Biogas to Renewable Natural Gas - Achieving Net Zero Energy

Abstract Overview: As industrial digesters and wastewater treatment plants become more efficient, the biogas industry has seen an increased interest in the production of renewable natural gas (RNG) for pipeline injection from biogas. We will specifically look at the installation at the City of Lincoln Nebraska's Theresa Street Water Resource Recovery Facility. This dual stage membrane technology provides increased methane recovery with a shorter payback period over cogeneration equipment. Abstract: The city of Lincoln has two Resource Recovery plants, the Northeast, and Theresa Street, which processes all the sludge for the city. They are rated for 5MGD & 27MGD respectively. The Theresa Street plant has three (3) egg shaped digester with a capacity of 1.1million gallons each, with a combination of municipal and industrial waste. The biogas was used in existing Waukesha engines (900kW) for the past 25 years, producing 40% of the electrical needs for the plant. They ran on untreated biogas and had reached the end of their useful life. In 2017 a study looked at four options: 1.) Replace with new engines, 2.) Send raw or treated biogas to a University of Nebraska Lincoln facility for boiler fuel, 3.) Produce RNG as vehicle fuel for the city bus fleet, 4.) Produce RNG for injection into the Black Hill utility pipeline. The final decision was for Option 4, RNG into the local Black Hills pipeline. The equipment & installation was publicly bid in March of 2018. Construction groundbreaking was in May of 2019 and Commissioning was completed November 9th, 2020. The upgrading equipment was purchased from a single system supplier, which consisted of Hydrogen Sulfide Removal, Gas Compression/Moisture Removal, Siloxane/VOC Removal, & a Two-Stage Membrane System for Carbon Dioxide Removal. The Hydrogen Sulfide consists of a single media vessel with pelletized iron hydroxide. This media is a full iron product and works through chemi-adsorption, pulling the sulfur into the pores of the media & reacting with the iron hydroxide. This static bed H2S Removal was chosen for its minimal impact of operations of the plant, as it does not need daily monitoring. The compression system is a LeRoi flooded screw compressor, which pressurized the gas to +200psig before sending it to the moisture removal system. In the moisture removal system, the gas is cooled to 40°F driving as much water out of the gas before reheating to 80°F. The reheating allows a 40°F separation from dew point, creating a relative humidity of approximately 25 percent, which is necessary for the Siloxane/VOC Removal System. Specific types of activated carbon were selected to remove the Siloxane and VOCs present at the time of start-up. It is imperative that gas is free of measurable siloxanes and VOCs prior CO2 Removal System, as to not foul the gas separation membranes. The CO2 is designed as a two-stage system where the first stage removes over half of the CO2, before passing the retentate onto the second stage. The second stage removes the remainder of the CO2, and the permeate is recycled back to the inlet of the compressor. The final gas quality meets the Black Hills specification Higher Heating Value of 950 Btu/cf min, CO2 < 2%, Oxygen <0.2%. The two-stage membrane system provides methane recovery of over 95%. The project cost was $8.6 million, with an additional $1 million provided to Black Hills as a 'transportation fee', paid monthly at $15,000. The forecast is to sell 100,000 dekatherms annually. The RNG is earning D5 RINS and Low Carbon Fuel Standard Credits in the California vehicle fuel market. Yearly revenue is estimated at $2.6 million. The city is currently working on getting a dual pathway approved for D3/D5 RINS to increase the overall revenue. With the sale of the RNG, the plant is exporting more BTUs than they are consuming (Electrical & Natural Gas) and consider themselves NET Zero! With great success comes some challenges. The project faced an early season snowstorm, and subsequent flooding stopping the construction. Then COVID restrictions caused project delays for material deliveries and work arounds for the contractors & subcontractors due the number of people on site, etc. There were also lessons learned regarding the upgrading equipment. Originally an air-cooled oil cooler was installed indoors, but the building heat load was calculated incorrectly and the HVAC could ventilate the room properly, the unit was relocated outdoors. Additionally, clearance above the indoor siloxane removal vessels was tighter than expected and heat tracing on an outdoor drain line was inadvertently turned off, which lead some freezing issues. In conclusion the plant is very happy with the choice to produce RNG, as it has similar O&M for gas cleaning when compared with cogeneration, with increased revenue. The system was designed for 2033 biogas production, and space is available for doubling the capacity of the site.