The Field Informs the Lab

In Part 1, we gave examples of how field studies can provide unpleasantly surprising results. Rocket stoves were designed to make a little less smoke and use substantially less fuel. So when the rocket stove was field tested by USAID the inventor, Dr. Larry Winiarski, was not surprised that the stove still made smoke. But the ARC team was surprised that it was not a real improvement over the open fire.

In 2011 the goals for cookstoves published by the Department of Energy asked that a stove use 50% less fuel and make 90% less PM2.5 to protect health when used indoors. Now in 2022 stoves are also supposed to address climate change, which means emitting less PM2.5 and hopefully making less than 8% black carbon. Field tests show that we need to make more improvements to meet these specific goals.

How are these reductions achieved in the lab?

  1. Use a chimney to reduce in-home concentrations of CO and PM2.5.
  2. In lab tests, approximately 850°C gases need to flow in tight channel gaps around the pot(s) to reduce the fuel used to cook by about 50%.
  3. Molecular mixing at 850°C (0.2 second residence time) can achieve something like a 90% reduction of PM2.5 (requires forced draft in a Rocket stove).
  4. This mixing reduces greenhouse gas emissions by about the same amount.

Natural-draft and forced-draft TLUD stoves burning pellets and forced draft Jet-Flame stoves burning dry sticks without bark get close to these reductions in the lab. Unfortunately, they frequently do not yet meet these goals in the field.

The lab has to move into the field to learn if current technology can accomplish modern goals. Let’s go!

Next week in Part 3: sometimes field tests show success.

Chart showing turn down ratios and firepower of 18 stoves
Firepower and turn-down ratio of 18 stoves, from “Test Results of Cook Stove Performance”

The ARC/EPA 2011 book “Test Results of Cook Stove Performance” compares performance and emissions, including turn down ratio and firepower, from survey of 18 stoves. Firepower is a measure of how much energy is released per unit of time. More energy is required to quickly boil water. Less energy is needed to simmer food.

The most effective cooking stove should be fuel efficient at both high and low power. The ratio between high and low firepower is called the turn-down ratio (TDR). It is a measure of how well the stove can be “turned down” from high to low power. 

A TDR of 2 means that half as much fuel was consumed while maintaining a simmer, compared to bringing water to boiling. Cooks usually appreciate a stove that is capable of both high-and low-power operation. Many foods will burn if the heat can’t be reduced enough.

It is interesting that the liquid-fueled stoves were generally low powered, at less than 2kW. Most of the wood burning stoves ranged from 8kW to around 6kW. In Mexico, gas stoves can have a hard time cooking tortillas.

The Mud/ Sawdust (TDR 3.9) and VITA (TDR 3.8) stoves had the highest Turn Down Ratio. The average for the other wood-burning stoves without chimneys was 2.4. The average for stoves with chimneys was 2.2. The Gyapa charcoal stove (TDR 2.8) scored slightly higher. 

The chart shows the average high firepower and the low firepower for each stove. It should be noted that in these tests the pot was uncovered, which increases the energy input needed to maintain the water at simmering temperatures.

Read more about the Chitetezo Mbaula project at unsustainablemagazine.com. Photo by Deogracias Benjamin Kalima

The Chitetezo Mbaula cookstove is distributed by United Purpose in Malawi with the goal of combating deforestation by replacing the traditional charcoal/firewood cooking stoves. In an effort to assist, ARC worked with stakeholders to see how small changes in the stove might translate into fuel and emissions reductions in lab tests. Of course, this information is only useful to researchers in the field as possible iterations. They determine if the changes might translate into practical conservation. The collaboration continues as possibilities are examined.

In its stock form, the stove achieved an average thermal efficiency of 22.5% during three modified laboratory based IWA 4.2.3 tests at high power. As the stove body got hotter, the thermal efficiency increased from 17.6% to 26.6%. The thermal efficiency Tier rating was 1, and PM2.5 emissions, at 1093.3 mg/MJd, gave a Tier rating of 0.

Simple Adjustments Make Some Performance Improvements

Three one inch in diameter holes were drilled through the back of the clay stove body with the intention of allowing more air into the charcoal bed. The pot gap on top of the stove was also reduced to 6mm. These two changes resulted in an average increase of thermal efficiency with char from 22.5% to 29.6%. 

The CO emissions factor per energy delivered to the cooking pot decreased from 10.45 g/MJd to 5.63 g/MJd, although at the same time the firepower decreased from 8.9 kW to 5.9 kW. Natural draft stoves with lower firepower tend to make less emissions. Since the time to boil (normalized to 75°C temperature rise) also increased from 22.4 minutes to 25.2 minutes, further study is needed to determine if the reduction in CO emissions also occurs at 22.4 min to boil. 

