Secondary Air Jets in Charcoal Stoves?

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Typical Charcoal Stove
Oorja FD TLUD

Charcoal stoves are batch loaded like TLUDs (top loaded up draft stoves), but they differ greatly in how air flow is used to encourage combustion. A charcoal stove has lots of primary air blowing up into the fuel. A TLUD uses a small amount of primary air to create a relatively constant amount of woodgas that is then combusted by a mixture of secondary air and flame above the fuel bed. The large amount of primary air in a charcoal stove tends to create the fire underneath the fuel, while the secondary air jets in a TLUD concentrate the fire above the batch of fuel.

If a TLUD allowed as much primary air as a charcoal stove into the bottom of the fuel bed, too much wood gas would be made and the TLUD would be smoky. I wonder if charcoal stoves could be patterned after TLUDs? Could a charcoal stove be top lit with jets of secondary air and flame on top of the batch of fuel? Would such an arrangement result in better performance? Would added secondary air in a charcoal stove maintain the fire on top of the fuel that would burn up the woodgas (predominantly CO in a charcoal stove)? Might this increase heat transfer efficiency since the temperatures (convection and radiation) might be hotter?

When Sam Bentson and Ryan Thompson studied how to improve charcoal stoves in the DOE 2011 project, they found that adding pre-heated natural draft jets of air did help to create more flame where it’s needed, right under the pot. Unfortunately, they also experienced that charcoal makes less flame than wood and covering the entire top of the fuel bed with flame (as in a TLUD) is difficult. Forced draft secondary air might be more successful, especially since charcoal stoves are usually short and do not generate needed velocities. To be most effective, the jets of air/fire should meet in the middle of the combustion chamber and cover the entire fuel bed. It seems to be best to use the minimum velocity necessary since too much air only cools the fire (Udeson, D.J., 2019, University of Washington).

We’ll try it and report on the results.

Charcoal Stove Design Principles (Ryan Thompson)

  1. Size the combustion chamber for the required task. Whatever fuel is loaded into the stove will be burned at a rate proportional to the amount of air made available to it. In most cases, more fuel means higher firepower because there is always excess air. For small amounts of food/water, where not much power is required, either load a small amount of fuel into the stove, or use a stove with a small combustion chamber. For cooking lots of food at once, use a stove with a big combustion chamber.
  2. Charcoal stoves can have a high turn-down ratio. If the primary air supplied to a charcoal fire is reduced close to zero, the fuel will still keep burning. When the air supply is increased, the firepower will also increase. Normally there is a spike in CO when the air supply is increased sharply, but it tends to stabilize once the firepower comes back up.
  3. Use pre-heated secondary air jets to burn up the CO. Most charcoal stove designs have air coming in from the bottom and sides of the fuel bowl. The air must pass through the burning charcoal before it gets to the top of the fuel where the CO is being emitted. If hot air is added above the charcoal, it is available to combust the CO. Try to keep the bottom of the batch of charcoal as cold as possible and burn down from the top of the batch of the fuel.
  4. Position the pot close to the charcoal. This maximizes heat transfer from radiation.
  5. Insulate the stove body. Insulate the stove body until a temperature of 620˚C or higher is achieved above the burning fuel. Insulation needs to be lightweight and trap still air. Examples of good insulation are: ceramic fiber, rock wool, wood ash, sheets of foil, etc.
  6. Use small channel gaps between the pot and the stove. The pot can be located very close to the top of the burning charcoal. A skirt with a 6mm channel gap around the pot helps increase the heat transfer efficiency from convection. Both convective and radiative heat transfer are important in a charcoal stove.
  7. Maintain a constant cross sectional area throughout the stove to start the design process and reduce as directed by experimentation under the emissions hood. This will help to keep the velocity of the draft as high as possible.
  8. Supply a large amount of primary air to assure sufficient firepower. Make the primary air door large enough to boil the water quickly. The door can be partially closed to reduce power when desired. The door has to be almost airtight to reduce firepower sufficiently when simmering in a pot with a lid.
Summertime_Solar_Cooking

Cooking With The Sun

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Maria Telkes NYWTS
Summertime_Solar_Cooking

 

Last week we shared info about retained heat cooking. It’s frequently paired with solar cooking for feeding folks with no earth-generated fuel expenditure at all. But does it work? The cooks at ARC think that it’s great  – in the summer time when we have lots of sunlight.

