AN ASSESSMENT OF PHOTOSYNTHETIC BIOFUELS AND ELECTROFUELS TECHNOLOGIES UNDER RATE-LIMITED CONDITIONS

Open Access
Author:
Tuerk, Amalie Lauren
Graduate Program:
Chemical Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
June 29, 2011
Committee Members:
  • Wayne Roger Curtis, Thesis Advisor
Keywords:
  • renewable energy
  • biofuels
  • chemolithoautotrophic growth
  • rhodobacter capsulatus
  • ralstonia eutropha
  • chlorella vulgaris
  • botryococcus braunii
  • mass-transfer limited growth
  • light-limited growth
Abstract:
This thesis investigates the yields and system productivities for two different biofuel production processes, where the kinetics of the systems are constrained by rate-limitations expected in commodity-scale systems. The system that is the subject of the first investigation is an experimental comparison of algal biomass and fuel productivities under light-limited growth, where it is hypothesized that system productivities will be a function of light-limited growth kinetics rather than the intrinsic maximum growth rate of an algal species. Biomass and algal oil productivities were determined for two algal species -- the fast-growing, lipid-producing Chlorella vulgaris and the slow-growing, isoprene hydrocarbon-producing Botryococcus braunii -- in a continuous high-density, light-limited trickle-screen photobioreactor. In a light-limited system, all light is being utilized to its fullest extent such that the algal growth kinetics are dominated by the rate of light available to them. Despite an order-of-magnitude difference in the growth rates of these two algae, biomass productivities in the system conditions differed only by 10%, and the slower-growing alga (B. braunii) actually achieved slightly higher productivities. However, the productivity of energy captured into both algal oils and biomass (determined by bomb calorimetry) were very different: B. braunii captured 2.2 times as much light energy into algal oils than C. vulgaris, and twice as much light energy into the total algal biomass (oil + non-oil cell mass). These results highlighted the importance of species selection based on energy conversion efficiency and not intrinsic growth rate. Because the lipid-synthesis kinetics of C. vulgaris are growth dissociated, a loop air-lift batch reactor run was executed to determine if higher lipid productivities could be achieved by the induction of lipid-synthesis under non-growth conditions, at both light-limited and non-light limited conditions. C. vulgaris biomass and oil productivity under light-limited conditions far exceeded those in the non-light limited conditions, which confirmed the hypothesis that maximum utilization of light (i.e., light-limited growth) was essential for maximal productivities. C. vulgaris productivity in the batch system peaked during light-limited growth, but fell below the performance of the continuous reactor system. While C. vulgaris lipid productivity in the light-limited batch system exceeded that found during the continuous growth, it did so by only 25%. Furthermore, the biomass and lipid productivities peaked simultaneously when the lipid content of C. vulgaris was still very low (prior to lipid accumulation induction). Although the lipid content of C. vulgaris increased from 6-22% by the end of the batch run, lipid productivities during this accumulation phase were still lower than those found during the active growth phase. This emphasized that lipid productivity and not lipid content is a more important metric for assessing the performance algal fuels technologies. The system that is the subject of the second investigation is a theoretical “Electrofuels” process, where gaseous H2, O2, and CO2 are the growth substrates for chemolithoautotrophic production of isoprene hydrocarbons by Rs. eutropha or Rb. capsulatus. The H2 and O2 are generated by abiotic electrolysis, using electricity that could be generated by solar photovoltaic cells, and serve as the electron donor and acceptor to provide energy for microbial metabolism. The yields, productivities, and process economics of such a system were assessed theoretically to provide insight and focus towards research areas with the highest possible impact for process feasibility. This was accomplished through the development of a modified theory of microbial energetics, based heavily upon the ‘Electron Balance’ approach originally developed by McCarty (1971; 2007); this method predicts maximum yields of microbial growth based on the relative energetics of energy generation (catabolic reactions) and cell synthesis (catabolic reactions). Employing this approach, the maximum theoretical yields of cell and product fuel were established on each of the gaseous substrates; expressions for realistic, net fuel yields for the system were then derived based on an assumption of kinetic limitation by gas-liquid mass-transfer in a commodity-scale continuous-growth bioprocess. This quantitative theoretical framework was utilized in the calculation of fuel yields, fuel productivities, and minimum operating costs for fuel production in a range of process conditions. The rate of mass-transfer to the bioprocess is a critical process parameter that determines the productivity of the system. Furthermore, it was established that a continuous growth-process siphons metabolic energy that could otherwise be used for fuel synthesis; very little of the energy contained in the H2 substrate was captured into fuel. Therefore, high-mass transfer, low-growth perfusion-type process development and genetic manipulation for the metabolic decoupling of fuel and growth processes are important research thrusts for Electrofuels process feasibility in order to maximize the amount of substrate hydrogen utilized towards fuel synthesis.