Biofuels major forms

It is useful to consider two major forms of biomass fuel mechanically processed and chemically processed. For mechanically processed biofuel in which the material is used essentially in its natural form (as harvested) direct combustion usually supplies heat for cooking, space heating or electricity production, although there are also small- and large-scale industrial applications for steam raising and other processes requiring low-to-medium temperature process heat. 

The polysaccharides in the raw materials need to be hydrolyzed before the sugar monomers canbe fermented to ethanol. Today, enzymatic hydrolysis is regarded as a method with great potential. One major obstacle to overcome is the high cost of cellulolytic enzymes. In 2001, the United States Department of Energy formed a contract with two commercial producers of cellulolytic enzymes in an attempt to achieve a 10-fold decrease in the cost of the cellulolytic enzymes (www.ott.doe.gov/ biofuels/research partnerships.html). 

This entry is organized into three major parts. The first identifies the biomass resources in the form of conventional forestry, agricultural crops and residue, and oil-bearing plants, among others. The second describes the conversion processes of bioresources into biofuels, and it is followed by the end product usage of biofuels in producing electricity in power plants. 

Abstract Cellulose is the most important biopolymer in Nature and is used in preparation of new compounds. Molecular structure of cellulose is a repeating unit of p-D-glucopyranose molecules forming a linear chain that can have a crystallographic or an amorphous form. Cellulose is insoluble in water, but can dissolve in ionic liquids. Hemicelluloses are the second most abundant polysaccharides in Nature, in which xylan is one of the major constituents of this polymer. There are several sources of cellulose and hemicelluloses, but the most important source is wood. Typical chemical modifications are esterifications and etherifications of hydroxyl groups. TEMPO-mediated oxidation is a good method to promote oxidation of primary hydroxyl groups to aldehyde and carboxylic acids, selectively. Modified cellulose can be used in the pharmaceutical industry as a metal adsorbent. It is used in the preparation of cellulosic fibers and biocomposites such as nanofibrils and as biofuels. 

Producing ethanol as a biofuel from the sugars in corn (glucose) has become a major industry in the grain belt. This process can be summarized as follows, where glucose is fermented to form ethanol and carbon dioxide. Use bond energies to estimate ATT for this reaction. 

The vast majority of enzyme biofuel cells is based on the electroenzymatic oxidation of glucose by glucose oxidase (GOX) and oxygen reduction by laccase, rarely, bilirubin oxidase, or even ascorbate oxidase. Usually two couples of redox mediators are involved in the functioning of the enzymatic biofuel cell. One is required to establish an electrical connection between the electrode surface and the reduced form of flavin adenine dinucleotide, the prosthetic center of GOX. The second couple, located at the cathode, allows the electron transfer from the electrode siuface to the copper center of laccase where the oxygen reduction takes place (Fig. 3.2). 

---