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