Clean Combustion Needs More Than Just Heat

Dr. Larry Winiarski wrote the ten rocket design principles

We have been having a lot of fun doing a modern literature search: Surfing YouTube. YouTube is often years ahead of the slower, but probably more accurate, information in peer reviewed journal articles. I suppose that many people are looking at both.

A shared misunderstanding seems to be that making the combustion zone hotter cleans up combustion. Yes, it is great to keep the temperatures around 900°C, which shortens the residence time needed to burn up the wood gas. However, just raising the temperature misses other necessary components that also move stoves closer to complete combustion. They are:

1. FUEL/AIR RATIOS: Fuel and air are needed for complete combustion.

2. MOISTURE: Biomass has to be relatively dry to burn.

3. MIXING: Turbulence needs to completely mix the fire, air and wood gas.

4. RESIDENCE TIME: Less time is required at higher temperatures to burn up the wood gas.

5. TEMPERATURE: Good – higher temperatures decrease the residence time. Bad –higher temperatures increase the rate of reactions possibly producing more wood gas than can be cleanly combusted.

6. METERING: As Dr. Winiarski wrote in his Rocket Design Principles: “Only make the amount of wood gas that can be combusted.”

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Electricity: Planning for Net Zero by 2040

www.weforum.org/agenda/2023/04/electricity-generation-solar-wind-renewables-ember/
  • Transitioning to carbon neutral electric generation would replace a big climate problem in the U.S., since about 60% of its electricity comes from burning natural gas. 
  • The World Energy Forum forecasts that around 40% of electricity could be from wind and solar doing most of the heavy lifting by 2040, enabling a net zero global future. 
  • Today hydropower provides about 16% of the world’s electricity, generating power in all but two U.S. states. 80% to 90% of our electricity at the lab comes from the wonderful Columbia River.
  • ARC is working to clean up combustion so renewable biomass (domestic switch grass, for example) could cook food and heat homes when fossil fuels are no longer available.
  • Reading a book at night in a warm house is a wonderful thing. Somebody is playing the piano… Dinner was great.
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African Mud Stoves with Chimneys

Damon Ogle was the Technical Director here at ARC

Damon Ogle and the ARC staff have a long history, starting in Central America and Mexico, listening to folks praising their stoves with chimneys. There are now millions of beautiful Latin American kitchens in which the dangerous smoke is transported out of the house, as it is in the USA/Europe. The Rocket stove can be about 50% more fuel-efficient compared to the open fire, so about half the smoke is made. But that is not good enough to protect health inside a home.

Although health-protecting chimneys are seen in Latin America and India, it’s rare to see chimneys in Africa.  

One simple African stove with chimney is seen above. A sunken pot (or pots) sits down near the fire exposing its bottom and sides to the flame. The pot seals into the hole and the smoke flows up the chimney, not into the lungs of the cook and her children. 

Since 1976, ARC has continued to work with local communities worldwide to try to save fuel and protect health. Trying to protect climate requires very clean combustion and we’re working on that, too.

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Earth Day 2023: The Future Can Be Fun!

fun-future-robert-crumb.jpeg
(R. Crumb)

If renewable switch grass, for example, was burned cleanly enough biomass could join solar, wind, hydro, and thermal energy as sustainable energy replacements in the post fossil fuel era. ARC estimates that an emissions rate of 0.3 grams/hour of PM2.5 would protect air quality in cities and meet the Paris Agreements when replacing natural gas.

Not all that hard to do…

 “Earth is likely to cross a critical threshold for global warming within the next decade, and nations will need to make an immediate and drastic shift away from fossil fuels to prevent the planet from overheating dangerously beyond that level, according to a recent report from the Intergovernmental Panel on Climate Change.

… It says that global average temperatures are estimated to rise 1.5 degrees Celsius (2.7 degrees Fahrenheit) above preindustrial levels sometime around ‘the first half of the 2030s,’ as humans continue to burn coal, oil and natural gas.

…Under the 2015 Paris climate agreement, virtually every nation agreed to ‘pursue efforts’ to hold global warming to 1.5 degrees Celsius. Beyond that point, scientists say, the impacts of catastrophic heat waves, flooding, drought, crop failures and species extinction become significantly harder for humanity to handle.” –Brad Plumer reporting in the New York Times, 4/21/23

On the other hand, transitioning to renewability accomplishes an almost universal dream of humanity.

