Burning “biomass”—trees, grasses, and other plant matter—to generate electricity has been considered a “clean” technology in some quarters. Currently, European countries do not count carbon dioxide emitted from biomass burning as part of their greenhouse gas (GHG) emissions. This is curious, given that burning biomass does emit carbon dioxide, as well as a small amount of methane. How renewable is biomass burning? Does it leave a “carbon debt” of carbon dioxide in the atmosphere?
A Simple Idea that Sounds Good
Biomass Burning is a seductive approach to our goal of minimizing GHG emissions in electric energy generation. The principle is simple: when you burn wood, or grass, the CO² emitted is recaptured by new growth. The tree that was chipped and pelletized to feed power plants is replaced by growth of a new tree that absorbs the carbon in a closed loop.
Voila! Net zero carbon! By burning biomass, we’re taking as much carbon dioxide out of the air as we’re putting into it. Yay!
Well, not quite. There are at least a couple of reasons why you do not get net zero emissions, and why biomass burning does not stack up well against hydroelectricity, wind, solar, and nuclear, none of which emits carbon dioxide or methane in their operations.
Reason #1: getting the fuel to the power plant.
The material has to be extracted from its source, converted into a usable form (pellets in the case of wood), then transported to wherever it’s burned. To extract it, convert it, and transport it to a power plant takes a lot of energy. So much so that, with current technology, the only means of doing so is with machines that burn fossil fuel. That includes such small machines as chain saws and such large ones as the ships by which the wood pellets get from U.S. forests to Europe, the primary consumers. (Germany, BTW, does not import wood pellets; they burn their own biomass.) So you have already emitted carbon before you even start burning the fuel.
Reason #2: carbon storage, and the time between emission and re-uptake
Taking up the carbon emitted from burning is far from instantaneous. At present, much of the biomass being burned is from wood pellets made from trees in the southeast U.S for use in Europe. The age of most of these trees is more than 20 years, so they’ve been storing carbon for at least 20 years (many of them up to 100 years). If the forest were to regrow at the rate it has been growing, it would take at least another 20 years for the emitted CO² to be taken out of the atmosphere—or to “sequester” it, in the parlance of energy wonks. That’s a long time to wait when we’re looking at the acceleration of global warming.
One answer to the issue of storage and re-uptake is to grow, harvest, and replant fast-growing trees such as poplar and willow, as well as grasses—switchgrass being the most often mentioned example. (We’ll get into the ecological consequences of this approach in a later post.) Meanwhile, you still have much of that carbon released by the previous, older trees hanging around—the ones you cut down to plant the new, faster-growing crops. Trees that, had they been left standing, would be sucking up tens of millions of tons of carbon without being asked to. In fact, older trees take up more carbon dioxide per unit of time than younger ones. See:
http://www.nature.com/nature/journal/v507/n7490/abs/nature12914.html
For more on the tradeoff between trees, grasses, and agricultural sources, see this biomass endorsement on the part of the Union of Concerned Scientists:http://www.ucsusa.org/clean_energy/smart-energy-solutions/increase-renewables/growing-energy-on-the-farm.html#.V8H4OfkwhD8
How Bad is This?
Not as bad as burning coal. Coal represents carbon accumulated over fifty million years plus. For all practical purposes, coal, unlike currently growing biomass, is non-recyclable. Still, biomass burning is emitting carbon dioxide. So the rate of sequestration of carbon is a key question. It is the rate by which the difference between carbon emitted and carbon sequestered—the difference being the carbon debt—is drawn down.
Back to the Issue of Lag Time
By far the most biomass being burned for electricity generation is wood. Since older trees take up more carbon per year than younger ones (cited above), longer rotation cycles yield the greater percentage advantage. This takes us back to the issue of lag time between burning the wood and taking the carbon up again. Over the course of hundreds of years the matter of lag time becomes negligible. But over the course of the next 30 years—the window of time we have to experiment with fuels before all climate hell breaks loose—the lag time becomes crucial.
There’s another non-negligible disadvantage with biomass burning. Coal is very concentrated carbon, and burning it is more efficient than burning wood. Per unit of energy produced, coal emits about a third less carbon dioxide than wood. (Natural gas has even greater efficiency, on the order of 60% less than wood.)
Let’s take a moment to note that all of the fuels being compared here—biomass, coal, natural gas—are for thermal power plants. That means they heat water to make steam that drives turbines. (Nuclear is also thermal, although the fuel emits no carbon dioxide.) Physics and the current state of engineering dictate that most present-day thermal power plants attain efficiencies of 30-40% (the theoretical limit for thermal generation is 50%). That is, only 30-40% of the energy in the fuel generates electricity. The combined-cycle design actually recaptures enough waste heat to boost overall efficiency to as much as 60%, and is being quickly adopted in new gas-fired plants.
For these reasons, there’s an argument that for at least the first 32 years of a biomass power operation, the carbon debt from burning trees is worse than burning fossil fuels. See this from the Partnership for Policy Integrity :
http://www.pfpi.net/?page_id=155
European Doubts
The Europeans are now reconsidering their rules that treat biomass on a par with solar, wind, hydroelectric, and geothermal in regard to GHG and climate. See this article by John Upton in Climate Central:
http://www.climatecentral.org/news/europe-aims-to-close-loophole-on-wood-energy-20591
We’ll save discussion of ecological and health impacts of biomass burning, aside from GHG emissions, for a later post.
So what about “carbon sinks?” There’s a lot of uncertainty about where carbon dioxide goes when it gets pulled out of the atmosphere—so-called “carbon sinks.” Currently, about 30% is absorbed by oceans, but where the rest is going remains a puzzle. Is much of it going into trees, and if so, what trees? Tropical trees? Old trees or young trees? Hardwoods or softwoods? Trees that are being pelletized for fuel, or trees in boreal forests whose growth is accelerating due to global warming? An article in science2.0 discusses the “mystery,” and how the Orbiting Carbon Observatory is expected to identify where the carbon sinks are, and how they perform. See:
http://www.science20.com/news_releases/where_does_co2_go_mystery_missing_sinks
Also see the Wikipedia entry for the Orbiting Carbon Observatory:
https://en.wikipedia.org/wiki/Orbiting_Carbon_Observatory
In case you really want to get into the weeds on the biomass problem, see see Renee Cho’s article in Columbia’s Earth Institute State of the Planet: http://blogs.ei.columbia.edu/2011/08/18/is-biomass-really-renewable/
and the aforementioned article from the Partnership for Policy Integrity, : http://www.pfpi.net/?page_id=155