Page 287 - Low Temperature Energy Systems with Applications of Renewable Energy

Biofuels conversion: energy-saving processes and use of biogas 273

bioreactor, kg; m g is mass of gas formed from the substrate due to anaerobic
fermentation, kg.
Taking into account the specific weight of the components in the bioreactor whose
density is uniform throughout the volume due to the mixing of the substrate, Eq. (7.2)
becomes:

dV S dV L
S
L
S
L
V S r þ V L r ¼ r $ $s þ r $ $s þ V G r G (7.3)
ds ds
where V S and V L are the volumes of the solid and liquid phases of the substrate at the
3
bioreactor inlet, m ; V G is volume of gas formed as a result of anaerobic fermentation
3 3
of the substrate, m ; r S and r L are substrate solid and liquid phase densities, kg/m ; and
3 dV S dV L
r G is biogas mixture density, kg/m . The terms r $ $s and r $ $s represent the
S ds L ds
change in the mass of solid and liquid matter of the substrate after its treatment in a
bioreactor.
The thermal regime in a bioreactor depends on the thermal insulation properties of
its external walls, the additional energy supplied to the bioreactor to provide thermal
stabilization, the regime of anaerobic fermentation, and any heat added to offset any
heat losses. The heat balance equation of a bioreactor, assuming the temperature fields
are uniformly distributed within its volume as provided by mixing of the substrate, is
given by the relationship:
Q b ¼ a t A b ðT b T env Þþ m b c m ðT b T env Þþ Q h Q g (7.4)
where Ǫ b is the heat entering the internal environment of the bioreactor, J; the first term
on the right, Q env ¼ a t A b (T b T env ), is the heat loss to the environment; a t is the total
2
heat transfer coefficient through the vessel and insulation of the reactor, W/(m K); A b
2
is outer surface area of the bioreactor shell, m ; T b is the bioreactor internal temper-
ature, S; T a is ambient temperature around the bioreactor, S; m b is mass of substrate,

biogas mixture and air in the installation, kg; c m is the given specific heat capacity of
the substrate, biogas mixture and air in the bioreactor, J/(kg K); the term m b c m
(T b T env ) ¼ m b c m DT represents the change in the thermal energy accumulated in the
bioreactor; DT is current temperature difference during anaerobic fermentation;
Ǫ h ¼ Ǫ T1 þ Ǫ T2 is additional total heat entering the substrate thermostabilization unit
(where Ǫ T1 and Ǫ T2 are the heat coming from the main heater and from the waste heat
exchanger 5, respectively), J; Ǫ g is the heat of the biogas mixture removed from a
bioreactor, J.
The bioreactor pressure influences the intensity of the fermentation process and,
consequently, the conditions and the amount of biogas formation. An increase in pres-
sure in the bioreactor has conflicting effects. On the one hand, increasing the total pres-
sure and the partial pressure of carbon dioxide is a favorable condition for biogas
production. On the other hand, increasing the pressure leads to an increase in the sol-
ubility of methane in water which inhibits its release into the gaseous phase. As a
compromise, most bioreactors operate at a pressure that is slightly higher than
atmospheric.