Wastewater treatment plants (WWTPs) are under increasingly strict regulations to limit nitrogen and phosphorous in effluent discharge since they can have deleterious effects on aquatic species. In WWTPs, ammonia in wastewater is converted to nitrites, then nitrates and finally nitrogen in biological nitrification-denitrification processes. Methanol is commonly used as an external organic carbon source for denitrification since it is readily available, relatively cheap, and residual methanol can be removed from the treated effluent by aeration. Denitrification using methanol exhibits high potential removal efficiency, high process stability and reliability, relatively easy process control, low land area requirement, and moderate cost [1]. However, high competing demand for methanol from utility chemicals manufacturing can result in ever increasing cost of methanol for denitrification. Additionally, safety concerns related to methanol storage and handling facilities might preclude the purchase and transport of commercial methanol to WWTPs, highlighted by the ban of methanol use for denitrification in New York City in the late 2000s.

The presence of anaerobic digesters in WWTPs offers an opportunity to use biogas as a feedstock for on-site methanol production. Biogas, a mixture of 50-70% methane, 30-40% carbon dioxide, along with sulfur and trace elements, is used only in about half of WWTPs in the United States with digesters, for digester heating and combined heat and power [2]; in the rest of the cases, biogas is flared. On-site conversion of biogas to methanol has the potential to mitigate risks from cost and supply volatility by reducing the dependence on external carbon sources, address safety concerns and reduce the overall carbon footprint of the WWTPs [3]. However, traditional methanol synthesis processes from steam-methane reforming-based syngas (CO+H₂) are not viable since they cannot be scaled-down economically for use at WWTPs. This provides an opportunity for the development of a cost-effective and modular closed-loop process for on-site production of methanol at WWTPs.

RTI has developed and demonstrated a cost-competitive, internal combustion engine-based, impurity-tolerant syngas generator at near-deployment scales. RTI’s pilot plant processes 50,000-60,000 standard cubic feet per day (SCFD) natural gas to produce syngas in a partial oxidation process. We have also integrated methanol production, with a 15% exhaust slipstream producing 1 barrel/day (BPD) of crude methanol. Instead of conventional process equipment that rely on economies of scale, the engine-based process uses economies of mass manufacturing to lower costs. The production of crude (75vol%) methanol alleviates safety concerns of handling, storage, and use. This paper will present technoeconomic evaluation of integrating the engine-based modular methanol synthesis technology in WWTPs for cost-effective on-site production.

A detailed biogas-to-methanol conversion process, including biogas cleanup, syngas production, conditioning and compression, and methanol synthesis sections, was simulated using Aspen HYSYS chemical process simulation software. Data from the operation of the engine and methanol synthesis pilot plant was used for reconciliation and validation of model performance. Several plant configurations were studied to optimize the biogas to methanol conversion process. Heat and mass balances, utility requirements, auxiliary power loads and equipment specifications were generated to calculate operating cost estimates. ASPEN Process Economic Analyzer (APEA) and vendor quotes were used to generate capital cost estimates. WWTP flow capacities determine the amount of external carbon needed and the biogas potential. Our analysis shows the engine-based methanol production process can meet methanol demand (~9 BPD) for WWTPs as small as 5 million gallons per day (MGD) capacities. Methanol costs are expected to be at par with commercial prices [4] from large-scale production at a 20 MGD scale, and much lower for larger WWTPs, up to 20% lower for a 30 MGD WWTP as an example. Our analysis indicates that the engine-based methanol production system is ideal for medium-sized WWTPs from 10-50 MGDs, with short payback periods and high returns on investment.

References

[1] H. H. Kung, MethanolProduction and Use Chemical Industries ; V. 57, 1994.

[2] United States Environmental Protection Agency, "Digesters at Water Resource Recovery Facilities," [Online]. Available:https://www.epa.gov/anaerobic-digestion/types-anaerobic-digesters#WRRFdi.... [Accessed April 2020].

[3] S. K. C. Yu-Chen Su, "Bioaugmented methanol production using ammonia oxidizing bacteria in a continuous flow process," Bioresource Technology, pp. 101-107, 2019.

[4] Methanol Institute, "Methanol Price and Supply/Demand," March 2020. [Online]. Available: https://www.methanol.org/methanol-price-supply-demand/.