A 2014 survey of biomass stoves for DOE showed that tight pot skirts are great!

We had a couple of days between jobs at the lab and decided to see if a simple Rocket stove manufactured in India, patterned after the BURN stove, could get better thermal efficiency. Low grade stainless steels, like 304, can’t withstand the hotter combustion chamber temperatures generated when insulated, so in the BURN stove room air is used to keep the steel cool enough to increase durability.

One of the key properties of any stainless steel alloy is its resistance to oxidation. High temperatures can compromise the oxidation resistance of steel alloys, leading them to become rusted and weakening their structural integrity.

As stated by AZO Materials, grade 304 stainless steel possesses “good oxidation resistance in intermittent service to 870°C and in continuous service to 925°C.” However, they warn that “continuous use of 304 in the 425-860°C range is not recommended if subsequent aqueous corrosion resistance is important.” In other words, you can expose grade 304 alloy steel to temperatures of up to 870°C for short periods of time without ill effect, and for extended periods of time in temperatures of up to 925°C. However, this can compromise the corrosion resistance of the metal, making it more susceptible to damage from exposure to moisture. (https://www.marlinwire.com/blog/what-is-the-temperature-range-for-304-stainless-steel-vs-316-vs-330)

When the low mass, uninsulated BURN Rocket stove has (1) 6mm high pot supports, (2) a pot skirt that creates a 6mm channel gap around a family sized pot, and (3) a fire that creates hot, tall flames that transport 800°C to 1,000°C gases to the pot, the thermal efficiency has been measured at around 52%. 

We lowered the pot supports in the simple Indian Rocket stove to (1) 6mm high and used a (2) 12cm high, 6mm channel gap pot skirt around a 25cm in diameter steel pot filled with 5 liters of water. Thinking that the simple Indian Rocket stove could use a 1,200°C thin walled refractory ceramic combustion chamber, (less than $1 from Shengzhou Stove Manufacturer), we (3) surrounded the combustion chamber with ceramic fiber insulation. (4) The fire was made from tiny sticks. Tiny sticks make hot, tall, dirty flames and use up the least amount of wood while making really hot gases. When burning tiny sticks, gas temperatures under the pot can be over 1,000°C. The 1,000°C gases heat water quickly and efficiently when 6mm channel gaps are used below and on the sides of the pot.

With these changes, the simple Indian Rocket stove scored an average of 56% thermal efficiency (3 tests to boil). 

If (1) 6mm pot supports, (2) 6mm pot skirts, (3) insulation, and (4) tiny sticks making 1,000°C gases had been used in the 2014 DOE stove survey the average scores would have been a bit higher. One lesson is that channel gaps and types of fires can have a big effect on heat transfer efficiency.

Go for those 1,000°C gases flowing right next to surfaces for high thermal efficiency. 

Add metering and mixing to 1,000°C gases with sufficient residence time and combustion efficiency is also improved.

Check out the heat transfer and combustion chapters in “Clean Burning Biomass Cookstoves, 2021” at www.aprovecho.org

Ryan Thompson and Sam Bentson spent a year exploring charcoal burning

Ken Newcombe and C-Quest Capital have seen firsthand how charcoal production has wiped out forests in Africa. Making charcoal is a very energy inefficient process (Aprovecho Institute, 1984a). Burning off the volatile compounds in wood consumes and wastes between 50% and 80% of the energy!  Ken recently observed that saving charcoal was not easy, and  that field tests indicated that traditional stoves were as fuel efficient as more modern stoves. So I asked Ken if he had read Sam’s paper* on how to save fuel in charcoal stoves, and Sam sent it to him.

Busy people are out in the world accomplishing things. There’s a ton of information on the Internet and lots of it is conflicting. Publishing information in journals and books sometimes seems to be a fairly ineffective way to reach actively engaged stovers. That’s why we publish this weekly newsletter, trying to disseminate data driven information. Maybe you can pass it to on to your colleagues?

After testing many charcoal stoves, Sam concluded that:

“Using the minimum load to complete the WBT [water boiling test] results in similar performance between most charcoal stoves, while filling the combustion chamber with fuel results in large differences on the same measures. Based on these laboratory results, limiting the size of the combustion chamber so as to prevent excess fuel loading may be an effective technique for decreasing fuel consumption while cooking.” *

The loaded charcoal is used up, so loading only the amount needed for the cooking process is very important for saving fuel.  At the same time, Sam and Ryan Thompson spent a year exploring charcoal burning and came up with an “all Tier 4” charcoal stove that was higher scoring than the other optimized wood burning stoves that ARC made for the US DOE.

CAD drawings and a chapter on charcoal describe what Sam and Ryan did and how to build the stove they invented. (Clean Burning Biomass Cookstoves, 2021,  www.aprovecho.org/publications-3).

