Donal F. Day
Biofuels generally are defined as fuels produced from recently derived organic matter versus fossil fuels, which are derived from ancient organic matter. In either case, solar energy is the original energy source. Concerns about increasing levels of atmospheric carbon dioxide released from burning fossilized carbon, mixed with the desire to secure national energy supplies, have driven research on alternative fuels.
References to biofuels mean plant-derived oils, alcohols or burnable fibers. The desired form of fuel, either solid or liquid, and the technical difficulty in converting a specific feedstock into a fuel control the production. The value of a biofuel is fixed essentially by the wholesale cost of the fossil fuel it replaces, whether diesel or gasoline, coal or natural gas.
Successful biofuels have to meet the same performance characteristics as fossil fuels. Will they deliver the same performance as the fuel they replace for the same cost? And they must be usable in the existing infrastructure. A problem facing biofuel production is that the energy density of the feedstock – the available energy per unit of mass – is small compared with fossil fuels. This means that large volumes of feedstock must be used to obtain the same amount of fuel energy as from a smaller volume of crude oil or coal.
Biofuel usage varies across the world. Byproducts concentrated at industrial processing sites, such as bark and sawdust in saw mills, are the largest commercially used biomass sources. For example, bagasse, the fiber remaining after juice extraction in sugarcane processing, provides the energy for processing cane sugar at the sugar mill, and surplus bagasse can be used to supply electricity for local grids. In developing countries, biomass, particularly fuel wood and charcoal, is mainly used in open-combustion devices for cooking and, to a lesser extent, space heating.
Available liquid biofuels are usually biodiesel and ethanol, and solid biofuels are wood and biomass pellets. Biodiesel production requires an oil crop, which traditionally is soy or canola. The oil is cracked to free up the fatty acids, which are then transesterified with an alcohol. These fatty alcohols are burned as biodiesel.
Because the demand for this fuel is high and the traditionally available feedstock supplies are limited, interest is growing to develop technologies for producing bio-oil from algae. Selected strains of algae can contain 50 percent oil by weight, compared with wild algae, which contain around 10 percent oil. Large areas of land are required for open-water algal farms. Algae also can be grown in bioreactors, but getting sunlight into a closed tank is a problem. A readily available carbon dioxide source also is required, and the algae must be separated from large volumes of water prior to oil extraction.
The United States is the world’s largest ethanol producer at 9 billion gallons per year, primarily from corn. This is about six percent of the United States’ gasoline consumption. The ethanol production process uses a traditional technology, where starch is converted to sugars through use of enzymes and is then fermented into ethanol by yeast. The ethanol is captured by distillation.
Bioethanol production from corn underwent a rapid expansion during the past few years based on a rapid run-up in oil prices and favorable government support. A fall in oil prices halted this expansion and in some cases led to closing production facilities. Increased production from these corn-fed plants will require higher fuel prices and expanded sources of feedstock – for example, higher-yielding corn varieties – to be profitable again.
The large volume of gasoline used in this country requires an enormous feedstock source to produce enough biofuel to affect the nation’s fuel supply. The largest available source is lignocellulose – biomass – which is estimated at 2.7 billion recoverable tons per year. This resource exists in diverse forms from grass to wood.
Considerable research has gone into defining methods of converting lignocellulose to useable liquid fuels. Alcohol from lignocellulosics can be produced by two basic processes –biochemical or thermochemical.
The biochemical approach requires technology for making the component sugars of biomass available for hydrolysis, converting the polymeric sugars into simple sugars and then fermenting them into ethanol. Biochemical processing depends on effective, low-cost methods for pretreating or reducing the complexity of the sugar polymers from the biomass and then technology for hydrolyzing these polymers to fermentable sugars. This approach potentially makes available a range of products derived from the lignin and minor sugar components of the biomass. It produces less alcohol per ton than thermochemical production and generates larger amounts of waste, which must be remediated.
The thermochemical approach involves burning the biomass in an oxygen-poor environment to produce a mixture of carbon monoxide and hydrogen. This synthesis gas can then be reformulated into alcohols. The thermochemical route is relatively feedstock-independent – it doesn’t matter what is burned – but the feed must be homogeneous and clean and have low moisture content. Agricultural material does not normally meet these criteria, so much effort must be put into feedstock preparation. This process has the advantage of producing about 1.6 times the amount of alcohol per ton of feedstock as that produced using the biochemical method. There is, however, less chance for producing higher-value products to enhance the profitability of the process. The biochemical process has more flexibility in terms of potential products but is also technically more challenging.
Cellulosic ethanol is considered to be a first-generation biofuel. Second-generation fuels are those which are in development but are not as close to commercialization. They also are produced either thermochemically or biochemically. Agriculture underlies the country’s push to biofuels (bio-ethanol, biobutanol, bio-oil), yet agricultural products are normally available only in a fixed window in time. Conversely, feedstock-independent flexibility in operation is a key to financial viability of any biorefinery.
The LSU AgCenter has a comprehensive research program designed to overcome technical hurdles blocking biofuel production in Louisiana. Projects range from finding multiple feedstocks suitable for year-round delivery and developing the tools for producers and processors to valuing these crops, developing the processing technologies for biofuels and finding supplemental, high-value byproducts from the process streams to improve the profitability of biorefineries.
The Audubon Sugar Institute conducts laboratory and pilot-scale research on potential alternative fuels ranging from producing ethanol from sugars and biomass at raw sugar mills and using starch-based products to produce butanol to systems for on-demand hydrogen production from water. The technology closest to commercialization after biodiesel is the production of ethanol from sweet sorghum, sugarcane bagasse and energy cane using resources at existing sugar mills.
Alkaline pre-treatment/hydrolysis options for lignocellulosic feedstocks have been successfully used at the Audubon Sugar Institute to reliably convert and ferment the cellulose in bagasse at a 30 percent solids loading with an 85-90 percent conversion rate to ethanol. This process has been extended from bagasse to sweet sorghum and energy cane.
Ethanol as a fuel has a distribution problem because its corrosiveness prevents it from being shipped in pipelines. Butanol, which also can be produced by fermentation of sugars, has higher fuel value and is noncorrosive. Audubon is looking to improve traditional butanol fermentation technology by increasing yields and productivity using traditional, sugar-containing feedstocks.
Biofuel production will not solve all our fuel problems, but in Louisiana, it is possible to produce sufficient biofuels to replace much of the gasoline used in this state. First-generation production facilities are being designed and built. Biofuels are in our future as a supplement to, but not a replacement for, fossil fuels.
Donal F. Day, Professor, Audubon Sugar Institute, LSU AgCenter, St. Gabriel, La.
(This article was published in the fall 2009 issue of Louisiana Agriculture.)