Dr. Tom Reed: “Smoking Backwards”

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Dr. Tom Reed: “Smoking Backwards”

Our beautiful friend Tom Reed (1926-2018) at a Winter Stove Camp

ONE: Starting in 1985, Tom invented the natural and forced draft Top Lit Up Draft stoves (TLUD). He would demonstrate one of the clean burning principles at Stove Camps by lighting the tip of a cigarette and then sucking on the burning end in his mouth. With practice, he did not burn his tongue.  Remarkable!

Sucking the woodgas through the burning end of the cigarette cleaned up a lot of the harmful gases and protected him from most of the smoke. Similarly, lighting the top of the fuel bed in a vertical cylinder pulled the made woodgas up through the charcoal bed and flame in a TLUD. (UP DRAFT)

TWO: In the same way, sticks of wood can be placed in a horizontal cylinder, lit under the short chimney of a Rocket stove, where the made woodgas is pulled through the burning combustion zone resulting in cleaner combustion. (HORIZONTAL DRAFT)

THREE: We tune stoves under the emissions hood to reduce emissions by combining the proper 1.) amount of metered fuel turned into woodgas, 2.) temperature, 3.) air/fuel ratio, 4.) mixing, 5.) 100% of woodgas pulled into the flame, and 6.) long enough residence time of woodgas in the combustion zone.

FOUR: Before iterating changes in the stove to reduce emissions, we try to optimize heat transfer efficiency so the least amount of biomass completes the task.

FIVE: Tom, we remember you so fondly! Thanks for the help!

Rocket Bread Ovens

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I ran across this old video when exploring Rocket stoves on YouTube. It is great to have both peer reviewed journal articles and places like YouTube. I hope that our publications library at aprovecho.org includes, perhaps even combines, helpful information from both academia and the field.

We baked with a high mass bread oven for years at the Aprovecho campus. We bragged about it. It was only when Dr. Winiarski built a Rocket bread oven next to the old stove that we experienced the differences. Then, it took less than a week for baking to switch to the Rocket oven. No more throwing logs into a mud/sand cave for hours to try to get up to temperature! Why try to heat up a thousand pounds of mud when you want to make 20 pounds of bread?

Larry’s Rocket oven got to 400°F in about 30 minutes and used hand fulls of twigs to bake enough bread for the hungry eco-scientists. I loved the Rocket bread ovens and helped Larry to build them in Mexico. We learned a lot from Larry including that using a new stove was a lot more convincing that talking about it. Why not let the new oven or stove speak for itself.  Then stand aside and watch when folks like it enough to improve the prototype? 

So nice to let the experts (the cooks) tune the cooking functions of a stove! So nice to empower their hard won expertise! So appropriate to sit back and learn.

red sign with white letters reading Wood Burning Prohibited

Clean Burning of Biomass

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red sign with white letters saying Wood Burning Prohibited

Scotland has banned the use of climate polluting home heating systems such as oil and gas boilers, and wood burning stoves (except in cases of need) in new construction. Heating homes creates one fifth of Scotland’s CO2e. The plan is to switch to electric heat pumps, hydrogen and tighter, better-insulated homes in an effort to achieve carbon neutrality by 2045. https://www.bbc.com/news/uk-scotland-68778757

Oil and gas (fossil fuel) burners create too much CO2. Burning renewably harvested biomass can emit close to zero CO2, but old stoves make too much black smoke which is ~2000 times worse for climate change compared to CO2 by weight.

Heating stoves that burn sustainable biomass cleanly enough to protect health and climate are starting to become available. Testing the new generation of stoves in use will show whether biomass can join solar, wind, and hydro as a useful renewable energy resource in the post fossil fuel era.

Maybe those red signs will become green?

Green sign saying Sure, Light Up
bed of charcoal in rocket stove

HOT!

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Boman reports that temperatures above 850°C, in a 5kW combustion zone combined with air rich and well-mixed conditions for 0.5 seconds in the post combustion zone, resulted in an almost complete depletion of particulate matter (Boman et al., 2005).

