As discussion circulates with respect to the United States dependence on foreign oil to fuel its economy, biofuels are readily being examined as viable, economic alternatives to oil. Biofuels, which are produced from various types of organic materials known as biomass, have become a recent topic of interest because the raw materials are local, abundant, and renewable.
Biomass is not only of interest as a fuel source, though. It is also readily becoming a raw material option for power and bio-based products such as chemicals, building materials, and plastic production. Currently, only a small portion of products produced in the United States come from plant-derived materials. But this may change as scientific and commercial process development look to other sources of raw materials for developing new, durable, high-performance materials.
So what is biomass?
Biomass is organic plant materials, such as corn leaves or stalks, which do not directly go into foods or consumer products. These plant materials contain a complex mixture of organic materials, commonly known as carbohydrates, fats, and proteins. The amounts of these organic components vary with the type of biomass used in the process.
The most abundant components of biomass are carbohydrates and lignin. The carbohydrates, known as cellulose and hemicellulose fibers, are polysaccharides which provide strength to the plant structure. The lignin forms covalent bonds to the polysaccharides, thereby cross-linking the fibers together.
Cellulose contains the six-carbon sugar monomer glucose. Hemicellulose contains both five and six carbon sugar monomers that include glucose, xylose, mannose, galactose, and arabinose. Xylose is the most abundant polysaccharide in hemicellulose. These polysaccharides are used in a conversion process to make ethanol.
How does the process work?
Ethanol is produced from biomass in a two-step process. First, the carbohydrates are hydrolyzed into their component sugars using either acid or enzymes. Then, yeast is used to convert the sugars into ethanol in a process similar to beer fermentation.
This basic process may sound simple, but there are multiple variables at each step that must be considered. Should the processor use acid or enzymes to hydrolyze their biomass? If they decide on acid hydrolysis, at what temperature and concentration should the reaction occur? Which enzymes or yeast should be selected to yield the most ethanol? Each of these variables has positive and negative aspects that must be evaluated to determine which process is best for the biomass source.
One aspect that makes ethanol production so appealing is that the cellulosic materials used in the process can be derived from so many different sources. Biomass sources range from agricultural wastes, grasses, forestry residues, and energy crops, to municipal and animal wastes.
The main sources of forest wastes are bark, sawdust, timber slash, mill scrap, rotten or dead wood and excess saplings and small trees removed during forest thinning processes. Common agricultural wastes that are being examined are corn stover (leaves, cobs and stalks of corn plants), bagasse (sugar cane waste), wheat and rice straw, nutshells and animal manure. Energy crops, which are characterized as fast growing shrubs or trees, are switch grasses, hybrid poplar and willow trees.
Characterizing your biomass
With such a wide range of biomass sources and production process variables, understanding the chemical composition of the material becomes an important issue. Several of the industry standard tests for characterizing biomass are described below:
• Total Solids: A way to determine the moisture content within the sample.
• Ash Determination: The amount of inorganic or mineral material present in the sample.
• Exhaustive Ethanol and Water Extractables: The removal of non-structural material from the biomass sample to prevent interferences during other analyses, as well as free sugar determination.
• Structural Carbohydrates: The determination of glucose, xylose, galactose, arabinose and mannose concentrations in the sample; used to determine cellulose and hemicellulose concentrations in the biomass.
• Acetyl Content: Acetic acid concentration in the sample, may also include formic and levulinic acid content depending on the feedstock.
• Lignin: Determination of the structural plant material that does not contribute to the sugar content in the sample.
• Starch Content: Represents the readily available source of sugar within some feedstocks.
• Ethanol Content: Analysis of fermentation broths using gas chromatography.
• Bomb Calorimetry: The determination of the sample’s BTU content.
image links:Biomass Sources