Fermentation Design and Gas Transfer Considerations for Biochemical Methane Conversion

Fermentation Design and Gas Transfer Considerations for Biochemical Methane Conversion

Kyle Stone, Matthew Hilliard, Q. Peter He, and Jin Wang

Department of Chemical Engineering, Auburn University, Auburn, AL, 36849, USA

Methane is an essential component of the global carbon cycle and one of the most powerful greenhouse gases (GHGs) with a global warming potential over 20 times that of CO₂. Yet, methane is a rich source of carbon and energy that is utilized for heat, fuel synthesis, and valued chemical production. In addition, there is a rapid expansion of global methane supply predominantly because of increased production of natural gas [1] and biogas [2]. Due to concerns about climate change, abundance of supply, and favorable economic conditions, there has been growing interest in biological processes as a sustainable and efficient way of converting methane to valuable products while reducing GHG emissions.

These biological processes utilize the largely aerobic bacteria called methanotrophs that consume methane as their sole source for carbon and energy. Due to their unique metabolism, methanotrophs can convert methane at ambient temperatures and pressures to many different products. Specifically, methanotrophs have the natural ability to produce methanol, valued proteins, biopolymers, lipids (for biodiesel), organic acids, sucrose, and ectoine [3,4]. With genetic modification, methanotrophs can also produce non-native products such as carotenoids, isoprene, 1,4 butanediol, and farnesene [3,4]. Several of these products can potentially be simultaneously generated with specifically designed processes [4].

However, all processes with methanotrophs face several challenges, such as clean sources of methane, biocatalyst modification, and process design. For process design, the most influential factor is efficient gas-liquid transfer (i.e., methane and/or oxygen gas transfer to the liquid medium where cells reside). The gas-liquid transfer of the poorly soluble methane and oxygen can be strongly affected by reactor design and mass transfer enhancing agents in the liquid medium.

In this work we review recent progress about effective gas transfer for bioprocesses with methanotrophs. One focus will be on the quantification of methane transfer rate and cellular uptake rate. We will also provide a detailed review and discussion of the bioreactor configurations and the influence of these systems on gas-liquid transfer. Finally, we will discuss gas-liquid transfer enhancement with promoting agents. Recommendations will be provided and potential research areas for methane conversion through methanotrophs will be discussed.

[1] International Energy Statistics: Natural gas proved reserves, (2015). http://www.eia.gov/cfapps/ipdbproject/IEDIndex3.cfm?tid=3&pid=3&aid=6.

[2] Biogas Opportunities Roadmap, U.S. Department of Agriculture, U.S. Environmental Protection Agency, U.S. Department of Energy, 2014.

[3] P.J. Strong, S. Xie, W.P. Clarke, Methane as a resource: can the methanotrophs add value?, Environ. Sci. & Technol. (2015).

[4] P.J. Strong, M. Kalyuzhnaya, J. Silverman, W.P. Clarke, A methanotroph-based biorefinery: potential scenarios for generating multiple products from a single fermentation, Bioresour. Technol. (2016).