Stove parts at SSM

A Hospitable Chinese Environment for Improved Cookstoves

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Stove parts at SSM
Ceramic combustion chambers and other stove parts stockpiled at SSM

Information summarized via Google AI:

In 2026, the Chinese carbon market (CCER) is generally a more hospitable environment for cookstove projects—defined as traditional improved biomass stoves that lack advanced digital monitoring—compared to UN or Western voluntary markets. 

While international markets now demand costly digital “metering” and stricter methodologies, the Chinese domestic market offers more flexibility for simpler projects. 

  • Protection from VCM Volatility: Traditional cookstove projects in Western markets are currently facing a “crisis of quality”. Large-scale studies in 2025 found that older projects likely over-credited by 900%, causing Western buyers to avoid them. 
  • The Chinese CCER market provides a sheltered environment where domestic compliance entities can still utilize these credits without the same level of global reputational risk.
  • Methodological Lag: While Western markets are shifting to metered and measured standards (e.g., Gold Standard’s new digital tools), China’s revamped CCER scheme allows for traditional improved efficiency stoves that fit into its broader “dual control” carbon and energy policies.
  • Price Floor: In 2026, “old-fashioned” credits in the global voluntary market have struggled to trade above $3.30/tCO2e. In contrast, CCERs are expected to maintain a higher floor ($10+) due to scarcity and mandatory offset needs within the Chinese ETS.

Tariffs

Recently many African countries removed tariffs on Chinese biomass cookstoves dramatically reducing cost to consumers. A widespread trend toward zero-rating clean cooking technologies has emerged to meet energy access goals. While China has eliminated tariffs on imports from 53 African nations, this does not automatically remove the duties African countries charge on Chinese-manufactured goods. 

Recent Policy Changes (2026)

Several African nations have implemented specific exemptions for clean energy equipment: 

Sierra Leone

: Under the 2026 Finance Act, the government has officially approved zero import duties on a wide range of clean cooking technologies, including improved biomass stoves and solar cookers.

Cameroon

: Effective in 2026, Cameroon has introduced customs exemptions for biofuel equipment, including industrial machinery used to produce pellets and eco-charcoal, to curb deforestation.

Kenya and Tanzania

These nations were identified in early 2026 as having the greatest improvement in clean cooking policy coverage, frequently utilizing tax incentives and duty remissions to lower costs for consumers. 

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.

Don’t Let The Flame Touch The Pot

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Zones of a Laminar Diffusion Flame www.chemistryviews.org/details/ezine/1393243/What_Makes_a_Candle_Flame/

As we iterate changes in natural draft TLUDs and Rocket stove prototypes, trying to reduce emissions while maintaining high thermal efficiencies, one design principle keeps on suggesting itself:

Don’t let the flame touch the pot.

The pot is ‘cold’ and if the flame has not burned itself out and contacts the surface, we instantly see CO and PM2.5 rise on the real time emissions screen. (See “Zone 3 and 4 boundary” comment in illustration above.)

Real time PM2.5 is not as accurate as the weight of particulates captured on a filter, but it can be useful. Changes can be done quickly in the prototype and resulting effects are seen right away on the computer screen. Fun!

When we get close to project goals, we switch to gravimetric PM2.5 and statistical confidence to make sure that measurements are accurate. 

In both TLUDs and Rockets, changes in prototypes that make sure the flame doesn’t touch the pot seem to be quite effective.

Give it a try?

Early Rocket Stove Research at Aprovecho Still Rings True

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Early Rocket Stove Research at Aprovecho Still Rings True

A summary of Global Modeling and Testing of Rocket Stove Operating Variations, Nordica A. Hudelson, K.M. Bryden, Dean Still Department of Mechanical Engineering, Iowa State University, Aprovecho Research Center, 2001

Photo: Karl Maasdam/OSU Foundation

In the summer of 2000, Aprovecho’s current Executive Director Nordica MacCarty spent a couple of months doing a series of 50 rocket stove tests. Nine variations of the basic rocket stove were tested, changing several parameters. The goal of this research was to determine the location and magnitude of heat losses from stoves to inform better design of efficient stoves. Her conclusions and recommendations are still valid twenty-four years later. 

From the paper:

Several important stove design parameters were varied for efficiency and loss comparisons. 

  1. The stove inlet diameter was an important factor… The chimney height had a drastic impact on the heat radiation from the flames to the pan. 

The most basic result of this series of three tests per stove shows that smaller inlets and shorter chimneys are more efficient, shown in the following chart: It should be noted that the smaller inlet stoves took a much longer time period to reach boiling than those with larger inlets. Thus while they are technically more efficient, they may not be ideal for field distribution as a stove will not be used if it does not perform according to the users expectations. 

  1. The gap between the top of the stove and the bottom of the pan influenced how much heat from the flue gasses and flames was transferred to the pan. 

