Engineering Ethics Blog: A Burning Question: Trees Into Electricity?


Ancient humans probably learned first about fire by watching a
forest burn.  One would think that in
this era of nuclear and solar energy, the very old-fashioned alternative of
burning wood for power is passé, but one would be wrong.  A recent article on the Wired website
points out that biomass-fueled power plants are enjoying a comeback both in the
U. S. and Europe, but for different reasons. 
And the reasons are controversial.

Burning wood releases the greenhouse gas carbon dioxide, so
other things being equal, generating electricity with solar or nuclear power is
ecologically friendlier for that reason alone. 
However, every tree on the planet has a natural life cycle, and before
humans came along, the fate of many trees was to perish in a lightning-ignited
forest fire.  We now know that such fires
are a normal way for forests to renew themselves, and nature is not taken by
surprise when a forest burns.  A few
years later seedlings have sprouted into trees and the scars are largely
healed. 

But in places like California, where residents of forested areas
have promoted fire-prevention efforts that allow a buildup of dead trees and
underbrush, the inevitable fires that nevertheless result can prove even more
devastating than if people had just left nature to itself.  So a movement has arisen in that state to cut
down dead trees and burn them in biomass plants, so much so that California
leads the nation in the number of biomass-to-electricity facilities.

At first glance, this looks like a win-win situation.  The forests are better managed with those
dead trees pruned away, the electric grid gets some much-needed power plants,
and the local job markets benefit through the creation of labor-intensive
logging and chipping activities.  But
critics point out that burning any kind of biomass has a carbon footprint we
could avoid, and the carbon sequestered in dead trees doesn’t contribute to
global warming.

I suppose somebody could get a grant to figure out exactly what
mix of benign neglect, active harvesting of dead or even living trees, and
biomass energy production would lead to the optimum of electricity and minimum
carbon footprints, but even if you could figure it out, other factors would
intervene before you could optimize things. 

Such factors include politics, both domestic and abroad.  In the Southeast U. S., where attitudes
toward forests are more commercial than esthetic, it turns out there is a
booming business in planting and harvesting pine forests to make wood pellets
for export to Europe.  In a controversial
decision, the European Union decided to designate biomass-fueled power plants as
renewable energy, and now European countries are importing lots of wood pellets
from the U. S. to burn for electricity.

Back when we lived in New England a couple of decades ago, a
friend of ours started a business selling wood-pellet stoves for home
heating.  As long as the pellets were
made locally, they were cheaper per heating unit than fuel oil, which was the
only alternative for many homes.  But somehow
I doubt that shipping wood pellets across the Atlantic is as cost-effective as
shipping oil, or even coal.  But it’s
renewable, and that label is valued increasingly by an ecologically-conscious
public willing to pay more for it.

If you consider the life cycle of a particular tree, there is a
good but not certain chance that it will perish in a forest fire some day.  In prehistoric natural forests, this fate was
probably more common than it is today in California’s fire-protected forests,
but as recent years have shown, it’s impossible to prevent all forest fires.  And when an artificially-protected forest choked
with dead trees and dry underbrush does catch fire, the resulting conflagration
can be a lot worse than if we had just walked away from the place a few dozen
years ago and let nature do its own burning at its own pace.  But people with million-dollar homes in the
middle of a forest don’t want to do that, and so you get the situation that
California faces now, where many forests resemble powder kegs waiting for a
match.

If you look at the situation from a sustainable-energy
perspective, it seems to me that biomass energy fits the description better than
many other so-called sustainable options. 
Over the long term, here’s what happens. 
Trees use sunlight, water, carbon dioxide, and a few other things to
make cellulose.  Either before or after
the tree dies, people come along and chip up the tree and burn it for power,
releasing the carbon dioxide back into the air. 
But other trees will come along some day and grab that same carbon dioxide
and repeat the cycle.  Sounds pretty
sustainable to me.

One practical problem in the way of going completely biomass for
our electricity is that biomass plants don’t scale very well.  Just as an example, the largest biomass plant
in Texas has a capacity of only 100 megawatts (MW).  The smallest natural-gas plant in Texas has a
capacity of 176 MW, and the largest can put out 2051 MW, comparable to the two
nuclear plants in Texas.  The fact of the
matter is that it takes a whole lot of wood chips to make not that much energy,
and so far, most biomass plants in the U. S. have been built not simply to
produce power, but to achieve other ends as well:  reduction of dead-tree mass, employment, and
so on. 

So we probably shouldn’t envision a future in which all our
power comes from burning trees.  There
just aren’t enough trees to go around for that. 
But in situations where labor, forestry policies, and politics coincide,
biomass energy can both make sense and do some good.  It’s not all good, but it’s not all bad
either, like most things in life.  And in
burning wood for fuel, we are doing something that humanity has done since the dawn
of time. 



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