Enhanced Methane and Hydrogen production in Reverse Membrane Bioreactors via Syngas Fermentation
Konstantinos Chandolias
Thesis for the degree of Doctor of Philosophy at the University of Borås to be publicly defended on November 29th 2019, 10:00 a.m. in room D207, University of Borås, Allégatan 1, Borås, Sweden.
Language: English
Faculty opponent is:
Associate Professor Hariklia N. Gavala
Department of Chemical and Biochemical Engineering
Technical University of Denmark-DTU, Lyngby, Denmark
PhD thesis is available at
Swedish Centre for Resource Recovery
University of Borås
SE-501 90 Borås, Sweden. +46(0) 33 435 4000
Abstract
The increase in the waste production and the energy demand worldwide stimulates the development of waste treatment processes, such as the anaerobic digestion. This biochemical process converts organic substrates into biogas, with anaerobic microorganisms. However, some types of substrates have low bio-degradability due to its recalcitrance or the presence of inhibitors. This can be solved by the coupling of anaerobic digestion with gasification, a thermochemical process that can convert organic substrates into syngas (H2, CO, and CO2) regardless of the substrate´s degradability. Consequently, syngas can be converted into biogas and other fermentative products via anaerobic digestion, in a process known as syngas fermentation. In comparison to the catalytic conversion of syngas, syngas fermentation has several advantages such as lower sensitivity to CO/H2/CO2 ratio and to syngas contaminants as well as higher product specificity.
The main goal of this thesis was to improve the syngas conversion rate into CH4 and H2 by addressing the cell washout, the cell inhibition by syngas contaminants, and the low gas-to-liquid mass transfer, which are major challenges in syngas fermentation. For this purpose, a reverse membrane bioreactor, containing a mixed culture encased in membranes, was used in various set ups. The membranes were used in order to retain the cells inside the bioreactors, to protect the cells against inhibitors, and to improve the gas holdup and gas-to-cell contact by decreasing the rise velocity of syngas bubbles. As evident from the results, the cell washout was successfully tackled during a continuous experiment that lasted 154 days. In addition, membrane bioreactors fed with the syngas contaminants, toluene and naphthalene, achieved approximately 92% and 15% higher CH4 production rate, respectively, compared with the free cell bioreactors. In order to improve the gas holdup and consequently the gas-to-liquid mass transfer of syngas, a floating membrane bed bioreactor was set up. This bioreactor contained membrane sachets, filled with inoculum that formed a packed floating membrane bed and achieved an increase of 38% and 28%
for the conversion rate of H2 and CO, respectively. Furthermore, the addition of a mixture of heavy metals improved the production rates and yields during the syngas conversion into fermentative H2.
Keywords: syngas fermentation; CH4; H2; cell washout; inhibitors; mass transfer
