Now, we have claims both for and against the carbon-neutrality of wood energy almost daily in the news. There are stories like these, that make woody biomass out to be as bad or worse than fossil fuels:
- WPIF: Use of biomass will increase carbon emissions, not reduce them
- Biomass boiler proposal slammed by University of Montana plant employees
- MA Proposes GHG Restrictions on Biomass Power
And on the other side, whenever a biomass project is moving forward, the sponsors always claim the carbon reduction benefits of using wood versus fossils fuels. Confused?
By far the most common headlines related to this controversy in this past year were those that read something like..."Study finds biomass power not carbon neutral." The "study" all these articles refer to is the "Manomet Study of Woody Biomass Energy" conducted in Massachusetts by the Manomet Center for Conservation Sciences. The study's most controversial conclusion, as stated by the Center upon the reports release, is that:
"The Manomet study, which is based on a comprehensive lifecycle carbon accounting framework...shows that using wood for energy can result in an initial “carbon debt” because burning wood releases more CO2 into the atmosphere per unit of energy than fossil fuels (oil, coal, or natural gas). But unlike fossil fuels, forests can grow back and recapture (or sequester) CO2 from the atmosphere. Over time, through accelerated forest growth, the carbon debt can be “paid off.” After the carbon debt is paid off, if the forest continues to grow, a “carbon dividend” is realized and the use of wood for energy then becomes increasingly beneficial for greenhouse gas mitigation. As a result, using wood for energy can lead to lower atmospheric greenhouse gas levels than fossil fuels, but only after the point in time when the carbon debt is paid off. Whether or not full carbon neutrality will be achieved in these circumstances will depend on if, when, and how the forest is harvested in the future.
The most innovative and policy-relevant finding in the report is the “debt-then-dividend” model that shows using forest biomass for energy can increase greenhouse gases for a period of time before it reduces them. The length of time it takes to pay off the carbon debt and begin accruing carbon dividends (i.e., greenhouse gas benefits) can vary widely, from five years to many decades. The length of time depends on a complex interaction of (1) the type of biomass energy used (electricity, heat, or combined heat and electricity); (2) the fossil fuel that biomass energy replaces (coal, oil, or natural gas); and (3) the degree the growth potential of the forest is realized by the landowners' forest management methods.I believe the report was well-intentioned and not purposely targeted at producing a conclusion that would cast biomass in a bad light. But the "innovative and policy-relevant" use of the "debt-then-dividend" carbon accounting model led to the common perception that woody biomass produces immediate negative consequences, in terms of greenhouse gases, relative to fossil fuels.
As an example, with an electric power plant that relies on biomass using whole trees from natural forests in the Massachusetts region—and not waste wood from tree work and landscaping that has different carbon cycle impacts—the carbon debt period is likely to last for at least 20 or 30 years before carbon benefits begin to be realized. In contrast, using forest biomass in thermal applications, such as heating municipal buildings or schools, has lower carbon debts and can provide carbon dividends for the atmosphere sooner, generally within 10 to 20 years."
The result was unfortunate...both the mass media and anti-biomass activists extrapolated the report's conclusions far beyond what the study authors claimed. It has resulted, among other things, in a significant perceptual transition in Massachusetts from one of "Mass. looks to forests for renewable energy source" in 2009 to "Once hailed, wood power fizzles in Massachusetts" in 2011. The last story illustrates the Manomet study's role in the debate...
The key phrase there is "a certain type of wood at large-scale plants". You'll recall that we have discussed that "size matters" in the debate of when and where to implement biomass projects. "Large-scale", which is the scale power producers like to work at, is not an economically or environmentally sustainable model for biomass."Wood power's problems came as the state changed its views on wood's carbon emissions...Advocates argue wood power is carbon neutral because the carbon released by burning wood is eventually reabsorbed by new forest growth. But opponents, led by the citizens group Stop Spewing Carbon!, say it's a dirty technology that releases much more carbon than trees can quickly absorb.A state-commissioned report last year by the Manomet Center for Conservation Sciences indicated that burning a certain type of wood at large-scale plants would give off more carbon emissions by 2050 than coal-fired plants."
But the comments made by the folks stopping the biomass projects hurt all potential biomass projects, not just the ill-conceived ones.
"Meg Sheehan of Stop Spewing Carbon! said wood power is so dirty, it should it should never have been part of the state's clean energy plan."Wrong, Meg. It definitely should be part of the plan. But primarily in the form of small to mid-size facilities, and more importantly, primarily in home heating, district heating, industrial heating, or combined heat and power facilities. That is where the new Massachusetts regulations get it right...they specify that the projects must attain at least 40% energy efficiency. That means essentially, that woody biomass power production must be attained through CHP projects, not the larger power-only facilities. Great idea.
