It’s fascinating to read the ISO 19867 Standards for cookstoves and I agree with a lot of it. Many of us in the ‘stove world’ were involved for years in creating those documents. One of the big improvements is testing stoves at high, medium, and low power while reporting the results with the firepower.

We forgot to do that in the International Working Agreement started in Peru at the Partnership for Clean Indoor Air meeting in 2011. Since stoves generally make fewer emissions at low power it was a temptation to reduce the firepower and achieve a higher score on the Tiers. Since firepower was not seen on the Tier scorecard, it was really not possible to compare performance. And as we know, people tend to like high power stoves. It’s so great that this problem has been fixed in ISO 19867.

How to get a Tier 5 score for emissions? Use a chimney.

Santa Claus in a chimney
Santa understands the importance of chimneys… Happy Holidays from the Aprovecho team!

The chimney transports most or all of the PM2.5 and CO out of the kitchen. Only “fugitive” emissions escape into the room. In ISO 19867 the fugitive emissions are used for the emission rate values. For unvented stoves, total emissions are used for the emission rate values. Just make sure that your chimney and stove do not leak.

It makes sense. Here in rural Oregon, unfortunately, smoke pours out of chimneys all day and night as folks stay warm with wood. Heating stoves can be very smoky! The airtight chimney and stove get essentially all of the smoke outside of the building where concentrations are and stove get essentially all of the smoke outside of the building where concentrations are diluted.

Now, of course, at ARC we try to combine high combustion efficiency with effective chimneys. We need to protect the quality of the outside air, as well. The combination is intended to protect indoor and outdoor air. If the outdoor air is polluted it is less effective in lowering harmful concentrations. Combustion efficiency is always great and to protect health it must increase when the outside air quality is degraded. In Beijing you don’t want to add one more milligram of smoke into the air!

An indoor/outdoor air quality planning tool.

Sam Bentson created an excel spreadsheet that explains how protecting indoor and outdoor air quality are related. You can download the spreadsheet here, and learn how to use it for project planning: aprovecho.org/portfolio-item/project-planning/

Chart showing how more air exchanges reduces indoor air pollution from cooking
Chart describing the influence of air exchange per hour rates on the concentration of PM2.5 in a 30 cubic meter room. Higher air exchanges equal lower PM2.5 concentrations.
Using the ISO box model, Sam Bentson has calculated how increased ventilation helps a classic Rocket stove (around 30 mg/minute of PM2.5) and a modern TLUD burning pellets (about 5mg/minute PM2.5) to protect health.

In the lab, we are used to thinking of the ISO Tiers as static, based on how much pollution enters a 30 cubic foot kitchen during four hours of cooking with 15 air exchanges per hour. However, in 2018 ISO published 19867-3 that further explains how, for example, increasing the air exchange rate (ACH) changes the Tier rating. Generally, doubling the air exchange rate cuts pollution (PM2.5 and CO) in half.

In a low ventilation situation (10 ACH), Tier 4 requires that the emissions of CO are lower than 2.2 grams per megajoule delivered to the pot (g/MJd). But in a higher ventilation condition (30 ACH) the stove can be three times dirtier, emitting up to 7 g/MJd, and still be in Tier 4. Cooking outside is often employed by the cooks we work with because smoke is bothersome and unhealthy.

ISO 19867-3 reports that studies of air exchange rates have found a lot of variation in ventilation, from 4 ACH in very tight buildings to 100 ACH outside in the fresh air. When I lived on a ranch in Mexico, most of the cooking took place outside under a veranda (which also made it easier to smell the wonderful homemade coffee brewing in the early mornings). When Sam Bentson carefully measured the ventilation rate under our veranda in Oregon he also found that when a gentle breeze was blowing (2 MPH) the air exchange rate per hour was around 100.

At 100 ACH, with so much dilution occurring outside, achieving Tier 4 for PM2.5 and CO is easier. In our experience, the most successful and cost effective interventions are situation dependent. We find that a combination of approaches to protecting health enables a welcome adaptability to the actual and interwoven circumstances.

Ornate chimneys at Hampton Court Palace, London
Ornate chimneys at Hampton Court Palace, London
Multiple ornate chimneys grace Hampton Court Palace.

