The last major engineering-related activity carried out by the ASP was a PETC-funded study, both laboratory and process design, for microalgae biodiesel production using power plant flue gases. The general arguments for use of microalgae in CO2 mitigation, from high productivity to relatively low water use were reviewed in several reports (Brown et al. 1991; Chelf et al. 1991). The authors concluded that the SOx and NOx impurities in flue gas would likely not have a major effect on algal cultures, and, indeed, that the NOx could not even provide the N requirements of the cultures. Prior work indicating an inhibitory effect of flue gases (for example Negoro et al. 1991) was probably due to the acidification of the media and resulting pH drop, because of excessive flue gas transfer.

To demonstrate that flue gases can be used for microalgae culture, NREL set up an experimental apparatus to supply controlled and measured amounts of such gases to the algal cultures. Figure III. D.1. shows a typical result, with no detectable difference between the simulated flue gas culture and the control gas (similar CO2 levels, but without SOx and NOx). With almost 1,000 times more CO2 than SOx in flue gas, alkalinity neutralization would not be a major problem except where high water reuse and low alkalinities in the water coincide. In general, flue gas supply to algal cultures should not present a major problem.

Aside from the experimental work, the ASP also carried out work on systems design and analysis for microalgae biodiesel production using power plant flue gases (Kadam 1994, 1995). The analysis was based on the production of essentially pure (liquified) CO2 from the flue gases of a 500-MW power plant, using conventional amine scrubbing processes, and its supply to a 100-km remote microalgae production facility. Delivered costs were estimated at $40.5/mt CO2. The microalgae pond system design was based on the prior effort of Neenan et al. (1986; see Section III. D.6.) and included site selection criteria for specific power plants in New Mexico. The summary of the system model inputs and outputs are provided in Table III. D.8., which summarizes current and long-term projections for such a process. Although there are some significant differences between this and other cost analyses (prior sections), overall these results agree that through long-term productivity increases such processes could achieve CO2 mitigation costs competitive with other options. This places the focus on long-term efforts for productivity enhancements.

I Publications:

Brown, L. M.; Zeiler, K. G. (1993) “Aquatic biomass and carbon dioxide trapping.” Energy Conv. Mgmt. 34:1005-1013.

Chelf, P.; Brown, L. M.; Wyman, C. E. (1991) “Aquatic biomass resources and carbon dioxide trapping.” Biomass and Bioenergy 4:175-183.

Kadam, K. L. (1994) “Bioutilization of coal combustion gases.” Draft Milestone Completion Report, Recovery & Delivery, National Renewable Energy Laboratory, Golden, Colorado.

Kadam, K. L. (1995) “Power plant flue gas as a source of CO2 for microalgae cultivation: technology and economics of CO2 recovery & delivery.” Draft Report, National Renewable Energy Laboratory, Golden, Colorado.

Kadam, K. L. (1997) “Power plant flue gas as a source of CO2 for microalgae cultivation: economic impact of different process options.” Energ. Convers. Mgmt. 38:S505-S510.

Zeiler, K. G.; Kadam, K. L. (1994) “Biological trapping of carbon dioxide.” Draft Milestone Completion Report, National Renewable Energy Laboratory, Golden, Colorado.

Table III. D.8. Summary of costs for microalgae CO2 mitigation.

‘COj recovery cost = $40/t ZC02 credit = $50Л CO2

3Based on credit at the following rate: lipid «= $240/t, protein = $120/t, carbohydrate = $ 120/t

III. D.10. Conclusions.

The cost analyses for large-scale microalgae production for fuels reviewed earlier evolved from the rather superficial analysis of the 1970s to the much more detailed and sophisticated studies during the 1980s, with some updates and advances during the present decade. The basic process did not change significantly from the conceptual designs first suggested by Oswald and Golueke (1960): very large open, shallow, unlined, mixed, raceway ponds. However, the design details have evolved significantly, and current engineering and cost analyses are much more realistic.

There are, of course, still some major uncertainties with these engineering studies. For the fundamental raceway design these are the issues of scale and the need for some type of pond lining. Current commercial microalgae production ponds are typically 0.25-0.5-ha in size, and are lined with plastics to prevent percolation and silt suspension and to allow pond cleaning. However, there are also examples of much larger and unlined raceway ponds in a commercial production facility, specifically the Earthrise Farms Spirulina plant in southern California, where two large (approximately 5-ha) unlined ponds are currently operating. Similar systems are also used in wastewater treatment. The City of Hollister wastewater treatment plant includes a single 7-ha raceway unlined pond mixed with an Archimedes screw. Even larger (>50 ha) unlined, unmixed ponds are also used for microalgae production in Australia for commercial production of Dunaliella, and in several countries in wastewater treatment. Thus, although some uncertainties remain (such as allowable channel width and wind fetch effects), in general the basic engineering designs and assumptions for the microalgae cultivation ponds appear well established.

