The Types of Biomass Available
10. Competitive on cost with food crops
The four that have been chosen for further study are Miscanthus, switchgrass, reed canary grass and giant reed. Some of the properties of these grasses are compared with short rotation coppice of willow and poplar in Table 4.4.
0 10 20 30 40 50 60
SRC/wood Straw MSW Switchgrass Miscanthus Giantreed Reedcanarygrass Coal Gas Oil
MJ/kg
Fig. 4.2. Energy content of fossil fuels, SRC and perennial grasses.
Table 4.3. Perennial grass species tested in the EU as an energy crop. (From Lewandowski et al., 2003.)
Photosynthetic Yields
Name Latin name pathway (t/ha/year)
Meadow foxtail Alopecurus pratensis C3 6–13
Big bluestem Andropogon gerardii C4 8–15
Giant reed Arundo donax C3 3–37
Cypergras, Galingale Cyperus longus C4 4–19 Cocksfoot grass Dactylis glomerata C3 8–10
Tall fescue Festuca arundinacea C3 8–14
Raygras, ryegrass Lolium sp. C3 9–12
Miscanthus Miscanthus sp. C4 5–44
Switchgrass Panicum virgatum C4 5–23
Napier grass Pennisetum purpureum C4 27 Reed canary grass Phalaris arundinacea C3 7–13
Timothy Phleum pratense C3 9–18
Common reed Phragmites communis C3 9–13
Energy cane Saccharum officinarum C4 27
Giant cordgrass/salt reedgrass Spartina cynosuroides C4 9/5–20 Prairie cordgrass Spartina pectinata C4 4–18
68Chapter 4
Table 4.4. Properties of biomass crops. (From Powlson et al., 2005; Lewandowski et al., 2003.)
Crop Poplar (SRC) Willow (SRC) Miscanthus sp. Switchgrass Reed canary grass Giant reed
Yield t/ha/year 7 7 12 (5–44) 10 (5–23) 8 (7–13) 5–23
(15–30) (10–25) (15–35)
Establishment time 3 years+ 3 years+ 3 years+ 2–3 years+ 1–2 years 1–2
Photosynthetic pathway C3 C3 C4 C4 C3 C3
Fertilizer Low/medium Low/medium Low Very low Medium Moderate Water supply Wet Wet Not tolerant to Drought tolerant Drought tolerant Drought tolerant
stagnant water
Pesticide Low Low Low Very low Low Low
Establishment costs High High Very high Very low Very low High Pest/disease – Beetle rust None None Some insect problems Few
Day/length Long Long Long Short Long Long
Plantation longevity 20 years 20 years 20 years 20 years 10 years n/a
Energy content GJ/t 15 15 17.6–17.7 17.4 16.5–17.4 17.3–18.8
Output GJ/ha/year 105 105 260–530 174–435 240–600 88–403
(262–525) (175–437) (262–613)
Miscanthus
In Europe research on perennial grasses started with Miscanthus. The genus Miscanthus contains 17 species and originates from East Asia and a hybrid Miscanthus
´ giganteus was first introduced into Europe in 1930 from Japan as an ornamental.
Miscanthus grows vigorously and can be harvested dry in one harvest. Miscanthus is a C4 metabolism grass which can reach 4 m in height and forms rhizomes. Different Miscanthus species have different rhizomes, which are persistent, with the oldest plantation some 18 years old. Miscanthus is wind pollinated with fan-shaped inflor- escences. Miscanthus can be grown on a wide range of soils but does not tolerate waterlogged soils. Most of the yields for Miscanthus reported in Europe have been determined using Miscanthus´ giganteus. Yields are variable with values in the range of 5–44 t/ha/year. It does however suffer from some problems of poor resistance to cold and high costs of propagation as rhizomes have to be used.
Switchgrass
Switchgrass is a native of the North American grasslands, a perennial C4 grass with a high yields on poor soils. Like Miscanthus it was introduced into Europe as an ornamental grass, but based on the data obtained in the USA it has been considered as an energy crop. It is perhaps the best choice as it is drought-tolerant, gives high yields and can be harvested once a year.
Reed canary grass
Reed canary grass is a species indigenous to Europe belonging to the Gramineae family.
It is adapted to low temperatures and short growing times. Reed canary grass is a C3 grass which grows to 3 m in height, is propagated by seed and is harvested once a year.
