(428d) The Cultivation of Algae and Its Conversion to Biodiesel

The Cultivation of Algae and its
Conversion to Biodiesel

Daniel
Choi, Spencer Glantz, Jasmin
Alsous, Warren D. Seider, Stuart W. Churchill

Department
of Chemical & Biomolecular Engineering,
University of Pennsylvania

A
presentation at the 2010 Annual Meeting under the title "The Production of
Alkanes from Algae" drew a large audience, even though it was on Friday
morning, and evoked considerable discussion. We have taken that discussion and
the report upon which that presentation was based as starting points for a
re-examination of the potential of cultivated algae as a source of biofuels.

The
possibility of substitution of biofuels for petroleum-based ones, even in small
part, has evoked considerable interest for economic, political, and environmental
reasons. There is a general recognition that the cultivation and processing of
algae into biofuels have several advantages with respect to other agricultural
crops in terms of the conversion of solar radiation into to usable forms of
energy-containing liquids and solids. First, the cultivation of algae does not
directly affect the food supply as does the diversion
of corn or soybeans. Second, algae grow by absorbing CO₂, and if converted to a fuel and burned, are CO₂-neutral. 
Third, the production of lipids per acre far exceeds that
of any other agricultural commodity, and thereby lends itself to different
applications than those of the low energy-density cellulosic ethanol derived
from corn and other such conventional crops. For example, the lipids in the
algae can be readily converted to biodiesel or conventional fuel-range
hydrocarbons. Fourth, it may be possible to grow algae with salty and
contaminated water and thereby on land unsuitable for traditional agriculture,
as well as to capture phosphorous from runoff.

The
aforementioned prior investigation was based on the production of 17,500 BPD
(225,000 lbs/hr) of biodiesel – a rate compatible in scale with those of petroleum
refining – on a site in flat, undeveloped land near the small town of
Thompsons, Texas, which is about 25 miles southwest from Houston, 30, 60, and
60 miles northward from three petroleum refineries on the Gulf Coast, and on a
branch of the Atchison, Topeka, and Santa Fe Railway. This area receives
insolation varying from the equivalent of 112 W/m² in January to 250W/m² in June, so the plant can be operated
year-round.  A near-by coal-fired power-plant
that generates 2500 MW of electricity produces sufficient CO₂ in its stack gas to meet that need for
algae cultivation. The prior investigation utilized three modules: the Simgae^TM
Algal Biomass Production System for the cultivation; the OriginOil^TM single-step technology for the
extraction of the algal lipids; and conventional catalytic hydrotreating for the conversion of the lipids into
n-alkanes (green diesel). It was concluded that the process might be
profitable, but that a capital investment of the order of 2.8 billion dollars
would be required – far too great in consideration of the many technical
and fiscal uncertainties. The present analysis is based on that same location
and rate of production of biofuel but examines improved methods of cultivation
and processing.

Our
objective was to identify modifications in the processing that would reduce the
capital investment significantly while improving the technology and maintaining
or decreasing the fixed and variable costs. Improved alternatives were sought
for all three of the modules that characterized the previous investigation. The
first consideration was reduction of the principal factor in the capital cost,
namely the acreage required for the cultivation of the algae. Two improvements
were identified in that respect. The first is the utilization of the
autotrophic-heterotrophic process of cultivation proposed by Miao and Wu in
2006 (Bioresource Technology), which involves a
photosynthetic stage but shifts most of the growth to a fermentation
stage.  In past decades, such a cultivation model has been economically
unfeasible, given that the most widely employed source of organic carbon in algal
fermentation has been glucose, a costly feedstock. However, in 2010, O'Grady (Bioprocess
and Biosystems Engineering) found that crude glycerol could serve as a
low-cost substitute for glucose for the cultivation of some heterotrophic cultures
of algae. In a complementary study, Heredia-Arroyo (Applied Biochemistry and
Biotechnology) found that, for a crude glycerol input of 83 pounds
per 1,000 lbs of algae, the biomass
productivity and cellular lipid contents observed during fermentation were
similar to those for algae grown in a glucose-rich media. At such a
concentration, roughly 35 million pounds of glycerol are required annually to
meet the 17,500 BPD biodiesel production rate. Given that 200 million pounds of
crude glycerol are produced annually as a by-product of the transesterification
process by which biodiesel is chemically synthesized, crude glycerol will
therefore be a readily available feedstock and was proposed as the substrate
for heterotrophic growth.