Jet-Flame and Pot Skirt Increase Efficiency, Reduce PM2.5

The Shengzhou Stove Manufacturer Jet-Flame was then inserted into the stove body with a metal Rocket combustion chamber. A 6mm channel gap metal skirt was also used around the 5 liter flat bottomed pot. With these changes, the stove achieved an average thermal efficiency of 47.7% during three laboratory tests at high power. As the stove body got hotter, the thermal efficiency increased from 44.9% to 52.3%. The IWA thermal efficiency Tier rating was 4. Since all of the tests scored within Tier 4, which is the maximum score under the ISO IWA, the 90% confidence interval of the Tier rating was 4 to 4. The PM2.5 emissions of the stove were 69.0 mg/MJd and the Tier rating was 3.

When tested in the field, ARC roughly estimates that emissions will be something like three times higher. This is a “rule of thumb” that is not meant to be an accurate guess but a reminder that many researchers have found that emissions in the field are much higher compared to lab test results! The lab test can point out theoretical “improvements” but only field testing can determine actual performance and practicality. On the other hand, if cooking takes place outdoors, as in the photo above, exposure to harmful smoke can be estimated to be dramatically reduced by the increased air exchange rates.

Last week we wrote about using the LEMS to tune up a stove, so it makes sense to share the actual results of a recent test series with you this week.

When ARC makes an Open Fire, we often use three bricks on end to hold up the pot. The bricks are 16cm high. It has been fascinating to experiment with the SSM Jet-Flame in the open fire to try and determine how fuel-efficient and clean burning the combination can be. Last month, we spent a couple of weeks changing one thing at a time and then completed nine 30-minute ISO high power tests on the close-to-optimized design.

Here are the test results:

test results chart

Description of the changes

  • We kept the pot height at 16cm above the top of the Jet-Flame.
  • Three rebar supports held up the pot replacing the heavier and bulkier bricks.
  • A short 6cm high by 18cm long FeCrAl fence kept the sticks on top of the combustion zone in the Jet-Flame.
  • A lightweight Winiarski 304 stainless steel “0.7 constant cross sectional area” stovetop increased the heat transfer efficiency from the hot flue gases into the pot.
  • Thermal efficiency was also improved with an 11cm high pot skirt creating a 6mm channel gap on the sides of the 26cm in diameter pot.
  • We learned that the sides of the open fire should be partially enclosed for best performance. A 5cm high opening at the lower portion of the sides of the open fire allowed fresh air to enter the combustion zone. 11cm of the upper portion of the sides of the Open Fire were enclosed with aluminum foil.
  • To make sure that there was no backdraft, a 7cm tall, 14cm wide and 7cm deep metal fuel tunnel was added on the outside of the sides of the partially enclosed Open Fire.

Photo of the experiment

Conclusion

It looks like a Rocket combustion chamber may not be needed to achieve Tier 4/5 results from an “Open Fire” when tested in a lab. A short fence that holds a single layer of sticks on top of the primary air jets seems to be as good.

PM2.5, CO2, CO and other metrics measured by the LEMS are displayed in a real-time graph during testing.

When enough data is available in real time, it is not hard to make progress improving a stove. The LEMS emission hood provides real time, holistic feedback as the experimenter makes changes. Watching all of the measures simultaneously makes it possible to tune a stove like a car. The goal is to burn biomass at high, medium, and low power with Tier 4/5 levels for thermal efficiency and emissions of CO and PM2.5.

It is easier in a forced draft stove.

Watching the real time PM2.5 data on a screen while turning two knobs that control the velocity/volume of primary and secondary air jets going into a combustion chamber quickly establishes a close to optimal compromise resulting in the lowest emissions. The stove “tune-up” process includes making sure that the temperature of the gases touching the pot stay as hot as possible, keeping the thermal efficiency high. Maintaining high temperatures in the combustion chamber is important as well. At the same time, the CO2 (a proxy for firepower) needs to stay around 5kW (high power), 4kW (medium power) and 3kW (low power). Since burning wood does not make much CO, the CO should stay low. The Oxygen (O) sensors (air/fuel ratio) warn us if the available Oxygen is too low.

Establishing a close to optimal balance of PM2.5, temperatures, CO, CO2, O is easier when working on a stove with metered fuel like a TLUD or a pellet burning heating stove. Consistent metering reduces disturbances in the rate of reactions (how fast the solid wood turns into woodgas). A standardized test, such as ISO 19867, provides data on fuel use, time to boil, thermal efficiency, firepower, emissions rates, etc.

After the real time balances look good, we use the more accurate pump and filter PM2.5 system with enough repetitions to establish statistical confidence. It usually takes two to four weeks to tune up a stove. After tuning up several similar stoves, the data can coalesce into time saving design principles. Data derived design principles are more likely to be predictive.

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