How can we achieve 850°C in a combustion chamber? 

Yellow flames are around 1,100°C. but: 

  1. Heat flows in one direction, from hot to cold.
  2. Mass, charcoal, and incoming air (primary and secondary) are usually much colder.

Elevating temperatures to 850°C in the combustion chamber is not easy but it is possible!

  1. Use a thermometer.
  2. Create hot fires.
  3. Reduce mass as much as possible.
  4. Replace mass with insulation.
  5. Inject pre-heated air into charcoal to create high temperatures, especially in the top of the pile. Charcoal is a good insulator and when the top of the fuel bed is not red hot the effect can be cooling.
  6. Minimize the pre-heated incoming air, maintaining a minimum 3 to 1 fuel/air ratio.
Three images - Icy tree limbs, a man with a chainsaw, a car full of logs

Ice Storm 2024!

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On January 14, ARC staff experienced a catastrophe when ice brought down about one third of the trees in our valley and we had no electrical power for seven days. Depending on the grid for heat and light is great when everything works. However, when the lights go out, a warm house becomes really important. Light is easy to make with photovoltaics, etc. Living for a week in the cold was a great reminder of the importance of a clean burning non-electric biomass heating stove. 

I lived without electricity on a ranch in Mexico for eight years where it was warm pretty much all of the time. Even so, on chilly nights we lit an open fire and sat around it, talking. The old folks pulled blankets around themselves, too.

I do not remember missing refrigeration or lights. It was not a big deal. At the ranch, people had lived for over a hundred years without them and much more energy went into trying to find juice for the little radio.

Going for a week without electricity reminded me that a warm house is important. In response, ARC is building health/climate non- electric pellet heating stoves as winter turns into spring.  If the creek don’t rise, we want to have these stoves in all buildings before next winter.

PS: I am fine reading a book by candlelight as long as my feet are warm.

thermal image of house juxtaposed with daylight image of same house

COP 28: Near-zero emissions in global building sectors

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thermal image of a house juxtaposed with daylight image of same house
Heat can constantly leak out of older homes. Photo: Gina Sanders

Aprovecho is investigating how to design and manufacture biomass-heating stoves that protect health and climate when burning renewably harvested biomass. Of course, staying warm depends on many factors including how much energy is being leaked from the building.

Net-zero buildings are usually tight and well insulated. A net-zero home can have a heating load of 10,000 to 15,000 Btuh (or ~3 to 4 kW) in a cold, northern climate. At COP 28, a minority of nations agreed to move towards net zero homes to reduce climate change by heating the better buildings with renewables. Green Building Advisor: 28 Countries Sign Buildings Breakthrough Agreement at COP28

Since the 1970’s, architects and engineers have learned how to dramatically reduce energy losses in buildings. Many net-zero homes take advantage of solar power to assist heating and create electricity. Solar gain helps a tight, well-insulated home to stay warm.

The United Nations found that buildings and construction account for 39% of total carbon emissions annually. Net Zero Homes: Your Guide to the Greenest Housing Option  If a new generation of very clean burning biomass heating stoves can protect health and climate, might they assist COP* countries to move towards near-zero emissions in global building sectors? *COP is the decision-making body of the UN Framework Convention on Climate Change.

From: EPA’s Lab Test Results for Household Cookstoves, Jim Jetter, 2012 

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Since 2012, optimized biomass cook stoves have been tested at ~50% thermal efficiency

The temperature of the hot gases flowing past the surface of the pot is increased by

  1. Creating as much flame (1,100C) as possible in a low mass, insulated combustion chamber.
  2. Decreasing the distance between the fire and the pot without making excess smoke.
  3. Not allowing external air to cool the combustion gasses.

In convective heat transfer, the primary resistance is the surface boundary layer of still air immediately adjacent to a wall. 

Increasing Temperatures, increasing exposed Area, increasing Radiation, increasing Velocity in a 6mm to 7mm channel gap (10cm or higher) pot skirt has been shown (up to 5kW firepower) in a 24cm or larger diameter pot to result in ~50% thermal efficiency. Reducing losses from the exterior of the pot skirt with refractory ceramic fiber insulation also increases thermal efficiency. 

