Plant Biomass as a Renewable Source of Biofuels ...

Lambert Academic Publishing, Germany, 2013

Plant Biomass as a Renewable Source of Biofuels and Biochemicals Michael Ioelovich Designer Energy Ltd, Rehovot, Israel E-mail address: [email protected] 1. Introduction Existence and further development of the present civilization requires expanded consumption of energy, chemicals and materials. Nowadays the main energy sources are fossil fuels – coal, petroleum and natural gas. Over 80% (about 420 EJ per year) of world energy demand are met by the combustion of fossil fuels [1-4]. In 2012 coal gives about 26% energy of the world. Crude oil – petroleum, provides about 33% of the world energy, while natural gas provides about 22% of the world's energy consumption (Fig.1). Being burnt in power stations and industrial boiler plants, the natural gas is also used by many people to prepare food and heat their homes. Combustion of the petroleum-based fuels and liquefied natural gases give more heat energy (40-45 MJ/kg) than combustion of coals (20-25 MJ/kg). Fossil coals usually contain increased content of sulfur, and their burning is accompanied with emission of sulfur dioxide gas that contributes to formation of acid rains polluting the environment. Simultaneously, the same fossil sources are feedstocks for the production of a huge range of organic chemicals and materials - solvents, adhesives, dyes, explosives, monomers, polymers, resins, fibers, films, drugs, etc. Unfortunately, the fossil sources are not reproduced in the nature; their reserves permanently are exhausted and run down. As is follows from the investigations, proven reserves of coals in the world are about 1000 Gt; and known reserves of the crude oil are estimated in 2-3 trillion barrels. Proven resources of the conventional natural gas (from gas fields and also accompanying gas from crude oil fields) are about 200 trillion cubic meters. 1


Recently, high volumes of unconventional shale gas were found. However, composition of the shale gas is complex, and the production of this gas is more expensive than of the conventional natural gas.

Petroleum Coal Natural gas Biofuel Others

Figure 1. Consumption of energy sources in the world

Use of the same limited fossil sources for production energy on the one hand, and chemicals and materials on the other hand, leads to an apparent contradiction and namely, the more burn the petroleum-based fuels, gas and coal, the less is left of the fossil raw materials for the production of chemicals and materials. No wonder, Dmitry Mendeleev and other scientists warned that burning of the petroleum is similar to that heating of the stove by means of the money. To eliminate the imbalance in the fossil organic sources, it is required the increased utilization of alternative sources of energy and raw materials. A considerable attention in recent years is given especially to plant biomass, which in contrast to the fossil sources is continuously renewed in the nature. As a result of the biosynthesis, a mass of the plants increases approximately on 1.5 trillion tons annually [5]. 2


Current technologies of the biofuels are based on transformation of glucosebased carbohydrates into bioethanol and vegetable oils into biodiesel fuel. At present, the main sources of carbohydrates for the industrial fermentation into bioethanol are juices of sugarcane and sugar beet, as well as starches of corn, wheat, potatoes and some others agricultural plants. In Brazil 100 percents of the ethanol are produced from juice of sugar cane. In USA approximately 85 percents of the total ethanol production relies on the corn grains. The typical representatives of the oil-crops are olive, rape, sunflower and some others plants. The bioenergy provides about 10% (55 EJ per year) of the global energy supply. Since carbohydrates and vegetable oils are required by the food industry, their use for production of the biofuels or biochemicals is limited. Moreover, further expansion of the production to higher volume of the bioproducts will cause shortage of land areas, exhaustion of the soil, deficit of food and feed products and increasing of their prices [6]. It was calculated that the production of sufficient ethanol to fuel an automobile traveling 10,000 miles would need about 1.5-2.0 acres of corn. If all automobiles were fueled with corn-based ethanol, then the most part of agricultural land area of USA would be needed to grow the corn to produce biofuel only. Federal office of Germany for the environment said that in the world is not enough agricultural land for cultivation specially the energy crops. An alternative way to obtain biofuels and valuable biochemicals without competing with food industry is use a non-edible biomass [7]. This biomass type involves residues of agricultural plants (e.g. stalks, husks, cobs, etc.), forest residues (e.g. sawdust, twigs, shrubs, etc.), waste of wood, textile, pulp, paper and cities, as well as some plant species (e.g. Miscanthus, Switchgrass, Bermuda grass, etc.). Agriculture, forestry, pulp and paper industry, as well as cities create vast amounts of lignocellulosic residues. Moreover, huge amounts of algae are not utilized yet and can be used as 3


