https://sciencetrends.com/developing-integrated-microalgal-biorefinery-fuel-production/
December 11, 2017
Developing An Integrated Microalgal Biorefinery For Fuel Production
by Yogendra Shastri
Concerns about climate change, primarily driven by the use of fossil
fuels, have driven exploration of renewable and sustainable sources of
energy. Biofuels obtained from biomass-based resources are potential
options to replace gasoline and diesel as transportation fuels.
Microalgae are promising biomass sources on account of their high
productivity, adaptability to diverse environmental conditions, ability
to sequester carbon dioxide, and utilize wastewater as a resource.
However, commercial production of microalgae-based fuel is still not a
reality due to techno-economic bottlenecks. To this end, an integrated
biorefinery, co-producing fuel, a low-value high volume product, with
other value-added products such as protein, reduced sugar, and polar
lipid, which are low volume high value, has been proposed. For such an
integrated biorefinery, identification of the best combination of design
and operational decisions among the alternatives is extremely challenging.
In this work, superstructure optimization has been used to address this
challenge. An optimization model considering the production of biodiesel
along with various value-added chemicals is developed. The various
processing stages considered include cultivation of microalgae,
harvesting, and drying, lipid extraction, followed by
transesterification to produce biodiesel. Lipids may also be
fractionated into individual classes. Lipid extracted microalgae may be
used for extraction of protein and reduced sugar. The constraints of the
model include a mass balance among the various steps, along with
equations governing equipment sizing, process synthesis, and economics.
Amount of solution and quantity of biomass cultivated are some of the
continuous variables whereas the number of equipment and number of
batches are examples of integer variables in the model. Selection of
alternatives is attained by binary variables. The optimization model is
a Mixed Integer Linear Programming (MILP) and is developed in General
Algebraic Modeling System (GAMS). The objective is to minimize the net
Annualized Life Cycle Cost (ALCC) of producing a fixed amount of biodiesel.
The model is applied to a scenario with 30 Mg/d production target of
biodiesel from phototrophic strain, with an upper bound on the demands
for other value-added products. The biodiesel ALCC is US $ 8.53/L and
reducing sugar is obtained as a co-product. This ALCC is 35% less than
that when biodiesel is the only product of the refinery. Growth,
performed in raceway ponds, is the most expensive step, accounting for
74% of the final biodiesel cost. The optimal flowsheet and operating
strategy for each step are also recommended. Options such as
supercritical lipid extraction and growth in photobioreactors are highly
efficient for individual steps but are not optimal from a biorefinery
perspective.
Several options to further reduce the biodiesel are explored.
Co-cultivation of the phototrophic and heterotrophic strains reduces the
ALCC by 10.2%. Optimal batch scheduling with infinite intermediate
storage coupled with debottlenecking reduces the net ALCC by 25%. The
integrated biorefinery is highly profitable if reducing sugar is the
main product with unlimited demand. In such cases, a shorter microalgae
growth cycle with higher carbohydrate fraction is preferred, and
biodiesel becomes a side product. The comprehensive integrated model
enables us with an informed decision-making for the microalgae biorefinery.
This study, optimization of integrated microalgal biorefinery producing
fuel and value-added products was recently published in the journal
Biofuels, Bioproducts and Biorefining.
