Finite Biomass Capacity Limits

Matt Tilghman
December 9, 2011

Submitted as coursework for PH240, Stanford University, Fall 2011

Fig. 1: Approximate land area required to supply 100% of the USA's electricity needs via various methods, [1-8]

For anyone who is optimistic about the human race, it should be clear that we will one day run out of fossil fuels. And according to the pessimistic, we must phase out the fossil fuels long before they expire. Renewable energy - be it for climate change mitigation or simply because we need new energy - will be an inevitable necessity if humans last long enough. There several viable contenders, such as sunlight, wind, and biomass. Supplying the world's energy budget using any of these would amount to a colossal undertaking. Regarding powering the world with solar energy, people are constantly calculating the total area required - a number on the order of 6.4 million acres. [1] Similar numbers for biomass are hard to find.

There are several ways to obtain the biomass needed for generating power. One way is to grow it oneself, on site. This typically requires a fast growing plant, such as algae or eucalyptus trees. Algae biomass is a very interesting and complicated topic, but certainly requires large plants with new and specific engineering and infrastructure for growing the algae. The goal of this report is to assess land limitations for low hanging "biomass fruit" - either currently existing biomass or easily grown biomass (little additional infrastructure). Thus, algae will not be discussed. Eucalyptus trees, however, can be grown with little research and infrastructure needed. Eucalyptus trees grow fast, and are coppicing, meaning they do not need to be replanted after harvesting. [2] A study showed that in California, eucalyptus could be grown and harvested at a rate of 8.7 metric tons per acre per year. [2] Eucalyptus biomass has an an average energy content of approximately 20 kilojoules per gram (kJ/g). [3] This means that an acre can provide approximately 175 gigajoules (GJ) per year, or 0.049 gigawatt hours (GWh) per year. Compare this to the roughly 4.2 million GWh (4200 terawatt hours) of electricity used in the United States in 2009. [4] This means that if such a type of biomass were to provide all the US's electricity, it would require roughly 86 million acres of eucalyptus. This area is only slightly smaller than the state of Montana. Such a colossal use of land for a single species would result in huge biodiversity issues. Furthermore, land that can support such large Eucalyptus plots is likely to be more valuable than land usable for solar electricity. And this number has not yet taken into account the conversion efficiency for biomass generators.

The other major way to obtain biomass fuels is to collect waste products, agricultural or otherwise. The largest agricultural crop in the US is corn, which amounts to approximately 430 billion pounds of corn stover (everything left over after harvesting the ear) waste per year. [5] Because of erosion and soil nutrient concerns, it is estimated that only about 140 billion pounds of this can be harvested sustainably. Energy content of corn stover can vary, but hovers around an average of 19 kJ/g, or about 8.5 megajoules per pound. [6] This means that over the course of a year, sustainably harvested corn stover could yield an energy content of approximately 1.2 billion gigajoules, or 330 terawatt hours. This is 8.7% of the annual electricity usage. We have not yet accounted for conversion efficiency, which in purely biomass power plants can be theoretically as high as approximately 40%, (but usually lower) which brings this number down to 3.5%. [7] Corn accounts for roughly 25% of crops grown, by area. [8] Using the admittedly rudimentary method of treating all crops as equal, this factor of four suggests all harvestable agricultural waste can amount to roughly 14% of the nation's electricity usage.

Clearly, biomass is not a stand-alone panacea for any encroaching energy or climate crisis. However, it can, and should, play a part in an overall combination of many small solutions. For instance, biomass may prove to be more valuable when used to create liquid fuels, instead of electricity. Solar or wind might be feasible ways to meet electricity demands, but purely electric vehicles represent significant environmental concerns. And purely electric air travel is next to impossible. Therefore, meeting CO2 emission reduction goals will certainly involve utilization of our country's biomass resources, especially for the transportation sector.

© Matt Tilghman. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.


[1] J. Turner, "A Realizable Renewable Energy Future," Science 285, 687 (1999).

[2] R. M. Sachs, D. W. Gilpin and T. Mock, "Short-Rotation Eucalyptus as a Biomass Fuel," California Agriculture, 34, No. 8, 18 (1980).

[3] K. J. M. Dickinson and J. B. Kirkpatrick, "The Flammability and Energy Content of Some Important Plant Species and Fuel Components in the Forests of Southeastern Tasmania," J. Biogeography 12, 121 (1985).

[4] "2011 Key World Energy Statistics," International Energy Agency, 2011

[5] R. L. Graham et al., "Current and Potential U.S. Corn Stover Supplies," Agron. J. 99, 1 (2007).

[6] L. O. Pordesimo et al., "Variation in Corn Stover Composition and Energy Content With Crop Maturity," Biomass and Bioenergy 28, 366 (2005).

[7] Faajj et al., "Gasification of Biomass Wastes and Residues for Electricity Production," Biomass and Bioenergy 12, 387 (1997).

[8] "Crop Production, 2010 Summary," U.S. Department of Agriculture, January 2011.