Pathways for producing liquid biofuels share many common features regardless of the biomass feedstock being used. All have a cultivation step, a collection or harvest step, and a processing or finishing step. Some land-based crops are used or being considered for use as biofuel feedstock because of their ability to produce oils, their ability to produce carbohydrates that are readily converted to fuels by microbial action, or their ability to fix carbon with low input. Oil-producing crops, such as soybean, jatropha, and camelina, are harvested and the oil is separated for subsequent processing. Sugar in sugarcane and starch in corn grain can be converted efficiently to ethanol by yeast and bacteria. Dedicated energy crops, such as poplar, switchgrass, and Miscanthus, are selected because of their growth with low inputs of nutrients or their ability to store carbon in soil (Tilman et al., 2006; Pyter et al., 2009; NRC, 2011). The lignocellulosic biomass can be converted chemically, thermochemically, or biologically to liquid fuels (NAS-NAE-NRC, 2009). There are algal biofuel systems analogous to each of these feedstock types. Analysis of the options in the cultivation, collection, and processing of algae is complicated by the vast number and complexity of options. As discussed in more detail in Chapter 4, these options affect the resource requirements needed to produce fuels. Analyzing all possible pathways in this report is not practical. However, the pathways can be grouped by the main features they share that are affecting resource use and energy balance. A few representative pathways are analyzed to illustrate the current state of the technologies and where advances are needed to reduce the resource requirements.
Trends observed in the science and technologies for other biofuel production are likely to occur in algal biofuel production as the latter develops as an industry. These trends include improvements in biomass production and total biomass processing discussed in Chapter 2, and the increasing comparative analysis of the full life-cycle impacts and requirements for various sources of alternative liquid fuels through the use of life-cycle assessments (LCAs) discussed in Chapters 1, 4, and 5. An additional trend is the move toward drop-in fuels that are compatible with existing infrastructure for petroleum-based fuels. Ethanol and fatty-acid methyl esters (FAME; or commonly called biodiesel) have compatibility and performance issues in vehicles that hamper their adoption (NAS-NAE-NRC, 2009; NRC, 2011). Current trends are moving toward production of pure hydrocarbon fuels or blendstocks that are compatible with existing fuel infrastructure and vehicle technologies (NREL, 2006).
The production of fuels and energy from algae is not an established industry and a variety of production systems have been proposed. Figure 3-1 is a simplified diagram that attempts to limit and group the potential steps in the algal biofuel production pathway. Each row of the diagram details a processing step or process option. Different combinations of cultivation and processing options have resulted in more than 60 different proposed pathways for producing algal biofuels.
As noted in Chapter 1, this study focuses on algal production systems that rely directly on photosynthesis (see Figures 3-1 and 3-2). Heterotrophic cultivation is, by design, outside the scope of this report. The exclusion of heterotrophic production from this report is not a judgment on the validity of these approaches but a reflection of the requirement to study photosynthetic algae as a feedstock for fuel production. This report examines pathways for producing liquid transportation fuels from algae. Gaseous power generation and hydrogen production are not discussed. Proteins are considered only as coproducts.
From the perspective of this study, the large number of possible designs of an algal biofuel pathway means that a small number of the most likely designs need be chosen