Our preferred solar oven was invented in 1953 by Mária Telkes. She was a Hungarian-American biophysicist, scientist and inventor who became well known for her work in solar energy. Her many inventions also included a solar home heating system and solar powered water desalinator.

We like Telkes solar cookers that are big enough to generate firepower that is similar to wood burning cookstoves. A solar cooker is so easy to use and so much cleaner! Of course it is different because a Telkes solar cooker is an oven that is outside on the porch of the kitchen. The cook stands in the shade from the roof and the solar cooker basks in the sun. Just point it at the sun, put the food in the oven, and you are free to do everything else.

Here’s how we do the simple math to create a powerful Telkes solar cooker. Let’s say, as rules of thumb, that:

  1. There are about 250 BTUs in a square foot of sunshine per hour.
  2. There are approximately 8,600 BTU in a pound of wood. Remember, wood is stored solar energy.
  3. Therefore it takes a solar oven with about 34 square feet of intercepted sunlight to equal the cooking power of one pound of wood burned in an hour.
  4. In a solar oven with the firepower of one pound of wood (burned in an hour) the intercepted sunlight should be about 6′ by 6′. This is the measurement at the top edges of the solar reflectors – the widest part of the cooker.
  5. About 1/3 of the energy cooks the food and about 2/3rds of the energy is lost. 
  6. In our 6′ by 6′ solar oven, 3,000 BTUs would boil 2 gallons of water in about an hour. The light weight pot is black, it has a tight lid, the oven is well insulated, and airtight. The glass is double glazed. The losses are minimized and the solar gain is optimized with large reflectors on all sides. Large amounts of food can be made every sunny day without using up any earthly resources. 

In our experience, solar cookers are great when they are big enough to do the cooking task in a reasonable amount of time. ARC cooks have used them in the summer to cook lunch and dinner for 20 people and it’s nice to have a no-fuss oven that needs little tending. Solar cooking is certainly more comfortable when cooks don’t have to deal with a hot fire on summer days. At the Aprovecho farm, the staff have used stored solar energy in the winter (biomass) and direct solar energy (sunlight) in the summer for cooking, heating water, etc.

Of course, everything is dependent on sunlight.

Retained Heat Cooking

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Use moisture proof insulation! Wet insulation doesn’t work well. Illustration from solarcooking.org 

While at ARC we focus on how to cook most efficiently with biomass, it is good to remember that some cooking can continue without consuming fuel. A retained heat cooker (RHC), also known as a Haybox, is a great way to save on fuel for appropriate cooking tasks such as simmering rice or beans.

How does a retained heat cooker (RHC) help when cooking? When food simmers, the fire replaces the constantly lost heat from the pot. If the heat were not lost but captured instead, then less fuel would be needed for cooking. Placing the pot of boiling food in an insulated container keeps the food hot enough to simmer it to completion. In the same way, a drafty and uninsulated house has to have a big fire in the heating stove going all the time to keep the house warm. Even if no fire is lit, the super-insulated, almost airtight house can stay warm for a long time.

After a pot of food boils, the contents are close to 100°C. When the hot pot is placed in a super-insulated, almost airtight box, the food finishes cooking, because the stored heat stays in the food. Once the pot is in the box, food cooks without further attention. The retained heat cooker, saves time, effort, and fuel, freeing the cook from long hours of watching the slow fire when simmering food.

