Author's School

School of Engineering & Applied Science

Author's Department/Program

Energy, Environmental and Chemical Engineering

Language

English (en)

Date of Award

Summer 8-21-2013

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Chair and Committee

Jay R. Turner

Abstract

ABSTRACT OF THE DISSERTATION

Modeling the Production of Microalgal Biodiesel

by

Mark Henson

Doctor of Philosophy in Energy, Environmental and Chemical Engineering

Washington University in St. Louis, 2013

Professor Pratim Biswas, Chair

Biodiesel produced from microalgal lipids is being extensively researched as an alternative to petroleum–derived diesel. Literature reports of prior modeling to estimate the likely energy and carbon footprints and manufacturing cost of microalgal biodiesel are inconclusive, with wide ranges for performance measures such as Net Energy Return and manufacturing cost. The goals of this research are to develop an integrated techno–economic life cycle inventory model of microalgal biodiesel production, create a base case that simulates proven manufacturing processes, identify potential barriers to technical, financial, and/or environmental viability, perform sensitivity analyses that identify the input parameters and modeling assumptions that have significant influence on key biodiesel performance indicators: KPI's), and perform case studies involving alternative microalgal properties, manufacturing processes and operating conditions, modeling time-scales, and geographic locations.

The model created to meet these objectives, called TELCIM, is the first publicly available, integrated techno–economic life cycle inventory model of microalgal biodiesel manufacture. It consists of a set of interlinked engineering, financial, and life cycle inventory models. It is implemented in Microsoft Excel®, and simulates a five–step manufacturing process consisting of microalgae cultivation, biomass harvesting, lipid extraction, lipid conversion to biodiesel, and anaerobic digestion of residual biomass. Material and energy flows are estimated using mass and energy balances and equipment performance equations. Operating and capital costs are estimated using standard accounting methods, and a cradle–to–gate life cycle inventory of energy and resource consumptions and pollutant releases is compiled. Detailed descriptions of TELCIM's component physical models, along with the derivation of their governing equations, are provided.

TELCIM was initially populated with data representing conventional microalgae cultivation and harvesting and vegetable oil extraction and conversion technologies deployed in a southern California location. The Net Energy Return for this case is below 1.0, the minimum threshold for long-term sustainability; the carbon intensity is similar to that of petrodiesel; and the manufacturing cost is uncompetitive with current transportation fuels. Detailed breakdowns show the contributions of each major process step and use category to these performance metrics, allowing identification of the major barriers to viability. Among the biggest obstacles is the large amount of energy used to dry the biomass to the 10% residual moisture content required by the conventional oilseed extraction process.

Two types of single–parameter sensitivity analyses are used to identify input parameters that have significant influence on the KPI's. Tornado plots reveal that biological properties including lipid fraction, intracellular water content, and growth rate are among the most influential with respect to one or more of the KPI's. Trend analyses of several of the anaerobic digester operating parameters show that factors affecting biogas production have much stronger influence on the KPI's than factors relating to nutrient recovery. Several variants to the base case were performed, including cases in which the microalga's growth rate and lipid content conform to R&D targets established by the National Alliance for Advanced Biofuels and Bioproducts; a hypothetical wet extraction process allows the drying step to be bypassed; and there is no anaerobic digestion step. The results from these cases suggest that one or more breakthroughs in cell biology and/or process engineering are necessary to make microalgal biodiesel a sustainable large–scale alternative to petroleum diesel. They also show that including anaerobic digestion in the manufacturing scheme improves the KPI's of greatest interest, and delivers an acceptable financial return on incremental investment.

TELCIM is a steady–state model, and the climatological data used in the base case represents annual average conditions. The effects of monthly variations in sunlight intensity are simulated under several facility design bases and operating strategies; it appears that designing for maximum sunlight intensity is more cost and energy effective, but necessitates underutilization of available carbon dioxide and reduces the biodiesel production rate. Enhancing the correlation between sunlight intensity and microalgal growth rate to account for the effect of light saturation on photosynthetic efficiency significantly dampens seasonal variations in biomass productivity. Analysis of hourly average versus daily average sunlight intensity indicates that daily average data is sufficiently precise if the long–term average biomass productivity is known; otherwise substantial error can be introduced if biomass productivity is estimated directly from sunlight intensity data, and daily average data is used instead of hourly average data. Five alternative manufacturing locations along the southern rim of the continental United States are simulated using local climatological inputs, including sunlight intensity. The only performance measure that differs significantly among these sites is water intensity, which is predicted to be much lower east of the Rockies due to higher precipitation rates.

Several supporting analyses are also presented in the dissertation, including a comparison of the composition of microalgal and soy lipids, which addresses the assumption that conventional oilseed extraction and conversion technologies will be effective with microalgal biomass as a substrate. The mechanisms of oil extraction, and the effects of residual water on extraction efficiency, are explored in an attempt to optimize evaporation energy load and oil yield. The impact of ambient temperature and relative humidity, dryer operating conditions, and dryer control strategies, on the energy burden imposed by biomass drying, are evaluated. And a multi–parameter sensitivity analysis explores the effect of potential interdependencies among biomass growth rate, intracellular water content, and lipid fraction on the KPI's.

Additional findings from this research include that microalgal properties affect virtually every step of the manufacturing process; TELCIM can be used to identify optimal strain characteristics for a given processing scheme. The bulk of the energy use and carbon dioxide emissions attributable to the cradle–to–gate manufacturing process occur within the biodiesel plant, but offsite emissions resulting from energy and raw material supply and transportation activities are not negligible. The main driver of Net Energy Return, carbon intensity and unit manufacturing cost is electricity usage, primarily for pumping fluids over long distances. Replacing evaporation losses can impose an enormous water burden, perhaps overwhelming local supplies, especially in the western United States.

TELCIM is a deterministic model; for a given set of input data it returns a set of numerical outputs without any measures of uncertainty or error. Recommendations for future work include an uncertainty analysis involving Monte Carlo simulations. They also include enhancing TELCIM to perform life cycle assessments, by assigning specific fates to byproducts and gauging the environmental impacts of resource usages and pollutant emissions. TELCIM can be modified to simulate alternative manufacturing schemes, such as feeding the biogas produced in the digesters to the central power unit, using the algae ponds for secondary wastewater treatment, and separate growth stages in which biomass production and lipid production are alternately promoted. Finally, the case studies simulating alternative manufacturing sites can be enhanced by using local operating and capital cost structures, especially for those sites in the southeastern United States that appear to have a more acceptable water intensity.

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Permanent URL: http://dx.doi.org/10.7936/K76W9851

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