For the harvesting, fuel processing, and media/nutrient recycling subsystem designs the cost analyses are perhaps less robust, based on often untested assumptions. However, overall, none of these appear to provide a likely major show stopper. Still, most of these issues require more R&D. One area where little work has been done is in the extraction of the algal oils. Although in the most recent studies the use of large three-phase centrifuges was recommended (Benemann and Oswald 1996), this requires further study.

Although no single design component or unit process in these engineering analyses has an overwhelming effect on costs, the cost projections are optimistic; therefore, there is relatively little scope for any further cost reductions. In most cases, engineering designs and specifications were based on the cheapest possible design and likely lowest costs. Also, the engineering design and system construction approaches were based on agricultural engineering practices, rather than those of chemical engineering, as agricultural materials and construction methods are more applicable, in addition to being of lower cost.

A major conclusion from the cost analyses is that there is little prospect for any alternative designs for microalgae production systems that would be able to meet the requirements of microalgae production for fuels. This is particularly true of closed photobioreactors, in which the culture is entirely enclosed, in greenhouses, plastic tubes or bags, or other transparent enclosures. The costs of even the simplest such system would likely be well above what is affordable for

fuel production processes. Even the simplest plastic sheeting cover over the ponds would much more than double total systems capital and operating costs. The simplest tubular photobioreactors are projected to have capital costs some ten times higher (e. g., $50/m2) than open pond designs (Benemann 1998). And, despite many proponents of such closed photobioreactors, current commercial microalgae production systems use exclusively open pond cultures, even for very high-value microalgae products. The few attempts at large-scale (> 1 t/yr) production of microalgae in closed systems have failed.

Of course, closed photobioreactors could have benefits in areas such as better control over environmental conditions (pH, temperature) and biological contaminants, and higher cell concentrations, reducing liquid handling and harvesting costs. Thus, it would be theoretically possible to grow algal strains not able to dominate in open ponds, at higher productivities and reduced harvesting costs, thereby making up for the higher costs of closed photobioreactors (which proponents assume to be only marginally higher than open pond systems). Closed systems of various types may find important applications in the production of the “starter culture” or inoculum that will be required to initiate and maintain large-scale open pond operations. This could be particularly important when genetically improved or genetically engineered algal strains are used.

At the other engineering design extreme are the very large (up to 100 ha) unmixed ponds used in the production of Dunaliella in Australia (and, until recently, also used for Spirulina production in Mexico). Such production processes are of even lower cost than the mixed raceway designs. However, due to hydraulic and CO2 supply limitations (among others), productivities are maximally only a few g/m2/d, a small fraction of those required for microalgae fuels production. Thus, there seem to be few practical choices in the basic engineering design of a raceway pond system. Even the mixing options are restricted; paddle wheels are overall more economical, flexible, and suitable than the alternatives (e. g., Archimedes screws, recirculation pumps, or air­lifts).

However, the most important issues raised in these economic and engineering analyses are not the engineering designs, or even the cost estimates, but the biological assumptions on which such designs are based. These have changed dramatically during the past 2 decades in one major aspect: productivity. Productivity projections have escalated from less than 50 mt/ha/y in the initial studies (e. g., Benemann et al. 1977), to almost 300 mt/ha/y (on an equivalent heat of combustion basis) in the most recent extrapolations (Benemann et al. 1993). In terms of photosynthetic efficiency, these improvements are from about 2% total solar energy conversion to a near-theoretical 10% efficiency. This dramatic increase in projected productivities was based on two main factors: first the significant advances in the state-of-the-art during these 2 decades, with significantly higher productivities than originally anticipated being measured in outdoor systems. And second, the clear necessity to achieve very high efficiencies for any sunlight-to-fuels process. Although there are theoretical, and practical, approaches to achieving such high efficiencies, they will without a doubt require relatively long-term R&D efforts (see Section IV. A.2.).

Productivity, in terms of solar conversion efficiency, is only one of the objectives of future R&D in this field. A related issue is that much of this productivity must be in the form of algal lipids, suitable for utilization and upgrading to fuels. Although some progress was made in this area in the laboratory, through physiological and genetic means (see Section III. B.5.d.; also Section II), this still will require considerable research. Another area that will require significant research is the development of a low-cost harvesting process. Again, the engineering and economic realities constrict the choices to the lowest-cost option, which would appear to be a simple settling process, followed by further mechanical concentration and processing.

The major conclusion of these analyses is that microalgae production for fuels is currently not limited by engineering designs, but by the many microalgae cultivation issues, from species control in large outdoor systems to harvesting and lipid accumulation to overall productivity. Future R&D must focus on these biological issues as a primary research objective, in the quest for low-cost production processes.