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More than 90% of the fuel ethanol produced in the United States is made from corn. Most analysts agree that the ability to significantly increase ethanol production using corn grain is limited, and that large increases will require the use lignocellulose resources such as agricultural residues, grasses, and wood. Ethanol is produced by fermenting plant carbohydrates with yeast. Plant carbohydrates are grouped as soluble sugars such as sucrose from sugarcane, storage carbohydrates such as starch from grains and tubers, and structural carbohydrates such as cellulose, hemicellulose, and pectin, which make up the plant cell wall. The principal carbohydrates contained in lignocellulose resources are the structural carbohydrates. These carbohydrates, along with proteins and lignin, form the complex matrix of plant cell walls that give plants structural stability and protection from the environment. In addition to cellulose, plant cell walls contain significant quantities of hemicellulose. Hemicellulose is composed principally of pentose sugars. Xylose is the principal pentose found in grasses and hardwood tree species, while mannose is the major pentose in softwood tree species. Producing fuel ethanol from lignocellulose resources involves four major steps—preparation of the feedstock, fermentation of the sugars, recovery of the ethanol, and handling of the coproducts.

Feedstock Preparation
The inherent stability and chemical complexity of cellulose increase the difficulty of breaking it down into glucose.  A number of pretreatment approaches are being explored to overcome this problem. The goal of any pretreatment technology is to alter or remove structural and compositional factors present in plant biomass that hinder the breakdown of the polysaccharides in the cell walls into fermentable simple sugars. Dilute sulfuric acid is commonly used as a pretreatment method, but it increases cost due to the need for reactors constructed of expensive steel materials, and results in the formation of unwanted salts that require neutralizing agents. Liquid hot water pretreatment with pH control effectively dissolves hemicellulose and lignin without the need for costly and potentially dangerous pretreatments and neutralizing agents, while minimizing degradation of the simple sugars.  Other pretreatment methods include steam explosion, ammonia fiber expansion, and other chemical solvents. Few of these pretreatment processes have been fully commercialized or tested at industrial scales.  Scaling pretreatment processes to commercial sizes, and the associated reactor design issues, at present remain a major barrier to the commercial production of fuel ethanol from lignocellulosic biomass. Following pretreatment, plant cell wall polysaccharides are more susceptible to the chemical or enzymatic hydrolysis that breaks them down into single sugars that can be fermented into ethanol.  Depending on the type and effectiveness of the pretreatment method, hydrolysis takes 24-48 hours to complete. In an effort to reduce the overall time needed to produce ethanol, the hydrolysis and fermentation processes are being combined in a process called simultaneous saccharification and fermentation (SSF). In SSF, the fermenting microorganism, such as yeast, and the enzymes that hydrolyze the polysaccharides, are both added at the same time so that the sugars are fermented as soon as they are available in a just-in-time process. SSF reduces ethanol production time and also reduces the amount of enzyme used because it produces fewer compounds that inhibit ethanol production.

Fermentation of sugars
Fermentation involves microorganisms which consume sugars as a food source. The fermentation process for producing fuel ethanol from cellulose is similar to that for corn grain. The major difference between using cellulosic feedstocks and starch feedstocks lies in the existence of relatively large amounts of pentose sugars (five carbon sugars such as xylose and arabinose) contained in the hemicellulose. These sugars also need to be fermented to make the overall process economically feasible. Existing strains of S. cerevisiae cannot directly ferment xylose, but can ferment xylulose through the pentose phosphate pathway. Other yeasts and bacteria can ferment xylose or xylitol, and efforts utilizing the tools of biotechnology are underway to develop industrial microorganisms capable of efficiently converting xylose to ethanol. Ethanol fermentation results in four major products: additional yeast cells from cell division, ethanol, carbon dioxide, and heat.

Ethanol recovery
The recovery of ethanol produced in cellulosic processes is expected to use technology similar to that used in corn ethanol or sugar cane ethanol facilities. These technologies include repeated distillation and condensation of the ethanol-water mixtures produced during fermentation, until the point is reached where the difference between the boiling points of water and ethanol ceases to exist. This occurs when the mixture is 95.6% ethanol and 4.4% water by weight. At this point, the ethanol and water both vaporize to the same extent and cannot be further fractionated by distillation. Final purification is then performed by the use of molecular sieves.

Co-product production and handling
Unlike processes that produce ethanol from corn, the residual solids produced in lignocellulose processes have little value as an animal feed, being high in lignin and low in fiber and protein. However the material can be used to produce the heat, steam, and electricity needed to run the ethanol production facility, with the excess electricity sold to the electrical grid.

Related Topics Pretreatment