Much ado has been made of late about the advantages and, increasingly, the potential disadvantages of using wood to create energy. Figuring out what’s what on that front has taken on new urgency, with renewable energy policies in Europe driving an unprecedented boom in the use of manufactured wood pellets to generate electricity.
The vast majority of that wood fuel is being culled from American forests, making pellet-makers fat and happy — projections suggest it could become a multi-billion dollar industry over the next ten years — but causing forest advocates and climate campaigners to grow increasingly alarmed.
On its face, wood-based energy would seem to be both renewable and climate friendly. Unlike coal, oil or natural gas, trees can be grown, harvested and replanted in perpetuity — at least in theory. And while planet-warming carbon dioxide is released when wood is burned to boil water, generate steam and drive a turbine, that CO2 is also absorbed by trees when they are growing, resulting in what would seem to be a wash as far as the climate is concerned.
Or so, again, goes the theory.
But a host of new studies, including one released on Monday by the Natural Resources Defense Council, suggest that it’s really not that simple, and that deriving any real benefits from wood-based power depends on an exceedingly long and complex list of variables, caveats and assumptions.
One key variable: Where does the wood come from?
In a new issue brief, Sami Yassa, a senior scientist with NRDC’s Land and Wildlife Program, attempted to model the precise carbon impacts of burning wood pellets made from varying amounts of whole trees, wastes from saw and paper mills, and tree tops and branches left over from routine logging operations.
Why does this matter? Well, it’s all about timeframes. A pile of sawdust or thin twigs left to rot on the ground will decompose and release stored carbon dioxide into the atmosphere relatively quickly, the thinking goes, so collecting such material and burning it doesn’t much accelerate that natural process. So long as new trees are being planted to slurp up atmospheric CO2, no consequential emissions are added to the atmosphere — making wood energy in this context look better than, say, electricity derived from coal, which can only deliver its stored CO2 upward.
But things get messy, according to NRDC, when whole trees are harvested specifically to generate energy — something that the booming market for wood pellets in Europe is already driving, advocates say.
In a nutshell, that’s because wood is far less energy dense than fossil fuels like coal and natural gas, so you have to burn a lot more of it, relative to fossil fuels, to get the same amount of energy output. This means that, in the early years of a hypothetical wood-based biomass power plant, the carbon dioxide emissions are comparable to, or worse than, even the worst climate-trashing coal plant. Eventually that carbon pollution will be cancelled out by new trees that are growing and absorbing CO2, but just how long is “eventually?”
That’s the key question, and according to the NRDC’s new wood pellet modeling effort, the answer depends a great deal on whether the wood pellets were manufactured mostly from scrap wood, or by cutting down whole trees. The analysis included modeling results for three representative scenarios: Wood pellets made of 70 percent, 40 percent, and 20 percent whole trees.
From the report:
The modeling shows that it will take approximately 55 years for forest regrowth to recapture enough carbon from the atmosphere to reduce the plant’s cumulative emissions below those of coal. At levels greater than 40 percent, pellets emit more carbon than coal for most of this period. In addition, as the percentage of whole trees increases above 70 percent (not shown in the figures), the level of carbon emissions continues to increase.
When whole trees make up 20 percent of the wood in pellets, emissions are slightly higher than natural gas and slightly lower than coal for a period of approximately 55 years, as shown in Figure 3. Even when whole trees make up as little as 12 percent of pellets, our modeling showed that burning pellets still produces emissions comparable to natural gas trend line for approximately 50 years.
Put simply: Wood-based power could be carbon neutral at some point down the line, but it will take a heck of a long time to realize, especially if whole trees are used in the process.
Now, whether that sounds like good news or bad news to you in the big scheme of things will depend on your perspective vis a vis the sensitivity of the climate to ongoing carbon pollution. As far as NRDC is concerned, 50 years is a dangerously long time to wait for the presumed beneficial results of any alternative to fossil fuels.
“This 5-decade time period is significant,” Yassa stated in the study. “Climate policy imperatives require dramatic short-term reductions in greenhouse gas emissions, and emissions from these pellets will persist in the atmosphere well past the time when significant reductions are needed.”
Of course, one obvious solution to all this is to avoid cutting down and burning whole trees in the production of energy. While some groups decry any sort of wood-based energy, many environmental groups — including the NRDC — suggest that a carefully regulated biomass industry that relies solely on branches and scrap wood and other similar debris, can be a key part of a low-carbon energy transition.
The biomass industry says they’re on the same page, and that the concerns being raised by the NRDC and other organizations over whole-tree harvesting are much ado about nothing. “Biomass power in the United States uses residues and byproducts, period,” Bob Cleaves, the president of the Biomass Power Association, declared in a sharply worded letter to the Washington Post in March. “There is no market to sell timber for fuel.”
Even if that were true now, critics say, the galloping demand for wood pellets in the UK and elsewhere will quickly exhaust the supply of forest residues and other woody debris, making the harvesting of whole trees for the purpose of energy production more or less inevitable without tougher regulations.
Wood pellet exports from the U.S. have more than doubled in just the last three years alone, accounting for more than $500 million of trade in 2014.
“Several studies have concluded that logging residuals alone may be unable to meet bioenergy demands in the region we modeled,” Yassa wrote, “and that pulpwood trees may need to be used to meet the increasing demand.”
In an email message, Yassa pointed to a December 2014 investigationconducted on behalf of the NRDC and the forest advocacy Dogwood Alliance. It showed photographs of what are described as “whole trees” being trucked into a North Carolina facility operated by pellet manufacturer Enviva — a major supplier to European markets.
On its Web site, Enviva offers this explanation: “The only whole trees that Enviva uses are either young commercial softwood thinnings, which are cut to ensure healthy growth of high-value timber, or in some cases small, diseased or deformed trees that do not meet specifications for sawlogs. In many places, there is no other market for this wood. Often, what may appear to be a whole tree is actually the top of a tree, which cannot be used to make the high-value wood products for which the trunks have been harvested.”
The truth will need to be sorted out quickly. Under current policies, the wood pellet market is projected to to continue exploding for at least the next decade.