COLIN M. BEAL, ROBERT E. HEBNER, MICHAEL E. WEBBER, RODNEY S. RUOFF, A. FRANK SEIBERT, and CAREY W. KING

5.1 INTRODUCTION

The aspiration for producing algal biofuel is motivated by the desire to:

(1) displace conventional petroleum-based fuels, which are exhaustible,

(2) produce fuels domestically to reduce energy imports, and (3) reduce greenhouse gas emissions by cultivating algae that re-use carbon dioxide emitted from industrial facilities. In theory, algae have the potential to pro­duce a large amount of petroleum fuel substitutes, while avoiding the need for large amounts of fresh water and arable land [1-3]. These attributes have created widespread interest in algal biofuels. In practice, however, profitable algal biofuel production faces several important challenges. The goal of the research presented in this paper is to examine and quantify the extent of some of those challenges with an eye towards identifying critical areas for advances in the development of algal biofuels.

For algae to be a viable feedstock for fuel production: a significant quantity of fuel must be produced, the energy return on investment (EROI) of the life cycle must be greater than 1 (and practically greater than 3 [4]), the financial return on investment (FROI) should be greater than 1, the wa­ter intensity of transportation using algal biofuels should be sustainable, and nutrient requirements should be manageable. This study examines these criteria for two cases using second-order analysis methods described by Mulder and Hagens [5], which include direct and indirect operating

expenses, but neglect all capital expense. Process-specific terminology is based on the reporting framework established by Beal et al. [6].

There are several energy carriers and co-products that can be produced from algae, such as renewable diesel, electricity, hydrogen, ethanol, phar­maceutics, cosmetics, and fertilizers [7-9]. While non-energy co-products might enable economic viability of algal biofuel products in the short term, large scale production would quickly saturate co-product markets. Thus, in the long term, production of domestic, renewable, low-carbon fuels as an alternative to conventional fuel sources remains the main motivation for researching large-scale algae production. Consequently, this research focuses on the energy products. While bioelectricity from algal feedstocks is one possible pathway for energy production, this work considers only the co-production of bio-oil (a petroleum fuel substitute) and bio-gas (i. e., methane, which is a natural gas substitute) because those two fuels are produced from the experimental process at UT and align more directly with displacing petroleum [10-12]. Further, both bio-oil and bio-gas are feedstocks that can be combusted within additional technologies to pro­duce electricity.

Because the intent of this research is to analyze and anticipate a mature algal fuels industry that does not yet exist, researchers have two options for conducting a process analysis as in this paper: (1) use data derived from experimental processes followed by scaling analyses (recognizing that lab-scale experiments are inherently sub-optimal) or (2) use estimated data from models of future commercial-scale systems. Both of these ap­proaches are used in this study. Firstly, an Experimental Case is described, which is based on unique direct end-to-end measurements (from growth through biocrude separations) performed in a controlled indoor/outdoor laboratory setting at The University of Texas at Austin. Secondly, a High­ly Productive Case is described, which is based an optimistic analytical model that incorporates the technology and pathways of the Experimental Case.

We encourage other researchers to present (life cycle) metrics of al­ternative algal technology pathways in the step-by-step manner we dem­onstrate. The reasons for presenting life cycle metrics at multiple stages are threefold: (1) easier facilitation of future life cycle assessment (LCA) harmonization and meta-analyses that can effectively compare many independent studies, (2) better tracking of technological progress over time, and (3) better comparison of competing technologies (e. g., capital intensive versus resource intensive). The benefits of LCA harmonization were demonstrated by Farrell et al. [13] in comparing net energy for corn ethanol. The National Renewable Energy Laboratory of the US Depart­ment of Energy tests and tracks photovoltaic cell efficiencies over time such that specialists and the general public can easily track the rate of progress, which is beneficial for the community as a whole. By doing so, one is able to observe the improvements that were made to photovoltaic cell designs over the course of research and development, providing a van­tage point for researchers and investors alike to gauge the progress in that energy production technology. The authors believe algal energy processes would benefit from similar indicators and analyses, and this manuscript presents its results in that spirit of tracking technological metrics starting at the experimental batch scale. Additionally, the calculation of multiple life cycle indicators (e. g., EROI, FROI, water use, resource consumption, land use, air emissions, etc.) from the same experimental or modeled pro­cesses provides congruent indicators that emphasize the real design trad­eoffs (e. g., water versus electricity inputs).

The work presented adds to research in the authors’ prior publications, which presented the second-order energy return on investment (2nd O EROI) analyses for an Experimental Case and a modeled Highly Produc­tive Case. In the previous work, the 2nd O EROI, which is a ratio of the energy output of a system to the energy input for that system, for these two cases was determined to be 9.2 * 10-4 and 0.22, respectively [14]. That study illustrated the energetic challenges associated with producing algal biofuel. The present study extends the previous work with five new analytical thrusts to determine (1) the partial FROI, (2) the second-order water intensity of transportation using the algal biofuels produced, (3) the nitrogen constraints, (4) the carbon constraints, and (5) the electricity re­source constraints for the Experimental Case and the Highly Productive Case, respectively. The cost, water, and resource results from this new work are presented in conjunction with the previously determined energy results. Thus, for our two cases (one experimentally measured and one analytically derived), this present research serves as a comprehensive and coherent evaluation of the algal biofuel process. It is important that LCAs demonstrate relationships among multiple metrics that are calculated. By reporting multiple metrics for the same algal energy processes, this paper presents an understanding of how one metric (e. g., water consumption) is linked to another (e. g., energy production). Although the Experimental Case is not representative of commercial biofuel production due to signifi­cant artifacts that are inherent to lab-scale (vs. industrial scale) production, it represents the first known end-to-end experimental characterization of algal biocrude production at relatively large scale (thousands of liters). While other experiments have been performed at similar scale, they did not conduct the comprehensive mass and material balances that are pre­sented here. Conversely, the data used for the Highly Productive Case are based on optimistic assumptions for operating within the specific produc­tion pathway in this study. To place the Highly Productive Case in context with other analyses that have been published, each assumption is com­pared with those from other studies in the literature.

Many prior studies have been performed, each with a slightly different focus: some have emphasized algal biomass productivity, estimated algal oil productivity per acre of land, or evaluated only a few constraints on al­gal biofuel production (e. g., energy requirements, cost, etc.) [15-18]. This paper takes the approach of considering many constraints simultaneously (energy, cost, water, and resources) to give a more complete assessment. To this end, quantitative targets are presented in the “Conclusions” that, if achieved, would enable algal biofuel production at large scale.