link to Rocket Stove 2020 YouTube video

How can burning wood, agricultural waste or even cow dung be a carbon neutral energy source? How do you start a fire without making a lot of smoke? How can a metal skirt around a cooking pot help with fuel efficiency? Dean Still has the answers for you in this new video.

Find out more about the Jet-Flame combustion accessory used in this video at www.jet-flame.com.

Computer screen shows emissions rates as lines on a graph

David Evitt, ASAT COO, and Sam Bentson, ARC GM, have been adding capacity to the Laboratory Emissions Monitoring System (LEMS). So far, four oxygen sensors, temperature probes in the fire and under the pot, and a velocity sensor give us a clearer picture of what’s going on in a stove. Knowing PM2.5, CO, CO2, and firepower at the same time, combined with the improved testing protocols in ISO 19867, is making us more confident that iterative improvement can be accomplished relatively quickly.

During a test, the LEMS screen shows real time emission rates of CO2 (blue line), CO (red line), and PM2.5 (black line).

Starting in early December, Dean Still and a research assistant will be doing 20 tests per week to create an optimized forced draft insert that cleans up the combustion of found biomass fuels and improves thermal efficiency in open fires, high mass, and Rocket stoves. A screen in the hood showing real-time data helps reduce the needed repetitions to achieve statistical confidence.  The 90% confidence interval has to be less than 1/3 of the range of the Tier that contains the conservative bound of the confidence interval. When Tier Confidence Interval Range is equal to or less than 0.33, the number of tests is deemed sufficient to meet the Aprovecho data repeatability quality standard (seven to nine tests each for high, medium, and low power are usually sufficient).

David is in grad school with Dr. Nordica MacCarty at Oregon State University and the ARC lab is supplying them with information. We’ll keep you in the loop as we make discoveries. Part of the goal is to keep the optimized insert as close to a $10 wholesale price as possible.

Here we go! Eco-Science marching forward!

YouTube Video explains the importance of mixing for clean combustion

In this video, Dean Still explains why mixing air into flame is important for cleaner combustion. He uses several Rocket Stoves to demonstrate the effects of both natural draft and forced draft secondary air jets. Which style is more effective? Watch to find out!

For a simple way to add mixing to a Rocket Stove, check out the Jet-Flame.

Ornate chimneys at Hampton Court Palace, London
Ornate chimneys at Hampton Court Palace, London
Multiple ornate chimneys grace Hampton Court Palace.

An Important Health Intervention

When cooking stoves are tested in the field the emissions of PM2.5 and CO are often higher than lab results (Roden et al., 2009). The wood can be wetter, the fire is made with less attention, and many real life variables create higher levels of pollution. It’s hard to imagine that unvented cookstoves for indoor use can be invented that will protect health when too much wet fuel is pushed quickly into the combustion chamber. Even modern cars make a lot of smoke when trying to combust bad quality gasoline.

Clean burning stoves require clean fuel just like automobiles. The sticks of wood need to be relatively dry and the metering of the sticks into the combustion chamber cannot happen too quickly. Perhaps batch fed pellet stoves will have more similar lab and field results if the pellets are well made, dry, and clean?

It’s illegal to install most types of unvented combustion devices in the United States and Europe. Even natural gas room heaters and gas cooking stoves are vented. For realistic protection of health, ARC consultants try to attach chimneys to biomass cookstoves whenever possible. When the stove smokes at least the pollution goes outside above the roof line where it becomes diluted.

Health Supportive Alternatives

Adding a chimney is not always a possibility. In these cases, it is helpful to move cooking out of the closed kitchen, for example under a veranda in the open air. Increasing air exchange rates by cooking under a veranda has been shown to dramatically lower concentrations of harmful PM and CO. Even opening the door and window in a test kitchen lowered the particulate matter 1-hour concentrations between 93% to 98% compared to the closed kitchen, and the CO 1-hour concentrations were 83% to 95% lower (Grabow et al., 2013).

