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%.