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Clean Burning Also Necessary for Biomass Home Heating

https://climatechangedispatch.com/wp-content/uploads/2019/12/wood-burning-stoves-germany.jpg

Since 1976, ARC has been investigating how to improve heat transfer and combustion efficiency in Low Middle Income Countries’ wood burning cook stoves. Emissions of Particulate Matter have been shown to kill millions of people annually. PM concentrations are frighteningly high in homes without chimneys but emissions into outdoor air are an increasing health/climate concern. Incomplete combustion in cooking and heating stoves is an obvious problem especially when compared to the very clean combustion in more mature technologies like automobiles. 

The EPA biomass heating standard allowing two grams per hour of PM to pollute the environment is very lenient. National standards in Europe also allow biomass stoves to endanger health/climate. Cook stoves are forced to burn much more cleanly by stricter WHO standards and ISO benchmarks.

The Guardian’s Environment Editor Damian Carrington reported in 2021, “Despite their severe impacts on air pollution and human health, domestic heating emissions are under-regulated in the EU, especially when compared to other sources such as traffic. Neither the EU EcoDesign requirements nor the more ambitious Nordic ecolabel succeed to keep particle emissions from new stoves within acceptable levels. In 2022 a new EcoDesign stove will be allowed to emit 60 times as much particulate matter as an old truck from 2006, and 750 times as much as a newer truck from 2014.”

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Biomass Pellets in the USA: Fuel Switching

Total Plants: 107Metric tons/year:11,188,200
The pellets are used for fuel.

Depending on the size of the home, winter heating with a pellet burning stove uses from 2 to 6 tons of wood pellets per year. If the average house burned 3 tons per year, 3,729,400 homes could be heated with pellets currently manufactured in the USA. There are 142,153,010 residences in the USA. biomassmagazine.com/plants/listplants/pellet/US/

Bill Gates has written that the climate crisis can be solved by developing least cost, renewable technologies to replace fossil fuels. (“How to Avoid a Climate Crisis”, 2021)     

How do fuel costs compare?

Fuel Oil #2       Cost per million BTU = $30.19

Electricity         Cost per million BTU = $35.17

Natural Gas      Cost per million BTU = $15.38

Wood Pellets   Cost per million BTU = $19.15

LPG/Propane   Cost per million BTU = $41.13

www.pelletheat.org/compare-fuel-costs

Fuel switching from natural gas to renewably harvested wood pellets or split logs or dried wood chips (only if they can be burned cleanly enough to meet the Paris Agreements) seems to include a relatively small Green Premium. Replacing LPG/Propane, electricity, and Fuel Oil #2 with wood pellets seems like a good deal. 

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50% Thermal Efficiency Depends on Several Factors Including the Surface Area of the Pot

Illustration from The Smithsonian’s explanation of how a boundary layer works 

A boundary layer of still air on the bottom and sides of a pot keeps the hot gases from actually contacting the surface and is a dominant factor in heat transfer efficiency.

  1. According to Newton’s Law, doubling the surface area doubles the heat transfer when the temperature and velocity of the gases are constant.
  2. In a Rocket stove at high power, the gases can be around 800C and the velocity can be around 1.2 meters per second.
  3. Keeping a constant cross-sectional area in the pathway the gasses take through the stove is important. Reducing the constant cross-sectional area channels under and around the sides of a pot to 0.75 of that area helps to keep the gases hot and flowing at highest velocity.
  4. The 0.75 cross sectional channels encourage the gases to thin the boundary layer increasing heat transfer.
  5. Pots have to have sufficient external area to achieve 50% thermal efficiency.
  6. In recent tests of optimized Rocket stoves, a pot with an area of around 800cm2 scored 34% thermal efficiency. Increasing the area to around 1000cm2 increased thermal efficiency to about 40%. In the same stove, a pot with 1200cm2 can be expected to result in above 45%. We use 26cm to 30cm in diameter pots with at least 5 liters of water to get closer to 50% thermal efficiency.
  7. Keep in mind that increasing the surface area of the water in a pot also increases the amount of steam, which makes bigger pots harder to bring to full boil without a pot lid.
  8. Thermal efficiency, when burning biomass, tops out (so far) at around 55%. The gases in the channels at the bottom and sides of the pot loose temperature and velocity resulting in an upper limit to heat transfer efficiency. 
  9. Raising the temperature and velocity of the gases will increase efficiency.
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sticks burning in rocket stove
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How To Achieve Close To Complete Combustion of Biomass

