Clean Burning Also Necessary for Biomass Home Heating

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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.”

Biomass Pellets in the USA: Fuel Switching

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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. 

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

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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.
sticks burning in rocket stove

How To Achieve Close To Complete Combustion of Biomass

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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.

Using Air Changes to Meet ISO PM2.5 Performance Targets

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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

Secondary Air in TLUDs and Rocket Stoves

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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.

Win a Rainbow Coalition of Friends

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The ARC lab is located in the Oregon woods where “hippies” and “rednecks” live on small farms in approximately equal numbers and share numerous points of view. I learned about these overlapping values when I accompanied my Dad, a Christian community organizer, to pot-luck meetings at nearby Granges, members of a farmers’ association organized in 1867. The one hundred and sixty two Granges in Oregon sponsor social activities, community services, and political lobbying.

My Dad was taught community organizing by Saul Alinsky in Chicago. He reminded his students to “Never go outside the expertise of your people.” https://www.goodreads.com/book/show/102748.Rules_for_Radicals  The people who have a problem have to be the ones to generate the solution. Mahatma Gandhi and E.F. Schumacher agreed. I watched my Dad as he listened and I admired his ability to help folks become aware that their constituency was a rainbow coalition.

Americans are a rich people but we often feel that life in the USA is getting worse and that this trend is out of control. Both “hippies” and “rednecks” can feel that nature is not being respected, that God has been forgotten, and that fighting for more money – being selfish – is largely responsible for the downward spiral. When many sorts of rural Oregonians visit our lab, they are happy that we are working to make renewably harvested biomass burn without making smoke.

Improving technologies can become a middle path that wins a rainbow coalition of friends.

Chart comparing energy output of 1 acre of grain vs 1 acre woodlot

Ethanol or Direct Burning for Heating Applications?

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The direct burning of biomass seems to be dramatically more efficient compared to ethanol for applications such as home heating, cooking, heating water, or drying clothes. It makes sense that not having to create alcohol from biomass would save energy. When the use of natural gas is decreased (due to climate change), burning biomass for heating seems like a fuel-efficient option that could reduce the extra burdens on electricity. 

One of my favorite reference books is “The Energy Primer” published in 1974. It has comprehensive review articles on solar, wind, water, and biomass energy. The following chart comes from a great article on biomass written by Richard Merrill. It shows that when renewable biomass is combusted, the efficiencies are much higher compared to making alcohol from biomass and then burning it.

The very clean burning of biomass allows efficient heating applications.

Chart comparing energy output of 1 acre of grain vs 1 acre woodlot

The Not-Stove Intervention

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Cooking outside

When the air exchange rate is doubled, the concentration of smoke is reduced by half. The average house in low middle income countries is estimated to have around 12 air exchanges per hour (ISO 19867-3). When ARC measured the air exchange rate outdoors in a gentle breeze it was found to be about 120 per hour. Exposure to smoke could theoretically be dramatically reduced by moving outside.

Changing stoves costs money and the transition to an improved stove is often a time consuming process. ISO reports that even stoves with chimneys (that often leak) only reduce concentrations of smoke in houses by around 75%. Experimenting with increasing the air exchange rate is probably the most cost-effective intervention to protect health. 

When we went to villages where women cooked outside, Dr. Winiarski would frequently ask the women if digging a well, etc. might be more important than new Rocket stoves. That is one of the things that we loved about Larry!

How To Make An Institutional Stove With Chimney

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Dr. Mouhsine Serrar and the Rocket institutional stove designed by Dr. Larry Winiarski

There are at least three ways to make institutional stoves with chimneys, all of which work well and save fuel and decrease emissions. Here they are:

  1. Shell Foundation supported the making of an eight-part video, a step by step guide to making a 50 to 100 liter institutional Rocket stove, with a heat resistant metal Rocket combustion chamber. It is a great stove with lots of successful field testing but it costs the most to construct because heat resistant metals like 410 stainless or FeCrAl are increasingly expensive. The super insulated combustion chamber requires these types of metal. 304 stainless will not last. https://youtu.be/VdhLWMW7IXA
  2. Cooking With Less Fuel: Breathing Less Smoke shows how to make the same institutional stove using bricks for the Rocket combustion chamber. Construction details can be found at aprovecho.org in the publications section. The book was written with the World Food Program in Rome. This is a less expensive stove that is slightly less fuel efficient at cold start but lasts longer and is easier to make in places where 410 stainless and FeCrAl are not available.
  3. Making a VITA style institutional stove without a Rocket combustion chamber is the least expensive way to create an institutional stove. The open fire under the pot is supported on a grate and the hot gases flow up the inside of the skirt, down the outside of the skirt and exit out the chimney placed below the bottom of the pot as in the Rocket stoves shown above. You can find a video we made about constructing the VITA stove at: https://aprovecho.org/video-gallery/

Lots of manufacturers do not use the chimney but we think that protecting health is very important. We try to follow Don O’Neals advice (HELPS International) to always include chimneys whenever possible, imagining our mothers cooking and getting ill from exposure to the harmful emissions without the protection of the chimney.