The production of biofuels Essay

 Several types of materials are found to be suitable for the production of biofuels. It must be stressed that it is not always possible to transfer the results of pretreatment from one type of material to another. Furthermore, one technology that is effective for a particular type of biomass material might not work for another material. All of the pretreatment methods discussed can lead to a high yield of glucose from cellulose as long as suitable feedstock and sufficient enzyme activities are used in hydrolysis. It is not the enzymatic accessibility that actually matters in the overall cost of biomass processing. However, the other factors such as enzyme dosing, total recovery of sugars (especially hemicellulose sugars), equipment and energy cost, and so forth, can vary dramatically among the different types of pretreatment technologies and will result in different overall process economics. Also, it is obvious that the solid substrates obtained from different pretreatment methods vary greatly in composition and properties, which shows that the optimal enzyme recipes could be very different for each of the substrates. An indepth understanding of the substrates and how they affect the enzyme functions is very important.

Various pretreatment processes for lignocellulosic biomass, and their advantages and disadvantages, are summarized in Table 5.1. The choice of the pretreatment technology used for a particular biomass is dependent on the composition of biomass and the byproducts produced as a result of pretreatment (Bensah and Mensah, 2009). These factors substantially affect the costs associated with a pretreatment method.

Several researchers have made a comparison of various pretreatment methods for lignocellulosic feedstocks (Wyman et al., 2005a,b Rosgaard et al., 2007Silverstein, et al., 2007 ). Rosgaard et al. (2007) evaluated the effectiveness of different types of pretreatment procedures, i.e., acid or water impregnation followed by steam explosion versus extraction with hot water, on wheat straw and barley. The pretreatments were compared after enzyme treatment using cellulase enzyme. The acid or water impregnation followed by steam explosion of barley straw was found to be the best pretreatment in terms of the resulting glucose concentration in the liquid hydrolysate after enzymatic hydrolysis.

Silverstein et al. (2007) examined the effectiveness of sulfuric acid, sodium hydroxide, hydrogen peroxide, and ozone pretreatments for the conversion of cotton stalks to ethanol. Solids from sulfuric acid, sodium hydroxide and hydrogen peroxide pretreatments showed significant lignin degradation and/or high sugar availability and therefore were hydrolyzed rapidly by cellulsae enzymes  Celluclast 1.5 L and Novozym 188 from Novozymes. Pretreatment with sulfuric acid resulted in the highest xylan reduction (95.2% for 2% acid, 121 C/15 psi, 90 minutes) but the lowest cellulosetoglucose conversion during hydrolysis (23.9%). Sodium hydroxide pretreatment resulted in the highest level of delignification (65.6% for 2% NaOH, 121 C/15 psi, 90 minutes) and cellulose conversion (60.8%). Pretreatment with hydrogen peroxide resulted in significantly reduced delignification (maximum of 29.5% for 2%, 121 C/15 psi, 30 minutes) and cellulose conversion (49.8%) in comparison to sodium hydroxide pretreatment. Ozone did not cause any significant changes in xylan