Engineering Biotechnology
Utilizing a new, biological method for treating waste streams and creating energy

Anaerobic digestion is a natural process that produces biogas (60% CH4, 40% CO2) from the decomposition of organic wastes. These wastes include but are not limited to, agricultural waste, animal wastes, and food wastes. In this raw composition, biogas is moderately useful – it can be combusted to generate electrical energy. However, if raw biogas could be upgraded to natural gas pipeline standards (>97 % CH4, <2 % CO2) it becomes an extremely valuable and versatile energy source. Furthermore, RNG is compatible with existing pipeline infrastructure. The upgraded gas can be directly injected into the grid. While physical-chemical technologies exist for biogas upgrading, they are costly, energy intensive, can often remove fractions of valuable methane, and do not mitigate COemissions.

Recently, a new, biological method for upgrading raw biogas to RNG standards. It relies on hydrogenotrophic methanogens (HMs), which reduce CO2 to CH4 when fed H2 as an electron donor: 

                                                                                  4H2 + CO2 → CH4 + 2H2O                                     𝞓Go = -130.4 kJ mol-1

This biological approach is advantageous compared to current upgrading technologies because it provides ultimate carbon sequestration while increasing the volumetric production of RNG. While this biological method has been conceptually validated, hydrogen mass transfer resistance from the gas to liquid phase is the leading cause for low volumetric production rates. A novel approach of utilizing this biological pathway is to use biotrickling filters (BTFs), which can offer a high specific area for biofilm growth, high density of biomass and are known for their high gas to liquid and gas to biofilm mass transfer coefficients.

My thesis work at Duke has focused on developing a one-stage, biotrickling filter utilizing HMs to achieve fast biogas upgrading rates while complying with RNG standards.

A proof of concept study conducted in 2015 demonstrated that a biotrickling filter could upgrade H2 and CO2 (80:20% vol.) at rates that were 5-30 times faster than previous reports. Using a robust mathematical model and a sensitive dissolved hydrogen sensor, it was possible to analyze the hydrogen mass transfer resistance in a biotrickling filter. Major findings suggested that reducing the liquid film thickness significantly enhanced the biogas upgrading rate. It was also demonstrated that this biotechnology could consistently upgrade various raw biogas mimics to RNG standards (>97 % CH4, <2 % CO2) at rates that were ten times faster than previous studies. Through conducting a paper scale up, it was realized that the fast upgrading rates achieved in the lab made the biotechnology economically competitive with current physical-chemical processes. In conclusion, my research has shown that a biotrickling filter is a promising solution for upgrading raw biogas to RNG at fast rates.

My current research interests involve the co-removal of CO2 and other raw biogas impurities, specifically H2S and siloxanes. At the given moment, I am investigating COand H2S co-removal in a biotrickling filter.

Wait...Where does the hydrogen come from?

This is an important, big-picture question I am frequently asked. Since this biotechnology is aiming to generate renewable natural gas, the hydrogen gas should also be sustainably sourced. As wind and solar energy generation continue to rise, issues with curtailment are becoming more prevalent. We need infrastructure to store excess wind and solar power at peak production times. One potential use for this excess energy could be creating hydrogen via electrolysis for biogas upgrading systems. This process is known as power-to-gas and is starting to gain traction as a viable energy storage/generation technique.