Plant matter can be converted into liquid fuel that can serve as a replacement for oil-based fuel. But how does it work and how much production is feasible?
Plants convert carbon dioxide and water into sugar via photosynthesis using energy from the sun. The sugar undergoes further transformations within the plant. Carbohydrates, compounds composed entirely of carbon, hydrogen and oxygen, are produced. They are large molecular chains made up of sugar links. Carbohydrates produced include starch, cellulose and hemicellulose. Lignin, a high molecular weight polymer is also produced.
To make biofuels, the idea is to rearrange the components of the biomass to yield a new compound that can serve as a fuel. Enzymes, proteins that catalyze biochemical reactions, are capable of facilitating the desired reactions in carbohydrates. Common and inexpensive enzymes exist for converting starch into ethanol. Practical enzymatic conversion of cellulose and hemicellulose is not yet available, but it is expected that genetic engineering will yield suitable new enzymes. Chemical and thermochemical processes for converting biomass (including lignin) into liquid biofuel also exist.
But ethanol is not the only fuel that can be made from biomass. For example, butanol, a fuel with more desirable properties than ethanol can be produced enzymatically and hydrocarbons can be produced thermochemically. Processes such as these are well known, but are not currently competitive with enzymatic ethanol production from carbohydrates. Also, some plants produce triglycerides that can be used as fuel, either directly or after chemical processing by transesterification.
So what limits biofuel production? First and foremost is the land required. Fortunately, estimating the energy in biomass is quite simple. Terrestrial plants have energy content (BTU per dry ton) that falls in a narrow range. Plant yields (dry tons per acre) also fall in a narrow range. So it is simple to calculate the range of possible energy contained in the biomass per acre of land involved. Then only a fraction of the plant energy remains in the biofuel produced from the biomass.
The usual metric for considerations of large energy production is the Quad, defined a one quadrillion or 1015 BTU. The land required for producing a Quad of energy in biofuel ranges from 10 to 45 million acres depending on assumptions about plant energy content, plant yields and biomass-to-biofuel conversion efficiency. The United States uses around 35 Quads of oil-based energy per year (27 Quads for transportation). Producing enough biofuel to replace current consumption of oil would require hundreds of million acres. The total area of the lower forty-eight states is 1900 million acres and cropland is 441 million acres. Either production of biomass for biofuel on dedicated land or collecting waste biomass for conversion requires more land than is feasible for replacing current oil use totally.
So now we know that biofuels cannot replace current oil use totally. In fact, they cannot even come close.
There are other issues related to biofuel use. These include competition with food production, building a new production and delivery infrastructure, effects on soil fertility, adaptation to using biofuel instead of oil-based fuel in engines and other equipment and cost.
Biofuel use can make a modest contribution to liquid fuel needs, it can help reduce imports, it can create domestic jobs, but it cannot begin to supply total future liquid fuel needs.
Learn more about the opportunities and options for solar, wind, hydropower, biofuels and other alternative energy sources. Download a free 41-page excerpt from Future Energy: Opportunities and Challenges, click this link.
About the Author
Tom Kerlin retired as head of the Nuclear Engineering Department at the University of Tennessee in 1998, after serving on the faculty for 33 years. His professional interests include instrumentation, nuclear reactor simulation, and dynamic testing for model validation. He has published extensively on these topics. In addition to his university service, Dr. Kerlin founded a spin-off company, Analysis and Measurement Services Corp., to provide the nuclear industry with the testing capability that he invented for safety system sensors. Dr. Kerlin’s method has been used hundreds of times in nuclear power plants in the U.S. and around the world. Upon retiring, Dr. Kerlin studied the literature on energy production and use and concluded that there was a need for a comprehensive book on our future options that even non-specialists would understand. His book is the result.