Environmental Engineering Reference
In-Depth Information
.
case C, when agricultural by-products are also digested
.
case D, when the full plant is digested; here, full plant means above-
ground biomass in the case of sugarcane, harvested above-ground
biomass minus trunk for oilpalm and total biomass for cassava.
Table 7.2 presents the chains analyzed. The main assumptions for the
calculations performed are presented in Table 7.3 and Fig. 7.6 summarizes
the results of the assessment on the value of AD for recovering the energy
contained in by-products of the biofuel industries. It is important to note
that the performed calculations follow equation 7.3 - that is, the added
value of AD within a contributive perspective.
As can be seen in Fig. 7.6, great differences are found among the chains
studied. When AD is not part of the chain, the contribution of by-products
to the energy flows within the chains appears crucial, constituting 41-68%
of the sum of all energy flows. In fact, energy in by-products including field
residues constitutes 51-71% of the total energy content in the crop.
Industrial by-products constitute a lower share of the crop energy as
compared with residues from crop production, except in the case of oil
palm. The proportion of the energy content present in the aerial biomass as
compared with the total energy fixated by the plant is 26%, 7% and 34% for
cassava, oil palm and sugarcane respectively. In the case of oil palm, the
energy content of the trunks, which are replaced every 25 years, has not been
considered. Due to the energy importance of by-products, when they are
added value (as in cascades B and C), much higher net energy outputs are
produced. The energy content in aerial biomass appears especially relevant
when comparing cascades B and C. As can be observed, the net energy
output of sugarcane and oil palm become positive only when this flow is
included.
Table 7.2
Biomass chains analyzed for the contribution of anaerobic digestion
General
description
Flows digested
Sugarcane Cassava
Oil palm
A Biofuel
None
None
None
B Biofuel + AD industrial
by-products
Vinasse +
bagasse
Vinasse + bagasse POME + fruit
residue + glycerine
a
C Biofuel + AD industrial
by-products and
biomass by-products
Vinasse +
bagasse +
trash
Vinasse + bagasse
+ leaves and stalks
POME + fruit
residue + glycerine
+ leaves
D Methane from full plant Full plant
Full plant
Full plant
a
POME = palm oil mill effluent.
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