Ideas

Biogenic Energy

October 26, 2017

A wood stove burns with pellet fuel. Photo © U.S. Department of Agriculture / Flickr through a CC BY 2.0 license

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.

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 COemissions to the inventories of emissions from various nations. Deforestation currently contributes about 30% to the rising concentrations of COin the atmosphere. In contrast, the uptake of CO2 by regenerating forests is long-term and not guaranteed.

Aerial view of habitat destruction caused by poorly executed clear cutting in the Mid-Atlantic Coastal Plain ecoregion of northeastern North Carolina. Photo © Mark Godfrey / The Nature Conservancy

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.

New growth in clear-cut hardwood forest areas along the Savannah River in Hampton County, South Carolina. South Atlantic Coastal Plain ecoregion. Photo © Mark Godfrey / The Nature Conservancy

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.

Forest biomass is not carbon-neutral in a meaningful time frame.

This post originally appeared on William H. Schlesinger’s blog Citizen Scientist, published by Duke’s Nicholas School of the Environment.

William H. Schlesinger

William H. Schlesinger is one of the nation’s leading ecologists and earth scientists and a passionate advocate for translating science for lay audiences. A member of the National Academy of Sciences, he has served as dean of the Nicholas School of the Environment at Duke and president of the Cary Institute of Ecosystem Studies. More from William H.

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2 comments

  1. I am curious as to the total amount of CO2 pollution not being calculated with respect to the production and transport of oil. These factors include manufacture and transport of the iron ore used in all the steel refining, production equipment , oil rigs, ocean tankers, pipelines, refineries, trucks, tools, tires, etc., as well as resultant CO2 from the shipping itself in ocean tankers and trucks. Another factor to be added would have to be a calculation pollution cleanup of spills such as the Gulf War, Exxon Valdez, Deep Water Horizon as well as the many others both large and small.
    Is this ever factored in?

  2. I am in Europe now, which likes to burn wood. In particular the wood from the US because they call it green and don’t need to obliterate their own forests. Americans are more than willing to oblige.
    My question has to do with the actual combustion of wood. Complete combustion is difficult enough with liquid fuels but seems like it would be more difficult with wood because of moisture content and other variabilities. The only numbers I can find are that wood yields about 400 ppm CO2 and 5 ppm N2O plus other things (H2O, NO2). I’m not a chemist and I’m kind of mixing complete and incomplete combustion. It seems like the 1% N2O should contribute significantly to emissions as it is 265 times as “effective” as CO2. Could you shed some light on the N2O emissions or am I just totally off base?