The potential of farmed algae as a biofuel has been recognized for some time because 1) its cultivation does not encroach on the food sector, 2) its productivity of lipids per acre far exceeds that of any other agricultural commodity, 3) the lipids can be converted catalytically to biodiesel or conventional fuel-range hydrocarbons, and 4) it grows by absorbing CO2, and if converted to a fuel and burned, is CO2-neutral.
The commercial cultivation and harvesting of algae, and their processing to produce hydrocarbons are drawing renewed attention because of several technological developments, including the identification of particular strains that are more rapid-growing, the identification of particular strains that contain a higher concentration of lipids, a new method of separation of the lipids from the raw algae, and new schemes of cultivation that allow continuous processing. The objective of this study has been 1) to formulate a preliminary integrated design for a commercial-scale process that takes advantage of these recent developments, and 2) to evaluate its long-term economic feasibility.
The conversion of algae to useful biofuels by virtue of these new developments and concepts can be thought of as occurring in three modules: 1) cultivation, 2) lipid extraction, and 3) lipid conversion to alkanes. For each of the three modules, there are a variety of approaches with widely varying claims as to cost effectiveness. We have attempted to evaluate the several methodologies for each module separately but at the same time to formulate an integrated process. As a basis for our analysis we chose the production of 20, 000 BPD (226,000 lbs/hr) of alkanes.
Location. The selection of a site proves to be a critical factor because of the need for 1) isolation from a population center, 2) a large relatively flat area with a sufficient and relatively constant insolation, 3) a moderate year-around temperature, 4) a nearby coal-fired power plant whose stack gas contains a sufficient quantity of CO2, 5) a sufficient source of saline water, 6) a nearby petroleum refinery as a market for the alkanes and as a source of hydrogen, and 7) a means of transportation of the lipids or alkanes thereto. Sites in Arizona and California that might meet these requirements were considered, but East Texas seemed to be more promising overall. We chose an undeveloped area adjacent to Thompson, Texas, a city in Fort Bend County with a population of 236, as the location of the projected algae farm. Thompson, which is about 25 miles southwest from Houston and 30, 60, and 60 miles northward from three petroleum refineries on the Gulf Coast, 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. The W.A. Parish Electric Generating Station in Thompson generates 2475 MW and produces sufficient CO2 in its stack gas to meet the needs for algae cultivation.
Cultivation. We identified three promising approaches for cultivation. The first utilizes open-air “raceway” ponds. They are hampered by contamination and low growth rates. The second utilizes a compact photo-bioreactor with cultivation of the algae inside transparent tubes with complex internals designed to optimize the rate of growth. This methodology requires accounting for the concentration of carbon dioxide, the exposure to light, the rate of mass transfer from the gas-phase to the liquid phase, and the possible use of nitrogen starvation to maximize lipid production at the expense of proteins. The third methodology is a hybrid one that utilizes a simple infrastructure modeled after agricultural practices combined with the ability to control certain parameters of the environment by means of cultivating the algae inside polyethylene tubes. We chose the latter and utilized the SimgaeTM process developed by the Diversified Energy Corporation as a prototype. As an algae strain we chose Nannochloropsis sp., a marine species with a lipid content of 31- 68% (dry wt.) and other favorable qualities.
A residence time of nearly 48 hours on average is required to double the amount of algae under the chosen conditions, which utilize a 50% recycle. The corresponding length of the 6-in tubes is 1250 ft with a flow rate of 0.023ft/s. Each field consists of 100 reactor beds, each with 16 tubes, and, together with manifolding and paths, occupies 33 acres. 1120 fields totaling 36,960 acres were required for the chosen rate of production of algae. The estimated value of the land is $37,000 per acre or $1,370,000,000 in all.
Nutrients in suspension and solution in the saline water are combined with the recycled algae at the inlet. The stack gas pumped from the power plant by hydraulic induction is mixed with air to reduce its CO¬2 ¬content from 32 to 6 wt%, and is introduced every 300 ft along the tubing. The traces of SO2 ¬ and NOx in the stack gas actually enhance the growth of the algae. The O2 produced by the growth of the algae is released by relief valves every 300 ft.
Lipid extraction. Historically, the desirable triglyceride fraction has been separated from the proteins, carbohydrates, phospholipids, nucleic acids, and water that make up the rest of raw algae by extraction with hexane. A new process called Quantum FracturingTM by OriginOil, and for which a patent has been applied for but not yet issued, is claimed, by a combination of pH modification, microwaves, and ultrasonic pulses, to rupture the wall of the algae cells while they are still in slurry form, and allow the lipids to escape, resulting in a 90% reduction in cost. The resulting slurry can then be separated by gravitational settling into a lipid layer, an aqueous layer, and a wet layer of biomass. The lipids are transported to a petroleum refinery for conversion into alkanes, the aqueous fluid is recycled, and the biomass is centrifuged and then drum-dried to reduce the residual water below 10 wt%. It may find a nearby market for livestock feed or other uses.
Lipid processing. Because the catalytic hydrotreating process chosen to convert the lipids to alkanes is similar to those employed in hydrocarbon processing and requires a source of hydrogen, it was decided to carry out this step at a petroleum refinery rather than at the site of cultivation and extraction. Thus either the lipids or the alkanes can be sold to the refinery The most feasible form of transport to the refinery is by rail in tank cars. Since railway shipments are intermittent while the lipid production and processing are continuous, it is proposed to provide for seven days of storage in tanks at the site of production and for two days of storage in tanks at the refinery.
The lipids react with hydrogen at 350 o C and 50 bars in downward flow through packed beds of granules impregnated with a NiMo catalyst. In the first step the triglyceride is hydrogenated and broken down into C3 hydrocarbons and components of free fatty acid. These intermediates are then converted into straight-chain alkanes by decarboxylation, decarbonylation, decarbonylation, hydrogenation, and the water-gas shift. The product is separated by means of a high-temperature flash drum, a low-temperature, three-phase flash drum, a steam-stripper, and an amine scrubber with intermediate heat exchange. The alkane product from this process is comparable to petroleum-based diesel fuels. Thus it can be readily incorporated into existing energy infrastructure as a diesel blending-stock or as a feedstock for other processing units in the refinery.
Conclusions. It appears to be technically feasible to produce alkanes from algae by means of a process such as the one described herein. On the other hand the economic feasibility of such a process is difficult to estimate with any degree of certainty and is not promising. The cost of the land and facilities for cultivation requires a huge capital investment and does not appear to be reducible. The Quantum Fracturing process of OriginOil has great promise but is unproven in a commercial sense, and the costs of licensing are unknown. The nutrients required for the growth of the algae are not currently available commercially on the required scale, and hence their ultimate cost is very difficult to estimate. The extent of the market and the price to be obtained for the large quantity of biomass which is produced as a byproduct is also uncertain. It is concluded that although this process is not profitable in the USA under present conditions it could become so, and should be kept on the shelf of possible alternative sources of energy.
Key Words. Algae, lipids, alternative energy, biotechnology