Illustration of how an electrostatic precipitator works
Illustration of how an electrostatic precipitator works
Electrostatic Precipitation: Smoke particles are negatively charged and attracted to positively charged metal plates that are automatically cleaned.

Both automobiles and the biomass industry rely on improving combustion efficiency and post combustion reduction of PM2.5 to achieve “clean burning.” It’s really hard to rely on the combustion chamber to burn up enough of the harmful smoke to protect health, especially in large scale applications. Of course, all efforts should be made to be as efficient as possible. The goal is to burn up everything! In cookstoves, with limited space and a pressing need to be affordable, the problem becomes more acute.

Biomass heating stoves are larger and can cost a lot more than cookstoves. Industrial technologies are even less constrained. For decades, home heaters have tried catalysts to reduce emissions. Factories have used a wider array of technologies including filtration, catalysts, and electrostatic precipitation. Chapter 8 in “Clean Burning Biomass Cookstoves, 2nd edition, 2021” includes explanations of these technologies. 

Generally, filtration can work very well to capture dust and smoke with reported efficiencies of up to 99% (Frisky, et al., 2001). Catalytic converters are placed into the hot exhaust path where temperatures are hot enough (above 426°C). They work well with CO (30% to 95%) but not so well to remove PM2.5 (30% to 40%) (Hukkanen, et al., 2012). The Swiss electrostatic precipitator (ESP) called the OekoTube has been measured to reduce PM2.5 by 80.2% to 97.7% (Brunner, et al., 2018). However, as in industrial uses, routine cleaning is necessary to remove creosote and other coatings that interfere with proper function. Unlike filters and catalytic converters, the low wattage ESP does not reduce the draft in the stove, which could be potentially advantageous. 

ARC has been experimenting with post combustion of PM2.5 since 2017 as a result of the EPA SBIR funded work to create a clean burning biomass heating stove. We believe that if ESP is to be useful, automatic self-cleaning must be included, as in some industrial products. The hope is to invent super clean combustion but it’s great that post combustion approaches already exist. On the other hand, forced draft mixing, which is relied upon for combustion efficiency in industry, is largely missing in both cookstoves and residential biomass heaters. Perhaps its addition will be sufficient to reach the goals of protecting health and carbon neutral fuel use with renewably harvested biomass?

Kuniokoa Stove, original top replaced with cast iron top.

It is more likely that close to 50% thermal efficiency will be achieved with a biomass burning stove when:

  • Small sticks are burned that produce tall, hot flames while using the least amount of wood.
  • A 30cm in diameter aluminum pot is used with a 14cm high pot skirt that creates a 6mm channel gap.
  • The stove top (with 6mm pot supports) weighs as little as possible. The narrow channel gaps in the stove top effectively deliver wasted heat from the hot gases into the stove top while increasing beneficial convective heat transfer into the pot, so less mass to hold the heat is better.
  • A grate helps the sticks to make tall, hot flames and reduces the made charcoal.

Starting with all of the above, we tested various Rocket stove combinations to try to determine the effect of mass in the combustion chamber. The Kuniokoa Rocket stove is the lightest Rocket stove in our museum – it is made from sheet metal without insulation. (A refractory metal combustion chamber lasts longer when uninsulated.) When tested at high power (4,645 watts) the thermal efficiency was 51.7%, PM2.5 was Tier 2, and CO was Tier 3. Thermal efficiency dropped to 46.1% when we exchanged the Kuniokoa sheet metal stove top (0.31 kilo) with a cast iron version (2.36 kilo).

A similar Shengzhou Stove Manufacturer (SSM) Rocket stove was tested with a refractory cement combustion chamber (2.7 kilo) surrounded with rock wool insulation. The stove top was made from lightweight 304 stainless steel. When tested at high power (4,816 watts) the thermal efficiency was 48.6%, PM2.5 was Tier 2, and CO was Tier 3. The refractory cement combustion chamber is heavier but it can be insulated because the material has a working temperature of 1,100°C.

When a SSM lighter refractory ceramic combustion chamber (1.2 kilo) was exchanged into the SSM Rocket stove with rock wool insulation and a lightweight 304 stainless steel stove top, the thermal efficiency (at 4,709 watts) rose to 51.4%, with Tier 2 for PM2.5 and Tier 3 for CO.

  • It may be that insulating a one kilo combustion chamber in a Rocket stove offsets the disadvantage of the higher mass when compared to uninsulated sheet metal.
  • In these tests, adding another kilo to the insulated combustion chamber in the SSM Rocket stove lowered thermal efficiency from 51% to 46%.
  • When the mass of the stove top was increased from 0.3 to 2.3 kilos, thermal efficiency dropped by about 5%.
Dos por Tres Stove with Chimney, photo courtesy of Proyecto Mirador

During an ETHOS panel discussion on Cooking, Health, and Climate, it was great to see that the Justa stove with chimney protected health so well. Chimneys are mandated by law in the USA/Europe/China and many other countries. When Mahatma Gandhi returned to India from England he introduced chimneys as a logical upgrade of kitchens.

The WHO (2018) listed five prescriptions to protect health:

  1.  Use only clean household energy when available 
  2. While waiting for gas, use technologies like low-emission biomass cook stoves
  3. Minimize the time children spend around smoky fires
  4. Increase ventilation
  5. Install a chimney

Functional chimneys are the historical first step to protect health.  It’s so pleasant to sit and chat in the clean kitchen when a Justa, Dos por Tres, or Patsari stove is being used! Following up with improved combustion efficiency helps to protect climate and outdoor air quality.

A Justa Stove with chimney, photo courtesy of Stove Team International

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

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.

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!

Dr. Tom Reed, Dr. Alexis Belonio, Dr. Paul Anderson and Kirk Harris have refined TLUD technology

A natural draft TLUD can be as clean burning as a forced draft TLUD burning wood pellets. On the other hand, a natural draft Rocket stove needs a fan to be clean burning.

Dr. Tom Reed’s forced draft (FD) Woodgas stove achieved an emissions rate of 2mg/min of PM2.5 with pellet fuel (ARC, 2015). The Kirk Harris natural draft (ND) TLUD emitted 0.7mg/min of PM2.5, when tested at Lawrence Berkeley National Lab with pellets.

WHO Intermediate Emission Rate Targets

Unvented stoveVented stove
PM2.51.75 mg/minPM2.5   1.75 mg/min
CO 0.35 g/minCO   1.45 g/min

ND TLUDs tend to be pretty tall to generate necessary draft. A FD TLUD can be shorter since the fan creates the draft. Taller ND TLUDs can be expensive to manufacture. Precise control of primary air enabled a 3 to 1 turn down ratio in the Harris ND TLUD, difficult to achieve in a FD TLUD. (The FD Mimi-Moto, for example, has two combustion chambers, small and large, to provide cooks with high and low power).

Modern ND TLUDs

  • The primary air is adjusted to control the rate at which pellets are turned into combustible gases.
  • The secondary air jets cover the fuel bed.
  • A hole in the concentrator plate above the secondary air jets forces the flame into a vertical cylinder.
  • The cylinder of flame then enters static devices that create further mixing of air, flame, and gases.
  • The flame is shortened and does not touch the bottom of the pot where gases can condense into smoke.
Static mixers in the Kirk Harris 0.7mg/min PM2.5 ND TLUD
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.”