7.1 BIODIESEL FROM MICROALGAE AND MICROBES

7.1.1 Marine and Aquatic Biotechnology

In mid-2007, a review in the journal Biotechnology Advances concluded that micro­algae appeared to be the only source of renewable biodiesel capable of meeting the global demand for petrodiesel transportation fuels.1 The main argument was that the oil productivity of selected microalgae greatly exceeds that of the best seed oil — producing terrestrial plants; although both life forms utilize sunlight as their ulti­mate energy source, microalgae do so far more efficiently than do crop plants. By then, three U. S. companies were developing commercial bioreactor technologies to produce biodiesels from “oilgae” (as the producing species have been termed): Greenfuel Technologies (Cambridge, Massachusetts), Solix Biofuels (Fort Collins, Colorado), and PetroSun (Scottsdale, Arkansas) via its subsidiary Algae Biofuels.

This is a very different scenario from that detailed in the 1998 close-out report on nearly decades of research funded by the U. S. DOE.2 This program had set out to investigate the production of biodiesel from high-lipid algae grown in ponds and utilizing waste CO2 from coal-fired power plants. The main achievements of the research were the following:

1. The establishment of a collection of 300 species (mostly green algae and diatoms), housed in Hawaii, that accumulated high levels of oils; some spe­cies were capable of growth under extreme conditions of temperature, pH, and salinity.

2. A much greater understanding of the physiology and biochemistry of intracellular oil accumulation — in particular, the complex relationships among nutrient starvation, cell growth rate, oil content, and overall oil productivity.

3. Significant advances in the molecular biology and genetics of algae,[62] including the first isolation from a photosynthetic organism of the gene encoding acetyl-CoA carboxylase, the first committed step in fatty acid biosynthesis.3

4. The development of large-surface-area (1000 m2) pond systems capable of utilization of 90% of the injected CO2.

Although algal production routes had the enormous advantage of not encroaching on arable land or other agricultural resources for food crops, the perceived problem in the 1998 report was the high cost of algal biodiesel relative to conventional automo­tive fuels, up to $69/barrel in 1996 prices; the higher the biological productivity, the lower the production costs, whereas using flue gas was more economical than buying CO2 supplies (figure 7.1). With crude oil prices then being $20/barrel or less, such production costs were disappointingly high (figure 1.3). Post 2000, oil prices three to four times higher allow a very different interpretation of the algal biodiesel option (figure 1.11).

The open-pond technology was not only the simplest but also the cheapest pro­duction choice. Closed-system production offered far more controllable growth envi­ronments for the algae, but the cost of even the simplest tubular photobioreactors were projected to have 10 times higher capital costs than open-pond designs. In addition, open-pond cultures had been commercialized for high-value algal chemical products— and any attempt at large-scale (>1 ton/year) closed-production systems had failed.2 Choices of location and species had dramatically increased productivity during the lifetime of the program, from 50 to 300 tonnes/hectare/year, close to the calculated theoretical maximum for solar energy conversion (10%). The report concluded, there­fore, that microalgal fuel production was not limited by engineering issues but by cultivation factors, including species control in large outdoor environments, harvest­ing methods, and overall lipid productivity. Encouragingly, the potential supply of industrial waste sources of CO2 in the United States by 2010 was estimated to be as high as 2.25 x 106 tonnes/year, with Fischer-Tropsch conversion plants from fossil

fuels (chapter 6, section 6.2) and gasification/combined cycle power facilities offering the largest amounts of CO2 at low cost prices (figure 7.2).

The era of high oil prices, interest in and research effort into algal sources of oils for biodiesel production has become more globally distributed. Typical of this recent change in scientific and technical priorities has been Chinese studies of Chlorella protothecoides — but with the cells grown heterotrophically[63] (using chemical nutrients) rather than photosynthetically with a source of CO2:

• Heterotrophically grown cells contained 57.9% oil, more than three times higher than in autotrophic (photosynthetically CO2-fixing) cells; chemical pyrolysis yielded an oil with a lower oxygen content, a higher heating value, a lower density, and a lower viscosity than autotrophic cell bio-oil.4

• With a corn powder hydrolysate as carbon source (rather than glucose), a high cell concentration could be achieved; the oil (55.2%) could be efficiently extracted with hexane as a solvent and converted to biodiesel by transesterification with an acid catalyst.5

• Optimization of the transesterification defined a temperature of 30°C and a methanol:oil molar ratio of 56:1, resulting in a process time of 4 hours.6

• The process could be upscaled from 5 to 11,000 l, maintaining the lipid content; hexane-extracted oil could be transformed to methyl esters using an immobilized lipase and with a transesterification efficiency of more than 98% within 12 hours.7

As a very fast-growing “crop” (in comparison with terrestrial species), microalgae can also be viewed (even if grown autotrophically in external environments) as a lignin-less biomass source, potentially capable of being used as the substrate for ethanol production as well as biodiesel.8