Cross- sectional illustration of a biomass cookstove with auto damper

Health: Reducing PM2.5 by ~90%

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Auto damper illustration from “Clean Burning Biomass Cookstoves”

Although airtight biomass heating stoves in the USA on average emit a lot more smoke than Southern Hemisphere cook stoves per unit of time, legally mandated chimneys move it outside where it is diluted enough to generally meet WHO standards. That’s great! Anyone who uses a wood burning heating stove knows that a chimney is necessary as a first step in the attempt to protect indoor air quality. Then, combustion efficiency must be high when density of wood stove use approaches urban levels, external air becomes stagnated, etc.

After decades of trying, research shows how hard it is to protect health with biomass cook stoves. It is not hard to design a cookstove with a chimney that achieves ~50% thermal efficiency when sufficient pot surface area is exposed to hot gas flow, but more than a chimney is needed for best protection.

I wonder if a vented airtight stove with sunken pot or pots (with or without griddle) would be “vale la pena,” worth the hassle? Dr. Kirk Smith spent ten years trying to protect health in Guatemala by removing smoke from houses with chimneys added to plancha cookstoves. The cooking pot would sit over a hole in the plancha for best heat transfer, but when the pot was removed from the stove smoke would pour into the kitchen through the open hole still harming health. He might say that an automatic damper must close when the pot is removed to keep smoke out of the room.

We have made a first attempt, illlustrated above.

See more details and drawings of an auto damper design on page 105 of Clean Burning Biomass Cookstoves, 2nd Edition, 2021. (Part 1, Main content)

I wonder if Kirk’s arduous experiment would have been more successful at protecting health if an auto damper had been installed?

ARC Assists CSIR-Ghana in Capacity-Building 

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In the first week of October, ARC Research and Development Engineer Jaden Berger visited CSIR-Ghana for capacity building training. The Council for Scientific and Industrial Research (CSIR) is the foremost national science and technology institution in Ghana.

The main focus of the visit was to teach them how to perform field testing using various sensor suites. CSIR was especially focussed on learning to perform KPTs (Kitchen Performance Tests) while using EXACT sensors from Climate Solutions Consulting. We also used other sensors CSIR already had: a PEMS (Portable Emissions Monitoring System) with a portable hood, an IAP (Indoor Air Pollution) meter, and an air quality sensor along with performing UCETs (Uncontrolled Cooking Efficiency Tests) during cooking to determine the efficiency of the stove.

Making observations of how cooks are using stoves.

Setting up a PEMS with a portable hood to measure stove emissions.

Testing was done at a secondary boy’s boarding school in Accra. The school cooks breakfast, lunch, and dinner for 3,000 students using a variety of improved and unimproved stoves. The stoves identified as the least efficient and highest emitters were the 12 wood stoves and 4 palm kernel stoves. (Palm kernels used to be considered agricultural waste from palm oil production but are now commonly used as fuel.) Several design meetings were held to determine a design that would increase efficiency, clean up emissions, and remove emissions from the room the cooks were in.

Palm kernel stoves in use for breakfast.

Weighing wood for three 24 hour-long KPTs.

Performing UCET measurements.

The next step is for CSIR to finalize a CAD model of the design along with some CFD analysis to predict if the prototype will work. ARC will then virtually meet with CSIR and their manufacturer to finalize the design and begin creating prototypes.

During the second week of the visit, ARC and CSIR worked on wrapping up older projects. This included gathering final data for a charcoal conversion efficiency study, creating a draft of the charcoal conversion efficiency protocol so that it can be published, and developing and using a durability protocol that is more applicable to conditions a stove will have to withstand in Ghana.

Taking measurements for creating a new durability protocol.

Overall, a successful trip with good progress made toward improving health and cooking conditions in Ghana.

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)

African Mud Stoves with Chimneys

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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.

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!

Cooking Outdoors as a Health Intervention

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Cooking over an open fire in Ghana. (Photo: Global Alliance for Clean Cookstoves)

The air in a kitchen has to be very clean to protect women and children from multiple diseases. Unfortunately, moderate amounts of smoke seem to damage health almost as much as higher concentrations. 

As exposure rises from zero, the chance that a child will get pneumonia increases sharply and then levels off so that indoor air with 200μg/m3 PM2.5 is almost as dangerous as air at 400μg/m3 (Burnett et al., 2014). The World Health Organization Intermediate Guideline for PM2.5 is 35μg/m3.

In order of effectiveness, when cooking in a kitchen, health interventions seem to be:

  1. Venting smoke up a functional chimney.
  2. Increasing the fresh air entering the kitchen to dilute smoke and gases. (When the outdoor air is clean and the air exchange rate is doubled, the indoor air pollution is reduced by half.)
  3.  Burning up almost all of the smoke in the stove.

