The Challenges and Benefits of Renewable Natural Gas Production: An Oregon Story

BACKGROUND The Metropolitan Wastewater Management Commission (MWMC) Eugene/Springfield Water Pollution Control Facility (WPCF) in Eugene, Oregon is a 35 MGD plant that generates 400,000 to 500,000 cubic feet per day of biogas from four (4) anaerobic digesters. For over 10 years the facility has operated a combined heat and power cogeneration facility, however changing priorities and the desire to fully utilize all their digester gas, caused the plant to investigate alternative biogas utilization strategies. This presentation will highlight technology selection, challenges with equipment procurement and construction, coordination with stakeholders, and lessons learned along the way. The Renewable Natural Gas (RNG) Upgrades Project (Project), commissioned in December 2021, is the first RNG facility at a Publicly Owned Treatment Works to come online in the state of Oregon. PROJECT OBJECTIVES The specific objectives of this project are to:

*Maximize beneficial use of biogas created by anaerobic digesters

*Avoid costly ongoing maintenance and upgrades to an existing internal combustion engine

*Produce RNG, consistent with the Federal Renewable Fuel Standards Program

*Reduce greenhouse gas emissions by up to 7,500 metric tons of CO2 annually

*Sell RNG to provide the MWMC with additional revenue TECHNOLOGIES CONSIDERED The following biogas upgrading technologies were considered as part of the initial screening for the Project:

*Water wash

*Pressure swing adsorption (PSA)

*Amine scrubbing

*Membrane separation FACILITY DETAILS The constructed Project includes hydrogen sulfide (H2S) and moisture removal, PSA technology to scrub the biogas to a high-BTU, natural gas quality product gas, and compressors to meet the gas utility's injection criteria. A tail gas stream comprised of carbon dioxide (CO2) and trace amounts of methane and volatile organic compounds (VOCs) not captured by the PSA is conveyed to a regenerative thermal oxidizer for destruction. A Receipt Point Facility operated by the gas utility, Northwest Natural, was constructed at the WPCF site to take transfer of the gas while simultaneously monitoring the quality, pressure, temperature, and quantity of product gas. The MWMC is working with Anew to register and broker the sale of renewable energy credits, including federal RINs and state carbon fuel standard credits. The Project is designed to utilize 100% of the biogas produced by the WPCF and upgrade the biogas to RNG for injection into the gas utility pipeline (approximately 300 scfm at 175 psi). LESSONS LEARNED Following are examples of the challenges that were encountered and associated lessons that were learned during the design, bidding, construction, and operational phases of the Project:

*It is important to maintain strong and balanced partnerships across all stakeholders, and encourage open lines of communication.

*There are safety concerns inherent with producing medium-pressure RNG at a WWTP. Provisions should be made to limit risks and educate plant staff.

*Look at pre-selecting RNG equipment to limit design scope; pre-procurement can be challenging due to the high cost of equipment.

*Having a focused member of the operations staff available for start-up is imperative.

*Consider providing a dedicated flare for off-spec gas rather than a dual-use flare.

*Test various engineered media for performance in removing hydrogen sulfide. CONCLUSION The Project was designed to have the capability to run continuously in response to fluctuating biogas availability, and automatically adjust operations to consistently meet project performance requirements. Plant staff have worked in collaboration with the Biogas Upgrading Equipment vendor Greenlane Biogas to fine-tune operation and troubleshoot issues, which has been critical to the overall success of the project. The Project had a capital cost of $14.5 million and an expected simple payback, which is highly dependent on RIN values) of 8 years based on an average uptime of 85%. Over the first two years of operation the uptime has fluctuated, resulting in monthly RNG production ranging from negligible to nearly 9,000 decatherms, but the Project is on track to provide the anticipated return-on-investment.