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Making Biofuel from Microalgae

So much potential coexists with so many scientific, environmental and economic challenges

Philip T. Pienkos, Lieve Laurens, Andy Aden

Vital Economic Questions

2011-11PienkosF6.jpgClick to Enlarge ImageThe technical hurdles described above have all been overcome to some degree (in the laboratory or in pilot facilities), but the biggest challenge to the commercial viability of algae-based biofuels is to carry out steps at low enough cost to produce a competitively priced biofuel. Analyses over the past 20 to 30 years predicting cost have ranged from what could be characterized as wildly optimistic (less than $1 per gallon) to more conservative but hardly encouraging (more than $40 per gallon). Until true production costs are understood, cost reduction from research and development gains are difficult to quantify. Therefore, NREL is attempting to establish baseline costs for producing algal biofuels using technology readily available today.

To do this, we apply experience and methodologies used previously for technoeconomic analysis of lignocellulosic, or woody, feedstocks for biofuels production. Technoeconomic analysis combines detailed conceptual process design with economic analysis to tie performance to cost. To do this, we have modeled baseline algae processes for both open-pond and closed photobioreactor systems. The models include growth; harvesting; concentration; lipid extraction and recovery; and conversion to fuel. The baseline models currently assume that spent algal biomass goes through anaerobic digestion to recover some of the energy value as methane. However, the models will allow alternative coproducts to be explored. The material- and energy-flow rates calculated from these models can be used to determine the size of equipment needed and to develop capital and operating costs for the algal biorefinery. In the end, the algal oil is assumed to be converted to diesel or jet blend stock.

2011-11PienkosF7.jpgClick to Enlarge ImageIn our recently published baseline analysis, fuel product at a 10-million-gallon-per-year facility could be produced for between $10 and $20 per gallon. Although many parameters affect the economics, we have identified two key cost drivers: lipid content and growth rate. With improvements to these and other parameters, as well as several coproduct scenarios, the potential for cost reduction is significant. Further enhancement of these models will come from using more experimental data from pilot operations underway across the country. Assumptions regarding the recycling of nutrients and water to reduce cost and improve sustainability will be tested for validity.

The composition of the biomass produced by a given process hugely influences the economics. The ratio and composition of proteins and carbohydrates in a given algae crop will determine the fate of the residual biomass once lipids are extracted. That can play a significant role in the overall process, perhaps even driving the development of alternative uses of residual algal biomass. For example, biomass with high fermentable sugar content could be converted into fuel ethanol, a product with more commercial value than methane.

One problem that also must be tackled is the large range in reported algal lipid content in the literature. The use of a wide variety of extraction methods and solvent types is part of the problem. The lack of a standard lipid-quantification procedure, differences in compatibility of the polarity of the solvents, differences in the polarity of the lipid molecules, and the accessibility of the lipids to solvent penetration all play a role. Inevitably, the extractable oil fraction will contain nonfuel components (such as chlorophyll and other pigments, proteins and hydrophobic carbohydrates). Thus it is necessary to assess the fuel fraction—the fatty-acid content of extracted lipids—within these oils. This is vital for accurately capturing productivity improvements. We must be certain that an observed increase in extracted lipids is not an artifact of the measurement process.

In this context, the algal-biofuels research community is moving away from extraction-based lipid quantification and toward a whole-biomass transesterification process, which gives an accurate yield of the potential fuel fraction. It does that by measuring only the fatty acids as methyl esters. Since the fatty acids are ultimately going to form the basis of the biofuel produced, this is an accurate measure of the total oil yield.

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