Page 193 - Materials Chemistry, Second Edition

Sustainability of (H 2 ? CH 4 ) by Anaerobic Digestion 181

necessary to run the bioreactor. A detailed analysis of the net energy production of
two-steps AD can be found in (Ruggeri et al. 2010) and only a brief introduction is
given here. The net energy production E net may be calculated as:

ð3Þ
E net ¼ E H2 þ CH 4 E h þ E hp þ E l þ E m þ E p
where
is the energy produced (MJ/L)
E H2+CH4
is the heating energy necessary to reach the working temperature (MJ/
E h
L)
is the heating energy necessary to reach the pretreatment temperature
E hp
(MJ/L) if a thermal pretreatment is present
is the thermal energy loss, which depends on the outdoor ambient
E l
temperature and the duration of the fermentation (MJ/L)
E m is the electrical energy consumed for mixing (MJ/L) if a mixing system
is present
is the electrical energy consumed for pumping (MJ/L)
E p
The calculation of the net energy production requires the evaluation of the heat
necessary to pretreat the organic refuse and the heat needed to keep the system at
the working temperature. The heat required to keep the fermenting broth at the
working temperature (T w ) is the sum of the heat necessary to warm up the feeding
biomass from the ambient outdoor temperature (T a )to T w and the heat lost from
the digester walls, which depends on the geography of the plant location, seasonal
variations and obviously on the night/day oscillations. Figure 5 offers an overall
view on the energies involved in the balance of an AD reactor, which is valid
either in the case of producing H 2 or CH 4 .
The energy balance of full-scale AD should be conducted in order to evaluate
the quantity of net energy produced from a carbonaceous substrate as a function
of two parameters, namely working temperature and the diameter of bioreactor.
In the following sections, each term of Eq. (3) will be explained.

Fig. 5 Global view on the E
energies involved in the mixing E
balance of an AD reactor production

E heating E loss

E pump