Page 69 - Biofuels Refining and Performance

52 Chapter Two

TABLE 2.1 Differences between C 3 and C 4 Plants
Plant
characteristics C 3 cycle type C 4 cycle type

Leaf anatomy Mesophyll (palisade and Krantz anatomy, bundle-
spongy type), no chloroplasts sheath cell with
in bundle-sheath cell chloroplasts
Chloroplasts Single-type Dimorphic
Carboxylase type Primary (Rubisco) Primary PEPCase in
mesophyll, Secondary
(Rubisco in bundle-
sheath cell)
Primary CO 2 acceptor RuBP PEP
Primary stable product 3-phosphoglyceric acid (3-PGA) Oxalocetate (OAA)
Ratio of CO 2 :ATP:NADPH 1:3:2 1:5:2
Productivity (ton/ha
yr) ~20 ~30

grown on a repetitive cropping mode for continuous and maximum
production of biomass. Grasses such as Bermuda grass, Sudan grass,
sugarcane, and sorghum are good candidates for energy generation from
biomass. A comparison of the characteristics of C and C plants, in
4
3
terms of leaf anatomy, is shown in Table 2.1.

2.4 Plant Types and Growing Cycles
Several plants have been proposed to be good sources of energy. These
include woody crops and grasses/herbaceous plants, starch and sugar
crops and oilseeds, fast growing trees such as hybrid poplars, shrubs
such as willows, and so forth. Energy crops can be grown on agricultural
lands not utilized for food, feed, and fiber. Farmers could plant these
crops along the riverbanks, along lakeshores, between farms and nat-
ural forests, or on wetlands. These crops could be a good source of alter-
nate income, reducing the risk of fluctuating markets and stabilizing
farm income. Woody plants, herbaceous plants/grasses, and aquatic
plants are different sources for biomass production. The type of biomass
selected determines the form of energy conversion process. For instance,
sugarcane has high moisture content, and therefore, a “wet/aqueous”
bioconversion process, such as fermentation, is the predominant method
of use. For a low-moisture content type such as wood, gasification, pyrol-
ysis, or combustion are the more cost-effective ways of conversion.
Characteristics of an ideal energy crop are mentioned below:

Low energy input to produce
Low nutrient requirements
Tolerance to abiotic and biotic stresses