If you’ve spent any time contemplating the curl of smoke
from a wood stove flue, you’ve probably wondered just what
was in that cloud and whether it contained pollutants
that might someday be recognized as harmful. You may even
have heard that some communities–such as Vail,
Colorado–have been forced to restrict wood burning to
avert health hazards from haze, fly ash, and carbon
monoxide.
But, until just a few months ago, there was very little
solid technical information about wood stove pollution–the specific kinds and
quantities of emissions produced by residential wood-fueled
appliances. Well, the facts are now beginning to come in,
and though it will be a few years before scientists
have a thorough grasp on the scope of the
problem, the initial results are, for the most part,
depressing.
A study completed in June 1980 (it was carried out by the
Monsanto Corporation, at the request of the Environmental
Protection Agency) has shown that wood stoves, although they
produce different sorts of heavy emissions from
those associated with automobiles or power plants, do pose
significant pollution problems. It seems that wood burners,
especially the airtight models increasingly favored by
consumers, are major producers of what scientists call
polycyclic organic matter (POM). In fact, the
results of the preliminary study suggest that
residential wood stoves contribute more POM to the
atmosphere than all other sources of combustion combined!
The term “polycyclic” refers to a molecule that contains at
least two closed atomic rings, but the most important
aspect of POM–as far as we consumers are
concerned–is that many of the compounds classed under
the heading are known to be mutagens or carcinogens. Two of
the most toxic examples are benzo(a)pyrene and
dimethylbenzanthracene … but there were actually 25 POM
compounds (or subdivisions of them) identified by the
Monsanto study, plus miscellaneous aldehydes and other
organics.
Efficient = Dirty
In an attempt to categorize the severity of the problem
under differing combustion conditions, the Monsanto
team–working at the Auburn University laboratories–tested
three different wood burning appliances using logs of green
and seasoned oak and pine, each at a variety
of draft settings. The results demonstrated conclusively
that the production of POM is a function of combustion
temperature … which–in most wood stoves–is
closely related to oxygen supply.
Therefore the fireplace insert that the group
tested–which burned its fuel supply relatively
rapidly, yielding efficiencies that peaked at roughly
25%–produced about 1/10 as much POM as did either a
baffled or unbaffled airtight. (In fact, the two airtight
stoves performed similarly, yielding efficiencies of
between 45 and 50%.) Quite clearly, limiting the rate of
combustion in a woodstove by reducing the air supply
increases POM emissions dramatically. In other
words, the more efficient the unit becomes, the more POM it
will produce.
But other factors also influence the rate of POM production
in a wood stove. The moisture content of the fuel, for
instance, has the direct effect of reducing combustion area
temperature since the liquid must be evaporated
before the wood can burn. While the tests showed little
difference in the pollution produced by burning
green–as opposed to seasoned–oak, burning green pine
caused roughly a three-fold increase in POM emissions over
that of either of the two oak samples or of the
seasoned pine. This result suggests that the resinous
content of some softwoods has a tendency to increase POM
emissions.
Though the POM results were the most significant
discoveries made by the study, the research also showed
that rates of carbon monoxide production are influenced by
many of the same factors as are POM emissions. Again,
airtight stoves seem to be far greater polluters than are
their less efficient brethren. (We should also note,
however, that though wood burners are not heavy producers of
nitrous oxides, the higher temperatures that occurred in
the fireplace insert did yield more of that
particular pollutant than was produced by the more
efficient stoves.)
What’s to Be Done?
We’re not suggesting that you abandon your wood stove, nor
is the EPA currently contemplating any specific
restrictions on the production or use of such appliances.
(In comparison, after a study in Sweden similar to our
EPA’s project yielded nearly identical results, wood burning
was–as of November 1980–strictly limited in
that country!) But we are suggesting that you
operate your wood burner in ways that tend to reduce
emissions!
The object of a clean stove-operating procedure is to
maintain the highest combustion temperatures possible. You
can help your stove accomplish this by using seasoned wood
(and, specifically, by not burning green pine!) and keeping
the draft at least partway open. (Ideally, this means
sacrificing–at least for the time being–a
smoldering all-night burn in return for cleaner air.
The choice is yours.)
You can also reduce POM and CO emissions by burning smaller
fires in your stove. Such an approach avoids the creation
of oxygen-starved areas in the firebox … spots in which
heavy emissions tend to be produced. Furthermore, even
though there are no conclusive results to date, there seems
to be reason to believe that lined (with ceramic,
soapstone, etc.) fireboxes maintain hotter combustion than
do “bare” chambers.
Salvation in Technology?
A number of different companies and individuals have
recognized the dangers of wood stove emissions, and are
developing new products in an attempt to solve the problem.
For example, the Corning Glass Works has introduced a catalytic converter–similar to the automotive
devices, but with much larger passages to accommodate lower
“exhaust” velocities and larger particles
–that may help strip flue gases of
pollutants while increasing overall stove efficiency.
Corning has no retrofit devices available at this
time, but Franklin Cast Products (a major importer of
Taiwan-made stoves) does have two wood burners–which,
at this writing, sell for about $1,000
apiece–equipped to use the Corning device as a wood stove catalytic converter.
Other research groups feel that the solution will involve
moving away from current airtight stove designs. Jotul of
Norway–while unwilling to divulge specific
information about work being carried out under its
$2,500,000 annual research budget–is rumored to be
working on a wood gas (pyrolysis) stove. Still another possibility is to use high-temperature
furnaces (such as Hampton Technology’s Jetstream), that
employ heat exchangers and storage systems. And, while
MOTHER EARTH NEWS is keeping a watchful eye on the corporate solutions
to our pollution problems, we’re also keenly attentive to
work being done by individuals (such as the development of
Larry Dobson’s sawdust stove, featured in MOTHER EARTH NEWS’ Guide to
Home Energy).
We’re confident that there are solutions to the problem,
and–in fact–MOTHER EARTH NEWS’ research staff is busy
working on a number of retrofit approaches to lessening
emissions. What’s more, we’re doing our best to encourage
others to apply their knowledge to the effort. And
when the solutions do become available, you’ll read about
them here!
