A cookstove production line at SSMNordica MacCarty

Tier 3 Biomass Stoves

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Photo of sheet metal machines factory in China
Biomass cookstoves are mass produced at Shengzhou Stove Manufacturer

To reach multiple, interlinked Sustainable Development Goals, the UN advises that “the share of the population mainly using improved cooking solutions like low-emission biomass stoves reaching Tier 3* or better needs to increase to 35 percent by 2030.” (ACHIEVING UNIVERSAL ACCESS AND NET-ZERO EMISSIONS BY 2050: A Global Roadmap for Just and Inclusive Clean Cooking Transition, United Nations, 2023)

Well, that would be a very welcome change. The stove community has been trying to bring improved stoves into use for decades. Luckily, we are starting to know a lot more about how to improve stoves and increase market share: To make Tier 3 stoves successfully compete with traditional stoves. Much better stoves must be as loved by cooks while they make profit for factories, distributors, retailers, and, if lucky, for carbon developer and markets.

It may be that the most important missing link has been that, without carbon revenue, Tier 3 stoves have been too expensive. However, since the prices of avoided tons of CO2 have been fluctuating, a lot of folks have been exploring ways to sell Tier 3 biomass stoves without this subsidy.

Over the years, stakeholders (including the DOE, Shell Foundation, the European Union) have commented that substantial price reductions are possible by using less expensive materials, with design changes, more efficient production at scale, tariff reductions, decreases in the cost of transportation, and distribution with higher volume sales. As 2026 begins, the combination of factors seems to be bringing market driven Tier 3 (or even Tier 4) stoves closer to reality. 

Getting better products in use is occurring on a massive scale globally.

Let’s include good stoves.

*Biomass cookstove Tiers of Performance range from Tier 0 (worst) to Tier 5 (best). They are determined by using a standard test sequence (ISO 19867) that establishes international comparability in measurement of cookstove emissions and efficiency. Tiered Metrics include thermal efficiency, and levels of CO, CO2 and PM2.5.

ISO 19867: Thermal Efficiency

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Boiling that five liters in 25 minutes max!

There have been many versions of Water Boiling Tests, including the 1987 International Standards, Shell Foundation, IWA, ISO 19867, Chinese, Indian and many others. The lab tests do not predict in-field use but are intended to compare results when variables are controlled. 

It can be amusing, in a sad way, to watch how the stove communities (heating and cooking) can get quite hot under the collar about how lab tests don’t accurately predict what users experience. I suppose there are some small rewards that accompany a historical perspective and having read the quite explicit introductions?

I like ISO 19867 and value testing stoves at high, medium, low power, etc. The recent grant has us attempting to upgrade performance in twelve natural draft TLUDs and Rocket stoves. When using ISO 19867, it’s interesting to see how much thermal efficiency is valued! Emissions of CO and PM2.5 are evaluated by the weight of the pollutant (gram or milligram) per megajoule delivered to the pot. To get a good score, thermal efficiency must be as good as you can get, while CO and PM2.5 must be reduced as much as possible, as well.

Not a bad idea! 

We are investigating a new way of making Rocket stoves and have tried it in two SSM stoves so far and are now trying it in a BURN stove. Going for highest thermal efficiency is pretty well understood and that’s nice when emissions and thermal efficiency are interrelated.

Lab Tests: Cooking and Heating Stoves

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Unfortunately, although introductions to lab tests warn that results do not predict actual performance, the recent use of lab data to earn carbon credits has made an unfortunate error more commonplace. For decades, introductions to lab tests have warned that only field-testing can determine actual efficiency, emissions, effectiveness, market validity, etc. The World Health Organization based their stove standards aimed at protecting health on field-testing for this reason. 

Lab tests are helpful when comparing performance to understand how fire might be more useful. Starting with the 1985 International Standards, test users were advised not to use lab data to predict actual performance. While improving other carbon methodologies, using field-testing to estimate reductions would dramatically improve the accuracy of offsets.  

Carefully performed lab tests tend to overestimate fuel efficiency and underestimate emissions. This has landed cook stoves and heating stoves in serious controversy. A lab tested Tier 4 cookstove can be Tier 2 in real life – or mistaken for a flowerpot. My first Rocket stoves were often used for this important function in Mexico. 

A lab tested 2 g/hr PM heating stove often emits a lot more smoke when the harmful pollutant is measured from chimneys in houses. In an effort to reduce confounding variables, lab tests show closer to optimal performance. Real life human beings tend to operate stoves with less care, wood is wet, life deserves attention, too.

