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.

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
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. 
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.

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