BIOCHEMICAL CONVERSION: ANAEROBIC DIGESTION - Biomass as a Sustainable Energy Source for the Future - page 464

Energy content of biogas
CH 4 + 2O 2 → CO 2 + 2H 2 O
Normal volume of 1 kmol gas = 22.4 m 3 :
CH 4 content of biogas = 60 vol.%:
Eficiency of electricity generation = 40 vol.%:
1 kWh corresponds to 3.6 MJ:

∆ G 0 = − 818 kJ.mol -1

CH4

E = 36.5 MJ.m 3 n,CH4
E = 21.9 MJ.m n , biogas
E = 8.8 MJ electricity .m n , biogas

E = 2.1 kWh electricity .m n , biogas

Energy production of anaerobic digestion
Substrate:100,000 t.y −1 ;150 m 3 , biogas .t -1
31.5·10 6 per year:
60 vol.% CH 4 ; 22.4 m 3 .kmol -1 :
818 MJ.kmol - 1 :
Eficiency of electricity generation = 40%:

φ V = 0.48 m 3 n , biogas .s −1

φ n = 0.012 kmol CH4 .s −1

P = 9.7 MJ.s -1 or MW total energy
P = 3.9 MJ.s -1 or MW electricity

CH4

FIGURE 14.12 Energy calculations for methane-containing biogas.

2.7 MW electricity

1.8 MW steam
2.2 MW hot water

CHP

Biogas

Manure and
industrial waste
135,000 (t.y -1 )

Anaerobic digestion
(35°C, 77 m 3 , biogas .t -1 )

Hygienization
(70°C)

P-recovery as
MgNH 4 PO 4
(1 kg p .t -1 )
N-recovery as
NH 4 NO 3
(4 kg N .t -1 )

Digestate

Organic
granulate

FIGURE 14.13 Example of an energeticallyautonomic anaerobicdigestion scheme for treatment
of 135,000 tonnes pig manure and industrial waste per year. Besides electricity, the CHP generates
high- and low-value heat (steam and hot water), which are used for substrate hygienization,
concentration of the nutrients recovered, and drying of the solid residue facilitating effective
combustion.