Focus on: Wood-based bioenergy

forest biomass
Forest biomass is the woody material, such as logging slash, that is gathered and burned in a controlled environment, such as a boiler. Photo: DNR.

Much of our nation’s energy (both liquid transportation fuels and electric power) is derived from “fossil fuels,” which include oil, coal, and natural gas.

There are several drawbacks to these energy sources:

  • They are non-renewable resources. Once existing deposits are used up, they are gone. Through new technology we have gotten better at finding and accessing more of these deposits, which has kept supplies plentiful, but ultimately they are finite.
  • Their use converts carbon stored the earth to carbon dioxide which is released into the atmosphere. Rising concentrations of carbon dioxide in the atmosphere changes global climate, with a myriad of consequences.
  • Prices are unstable and usually climbing, impacting all areas of our lives and economy and our nation’s foreign policy.
  • Extraction (e.g. mining, offshore drilling, fracking, etc.) can harm the environment, especially if there is an accident.

The advantages of bioenergy is that it can be renewable, locally produced, and possibly ‘carbon neutral,’ though there is disagreement on this last point. It also can utilize what would otherwise be waste products, thus solving two problems at once: disposing of waste disposal and finding a reliable energy source. Bioenergy is of particular interest in the forestry sector because forests are a huge potential source for the needed raw material (referred to as ‘feedstock’).

Types of Bioenergy

There are two key types of forest-based bioenergy. They are often mistakenly referred to interchangeably, when in reality they represent two very different approaches with different implications. The first type is usually referred to as biomass. This is where woody material is gathered and burned in a controlled environment, such as a boiler. The heat from burning biomass can be used to heat a building or support an industrial process, but also to produce electricity by heating water to generate steam to drive a turbine. When both heat and electricity are desired outputs, the process is called cogeneration because it is simultaneously generating two different usable forms of energy.

The other type of forest-based bioenergy is biofuel. This is where woody material is chipped up and processed in a biorefinery where it is chemically converted to a combustible liquid fuel that can be used to power internal combustion engines. The refining process is fairly complex.

With either biomass or biofuel, the energy is captured through a combustion process, thus releasing carbon dioxide into the atmosphere just like burning fossil fuels. However, there is a fundamental difference. With wood-based sources of energy, carbon from the atmosphere is sequestered in the trees, released back into the atmosphere by burning the wood, re-sequestered in new trees, re-released by burning that wood, etc. This is why wood-based bioenergy is often referred to as ‘carbon neutral,’ because it is a sustainable cycle that results in no net change in atmospheric carbon when looked at over the course of a rotation of trees.

In contrast, the carbon released into the atmosphere from burning fossil fuels came from deep in the earth. Granted, this carbon also originated in the atmosphere at some point. However, this is only carbon neutral if looked at over a time span of millions of years rather than a few decades (or even just a couple years for hybrid poplar).

Carbon Neutrality Questioned
The carbon neutrality of wood-based bioenergy has been called into question by some scientists for several reasons. One reason is that the production of wood-based bioenergy includes carbon emissions (such as from trucking the material from the forest to the energy plant, running the biofuel refining process, and so on). These production activities can cause more total carbon to be released over the course of a cycle than what is sequestered in the trees. Others say that claims of carbon neutrality claims do not account for the fact that harvesting woody material can disturb the ground, which releases carbon that is stored in the soil.

A recent report that is particularly critical of wood-based bioenergy asserts that the carbon re-sequestered from new trees is only a fraction of what is released, thus being far from carbon neutral. Since woody material produces much less energy per unit of carbon compared to fossil fuels, the report suggests that wood-based bioenergy is actually worse than fossil fuels relative to carbon emissions. In contrast, reports supporting wood-based bioenergy assert that the process is nearly carbon neutral and that by displacing fossil fuels there is a significant net benefit relative to carbon emissions.

Assumptions matter
How can different reputable scientists come to such different conclusions about wood-based bioenergy? It is all a matter of the assumptions used in the analysis. For instance, the report showing that the carbon re-sequestered in new trees is far less than what is released is looking at a very short time horizon that doesn’t allow enough time for trees to re-grow. In contrast, analyses showing equivalent sequestration and release are looking at it over the course of a compete rotation of trees. The timeframe considered makes a profound difference.

There can be several other key differences in assumptions when analyzing carbon emissions from wood-based bioenergy. For instance, there can be different assumptions about the amount of soil disturbance during harvest or the quantity of emissions from transportation and production. There can also be differences in the types of wood material that are assumed to be harvested for bioenergy. Harvesting old-growth trees vs. young plantation trees vs. residuals from lumber production has profoundly different implications.

Another key difference can be the assumptions about what would otherwise happen to the woody material if not used for bioenergy. Some analyses may assume that the carbon would otherwise stay sequestered in the wood indefinitely. Others may be looking at material like logging slash that would otherwise decompose (releasing the carbon) quickly. Still others may be looking at trees thinned out of overstocked forests that otherwise would likely have burned in uncontrolled wildfires. Again, these assumptions result in profoundly different conclusions.

Not Just Neutrality
Factors beyond carbon neutrality should also be considered. Removing woody material from the forest could negatively impact wildlife or result in nutrient losses (though most of the nutrients are in the tree needles, so if the needles are left and only the woody parts are removed there may not be much of a nutrient issue). Or removing woody material could positively impact wildlife, forest health, and fire risk if it involves thinning overstocked stands. A bioenergy market could make thinning and forest health improvements more economically feasible. Or it could provide an economic motivation for over-harvesting.

To summarize, the benefits of wood-based bioenergy all depend on what is harvested, where it is harvested from, how it is harvested, how it is transported, how it is utilized as an energy source, the time horizon considered, and the alternative fate of the feedstock material. The question society will need to address is whether, despite some potential drawbacks, an energy portfolio that includes wood-based bioenergy is a better long-term strategy than other viable alternatives. This may ultimately be a question more of social values than of scientific analysis.

By Kevin W. Zobrist
WSU Regional Extension Specialist, Forest Stewardship

This is the final essay of a four-part series on carbon and forestry. The other  articles are: