It’s a common-sense tenet of the case against fossil fuels that burning coal, especially, but also oil and natural gas is a threat to earth’s climate because it disrupts the carbon cycle.

You know the reasoning:

Since the beginning of widespread industrialization, carbon locked up in the earth for millennia has been returned to the atmosphere on a timescale marked off in mere decades. The rate at which the world’s forests can recapture it, in the form of CO 2 , can’t accelerate to match.

Which leads to a second, sensible-sounding tenet: Growing and burning wood as an alternative to fossil fuels must by definition be carbon-neutral, or nearly so, with each felled tree’s contribution of carbon emissions balanced by a growing tree’s recapture.

And it might work that way on the scale of a campfire or a woodstove capable of heating a home. But burning wood on an industrial scale is a different matter, and one increasingly under challenge for its environmental shortcomings.

‘Wood is worse than coal’

A recent story in the Washington Post, discussing the surge in wood-pellet production in the southern U.S. to meet the demand of new-style power plants across Europe, summarized it succinctly:

A number of independent experts and scientific studies — including a new analysis released Tuesday — are casting doubt on a key argument used to justify the cutting of Southern forests to make fuel. In reality, these scientists say, Europe’s appetite for wood pellets could lead to more carbon pollution for decades to come, while also putting some of the East Coast’s most productive wildlife habitats at risk.

“From the point of view of what’s coming out of the smokestack, wood is worse than coal,” said William H. Schlesinger, the former dean of Duke University’s Nicholas School of the Environment and Earth Sciences and one of nearly 100 scientists to sign a letter to the Environmental Protection Agency last year asking for stricter guidelines on using biomass to generate electric power. “You release a lot of carbon in a short period of time, and it takes decades to pull that carbon back out of the atmosphere.”

And this should be of perhaps special concern to Minnesotans as well, because plans for greenhouse-gas reduction with a possible wood-burning component are under consideration by the state Commerce Department and Environmental Quality Board.

Certainly it’s of concern to the Sierra Club’s Northstar Chapter and to Jim Hawkins, a volunteer who has devoted prodigious effort to analyzing the issues – and to warning state planners away from wood-fired boilers, while urging them to consider switchgrass as the better biomass solution.

Propping up pellets

The Post story, by Joby Warrick, explains that the surging popularity of wood pellets as power-plant fuel in Europe is driven by government subsidies aimed at cutting coal consumption and carbon emissions.

Although pelletized wood costs about twice as much per ton as coal, sometimes more, and yields a lot less heat when burned, subsidies make the pellets “affordable” for electric power production. Another advantage: Boilers built to be fired with coal can be switched to pellets with minimal retrofitting.

Meanwhile, a wood-pellets industry that barely existed a decade ago “is reshaping Southern landscapes from coastal Virginia to the Gulf of Mexico,” with pellet mills running full tilt in seven states and more on the drawing board; exports doubled between 2012 and 2014.

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Although pellets can be made from lumber scraps and sawdust, or low-quality timber with no other productive use, Warrick found that European demand is being fulfilled in significant portion with the harvest of both mature hardwoods and pine, some of the latter from plantations and some not.

And that’s what matters most to environmental advocates like the Natural Resources Defense Council, which published an issue brief on the subject last month.

“Under the right circumstances, true wood waste could serve as a low-carbon option for producing pellets,” NRDC acknowledges. “For example, sawdust and chips from sawmills that would otherwise quickly decompose and release carbon anyway can be a low-carbon source.”

But if whole trees contribute much of the wood made into pellets, the picture changes: at 70 percent whole-trees content, cumulative CO2 emissions per megawatt-hour of power production significantly exceed the emissions from a coal plant for six decades, and from a natural-gas plant for nearly seven.

At 40 percent whole trees, the wood-burning still produces more CO2 than goal or gas, and for about the same time periods, but the amount of the excess is smaller.

