How far can wood fuel take us?

There was an interesting discussion around the lunch table at the solar startup where I work the other day, and someone mentioned that if we switched over entirely to wood heat, the nation would be entirely deforested in just a few years. Having grown up in Maine, the most heavily forested of the 50 states, this is a bit hard for me to imagine, so this evening I poked around a bit on the internet to get a sense for the orders of magnitude involved.

To clarify, I don’t view it as realistic for every household to burn splits of stovewood as primary heat – most people won’t be willing to go to the trouble, people generally aren’t home regularly enough, and in towns of even modest size the air pollution would be intolerable. But for a while I’ve been intrigued by the prospect of wood pellets – wood or wood waste that has been extruded into little cylindrical hunks about the size and shape of chicken feed (for those not familiar, about 3/16-1/4″ diameter by 3/8-1/2″ long as best I remember). The result is a fuel that is much cleaner and more convenient than stovewood; pellet stoves have hoppers that take a whole sack of pellets at a time, enough to last a day or more, and an auger mechanism feeds them into a controlled combustion chamber at a controllable rate. The pellets are transported in sacks on pallets. The local garden store sells pallets of pellets for $225/ton, which compares favorably to fossil alternatives with the exception of coal. The pellet industry trade group reports annual production in N. America of 1.1 million tons. Apparently pellets are a common heating fuel in Scandinavia, and you can buy furnaces that run on them in the US. With a bit of technical ingenuity applied to get around the inconvenience of having to grunt sacks of pellets around, it’s possible to imagine them becoming a significant source of residential heat.

So, what about the resource? The US has about 750 million acres of forested land, and the number that I heard back home in Maine for maximum sustainable yield from well-managed forest land is about 0.5 cords per acre per year. This yield is confirmed by a paper I found on forestry in MN, which estimated a maximum sustainable yield of 7M cords per year from 14.8M acres of woodlands. In my home state of Maine, the actual yield recently has been in the neighborhood of 6M cords per year, and though the state has 17.7M acres of forest land, a great deal of it is not actively managed.  Some of that forest is in wilderness areas and places that we wouldn’t want to go sawing into, and some places (like the mountain west) surely have lower productivity due to poor soils and lack of water, but others (like the south and northwest) must have higher productivity, so it’s probably a decent number to work with.    So, we estimate the sustainable production limit at half of 750M or 375M cords per year.  Poking around the internet I found an estimate of annual US timber production at 132M cords per year, so that leaves something like 240M cords of potential resource.  I believe there are about 100M households, meaning at most about 2 cords per household (once you account for wilderness and such).

It just so happens that two cords is nearly the amount that Alexis and I burned, along with about one tank of heating oil (which contains about 20% less energy than the two cords of wood) to heat our small (1200sf, almost zero solar gain) poorly insulated (brick) house here in NH.  Most people live in houses substantially bigger than ours, though many also live in much warmer climates.  It seems safe to say we couldn’t replace all of our residential heating with wood pellets on a sustainable basis.  But at least it’s on the right order of magnitude – and with the construction business in the toilet it’s not hard to imagine a good part of that 132M freeing up until the economy recovers.  All in all, it wouldn’t surprise me to see an upsurge in wood pellet usage as oil and gas costs jump sharply higher.  Anecdotally I hear that the market for stovewood has perked up already as oil prices have spiked.  Good passive solar design is a no-brainer for new construction and remodeling in suitable locations, and we might even see active solar come back, but we are going to have to find some other way to stay warm in the lean times to come.


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