Approximately 50% savings in time and fuel savings can be expected. The rice or stew won’t burn and the cook can make dessert! Because the fuel is only used for boiling food, cooking with a Haybox creates much less pollution, helping to clean up the air in the kitchen. In tests of 18 stoves, using a retained heat cooker reduced, on average, CO emissions by 56% and PM emissions by 37% . (Test Results of Cook Stove Performance, 2011)

RHCs have been used for hundreds of years. They can save time and effort that can be devoted to other tasks. The attraction begins with convenience. The fuel savings and decrease in harmful emissions add to the benefits of retained-heat cooking. More information on Hayboxes can be found in the EPA’s “Guide to Designing Retained Heat Cookers.”

0.75 Constant Cross Sectional Area: A Winiarski “Rule of Thumb”

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Uganda 2-pot rocket style stove

The Uganda 2-pot stove that is described on page 26 in the EPA publication “Test Results of Cook Stove Performance” is a natural draft stove that also uses much less fuel to cook and protects health. The document can be found at: https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P100EKU6.TXT.  Has it been a while since you looked at this book, a comparison of 18 cook stoves? The 2011 book started our surveys with the emissions hood and Test Kitchen, trying to quantify comparisons of fuel use and emissions from available stoves.

Dr. Nordica MacCarty’s paper with comparisons of 50 stoves is a much more complete survey. See: “Fuel use and emissions performance of fifty cooking stoves in the laboratory and related benchmarks of performance” (MacCarty, et al, 2010) https://www.sciencedirect.com/science/article/pii/S0973082610000311

The Uganda 2-pot stove has a Rocket combustion chamber. The hot gases made by the fire pass through narrow, insulated channels around the first pot, which is sunk into the stove. The gases then flow through an insulated tunnel and are forced into narrow channels around the second pot before exiting the chimney. The pots fit tightly into holes in the sheet metal top, preventing smoke from escaping into the kitchen. This stove is fast to boil and, because of the sunken pots, uses less wood than most stoves with chimneys. 

When we build and test stoves we often reflect on Larry Winiarski’s advice that helped to improve the Ugandan stove. Larry advised us that in a 2-pot horizontal stove, channel gaps around the pots that are 0.75 constant cross sectional area are a good compromise between maintaining needed draft and increasing heat transfer efficiency. The cross sectional area of the Ugandan fuel entrance in the Rocket combustion chamber was about 16 square inches so we made the channel gaps all the way to the chimney at 0.75 times 16 square inches. We use Larry’s “rule of thumb” and tests remind us how well Larry knew stoves. He had a good touch.

Constant cross sectional area through a cookstove.


Forced Draft and High Mass Combustion Chambers

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  • Oorja FD TLUD
  • CQC w/FD Rocket

Years of experimenting with stoves at ARC taught us that a high mass combustion chamber absorbs a lot of heat that could be going into the cooking pot. That led us to presume that efficient combustion chambers had to be lightweight, which are harder to make. Once we started experimenting with forced draft, we were surprised to learn that adding forced draft (FD) to a TLUD or a Rocket stove increases the temperature of the gases and largely overcomes the negative effect of a high mass combustion chamber.

The Oorja FD-TLUD and the FD-CQC mud brick stoves generate temperatures of around 1,000°C in the combustion chamber. The option to use a high mass combustion chamber lowers cost and dramatically increases durability when designing forced draft, health protecting, affordable, clean burning, carbon neutral stoves.

The combustion chamber in the pellet burning FD-Oorja that we have in the lab is made from castable refractory ceramic and is over 20 years old. The retail price of the 400,000 British Petroleum Oorja stoves sold in India was around $18. The CQC Rocket combustion chamber is less expensive and is manufactured from sand, clay, and cement. With the addition of forced draft via a Jet-Flame, it reaches Tier 4 for  thermal efficiency and PM 2.5.

To replace health protecting stoves that use natural gas (a fossil fuel) it seems likely that FD-TLUDs and FD-Rockets can be built with lower cost and 10 year durability combustion chambers. The renewably-harvested-biomass fueled stoves need to be manufactured and field tested, and there is a lot of ground to be covered (an understatement), but it’s great that harder-to-make low mass combustion chambers may not be necessary.