*(Energy for Sustainable Development, 2013, Bentson, Still, Thompson, Grabow)

Dr. Larry Winiarski
Dr. Larry Winiarski
Dr. Larry Winiarski, 1940-2021

Dr. Larry Winiarski, the Technical Director of Aprovecho Research Center (ARC), died this past week at the age of 81. In the 1980’s and 90’s, Dr. Sam Baldwin defined how to improve heat transfer efficiency in biomass cook stoves (pot skirts, etc.), Dr. Tom Reed created the TLUD, and Dr. Winiarski invented the Rocket stove. The saying “We stand on the shoulders of giants” certainly applies to the stove community.

Larry led teams from ARC around the world starting in Central America, where the plancha stove was evolved, after he found that a floor tile called a baldosa made a long lasting and relatively low mass combustion chamber that was surrounded by wood ash, a great natural source of refractory insulation. Larry discovered that Rocket type stoves, like plancha stoves, can be described by ten design principles and that these simple engineering principles could be taught to indigenous people, mostly women, who were the experts in using the stoves. My memories of Dr. Winiarski, who was born in Nicaragua, are often about him having a wonderful time speaking Spanish as stoves were constructed and flavorful food prepared.

It is not an exaggeration to say that Larry had a heart of gold. He picked up sick kids and walked from the city dump in Managua to a distant hospital. He slept on cement floors for months at a time in Haiti. Larry lived as others lived in Africa for years and because of his character was loved and respected in villages worldwide. His Rocket stove found a place in people’s homes in the same way that Larry was cared for, accepted, and loved by strangers. Larry is missed by thousands of friends and he was blessed with a life well lived.

There will be a Celebration of Larry’s Life on Saturday, August 28, 2021 at 1:30 PM, at Colgan’s Island, 79099 Hwy 99 N in Cottage Grove, Oregon.

Smoky kitchen with no chimney vs. kitchen with chimney and clear air
Reducing/removing smoke from the kitchen improves health.
Adding a chimney is one recommended solution.

In 2014, the World Health Organization (WHO) published intermediate and final indoor air guidelines for vented and unvented biomass cooking stoves. Their strong recommendation directed governments and implementers to advocate technologies and fuels that are proven to protect health. The WHO advises implementers that to protect health the cook stove intervention should not exceed the following air pollutant emission rates in actual use:

WHO Intermediate Emission Rate Targets:

Unvented stove Vented stove
PM 2.5:  1.75 mg/minPM 2.5 7.15 mg/min
CO:   0.35 g/minCO:  1.45 g/min

Many newer biomass cookstoves with chimneys meet the WHO Targets of 7mg/minute for PM2.5 and 1.45 g/minute for CO when tested in the laboratory. Adding chimneys to cook stoves makes them more costly, but ARC designers are relieved to have a “line drawn in the sand.” As seen in Sam and Shikhar’s video last week, experiments in the ARC Test Kitchen showed that a natural draft hood can also be a big help in protecting health.

Protecting outdoor air quality is equally important. Dr. Nordica MacCarty (Oregon State University) and Ken Newcombe (C-Quest Capital) will be investigating effects on indoor and outdoor air when the combination of a natural draft earthen hood and a CQC earthen stove with Jet-Flame is used in houses in Malawi. Ken and CQC are invested in protecting health and have funded the study.

Industrial technologies routinely achieve strict standards for combustion efficiency and further reduce emissions with post combustion techniques. Introducing these well-known applications into cook stoves seems a logical progression. Clean up combustion and, at the same time, clean up the indoor and outdoor air. We have been very successful doing this in the US and Europe, and China is now on the same path.

The WHO vented stove Emission Rate Targets are based on 75% of the smoke and gases being removed up the chimney and out of the house. In their review of field studies, an average of 25% of the smoke and gas remained in the kitchen. Almost none of the residential biomass heating stoves in the United States meet the WHO Targets for PM 2.5 but the chimney transports the smoke outside where it is diluted by clean outdoor air to safe levels of concentration.

Meeting emission targets is a necessary and ever present goal. At the same time, wood burning stoves can be improved in many other ways. Improving the smoky mud stove to use less fuel is not a complete cure but is very helpful, benefitting the user who either pays for the fuel or has to collect it. The functional chimney makes a tremendous difference by sending smoke and gas out of the kitchen, making it a more pleasant and healthy environment. Making the high mass stove safer results in fewer burns. The list of improvements goes on and on – making the stove better at cooking local foods, increasing the number of air exchanges per hour in the kitchen, moving the kitchen outdoors, etc.

In the real world, positive changes are hard to accomplish but are always great.