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.

Jaden and the trainees in the Ethiopia lab

What a Great Year!

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Regional Testing and Knowledge Centers

ARC just received a grant to hire full time assistance for the Regional Testing and Knowledge Centers (RTKC). Many of the RTKCs use our emission equipment. Jaden has just returned from Tanzania where she was setting up a new lab and Travis is going to Mozambique soon to do the same thing. Over 100 emission boxes have been sold worldwide.

The Clean Cooking Alliance lists 38 RTKCs. What an amazing resource!

It’s great to have a dedicated person available so anyone with a problem can get immediate help. The new hire will learn by testing every day, crunching the numbers with Jaden’s Python software, cleaning the equipment, calibrating it, etc. Sam and Jaden are here to help with complicated problems.

Maybe Travis, Kim and I can help with iterative development of improved stoves?

What a great year!

AI: Great but Stuck in a Box

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It’s great to have colleagues whose opinions are trusted. Dr. Larry Winiarski was usually right, by which I mean, that when we built and tested his inventions they might not have been perfect but were significant improvements, good starting places for further development. I always appreciated talking to Dr. Tom Reed, inventor of the TLUD, for the same reasons. Hui Yang Shen, the head of Shengzhou Stove Manufacturer, has been an almost-always-right resource about manufacturing. In 1987, Dr. Sam Baldwin wrote a book on cook stoves that I still refer to frequently.

In the same way, Google’s Gemini AI has become an interesting resource for me. It has been fascinating to encounter such an accessible way to find another reasonable opinion. One question to AI every morning has become part of my routine although I don’t have time for more. Google AI gives many references for each summary and I try to at least scan them. 

At ARC, we have the great advantage of doing experiments to learn. Reading what others have written is interesting, however I much prefer and believe that progress is faster when doing iterative development of prototypes under an emissions hood.  As importantly, going into the field to do R&D with cooks, distributors, manufacturers, etc. is necessary to make things that work. Reading alone cannot get you to success but it is a sturdy third leg of the stool: Literature searches, lab and field R&D.

Glad whenever I get out of the box. 

Cleaning up combustion: TOO COLD!

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In the middle of August, we had three stoves under development using the three emission hoods (two-forced draft, one natural draft). Sam and Travis built the natural draft hood to help improve heating stoves. Sam is thinking about making another one, number four, since we are working on more heating stoves and things can get a bit backed up.

During testing teams often trade observations during the day and sometimes, when the air temperature gets a bit elevated, I’ve heard folks exclaim: “WOW, FIRE HOT!”

What makes fire colder? Things like:

  1. The air entering the combustion chamber (~30C in August)
  2. Surfaces that absorb heat, like thermal mass or
  3. Pots of water (~100C)

Thermometers are very helpful when trying to improve performance! Both combustion and heat transfer efficiency increase when temperatures rise. Taking a stove’s temperature(s) can facilitate making helpful changes in a prototype. 

WOW, look at the temperature, IT’S HOT!

Lowering Emissions in a Short, Natural Draft TLUD

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Lowering Emissions in a Short, Natural Draft TLUD

The great thing about 12” high tincanium natural draft TLUDs is that they are cheap, short and powerful. Dr. Tom Reed had a dream that a billion TLUDs would protect health, save wood, etc. We wondered if an all Tier 4, affordable, stainless steel ND-TLUD might help?

The first step was to test the baseline ND-TLUD. It’s great for camping outdoors but not close to Tier 4 for PM2.5, CO and Thermal Efficiency. We are now trying three ways to achieve all Tier 4:

  1. A nothing fancy prototype. Control primary and secondary air and don’t let flames touch the bottom of the pot.
  2. A static mixer prototype. 
  3. A better concentrator ring prototype.

Usually when we make one change a day in a prototype and test the effects under the emission hood it takes about a month to get close to an “optimal” solution. Doing one iteration a day leaves us time to do all the other work here at the lab. 

Looking forward to sharing test results! 

Learning From People

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A rat proof box

When Dr. Larry Winiarski and I lived for a season at Rancho San Nicolas in Baja California Sur, Mexico we wanted to be helpful. We built cook stoves, solar cookers and other delightful inventions that we found interesting. But Larry was an insightful fellow and he started to listen to folks and ask them what problems they wanted to solve.

We had such a great time at the ranch, people were so nice, but Larry learned more than I did because he was born in Honduras and was more fluent in Spanish. It took me a couple of years to be able to understand what folks were saying. Speaking the language is important for many reasons!

After a while, Larry and I started working with the rancheros making rat proof boxes from available wire reinforced cement. Steel would have been great but was much too expensive and you couldn’t get it anyway. Rats were eating so much food and storage was a major problem. Stoves were great but homemade rat proof boxes took off like hot cakes. 

We started making boxes and became more helpful. Folks loved Larry anyway but now he wasn’t just good company. After a couple of years, I started fixing holes in boats.