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Rebuttal: What Determines CO2 Neutrality in Bioenergy Combustion?

The recently released research in Geophysical Research Letters suggests that if 10 percent of the world's energy needs were fulfilled by biofuels from dedicated crops then the consequence would be that global greenhouse gas levels would increase. The foundation of their model is that deforestation, in order to make room for energy crops, eliminates CO2 absorbing forests. They also note that the increased release of nitrous oxide from fertilization will have GHG effects.

Find a summary of the study here

The research is important and focuses our attention to what should be the absolute constraint on any strategy for using biomass for energy:  The net stock of carbon in the atmosphere and oceans cannot be increased by the combustion of biogenic materials. 

In other words, that carbon cycle has to be complete and should be more or less contemporaneous.  The stock of carbon released from combustion should be absorbed contemporaneously by new growth sufficient to fully mop up that carbon.

This carbon cycle has to be viewed in the context of our current systems of mining (oil, gas, and coal extraction), transporting, refining, and delivering fossil fuels; all of which requires fossil fuels.  Any deliverable fuel will require this carbon overhead.  Until we work out how to deliver products without the use of fossil fuels, it is the combustion of the fuel and its carbon impacts that matter most. 

It is also important to pay attention to the energy usage pathways when considering renewable strategies.  There are three primary uses for energy: electricity, transportation, and heating (thermal).  Across the U.S., each of those pathways uses about one third of the total energy used.

We may be able to generate some proportion of our electricity needs from wind and solar and eliminate a lot of that carbon emission overhead.  Natural gas may lower the net increase of carbon per unit of energy generated.  But those technologies are specific for electricity generation. 

Perhaps transportation, which currently relies on liquid fuels, can be converted to batteries and CNG (both from natural gas and biogas) in the medium term and that may lower GHG emissions per mile traveled.  However, the MIT research referenced at the beginning of this article suggests that the production of liquid biofuels, at least from newly cleared cropland, has its limits.

Thermal energy is different.  Thermal energy is used in many manufacturing processes.  But what is often forgotten is that in those locations that experience winter, thermal energy is needed to keep buildings warm.  The vast majority of thermal home and business demand in the colder regions is satisfied with a combustion system that uses natural gas, heating oil, or propane.

In some locations, low temperature thermal energy is utilized with pumps to capture heat below the earth’s surface.  But tens of millions of homes and businesses burn fuel and will continue to burn fuel for decades to come.

Across the Northeastern and Midwestern states, there are more than 20,000,000 homes that are not connected to natural gas, and instead use heating oil or propane for fuel. Most of those homes are in rural locations that will never have natural gas infrastructure. 

Many EU countries use fuels derived from sustainably managed forests and have done so for years.  For example, in Upper Austria nearly three quarters of all new heating systems in homes and small businesses are fueled with wood pellets made in Austria. But for this strategy to have any environmental benefit validity, the fuel supply has to be sustainable and carbon neutral – if our forests are properly managed and truly renewed (as should any renewable energy!) then we can satisfy both criteria. 

Think of how our fisheries are managed.  Somehow, in a given year X tons of fish can be harvested in order to not deplete the stock of the fishery.  Now imagine this same exercise with a 100,000-acre forest.  Working out the sustainable harvest for a forest is much easier than a fishery, as the forest is easily bounded, observed, and measured. 

Suppose that the scientists and foresters determine that this 100,000-acre forest grows at the rate of one ton per acre per year.  That means that every year the forest grows 100,000 tons of new wood.  That also means that the maximum harvest would be 100,000 tons of wood.  To be conservative and perhaps account for other sustainability criteria, the harvest limit is set at 80,000 tons per year.  That means that every day, the forest creates about 275 tons of new wood.  Given our limit, about 220 tons are removed.  If that 220 tons is used as fuel, the carbon released is more than netted out by the capture of carbon in the new 275 tons that is created that same day.  The cycle is complete and contemporaneous.

There are more benefits than just environmental. The conversion of homes and businesses from fossil fuel to regionally produced refined wood fuels produce three significant positive economic pathways. 

First, the supply chain for wood derived fuels creates and supports jobs and commerce in the forest products sector.  Given the changes in demand for printed media that are ongoing, this is a very important benefit to the economic well being of areas that depend on forest industries.

Second, pellet fuel is about half the cost of heating oil.  This lowers the annual bill for heating, which leads to an increase in disposable income that has positive economic effects.

And finally, every dollar spent on a gallon of fossil fuel purchased for heat sends most of that dollar out of the region – and a substantial portion out of the country.  Buying fuel made regionally stops the export of money and jobs keeping both benefits domestic.

There are obviously limits to what the forests can sustainably supply.  But if we do not cross that boundary we can still convert millions of homes and businesses off of reliance on fossil fuels for heating. 

The GHG benefits are significant.  Every million BTU made from heating oil adds a net of 160 pounds of CO2 from combustion.  Wood derived fuels, depending on the ratio of growth to harvest, are carbon neutral or even may be carbon negative in combustion. 

The sector is not waiting for a breakthrough in technology to make it affordable and reliable. The fuel is already much cheaper than the alternatives.  There are more than a million fully automatic pellet fueled central heating systems (not pellet stoves) in Europe that are heating homes and businesses cleanly and reliably.  The economic benefits suggest that our policy makers should pay attention to biomass thermal and make sure the sector gets the attention and respect it deserves. 

Lead image: Sun rays in forest via Shutterstock

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Volume 18, Issue 4


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