https://www.jet-flame.com/wp-content/uploads/2019/10/Bricks.jpg
The Jet-Flame pushes jets of primary air into the fire to aid combustion.
  1. When a wooden stick is burned a lot of smoke is produced but the made charcoal at the tip of the wooden stick does not make much smoke.
    Rocket Stove: Push the sticks in slowly so the charcoal at the tip is burning.
    TLUD: Charcoal covers the slowly burning fresh wood.
  2. If the stove begins smoking, the solid wood is being turned into gas too quickly, too much wood gas is being produced and un-combusted fuel is escaping.
    Rocket Stove: Pull the sticks back until just the tips are burning.
    TLUD: Reduce the primary air.
  3. Mixing the smoke, gases, flame, and air reduces emissions.
    Rocket Stove and TLUD: Cut up the laminar flames with static mixing devices or jets of primary or secondary air. Aim the jets of secondary air into the flame and adjust the velocity of the jets to completely cover the burning fuel. Primary air jets can also achieve close to complete combustion. Excess velocity in primary or secondary jets is detrimental when it reduces the combustion temperature.
  4. For close to complete combustion the temperature in the combustion zone needs to be 850C or above. The woodgas and air and flame have to be thoroughly mixed. The residence time needs to be 0.2 seconds or more. Reduce the amount of woodgas entering the combustion zone until close to complete combustion is achieved. Biomass fuels with 15% or lower moisture content are easier to burn.
  5. It is necessary to tune the stove under an emissions hood to achieve close to complete combustion. Change one variable at a time and test until significance is achieved.
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Using Air Changes to Meet ISO PM2.5 Performance Targets

Almost 20 years ago, ARC was invited to a conference in Bonn, Germany to consider standards for cook stoves including emissions. In preparation, we met with Dr. Kirk Smith and his protégés at the University of California at Berkeley. We discussed the beneficial effect of increased air exchanges that decrease the concentrations of CO and PM2.5, helping a lot to reduce human exposure.

When the Air Change per Hour Rate (ACH) is doubled the concentrations of PM2.5 are cut in half, if the other variables stay constant.

It’s great that the air change rate is included in the ISO Voluntary Performance Targets! Stove project managers can measure the ACH where folks are cooking and use the information to get a better idea of what combustion efficiency is needed to protect health. Cooking outside or in kitchens helps a lot to reduce exposure to CO and PM2.5 as shown in the following chart:

ISO Tier 3 and 4 PM2.5 ISO Voluntary Performance Targets (with Air Change Rates)

Tier PM2.5 Emissions (milligram/ megajoule delivered) 21ACH ~inside42ACH ~half walls84ACH ~outside
4≤62≤~124≤~248
3≤218≤~436≤~872
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Secondary Air in TLUDs and Rocket Stoves

http://stoves.bioenergylists.org/stovesdoc/Reed/Pics%20in%20files.jpg
Forced draft mixing with 2nd air jets in Dr. Tom Reed’s WoodGas Stove at around 1,000C

Forced draft mixing with preheated jets of primary air reduced emissions of PM 2.5 by around 90% in our stove tests with the Jet-Flame. Would adding secondary air jets further decrease emissions?

Secondary Air Works in TLUDs

Lefebvre, Vanormelingen, and Udesen examined secondary air jets air in cylindrical combustion chambers and describe most successful patterns of penetration depth. Jet penetration lengths approaching the middle of a cylindrical combustion chamber resulted in a maximum reduction of PM2.5 emissions. An increase in the number of jets created more thorough mixing. It was important to have the jets meet in the middle, but with minimal necessary force, to ensure highest temperatures and highest velocity of hot gases to the pot.

Forced draft secondary air jets can decrease the upward draft in a cylinder. Jets of air aimed horizontally into the flame most efficiently create mixing. But even when aimed upwards toward the pot they create a ‘roof of air’ that slows the draft by creating a high-pressure front.

Regardless of the velocity of secondary air flow rates, or the angle at which air is injected into the combustion chamber, supplying secondary air also tends to significantly lower the temperature. For this reason, using a minimal amount of air was found to be best. There is a reported balance resulting in optimized mixing, draft, residence time, and temperature. (Lefebvre, 2010) (Vanormelingen, 1999) (Udesen, 2019)

How Do We Add Secondary Air Successfully to Rocket Stoves?

One obvious difference between TLUDs and Rocket stoves is the large fuel door in the side of the Rocket stove. A TLUD is an open topped cylinder with a small amount of primary air entering the batch of fuel from below the packed fuel bed. In the TLUD, the fuel is initially dropped into the cylinder, while in a Rocket stove horizontal sticks are pushed into the combustion zone through a fuel door. The pressure/volume of secondary air jets introduced into a Rocket stove may be limited because the high-pressure front can create a backdraft that sends smoke out of the fuel door.

Supported by funding from The Osprey Foundation, ARC is currently experimenting to determine: 1.) How much pre-warming can be achieved and 2.) What is the most effective pressure/volume for secondary air jets in a forced draft Rocket stove.

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