The previous sections have alluded to a number of potential fuel products from algae.

The ASP considered three main options for fuel production:

• Production of methane gas via biological or thermal gasification.

• Production of ethanol via fermentation

Production of biodiesel

A fourth option is the direct combustion of the algal biomass for production of steam or electricity. Because the Office of Fuels Development has a mandate to work on transportation fuels, the ASP did not focus much attention on direct combustion. The concept of algal biomass as a fuel extender in coal-fired power plants was evaluated under a separate program funded by DOE’s Office of Fossil Fuels. The Japanese have been the most aggressive in pursuing this application. They have sponsored demonstrations of algae production and use at a Japanese power plant.

Algal biomass contains three main components:

• Carbohydrates

• Protein

• Natural Oils

The economics of fuel production from algae (or from any biomass, for that matter) demands that we utilize all the biomass as efficiently as possible. To achieve this, the three fuel production options listed previously can be used in a number of combinations. The most simplistic approach is to produce methane gas, since the both the biological and thermal processes involved are not very sensitive to what form the biomass is in. Gasification is a somewhat brute force technology in the sense that it involves the breakdown of any form of organic carbon into methane. Ethanol production, by contrast, is most effective for conversion of the carbohydrate fraction. Biodiesel production applies exclusively to the natural oil fraction. Some combination of all three components can also be utilized as an animal feed. Process design models developed under the program considered a combination of animal feed production, biological gasification and biodiesel production.

The main product of interest in the ASP was biodiesel. In its most general sense, biodiesel is any biomass-derived diesel fuel substitute. Today, biodiesel has come to mean a very specific chemical modification of natural oils. Oilseed crops such as rapeseed (in Europe) and soybean oil (in the U. S.) have been extensively evaluated as sources of biodiesel. Biodiesel made from rapeseed oil is now a substantial commercial enterprise in Europe. Commercialization of biodiesel in the U. S. is still in its nascent stage.

The bulk of the natural oil made by oilseed crops is in the form of triacylglycerols (TAGs). TAGs consist of three long chains of fatty acids attached to a glycerol backbone. The algae species studied in this program can produce up to 60% of their body weight in the form of TAGs. Thus, algae represent an alternative source of biodiesel, one that does not compete with the existing oilseed market.

As a matter of historical interest, Rudolph Diesel first used peanut oil (which is mostly in the form of TAGs) at the turn of the century to demonstrate his patented diesel engine2. The rapid introduction of cheap petroleum quickly made petroleum the preferred source of diesel fuel, so much so that today’s diesel engines do not operate well when operated on unmodified TAGs. Natural oils, it turns out, are too viscous to be used in modern diesel engines.

In the 1980s, a chemical modification of natural oils was introduced that helped to bring the viscosity of the oils within the range of current petroleum diesel3. By reacting these TAGs with simple alcohols (a chemical reaction known as “transesterification” already commonplace in the oleochemicals industry), we can create a chemical compound known as an alkyl ester4, but which is known more generically as biodiesel (see the figure below). Its properties are very close to those of petroleum diesel fuel.

+

HCOH

I

HCOH

I

HCOH

1 molecule of glycerol

Commercial experience with biodiesel has been very promising5. Biodiesel performs as well as petroleum diesel, while reducing emissions of particulate matter, CO, hydrocarbons and SOx. Emissions of NOx are, however, higher for biodiesel in many engines. Biodiesel virtually eliminates the notorious black soot emissions associated with diesel engines. Total particulate matter emissions are also much lower6,7,8. Other environmental benefits of biodiesel include the fact that it is highly biodegradable9 and that it appears to reduce emissions of air toxics and carcinogens (relative to petroleum diesel)10. A proper discussion of biodiesel would require much more space than can be accommodated here. Suffice it to say that, given many of its environmental benefits and the emerging success of the fuel in Europe, biodiesel is a very promising fuel product.

High oil-producing algae can be used to produce biodiesel, a chemically modified natural oil that is emerging as an exciting new option for diesel engines. At the same time, algae technology provides a means for recycling waste carbon from fossil fuel combustion. Algal biodiesel is one of the only avenues available for high-volume re-use of CO2 generated in power plants. It is a technology that marries the potential need for carbon disposal in the electric utility industry with the need for clean-burning alternatives to petroleum in the transportation sector.