Environmental Engineering Reference
In-Depth Information
A more detailed description of energy balances of AD facilities is given in
Chapter 9 of this topic.
7.4
Contribution of anaerobic digestion to the energy
balance of biofuel chains
In recent years, legal restrictions on bioenergy production of liquid fuels
have been witnessed in different regions of the world. These have taken the
form of fixed targets for the contribution of biofuels to overall automotive
energy consumption or compulsory blending regulations of gasoline and
diesel with fixed proportions of bioethanol and biodiesel respectively.
Although bioethanol and biodiesel are attractive energy carriers due to
their high energy density and liquid character, their perceived environmental
and social-economic advantages are being questioned. The main criticisms
are directed towards their limited energy gains, the need for fossil fuels or
part of energy revenues for their production and the land required for
biomass cultivation, which generates undesirable competition with food and
other uses of land.
Anaerobic digestion can positively impact the energy balance of both
chains via its direct contribution in terms of energy in the form of methane or
in its indirect contribution via the replacement of fertilizers, water and extra
energy coming from the digestate. Concerns regarding AD technology focus
on the fact that the fuel produced is a low energy density gas and not a liquid
like biodiesel or bioethanol, implying that higher storage volumes are
required. Another constraint is possible emissions of GHG gases if
technological units are not managed adequately (Baldassano and Soriano
2000). However, as recently shown by Tilche and Galatola (2008), biogas may
make a considerable contribution to GHG emissions reductions, particularly
if used as a biofuel. The potential contribution of AD to GHG reduction as
computed for 27 EU countries on the basis of their 2005 Kyoto declarations is
of the order of magnitude of 3.9
￿ ￿ ￿ ￿ ￿ ￿
10 9 m 3 CH 4 yr 1 . The sum of bio-methane
from landfills and from sewage sludge corresponds to about 380 PJ yr 1 .If
also considering energy crops, biogas has the potential of covering almost
50% of the 10% biofuel target of all automotive transport fuels for 2020,
without implying a change in land use (Tilche and Galatola 2008).
The following examples highlight the role of AD for specific biomass
chains in the case of bioethanol production from sugarcane and cassava and
biodiesel production from oil palm using the methodology proposed earlier.
The cases are examined using Colombian boundary conditions and compare
the added value of AD to specific biomass chains for:
.
6
case A, when AD is not present
.
case B, when industrial by-products are processed in the digestor
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