Environmental Engineering Reference
In-Depth Information
Table 2.2 Methane yields of some common energy crops (plants and
plant parts)
Methane yield (m
3
/VS)
Energy crop
Maize (whole crop) 205-450
Grass 298-467
Clover grass 290-390
Hemp 355-409
Sunflower 154-400
Oilseed rape 240-340
Potatoes 275-400
Sugar beet 236-381
Fodder beet 420-500
Barley 353-658
Triticale 337-555
Alfalfa 340-500
Ryegrass 390-410
Nettle 120-420
Straw 242-324
Leaves 417-453
Source: Data compilation after Murphy et al. (2011).
and parts of plants, have been tested and proven to be suitable as biogas
feedstock. These include maize, various grasses, various cereals, beets,
potatoes and sunflowers, as shown in Table 2.2.
Some newly introduced energy crops in central Europe include Silphium
perfoliatum, Sorghum bicolor, Sorghum Sudanese and Helianthus tuberosus.
Mixed cropping with different plant species has also been considered.
Woody crops are currently not used for AD, as biomass containing a high
percentage of lignin cannot be effectively decomposed by anaerobic
microorganisms. Ongoing research is currently focusing on delignification
pre-treatments and technologies for efficient utilization of woody biomass as
feedstock for biogas production (Al Seadi et al., 2008).
The use of energy crops as a feedstock for biogas implies some specific
technologic steps prior to digestion: harvesting, pre-processing and storage/
ensiling. Herbaceous energy crops like grass, sunflowers, maize and sugar
beet are normally used fresh or as silage. Maize is the most frequently used
energy crop in the majority of existing biogas plants (Murphy et al., 2011); it
can be harvested with ordinary combine harvesters that simultaneously
harvest and chop the whole plant for subsequent ensiling. Crops for biogas
can be immediately fed to the digester or stored as silage for year-round
availability. Grass crops can be harvested 3-5 times per season (Fig. 2.1).
Murphy et al. (2011) have indicated that the composition of crops and thus
their suitability as AD feedstock varies with the stage of maturity. In
general, cellulosic content increases with maturity, negatively affecting the
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