Giant reed
Giant reed is also an indigenous species belonging to the Gramineae family. It is a tall perennial C3 grass which can reach heights of up to 8–9 m. The giant reed tolerates a variety of conditions but prefers well-drained soils. It is propagated by rhizomes rather than seed and the yield can reach 100 t/ha/year under optimum conditions.
All perennial grasses are regarded as drought tolerant, require few inputs and grow on poor land. However, establishment of these crops is not easy and can vary greatly. Yields of biomass can also vary and appear to be related to nitrogen and water availability. Switchgrass requires as much water as traditional crops and is responsive to nitrogen fertilizer but too much usage will give a problem of lodging.
C3 and C4 metabolism
One of the important characteristics of some of the perennial grasses is the possession of C4 metabolism rather than C3. C4 and C3 metabolism refer to the pathways used
to assimilate carbon dioxide during photosynthesis. C4 plants are more efficient at higher light and temperatures compared to C3 plants. The C4 plants have a lower moisture content, require less fertilizer input and are twice as efficient with water. C4 assimilation of carbon is theoretically 350 kg/ha/day compared with 200 kg/ha/day for C3 plants (Venturi and Venturi, 2003). All these features make C4 plants more suitable for biomass fuel planting than C3 plants.
The development of the C4 metabolism of carbon dioxide assimilation evolved from the Calvin cycle in C3 plants to avoid the loss of carbon dioxide through photo respiration.
The fixation of carbon dioxide during photosynthesis takes place in three stages. The addition (carboxylation) of carbon dioxide to ribulose-1,5-bisphosphate (Fig. 4.3) is fol- lowed by the reduction of 3-phosphoglycerate to glyceraldehyde-3-phosphate. Ribulose-1, 5-bisphosphate is then regenerated from glyceraldehyde-3-phosphate. The first step of the Calvin cycle is catalysed by the chloroplast enzyme ribulose bisphosphate carboxylase/
oxygenase known as rubisco. However, another property of the rubisco enzyme is to catalyse the oxygenation of ribulose-1,5-bisphosphate which is the start of light-dependant oxygen uptake and carbon dioxide release, known as photorespiration, which reduces plant yield.
Regeneration
Glyceraldehyde-3- phosphate
Sucrose starch
Carbon dioxide
ADP
ATP
Reduction
NADPH + ATP
ADP + NADP Ribulose-1,5-
bisphosphate
3-Phosphoglycerate
Fig. 4.3. The Calvin cycle involves the fixation of carbon dioxide through photosynthesis (C3 metabolism).
In the C4 metabolism two different types of cell are involved in photosynthesis, the mesophyll and bundle sheath cells (Fig. 4.4). In the mesophyll cells carbon dioxide is used to carboxylate phosphoenolpyruvate (PEP) forming oxaloacetate. The oxalo- acetate is converted to malate (C4) and this is transferred to the bundle sheath cell where the malate is converted into pyruvate and carbon dioxide. The carbon dioxide is then used in the Calvin cycle. The C4 cycle has a higher energy demand but the cycle reduces photorespiration and water loss. The phosphoenolpyruvate (PEP) carbo xylase enzyme has a high affinity for the carboxyl ion such that it is saturated and in equilibrium with carbon dioxide gas. Oxygen is not a competitor in the PEP
Carbon dioxide
HCO3–
Malate (C4)
Phosphoenol pyruvate
(C4) Oxaloacetate
Mesophyll cell
Malate (C4)
Pyruvate (C3)
Calvin cycle CO2
Chloroplast
Metabolites
Pyruvate (C3)
Mitochondrion
Bundle sheath cell
Fig. 4.4. C4 metabolism in plants involves two types of cells where carbon dioxide is used to carboxylate phosphoenol pyruvate forming oxaloacetate, which is converted into malate.
The malate is transferred to the bundle sheath cells where it is split into pyruvate and carbon dioxide. The carbon dioxide is used in the Calvin cycle.
carboxylase reaction because the substrate is a carboxyl ion. The high activity of the PEP carboxylase allows the plants to reduce the stomatal opening, reducing water loss, while fixing carbon dioxide at an undiminished rate. The high concentration of carbon dioxide in the bundle sheath cells allows the cells to carry out photosynthesis at high temperatures.
Animal and municipal waste
Animal wastes consist of excess slurry and dung from cattle, chickens and pigs. These wastes can be used to generate biogas through anaerobic digestion and there are a number of farm-sized units available. There are also a number of small electricity power stations which run on chicken slurry from large battery chicken farms.