Carbon cycle. NASA image.
George Harvey
Dartmouth College has committed to being 100% free of fossil fuel use, and so it is examining a switch from heating with number 6 oil to use of biomass, which would allow it to continue to use most of the current heat distribution system. The idea that it could use biomass has created quite a stir, however.
Respected scientists and alumni are objecting that there are better ways to do things. They say that burning biomass does not reduce carbon emissions and creates pollution. They say that we have better, less carbon-intensive, and less costly sources of heat. And they say that the forests need to grow to draw down carbon dioxide from the air. They are putting the biomass on hold as they look into other options, but I believe they may be missing important details that should be included in the calculations.
Biomass is part of a natural carbon cycle. Living things are composed mostly from water and carbon dioxide (CO₂). They go through various processes, and they are ultimately decomposed, mostly into water and CO₂. The natural carbon cycle contrasts with the fossil fuel cycle, which burns fossils that are not part of the natural cycle and emits CO₂ that builds up in the atmosphere. Where the natural carbon cycle kept CO₂ rather steady at about 280 parts per million (ppm), the fossil fuel carbon cycle has pushed it up to over 410 ppm.
We should look at what happens to a tree that dies and falls in the forest. Nature taking its course means it will decompose. It can be eaten by insects, such as termites, or decomposed by fungi or bacteria. When fungi decompose wood, they release CO₂, but termites and some bacteria also release large amounts of methane (CH₄).
We can contrast this with burning wood. Clean combustion, which can be done by a number of means, releases the smallest amounts of pollutants possible, with almost no particulates emitted. Ash is captured. Soot is practically non-existent. What comes out is CO₂ and water. A modern stove or industrial combustion unit is very different from an old “airtight” (typical of the 70s and 80s) that produced large amounts of creosote.
Please notice that where CH₄ can be emitted in large amounts by decomposition, none comes from clean biomass combustion. And since CH₄ is a much more powerful greenhouse gas than CO₂, it may be environmentally preferable to burn wood than to let it rot, in terms of greenhouse gases.
This brings up the question of where Dartmouth would get its fuel. To find this out, I obtained a copy of the college’s own report, “Dartmouth College Biomass Fuel Supply Assessment” (DCBFSA). According to that document, about 45,000 to 60,000 tons of green wood will be needed each year. All of this wood can be obtained within a relatively short distance.
Some people may exclaim at the amount of wood needed. As a matter of comparison, the New York Times used 104,000 tons of paper in 2018, and a ton of paper requires about two tons of green wood, if none of it is recycled. Accordingly, heating the buildings at Dartmouth could require a quarter of the amount of wood needed for the print edition of the New York Times.
More to the point, according to DCBFSA, forestry in the Dartmouth woodshed produces about 1.26 million green tons of wood each year, producing a total of 1.8 million green tons of forest waste, most of which is unharvested. “Dartmouth’s proposed wood use represents 3.3% or less of this volume,” the report says.
Leaving that wood in the forest would not only produce its own carbon emissions, it could also produce fire hazards. And those hazards could be amplified as forests are under increasing pressure from climate change, which threatens nearly all species of trees in Vermont and New Hampshire. Maintaining forests will require careful forest stewardship, removing dead wood, cutting dying and infected trees, and disposing of them. And that will mean, very probably, burning a large multiple of the wood Dartmouth College envisions using.
It happens that Dartmouth College is taking the scientists who warned them about using biomass seriously and is examining the whole question again. This brings us to the question of what the alternatives are.
The heating systems at Dartmouth are designed around combustion. Ductwork can continue to be used, providing combustion continues. This probably means a choice of continuing the use of fossil fuels or converting to biomass.
The other choices, while renewable, would require installation of new heating systems, ignoring the ducts that are in place. This can be done with geothermal or air-source heat pumps, which would have to be installed in large numbers, at great expense. The heat pumps would have to be powered by renewable resources, and this almost certainly would mean wind-power, which many people object to, or solar photovoltaics, which would cover a lot of land. Regardless of which path the college takes, it would be far more expensive than converting to biomass.
One other issue with renewable energy other than bio-mass is that some of the buildings would have to have their heating efficiency updated for heat pumps to be used efficiently. This would increase the cost of the change-over considerably. While this job will have to be done in the future, avoiding use of bio-mass adds it to current costs.
We have gone a long way with all of alternative technologies, and they all have their place. They are efficient and operate at low cost. But we should recognize that one of the technologies, carefully engineered biomass, has its place along with the others. And Dartmouth College may be a prime example of what that place looks like.
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