The stove top to pan gap was varied from a standard of 1” down to ½” and then ¼” for comparison on the 4.5” diameter stoves. An almost linear change in efficiency was observed, increasing by about 9% when the gap was reduced from 1” to ¼”. An important consideration, however, is that the ¼” gap sometimes caused the fire to burn out the top front of the feed magazine because not enough air was able to be drawn through the decreased gap to create the proper draft.

  1. The amount of insulation in the stove influenced how long it took to heat up the stove and thus affected the efficiency. 

An unexpected discovery from these experiments is the effect of insulation on the efficiency of the rocket stove. First, it was shown that adding perlite insulation increases efficiency by about 2 to 5 percent over an uninsulated 4.5” diameter stove. However, super insulating the stove with two layers of fiberglass blanket insulation does not increase efficiency, but instead caused performance to actually decrease by as much as 3% for the 9” high stove. This is most likely due to the fact that adding fiberglass insulation increases the mass of the stove, thus it takes a longer time frame to heat up the stove and insulation.

  1. The use of a skirt around the pan increased heat transfer around the perimeter of the pan. 

It was shown that use of a skirt has the most profound impact on stove efficiency. The 4.5” diameter stoves with a 1” stove to pan gap were each run first without a skirt, then with a ¼” gap uninsulated skirt, then a ¼” gap insulated skirt, and finally a tight insulated skirt. Addition of the uninsulated skirt caused efficiencies to increase by 10%, and insulating that skirt caused an additional 10% rise! The stove with 9” chimney rose from 21% to 39% simply by use of an insulated skirt.

Losses 

Heat losses in different forms from different areas of a stove should be minimized in order to maximize the amount of heat transferred to the water. On average for all tests, convection accounted for 77%, radiation for 12%, and storage for 11% of total losses from the stove. For the pan, convection accounted for 92% of the losses, radiation for 6%, and storage for about 2%. 

Conclusions and Recommendations 

This series of fifty tests on varying operating setups of the rocket stove showed the following: 

A smaller inlet diameter results in higher efficiency, lower combustion gas losses, higher stove and pan losses, higher percent oxygen remaining, and lower air-fuel ratios. 

A shorter chimney results in higher efficiency, slightly lower combustion gas losses, higher stove and pan losses, lower percent oxygen, and a lower air-fuel ratio. 

Medium (perlite) insulation provides the highest efficiency and combustion gas losses, while increasing levels of insulation generally decreases stove and pan losses, percent oxygen, and airfuel ratios. 

Decreasing stove to pan gap increases efficiency, decreases combustion gas losses, increases stove and pan losses, and decreases percent oxygen and air-fuel ratios.

Use of a skirt with increasing degrees of tightness and insulation increases efficiency, decreases combustion gas losses, decreases stove and pan losses, decreases percent oxygen, and decreases air-fuel ratios. 

Thus, an ideal Rocket stove theoretically would have a small inlet, short chimney, perlite insulation, a small stove to pan gap, and an insulated skirt to provide maximum efficiency, minimal losses, and more complete combustion of the fuel.

Cook Stoves for Ethos

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An open fire Jet-Flame?

We have been at Shengzhou Stove Manufacturer this November working on cookstoves for display at the annual ETHOS conference including:

  • Natural draft TLUD
  • Forced draft TLUD
  • Natural draft Rocket stoves
  • Open fire Jet-Flame

If stove projects do not have capable stoves, then project goals will not be met in the same way that some carbon credit projects fail to meet their impact goals when evaluated by fact checkers.

When I think about stoves and factor in low PM2.5 and Black Carbon, I worry that only found fuels will be available in large enough supply to make a significant difference. And the only technology that I imagine can cleanly burn biomass covered with bark is the open fire Jet-Flame.

So, as we get ready for ETHOS, we are trying to reduce costs, etc. in the Jet-Flame while we also concentrate on the three other stoves listed above.

A Clean Open Fire?

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Illustration from: Birzer et al. (2013) “An analysis of combustion from a top-lit up-draft (TLUD) cookstove”, Journal of Humanitarian Engineering 2(1):1-8 DOI:10.36479/jhe.v2i1.11

When Dr. Tom Reed and Dr. Larry Winiarski added short, narrow chimneys above the fire in TLUDs and Rocket stoves, they increased draft and created laminar flow flame in the tubes that could bring higher temperature gases to the pot, increasing thermal efficiency. It also made it harder to add mixing to the stoves since the column of flame in the short chimney resisted injection of air.

Laminar flow relies on molecular diffusion, leading to poor and slow mixing of fuel and air. Combustion efficiency can be lower because incomplete mixing may leave unburned fuel resulting in higher emissions when compared to turbulent conditions. Laminar flame is a smooth ordered flow with reactants moving in parallel layers. The flame front is relatively stable as in a candle.

