By Norbert Senf
The Wood Stove Decathlon was a
historic event. It was the first ever attempt to gather a collection of stoves
in the field (literally, in this case) and test them for particulate matter
(PM) emissions.
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| Norbert Senf, right, with Neils Wittus, center, and John Ackerly |
For something untried until now,
the side-by-side field-testing can claim several firsts.
It successfully
compared stoves within a surprising range of categories including masonry
heaters as well as retrofit kits. The project was a success not only as a media
event but also in advancing the real world testing of wood burning stoves.
Cordwood is an extremely complicated
fuel to get repeatable data with because it is so inherently variable. To add
to the challenge, PM is particularly difficult to measure, even in a
laboratory. While the test results from the Decathlon were not sufficient to
provide PM numbers that allowed comparison with EPA numbers, they did allow a
ranking of the stoves against each other. This is a substantial achievement in
itself.
PM is the wood fuel pollutant of
greatest interest since it causes the most public health concerns. Carbon
monoxide (CO) is another pollutant. It
is created by incomplete combustion like PM, but it is much easier to measure.
It is generally not considered a health hazard in low atmospheric concentrations
outside of densely trafficked urban areas, and eventually oxidizes to CO2 on
its own.
Due to new wood burning emissions regulations in Germany, two new portable instruments for measuring PM in the field were recently developed there. Fortunately, this happened just in time for the Decathlon to try them out. The instruments are limited to the 15-minute test cycle that is mandated in the German regulation, and therefore can only measure what happens during a portion of the burn. Measuring an entire test cycle will certainly be a goal for future Decathlons.
Common wisdom holds that low
carbon monoxide (CO) emissions, which are easy to measure, will also ensure low
PM, which is difficult to measure. The contest results did not bear this out.
The stove with the lowest PM had the second highest CO. For the stove with the
lowest CO, there were 4 stoves with lower PM. To be sure, the data set is
limited. The 15-minute test window did not allow for average values to be
measured over the burn of an entire fuel load.
Repeatability is one of the most
important measures of data quality. Since each stove in the Decathlon received
two (in theory) identical test runs, we can get a brief glimpse here, as well.
Discarding obvious outliers, we see a coefficient of variation (CV) in repeat
runs of 43% on PM, 40% on CO and 7% on efficiency. This compares favorably with
EPA inter-laboratory repeatability studies, where the two stoves with the
largest data sets both came in at 44% CV on PM. For masonry heaters, an MHA
(Masonry Heater Association) laboratory study on repeatability with dimensional
lumber fuel cribs yielded 10% CV on PM, 1.5% on CO, and 0.26% on efficiency.
The repeatability metric provides
a useful baseline for judging data quality in future decathlons. There is an
ongoing fueling protocol debate in the testing community between the
repeatability achievable with fuel cribs, and the real world randomness of
cordwood. EPA testing is currently done with cribs. To get a repeatable EPA
cordwood number may require running a large number of (expensive) laboratory
test runs and taking an average. To date, very little work has been done to
provide data for either side of the debate.
All in all, the Wood Stove
Decathlon was a great effort towards advancing our knowledge about how wood
stove emissions compare in the real world. This was particularly valuable to
see for different classes of appliances with no commonly defined EPA testing
methods.
Valuable lessons and insights were
had for designing a future challenge. Seeing the complex testing issues play
out in real life was a unique educational opportunity for contestants,
organizers, judges, regulators and the testing community itself.
Norbert Senf was one of the ten judges at the Wood Stove Decathlon. He joined early efforts to write
codes and standards, and was a founding member of the Masonry Heater
Association of North America (MHA). He currently chairs the MHA Technical
Committee.