An Important Health Intervention

When cooking stoves are tested in the field the emissions of PM2.5 and CO are often higher than lab results (Roden et al., 2009). The wood can be wetter, the fire is made with less attention, and many real life variables create higher levels of pollution. It’s hard to imagine that unvented cookstoves for indoor use can be invented that will protect health when too much wet fuel is pushed quickly into the combustion chamber. Even modern cars make a lot of smoke when trying to combust bad quality gasoline.

Clean burning stoves require clean fuel just like automobiles. The sticks of wood need to be relatively dry and the metering of the sticks into the combustion chamber cannot happen too quickly. Perhaps batch fed pellet stoves will have more similar lab and field results if the pellets are well made, dry, and clean?

It’s illegal to install most types of unvented combustion devices in the United States and Europe. Even natural gas room heaters and gas cooking stoves are vented. For realistic protection of health, ARC consultants try to attach chimneys to biomass cookstoves whenever possible. When the stove smokes at least the pollution goes outside above the roof line where it becomes diluted.

Health Supportive Alternatives

Adding a chimney is not always a possibility. In these cases, it is helpful to move cooking out of the closed kitchen, for example under a veranda in the open air. Increasing air exchange rates by cooking under a veranda has been shown to dramatically lower concentrations of harmful PM and CO. Even opening the door and window in a test kitchen lowered the particulate matter 1-hour concentrations between 93% to 98% compared to the closed kitchen, and the CO 1-hour concentrations were 83% to 95% lower (Grabow et al., 2013).

Hundreds of years ago in Europe chimneys were developed as a first step to take smoke and gases outside of the kitchen. In the United States millions of wood burning heating stoves are used indoors every winter. Chimneys transport the pollution outdoors where it is mixed with the outside air.

References

Roden, C. A., Bond, T. C., Conway, S., Osorto Pinel, A. B., MacCarty, N., & Still, D. (2009). Laboratory and field investigations of particulate and carbon monoxide emissions from traditional and improved cookstoves. Atmospheric Environment, 43(6), 1170–1181. https://doi.org/10.1016/j.atmosenv.2008.05.041

Grabow, K., Still, D., & Bentson, S. (2013). Test Kitchen studies of indoor air pollution from biomass cookstoves. Energy for Sustainable Development, 17(5), 458–462. https://doi.org/10.1016/j.esd.2013.05.003

Box fan with filter reduces PM2.5

ARC has built and used two Test Kitchens and now, has by far the best one, built by Andy McClean and the three summer interns. We are looking at the distribution of smoke and the effect of opening doors and windows. Past tests with the early test kitchens showed that smoke stratified by height with the highest concentrations near the ceiling. Makes sense to cook near the floor! Opening the door dramatically lowered both PM and CO. (See:  Test Results of Cook Stove Performance, page 68)

Test kitchen
Andy McClean, Chuang Li, Katie Cushman, and Jon Au built the test kitchen.

The new Test Kitchen is a bit of an improvement. It has a volume of 30 cubic meters and the air exchange rate is controlled by 48 electric fans that are evenly spaced around the top perimeter of the building. Openings (48) along the bottom perimeter let dilution air in the test kitchen.

For the first experiment in the test kitchen the air exchange rate was determined using the tracer gas decay method (CO from a charcoal stove). The speed of the fans was adjusted until 15 ACH was measured. The smoke distribution while conducting water boiling tests was measured using 30 light scattering based PM detectors (HAPEx) that were hung from the ceiling and evenly spaced throughout the volume. The emissions rate of the rocket stove was measured using an ARC PEMS that was fitted with a partial capture probe and a gravimetric system for measuring PM2.5 (new development!). The gravimetric measurement of PM2.5 was used to calibrate the HAPEx. The tester fed the fire from an opening in the side of the test kitchen so as not to be exposed to the smoke from the fire. The test kitchen was built inside a large barn that had a fan-controlled cross ventilation. The background smoke concentration was recorded during the tests.

Results from the preliminary analysis show that there was vertical stratification, that the smoke in the room was evenly mixed from side to side, and that the average room concentration was similar to that predicted by a single zone box model. Stay tuned for more results!