High temperatures in the combustion chamber seem to have both positive and negative effects on emission rates of biomass. Higher temperatures lower the residence time needed for more complete combustion. At the same time, especially with dry wood, the rate of reactions (how much wood gas is being made per unit of time) is increased. If wood gas is made too quickly, some of it can escape unburned. Our experience has been that in a hotter combustion chamber the fuel must be metered into the fire more slowly to lower emissions. In Rocket and TLUD stoves, the rate of reactions must be controlled to eliminate smoke.

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 the time needed at 700°C for biomass particles (Li, 2016). At 900°C, a residence period of 0.5 seconds resulted in close to complete combustion of well mixed CO and PM 2.5 (Grieco and Baldi, 2011; Lu et al., 2008; Yang et al., 2008).

Boman (2005) reports that temperatures above 850°C in a 5kW combustion zone combined with air rich and well mixed conditions for 0.5 seconds resulted in an almost complete depletion of particulate matter. The use of insulation in Rocket stoves can create a combustion zone with temperatures above 1,000°C. Forced draft TLUDs can generate similar temperatures in the secondary air mixing zone above the fuel bed.  Interestingly, when temperatures are around 850°C the near complete combustion of well-mixed carbon monoxide and particulate matter seems to require short residence times. Both forced draft Rocket and TLUD stoves can minimize the emissions of products of incomplete combustion even though the residence time is very limited.

Recently, we ran a series of fifteen experiments trying to optimize performance in a low mass Rocket stove with Jet-Flame. Since we were testing with dry wood we had to be careful not to over insulate the combustion chamber. Insulation made the whole length of the stick catch on fire increasing the rate of reactions and firepower.  When more than 8cm of the stick was burning more mixing was needed to achieve close to complete combustion. When only the tips of the sticks were on fire the metering of woodgas into the fire was slower and less mixing was required.

As seen in the following graph, the emissions of CO were again shown to be reduced at higher temperatures. As a rule of thumb when designing a stove, we try to create temperatures above 700°C about 6cm above the fire, at a minimum.

Testing a Chitetezo Stove with a Jet-Flame

What do I like most about Aprovecho Research Center? We have created a “culture of daily experimentation.”

Scheduling two to three experiments every morning means that we can try less-likely-to-succeed variations. That’s great because experiments that don’t succeed in improving a stove often reveal interesting and possibly unexpected results. And that can lead to new insights. “Accident favors the prepared mind,” Pasteur reminds us.

Nordica MacCarty, our new Executive Director, recently suggested that it would be a good idea to see what happens when the Jet-Flame is used without a Rocket combustion chamber. We have been working with Christa Roth and Christoph Messinger from GIZ, investigating what happens when the Jet-Flame is used in the Chitetezo stove. It was easy to continue the investigation into the effects of primary air jets blowing up into the fuel. We removed the rocket-style combustion chamber that we were testing, and put a short metal fence around the Jet-Flame to keep the sticks on top of the air holes. A SuperPot was used and set directly on top of the stove.

It’s been very interesting to try different sized sticks with the Jet-Flame. Generally, small sticks burn faster and make a lot of flame, but also more smoke. Two 4 cm x 4.5 cm sticks burn more slowly and emit less smoke but the firepower is lower. With no insulation around the combustion zone, we tried burning four sticks that were 2 cm x 2.5 cm. The results were surprisingly good: Tier 4 for thermal efficiency, Tier 4 for PM2.5, and Tier 5 for CO.

The size of the sticks made a significant difference on all three measures, which makes sense and has been seen in natural draft stoves. But it’s noteworthy that the size of the sticks results in big differences with the Jet-Flame. Following up on Nordica’s suggestion made for a more interesting week here at the lab. Completing ten to fifteen half-hour experiments weekly makes it more likely that progress is being made.

Reading/learning in front of the computer is important, and OK for a while, but we also like to get into the lab to try and create/learn something new.

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.

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. 
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

Ryan Thompson and Sam Bentson spent a year exploring charcoal burning

Ken Newcombe and C-Quest Capital have seen firsthand how charcoal production has wiped out forests in Africa. Making charcoal is a very energy inefficient process (Aprovecho Institute, 1984a). Burning off the volatile compounds in wood consumes and wastes between 50% and 80% of the energy!  Ken recently observed that saving charcoal was not easy, and  that field tests indicated that traditional stoves were as fuel efficient as more modern stoves. So I asked Ken if he had read Sam’s paper* on how to save fuel in charcoal stoves, and Sam sent it to him.