Woodgas

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Dr. Tom Reed frequently talked about selling a billion clean burning woodgas stoves. Now that the Biden administration is pointing out that natural gas (and electricity made from fossil fuels) are things of the past, it can be imagined that using woodgas to cook may become a larger part of a post-fossil-fuel future.

“On January 27th, the President announced a series of climate actions that may well mark the beginning of the end of the fossil-fuel era…There’s a shock-and-awe feel to the barrage of actions, and that is the point: taken together, they send a decisive signal about the end of one epoch and the beginning of another. And that signal, most of all, is aimed at investors: fossil fuel, Biden is making clear, is not a safe bet, or even a good bet, for making real money. Coal, oil, and gas are the past, not the future.”

-Bill McKibben, The New Yorker, January 28, 2021

Unfortunately, using biomass for cooking is a difficult replacement because:

  • Smoke from cooking with wood is very dirty, damaging human health. 
  • Smoke is something like 680 times worse for climate change compared to CO2 by weight. (Roden, Bond, et al, 2008)
  • The wood fuel needs to be renewably harvested.
  • Although how to manufacture clean burning, carbon neutral biomass stoves is better understood, stoves need to be affordable to capture substantial market shares.
  • In the Millennium Villages, a retail price of something like $10 has been recommended to sell stoves directly into the market. (Adkins, Tyler, al, 2010)

Can affordable, clean burning, carbon neutral stoves be manufactured and sold?

In 2021, there seem to be at least two popular design options. Both rely on inexpensive, long lasting, refractory ceramic combustion chambers and prepared fuels: 

  • Natural draft or forced draft TLUDS, burning pellets
  • Forced draft Rockets, burning dry sticks of wood

Next week, we’ll explore these options.

sticks burning in rocket stove

Metering, Mixing, Temperature, Time

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sticks burning in rocket stove

Here are lessons learned that summarize what we consider to be the ‘state of the art’ in 2021. Please remember that, as scientists, we do not consider these hypotheses to be true. The concepts are evolving and, like all knowledge, will coalesce into a state of greater accuracy as experience, experiment, and analysis shapes understanding.

Optimize the heat transfer efficiency first. 

Then adjust the combustion efficiency of the stove.

  • The fire generates high enough temperatures for a metered amount of wood gas to combust. The stove body reduces temperatures as little as possible.
  • An appropriate amount of wood gas is made. The rate of reactions (how fast the wood is turned into burnable gas) is controlled by adjusting the primary/secondary air (TLUD) or by metering the fuel (Rocket). 
  • The stove creates molecular mixing with sufficient time for combustion to occur. At 850°C, with about one meter per second velocity, the residence time needed for close to complete combustion can be very limited.
  • Even at 900°C, with lots of mixing and residence time, the fuel must be metered at the proper rate. Increasing the rate of reactions too quickly can overwhelm the ability of the combustion chamber to combust all of the wood gas.
  • A zone of mixing of air, gases, smoke, and flame is created using natural draft or forced draft jets of primary and/or secondary air. To date, creating sufficient mixing in a Rocket stove requires forced draft jets of air but natural draft jets work well in a TLUD burning pellets.
  • It is possible that forced draft jets of air are most successful in reducing emissions in a Rocket stove when directed up from the bottom into the fire.
  • In a TLUD stove, jets of secondary seem to be most successful when aimed into the fire, usually just above the batch of fuel.
  • As a rule of thumb, the cooling effect of the primary or secondary air jets decreases thermal efficiency. Primary air tends to have less of a cooling effect than secondary air. 
  • Secondary air jets create a high pressure zone that reduces the draft, limiting the amount of velocity.
  • Secondary air jets are very successful in a cylindrical combustion chamber with a closed bottom, as in a TLUD. However, the cooling effect and lowered velocity of draft in the Rocket stove makes it more difficult to apply this technique with the open fuel door in the side of the stove.
  • Use the emissions hood to tune the stove.
  • The amount of flame, air, and wood gas entering the zone of mixing is adjusted under the emissions hood until close to optimal combustion efficiency is obtained. 