Finally, let's put to bed this concept that wood is not good from a carbon standpoint. Two excellent counterpoints to the Manomet technique of "debt-then-dividend" have been produced. First, there was the peer-reviewed analysis performed by Dr. Jay O'Laughlin, Director of the Natural Resources Policy Analysis Group at the University of Idaho, entitled "Accounting for Greenhouse Gas Emissions from Wood Bioenergy", in which a rigorous review of the assumptions made in the Manomet study resulted in several insightful conclusions:
First, from the Executive Summary:
"The “debt-then-dividend” model is flawed by time and space restrictions. The carbon cycle does not begin at the time a tree dies, rather it is continuous; wood utilization requires many, many stands sustained over a long period of time, not one stand over four decades as in the Manomet Center study report. The study report also purposely ignores wood products carbon pools and the benefit of avoided GHG emissions from substituting wood products for concrete and steel, which consume large amounts of fossil fuel energy in their production. The benefit of wood substitution is that fossil fuels stay in the ground, and their emissions are avoided.
Although the Manomet Center study report recognizes that “all bioenergy technologies―even biomass electric power compared to natural gas electric―look favorable when biomass ‘wastewood’ is compared to fossil fuel alternatives” (MCCS 2010a, p. 110), analysis focuses on whole-tree biomass harvesting. The report perplexingly claims that until trees regrow and recapture carbon from the atmosphere, coal is a better choice than wood for producing electricity. The study report also rejects the accepted convention that burning biomass to create energy results in a zero net GHG emissions increase; i.e., the rest of the world considers bioenergy is a low-carbon source of renewable energy, but the Manomet Center report does not."Then, from the body of the report:
"The problem of accounting for forest carbon is selecting the appropriate time framework. Wood is produced by a continuous cycling of carbon between terrestrial ecosystems and the atmosphere, and choosing a short time period is problematic. As one reviewer of this report noted, policy for wood bioenergy needs to consider the long term as well as the short (R. Sedjo, review comments; Sedjo 2010). As another reviewer wrote, “The Manomet Center’s approach over-complicates a relatively simple truth, which is that a forest can be thought of as a storage device for solar energy. Over a long cycle, the energy can be realized in various ways, such as a forest fire, biological deterioration, or by harvesting the biomass and burning it in a furnace.
Regardless, over a long period, the storage device will alternatively store energy and give up energy again and again. When we harvest and burn the biomass, we incur the cost of harvest and transport, but that is the only external energy component to an otherwise natural cycling process” (R. Harris, review comments). In essence, the Commonwealth of Massachusetts had it right in 2008...The Manomet Center’s study report is being used to lead the Commonwealth down another path."The Manomet Center responded to these criticisms in a rebuttal statement, defending the methodology and failing to yield to even a single point of Dr. O'Laughlin's. And there the issue stood, until this week, with a new analysis of the Manomet original study and their rebuttals. Dr. William Strauss, president of FutureMetrics, a bioenergy consultancy, has released a paper entitled "How Manomet got it Backwards: Challenging the “debt-then-dividend” axiom." The paper contains not only the theoretical argument against the "debt-then-dividend" carbon accounting technique, but also graphs illustrating the original points made by Dr. O'Laughlin relative to why forest-based carbon accounting needs to be analyzed over the long-term through multiple cycles. Dr. Strauss' conclusions sum up the issue:
"The Manomet study has an embedded axiomatic assumption regarding carbon accounting. The foundation of the study is a “debt-then-dividend” flow for carbon accounting. This paper has shown that this so-called axiom is a flawed basis for a proper understanding of the carbon benefits that wood-to-energy can provide. In fact, Manomet gets it backwards. There is no debt if the forest system has been in growth-to-harvest equilibrium or has a growth-to-harvest ratio greater than one and the forest is managed sustainably so that the net stock of biomass does not deplete. This is true in aggregate for Maine and it is true in other locations.
Wood-to-energy from sustainably managed forests, as this paper has shown and as all of Europe has codified in its carbon accounting rules, can provide net zero carbon emission or even positive carbon sequestration if the woody biomass stock is not depleted or grows over time."That's the essence of wood's role in the carbon cycle. It is carbon neutral, because it's use adds no new carbon to our atmosphere. Carbon accounting tricks aside, wood is a renewable alternative form of energy available to offset some portion of our fossil fuel consumption. Massachusetts, and our other heavily-forested states where biomass remains largely under-utilized, are well-advised to include biomass energy in its most efficient forms in their Renewable Portfolio Standards, and any other state initiatives and policy designed to reduce greenhouse gases.