Busy people are out in the world accomplishing things. There’s a ton of information on the Internet and lots of it is conflicting. Publishing information in journals and books sometimes seems to be a fairly ineffective way to reach actively engaged stovers. That’s why we publish this weekly newsletter, trying to disseminate data driven information. Maybe you can pass it to on to your colleagues?

After testing many charcoal stoves, Sam concluded that:

“Using the minimum load to complete the WBT [water boiling test] results in similar performance between most charcoal stoves, while filling the combustion chamber with fuel results in large differences on the same measures. Based on these laboratory results, limiting the size of the combustion chamber so as to prevent excess fuel loading may be an effective technique for decreasing fuel consumption while cooking.” *

The loaded charcoal is used up, so loading only the amount needed for the cooking process is very important for saving fuel.  At the same time, Sam and Ryan Thompson spent a year exploring charcoal burning and came up with an “all Tier 4” charcoal stove that was higher scoring than the other optimized wood burning stoves that ARC made for the US DOE.

CAD drawings and a chapter on charcoal describe what Sam and Ryan did and how to build the stove they invented. (Clean Burning Biomass Cookstoves, 2021,  www.aprovecho.org/publications-3).

*(Energy for Sustainable Development, 2013, Bentson, Still, Thompson, Grabow)

General Manager Sam Bentsen is happy about some LEMS test results.

Why test a stove?

Most of the time, our lab uses testing for product development. If we did not test a stove prototype we would be guessing whether it met expectations. Testing in the lab gets us ready for the field testing of prototypes. Then, customers take the prototype and make it work. The factory and distributors frequently ask for design changes as the product gets closer to shelves. From initial design to market usually takes about a year of testing/iteration/development.

Recently, a factory in Africa asked us to design a $10 wholesale, pellet burning forced draft TLUD prototype that achieves Tier 4 for thermal efficiency, CO, and PM2.5. The stove has to last ten years with scheduled maintenance and require as low wattage as possible.

We had tested the Oorja several times during surveys of commercially available stoves. ARC published the results in books and papers trying to inform the public how stoves compare on various measures of performance. We were trying to make available a Consumer’s Report on stoves (see list below). We knew that the Oorja stove met the Tier rankings and that it used a high mass, low cost, durable combustion chamber. We tried a castable refractory in the lab and we also found several manufacturers that make inexpensive ceramic combustion chambers.

The factory wants a high-powered stove to meet the needs of cooks in their region. Protecting health is also a major concern. Delivering a design that can be made for $10 is also very important. All the interconnected partners in the business plan have to make a healthy profit to bring a “Tier 4” technology to the public. The designer is only the first step in a web of stakeholders.

After all of the necessary parts were combined in the lab, testing with the LEMS (Laboratory Emissions Monitoring System) started. Many iterations were needed to get close to optimal performance. Adjusting the primary and secondary air at high power took experimentation. In several weeks of daily testing, the prototype was repeatedly achieving best scores. A CAD drawing was made and the design was sent to the factory. The factory is making their version of the stove, we will test it here and make adjustments if needed, and then field testing of the prototypes will begin, including home trails and test sales in stores.

Does it sound like a lot of work? The payback to know, rather than guess, that the product can be successfully sold. It’s great to make data based decisions, and a careful approach attracts investment. Failing miserably with products we loved (and lost money on) has made ARC consider external input carefully, especially from field testing.

Cook Stove Performance Reports:

Photo from the BURN Newsletter, May 2021
Photo from the BURN Newsletter, May 2021
Photo from the BURN Newsletter, May 2021

In a recent BURN newsletter it was announced that the natural draft Kuniokoa stove with a new pot skirt achieved 51.3 % thermal efficiency. That’s Tier 5, the highest score on the voluntary tiers of performance. Achieving great thermal efficiency involves improving heat transfer efficiency which is summarized in the acronym TARP-V: Increase Temperature, Area, and Radiation, use narrow channel gaps to achieve Proximity, and increase the Velocity of the gases flowing past the pot without decreasing the temperature.

Making a clean burning fire does not help very much to increase thermal efficiency. Even 97% combustion efficiency is very smoky.