On Minnesota’s drawing board

For some time now, the Minnesota Environmental Quality Board has been leading a project called Climate Solutions and Economic Opportunities (CSEO) to evaluate strategies for reducing greenhouse gas emissions in the state, especially from the electric power sector.

One part of the CSEO initiative, being undertaken in partnership with the state Commerce Department, is exploring gains that can be achieved with the technologies of combined heat and power (CHP), in which the heat created in power generation can be captured and put to use instead of thrown out with the cooling water.

CHP plants are especially suitable to firing with wood or other biomass, as well as with natural gas, and according to a draft action plan issued at the end of March for Minnesota’s future efforts, biomass-fired CHP projects are getting significant attention from state planners.

But the plan is peculiarly lacking in a definition of what is meant by “biomass,” and Anna Henderson, chief planner on the project, declined an invitation to discuss it with me. I was able to confirm with the Commerce Department’s Libby Caulum, however, that the CHP analysis is indeed looking at wood-fired alternatives, as well as natural gas.

In comments prepared for the CSEO/CHP review process, Hawkins argues for the Sierra Club that making electricity from round wood – an industry term for what you and I might call felled trees, as distinct from logging slash and mill waste, creates a permanent increase in atmospheric carbon.

Drawing on the work of Bjart Holtsmark, a Norwegian research economist whose specializes in natural resource and environmental subjects, Hawkins puts the case this way:

The typical carbon-cycle model assumes that a stand of trees is cut, burned and allowed to regrow to maturity. But using round wood for for significant CHP generation requires increased harvesting, year after year, for as long as the power plants operate.

Yes, the carbon released in Year 1 would start to be be recaptured in Year 2 and this would continue through, say, Year 100 and beyond; Hawkins notes that forests continue to capture and store carbon for decades after what a forester would define as maturity.

But Year 2 would also bring new harvests and new carbon releases in a cycle that rolls over from year to year, persisting until all harvesting of wood for power came to an end. Until that point, the forest simply couldn’t catch up.

Perpetual harvest, widening gap

Moreover, the increase in atmospheric carbon that could be be calculated for any pair of forest plots, one harvested and one replacing it, wouldn’t even begin to level off until the replacement passed the halfway point to maturity.

And the gap between carbon released and carbon recaptured would likely widen from year to year if new timber was cut before full maturity, in order to maximize power production – which is not just a theoretical possibility:

The Minnesota DNR recently decided to lower the rotation age for even age aspen/BAM to 45 years compared to a natural rotation age of 70; and to reduce it for red pine plantations to 60-70 years from 112 years. Thus, in the future, plots that were previously allowed to reach maturity before harvest now would be cut before maturity. The harvested parcels would never again attain maturity and, therefore, never re-capture all of the initial pulse of CO2 and the stock of carbon in living trees in the forest would be permanently reduced. A policy of harvesting before maturity, thus, leads to a permanent increase in atmospheric carbon. The effect of intensive commercial thinning is to lower the average rotational age of the parcels thinned without appearing or even intending to do so. If 30% of commercial quality wood is extracted from a parcel by thinning… the residual stand would need to grow in volume by about 43% in the remaining years to maturity in order to recoup the wood removed by thinning and to capture the pulse of CO2 released by burning the extracted wood. A 43% growth spurt of this magnitude is unlikely to occur in the slow growing boreal forest.

The better idea for carbon management, Hawkins says, is to let forests capture carbon for as long as they can and then, perhaps, to extend the storage time by making long-lived products from the timber, like houses or furniture.

And the better idea for biomass-based electric power, he said, is to grow, harvest and process switchgrass as a source of both pelletized fuel and biogas, using sensitive lands already enrolled in the Conservation Reserve Program along with other, non-forested northern acreage that isn’t good for much else.

This conjecture is based in part on University of Minnesota ecologist David Tilman’s work on the prodigious carbon-sequestering capabilities of perennial grasses. It’s an appealing argument, and I’ll revisit it in detail as the issue of biomass CHP for Minnesota goes forward.