60% thermal efficiency has been demonstrated in the lab.

Helpful links:

From: EPA’s Lab Test Results for Household Cookstoves, Jim Jetter, 2012

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Key findings compared with the 3-stone fire:

  • Most stoves that were tested had better thermal efficiency, but some did not.
  • Compared with the 3-stone fire, many stoves that were tested had better combustion efficiency, but many did not.
  • A natural-draft TLUD stove (ARC) had very high efficiency with processed, wood-pellet fuel with low-moisture content.
  • Some forced-draft (fan) stoves had very low emissions – but not all fan stoves did.
  • Most natural-draft stoves that were tested showed a bigger improvement (lower emissions) over the 3-stone fire with high moisture fuel than with low-moisture fuel.
  • A natural-draft TLUD stove (ARC) had very low emissions – but required processed, wood pellet fuel with low-moisture content.
  • Two rocket stoves were tested at a “medium power” level – and had lower emissions (per energy delivered to cooking pot) than at maximum power.
  • Charcoal stoves had high emissions of CO and high emissions of PM during start-up.

From The WHO on Lower Emission Solid Fuel Stoves

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In 2014, the World Health Organization (WHO) issued the first-ever health-based guidelines on clean fuels and technologies for household cooking, heating and lighting: INDOOR AIR QUALITY GUIDELINES: HOUSEHOLD FUEL COMBUSTION 2014

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Adding forced draft and chimneys to biomass cookstoves helps to meet WHO IAQ guidelines

From section 5.4.1 Roles of clean fuels and lower emission solid fuel stoves

“As recognized in these guidelines, and specifically in Recommendation 2, which addresses policy during transition, improved solid fuel stoves will continue to make an important contribution to the needs of a substantial proportion of lower income and rural homes where primary use of clean fuels is not feasible for some time to come. Work to develop substantially improved solid fuel stoves should continue in parallel with, but not hinder or displace, efforts to encourage transition to clean fuels. The contribution of solid fuel stoves to the mix of devices and fuels promoted will depend on the completeness of combustion that can be achieved when such technologies are in everyday use (as demonstrated through emissions testing), and the consequent reductions in health risks.” (pg.62)

Mixing with Primary and Secondary Jets of Air

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https://tse1.mm.bing.net/th?id=OIP.R4SW_2vN8wfW-9688ssYKgHaGL&pid=Api&P=0&h=220

Regardless of the velocity of secondary air, flow rate, or the angle at which air is injected into the fire, secondary air tends to lower the temperature of gases. Researchers have found that injecting secondary air into the side of the flame in a Rocket stove results in most effective mixing.*

The Jet-Flame, on the other hand, blows primary air jets up into the bed of made charcoal below the burning sticks of wood, creating a “mini blast furnace.” The jets of primary air increase the temperature in the charcoal, frequently resulting in higher temperatures in the combustion chamber. The mixing function is up into the fire, not into the side as with secondary air jets.

Boman et al., 2005 report that temperatures of 850C or above are needed for close to complete combustion in short residence times, as in a cookstove. Since excess air lowers temperatures, using the minimal volume of air in secondary air jets to achieve thorough mixing seems preferable. Researchers have recommended that the jets should penetrate into the middle of the flame but not enter into each other. (*Lefebvre and Ballal, 2010; Udesen, 2019; Vanormelingen and Van den Bulck, 1999).

Unfortunately, raising the temperature of pre-heated secondary air by a lot more than ~ 100C seems to be difficult. Cookstove combustion chambers are usually small, limiting the area exposed to high temperatures. The heat transfer efficiency is much lower from degraded temperatures further from flame.

 Residence time and temperature are easily measured. However, “thorough mixing” has not been defined and is not yet measured in our experiments. We infer that the woodgas/air/flame was thoroughly mixed when the emissions of PM2.5 and CO are close to zero as measured with the LEMS emissions hood.