EFFECT OF LONG-TERM OPERATION ON MFC PERFORMANCE AND THE PERFORMANCE OF A SCALE-UP CONTINUOUS FLOW MEC WITH AN EXAMINATION OF METHODS TO DECREASE CH4 PRODUCTION
Open Access
- Author:
- Rader, Geoffrey Kermit
- Graduate Program:
- Environmental Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- April 06, 2010
- Committee Members:
- Bruce Ernest Logan, Thesis Advisor/Co-Advisor
Bruce Ernest Logan, Thesis Advisor/Co-Advisor - Keywords:
- methane
hydrogen
microbial electrolysis cell
microbial fuel cell - Abstract:
- Cellulose dark fermentation is a sustainable method for bio-hydrogen production, but much energy is leftover from the process effluent as soluble fermentation endproducts. Microbial fuel cells are systems where fermentation endproducts or other organic and inorganic sources are oxidized by exoelectrogenic microbes to produce electricity with concurrent water purification. MFCs have been used effectively with a variety of substrates, but few studies have examined the stability of these systems over long periods of operation. It is shown here that MFC reactors—except those fed formic acid—still produced high power densities after one year of operation, but that these power densities decreased over time due to cathode deterioration. The degree of this loss in power density was specific to the electron donor, with the greatest power loss (54.3%) occurring in ethanol fed MFCs. The presence of a cathodic biofilm increased Coulombic efficiencies, but decreased power densities in MFCs fed fermentation endproducts. Microbial electrolysis cells (MECs) utilize exoelectrogenic microbes for the oxidation of organic and inorganic substrates. Unlike MFCs, MECs require a small applied voltage to enable hydrogen evolution at the MEC cathode. MECs have also been shown to be effective with various substrates, but most studies still use platinum catalyst cathodes and very small reactors (<100 mL). It is shown here that a 2.5 L continuous flow microbial electrolysis cell produced a current density of 1.18 A/m2 and a maximum volumetric hydrogen production rate of 0.53 m3/m3/d without precious metal cathodes. The current density of 1.18 A/m2 is slightly lower than might be expected based on other studies, and may be due to increased electrode spacing and the presence of electrode separators. This study provides further evidence that minimizing electrode spacing is likely the most important factor in achieving high current. The continuous flow reactor was shown to be especially prone to methane production as gas produced after 15 days of operation was greater than 90% methane. This rapid transition to nearly complete methane production was likely due to a lack of periodic air exposure during continuous flow operation. Several approaches have been suggested in literature to reduce methane production in MECs. It is shown here that beyond an applied voltage of ~0.6 V, MEC methane production was not dependent on applied voltage or hydrogen production rate, as methane production rates stayed approximately constant above this applied voltage. An increase in temperature from 20 °C to 30 °C increased hydrogen and methane production approximately equally. Reactor air exposure over one day as well as a high applied voltage to the stainless steel cathode did not reduce methane production. In MFC tests, power density was hindered by a low pH solution. The combination of a low pH solution and a steel cathode in MEC operation almost prevented gas production completely. Long-term addition of a 2-Bromoethanesulfonic acid—a known chemical methanogenesis inhibitor—was shown to inhibit methanogenesis, but once it was omitted from the feed there was no long-term inhibition of methanogenesis.