How can your stove get around 50% thermal efficiency?

  1. The BURN stove is very light weight, weighing in at around 3 kilos. Thermal mass in the stove body absorbs heat from the fire lowering the temperature of the gases trying to heat the water in the pot. MAKE THE GASES AS HOT AS POSSIBLE!  Hotter gases in narrow channels flowing past the bottom and sides of the pot thin the boundary layer of still air next to the pot and result in better heat transfer efficiency. The insulation in the BURN stove is 15mm of trapped air – a cylinder surrounds the riser in the Rocket combustion chamber.
  2. The channel gaps on the bottom and sides of the pot can be 6mm. 10cm or higher pot skirts are better. It’s great if the pot skirt is as high as the water level in the pot.
  3. Small, kiln dried sticks make a lot of flame (and smoke). Small sticks (we used 1cm by 2cm in a recent test) create hotter fires and gases, using less fuel compared to burning larger sticks. The hotter gas temperatures get a higher percentage of the heat into the pot.
  4. A big pot has more surface area and can be a better heat exchanger. Dr. H. S. Makunda found that larger pots (32cm in diameter) could score in the 50% range, while smaller pots (25cm) tended to get around 40% thermal efficiency. (H. S. Mukunda, CURRENT SCIENCE, VOL. 98, NO. 5, 10 MARCH 2010). 
  5. Don’t make a big fire. A moderate fire (3 to 5Kw) is better matched to family sized pots. (Prasad, Some studies on open fires, shielded fires and heavy stove, Eindhoven, 1981
  6. A hot start test usually adds something like 5% to the thermal efficiency. A cold start test transfers more of the heat from the fire into the stove body.

We built a Rocket stove that combined these characteristics. The stove top had 6mm high pot supports and the 6mm channel gap pot skirt was 10cm high. The pot had a diameter of 30cm. We used very light weight ceramic fiber insulation around the combustion chamber. The stove weighed 2.9 kilos and was 24cm high and 32cm wide. We tested it by burning five kiln dried 1cm by 2cm sticks in a hot, small fire that started quickly. The Rocket stove smoked like crazy at a firepower of around 4.5Kw, but the thermal efficiency from one high power, hot start test was 52.7%.

This week we will see what happens when we use the same Rocket stove/big pot with a Jet-Flame that should increase the Temperature of the gases and their Velocity.

When this sort of fire is maintained, a high mass Rocket stove can get close to Tier 3 for CO (less than 7.2g/MJd) and PM2.5 (less than 218mg/MJd).

Testing stoves means that many, many hours are spent watching the flames and pushing sticks of wood into the fire. We watch the real time emissions, water temperature, excess air, and temperatures in the combustion chamber on a computer screen as the testing continues. After hundreds of hours it becomes obvious that clean combustion has a certain “look.” For instance, when there is a lot of flame above the sticks the real time CO goes down on the computer screen. Without flame above the fuel, the gas rises up and is not combusted. That’s why charcoal can be dangerous, because burning charcoal does not create a lot of flame above the fuel.

When the wood sticks are changed into charcoal, the PM2.5 is dramatically reduced, as well. Pushing the unburned sticks into the fire creates smoke. Pushing the sticks in quickly makes a lot of smoke and pushing the sticks in slowly makes less smoke. Charcoal does not make much smoke and that’s one of the reasons that people like cooking with it. Feeding a fire is a compromise, as firepower and PM2.5 tend to rise together. A rocket stove can look OK as long as the sticks are fed slowly into the fire at less than about 2.5 kW. But at high power, Rockets start to smoke like crazy.

How can these two experiences be factored into a mathematical model of combustion? By using a video camera hooked up to a computer program? Residence time and temperature are easily measured, but the extent that turbulence occurs is not easily quantified. Engineers get past these sorts of problems by figuring out how to optimize mixing (using jets of air, for example), but a mathematical model is more easily filled in with numbers for other sorts of phenomena, such as the excess air ratio.

Both experiments and mathematical modeling shed light on how to make better stoves and hopefully complement each other. Arguments have been known to happen. I have been trying to figure out how to make clean burning biomass fires since 1989, and one thing has not changed since Dr. Winiarski started me on this path. I continue to be happier trying to answer questions that begin with the word “What” instead of “Why.”