appropriate feedstock for productions energy or chemicals. The not-edible plant raw materials are related to abundant, renewable and inexpensive biomass type. Total amount of such biomass that is accumulated annually in the world is estimated in 10 billion tons at least. Only in USA annual accumulation of the biomass is about 1 billion tons [8]. To generate the heat energy, the non-edible plant biomass can be burned directly or after its conversion into carbonized solid fuels (charcoal) or liquid biofuels (bioethanol, biodiesel, bio-oil). Another approach is use the biomass as feedstock for production of valuable biochemicals, such as organic solvents, organic acids, monomers, resins, polymers, etc. In this paper, structure, composition and properties of the non-edible plant biomass of various origins are described. Besides, some methods of thermal, chemical and biological treatment of the biomass are discussed. The potential of the plant biomass for production of biofuels and biochemicals is analyzed in order to choose the most efficient directions for its utilization.

2. Non-edible plant biomass 2.1. Structure of the biomass Any plant biomass is ensemble of multitude of plant fibers bonded with each other in the lateral direction by means of lignin-hemicellulosic “glue”. To isolate the individual fibers, the biomass undergoes by biological or chemical maceration in combination with mechanical or ultrasound treatment. One fiber is an elongated vegetable cell. The fibers can contain pores and structural defects. Hollow capillary – lumen, extends along the fiber. Fibers of biomass of have a different shape and size [9-11]. The biomass of softwoods consists mainly of fibers or cells called tracheids (Fig. 2, 3a). Length of the tracheids is 3-7 mm and diameter 20-40 μm. Width of the cell wall is 2-5 μm. 4


The biomass of hardwoods can consist of various types of fibers or cells – libriform (70-90%), as well as vessels and tracheids. Libriforms are short spindle-shaped fibers with length of 0.5-2 mm and average diameter of about 20 μm (Fig. 3 b). Vessels have a wide lumen with diameter of 50-90 μm (Fig. 3 c). Twigs of the trees and shrubs have the similar anatomic structure. The biomass of agricultural plants and grasses contain mainly libriform fibers and vessels [12].

Figure 2. Joint tracheids of spruce wood

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(a)

(b)

(c)

Figure 3. Shape of tracheid (a), libriform (b) and vessel (c)

Fibers of cotton and bast plants are long (20-40 mm) and have a specific shape. Cotton fibers are twisted, while fibers of bast plats (flax, ramie, etc.) are straight and round (Fig. 4, 5).

Figure 4. Twisted fibers of cotton

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Figure 5. Straight fibers of flax

The wall of any plant fiber is built from external primary P and inner secondary S walls [9-11]. The thin P-wall (about 100-200 nm) has loose net of fibrillar bundles. The S-wall has thickness of 2-4 µm and composed of three layers S1, S2 and S3 (Fig. 6). The layers S1 and S2 have nanothickness. The dominating S2-layer built of cellulose fibrillar bundles orientated mainly along the fiber axis [13-15]. The fibrillar bundles of the cell wall consist of elementary cellulose nanofibrils.

S2

S1

L

S3

Figure 6. Layer structure of S-wall of the plant fiber (L is Lumen)

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2.2. Composition of the biomass The chemical composition of plant biomasses was determined by conventional methods of chemical analysis [16, 17]. Percentage of lignin was analyzed my means of standard TAPPI procedure T222. The content of holocellulose was measured after delignification of the biomass with sodium chlorite. The obtained holocellulose sample was hydrolyzed with boiling 1.5% hydrochloric acid for 2 h. The content of cellulose was calculated from the dry residue remained after hydrolysis of the holocellulose, while the content of hemicelluloses was measured from weight loss of the hydrolyzed holocellulose sample. All plant biomasses and their wastes contain three main polymeric components – cellulose, hemicelluloses and lignin (Table 1, 2). The other components of the biomass and its waste can be ash, proteins and extractives (waxes, fats, oils etc.). Table 1. Typical chemical composition of the dry biomass Biomass

Cellulose, %

Hemicelluloses, %

Lignin, %

White cotton

94-96

1-2