While possible variations in the process of converting lignocellulosic biomass to ethanol are virtually endless, it can most simply be described as the in­tegration of five unit operations: (1) desizing, (2) thermochemical pretreat­ment, (3) enzymatic hydrolysis, (4) fermentation, and (5) ethanol recovery (Fig. 1). In the first step of the process, the delivered biomass must be made uniform in size to facilitate handling and transport via conveyor or screw drive and to provide a more consistent surface-to-mass ratio for thermo­chemical pretreatment. The pretreatment step is typically a short — (minutes) to long-term (hours) exposure to extremes of temperature (150-200 C), pH (<2 or >10) and pressure (2-5 atm) and may additionally involve a rapid pressure release that facilitates chemical infiltration and fiber explosion. Ide­ally, pretreatment produces a disrupted, hydrated substrate that is accessible to enzymatic attack, but avoids both the production of sugar degradation products and fermentation inhibitors. As discussed below, some pretreat­ments solubilize hemicellulose to oligomeric and/or monomeric sugars com­prised largely of pentoses that can be fermented independently or together with the glucose released from the cellulose fraction. In the next step, the pH is adjusted and enzymes are added to initiate cellulose hydrolysis to fer­mentable sugars. With pretreatments that do not solubilize the hemicellulose fraction, additional enzymes may be required to hydrolyze the hemicellulose

polymer. Hydrolysis typically is performed at pH 5 and 50 °C for 24-120 h, followed by addition of a fermentation organism to begin production of ethanol. In many cases (as described below) fermentation is initiated long be­fore hydrolysis has completed, since both the extent and speed of ethanol pro­duction can often be increased by combining the hydrolysis and fermentation steps. In the final step, ethanol is recovered via distillation, and remaining organic waste is burned for production of heat and/or power.

2.1