The
second modification is an increase of the diameter of the tubes through which
the algae flows as a suspension from 6 inches to 12 inches. The improved
rate of cultivation results in a 7.5-fold decrease and the larger diameter
in a roughly 1.5-fold decrease, for an overall 11-fold decrease in the required
area, and thereby in the cost of the land.

The
autotrophic-heterotrophic process of growth is dependent on the use of a
different species of algae, namely Chlorella protothecoides, in place of the Nannochloropsis
used in the prior study. This strain requires the use of fresh rather than
saline water and puts a premium on its recovery and reuse.

A better
understanding of the Single-Step Extraction process developed by OriginOil^TM
for the separation of the lipids from the algae was recently made possible by the
publication of several patents that describe in detail the technologies
involved, particularly quantum fracturing and electromagnetic pulsation. Given
this new information, the equipment required for Single-Step Extraction could be
designed and analyzed in depth, permitting a more accurate economic estimation
of the variable and fixed costs associated with the second module, namely of
6¢/lb compared to 56¢/lb for conventional solvent- or mechanical-extraction.

As an
alternative to the catalytic hydrotreating process
used in the prior analysis, we investigated a lipid-processing module based on
catalytic transesterification. In this process algal
lipids are converted to biodiesel, which consists of fatty acid methyl esters,
as opposed to the n-alkanes that constitute "green" diesel. A
byproduct of this reaction is crude glycerol. A plant to carry out this latter
process was designed. The production of pure biodiesel involves two reactors in
series with interstage glycerol removal, followed by a
separation train that requires vacuum distillation, neutralization and water-washing. This process was simulated using Aspen PLUS^TM
software,  and
the composition of the biodiesel-product stream was found to meet the ASTM D6751
Standards for biodiesel fuel. The results were compared directly with those
published for a similarly sized catalytic hydrotreating
plant and found to be superior, especially when considering the potential
recycle of glycerol for use in the fermentative phase of algal cultivation.

The
combination of these three revised modules results in an overall process that
reduces both the costs and the technical uncertainties, although some
significant uncertainties remain. In particular, pilot-scale tests are
essential to be sure that some unacceptable aspect has not been overlooked or
its impact under-estimated.

One major
uncertainty is the marketability and the price of the 1.9 pounds of dry biomass
produced as a byproduct for every pound of biodiesel. The overall process
appears to be profitable even without selling the biomass, but if not sold it
would then pose a problem of disposal. The production of biomass from the
operation described herein is less than 2% of the national demand for animal
feed, and therefore its sale would not appear to disturb the market. In
addition biomass from algae has a higher protein content than any of the common
agricultural sources of animal feed, and, on that basis, might even
support a premium price.  However, the proof of its acceptability
by animals and the absence of any side-effects
will require tests and therefore some time. There are also other potential
applications for the biomass, for example, in the pharmaceutical and
power-generation industries.

An
economic analysis based on the revised modules described above suggests that at
the current market price of $3.30 per gallon for pure biodiesel fuel, the
process might be profitable, with net earnings of almost $340 million annually.
Even more promising is the reduction in the capital investment to $1.2 billion,
which is almost 60% less than that estimated for  the prior process. The very
tentative economics for the revised process are favorable and translate to a
Return on Investment (ROI) of 32% and an Investor's Rate of Return (IRR) of
35%.