Hundreds of years ago in Europe chimneys were developed as a first step to take smoke and gases outside of the kitchen. In the United States millions of wood burning heating stoves are used indoors every winter. Chimneys transport the pollution outdoors where it is mixed with the outside air.

References

Roden, C. A., Bond, T. C., Conway, S., Osorto Pinel, A. B., MacCarty, N., & Still, D. (2009). Laboratory and field investigations of particulate and carbon monoxide emissions from traditional and improved cookstoves. Atmospheric Environment, 43(6), 1170–1181. https://doi.org/10.1016/j.atmosenv.2008.05.041

Grabow, K., Still, D., & Bentson, S. (2013). Test Kitchen studies of indoor air pollution from biomass cookstoves. Energy for Sustainable Development, 17(5), 458–462. https://doi.org/10.1016/j.esd.2013.05.003

Dean Still explains time and temperature in a Rocket Stove in a YouTube video

Dean Still and Sam Bentson have started collaborating on a series of videos that explain the basics of how Rocket Stoves work, so that stove designers and stove users can get the best performance out of this popular stove design. In this first installment, “Time and Temperature,” Dean explains the importance of high combustion temperature in a Rocket stove where there is limited time to burn up smoke particles. He demonstrates how the Jet-Flame (www.jet-flame.com) helps to increase combustion temperature by blowing air under the fire.

Be sure and subscribe to Sam’s YouTube channel so you never miss an episode! New videos will be added every other week.

Sam Bentson trains Bernard Kabera and colleagues to use the new stove lab equipment

Aprovecho’s General Manager Sam Benston recently returned from a trip to Rwanda, where he helped to set up a new ISO compliant cookstove lab. Here are some photos and information from Sam about his work there:

I was installing the LEMS (Laboratory Emissions Monitoring System) and PEMS (Portable Emissions Monitoring System) and the rest of the new ISO 19867 cookstove laboratory at the Rwanda Standards Board in Kigali. The lab started as an empty room full of equipment in boxes. I trained the laboratory staff on the set-up and use of the equipment for cookstove evaluations according to ISO 19867. Shortly after I left there was a Grand Opening to celebrate on the ISO’s World Standards Day. Here is a twitter link with photos:  https://twitter.com/REMA_Rwanda/.  Our new PEMS with the battery powered gravimetric system is visible.

The PEMS is visible here at the launch of the Cook Stove testing lab in Kigali
The PEMS is visible here at the launch of the Cook Stove testing lab in Kigali.
Photo via @REMA_Rwanda

Aprovecho provides a turnkey cookstove testing laboratory which is useful for cookstove performance certification, design, and basic research. The lab is centered around the ARC manufactured LEMS. It consists of a gas and particle analyzer with a pump and filter PM2.5 sampler, an emissions collection hood, and a dilution tunnel. The LEMS is the result of 20 years of development that started due to the lack of affordable and easy to use equipment suitable for cookstove emissions monitoring.

Testing a stove under the new LEMS hood.
Testing a stove under the newly installed LEMS hood.

Aprovecho develops its equipment as the need arises during research and development activities that occur in its laboratory. Aprovecho’s ability to commission the other instruments that makeup a cookstove testing laboratory is the result of a similar depth of experience.

Bernard Kibera and colleagues training to use the new stove lab equipment
Mr. Bernard Kabera and colleagues training to use the new stove lab equipment.
Sam Bentson trains Bernard Kibera and colleagues to use the new stove lab equipment
Sam Bentson trains Mr. Bernard Kabera and colleagues to use the new stove lab equipment.

It was remarkable to observe how the Rwandan people have protected themselves against COVID. It was a great honor to be part of their community at this time.

–Sam Bentson

Sad cooking pot on a stove
Two cooking pots
Mind the Gap!

Here are the TLUD (Top-Lit Up Draft Stove) derived heat transfer principles that ARC designers use when designing and improving stoves. They are just as important for Rocket stoves as TLUDs:

T: The temperature of the hot gas contacting the pot or griddle should be as hot as possible.

