Currently, the majority of liquid fuels are petroleum-based and thus non-renewable. Biofuel, namely bioethanol, is gaining market share as a sustainable alternative. First generation biofuel is produced using yeast to ferment plant-derived simple sugars and starch, whereas second generation or “advanced” biofuel requires pretreatment and hydrolysis of the plant biomass in order to gain access to the sugars contained in the cellulose, which comprises roughly 40-60% of the biomass. In both cases, sugar is the source of carbon and electrons for fermentation. Crops like maize and sugarcane grown for first generation bioethanol compete for land, and the enzymes required for second generation bioethanol are expensive.
My research combines bioprocess engineering and microbiology to examine syngas fermentation, an alternative process that uses synthesis gas (“syngas”) as the sole input of carbon and electrons to the fermentation process. Syngas is composed of carbon monoxide, hydrogen, and carbon dioxide. Because it can be produced by gasification of any type of biomass or industrial processes like steam reforming of natural gas, it is an extremely flexible feedstock.
Under anaerobic conditions, the bacterium Clostridium ljungdahlii and some other closely related species are able to ferment syngas. These bacteria live in one of two states depending on pH. When they are in an acidogenic phase they produce acetic acid and when they are in a solventogenic phase they produce ethanol. The process can be tailored to produce more energy dense alcohols like butanol.
We have optimized growth parameters over two years and I am comparing several different bacteria in a two-stage continuous fermentation bioreactor system at lab-scale in order to determine whether one species is most productive and what ethanol production rate can be achieved while maintaining a high ethanol to acetic acid ratio. The data collected from the comparison runs will help determine if the process is mass-transfer limited or nutrient limited and what our specific carbon monoxide consumption rate is per gram dry weight of biocatalyst. I have also sequenced the genomes of interest and we are investigating the important proteins to gain more insight to the process.