Issues Magazine

Biodiesel from Microalgae

By Kriston Bott and Sasi Nayar

South Australian researchers have adopted a business approach to biofuels. Their model presents microalgae as a promising second-generation biofuel feedstock for biodiesel production, not least because of its additional high-value bioproducts.

In Australia, fossil fuels provide the bulk of the energy being consumed exponentially by our growing population. Recent growth in fossil fuel prices has created increased interest in biofuels to meet growing energy needs. Environ­mental and fuel security issues are additional drivers for the current interest in this type of fuel source.

Biofuels are typically categorised as renewable alternative fuels that are produced from biomass. Biofuels include ethanol, biogases such as hydrogen and methane, bio-oil and biodiesel.

Biodiesel in particular is a clean-burning alternative to fossil diesel fuel. It is highly biodegradable, non-toxic, environmentally friendly and can be cost-competitive. Currently, biodiesel is produced from oils of animal and plant origin including canola, soybean, palm oil, rapeseed oil and non-food oil crops, tallow (animal fat) and used frying oils.

The term biodiesel refers to products produced by the reaction of fatty acids with alcohols in the presence of a catalyst to make fatty acid methyl esters, a process known as transesterification. The by-product of this process is glycerol.

Feedstock prices are the single largest component of biodiesel production costs, so sourcing a reliable supply of quality feedstock at an economical price is essential for sustainable biodiesel production. In addition, large-scale production of biodiesel with existing terrestrial plant feedstocks requires large areas of cropping land that can compete with land-based agriculture traditionally used to grow food. There is thus an urgent need to underpin the economic and sustainable growth of the biodiesel industry through the development of alternative, non-crop, non-food feedstocks for biodiesel production. One such feedstock source that shows particular promise is microalgae.

Microalgae are typically defined as single-celled microscopic plants. Like higher plants, they use the Sun’s energy to produce chemical energy. Microalgae are highly efficient at photosynthesis because they are typically suspended in water, allowing immediate access to essential requirements including nutrients and carbon dioxide. Through photosynthesis, carbon dioxide is fixed to form carbohydrates and these are then converted in the microalgal cell to a variety of complex organic molecules including lipids (such as fats and oils). It is these lipids that can be turned into biodiesel.

Lipids function in microalgal cells as membranes, as storage products, as metabolites and as sources of energy. The total lipid content of microalgae typically ranges from 1% to 40% of dry weight, but in some species it can be as high as 60–70%. The highest fraction of lipids found in microalgal cells is in the form of neutral lipids. Neutral lipids consist of a glycerol molecule joined to one, two or three molecules of a fatty acid. From the perspective of renewable fuel production, these neutral lipids are of the greatest interest because of their prevalence and their similarity with vegetable oils.

Microalgae have been grown in mass culture since the 1940s, and are currently commercially produced for aquaculture and for human consumption. These organisms have a wide range of physiological and biochemical characteristics, many of which are rare or absent in other organisms. To date only a few hundred species have been assessed for their chemical content and only a handful of species have been produced at industrial levels. They are an extraordinarily diverse group of organisms that can grow in waters ranging from freshwater to saltwater.

The capability of microalgae to be cultured in a wide variety of water sources, including saline water, is of critical relevance in a country such as Australia, where there is increasing demand for limited supplies of fresh water from urban and rural users. The microalgae can also utilise nutrients from a wide range of sources, including waste from agricultural and urban activities. Microalgae are therefore potentially a highly productive and environmentally friendly feedstock for biodiesel production.

Microalgae exhibit unique properties that make them well-suited for use in commercial-scale biodiesel production. Many species display rapid growth and high productivity, with some capable of being manipulated to accumulate substantial quantities of lipids, often greater than 60% of their biomass. Photobiological production of fuels from microalgae is believed to be one of the most important avenues towards establishing a significant source of renewable energy supply. While the mechanism of photosynthesis in these organisms is similar to that of higher plants, they are more efficient converters of solar energy due to their simple cellular structure. For these reasons, microalgae have the capacity to produce 30 times the amount of oil per unit of land compared with current terrestrial oilseed crops.

Optimising lipid production by microalgae is essential for the economic viability of the production of biodiesel from microalgae. To develop liquid fuels from algal biomass, it is necessary to understand the effects of environmental parameters on the production of lipids by these organisms.

Critical environmental factors identified as regulators of lipid production are nutrient deficiency, salinity, temperature and light. Although previous experiments have not pinpointed any single environmental parameter to influence oil production, there is a potential for further research to investigate lipid synthesis enhancement in microalgae by manipulating a combination of parameters.

To date, microalgal production systems designed for biodiesel production have not progressed beyond small-scale research systems and a few larger scale trials. Production systems trialled so far for biodiesel production include open pond systems and closed culture systems.

Most of the earlier studies suggest that open pond systems may be the only cost-effective production method due to the low cost requirements associated with biodiesel production. Open pond systems are cheaper to run than closed systems but have so far only been effective for a limited number of species.

Most current commercial-scale microalgal production systems require highly selective environments to minimise contamination by non-target species. In open pond monoculture systems this is usually achieved by maintaining extreme culture environments, such as high salinity, high pH and high nutritional status. Lack of control of light and temperature in an open system often results in low cell densities and system productivity.

Given these drawbacks with open production systems, the necessity to achieve high cell concentrations and the need to maintain a monoculture of microalgae that grow under non-extreme culture conditions, closed culture systems in the form of enclosed photobioreactors have been developed. The main issue with commercial production of microalgae in closed systems is the high capital and operating costs associated with this type of production, so they may be more suited to the production of high-value products, especially for biotechnological applications. Heterotrophic bioreactors could be an option for the production of high-quality biodiesel alongside high-value nutraceuticals from microalgal lipids, but the economics of large-scale production of low-value, biodiesel-only feedstock may prove not to be feasible.

Given the costs and benefits associated with the two types of systems, the ideal approach will be to use the beneficial aspects of the two systems to make algal biomass production technically and economically viable. Such a system utilises closed photobioreactors for the inoculum and open ponds for the grow-out phase to give the mass culture a competitive edge over contaminant species, while at the same time bringing the cost of production down. Such a “hybrid system” has been used effectively by commercial nutraceutical companies in the United States.

Higher costs and efficiencies associated with the harvest and extraction of lipids from microalgae is the bottleneck associated with the mass production of microalgae. The ability to harvest microalgae depends largely on the organism’s size, because this determines its ability to settle and be filtered. Solvent extraction is one of the most effective separation methods. However, unless the solvent can be recovered, this process may not be feasible for the commercial production of biodiesel. A key factor to consider in bringing down the cost of production of biodiesel from microalgae is the recovery of other high-value co-products and by-products during the production process.

To meet the longer-term and larger-scale feedstock requirements, the Algal Production Group of the South Australian Research and Development Institute has focused on bioprospecting and selecting native high-lipid microalgal strains with good growth rates as feedstock for biodiesel production.

We have formed a research partnership with the Biofuels and Renewable Energy Centre of Flinders University to enable commercially competitive and sustainable production of biofuels from microalgae. The principle focus of this research is a “biorefinery-based approach” that integrates bioprocessing and chemical processing to produce high-value bioproducts and chemicals concurrently with biodiesel production.

This approach will diversify the revenue stream to provide a sustainable business model. This is expected to resolve microalgal feedstock production cost issues and promote profitability from innovative value-adding technologies, resulting in new biochemical industries and a vibrant biodiesel industry.