A: Expose as much of the surface area of the pot or griddle to the hot gases as practical.

R: Increasing heat transfer by radiation is important. Move the zone of combustion as close to the surface to be heated without increasing harmful emissions.

P: Optimize the proximity of the hot gases to the pot or griddle by reducing the channel gap without reducing the velocity of the gases. Reduce the thermal resistance with appropriately sized channel gaps under and at the sides of the pot. Match the firepower to the channel gap size and to the size of the pot or griddle.

V: In convective heat transfer, the primary resistance is in the surface boundary layer of very slowly moving gas immediately adjacent to a wall. Increase the velocity of the hot gas as it flows past the pot without reducing the temperature of the gases. As a rule of thumb, heat transfer efficiency can double when the velocity of the hot gases also doubles (N. MacCarty, et al, 2015).

Man at chalkboard
When analyzing a system, try to improve the least efficient part first. 

There are three types of heat exchangers generally used to capture the heat produced in a combustion chamber.

The hot flue gases can:

  1. Heat mass, like heavy stone or masonry
  2. Heat water which then warms the house or…
  3. The easiest and least expensive route – make the hot stove gases efficiently heat the air inside the room

In modern houses with limited air exchange rates heating the air has become the popular option. High mass heat exchangers were created in the days of drafty houses when heating air was a losing proposition. Old houses had air exchange rates of more than 10 exchanges per hour. All the air in the house was replaced ten times or more every hour! It didn’t make sense to heat air that would quickly be outdoors.

Heat exchangers increase heat transfer to the room by making sure that the hot flue gases leaving through the chimney are as cool as possible. Even a smoldering fire turns about 90% of the wood into heat. But, heat transfer efficiency (heat delivered to the room) can be less than 20% in poorly designed systems. As the cartoon shows, a little improvement in heat transfer equals impressive increases in fuel efficiency.

Retaining Heat is Part of the Equation

We cook beans (and other long simmering foods) at Aprovecho using a “haybox.” The pot of food is boiled for ten minutes on a stove and then placed in a well-insulated, airtight box. The beans inside the pot get soft and palatable because the retained heat is sufficient to finish cooking them. We end up using a great deal less fuel because the haybox has improved the heat transfer into the pot. (It’s also a much easier cooking method!)

How a haybox works
A Haybox cooks beans by keeping the heat in the pot. When cooking on a stove, the heat needs to be constantly replaced, using more fuel.

The reason that beans are usually simmered over a fire for a couple of hours is because the pot constantly loses heat to room air. The reduced flame underneath the pot replaces the lost heat.

In the same way, a furnace or a wood replaces the heat in our houses because the house allows the heat to constantly leak away. The house loses heat and the burning wood replaces it. If the house loses a lot of heat, we use a lot of wood per season. If the house loses less heat, we can save trees and are better stewards of this precious resource. If the house loses very little heat, the stove is frequently not even lit because energy in sunlight and interior sources of heat are now equal to the heating demand.

Jet-Flame cross section

Dr. Larry Winiarski would remind me to imagine the languid rising of smoke from a cigarette when thinking about the velocity of natural draft gases in the Rocket stove.  I remember Larry saying that rising smoke is sexy, contemplative, and slow.

Sam Bentson, General Manager of ARC, and Chenkai Wang, Division Business Manager of SSM, spent months designing an inexpensive 2 Watt fan that developed a pressure of 0.75 inches of water column to blow high enough velocity air jets into a Rocket stove fire to increase mixing and combustion efficiency.

When Sam measures the dynamic pressure in the chimneys of household natural draft rocket cooking stoves he finds less than 0.01 inches of water column. Sam estimated, using the Archimedes principle, that a 10” inch in diameter chimney pipe at 700°C for its entire length would have to be 15 meters tall to generate 0.50 inches of water column. It’s amazing how powerful a little electric fan can be!

