Mixing with Primary and Secondary Jets of Air

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Regardless of the velocity of secondary air, flow rate, or the angle at which air is injected into the fire, secondary air tends to lower the temperature of gases. Researchers have found that injecting secondary air into the side of the flame in a Rocket stove results in most effective mixing.*

The Jet-Flame, on the other hand, blows primary air jets up into the bed of made charcoal below the burning sticks of wood, creating a “mini blast furnace.” The jets of primary air increase the temperature in the charcoal, frequently resulting in higher temperatures in the combustion chamber. The mixing function is up into the fire, not into the side as with secondary air jets.

Boman et al., 2005 report that temperatures of 850C or above are needed for close to complete combustion in short residence times, as in a cookstove. Since excess air lowers temperatures, using the minimal volume of air in secondary air jets to achieve thorough mixing seems preferable. Researchers have recommended that the jets should penetrate into the middle of the flame but not enter into each other. (*Lefebvre and Ballal, 2010; Udesen, 2019; Vanormelingen and Van den Bulck, 1999).

Unfortunately, raising the temperature of pre-heated secondary air by a lot more than ~ 100C seems to be difficult. Cookstove combustion chambers are usually small, limiting the area exposed to high temperatures. The heat transfer efficiency is much lower from degraded temperatures further from flame.

 Residence time and temperature are easily measured. However, “thorough mixing” has not been defined and is not yet measured in our experiments. We infer that the woodgas/air/flame was thoroughly mixed when the emissions of PM2.5 and CO are close to zero as measured with the LEMS emissions hood. 

Metering!

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Watching a Rocket stove or a pellet stove (as above), it becomes obvious that metering the fuel is a primary factor in achieving close to complete combustion. When too much fuel is introduced into the combustion chamber, the emissions of smoke increase almost immediately.

For the clean burning of biomass, the controlled metering of fuel seems to be as necessary as it is in the engine of an automobile. The rate of reactions (how fast the solid biomass is being converted into wood gas) is then matched with the corresponding amounts of Time, Temperature, and Turbulence required to minimize CO and PM2.5.

ARC has added Metering to Time, Temperature, and Turbulence while unsuccessfully searching the thesaurus for a synonym that starts with the letter T. Maybe someone can succeed where we have failed?

African Mud Stoves with Chimneys

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Damon Ogle was the Technical Director here at ARC

Damon Ogle and the ARC staff have a long history, starting in Central America and Mexico, listening to folks praising their stoves with chimneys. There are now millions of beautiful Latin American kitchens in which the dangerous smoke is transported out of the house, as it is in the USA/Europe. The Rocket stove can be about 50% more fuel-efficient compared to the open fire, so about half the smoke is made. But that is not good enough to protect health inside a home.

Although health-protecting chimneys are seen in Latin America and India, it’s rare to see chimneys in Africa.  

One simple African stove with chimney is seen above. A sunken pot (or pots) sits down near the fire exposing its bottom and sides to the flame. The pot seals into the hole and the smoke flows up the chimney, not into the lungs of the cook and her children. 

Since 1976, ARC has continued to work with local communities worldwide to try to save fuel and protect health. Trying to protect climate requires very clean combustion and we’re working on that, too.

50% Thermal Efficiency Depends on Several Factors Including the Surface Area of the Pot

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Illustration from The Smithsonian’s explanation of how a boundary layer works 

A boundary layer of still air on the bottom and sides of a pot keeps the hot gases from actually contacting the surface and is a dominant factor in heat transfer efficiency.

  1. According to Newton’s Law, doubling the surface area doubles the heat transfer when the temperature and velocity of the gases are constant.
  2. In a Rocket stove at high power, the gases can be around 800C and the velocity can be around 1.2 meters per second.
  3. Keeping a constant cross-sectional area in the pathway the gasses take through the stove is important. Reducing the constant cross-sectional area channels under and around the sides of a pot to 0.75 of that area helps to keep the gases hot and flowing at highest velocity.
  4. The 0.75 cross sectional channels encourage the gases to thin the boundary layer increasing heat transfer.
  5. Pots have to have sufficient external area to achieve 50% thermal efficiency.
  6. In recent tests of optimized Rocket stoves, a pot with an area of around 800cm2 scored 34% thermal efficiency. Increasing the area to around 1000cm2 increased thermal efficiency to about 40%. In the same stove, a pot with 1200cm2 can be expected to result in above 45%. We use 26cm to 30cm in diameter pots with at least 5 liters of water to get closer to 50% thermal efficiency.
  7. Keep in mind that increasing the surface area of the water in a pot also increases the amount of steam, which makes bigger pots harder to bring to full boil without a pot lid.
  8. Thermal efficiency, when burning biomass, tops out (so far) at around 55%. The gases in the channels at the bottom and sides of the pot loose temperature and velocity resulting in an upper limit to heat transfer efficiency. 
  9. Raising the temperature and velocity of the gases will increase efficiency.

