by Samuel Morton III and Mark Crocker, Center for Applied Energy Research -
University of Kentucky, USA


INTRODUCTION
Low-cost, high capacity processes for the conversion of biomass into fuels and chemicals are essential for expanding the utilization of carbon neutral processes and reducing dependency on fossil fuel resources. In this context, algal biomass exhibits many characteristics that make it a promising source of renewable energy. Algae are the fastest growing photosynthesizing organisms, possessing growth rates which are far superior to conventional crops (36 hour doubling times are feasible for select algae). Furthermore, as shown in Table 1, algae have higher oil content per mass than other sources of biomass, with some species having been shown to consist of over 50% oil by mass.
This coupling of fast growth rate and high oil content renders algae a potentially ideal source of bio-derived oil. Estimates of the potential for commercial bio-oil production from algae vary, with some as high as 140,000 liter/hectare/ year; however, these estimates are extrapolations based on small scale laboratory data and as such are largely unreliable. That said, algae have potential oil production rates which are at least an order of magnitude higher than terrestrial biomass.
Algae have additional benefits that potentially make them even more attractive for oil production than other fuel oil crops. The primary nutrient required for the growth of microalgae, besides sunlight, is CO2. In principle, CO2 is readily available from the flue-gas of power plants, in which case the waste CO2 is essentially recycled into oil and residual algal biomass. In effect, this would provide power plants with a means of reducing their CO2 emissions. Indeed, a recent study (Doucha, 2005) employing flue-gas from a natural gas fired boiler suggests that up to 50% CO2 removal can be attained. Another recent study has placed this figure as high as 82% for sunny days, with 50% CO2 removal on cloudy days (Vunjak- Novakovic, 2005).

LARGE-SCALE CULTIVATION OF ALGAE
Although algae possess considerable promise for the production of biofuels and utilization of waste CO2, there are significant difficulties and limitations which must be overcome before large scale algal biomass production can be realized. These limitations are not con- fined simply to algae cultivation.
Any future industrial scale production process can be subdivided into at least four main stages (see Figure 2), comprising: (1) algae cultivation, (2) algae harvesting, (3) oil extraction, and (4) biofuel production. For each step, a range of technical, economic, and environmental improvements are required prior to large-scale implementation.

The majority of these limitations relate to the economic costs associated with the inefficient utilization of energy and nutrients. Of these stages, algae harvesting and oil extraction are the least developed; further, both have a wide range of process options, each with unique process impacts and technical hurdles.
The mass cultivation of algae can occur in either an open culture system (pond) or a closed loop system photobioreactor (PBR). The selection of an open or closed culture system revolves around a number of critical system parameters. These parameters include (1) the microalgae to be cultured, (2) the anticipated carbon source, (3) the accessibility to required resources, and most critically (4) the cost of construction, operation, and maintenance of the algae culture system. There are a number of reviews in the scientific literature that provide an excellent discussion of the primary and secondary types of culture systems that have been proposed and/or investigated over the last 75 years. The earliest of these reviews focus on work that occurred during the time period surrounding the Second World War, primarily for the purpose of food production.

Organism Selection
Nearly all algae are considered members of the Kingdom Protista (cyanobacteria are the exception, belonging to the Kingdom Monera) and as such have many of the properties of bacteria (cellular nature and structure) and plants (cell wall thickness and photosynthetic abilities).
Algae are primarily divided according to significant common physical characteristics: color - rhodophyta (red algae), chlorophyta (green algae) - cellular/extracellular structures, extreme environmental tolerance, and growth behavior (colony forming, mobile). Large volumes of work exist focused on a wide range of organisms, culture conditions, and manipulations of the culture process to induce a desired outcome (be that growth, product formation, biomass type, etcetera).
Table 2 provides a list of commonly discussed microalgae and the approximate distribution of proteins, carbohydrates, and lipids for each species.

Carbon Dioxide Source
A second area of technical concern for the large scale culture of algae is the carbon dioxide source. There are essentially two sources of carbon dioxide available: (1) atmospheric carbon dioxide (at a current level of nearly 390 ppm) or (2) carbon dioxide from a large volume point source such as a fossil fuel power plant (up to 14% by volume). The primary hurdle in introducing carbon dioxide to an aqueous culture system is the low carbon dioxide solubility in water at temperatures compatible with optimum algae growth. There are essentially three methods of introducing carbon dioxide to the culture system: (1) natural diffusion of the gas into the liquid, (2) forced introduction of the gas via sparging or bubbling, and (3) presaturation of the liquid with carbon dioxide using a low pressure mixing device.
There are a number of operational concerns with each of the above methods; however, they all share one significant hurdle in the tremendous volumes of gas that must be processed and the vast quantity of water required to contain the carbon dioxide at the low solubilities expected. The advantages of one method over the other are connected to the algae culture system selected (sparging and natural diffusion are ideal for open ponds, while bubbling and presaturation are ideal for PBRs).