Primary Air Jets

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Champion Forge, circa 1920

The blacksmith’s forge is probably the most familiar technology that blows jets of primary air up into charcoal or coal, resulting in the high temperatures needed to shape or melt metals. A forge needs air at high pressure (10” water column) to do its work. A fan (usually a radial pressure blower) capable of moving air against significant resistance is required. You don’t need a big volume of air. What is needed is pressure to keep the air moving and to create molecular mixing of the woodgas and flame.

The Jet-Flame stove accessory
The Jet-Flame Stove Accessory

The SSM Jet-Flame

The Jet-Flame uses the same technology as a forge. It required a year of R&D to create a low cost, 1.5 Watt fan that delivers sufficient pressure into a Rocket stove fire. Thirty 2mm jets of air at 0.4 to 2 inches of water pressure blow up into the charcoal under the burning sticks of wood. When the pressure rises and the vertical jets of air penetrate further into the charcoal and fire, the following effects are seen:

• Temperatures rise in the combustion chamber, resulting in higher thermal efficiency as hotter gases flow past the pot. The higher temperatures also result in lower PM2.5 and CO. However, proper metering of the fuel, mixing caused by turbulence, and sufficient residence time are as important for decreased emissions of PM2.5 and CO. With higher temperatures, the related measures of firepower and CO2 also rise, while the fuel/air ratio decreases – as the fire increases, more oxygen is consumed. Increasing the pressure also increases firepower even when a constant fuel load (usually two 4cm by 4.5cm sticks) is being burned. Larger sticks have a lower surface area to volume ratio and make less smoke. When the outer surface of the sticks are covered with charcoal, emissions decrease as well.

• In a Rocket stove that has a large fuel opening, natural draft pulls room air into the combustion chamber. With forced draft adding more air, the average Lambda in a Rocket stove tends to stay above two times stoichiometric. The jets of air blowing into the charcoal result in higher temperatures as pressure increases resulting in temperatures over 1,000°C even at 2 to 5 Lambda. Secondary air jets blowing into the fire, on the other hand, have a cooling effect. In a Rocket stove with a Jet-Flame, the varying fuel/air ratios are not related to the emission rates of PM2.5 and CO.

• Higher temperatures caused by greater pressure also have detrimental effects. The percentage of black carbon is higher when temperatures/firepower/CO2 become elevated – hot yellow flames cause the formation of black carbon. The lifespan of affordable refractory metals is greatly decreased by very high temperatures such as 1,000°C. A durable refractory ceramic material is better suited to higher operating temperatures. Replacing metal combustion chambers with low mass, refractory ceramic was recommended by the 2011 DOE biomass stove conference. https://www1.eere.energy.gov/bioenergy/pdfs/cookstove_meeting_summary.pdf

• Lowering the firepower by adjusting the pressure in the air jets assists the metering of fuel to achieve a 3 to 1 turn down ratio. When the pressure is too high, the fire can be blown out when the charcoal has disappeared.  A layer of charcoal under the fire (or charcoal coating the sticks) helps to maintain the fire and lower emissions as charcoal emits much less PM2.5 compared to wood. The levels of CO tend to rise when the flame above the sticks decreases. The amount of flame above the sticks seems to be related to lower emissions of CO and PM2.5. The amount of flame above the burning sticks, the metering of the fuel, and the mixing of the wood gas are not as easily quantified as temperature and residence time but are as important for more complete combustion.

Smoke In The Air!

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pdf available at: deohs.washington.edu/sites/default/files/AirFilterInfographic_FINAL.pdf

I hope that you are living in a smoke free environment! There’s a forest fire about 40 miles east of the lab that floods our valley with smoke when the wind slows down. I just looked up and noticed that it was getting hard to see Blue Mountain, a sure sign that the northwesterly wind wasn’t pushing hard enough to clear the skies. It reminded me of living on the Coromandel Coast in India where a blue sky was unlikely even at the beach.