Jet-Flame cross section drawing
The 2020 Jet-Flame

Kirk Harris writes that he has wondered how an exceedingly small pressure variance could drive the tall flames that we see in some stoves. He envisions fire gas as having exceptionally low density, very light weight, with very low inertia.  Kirk thinks of fire as like a “hole in the atmosphere”, easy to push around. His stoves use static mixers and small velocity induced natural draft pressure differences to mix flame that has been divided into thin sheets. Using these approaches, the Harris TLUD stove achieves less than 1mg/min for PM2.5 when burning pellets.

The Jet-Flame, on the other hand, uses very high-pressure jets of air that blast up into charcoal and then mix wood gas and air as the jets pass through sticks on fire. The bottom air technique requires the equivalent pressure of a 15-meter-high, extremely hot chimney to lower emissions to about the same degree as the Harris stove. It is interesting to think of these two stoves side by side, representing quite different approaches to clean burning. The Harris stove is gently manipulating flame as easy to move around as a hole in the atmosphere while the Jet-Flame is dynamic, a bit loud, creating hot jets of air that drill holes in burning wood.

detail of burning mimi-moto cookstove

The Mimi Moto forced draft TLUD achieves around 1-2mg/min PM2.5 at high power without an appreciable amount of residence time, as seen below. The jets of forced air create a downward flow of flame but there is only 7cm between the top of the fuel bed and the bottom of the pot when starting the stove.

mimi-moto cookstove test
Using a type-K thermocouple the combustion zone measured around 950°C.
detail of burning mimi-moto cookstove
The combustion zone is only 7cm deep when the combustion chamber is full of pellets.

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 that at 700°C for all studied biomass particles. (Li, 2016) At 900°C, a residence period of between 0.6 to 1 second resulted in close to complete combustion of well mixed CO and PM 2.5. (Lu, 2008, Yang, 2008, Grieco, 2011). Boman (2005) reports that high temperature (>850°C) in a 5kW combustion zone combined with air rich and well mixed conditions for 0.5-1.0 second in the post combustion zone resulted in an almost complete depletion of particulate matter. Interestingly, when temperatures are around 900°C the near complete combustion of CO and PM requires only short residence times of 0.5 second. During such conditions, the residence time in the post-combustion zone is of minor importance for minimizing the emissions of products of incomplete combustion. For optimal results, a residence time of 0.5 seconds is suggested.

The Mimi Moto forced draft TLUD is clean burning at 950°C with very limited combustion time. Perhaps the combination of 1.) Metering the right amount of wood-gas into the combustion zone 2.) Coupled with molecular mixing 3.) At around 950°C reduces the need for 4.) Longer combustion times?

References

C. Boman, A. Nordin, R. Westerholm, M. Öhman, D. Boström.
“Emissions from small-scale combustion of biomass fuels- extensive quantification and characterization.” Umeå University, 2005.

H. Lu, W. Robert, G. Peirce, B. Ripa, and L. L. Baxter. “Comprehensive study of biomass particle combustion.” Energy Fuels, 22, pp. 2826-2839, 2008. doi:10.1021/ef800006z

Y. B. Yang, V. N. Sharifi, J. Swithenbank, L. Ma, L. I. Darvell, J. M. Jones, et al.
“Combustion of a single particle of biomass.” Energy Fuels, 22, pp. 306-316, 2007. doi:10.1021/ef700305r

E. Grieco, G. Baldi. “Analysis and modelling of wood pyrolysis.” Chemical Engineering Science, 66 (2011), pp. 650-660 

E. Hroncova, J. Ladomersky, j. Valicek, L. Dzurenda. “Combustion of Biomass Fuels and Residues: Emissions Production Perspective.” Developments in Combustion Technology, 2016 DOI: 10.5772/63793

J. Li, M. C. Paul, P. L. Younger, I. Watson, M. Hossain, S. Welch. “Prediction of high-temperature rapid combustion behavior of woody biomass particles.” Fuel, Vol. 165, (1 February): 205-214, 2016.
doi:10.1016/j.fuel.2015.10.061