Primary Resources Required for Growth
As with any living organism, algae have evolved over time to utilize environmentally present nutrients to maximize growth, cellular durability, and prolificacy. Owning to the plant-like nature of algae, there is the requirement for a range of elements to be present and accessible in the growth environment of a particular algae cell.
Regardless of the individual organism and media developed relative to its growth, a few basic elements are needed: carbon, nitrogen, phosphorous, sulfur, magnesium, and iron. Carbon is required as a fundamental building block of cellular material and is acceptable in a range of forms (carbon dioxide, carbonate, acetic acid, and sugars).
Nitrogen is another of the required fundamental building blocks of life. Nitrogen limitation has been shown to increase the production of lipids at the expense of protein formation and growth rate. As with carbon, nitrogen can be utilized from a range of different sources (nitrates, nitrites, ammonia, and urea). Phosphorous is another required building block and is incorporated into the synthesis of nucleic acids and/or is used to facilitate
energy transfer activities. The primary source of phosphorus used by algae is in the form of phosphate ions. As with nitrogen depravation, phosphorous limitation also results in a general increase in lipid production and a decrease in other cellular components, as well as growth rate. Sulfur is also required for cellular viability. It is utilized in the synthesis of certain essential amino acids and is normally supplied in the form of an inorganic sulfate. Magnesium is critical for algae as it is the central atom in the chlorophyll module used for the conversion of light to energy via photosynthesis. Iron is also a critical element for the efficient growth of algae. Iron serves several purposes in the cell, such as nitrogen assimilation, facilitating metabolism, and the synthesis of chlorophyll.
It has been hypothesized that the reason the oceans are not prolific in surface algae is due iron deficiency. In addition to these critical macronutrients, a range of trace elements is required and must be incorporated cautiously so as to avoid toxicity effects on the organism to be cultured. These required elements are the same as required for modern agriculture and the availability of a range of sources is assumed to exist in most parts of the world. Additionally, these elements can be derived from other non fertilizer sources such as sea water, agricultural streams, animal wastes, and other wastewaters.

Cultivation Scheme
The most common type of algae culture system is the open outdoor pond scheme. This is an obvious choice in that it most closely resembles the natural evolutionary environment for most microalgae. Despite the simplicity of the open pond concept and the maturity of the technology, significant energy and resources have been expended in the development of closed-loop outdoor PBRs. Companies such as Greenshift Corporation, Phycal, Seambiotic, Sapphire Energy, Solazyme, and Solix Biofuels are highly publicized recent actors in the development of production scale systems for algae cultivation in the United States, though the methods and utilization strategies for the biomass are varied.
A comparative list of selected performance parameters for generic open pond and closed-loop PBR systems is shown in Table 3. A pair of critical parameters, scale-up and cost, are highly dependent on the overall process purpose and the specific technology (and materials) utilized, and as such are very difficult to reliably quantify.

It is these two parameters that have proven to be significant hurdles to the successful construction and operation of large scale algae production facilities. To date, there are few examples of successful PBR systems that have been able to overcome process design problems and product value fluctuations. With few exceptions, only the open pond systems have been successfully scaled to large commercial ventures from which reliable long term performance data can be determined.

ALGAE HARVESTING
The isolation of algae from their culture medium is challenging for two main reasons: (i) their small size (typically 3-10 microns), and (ii) the low concentrations in which they can be grown (<5 g algae/L water). A compounding problem is the sensitivity of algal cell walls in many species to damage in high shear processes (e.g. centrifuging), which can result in leaching of the cell contents. To date, three main methods have been developed for their isolation, comprising filtration, centrifuging and flotation.
Filtration is normally performed using a cellulose membrane, a vacuum being applied in order to draw the liquid through the filter. Although this method is simple, the membrane tends to become clogged, rendering the process extremely time consuming. Centrifuging, in a continuous or semi-continuous process, appears to be more efficient in this regard, however, it is extremely energy intensive and cannot readily be scaled up to very large applications.
The third option, flotation, utilizes a bubble column. Gas is bubbled through the algal suspension, creating a froth of algae which can be skimmed off. A fourth approach, based on flocculation and sedimentation of the algae, has to date been little studied, although it would seem to offer a promising, low cost approach to algae harvesting.
The extent to which the water content of the resulting algae paste must be reduced depends largely on the method used for the subsequent oil extraction step. Ideally, drying to ca. 50% water content is required in order to produce a solid material that can be easily handled. Given the fact that algae paste, as obtained by centrifuging or filtering, typically consists of ca. 90% water, drying algae is an energy intensive proposition. Solar drying appears to be the main approach that has been considered to date. Solar drying is used commercially for drying grains and timber, and is inherently inexpensive; however, drying large quantities of algae would necessitate the use of a considerable area of land.

