Linda Benedict, Salvi, Deepti A, Aita, Giovanna
Giovanna M. Aita and Deepti Salvi
Biorefinery technology is a term coined in the 1990s to describe the fabrication of fuels, solvents, chemicals and plastics from renewable materials. By 2020, the United States is aiming to have at least 25 percent of organic-carbon-based industrial chemicals and 10 percent of liquid fuels from a bio-based industry. This challenge will require renewable resources – such as lignocellulosics, algae and municipal waste – to be produced at lower cost, in higher quantities and with properties suitable for efficient conversion.
A recent study conducted by the U.S. Department of Energy suggests that 1.3 billion tons of biomass are available in the United States each year to produce 130 billion gallons of liquid fuels, including ethanol, mixed alcohols, biodiesel, "green" gasoline and "green" diesel. The study also concluded that sufficient biomass is available to supply the raw materials now required for the production of industrial chemicals such as resins, polyesters, emulsifiers, "green" solvents and fuel additives.
Lignocellulosic biomass includes any plant material produced by photosynthesis. Potential sources of lignocellulose include agricultural residues such as corn stalks and wheat and rice straw, agricultural byproducts such as corn fiber, rice hulls and sugarcane bagasse, and energy crops such as switchgrass, sweet sorghum, high-fiber sugarcane and miscanthus grass. Lignocellulosic biomass as a source for biofuels and chemicals does not disrupt the food, fiber and feed chains and needs no major changes in energy requirements and agricultural practices. It can be sufficiently abundant to provide a major resource for making commodity-based fuels and chemicals (Figure 1).
Lignocellulose is made of three major components – cellulose, hemicellulose and lignin. Lignin is a highly complex material that gives plants strength and protection from the environment. Cellulose and hemicellulose are sugar polymers or chains of multiple sugars wrapped in a sheet of lignin. Table 1 shows the chemical composition of some lignocellulosic materials.
The ability to make fuels and other value-added products from lignocellulose relies on the separation and breakdown of lignin and sugar polymers into each of their components. In nature, lignocellulosic biomass is degraded over the years by natural forces and by microbes. In a biorefinery, however, breakdown of feedstocks must occur in a matter of hours or days. A number of technologies have been developed for improving the breakdown of lignocellulosic biomass. Available technologies include:
The LSU AgCenter’s Audubon Sugar Institute is working on producing fuels and value-added materials from renewable sources. In south Louisiana, raw sugar mills operate only three months of the year. The long-term research goal is to use the sugar mill year-round for producing not only sucrose, molasses and bagasse but also alternative products, without interfering with food-grade sugar production. Ethanol production from sugarcane bagasse, sorghum and energy cane has been an area of extensive research at Audubon. Audubon’s technology uses dilute ammonia at high temperature and pressure to separate and break down the main components of lignocellulose. Ammonia-based pretreatments with or without heat and/or pressure break the links between lignin and sugar polymers, resulting in pore formation and swelling of biomass and enhancing further chemical and/or biological degradation (Figure 2).
Once the biomass has been treated with ammonia, the lignin, cellulose and hemicellulose product streams are generated. Cellulose and hemicellulose are further broken down by the addition of enzymes to glucose and xylose. Glucose is fermented to ethanol by the addition of a nonpathogenic and nongenetically modified yeast (Figure 3). Xylose is being studied for the production of compounds other than ethanol. Audubon’s technology can yield 55 gallons of ethanol per dry ton of biomass from cellulose only. The target set by the U.S. government is 100-110 gallons per dry ton of biomass from both cellulose and hemicellulose.
Synthesis of nontoxic and biodegrad able polyesters from sugarcane is another promising area of research being conducted at Audubon. Investigation is under way to isolate precursors from molasses and lignin found in pre-treatment wastes associated with the production of ethanol from ammonia-treated grasses – bagasse, sorghum and energy cane. The goal is to use these synthesized polymers as the basis for direct regeneration of tissue.
Deconstruction of lignocellulose followed by chemical or fermentation processes is expected to be a near-term and the most practical pathway to fuels and chemicals. Nevertheless, its success is highly dependent on overcoming biomass recalcitrance, detoxifying waste streams and developing new processes that can be applied commercially at low costs.
Giovanna Aita, Assistant Professor, and Deepti Salvi, Postdoctoral Researcher, Audubon Sugar Institute, LSU AgCenter, St. Gabriel, La.
(This article was published in the fall 2009 issue of Louisiana Agriculture.)