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.

Cooking over an open fire in Ghana. (Photo: Global Alliance for Clean Cookstoves)

The air in a kitchen has to be very clean to protect women and children from multiple diseases. Unfortunately, moderate amounts of smoke seem to damage health almost as much as higher concentrations. 

As exposure rises from zero, the chance that a child will get pneumonia increases sharply and then levels off so that indoor air with 200μg/m3 PM2.5 is almost as dangerous as air at 400μg/m3 (Burnett et al., 2014). The World Health Organization Intermediate Guideline for PM2.5 is 35μg/m3.

In order of effectiveness, when cooking in a kitchen, health interventions seem to be:

  1. Venting smoke up a functional chimney.
  2. Increasing the fresh air entering the kitchen to dilute smoke and gases. (When the outdoor air is clean and the air exchange rate is doubled, the indoor air pollution is reduced by half.)
  3.  Burning up almost all of the smoke in the stove.

 Unvented Rocket stoves, and other ‘moderately clean burning’ stoves (such as a carefully tended open fire with pot skirt), emit much too much smoke and gas to protect health in houses. 

Cooking outside, especially upwind of the fire in a bit of breeze, is highly effective in lowering harmful concentrations of PM2.5.

Cooking outside seems to be a first choice intervention, when applicable. Even ‘moderately clean burning biomass stoves’ can be used when the cook is upwind of the fire in a bit of a breeze, meeting the WHO Intermediate Guideline for PM2.5. 

Of course, cooking with a low emission stove is preferable, when possible!

When Dean Still came to Aprovecho in 1989, Dr. Larry Winiarski asked him to compare the thermal efficiency of the Lorena stove and the Three Stone Fire. The testing revealed a problem for the ARC staff when our Lorena used three times more fuel than a carefully operated open fire! 

It’s surprising to learn how efficient a three stone fire can be!

Half of the staff, who had written books about the Lorena and taught thousands of people about their invention, were never convinced that a problem existed. The other half were embarrassed and became fervent believers in Dr. Kirk Smith’s famous saying that “You get what you inspect, not what you expect.”

Making a public mistake pushed a reconstituted ARC to proceed more slowly, to challenge speculation, and to try to generate reliable data. We learned that a lot of local knowledge is required to take successful products to market. Evidence can help to overcome inventors’ pride, cognitive dissonance, and the financial cost of changing directions. At the same time, inventor’s pride, cognitive dissonance, and the cost of changing direction also influence decision making.

Traditional three stone fire
Use of traditional three stone fire in one Rakhine village.  ©FAO/Myanmar

As with any tool, the skill of the operator determines how well the work is accomplished. It takes years to learn how to use a hammer or shovel. The Three Stone Fire can be effective and clean or it can be very dirty and wasteful. In some kitchens, large fires use a lot of wood and make a great deal of smoke. Small fires are also made that cook food relatively cleanly. 

Watching indigenous experts cook with fire has led to a better understanding of improved biomass fuel use. Cooks who are trying to conserve wood tend to burn the wood at the tip of the stick making flames. Knowledgeable cooks only need a small, hot fire close to the pot to boil water. 

Improving upon a well-made Three Stone Fire has been more difficult than expected. Learning from expert users helped teach engineers how to make better stoves. Well-constructed Three Stone Fires protected from the wind and tended with care, score between 20% and 30% thermal efficiency. Open fires made with moister wood and operated with less attention can score as low as 5%. 

When Tami Bond achieved 33% thermal efficiency with a Three Stone Fire, ARC started to depend on the pot skirt with a 6mm channel gap to help folks use less fuel to cook food. Expertise with the Three Stone Fire is an important skill that empowers the cook and has to be respected.

Smoggy NYC, photo by urbanfeel on flickr
Smoggy NYC, photo by urbanfeel on flickr
photo by urbanfeel on flickr

Several articles have pointed out that using biomass-heating stoves can result in health problems in densely populated areas. We are working with friends at the EPA to think about how we might define PM2.5 emission rates for residential biomass heating stoves that would protect health in densely populated cities. 

When the population density goes up (more people are generating pollution), the emission rate has to go down (the stoves have to be cleaner).

