Newsletters | Page 22 of 31

Setting Up a New Lab in Rwanda

October 23, 2020/0 Comments/in International Work, LEMS, New Product

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.
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 PM_2.5 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.

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

TARP-V: Optimizing heat transfer efficiency

October 15, 2020/0 Comments/in Cleaner Burning Technologies, Rocket Stoves

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).

Maurice Van

Heat Exchangers for Heating Stoves

October 9, 2020/0 Comments/in Heating Stove

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

The hot flue gases can:

Heat mass, like heavy stone or masonry
Heat water which then warms the house or…
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.

Flame and Flow Impressions

October 2, 2020/0 Comments/in Uncategorized

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.

Metering, Mixing, Temperature, and Time

September 25, 2020/2 Comments/in Cleaner Burning Technologies

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.**

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

A Tier 5 Rocket Stove

September 17, 2020/0 Comments/in Cleaner Burning Technologies, New Product, Rocket Stoves

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.

The Jet-Flame stove accessory — *The Jet-Flame*

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.

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.

Setting Up a New Lab in Rwanda

TARP-V: Optimizing heat transfer efficiency

Heat Exchangers for Heating Stoves

Retaining Heat is Part of the Equation

Flame and Flow Impressions

Metering, Mixing, Temperature, and Time

References

A Tier 5 Rocket Stove

Smoke!

An “Intervention Ladder”

What’s New at ARC?

The SSM Jet-Flame in the C-Quest Capital Clay/Sand Rocket Stove

Aprovecho Research Center