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

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

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

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.

A Recent History of the Rocket Stove: 2009

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.

Learning From The Field, Part 2

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.

Exploring Horizontal Gasifiers

What happens if a bunch of sticks are bundled together, lit at the tips, and inserted into a well-insulated horizontal enclosure that enters a Rocket stove providing draft?

Well, it’s hard to get everything to work well. As Dr. Winiarsky pointed out “Gasifiers are finicky.”

But, when all of the tips of the bundle of sticks are lit at the same time, and there is the right amount of primary and secondary air, then the fire moves horizontally in the well insulated enclosure away from the Rocket stove and emissions can be decreased.

Working with the Gates funded Global Health Labs in 2017, ARC experimented with various horizontal gasifier prototypes and sometimes the results (Tier 4 for PM2.5) were encouraging.

ARC decided to explore the development of primary air jets up into the fire as a more promising technique and left the horizontal gasifiers on the shelf. But who knows, someone may find the concept interesting and continue the investigation of the horizontal gasifier and make it less finicky?

An SSM Jet-Flame Optimized “Open Fire”

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.

Mass, Insulation and Thermal Efficiency

Kuniokoa Stove, original top replaced with cast iron top.

It is more likely that close to 50% thermal efficiency will be achieved with a biomass burning stove when:

  • Small sticks are burned that produce tall, hot flames while using the least amount of wood.
  • A 30cm in diameter aluminum pot is used with a 14cm high pot skirt that creates a 6mm channel gap.
  • The stove top (with 6mm pot supports) weighs as little as possible. The narrow channel gaps in the stove top effectively deliver wasted heat from the hot gases into the stove top while increasing beneficial convective heat transfer into the pot, so less mass to hold the heat is better.
  • A grate helps the sticks to make tall, hot flames and reduces the made charcoal.

Starting with all of the above, we tested various Rocket stove combinations to try to determine the effect of mass in the combustion chamber. The Kuniokoa Rocket stove is the lightest Rocket stove in our museum – it is made from sheet metal without insulation. (A refractory metal combustion chamber lasts longer when uninsulated.) When tested at high power (4,645 watts) the thermal efficiency was 51.7%, PM2.5 was Tier 2, and CO was Tier 3. Thermal efficiency dropped to 46.1% when we exchanged the Kuniokoa sheet metal stove top (0.31 kilo) with a cast iron version (2.36 kilo).

A similar Shengzhou Stove Manufacturer (SSM) Rocket stove was tested with a refractory cement combustion chamber (2.7 kilo) surrounded with rock wool insulation. The stove top was made from lightweight 304 stainless steel. When tested at high power (4,816 watts) the thermal efficiency was 48.6%, PM2.5 was Tier 2, and CO was Tier 3. The refractory cement combustion chamber is heavier but it can be insulated because the material has a working temperature of 1,100°C.

When a SSM lighter refractory ceramic combustion chamber (1.2 kilo) was exchanged into the SSM Rocket stove with rock wool insulation and a lightweight 304 stainless steel stove top, the thermal efficiency (at 4,709 watts) rose to 51.4%, with Tier 2 for PM2.5 and Tier 3 for CO.

  • It may be that insulating a one kilo combustion chamber in a Rocket stove offsets the disadvantage of the higher mass when compared to uninsulated sheet metal.
  • In these tests, adding another kilo to the insulated combustion chamber in the SSM Rocket stove lowered thermal efficiency from 51% to 46%.
  • When the mass of the stove top was increased from 0.3 to 2.3 kilos, thermal efficiency dropped by about 5%.

Temperature, CO & PM 2.5

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.

Video: Lighting a Fire With The Jet-Flame – No Smoke

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: