. Samuel Baldwin at the Department of Energy (who wrote the Bible on cook stoves in 1987) organized a two-day 100 person conference to identify how cook stoves could be improved and manufactured. Key recommendations were:

  •  At least 90% emissions reduction and 50% fuel savings are appropriate initial targets for biomass cook stoves. 
  • Multiple stove designs will be needed to accommodate a variety of cooking practices, fuels, and levels of affordability.
  • Technical R&D should guide and be guided by field research, health, social science, and implementation programs. At every stage, laboratory and fieldwork should be integrated into an iterative cycle of feedback and improvement.
  •  The cost and performance tradeoffs associated with the use of processed versus unprocessed fuels should be explored. While processed fuels can improve stove emissions and efficiency, the processing adds additional costs and these fuels may require a fuel distribution system.

From 2013-2015, ARC received a grant from DOE and spent three years establishing a baseline of stoves in use and then improved five types of stove prototypes with the iterative development process using the LEMS emission hood. The lab testing showed how combustion and heat transfer could be improved in those five types of stoves with the hope that field testing would evolve useful products that use less fuel and make less smoke. A book was written: Clean Burning Biomass Cookstoves, (2015) available on the publications page. The book was updated in 2021.

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.

Kirk Harris has been investigating TLUDs for decades and, as far as I know, his natural draft TLUD burning pellets achieved the lowest natural draft recorded score for PM2.5:  0.7mg/minute at high power (Lawrence Berkeley National Laboratory). This video shows Kirk in China at Shengzhou Stove Manufacturer where Mr. Shen built a copy of his stove to start the process of possibly manufacturing it.

The fascinating aspect in the video is how fast the flame is swirling, keeping the flame below the level of the pot and increasing dwell time.

Adding a fan shaped static mixer between the hole in the concentrator ring and the bottom of the pot has become commonplace in various TLUDS since Kirk invented the technique. We recently added a fan shaped static mixer in a natural draft TLUD to get rid of creosote. The tars were burned up in the hot, swirling flame.

Keeping the flame below the cold surface of the pot is always helpful and can be achieved with the Jet-Flame and in both natural draft and forced draft TLUDs.

To last long enough for commercial/carbon success, the combustion chamber has to be made with cast refractory ceramic. Making the static mixer and combustion chamber from cast refractory ceramic dramatically increases longevity. The Oorja stove in our lab has lasted for about 20 years!

I imagine Dr. Tom Reed smiling in heaven as the stove community moves closer to optimization with:

  • clean burning pellets 
  • a well-engineered TLUD 
  • a refractory ceramic combustion chamber 

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.