Turbulent flow, on the other hand, is chaotic with fluctuating eddies that mix fuel and air together. The combustion is characterized by wrinkled, chaotic, and often shorter flames. The improved mixing results in more fuel coming into contact with the air and usually higher combustion efficiency. Turbulent flow can be jumpy and irregular with vortices. Mixing creates an intermingling of reactants and products. The enhanced mixing effectively wrinkles and increases the surface area of the flame front. The overall propagation rate is increased because chemical reactions occur across a larger area.

When the short chimneys were removed in existing TLUDs and Rocket stoves, several interesting things occurred. The draft was significantly reduced, assisting the flames to remain short. Insulating the inner surface of the stove body helped to deliver hot gases to the pot. However, it became harder to achieve Tier 5 (50%) thermal efficiencies. 

The slow speed, turbulent flame conditions seem to dramatically reduce the emissions of PM2.5 in both stoves. Several TLUDs and several Rocket stoves without short chimneys have been built and tested under the emissions hood. A complete summary of results, with CAD drawings of the stoves, is being prepared.

a pot of cooked rice

Turn Down Ratio

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One of the things that I like about the IWA Water Boiling Test is that it mimics the boiling and simmering functions of cooking food. Wood burning stoves are notorious for being too slow to boil big pots of water and for burning things like rice because they can’t be turned down enough to simmer.

At Aprovecho the farm, where we agreed to cook with wood, I ate a lot of burnt food! It was not always the fault of the stove.

Boiling and Simmering

How fast should a stove boil five liters of water? At ARC, we have a rule-of-thumb that if a stove takes longer than 25 minutes to boil five liters of water, people will usually not like it.

The IWA requires that a stove simmer water at between 97C and 93C for forty-five minutes. Rice cooks at those temperatures but it does not burn. It’s better to stay at the lower end of the range so tomato sauce does not taste smoky.

Nice to have guidelines when trying to make stoves. Of course, cooks in the project area may be a lot more specific and perhaps more exacting!

What’s Cooking at Aprovecho

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Aprovecho Visits Tanzania

In September, Jaden traveled to Tanzania to help set up a LEMS at the Bureau of Standards. Tanzania plans to set minimum standards for biomass stoves on the market, ensuring that more clean and efficient stoves are sold. She also visited Tango Energy, a stove manufacturer with whom Aprovecho has worked on stove design. She got to work on building knowledge with them and visited a project site to talk to households about their experience with improved stoves. We are excited to establish such great connections in Tanzania and hope to work with them in the future.

Increased Support for RTKCs

Regional Testing and Knowledge Centers (RTKCs) help us with testing, design, fieldwork and valuable knowledge of local cooking practices. Without them, we would not be able to make the global impact we do today. With our new grant funding, we will be supporting RTKCs in an effort to increase their testing and design capabilities, as well as building a better-connected community of testing centers and their researchers.

Test Results To Be Published

We performed a field study in Oregon on cordwood heating stoves, with the goal of understanding how people use these stoves so we can make cleaner-burning designs that still meet user needs. The results from that study are on their way to publication. This paper combines anthropology and engineering, using survey data, semi-structured interviews, and emissions data to find common usage patterns and identify high-emissions events. This work has helped immensely in our development of new cordwood heating stoves. The full paper has been accepted for publication in the Journal Energy Research and Social Science with the title: Considering the user: An integrated assessment of residential wood heating practices in the United States and implications for wood heater design. We will publish a link on our publications page, once it is available.

Varying sized sticks cut from douglas fir lumber.

Stick Size: CO, PM2.5 and Thermal Efficiency

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Varying sized sticks made from douglas fir lumber.

The diameter of sticks from the same species of wood (we use Douglas fir at ARC for testing) seems to have a dramatic effect on emissions and thermal efficiency. We used to use small diameter sticks of wood and experienced high thermal efficiency, low CO and high PM2.5. Small sticks make a lot of flame as the wood burns quickly and minimizes the burning of charcoal. Charcoal is known to make lots of CO but little PM2.5. Lots of flame may result in high temperature gases that increase thermal efficiency.

Trying to decrease PM2.5 in a wood burning stove has pushed us to try burning bigger diameter sticks. Maybe more charcoal is then burning, which reduces PM2.5 but increases CO?  It also seems harder to achieve 50% thermal efficiency. 

It’s beginning to look like one of the most effective strategies to achieve project goals is to adjust the diameter of the burning sticks.

It is great to do standardized testing! CO was never a problem when we used small sticks. Now, using bigger sticks (1” by 1.5” in diameter) we struggle to get Tier 3 for CO but PM2.5 seems to be lot better. Without standardized testing, the influence of changes like the diameter of sticks might escape unnoticed.

Some days I love science!

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.