Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 211

Biomass can be converted to thermal energy, liquid, solid, or gaseous fuels, and other chemical products through a variety of conversion processes. These processes include physical conversion to densified fuels (e.g., pellets or cubes), thermal conversion through combustion or pyrolysis, chemical conversion, and microbial conversion or fermentation. The abundance of plant organic constituents and other physical and chemical characteristics vary significantly by plant type and the proportions of plant components such as leaves, stems, bark, and twigs in the feedstock.

The design of a biomass power or ethanol facility requires careful consideration of the effects that feedstock characteristics and composition have on the conversion process. Generating electricity and useful heat energy is most frequently done by direct combustion of biomass in a boiler. Energy content, moisture content, and chemical makeup are among the most important biomass characteristics affecting combustion processes. Biomass gasification yields a combustible gas that can be used to generate electricity. Particle size, energy content, moisture content, and volatiles are among the biomass characteristics affecting gasification.

The technology that is closest to commercialization for converting biomass to ethanol involves extracting complex carbohydrates, primarily cellulose and hemicellulose, from the biomass feedstock and reducing these components to simple sugars (a process called hydrolysis).

Cellulose is a long polymer of glucose, and it serves as a structural component of plant cell walls. It is often found in a composite mixture with hemicellulose and lignin. Hemicellulose is a polymer containing a variety of simple sugars and is more soluble than cellulose. Lignin is a ring-type carbohydrate that acts as cement in plant walls. The lignin portion of biomass is not converted to simple sugars through hydrolysis, but lignin can be burned to generate process heat and electricity.

Extracting the carbohydrates may involve “steam explosion” of the cell walls or dissolving the organic constituents with acids, enzymes, or organic solvents. Sugars resulting from hydrolysis are then converted into ethanol through microbial fermentation. The bulk and biochemical composition of the feedstock largely determine ethanol yield because these traits affect the hydrolysis and fermentation processes.