Secondary Air Injection

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Dr. Reed’s Alpha Limited stoves

Lefebvre, Vanormelingen, and Udesen examined secondary air jets air in cylindrical TLUD combustion chambers and described the most successful patterns and penetration depths. They reported that air jet penetration lengths approaching the middle of flame in a cylindrical combustion chamber resulted in a maximum reduction of PM2.5. An increase in the number of jets also created more thorough mixing. They advised that it was important to have the jets meet in the middle of the flame, but with minimal necessary force, to ensure highest temperatures and highest velocity of hot gases to the pot. (Lefebvre, 2010) (Vanormelingen, 1999) (Udesen, 2019)

It has been interesting to test Tom Reed’s larger Alpha Limited stove. More powerful secondary air jets forcefully meet in the middle, forcing the flame to create a cyclone that plunges down into the cylindrical combustion chamber. As reported in last week’s newsletter, in one 94-minute cold start high power test with Doug fir pellets, the larger WoodGas stove and insulated SSM SuperPot, achieved 58% thermal efficiency with a white filter and very low PM2.5.

Perhaps downward cyclonic mixing, caused by more powerful secondary air injection, might be another clean burning technique when combustion temperatures stay high enough? 

The Winiarski stove top that was added seemed to reinforce the downward cyclone. Larry always pushed better heat transfer efficiency in his pot supports, etc.

Tom probably invented using very little primary air and a lot more secondary air in forced draft TLUDs. So much easier to improve performance since Tom and Larry did a lot of the work!

Improving Market Based Products to Reduce Wood Use and Emissions

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Adjustable pot skirt can help save fuel

 Two dollar and fifty cent SSM adjustable pot skirt

Aprovecho staff travel around the world assisting stove projects. We experience that almost all biomass stoves in Low Middle Income Countries markets are sold for $10 USD or less. It seems to us that more expensive stoves are supported by carbon revenue, or sold in cities to the middle or upper classes. 

Depending on carbon revenue works well when prices do not fall, or when events don’t restrict trade. Aprovecho has learned a lot about the carbon market and helps projects to make best use of opportunities.

Aprovecho also develops market-based products trying to create sustainable businesses independent of carbon. Dr. Winiarski was a great proponent of market-based solutions that reduce fuel use and harmful emissions. He pointed out that improved thermal efficiency can be added to traditional stoves sold in markets without increasing cost. Week-long Partnership for Clean Indoor Air seminars (2002-2012) in Asia and Africa reduced fuel use by an average of ~ 30% with simple changes.

The size of the combustion chamber in a charcoal stove has been shown to be the most significant factor in fuel use. Maybe reducing the size of the combustion chamber in a traditional charcoal stove currently being sold would end up saving fuel in real life? See: The influence of initial fuel load on Fuel to Cook for batch loaded charcoal cookstoves (Bentson et al, 2013)

The $2.50 USD SSM adjustable pot skirt has reduced fuel use by 20% to 25%. Aprovecho hopes that factories/stakeholders can improve popular stoves by applying easy-to-teach changes.

There are many no extra cost improvements that are incremental first steps. 

They tend to be affordable and market based, which made Dr. Winiarski happy.

Achieving Turn Down Ratio in Cooking & Heating Stoves

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Do not burn the rice!

A stick fed cook stove can achieve a ~three to one turndown ratio (TDR) by burning more or fewer sticks per unit of time. In a low mass Rocket cook stove, ~five small sticks can boil the water/food and ~two to three small sticks can simmer it (without, hopefully, burning the rice).

The gas burner in a conventional furnace comes on when heat is needed and turns off when the thermostat indicates that the room is warm enough. The old style gas heater is either on or off.

A 10 to 1 TDR modern gas furnace can more economically run at higher and lower firepowers. Insulated, airtight homes can use more BTUs to heat water than to warm the home! Leaky houses can require a lot more energy to replace constant losses. (Reminds me of constantly bailing an old boat I used to own before it sank).

Batch fed, automated pellet heating stoves can have an adaptable ~five to one turndown ratio, burning 5 pounds of fuel per hour or one pound.

In cook stoves (and heating stoves), effective TDR can be achieved in several ways:

  1. The operator puts more or less wood into the combustion chamber (Rocket)
  2. Decreasing air entering into the stove slows the rate of combustion (TLUD)
  3. Simmering with just the made charcoal provides lower firepower  (T-CHAR)

Trying to widen TDR while maintaining very low emissions, very low Black Carbon ratios, in affordable products, makes life fun!

Over the Moon

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A couple of weeks ago, Claudia Truesdell and Shane Washburn from Google’s X – The Moonshot Factory visited us for the day. We made lunch on an earthen Rocket stove and on a metal pot skirt stove with Jet-Flame while doing a field test that generated some numbers. Google’s X was interested in what we have learned since 1976. 

Our staff had various things to share. I found myself explaining how to improve fuel efficiency in high mass stoves. Maybe ~95% of biomass stoves in use are high mass (?), made from earthen mixtures. 40% clay and 60% sand works well, especially when a relatively dry mix is pounded into a form. Metal is hard to find, expensive when purchased, and even stainless steel doesn’t last for a very long time near the fire. Improving earthen stoves might help lots of people.

When we did seminars with the Partnership for Clean Indoor Air, we showed that fuel use could be reduced by a minimum of 30% by:

  • Moving the flame further away from the mass walls.
  • Lowering the pot closer (but not too close) to the fire to increase the temperature of gases contacting the pot .
  • Using a constant cross sectional pot support to increase heat transfer efficiency to the bottom of the pot.
  • Adding an adjustable pot skirt that, especially when tight, forces the hot gasses closer to the sides of the pot.
  • Nowadays, we suggest adding a Jet-Flame to improve both heat transfer and combustion efficiency.
  • Most importantly, as recommended by Mahatma Gandhi, adding a functional chimney!