Biochemical Conversion
Involves the use of enzymes, bacteria or other microorganisms to break down biomass into liquid fuels.The use of micro-organisms for the production of ethanol had been around for a long time. However, such organisms have only become regarded as biochemical "factories" for the treatment and conversion of biological materials recently. Fermentation technologies, with the assistance of biological engineering, are leading to breakthrough processes for creating fuels and fertilizer, and other products useful in agriculture. Anaerobic digestion and fermentation are important biochemical conversion technologies.
Anaerobic Digestion
Anaerobic digestion is the use of microorganisms in oxygen-free environments to break down organic material. It is widely used for the production of methane- and carbon-rich biogas from crop residues, food scraps, and manure (human and animal). It is also frequently used in the treatment of wastewater and to reduce emissions from landfills.
‘Anaerobic digestion involves a multi-stage process. First, bacteria are used in hydrolysis to break down carbohydrates, for example, into forms digestible by other bacteria. The second set of bacteria convert the resulting sugars and amino acids into carbon dioxide, hydrogen, ammonia and organic acids. Finally, still other bacterias convert these products into methane and carbon dioxide. Mixed bacterial cultures are characterized by optimal temperature ranges for growth. These mixed cultures allow digesters to be operated over a wide temperature range, for example, above 0° C and up to 60° C. When functioning well, the bacteria convert about 90% of the biomass feedstock into biogas (containing about 55% methane), which is a readily useable energy source.’ (Citation T-28)
After the digestion process, solid remnants of the original biomass input are left over, and this by-product, or digestate, has many potential uses. It could be used as animal bedding, low-grade building products like fiberboard, and fertilizer (should be first chemically assessed for growth-inhibiting factors and toxicity).
Citation I-99
Fermentation is the use of yeasts to convert carbohydrates into alcohol – most notably ethanol, also called bioethanol. The total process involves several stages. In the first stage crop materials are pulverized or ground and combined with water to form a slurry. Then, heat and enzymes are applied to break down the ground materials into a finer slurry. In addition, other enzymes are added to convert starches into glucose sugar. The sugary slurry is then pumped into a fermentation chamber to which yeasts are added. After about 48-50 hours, the fermented liquid is distilled to divide the alcohol that the solid materials left over.
Corn grain is the primary feedstock in ethanol production in the U.S.; about 2.8 gallons of ethanol is produced from one bushel of corn. A by-product of the corn-to-ethanol process is spent grains. These spent grains are dried and can be used as feed for livestock – termed Distillers Dried Grains, or DDGs.
‘Cellulosic ethanol production by fermentation is more complex than conversion of starch or sugar components of plants. Cellulosic ethanol production involves use of wood, grasses? and the stems, leaves and stalks of non-grass plants. Lignocellulose, the structural material of plants, must first be broken down into sugars before being fermented into ethanol. Molecules of cellulose, hemicellulose and lignin – the components of lignocellulose, have strong chemical bonds and are difficult to separate. Mechanical pre-treatment and enzymatic are necessary to breakdown lignocellulose. As a result, at present, conversion of lignocellulosic materials into ethanol is less cost-effective than conversion of starch and sugar crops to ethanol.’
Fermentation
Biofuel/Biomass: Making Ethanol from Crops
Citation V-5