Syngas fermentation can be used in the production of biofuels and biobased chemicals utilizing the gases produced by gasification of lignocellulosic biomass, coal, petcoke and waste materials, or from gases present in industrial waste streams. These gases contain carbon monoxide (CO), hydrogen (H2), and carbon dioxide (CO2), which can be converted to alcohols, organic acids, and chemicals using microorganisms such as acetogenic bacteria (acetogens). Commercial biological gas conversion processes require stable operation, high substrate conversion, product specificity and productivity.
One of the main issues in this gasification-fermentation technology is stabilizing the fermentation process. Syngas fermentation can become unstable due to pH sensitivity and substrate inhibition. Hence, Dr. Hasan Atiyeh, Associate Professor, Department of Biosystems and Agricultural Engineering (BAE) at Oklahoma State University (OSU) in collaboration with Drs. Ray Huhnke (BAE, OSU) and Randy Phillips (White Dog Labs, Inc., New Castle, Delaware), developed and obtained a U.S. patent (10,017,789) on a novel control method to optimize gas supply to maintain constant pH for a stable continuous fermentation required for commercial production of alcohols.
“The pH of the medium is a highly sensitive indicator of optimal supply of CO and H2 to acetogens to make alcohols such as ethanol,” Atiyeh said. Atiyeh’s research team instrumented a 3-L continuous stirred tank reactor (CSTR) in which the gas flow was automatically adjusted by a proportional-integral-derivative (PID) process controller to control the pH in the fermentation broth.
“Syngas feed to fermentation using Clostridium ragsdalei was continuously adjusted under PID control to keep fermentation pH at 4.8 for 1400 h and 1500 h in two continuous runs with 1-2 g/L acetate and up to 25 g/L ethanol produced,” Atiyeh said. “Over 95% of the CO and H2 were converted into alcohol by the acetogen during continuous syngas fermentation. Ethanol production more than doubled using our novel control method compared to the conventional process.”
Atiyeh said, “The development of automatic control of syngas feed rate maintained constant pH, increased stability, ethanol selectivity and concentration, and this demonstrates the potential for applying this novel control method in commercial syngas fermentation.”
Research team’s invention enables future researchers and industry to implement this tool for various syngas fermentation bioreactors. This work also provides valuable guidance towards designing large scale bioreactors with increased alcohol productivity and syngas utilization, and further develops the hybrid gasification-syngas fermentation conversion of biomass to liquid fuels and chemicals that will likely make this hybrid technology economical.
Funding was provided by the Department of Transportation-Research and Innovative Technology Administration through the South Central Sun Grant Program.