Development of a High Performance, Biological Trickling Filter to Upgrade Raw Biogas to Renewable Natural Gas Standards

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2019

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Upgrading raw biogas (~60% CH4, 40% CO2, 1000-5000 ppmv H2S) to renewable natural gas (RNG) (> 97% CH4, < 2% CO2, < 4 ppmv H2S) for injection into the grid is a desirable endeavor. RNG would allow for a clean alternative to natural gas derived from fossil origin, and it also have a versatile use as a transportation fuel and source of heating energy. Current physical-chemical technologies, such as pressure swing absorption and organic chemical scrubbing, can successfully upgrade raw biogas to meet RNG standards (1,2). However, they are energy intensive, costly, and can remove fractions of methane gas along with the impurities. Recently, biological biogas upgrading technologies have emerged as a promising solution for converting raw biogas to RNG. The method relies on hydrogenotrophic methanogens to reduce the CO2 fraction of raw biogas to CH4 using H2 as the electron donor. This method is advantageous compared to traditional biogas upgrading methods because is sequesters carbon emissions while increasing the volumetric production of methane. While early studies on biological biogas upgrading in continuously stirred tank reactors were conceptually validating, hydrogen mass transfer resistance from the gas-to-liquid phase prevented fast upgrading capacities from being realized. Slow biogas upgrading rates hinder the economic feasibility of the process. Furthermore, these studies only focused on CO2 removal when in reality, other impurities, such as corrosive H2S, must also be removed before RNG injection into the natural gas pipeline.

The overall objective of this thesis research is to develop a biological trickling filter reactor that can upgrade biogas to RNG standards at fast upgrading capacities while biologically co-removing H2S. A biological trickling filter was chosen for this investigation because they are characterized by a high specific surface area for biofilm growth, high biomass density, and are known for their high overall mass transfer coefficients; all factors that contribute to high conversion rates. A proof-of-concept study validated that this approach could achieve upgrading rates that were 5 – 30 times faster than other bioreactor configurations. This finding supported further studies that aimed to investigate hydrogen mass transfer resistance specifically in a biological trickling filter reactor. This was accomplished using a highly sensitive dissolved hydrogen sensor, which collected concentrations in real-time. Using this sensor, experiments were conducted to assess mass transfer resistance in the gas and liquid films. It was discovered that there was no external resistance in the gas-film. Furthermore, the liquid phase was a main barrier for mass transfer and reducing the liquid film thickness can significantly improve biogas upgrading capacities by 20%.

In addition to laboratory experiments, a robust and conceptually correct mathematical model was developed for a biogas upgrading biological trickling filter. The model was used to provide deeper insight into process fundamental and identify biological versus mass transfer limitations in the bioreactor. The model successfully replicated complex experimental findings and confirmed that liquid transport through the bioreactor bed was faster than the rates of mass transfer and biological conversion. A sensitivity analysis revealed that the model was most sensitive to the empty bed contact time and the maximum rate of reaction. Interestingly, the mass transfer coefficient for the liquid film (kLa) did not significantly improve the biogas upgrading rate for the bioreactor. This is because the model predicts that the bulk of hydrogen mass transfer occurs from the gas to non-wetted biofilm phase.

Concluding mass transfer resistance testing and process optimization, it was demonstrated that the engineered bioreactor could successfully upgrade various biogas compositions to RNG standards. The rates achieved for these experiments (10 – 20 m3CH4 m-3 d-1) were 1.5 – 25 times faster than other comparable research studies. To determine the economic feasibility of this technology, a paper scale-up cost analysis was conducted to estimate the investment and operation costs of a biological trickling filter upgrading raw biogas (60% CH4, 40% CO2) to RNG (> 97% CH4 < 2% CO2). This was accomplished by using experimental findings to scale the dimensions and determine heating and cooling requirements based on seasonal temperatures. Cost estimates for parts were acquired through vendor quotes. The cost analysis showed that the bioreactor is economically feasible however, the H2 acquisition cost was ~ 650% of the bioreactor investment cost. This is because H2 was acquired from the electrolysis of excess wind and solar energy and the cost of the hydrolyzer was ~ $1,000,000. Despite this significant cost, the total amortized cost of the biological biogas upgrading system was comparable to current physical-chemical upgrading technologies.

The final study of this thesis investigated the potential to biologically co-treat CO2 and H2S using nitrate as the terminal electron donor. Since the addition of nitrate favored undesired oxidation-reduction reaction pathways with hydrogen, a method was developed to map electron transfers. The effect of nitrate on methanogensis was tested with and without sulfur oxidizing bacteria. Under both conditions, nitrate had a negative impact on methanogenesis and ultimately, prevented co-treatment from being achieved. While attempting to co-treat H2S and CO2, it was discovered that dissimilatory nitrate reduction to ammonium was favored over denitrification. The electron balance confirmed that a competition for electrons from hydrogen did exist. This competition required N:S feeding ratios upwards of 16:1, which far exceeded the theoretical ratios of (4:1) for denitrifying bacteria. While the high nitrate loading rates allowed for high H2S removal efficiencies (98%), they inhibited methanogenesis so that carbon dioxide removal efficiencies did not meet RNG standards. Thus, future work should focus on alternative electron donors for sulfur oxidation and quantifying methanogenesis inhibition caused by sulfur-oxidation/denitrification pathways.

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Dupnock, Trisha Lee (2019). Development of a High Performance, Biological Trickling Filter to Upgrade Raw Biogas to Renewable Natural Gas Standards. Dissertation, Duke University. Retrieved from https://hdl.handle.net/10161/20084.

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