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Biogenic Energy
by -- October 3rd, 2017

As long as the Clean Power Plan is still in place, electric utilities are looking for ways to clean up their carbon dioxide emissions to the atmosphere and prevent global changes in climate. Some utilities are switching to natural gas, which emits lower amounts of carbon dioxide (CO2) per unit of energy generated.  However, natural gas is not without its own problems (see http://blogs.nicholas.duke.edu/citizenscientist/natural-gas-its-all-about-the-leak-rate/).

Humans used firewood for thousands of years, during a period with little change in the concentration of CO2 in the atmosphere.  So some utilities are looking toward the burning of wood, also known as biomass or biogenic energy, to generate power. This is an interesting “throw back” to an earlier time when firewood was used widely for heat, light, and early industrial processes.  But, there is a reason why we switched to coal, which has greater energy content per unit weight.

It is natural to think that burning firewood would have a minor effect on atmospheric CO2.  After all, as trees grow back, they remove CO2 from the atmosphere. Thus, unlike fossil fuels, trees might be regarded as “carbon-neutral.”  The regionally moribund forest products industry sees biomass as a way to recreate jobs and profits, and a trove of biomass energy companies has a lot at stake financially.

However, there are several fundamental misunderstandings about the carbon neutrality of biomass. The first is that the CO2 released from the burning of biomass is somehow “different” from the CO2 from fossil fuels.  This is not at all true: the CO2 molecule from both sources has identical structure, mixes into the atmosphere with other CO2, and easily circulates from pole to pole and ground to stratosphere during its 3-year residence time in the atmosphere.

The argument that the CO2 from biogenic fuels has no impact on the concentration of atmospheric CO2 is also untrue.  When biomass is burned, it emits CO2 immediately.  If a lot of it is burned, it emits a lot of CO2.  That is why we keep track of tropical deforestation and add its CO2 emissions to the inventories of emissions from various nations.  Deforestation currently contributes about 30% to the rising concentrations of CO2 in the atmosphere. In contrast, the uptake of CO2 by regenerating forests is long-term and not guaranteed.

Consider a simple analogy: colleges do not ignore the 4-year costs of educating students for a B.A. degree, with the assumption that, of course, their increased earning power will result in generous contributions to the alumni fund in future years, paying for the up-front costs of their education.  The upfront costs are charged as tuition, just as they should be for impacts of CO2 on the Earth’s atmosphere.

We can also reverse the accounting. Suppose I buy a piece of vacant farmland and plant trees, what is known as afforestation.  Under the Clean Power Plan, I would get credit for their carbon uptake immediately, measured by changes in the carbon inventory on the land. The carbon uptake would not be discounted by some hypothetical possibility that this tract of forest might later be harvested or burned, putting the CO2 back in the atmosphere.

The easy way to deal with this controversy is to assign the biomass that power plants desire to burn into categories or “bins” with different recovery times.  The first bin would include carbon-neutral biomass contributed by the use of grasses, crop residues, and wastes as fuels.  These plants would regrow within a year. In the last bin would be mature trees, which would take perhaps 100 years to recover the carbon emitted to the atmosphere from their combustion.  A power plant would get lower rewards for switching to wood derived from mature trees.

Knowing the origin of the biomass that is burned should not be difficult; you can’t move biomass more than about 50 miles before the energy it contains is equivalent to the energy needed to haul it.  Power plants will obtain their biomass close to home.

The Environmental Protection Agency is currently grappling with this controversy, but it is not rocket science.  Forest biomass is not carbon-neutral in a meaningful time frame.

 

References

Cornwall, W. 2017.  The burning question.  Science 355: 18-21

Galik, C.S. and R.C. Abt. 2015.  Sustainability guidelines and forest market response: An assessment of European Union pellet demand in the southeastern United States.  Global Change Bioenergy doi: 10.1111/gcbb.12273

Jonker, J.G.G., M. Junginger, and A. Faaij. 2014.  Carbon payback period and carbon offset parity point of wood pellet production in the southeastern United States.  Global Change Biology Bioenergy 6: 371-389.

Ocko, I.B, and 8 others. 2017.  Unmask temporal trade-offs in climate policy debates.  Science 356: 492-493.

Ter-Mikaelian, M.T., S.J. Colombo and J. Chen. 2015.  The burning question: Does forest bioenergy reduce carbon emissions?  A review of common misconceptions about forest carbon accounting.  Journal of Forestry 113: 57-68.

5 Comments

  1. Paul Harcombe
    Oct 3, 2017

    Yes, but if the power company owns 100 ac of forest land, and harvests it sustainably such that harvest in any year doesn’t exceed net wood production, that 10 ac would be carbon neutral on an annual basis.

    • Dylan Gillis
      Oct 3, 2017

      I think by “forest biomass” he means ‘forest biomass for energy”. At this point we need a net gain in wood (tree biomass) production, and forest biomass for energy would constitute a net loss of carbon overall, regardless of any specific 10 or 100 acre lot in question.

  2. Josh Kelly
    Oct 3, 2017

    I agree that producing electricity from biomass contributes to global warming because of the inefficiency of hauling wood to generators, converting the wood to heat, and converting the heat to electricity. However, I have heard that using wood to heat in efficient wood stoves or in physical plants for schools, hospitals, or office buildings does approach carbon neutrality depending on the fuel costs of harvest and hauling. Can you address the efficiency of wood for thermal heating?

    • Bill Schlesinger
      Bill Schlesinger
      Oct 3, 2017

      My blog focuses on the neutrality of biomass vis-à-vis carbon dioxide emissions to the atmosphere. If you burn long-lived biomass, like wood, then the neutrality is only achieved over long time periods.

  3. Howard S. Neufeld
    Oct 3, 2017

    Thanks for a very succinct and lucid discussion of this topic. I particularly liked the statistic that if you move your energy source more than 50 miles, you use up its content in moving it.

    Along these same lines, my honors class on the future of human civilization discussed the PNAS article by Schramski et al. (Human domination of the biosphere: Rapid discharge of the earth-space battery foretells the future of humankind, published in 2015). This somewhat apocalyptic article says we need to drastically cut down on human appropriation of biomass, or else the consequences will be drastic, dictated by the inviolate laws of thermodynamics.

    What is interesting is how little discussion this article has generated – it hasn’t been cited as much as I would have expected. I wonder if anyone else remembers reading this article and what opinion they have of it? I’d be interested in hearing of any such opinions. Thanks.

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