What emission rate for PM2.5 would protect personal health if 1/3 of the folks in New York City replaced the natural gas used for residential heating with biomass?

Very roughly, using an EPA outdoor air pollution model, a biomass-generated PM2.5 emission rate of around 0.3g/h looks like it might work in NYC. That’s the emission rate of a good pellet stove.

To accurately make predictions, a model of the air circulation in a city can be generated. Great for planning. For a description of the EPA model, see Chapter 5 “Protecting Health” in Clean Burning Biomass Cookstoves, 2021.

Yesterday morning I was on an ETHOS panel discussing stoves, health and climate change. I loved the discussion and was filled with hope that facing the end of the fossil fuel era might catalyze better use of resources.  Living sustainably has been a dream of mine since I was 15 years old in 1967. Like many people I have been living with this dream for many years in a world that has not been committed to renewable energy.

Aprovecho (started in 1976) has tried to create better understandings of farming, forestry, and appropriate technology (focusing on biomass stoves), and helped me to investigate fire. Working with Dr. Larry Winiarski was a blessing in many ways, especially by showing me the utility of the scientific method.

It was great to be able to direct folks at ETHOS to our 2020 revised book “Clean Burning Biomass Cookstoves” that contains most everything that we’ve learned since the publication in 2015. Download here: Clean Burning Biomass Cookstoves, 2nd Edition, 2021. These newsletters are another “closer to real time” update.

The ETHOS Conference continues online through Friday (Jan. 28). You can see the agenda, watch pre-recorded presentations and register to attend the live seminars/discussions at ETHOS 2022. I think you will find it well worth your while!

-All best, Dean Still, Research Director

Would it be helpful to evaluate cookstoves with multiple metrics?

ISO 19867-1:2018 is a set of laboratory test protocols for evaluating cookstove performance, published by the International Organization for Standardization. Their Voluntary Performance Targets are a set of baseline criteria defined as tiers – Tier 0 is worst, Tier 5 is best. These ISO Targets measure Thermal Efficiency, Exposure (PM2.5 and CO), Safety, and Durability.

In 2020 The World Bank suggested the addition of Convenience, Availability, and Affordability. Since President Biden recently announced the “end of the fossil fuel era” ARC is adding Climate to the list, resulting in the nice looking mandala seen above.

Is it possible that cookstoves that score well enough on these eight metrics might be more successful interventions? We think that combining technical and contextual measures will help in the design/manufacture/sales of consumer based products. (ARC has called for affordability to be included for decades.)

Since LPG, propane, alcohol derived from fossil fuels, and a large percentage of electric generation emit dangerous amounts of CO2 causing climate change, it may be that clean burning, carbon neutral biomass stoves will rank higher on an “energy ladder,” especially when multiple factors are considered.

Chart comparing energy output of 1 acre of grain vs 1 acre woodlot

One of my favorite reference books is “The Energy Primer” published in 1974. It has comprehensive review articles on solar, wind, water, and biomass energy. The following chart comes from a great article on biomass written by Richard Merrill. When I taught semester courses to college students at ARC, I tried to give students useful comparisons so they would be able to estimate the potential success of alternate technologies (unfortunately, fads that are bound to fail are all too prevalent in the green culture).

How much renewable energy can be grown on an acre of land? Can a family create an energy budget based on yearly production? As seen below, there are big differences in the amounts of energy that can be produced by a one acre grain field or one acre woodlot.

Energy output of 1 acre of grain vs. 1 acre woodlot, from “The Energy Primer”

One acre of hay yields something like 29 million BTUs per year. One acre of trees is better, producing an estimated 42 million BTUs per year.

If the hay is turned into alcohol the yield is greatly reduced (6 million BTU/year) and the average yield of 3.5 tons per acre of trees is approximately 8.5 million BTU/year.

If the hay is fed to cows and the manure is turned into methane the energy content is 15 million BTU/yr.

Burning biomass for heating and cooking can be a lot more efficient than making alcohol or methane to be used for the same purpose.

