Page 192 - Materials Chemistry, Second Edition

180 B. Ruggeri et al.

For a sustainable energy point of view, it is necessary to energetically valorize
the volatile fatty acids (VFAs) and other residue compounds present at the end of
the first anaerobic step, which produces H 2 and VFAs as acetogenic fermentation.
This valorization also permits the waste materials to be degraded as much as
possible; the most adequate way is to use VFAs as a substrate for metanogenes to
produce methane.
The energy analysis can be applied for only the H 2 or CH 4 production or for
both AD processes in series to produce H 2 and CH 4 , respectively.
The results of these analyses show that the net energy balance of a bioreactor
producing H 2 in almost all conditions is never in the positive range (Ruggeri et al.
2010). On the contrary, two-steps (H 2 plus CH 4 ) in series show an increase in the
produced energy and, consequently, the net energy balance becomes positive. In
fact, from a thermodynamic point of view, during H 2 -fermentation from glucose,
only one-third of the energy available is converted to H 2 , the other two-third
remains occluded in the form of fatty acids. Therefore, one can obtain a positive
net energy balance from an energy valorization of the end-liquid metabolites that
accompany the H 2 production, due to the increment in the energy production.
Temperature and pH play an important role on fermentative hydrogen production.
Many studies (Akutzu et al. 2009; Wang and Wan 2008b;Muetal. 2006; Zhang
and Shen 2006; Nath and Das 2004; Hawkes et al. 2002; Lee et al. 2006) have
shown that, in an appropriate range, increasing the temperature can increase the
ability of hydrogen forming bacteria and archea bacteria to produce hydrogen and
methane, during fermentation. Temperature is the most important parameter, from
an energetic point of view, because it influences not only the energy produced, but
also the energy necessary to run the bioreactor. Therefore, the temperature is the
key parameter in the net energy balance of the technology. The present sustain-
ability analysis of the AD of organic refuse is based on results experimentally
evaluated by conducting test runs with market refuse pretreated with 2N NaOH at
pH = 12 for 24 h (Bettoli 2010).

3.2 Net Energy Production in Anaerobic Digestion Process

The net energy produced in an anaerobic digestion process is the difference
between the energy produced in the form of biofuels (H 2 and/or CH 4 ) and the
direct energy used to run and maintain the system. The production of renewable
energy (e.g., biofuels) in fact requires energy expenditure, as any other process.
Pretreatment units also represent an expensive energy cost. To perform the energy
balance in the present case, all the energy quantities have been evaluated in energy
units per unit volume of bioreactor (MJ/L). Many factors can influence the net
energy balance of anaerobic digestion such as the type of feedstock, environ-
mental, geographical, and operational conditions.
In order to calculate the net energy, it is necessary to consider the energy
balance of the anaerobic bioreactor, including the thermal and the electrical energy