Maybe the test warnings should have been highlighted in green?

International Standards, 1985

“This is a laboratory test…while it does not necessarily correlate to actual stove performance, when cooking food, it facilitates the comparison of stoves under controlled conditions with relatively few cultural variations.” (Testing the Efficiency of Wood Burning Cookstoves, International Standards 1985).

(ISO 19867-1)

“This document provides a standard test sequence that can be used to compare the performance of various cookstove types, cookstove fuels, and cooking practices under controlled laboratory test conditions, as specified in this document. For evaluation of the performance and predicted outcomes of a cooking system in the field [comprising cookstove(s), fuel(s), cooking vessel(s), kitchen, ventilation, and user(s)], ISO 198691 applies.”

https://www.iso.org/obp/ui/#iso:std:iso:19867:-1:ed-1:v1:en

A Multi-tiered Framework for Evaluating Cooking Systems

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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.

International Support for Cook Stoves and Climate!

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WASHINGTON, DC, April 22, 2021 — At the Leaders Summit on Climate hosted by President Biden, the U.S. government pledged to help countries achieve their climate ambitions through expanding access to clean cooking.

“Providing clean energy to households is critical to achieving global climate and sustainable development goals,” said Helena Molin Valdés, Head of the Climate and Clean Air Coalition Secretariat. “Smoke from fireplaces, cook stoves, and lighting is responsible for more than half of human-made black carbon emissions and millions of premature deaths from household air pollution. The Climate and Clean Air Coalition partners welcome the U.S. government’s re-engagement in the issue and look forward to cooperating to put in place solutions that improve lives and protect the planet.” 

The United Nations Foundation’s Clean Cooking Alliance

Would it be helpful to add climate metrics to the health-based ISO 19867? Currently, Scoring Tier 5 for emissions of PM2.5 and CO means that safety is assured in average households. As the score decreases from Tier 5 to Tier 0 the estimated amounts of ill health from breathing smoke and gas increase.

During ISO 19867 testing under an emissions hood, the fuel use, thermal efficiency and emissions of CO2, CO, and PM2.5 are measured and the data is used to determine a ranking on the voluntary tiers of performance. ARC multiplies lab data by a factor of three to estimate in field emissions. Usually in cook stoves, the CO2 has by far the largest effect on climate change. However, PM (black to white in color), CO, methane (CH4), non-methane hydrocarbons (NMHC), and nitrous oxide (N2O) also have varying amounts of climate forcing potentials. Currently, CO2, PM2.5, and CO are measured as a part of ISO 19867. ARC also determines the amount of black carbon in every test (using a filter) of PM2.5. Adding methane and non-methane hydrocarbons to the measured gases is not difficult. In fact, Sam is working on adding them to the LEMS right now.

The effects of inhaling particulate matter have been widely studied in humans and animals. They include asthma, cardiovascular disease, and premature death. Particles can also have an extremely strong effect on the atmosphere by absorbing and/or scattering the sun’s incoming radiation, depending on their color. The black particles have an approximate warming potential by weight of 680 times that of CO2 (Roden and Bond, 2006; Bond and Sun, 2005).

Total global warming impact, grams CO2 equivalent on a 100-year time-frame, per liter of water boiled and simmered for 30 minutes, normalized for starting temperature and fuel moisture content. Inclusive of CO2 and all PICs.

When we studied the global warming impact of five cook stoves burning biomass, CO2 was shown to be the major component, as seen above. At the time (2008), estimates of the various warming potentials were:

CO2….1  (IPCC, 2007)
CO….1.9  (IPCC, 2007)
CH4….25  (IPCC, 2007)
NMHC….12  (Edwards and Smith, 2002)
N2O….298  (IPCC, 2007)
PM – Black….680  (Roden and Bond, 2006; Bond and Sun, 2005)
PM – White….-50 (Estimate – Bond, 2007)

Warming potential, 100-year, CO2 equivalents

  • When biomass is harvested sustainably, the CO2 emissions from biomass-burning are considered to be greenhouse-neutral.
  • Although N2O is a strong climate-forcing constituent, emissions from the wood- and charcoal-burning stoves were very low, contributing less than 1% to the overall warming potentials.
  • The data suggests that there are biomass stoves that can be designed to (1) reduce the fuel used to cook, (2) reduce health-damaging emissions and (3) address climate change. The considerable differences in climate-changing emissions from the stoves in this study should be noted. Large-scale use of cleaner burning stoves might well reduce global warming effects, especially when the biomass is harvested in a “carbon neutral” manner. (N. MacCarty, D. Ogle, D. Still, T. Bond, C. Roden, Energy for Sustainable Development, 2008)