Secondary Air in TLUDs and Rocket Stoves

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Forced draft mixing with 2nd air jets in Dr. Tom Reed’s WoodGas Stove at around 1,000C

Forced draft mixing with preheated jets of primary air reduced emissions of PM 2.5 by around 90% in our stove tests with the Jet-Flame. Would adding secondary air jets further decrease emissions?

Secondary Air Works in TLUDs

Lefebvre, Vanormelingen, and Udesen examined secondary air jets air in cylindrical combustion chambers and describe most successful patterns of penetration depth. Jet penetration lengths approaching the middle of a cylindrical combustion chamber resulted in a maximum reduction of PM2.5 emissions. An increase in the number of jets created more thorough mixing. It was important to have the jets meet in the middle, but with minimal necessary force, to ensure highest temperatures and highest velocity of hot gases to the pot.

Forced draft secondary air jets can decrease the upward draft in a cylinder. Jets of air aimed horizontally into the flame most efficiently create mixing. But even when aimed upwards toward the pot they create a ‘roof of air’ that slows the draft by creating a high-pressure front.

Regardless of the velocity of secondary air flow rates, or the angle at which air is injected into the combustion chamber, supplying secondary air also tends to significantly lower the temperature. For this reason, using a minimal amount of air was found to be best. There is a reported balance resulting in optimized mixing, draft, residence time, and temperature. (Lefebvre, 2010) (Vanormelingen, 1999) (Udesen, 2019)

How Do We Add Secondary Air Successfully to Rocket Stoves?

One obvious difference between TLUDs and Rocket stoves is the large fuel door in the side of the Rocket stove. A TLUD is an open topped cylinder with a small amount of primary air entering the batch of fuel from below the packed fuel bed. In the TLUD, the fuel is initially dropped into the cylinder, while in a Rocket stove horizontal sticks are pushed into the combustion zone through a fuel door. The pressure/volume of secondary air jets introduced into a Rocket stove may be limited because the high-pressure front can create a backdraft that sends smoke out of the fuel door.

Supported by funding from The Osprey Foundation, ARC is currently experimenting to determine: 1.) How much pre-warming can be achieved and 2.) What is the most effective pressure/volume for secondary air jets in a forced draft Rocket stove.

How To Make An Institutional Stove With Chimney

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Dr. Mouhsine Serrar and the Rocket institutional stove designed by Dr. Larry Winiarski

There are at least three ways to make institutional stoves with chimneys, all of which work well and save fuel and decrease emissions. Here they are:

  1. Shell Foundation supported the making of an eight-part video, a step by step guide to making a 50 to 100 liter institutional Rocket stove, with a heat resistant metal Rocket combustion chamber. It is a great stove with lots of successful field testing but it costs the most to construct because heat resistant metals like 410 stainless or FeCrAl are increasingly expensive. The super insulated combustion chamber requires these types of metal. 304 stainless will not last. https://youtu.be/VdhLWMW7IXA
  2. Cooking With Less Fuel: Breathing Less Smoke shows how to make the same institutional stove using bricks for the Rocket combustion chamber. Construction details can be found at aprovecho.org in the publications section. The book was written with the World Food Program in Rome. This is a less expensive stove that is slightly less fuel efficient at cold start but lasts longer and is easier to make in places where 410 stainless and FeCrAl are not available.
  3. Making a VITA style institutional stove without a Rocket combustion chamber is the least expensive way to create an institutional stove. The open fire under the pot is supported on a grate and the hot gases flow up the inside of the skirt, down the outside of the skirt and exit out the chimney placed below the bottom of the pot as in the Rocket stoves shown above. You can find a video we made about constructing the VITA stove at: http://aprovecho.org/video-gallery/

Lots of manufacturers do not use the chimney but we think that protecting health is very important. We try to follow Don O’Neals advice (HELPS International) to always include chimneys whenever possible, imagining our mothers cooking and getting ill from exposure to the harmful emissions without the protection of the chimney.

The Jet-Flame in a home made CQC Rocket Stove

Jet-Flame Research Results From Six Types of Biomass Cookstoves

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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

Smokestacks belch out smoke, spelling out CO2 in a blue sky. A Euro symbol floats to the right.

A Recent History of the Rocket Stove: 2022

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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!

A Recent History of the Rocket Stove: 2016-2021

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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.

A Recent History of the Rocket Stove: 2011-2015

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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.