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FUELS FROM CROPS                     291

               Different species of Euphorbia of family Euphorbiaceae serve as the petroplants that
             have the potential to be a renewable substitute for the conventional petroleum sources.
             Latex of Euphorbia lathyrus contains fairly high percentage of terpenoids. These can be
             converted into high grade transportation fuel. Similarly the carbohydrates (hexoses) from
             such plants can be used for ethanol formation.
               Sugarcane (Saccharum officinarum, family Gramineae) is the main source of raw mate-
             rial for sugar industry. The wastes from sugar industry include bagasse, molasses, and press
             mud. After extracting the cane juice for sugar production, the cellulosic fibrous residue that
             remains is called bagasse. It is used as the raw material (biomass) and processed variously
             for the production of fuel, alcohols, single cell protein as well as in paper mills. Molasses
             is an important by-product of sugar mills and contains 50 to 55 percent fermentable sugars.
             One ton of molasses can produce about 280 L of ethanol. Molasses is used for the produc-
             tion of animal feed, liquid fuel, and alcoholic beverages.
               Sugar beet (Beta vulgaris, family Chenopodiaceae) is yet another plant which contains
             a high percentage of sugars stored in fleshy storage roots. It is also an important source for
             production of sugar as well as ethanol.
               Raising crops like sugarcane, sugar beet, tapioca, potato, maize etc. purely for produc-
             tion of ethanol is described as energy cropping, and the crops are called energy crops.
             Cultivation of plants (trees) for obtaining fuel/firewood is described as energy plantation.
               Other plants, such as Euphorbiaceae plants like Euphorbia lathyris, milkweed (Asclepia
             speciosa), and the tree legume (Copaifera multijuga), produce hydrocarbons which can
             be converted into and used as diesel, called biodiesel (Table 9.1). In addition, some fresh-
             water and marine algae are also known to accumulate hydrocarbons. Some algae like
             Chlamydomonas and anaerobic bacteria like Clostridium produce hydrogen gas, which
             can be used as a pollution free fuel. Long-term research efforts are needed to develop these
             organisms as sources of bioenergy.
               The euphorbeans and milkweeds can be grown in relatively dry environments on lands
             not suited for crop production; this makes them highly attractive sources of biofuels. The
             Euphorbias are relatives of plants used to produce rubber; they produce a latex, which has
             about 30 percent hydrocarbons emulsified in water.
               However, hydrocarbons, as such, are not usually produced from crops, there being insuf-
             ficient amount of the hydrocarbons present in the plant tissue to make the process eco-
             nomic. However, biodiesel is produced from crops thereby offering an excellent renewable
             fuel for diesel engines.

             TABLE 9.1  Typical Plants Used as a Source of Energy

             Nature of                           Estimated annual   Predominant mode
              biomass         Plant species      production (tons)    of energy use
                                                         10
             Wood       Butea monosperma, Casuarina  1.3 × 10    Firewood (ca. 50%
                          equisetifolia, Eucalyptus globulus,         of harvest)
                          Leucaena leucocephala, Melia
                          azadirachta, Tamarix dioica
             Starch     Cereals, millets, root, and tuber    1.9 × 10 9  Bioethanol
                          crops, e.g., potato
                                                         8
             Sugar      Sugarcane, sugarbeet       1.2 × 10     Bioethanol, e.g., in
                                                                 Brazil
             Hydrocarbons  Euphorbia lathyris, Aslepia speciosa,    —  Biodiesel
                         Copaifera multijuga, alage
             Wastes     Crop residues, animal/human refuge,    —   Biogas
                          sewage, etc.
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