Бурение грунтовых зондов, установка энергетических колодцев

3.1 Introduction

In theory, any form of plant material with cellulose and hemicel — luloses is suitable for cellulosic ethanol production. Lignocellulosic biomass is the general term used to describe the plant matter com­posed of polymeric compounds cellulose, hemicellulose, and lig­nin. These complex components form the structure of plant leaves, stalks, trunks, branches, and husks. Cellulose can also be obtained from plant products such as paper in the form of waste paper and cardboard. Cellulosic feedstocks contain sugars within their cellu­lose and hemicellulose, but they are more difficult to convert bio­chemically into ethanol than starch — and sugar-based feedstocks. Cellulose resists being broken down into its component sugars. This resistance to breakdown, or recalcitrance, is discussed in more detail in the biomass structure section in Chapter 4, and pretreat­ment sections in Chapter 5 of the book.

The key advantages of these feedstocks over starch- and sugar — based feedstocks are that they are much more abundant and thus can be used to produce substantial amounts of ethanol to meet global fuel demand. They can also be waste products, or in the case of trees and grasses, can be grown specifically for ethanol produc­tion on marginal lands not suitable for other crops. Less fossil fuel energy is required to grow or collect these feedstocks and convert them to ethanol. Most importantly, these materials are not used as human food products.

However, limitations on cellulosic feedstock quantities do exist. For example, limits must be placed on the amount of crop residue removed to protect lands from erosion and to sustain soil organic carbon. The U. S. Department of Agriculture’s Renewable Energy Assessment Project determines the amount of residue needed to protect the soil resource, comparing economic implications of using stover as a bioenergy feedstock versus a source of carbon to build soil organic carbon, and provides harvest rate recommendations.

While considering the non-gasification routes to cellulosic etha­nol, the important component for bioethanol production is the combined cellulose-hemicellulose fraction or polysaccharides. The exact composition of biomass depends on plant material, however, typical percentage compositions of the two polysaccharide compo­nents are: cellulose 35-50%, and hemicellulose 20-35%. Cellulose is a glucose polymer linked by в-1,4 glycosidic bonds. The basic building block of this linear polymer is cellobiose, a glucose-glu­cose dimer. Hydrolysis of cellulose results in individual glucose monomers. This hydrolysis process is also known as saccharifica­tion. The other polysaccharide component is hemicellulose, which is a short, highly-branched polymer of five-carbon (C-5) and six — carbon (C-6) sugars. Specifically, hemicellulose contains xylose and arabinose (C-5 sugars) and galactose, glucose, and mannose (C-6 sugars). Hemicellulose is more readily hydrolyzed compared to cel­lulose because of its branched, amorphous nature. A major product of hemicellulose hydrolysis is the C-5 sugar xylose. The third major component in the biomass is lignin, which accounts for 15-30% of total feedstock dry matter. Lignin is composed of poly phenolic structural units, and is the largest non-carbohydrate fraction of lig — nocellulose. Unlike cellulose and hemicellulose, lignin cannot be uti­lized in the fermentation processes. Lignin is a complex 3D polymer with several types of structural units. Some of the major structural units in lignin polymers are shown in Figure 3.1. A representative section of the lignin structure is shown in Figure 4.8 as well.

In addition to those shown in Figure 3.1, there are other minor organic and inorganic components and their concentrations are highly dependent on the source of the biomass. These are organic

compounds like resins, fatty acids, phenolics, phytosterols, lipids, amino acids, chlorophyll, and pigments. Then there are inorganic components, and the inorganic content can be qualitatively and quantitatively analyzed after ignition of the biomass, and the amount of residue or ash corresponds to the inorganic materials. Ash is the residue remaining after ignition or dry oxidation of herbaceous bio­mass above 575-600°C, and can be 3-10% of total feedstock dry mat­ter. It is composed of minerals such as silicon, aluminum, calcium, magnesium, potassium, and sodium in the oxide or carbonate forms.

Higher cellulose and hemicellulose containing biomass with lower lignin contents yield more ethanol via aqueous-phase biomass hydrolysis route or non-gasification route, since these plant materials give higher amounts of C-5 and C-6 sugars during the saccharifica­tion. Lignin and other minor organic inorganic substances are sepa­rated from the sugar solution before the fermentation, because some of these components may inhibit the fermentation process.

In the gasification path to cellulosic ethanol all carbon in the bio­mass is burned in a controlled oxidation process to a mixture of CO, H2 and CO2. Therefore, in theory, all carbon in cellulose, hemicel — lulose and lignin can be converted to ethanol.

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