Page 194 - Materials Chemistry, Second Edition

182 B. Ruggeri et al.

3.2.1 Energy Production
The produced energy is the total energy embedded in the produced gas, i.e., the
energy contained in the amount of hydrogen and/or methane retrieved from a
single batch run, with reference to the reactor volume and it can be calculated as:
ð ð ð Þ ð4Þ
E produced ¼ F P H 2 T w Þ H H 2 þ P CH 4 T w Þ H CH 4
where
P H2 (T w ) and P CH4 (T w ) are the specific productions of H 2 and CH 4 , respec-
tively, and refer to the amount of gas produced during
3
a single-batch run. They are expressed as Nm of
H 2 /CH 4 per unit of fermenting broth, which depends
strongly on the working temperature.
3
are the LHV (10.8 and 36.18 MJ/Nm , respectively)
H H2 and H CH4
F is the filling coefficient of the reactor, which is usually
equal to 90 % of the available volume.

3.2.2 Heating Energy

The energy required to warm up the fermenting broth mainly depends on its
specific heat, the difference between the outdoor ambient and the working tem-
perature of the bioreactor, and the efficiency of the heating system. The heating
energy per unit volume of bioreactor can be calculated as follows:
E h ¼ðq c p DT FÞ =g ð5Þ

where
3
q is the biomass density (kg/m )
-1
is the specific value of fermenting broth heat (kcal kg -1 °C )
c p
according to the season (°C)
DT = T w - T a
g is the global efficiency of the system to furnish heat taking into
account g comb and g heat exc
q and c p have been considered equal to those of water. The difference between
the working temperature and the outdoor ambient DT was considered for different
seasonal conditions, i.e., summer and winter conditions. To calculate the global
efficiency of the warming system, a global combustion boiler efficiency was
considered g: combustion efficiency (g comb & 0.8) and heat exchanger efficiency
(g heat exc & 0.6) were multiplied to obtain the global efficiency (g & 0.48) nec-
essary to furnish the heat required.