1.2 Process Overview

The process described here uses co-current dilute-acid pretreatment of lignocellulosic biomass (corn stover), followed by enzymatic hydrolysis (saccharification) of the remaining cellulose, followed by fermentation of the resulting glucose and xylose to ethanol. The process design also includes feedstock handling and storage, product purification, wastewater treatment, lignin combustion, product storage, and required utilities. The process is divided into nine areas :

• Area 100: Feed handling. The feedstock, in this case milled corn stover, is delivered to the feed handling area from a uniform-format feedstock supply system. Only minimum storage and feed handling are required. From there, the biomass is conveyed to the pretreatment reactor (Area 200).

• Area 200: Pretreatment and conditioning. In this area, the biomass is treated with dilute sulfuric acid catalyst at a high temperature for a short time to liberate the hemicelluloses sugars and break down the biomass for enzymatic hydrolysis. Ammonia is then added to the whole pretreated slurry to raise its pH from ~1 to ~5 for enzymatic hydrolysis.

• Area 300: Enzymatic hydrolysis and fermentation. Enzymatic hydrolysis is initiated in a high-solids continuous reactor using a cellulase enzyme prepared on-site. The partially hydrolyzed slurry is next batched to one of several parallel bioreactors. Hydrolysis is completed in the batch reactor, and then the slurry is cooled and inoculated with the cofermenting microorganism Zymomonas mobilis. After a total of five days of sequential enzymatic hydrolysis and fermentation, most of the cellulose and xylose have been converted to ethanol. The resulting beer is sent to the product recovery train (Area 500).

• Area 400: Cellulase enzyme production. An on-site enzyme production section was included in this design. Purchased glucose (corn syrup) is the primary carbon source for enzyme production. Media preparation involves a step in which a portion of the glucose is converted to sophorose to induce cellulase production. The enzyme-producing fungus (modeled after Trichoderma reesei) is grown aerobically in fed-batch bioreactors. The entire fermentation broth, containing the secreted enzyme, is fed to Area 300 to carry out enzymatic hydrolysis.

• Area 500: Product recovery. The beer is separated into ethanol, water, and residual solids by distillation and solid-liquid separation. Ethanol is distilled to a nearly azeotropic mixture with water and then purified to 99.5% using vapor-phase molecular sieve adsorption. Solids recovered from the distillation bottoms are sent to the combustor (Area 800) while the liquid is sent to wastewater treatment (Area 600).

• Area 600: Wastewater treatment. Plant wastewater streams are treated by anaerobic and aerobic digestion. The methane-rich biogas from anaerobic digestion is sent to the combustor (Area 800), where sludge from the digesters is also burned. The treated water is suitable for recycling and is returned to the process.

• Area 700: Storage. This area provides bulk storage for chemicals used and produced in the process, including corn steep liquor (CSL), ammonia, sulfuric acid, nutrients, water, and ethanol.

• Area 800: Combustor, boiler, and turbogenerator. The solids from distillation and wastewater treatment and the biogas from anaerobic digestion are combusted to produce high-pressure steam for electricity production and process heat. The majority of the process steam demand is in the pretreatment reactor and distillation columns. The boiler produces excess steam that is converted to electricity for use in the plant and for sale to the grid.

• Area 900: Utilities. This area includes a cooling water system, chilled water system, process water manifold, and power systems.