Environmental Engineering Reference
In-Depth Information
Similar energy outcomes as cascade C are produced when the total plant
is assumed to be digested, as in cascade D. In this case energy output
fluctuates between 90 and 210GJ ha
1
yr
1
. Cassava and oil palm deliver
similar results whereas sugarcane offers the highest energy outcome. Due to
the limited digestibility assumed for aerial biomass and lignocellulosic
residues like bagasse, the energy content left in by-products from AD is still
significant. If this energy is to be harvested via combustion, for example, as
proposed for sugarcane by van Haandel (2005) or performed nowadays for
fresh fruit bunches in Malaysia, the net energy output of cascade B in the
case of oil palm and sugarcane becomes positive whereas cascades C and D
can almost double their energy output.
The energy balances of the cascades were calculated using equation 7.3.
The energy balances for cascades A not benefiting from AD are 72, 128,
45GJ ha
1
yr
1
for oil palm, sugarcane and cassava, respectively. Extra
energy outputs for the different crops fluctuate between 44 and
144GJ ha
1
yr
1
when only industrial by-products are recovered (i.e. cascade
B), whereas when all by-products are valorized using AD, benefits can
increase to 71-290GJ ha
1
yr
1
. When the full digestion of the crop is
considered, 89-296GJ ha
1
yr
1
extra net energy outputs result as compared
with current biofuel systems being promoted. Such energy still has to be
upgraded for final use; in the case of grid injection, this would mean about
15% of the energy content of the biogas produced. If the extra energy
recovered from the biomass is expressed as land savings, a minimum saving
of one hectare per hectare of land invested could be the case in the least
ambitious scenario, which is when only industrial by-products are valorized
via AD. In other words, half of the land demanded would be needed to
provide the same energy output. Savings from the other systems are even
greater when AD is used to valorize the whole crop for energy purposes. In
this case, about two times more energy is produced as compared to
bioethanol or biodiesel systems, meaning that only 30-35% of the area used
to produce the biofuels would be needed to deliver the same energy output.
The added value of AD to biomass chains is also important in terms of
nutrient recovery. In the case of cassava, 25-30% total nitrogen, 45-55%
total phosphorus and 55-60% total potassium is removed in the root
harvest (Howeler 2001) and is therefore expected to be found in the by-
products of bioethanol processing, i.e. vinasse, bagasse and fresh fruit
bunches/peels. The case of sugarcane portrays a different scenario. In this
case, only a minor portion of the nitrogen remains in the aerial biomass, i.e.
10% of the fertilizer applied, the rest being found in the vinasse and bagasse
with the majority (80%) in the bagasse. In contrast, phosphorus is mainly
found in the vinasse, which can supply 60% of the fertilizer demand whereas
bagasse contains only 8% of the phosphorus (Kee Kwong et al. 1987; van
Haandel 2005). AD seems to be advantageous over other technological
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