Our forests as fuel? The devil is in the details

Biomass project in Lambert Creek, Oregon. Courtesy BLM.

Some folks argue that burning trees as an energy source — either for heat or electricity — is a “carbon neutral” resource — one that takes away as much carbon as it releases. It seems logical — new trees grow in the place of those that were cut down, and the new ones can absorb whatever carbon was released when the original tree was cut and later burned.

However, as with many things, the devil is in the details.

When a tree is cut down and burned it releases the carbon it has stored up over its lifetime. That carbon — taken out of the tree and released into the air — is called a “carbon debt.” Like any debt, carbon debts must be repaid in order to balance the account.

Unfortunately it often takes a long time to repay that debt. For the first few years after a timber harvest, the source forest may actually add to the carbon debt, as logging slash and trees damaged during harvest decompose and as disturbed soils release some of their carbon. Afterward, depending on the forest and the severity of harvest, it may take many decades to recover the former carbon stores.

The source of wood fuel determines the size of carbon debt — and here is where biomass materials vary greatly.

Limbs, tree-tops, and the leftover chips and sawdust from lumber mills are known as “biomass waste” since it doesn’t have any other use. Waste materials decay and release their carbon fairly quickly, even if we didn’t use them for energy, so they impose a minimal carbon debt. However, gathering tops and limbs during a commercial harvest involves either hauling out whole trees or using specialized bundling equipment, both of which can damage uncut trees and soils, and threaten already stressed forest ecosystems. Nationwide, 98.5% of mill wastes are already being utilized for power and heat generation. Furthermore, gathering and transporting these materials is expensive, and uses more fossil fuels that generate even more greenhouse gases.

Despite the interest in using waste wood, there just isn’t enough that would be practical and economical to use. In order to meet federal renewable energy standards (fuel and electricity) on target with the 2025 deadline, approximately 45% of the biomass used to meet it will likely come from whole, live trees.

The Wilderness Society has examined what an increase in the need for biomass would mean for the Northeast — an area where forests are now expanding after intensive clearing for agriculture across much of the region. In some places, mature forests, rare over the past century, are beginning to return, providing homes and food to black bears and martens and other deep-woods wildlife.

As maps in the slideshow indicate, expanding biomass energy facilities will draw trees from all across the Northeast and the Mid-Atlantic. In some places, multiple facilities will compete for wood. While this won’t mean clear-cutting every tree from Maine to Maryland, it could put a heavy strain on our forests and threaten the clean water, wildlife habitat and many other benefits that our forests provide. Careful scrutiny of wood supply projections should be part of every plant permitting process, and the cumulative demand from all facilities should be considered.

Even if forests can sustain greatly expanded biomass harvesting, using them to replace all fossil energy sources indiscriminately may not actually help the climate. Burning wood actually releases more greenhouse gases per unit of useful heat than most fossil fuels. A report released last month by the Manomet Center for Conservation Sciences shows that the ability for biomass to repay a carbon debt depends widely on a variety of factors. How the wood is cut, what energy source the wood replaces, how it is converted to useful energy, and the time-frame considered all factor into the carbon impact of using trees as fuel.

Biomass power plant. Courtesy NREL.The Manomet study found that when biomass is sourced from current timber operations that cut a bit more heavily, and replaces an oil furnace to generate heat, it could repay the initial carbon debt in as little as 7 years. However, biomass harvested the same way and used to replace natural gas to generate electricity would still have unpaid debt remaining in 90 years.

What this means is that switching from our high-emissions fossil fuel sources to biomass does not result in instant greenhouse gas reductions. It takes slow and steady regrowth of forest over many years before we start to see real reductions in greenhouse gases, and for the longest repayment scenarios we may not have that time before catastrophic climate changes overtake us.

We’ve noted before that biomass needs to be done right — with accurate accounting of carbon debts and credits.

The Wilderness Society is working hard to educate policymakers about the science behind using our forests as fuel. Biomass from trees can be a low-carbon energy source if done properly — with proper accounting through the entire life-cycle from forest to combustion using the best scientific methods and practices.

Without those, the clean water, habitat and other benefits that forests provide could be in jeopardy, and there will be even more carbon in the atmosphere — two debts that cost too much.

photo:
Biomass project in Lambert Creek, Oregon. Courtesy BLM.
Biomass power plant. Courtesy NREL.

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