Rocket Stove 2021 - Pot Skirts

In this video, Dean Still explains why a pot skirt – a sheet of metal wrapped around the cooking pot – is a simple yet important way to improve the fuel efficiency of a rocket stove. He also explains how to calculate the appropriate distance between the skirt and the pot. Stay tuned to the end of the video to find out who is causing all the ruckus in the background…

Helpful references:

simplified diagram of constant cross sectional area
Simplified drawing of the concept of constant cross sectional area.

This is a very simplified illustration of what “constant cross-sectional area” means. The top circle represents the cross-sectional area of a stove riser. The bottom ring shows the same area translated into the space around a pot. It’s important to keep the cross-sectional area that the hot gasses flow through consistent, so they don’t slow down. Hot, fast flowing gasses transfer heat most efficiently. 

graph helps calculate proper skirt gap for best heat transfer efficiency
Chart for calculating channel gaps, from Dr. Samuel Baldwin’s “Biomass Stoves: Engineering Design, Development, and Dissemination.” 1987, Volunteers in Technical Assistance.

This is the chart for determining efficient channel gaps, explained towards the end of the video. It was developed by Dr. Samuel Baldwin in 1987.

Here is the Ten Stove Design Principles poster referred to in the video. Many more helpful documents are also linked on the Publications page.

sticks and charcoal start to combust in a rocket stove

The Jet-Flame was developed from combustion concepts used in fluidized beds and TLUDs.

Fluidized Bed

fluidized bed combustion diagrams

“In its most basic form, fuel particles are suspended in a hot, bubbling fluidity bed of ash and other particulate materials (sand, limestone etc.) through which (under air) jets of air are blown to provide the oxygen required for combustion or gasification. The resultant fast and intimate mixing of gas and solids promotes rapid heat transfer and chemical reactions within the bed.”   https://en.wikipedia.org/wiki/Fluidized_bed_combustion

Top Lit Up Draft

diagram explaining how a top loaded up draft stove works

The TLUD uses under air flowing up through the fuel to transport wood gas into the hot layer of charcoal and flame above the fuel assisting more complete combustion efficiency.

Cleanly Starting the Jet-Flame

High velocity under air jets blow up into the lit charcoal placed on top of small sticks of wood. When the charcoal and wood are on fire, long pieces of wood are pushed into the made charcoal to start a Rocket Jet-Flame without making visible smoke. The sticks of wood are burned at the same rate as the continual production of charcoal creating a cleaner combustion process related to a fluidized bed and the TLUD.

sticks and charcoal start to combust in a rocket stove

Charcoal over wood is lit.

bed of charcoal in rocket stove

The charcoal becomes superheated with jets blowing up into the pile.

sticks burning in rocket stove

After 30 seconds, long sticks of wood are pushed against the burning charcoal creating flame.

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!

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).

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

Sam Bentson and David Evitt with the new Jet-Flame
Sam Bentson and David Evitt with the new Jet-Flame
Sam Bentson, ARC Lab Manager, and David Evitt, ASAT COO, developed the Jet-Flame with Shengzhou Stove Manufacturer and Dr. Dan Lieberman and Dr. Mike Barbour at the Gates funded Global Health Labs

The cast iron Jet-Flame sends 30 jets of pre-heated air up into the burning charcoal and wood in an open fire, sand/clay stove, or in a Rocket stove. It is patterned after industrial burners that position jets of primary air underneath the fuel bed to clean up combustion. Both Underfeed Stokers and Fluidized Bed Boilers use primary air that enters the fuel bed from underneath the fire.

In 2013, with DOE funding, ARC built a bottom-air-only prototype stove and has been experimenting with improving the technique, resulting in the Jet-Flame combustion chamber accessory manufactured by SSM in China. There are several advantages in a bottom-air-only approach. The jets of air flow into the fuel bed from holes in the floor of the combustion chamber. Since the pre-heated air flows vertically, back-drafting out of the fuel door in a Rocket type stove is easier to overcome. The jets of air super-heat the charcoal layer underneath the sticks of wood. The hot jets of air emerge from the charcoal and pierce the laminar flames emitted by the wood creating turbulent eddies that stir up the flames to enhance the speed of mixing and combustion. The turbulent combustion zone creates short, intense flames that burn the fuel more completely before they cool off too much to sustain combustion. The increased velocity of the higher temperature flue gases also improves heat transfer efficiency.

Winiarski sunken pot Rocket stove with chimney
Winiarski sunken pot Rocket stove

When the Winiarski sunken pot Rocket stove with chimney is combined with the Jet-Flame the increase in combustion efficiency results in a truly improved stove with the ability to protect health. Since the stove and chimney do not leak in lab tests the stove does not emit fugitive emissions into the kitchen. The stove achieves all ISO 19867 Tier 5 ratings for both thermal efficiency and emissions of CO and PM2.5.

In 2004, ARC was hired by the Shell Foundation to bring the Rocket stove to India. Protecting health was a component of the project. Unfortunately, the natural draft Rocket stove was not clean enough burning to accomplish the task. Higher temperatures and a lot more mixing were needed. We wish that, when asked for a health protecting stove, we had been this far along. It has taken a while to make some progress. 

Test results of the Jet-Flame with a vented Rocket stove.
Test Results of the vented (with chimney) sunken pot Rocket with Jet-Flame