OIL EXTRACTION
Oil extraction from algae is a much debated topic - several methods exist and each has its advantages and drawbacks. The three main methods applied to date are (i) expeller/press, (ii) solvent extraction, and (iii) supercritical fluid extraction. The expeller/press method, while being simple, requires dried algae and typically recovers ca. 70-75% of the oil. In contrast, solvent extraction is more complex but is able to recover nearly all the oil (>95%). If wet algae is used then a water miscible co-solvent is necessary; this co-solvent is typically also required in order to lyse the cells (i.e. open the cells to expose their contents), although other methods are available to do this, such as sonication or acidification. Finally, supercritical fluid extraction uses supercritical CO2 as the extraction solvent; while this method is able to recover almost 100% of the oil, it requires high pressure equipment.
In most scenarios, the solid recovered from the oil extraction process (algae cake) will be used as animal feed or as a feedstock for fermentation to ethanol, or anaerobic digestion to biogas. The ability to use the cake as feed will depend on its nutritional content and, in the case that coal-derived flue gas is used as the CO2 source for algae cultivation, whether there is any contamination by heavy metals (Hg, As, etc.).

BIOFUEL PRODUCTION
The production of biofuels from algae oil is another area in which there are several process options with no clear winner at present. Currently, there are three commercial routes for the production of liquid transportation fuels from vegetable oils and animal fats, and - by extension - from algal oils: (1) transesterification to biodiesel; (2) thermal cracking to gasoline; and (3) hydroprocessing to diesel and jet fuel. Of these, biodiesel (fatty acid methyl esters, FAME) is commercially the most important. However, while biodiesel has good lubricity and a high cetane number, it suffers from drawbacks such as poor storage stability, low energy content (roughly 8% lower than pure hydrocarbon diesel), unfavorable cold flow properties and engine compatibility issues (leading some engine manufacturers to void warranties on new vehicles using high levels of biodiesel). Consequently, there is interest in the conversion of algal and vegetable oils and animal fats to hydrocarbon fuels as an alternative to biodiesel.
Thermal cracking of fats and vegetable oils for the production of liquid fuels has a long history, particularly in those areas of the world that lack petroleum deposits.
Acid- and base-catalyzed cracking has also been investigated for this purpose. However, these processes are generally rather unselective, affording a wide variety of products (including light products which are typically of low value), with the yield of the desired gasoline fraction being not more than 45%.
A more attractive option exists in the form of hydroprocessing, which employs conditions similar to conventional hydrotreating for the removal of oxygen from the constituent triglycerides. Recent studies have shown that oxygen extraction can proceed via a number of parallel pathways which include hydrodeoxygenation (–H2O), decarboxylation (–CO2) and decarbonylation (–CO) reactions; of these, hydrodeoxygenation represents the dominant pathway. Based on this approach, several companies (UOP, Neste, Dynamic Fuels) have recently developed commercial processes for the production of paraffinic fuels from vegetable oils and animal fats. Given that these fuels closely resemble petroleum diesel in their properties and can be essentially used as a drop-in replacement, hydroprocessing is likely to be an important technology for future conversion of algal oils to transportation fuels.

OUTLOOK
The previous sections have provided a general review of the technical issues related to the production of algal biomass as an alternative energy source. It should be evident from this discussion (as well as others in the literature) that significant issues remain before the effective commercial implementation of such a process can proceed.
To this end, the United States government has recently promoted a range of research efforts to address these concerns and to bring commercial demonstrations of successful technologies to fruition. Two ongoing efforts to provide a roadmap for algae production technologies should be released in the middle to end of 2009.
The first, Algae Biofuels & Carbon Recycling (draft available online) was spontaneously conceived and produced by a consortium of active algae research groups led by Utah State University. The second effort, National Algal Biofuels Technology Roadmap (draft available online), is federally funded and being directed by DOE’s Energy Efficiency and Renewable Energy (EERE) Biomass Program. These efforts should have significant impacts on the long term outlook of large-scale microalgae production.