Burning up smoke is not all that difficult to do: just thoroughly mix the smoke into the flame. But that doesn’t happen in a forest fire (or in a three stone fire). The smoke and flame go in different directions. The industrial reduction of PM2.5 often depends on both improved combustion efficiency and the post-combustion filtration/scrubbing of emissions. When I can’t see Blue Mountain anymore, I switch on a box fan that has a 20” by 20” by 1” furnace filter taped onto the inlet side of the fan. The fan pulls the dirty air in my office through the filter and the PM2.5 is removed from the air that I’m breathing.

Simple cooking enclosure

We wrote a paper describing how the same fan and filter reduced PM2.5 when installed in a hood over the stove. We used a washable filter and hoped that the combination of a clean burning stove with post combustion filtration of smoke might help to protect inside and outside air quality. Check out the paper: Still, D. K., Bentson, S., Murray, N., Andres, J., Yue, Z., & MacCarty, N. A. (2018). Laboratory experiments regarding the use of filtration and retained heat to reduce particulate matter emissions from biomass cooking. Energy for Sustainable Development, 42, 129–135. https://doi.org/10.1016/j.esd.2017.09.011

Four Simple Changes = 56% Thermal Efficiency

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A 2014 survey of biomass stoves for DOE showed that tight pot skirts are great!

We had a couple of days between jobs at the lab and decided to see if a simple Rocket stove manufactured in India, patterned after the BURN stove, could get better thermal efficiency. Low grade stainless steels, like 304, can’t withstand the hotter combustion chamber temperatures generated when insulated, so in the BURN stove room air is used to keep the steel cool enough to increase durability.

One of the key properties of any stainless steel alloy is its resistance to oxidation. High temperatures can compromise the oxidation resistance of steel alloys, leading them to become rusted and weakening their structural integrity.

As stated by AZO Materials, grade 304 stainless steel possesses “good oxidation resistance in intermittent service to 870°C and in continuous service to 925°C.” However, they warn that “continuous use of 304 in the 425-860°C range is not recommended if subsequent aqueous corrosion resistance is important.” In other words, you can expose grade 304 alloy steel to temperatures of up to 870°C for short periods of time without ill effect, and for extended periods of time in temperatures of up to 925°C. However, this can compromise the corrosion resistance of the metal, making it more susceptible to damage from exposure to moisture. (https://www.marlinwire.com/blog/what-is-the-temperature-range-for-304-stainless-steel-vs-316-vs-330)

When the low mass, uninsulated BURN Rocket stove has (1) 6mm high pot supports, (2) a pot skirt that creates a 6mm channel gap around a family sized pot, and (3) a fire that creates hot, tall flames that transport 800°C to 1,000°C gases to the pot, the thermal efficiency has been measured at around 52%. 

We lowered the pot supports in the simple Indian Rocket stove to (1) 6mm high and used a (2) 12cm high, 6mm channel gap pot skirt around a 25cm in diameter steel pot filled with 5 liters of water. Thinking that the simple Indian Rocket stove could use a 1,200°C thin walled refractory ceramic combustion chamber, (less than $1 from Shengzhou Stove Manufacturer), we (3) surrounded the combustion chamber with ceramic fiber insulation. (4) The fire was made from tiny sticks. Tiny sticks make hot, tall, dirty flames and use up the least amount of wood while making really hot gases. When burning tiny sticks, gas temperatures under the pot can be over 1,000°C. The 1,000°C gases heat water quickly and efficiently when 6mm channel gaps are used below and on the sides of the pot.

With these changes, the simple Indian Rocket stove scored an average of 56% thermal efficiency (3 tests to boil). 

If (1) 6mm pot supports, (2) 6mm pot skirts, (3) insulation, and (4) tiny sticks making 1,000°C gases had been used in the 2014 DOE stove survey the average scores would have been a bit higher. One lesson is that channel gaps and types of fires can have a big effect on heat transfer efficiency.

Go for those 1,000°C gases flowing right next to surfaces for high thermal efficiency. 

Add metering and mixing to 1,000°C gases with sufficient residence time and combustion efficiency is also improved.

Check out the heat transfer and combustion chapters in “Clean Burning Biomass Cookstoves, 2021” at www.aprovecho.org.