Environmental Engineering Reference
In-Depth Information
production account for approximately 20% of the U.S. corn crop and 15% of the sorghum crop. The
fermentation process of starch to ethanol is similar for all grains. Essentially, starch (a polymer of
glucose) is enzymatically converted to sugar, which is then fermented to produce ethanol. There are
two methods of ethanol production from corn, namely, wet mill and dry grind processes (Nichols
and Bothast 2008). In the dry grind process, saccharification and fermentation occur simultaneously
after the dry grinding of corn. In the wet mill process, saccharification and fermentation are car-
ried out in separate steps. Starch in the wet mill process is fairly pure, allowing for the separation
of other components such as protein, lipids, vitamins, and fiber. Although ethanol production from
starchy grains works well, it could result in competition between energy and food production and
may not be sustainable in the long run (Yuan et al. 2008; Potters et al. 2010), hence, the drive to
produce second-generation biofuels.
Plant cell walls are mostly composed of cellulose, lignin, hemicelluloses, and pectin. These
compounds are integrated together to form a strong backbone of the cell wall that maintains the
structural and physiological integrity of the cell. Because cellulose and hemicellulose are polysac-
charides, they can be broken down into simple sugars and used for the fermentation of alcohol.
However, cellulose microfibrils are embedded in a matrix where lignin is a part, which resists
degradation. Lignin is composed of different subunits, namely,
p
-hydroxyphenyl (H), guaiacyl (G),
and syringyl (S). The bonds among the polymers are less reactive; therefore, a single enzyme cannot
degrade them all. Lignification is correlated with secondary wall thickening (Weng et al. 2008).
Energy from this source is appealing in that it is portable and compatible with the current fuel
infrastructure (Rubin 2008). Pretreatment of the complex cell wall is essential to achieve enzymatic
removal of sugars from the cell wall.
The general process of cellulosic ethanol production includes pretreatment, enzymatic hydroly-
sis, fermentation, and distillation and has been described in detail in Mosier et al. (2004).
3.3 BIoethanol
The steps in the process of cellulosic ethanol production are presented in Figure 3.1. There are
various pretreatment methods, including steam explosion, liquid hot water, pH, dilute acid, con-
centrated acid, alkaline-based treatments of lime, and ammonia fiber expansion used in cellulosic
ethanol production (Yang and Wyman 2008; Zhu and Pan 2010). In the steam explosion method,
superheated steam approximately 160-260°C kept under high pressure (100-700 psi) is exposed to
lignocellulose for a short period of time followed by a flashing process to release the steam in an
explosion. This causes the lignocellulose to open and expose the cellulose, allowing for increased
digestibility. The sugars are found in the liquid stream, but because of the nature of this process, sev-
eral compounds (furfural, 5-hydroxymethylfurfural) that inhibit fermentation are formed (Abogbo
and Coward-Kelly 2008; Lu and Mosier 2008; Brethauer and Wyman 2010).
In a hot water pretreatment, the explosive decompression of the steam explosion is replaced by
controlled cooling to keep the water in the liquid phase throughout (Lu and Mosier 2008; Yang and
Wyman 2008). Advantages of this method include making complete hydrolysis of hemicelluloses
possible and making treated material highly digestible during enzymatic saccharification. The
controlled pH liquid hot water treatment is a modified version of the hot water (140-220°C, for
10-30 min) pretreatment and provides greater control of the chemical reactions that occur during
pretreatment. An advantage of this procedure is that it minimizes the formation of degradation
products (Lu and Mosier 2008; Yang and Wyman 2008).
Dilute mineral acids such as hydrochloric acid, phosphoric acid, nitric acid, and sulfuric acid
have been studied for their efficacy as a pretreatment in cellulosic ethanol production. Sulfuric acid
hydrolysis seems promising because of its low cost and effectiveness (Lu and Mosier 2008; Yang
and Wyman 2008). For effective acid hydrolysis, ideal operation conditions may be 0.5-1.4% (w/w)
sulfuric acid treatment, 100-250 g/L biomass solid loading, and a residence time between 3 and
12 min at 165-195°C. A downside of this pretreatment is that it produces compounds that can inhibit
