2. Methods
A process concept was developed based on current state of the
art algae harvesting technologies. Two hypothetical production
scenarios were investigated: Scenario 1 used energy intensive
algae harvesting techniques dissolved air flotation (DAF) followed
by centrifugation to achieve 30% ds algae paste. In Scenario 2 less
energy intensive harvesting techniques flocculation/clarification
followed by belt pressing were used to achieve 20% ds algae paste.
A model was developed to estimate the material flows of the
processes; associated energy requirements for processing the algae
were then estimated based on values reported in published literature.
Operating parameters of a typical cane sugar mill in Louisiana
were used as inputs to the model. The CO2, energy and water resources
that could be made available for algae production from
the mill were then modeled based on the assumptions listed in
Table 1. The numbers in parenthesis indicate the typical ranges
found in the referenced literature. The main outputs of the model
with which the scenarios were compared were net energy ratio,
ER = (energy produced/energy consumed), and biodiesel production
(L/yr).
In general, algal biodiesel production can be described in the
following steps: (1) algae cultivation (includes paddlewheel mixing
and water pumping); (2) CO2 capture and delivery to the
ponds; (3) harvesting, also called dewatering; (4) drying; (5) oil
extraction; and (6) transesterification of the oil into biodiesel. A
compilation of published values of the energy requirement for each
step is shown in Fig. 2 (Chisti, 2008; Benemann, 1996; Sazdanoff,
2006). The values used in this study are shown in black.
Variation in the energy demand of each step reported by the
different authors adds uncertainty as to the feasibility of algal biofuels.
One important parameter – the energy content of the algae – can
range between 5 and 8 kWh/kg dry wt. depending on species and oil
content (Lardon, 2009). Therefore, in order for algal biodiesel production
to be thermodynamically feasible, the amount of energy
required to produce the algae and process it into useable fuel must
be less than this amount.
Fig. 2 shows that the total amount of energy required to produce
algae and convert it into algal biodiesel (the sum of all 6
steps) is less than the total amount of energy contained in the algae
using the assumptions of this study, indicating that the process
described is thermodynamically feasible. Since drying is the major
energy consuming step, and has the largest variation between
authors, the key to algal biofuel feasibility is ensuring that the energy
required in this step is well defined and minimized.