 Unvented Rocket stoves, and other ‘moderately clean burning’ stoves (such as a carefully tended open fire with pot skirt), emit much too much smoke and gas to protect health in houses. 

Cooking outside, especially upwind of the fire in a bit of breeze, is highly effective in lowering harmful concentrations of PM2.5.

Cooking outside seems to be a first choice intervention, when applicable. Even ‘moderately clean burning biomass stoves’ can be used when the cook is upwind of the fire in a bit of a breeze, meeting the WHO Intermediate Guideline for PM2.5. 

Of course, cooking with a low emission stove is preferable, when possible!

Natural Gas, Methane and Nitrogen Oxides

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I didn’t realize that natural gas is mostly methane which is about 84 times worse for climate change compared to CO2.

As reported in the recently published study “Methane and NOx Emissions from Natural Gas Stoves, Cooktops, and Ovens in Residential Homes,” methane leaks are bad news for both environmental and personal health. (Lebel, et al, Environ. Sci. Technol. 2022, 56, 4, 2529–2539)

Methane risks

The abstract in the Lebel article states:

  • Natural gas stoves in >40 million U.S. residences release methane (CH4) ─a potent greenhouse gas, through post-meter leaks and incomplete combustion.
  • Using a 20-year timeframe for methane, annual methane emissions from all gas stoves in U.S. homes have a climate impact comparable to the annual carbon dioxide emissions of 500,000 cars.
  • In addition to methane emissions, co-emitted health-damaging air pollutants such as nitrogen oxides (NOx) are released into home air and can trigger respiratory diseases.
  • Our data suggest that families who don’t use their range hoods or who have poor ventilation can surpass the one hour national standard of NO2 (100 ppb) within a few minutes of stove usage, particularly in smaller kitchens.

Justa, Dos por Tres, Patsari Stoves!

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Dos por Tres Stove with Chimney, photo courtesy of Proyecto Mirador

During an ETHOS panel discussion on Cooking, Health, and Climate, it was great to see that the Justa stove with chimney protected health so well. Chimneys are mandated by law in the USA/Europe/China and many other countries. When Mahatma Gandhi returned to India from England he introduced chimneys as a logical upgrade of kitchens.

The WHO (2018) listed five prescriptions to protect health:

  1.  Use only clean household energy when available 
  2. While waiting for gas, use technologies like low-emission biomass cook stoves
  3. Minimize the time children spend around smoky fires
  4. Increase ventilation
  5. Install a chimney

Functional chimneys are the historical first step to protect health.  It’s so pleasant to sit and chat in the clean kitchen when a Justa, Dos por Tres, or Patsari stove is being used! Following up with improved combustion efficiency helps to protect climate and outdoor air quality.

A Justa Stove with chimney, photo courtesy of Stove Team International

Smoke In The Air!

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pdf available at: deohs.washington.edu/sites/default/files/AirFilterInfographic_FINAL.pdf

I hope that you are living in a smoke free environment! There’s a forest fire about 40 miles east of the lab that floods our valley with smoke when the wind slows down. I just looked up and noticed that it was getting hard to see Blue Mountain, a sure sign that the northwesterly wind wasn’t pushing hard enough to clear the skies. It reminded me of living on the Coromandel Coast in India where a blue sky was unlikely even at the beach.

Burning up smoke is not all that difficult to do: just thoroughly mix the smoke into the flame. But that doesn’t happen in a forest fire (or in a three stone fire). The smoke and flame go in different directions. The industrial reduction of PM2.5 often depends on both improved combustion efficiency and the post-combustion filtration/scrubbing of emissions. When I can’t see Blue Mountain anymore, I switch on a box fan that has a 20” by 20” by 1” furnace filter taped onto the inlet side of the fan. The fan pulls the dirty air in my office through the filter and the PM2.5 is removed from the air that I’m breathing.

Simple cooking enclosure

We wrote a paper describing how the same fan and filter reduced PM2.5 when installed in a hood over the stove. We used a washable filter and hoped that the combination of a clean burning stove with post combustion filtration of smoke might help to protect inside and outside air quality. Check out the paper: Still, D. K., Bentson, S., Murray, N., Andres, J., Yue, Z., & MacCarty, N. A. (2018). Laboratory experiments regarding the use of filtration and retained heat to reduce particulate matter emissions from biomass cooking. Energy for Sustainable Development, 42, 129–135. https://doi.org/10.1016/j.esd.2017.09.011

Smoky kitchen with no chimney vs. kitchen with chimney and clear airLance MacCarty

Meeting the World Health Organization Emission Rate Targets

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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) will be investigating effects on indoor and outdoor air when the combination of a natural draft earthen hood and an earthen stove with Jet-Flame is used in houses in Malawi.

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.