At ARC, after decades of “living on the land”, we think that one or two acres of biomass for energy and five acres for food is a good place to start calculations when planning for a secure and happy family. It’s amazing to own land!

A St. Ayles skiff, my favorite boat

Before I met Dr. Larry Winiarski I was a boat builder, but I had already realized that my love for making boats was mostly supported by rich people. And when my friends and I built a 36’ ocean going sailboat it was great but after several years of exploring it started to be a bit self-serving. When Larry showed me that my carpentry skills could help him develop Rocket stoves to try to help people, I ended up being much happier.

Since Appropriate Technology is intended to be affordable, experiments do not cost very much. Low cost experiments enable anyone to improve necessary things like wood burning stoves. Using my skills to try to address a real problem was a lot more fulfilling. Including users in the process meant that I spent a lot of time with cooks and manufacturers who are the real experts. In India I lived in 18 villages working with groups of women who created the short Rocket stove now built around the world.

I wish that I had met Larry in grade school! Knowing that anything I learned could be useful would have made a big difference. Just reading and learning without an intended purpose seemed to me to be rather meaningless.

Doing experiments every day on stoves has helped me as a person. I had also made science toys and sold them at craft fairs, but again even though I loved making the crafts I ended up feeling unfulfilled just entertaining people. I wanted to do something that was more helpful. Finding a good problem to try to solve has helped me a lot. I include finding a good problem in my prayers for lots of people.

There are hundreds of good problems for folks to work on in Appropriate Technology. I’m thinking about teaching a class to local students in the hope that the meaning Larry passed on to me could work for them as well. If you would like to solve a problem, we can suggest many possibilities.

Wishing you all the best in the coming year,
Dean Still

In our Nov. 24 newsletter, we shared a basic description of how wood burns from Samuel Baldwin’s book “Biomass Stoves: Engineering Design, Development, and Dissemination” (1987). Here are more details about the process from the same book:

“The temperature of the hot gas above the wood is typically around 1100ºC and is limited by radiant heat loss and by mixing with cold ambient air. As the volatiles rise they react with other volatile molecules forming soot and smoke and simultaneously burning as they mix with oxygen. Some 213 different compounds have so far been identified among these volatiles. If a cold object, such as a pot is placed close to the fire, it will cool and stop the combustion of some of these volatiles, leaving a thick black smoke.”

“Overall, these burning volatiles account for about two-thirds of the energy released by a wood fire. The burning charcoal left behind accounts for the remaining third. Because the volatiles are released as long as the wood is hot, closing off the air supply stops combustion alone. The heat output of the fire is then reduced but the wood continues to be consumed for as long as it is hot, releasing unburned volatiles as smoke and leaving charcoal behind.”

“As the topmost layers gradually lose all their volatiles only a porous char is left behind. This hot char helps catalyze the breakdown of escaping volatile gases, producing lighter, more completely reacting gases to feed the flames. In some cases, the volatiles cannot easily escape through this char layer. As they expand and force their way out, they cause the burning wood to crack and hiss or spit burning embers.”

“The char layer also has a lower thermal conductivity than wood. This slows conduction of heat to the interior and thus slows the release of volatiles to feed the flames.”

“At the surface of the char, carbon dioxide reacts with the char’s carbon to produce carbon monoxide. Slightly further away (fractions of a millimeter) the greater oxygen concentration completes the combustion process by reacting with the carbon monoxide to produce carbon dioxide. The temperature near the surface of the burning charcoal surface is typically about 800ºC. The endothermic (heat absorbing) dissociation of carbon dioxide to carbon monoxide and oxygen, and radiant heat loss, limit higher temperatures.”

“When all the carbon has burned off only mineral salts remain as ash.  This ash limits the flow of oxygen to the interior and so limits the combustion rate. This is an important mechanism controlling the combustion rate in charcoal stoves.”

“The entire process uses about 5 m³ of air (at 20ºC and sea level pressure) to completely burn 1 kg of wood. To completely burn 1 kg of charcoal requires about 9 m³ of air. Thus, a wood fire burning at a power level of 1 kw burns 0.0556 grams of air per second. Additional excess air is always present in open stoves and is important to insure that the combustion process is relatively complete.”