How Energy Is Produced - The Renaissance of Renewable Energy - page 131

present. Although humans are known to occasionally produce and release methane, this is
associated with indigestion and is not a standard feature of our digestive process.

Table 4.6. Methane yields from digestion of various plant materials

CH 4 yield m 3 /t CH 4 yield m 3 /ha

Feedstock Crop yield t/ha

Maize (whole crop) 9-30 205-450 1,660-12,150

Wheat (grain) 3.6-11.7 385-425 1,245-4,500

Barley 3.6-4.1 350-660 1,445-2,430

Grass 10-15 300-470 2,680-6,300

Miscanthus 8-25 180-220 1,290-4,900

Sunflower 6-8 155-400 830-2,880

Oilseed rape 2.5-7.8 240-340 540-2,390

Peas 3.7-4.7 390 1,300-1,650

Potatoes 10.7-50 270-400 2,660-18,000

Sugar beet 9.2-18.4 235-380 1,954-6,310

Note: High net energy yield per hectare is an indispensable prerequisite for an
economic operation of a digestion plant.
Sources: Braun et al. ( 2009 ) , Murphy et al. ( 2011 ) .

If we place wet biomass in a warm closed tank, we reproduce the conditions in a cow's
intestine. So far, biogas tanks are far less efficient than their natural precursors. The best
artificial anaerobic digesters convert biomass to biogas at a rate three times lower than
a cow's intestine (and fifty times lower than a termite's!). Moreover, up to now it has
not been possible to effectively breed bacteria that digest lignin and cellulose, which is