During our
recent trip to Val d’Orcia, we visited a storage facility for the grains
produced in the area. But one of the giant warehouses we peeked into held
something other than grain- it housed a huge pile of grass pellets. To many,
these probably looked like something you would feed to an animal, but they
reminded me of home and of the wood pellet stove that we use to heat our living
room. So what is a pellet stove, and what is the big deal with these
weird-looking bits of wood?
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| Pellets: food or fuel? |
Generally,
pellet stoves are fueled by wood pellets, which are made of sawdust and other
wastes from logging and milling of trees. But they can also be made of
rice-husks, grasses, and other biological plant waste, like the ones in Val
d’Orcia. To make the pellets, wood material is dried, milled, compressed, and
formed into pellet shapes, with the lignin in the wood acting as a natural
“glue” that holds the pellets together. The most important step here is the
drying process, which gives pellets better storage and combustion properties
than traditional wood. The final pellets contain between 6-10% moisture. Wood
pellets are easy to transport, burn more efficiently, are easy to use, and
represent a renewable source of energy. Pellets are used for small heating
stoves as well as larger central furnace systems and for other heating
appliances, although they are used as a primary heating source mainly in
certain European countries.
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| Example of a wood pellet stove |
Wood pellets are viewed as an
environmentally-friendly fuel source in comparison with fossil fuels, since they emit little to no greenhouse gases.
Specifically, a study done in Sweden, the most prominent user of wood pellets for
heating, looked into emissions arising from the use of wood pellet stoves.
After performing tests both in a laboratory and in residential settings, the
study concluded that pellet stove emissions are generally low, but identified a
few major components of those emissions.
The main compounds that are found
in wood pellet emissions are carbon dioxide, carbon monoxide, aromatic
hydrocarbons, small volatile hydrocarbons, and antioxidant methoxyphenols.
Carbon dioxide (CO2) is a known greenhouse gas that was found to
compose 6-23% of smoke emissions from pellet burners. But the study argued that
the release of CO2 is balanced by the fact that pellets are made of
plants, and the CO2 released when burned is equal to what was once
consumed by the plants during their life. This theory, however, has come under
fire in recent years, and a June 2010 report for the Massachusetts Department
of Energy stated that burning wood pellets does indeed release a large amount
of CO2 into the atmosphere, which should not be ignored in
consideration of pellet use. In addition to CO2, carbon monoxide
(CO) is often released during pellet burning, usually due to
incomplete and inefficient combustion, which occurs when machines are used on
“half effect,” meaning not at full energy capacity.
Aromatic hydrocarbons, which consist
of one or more benzene rings, are also hazardous compounds that are released
during the burning of pellet fuel. Benzene, the simplest of these aromatic
hydrocarbons, is a known carcinogen, along with naphthalene. It was found that
the relative abundance of these hydrocarbons increases with increasing
combustion temperature in pellet burning appliances. However, the
concentrations of these compounds were very low (much lower than sitting in a
room with secondhand smoke) and it was argued that since they are emitted from
chimneys at higher elevation, they do not pose a threat to humans.
The volatile organic compounds
found in the emissions consist of small organic molecules such as methane, ethane, and
ethyne. Methane is another known greenhouse gas, having a global warming
potential over 20 times that of CO2. Like many of the other harmful
compounds in pellet stove emissions, methane is released in relatively low
amounts (0.5-30 ppm), which corresponds to less than 5% of the global warming
potential of burning fossil fuels. Of the other volatile organic compounds
released, alkenes are also dangerous because of their photo-oxidant potential,
and some can be genotoxic. This means that in the presence of UV radiation,
these compounds can attack cells of the body, including DNA, by causing radical
chain reactions.
Luckily, to combat this
photo-oxidation effect, the final group of compounds in pellet fuel emissions
is antioxidant methoxyphenols. These compounds are derived from lignin, which
makes up 30% of the composition of wood. When wood is burned at around 400°C,
lignin decomposes into methoxyphenols with either a guaiacyl or syringyl
structure, depending on whether it is hard or soft wood. At higher
temperatures, lignin is broken down further into simple phenols, and at 800°C
and above, aromatic hydrocarbons are formed. The methoxyphenols formed from
incomplete (low temperature) combustion are antioxidants that protect the
body’s cells from radical attack.
To summarize, pellet-burning
appliances release various compounds in their emissions that are both harmful
and helpful. In general, the concentrations of these compounds are low and wood
pellets are generally seen as a good alternative source of energy. However,
there are concerns about CO2 emissions, as well as the availability
and sustainability of wood pellets as a fuel source. It will be interesting to
see if the popularity of pellet stoves grows in the coming years, but for now I
will be perfectly happy sitting in front of my nice warm pellet stove during
the coming New England winter!
References:
http://www.fpl.fs.fed.us/documnts/fplrp/fpl_rp656.pdf
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