“Going Faster” on ISO 19867

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Many years ago, Kirk Smith hired Aprovecho to help Rob Bailis from U. C. Berkeley update and add emissions to the Water Boiling Test in the 1985 International Testing Standards. The Water Boiling Test (WBT) measured in the lab how much wood was used at full power and when simmering water. The writers of the International Testing Standards defined the purpose of the WBT as: “While it does not correlate to actual stove performance when cooking food, it facilitates the comparison of stoves under controlled conditions with relatively few cultural variables.”

The 1985 Kitchen Performance Test (KPT) measured fuel use in actual households, and the Controlled Cooking Test (CCT) was a bridge between the WBT and the KPT. ARC uses the Controlled (or Uncontrolled) Cooking Test to develop stoves with local committees of all stakeholders, as recommended by Sam Baldwin. In this test, locals cook with their own fuel, pots, and cooking practices, hopefully at Regional Testing and Knowledge Centers under the total capture emissions hood. Using the WBT in the lab has been a good tool for ARC to improve heat transfer and combustion efficiency. The cooks, marketers, manufacturers and funders in the project have to make the stove. It must work for users. They are experts.

We now use the new, updated Water Heating Test (ISO 19867) to improve heat transfer and combustion efficiency in the lab and it’s great. We are directed to try to use the type of wood, pot, and cooking practices from the intended project location. ISO 19867 also has us test the prototypes at high, medium, and low power to learn more about performance. As said, there are many other variables that can only be learned from the local cooks and everyone involved in the project. How much the can stove cost, that chapatis have to be toasted in the fuel door, that cooks in southern India sit cross legged so the stove must be pretty short, etc. is information that is obviously necessary and field based. The idea is that lab tests inform the prepared mind of the engineer who then works hand in glove with the project stakeholders in their location to make an effective product.

Kelsey Bilsback from Colorado State University advised that lots of times stoves in actual use are operated at exceedingly high fire powers. We agree! When applicable we use very high power (and relatively untended fires with sticks gathered from the forest). We are trying to find out whether a biomass stove burning found fuels can be clean burning at the equivalent of 85 MPH.

Thanks, Kelsey! Good idea!

It is easy to get a Tier 5 score for PM2.5 and CO!

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It’s fascinating to read the ISO 19867 Standards for cookstoves and I agree with a lot of it. Many of us in the ‘stove world’ were involved for years in creating those documents. One of the big improvements is testing stoves at high, medium, and low power while reporting the results with the firepower.

We forgot to do that in the International Working Agreement started in Peru at the Partnership for Clean Indoor Air meeting in 2011. Since stoves generally make fewer emissions at low power it was a temptation to reduce the firepower and achieve a higher score on the Tiers. Since firepower was not seen on the Tier scorecard, it was really not possible to compare performance. And as we know, people tend to like high power stoves. It’s so great that this problem has been fixed in ISO 19867.

How to get a Tier 5 score for emissions? Use a chimney.

Santa Claus in a chimney
Santa understands the importance of chimneys… Happy Holidays from the Aprovecho team!

The chimney transports most or all of the PM2.5 and CO out of the kitchen. Only “fugitive” emissions escape into the room. In ISO 19867 the fugitive emissions are used for the emission rate values. For unvented stoves, total emissions are used for the emission rate values. Just make sure that your chimney and stove do not leak.

It makes sense. Here in rural Oregon, unfortunately, smoke pours out of chimneys all day and night as folks stay warm with wood. Heating stoves can be very smoky! The airtight chimney and stove get essentially all of the smoke outside of the building where concentrations are and stove get essentially all of the smoke outside of the building where concentrations are diluted.

Now, of course, at ARC we try to combine high combustion efficiency with effective chimneys. We need to protect the quality of the outside air, as well. The combination is intended to protect indoor and outdoor air. If the outdoor air is polluted it is less effective in lowering harmful concentrations. Combustion efficiency is always great and to protect health it must increase when the outside air quality is degraded. In Beijing you don’t want to add one more milligram of smoke into the air!

An indoor/outdoor air quality planning tool.

Sam Bentson created an excel spreadsheet that explains how protecting indoor and outdoor air quality are related. You can download the spreadsheet here, and learn how to use it for project planning: aprovecho.org/portfolio-item/project-planning/