Four years ago, the City of Longmont, Colo., embarked on the U.S. Front Range’s first project to convert biogas to renewable natural gas (RNG) for city-owned vehicle fueling.
What began as an effort to make better use of the city’s wastewater treatment plant (WWTP) biogas has culminated in a regional asset that bolsters the community’s sustainability efforts and quality of life.
From 2018 to 2020, Longmont equipped its WWTP with a state-of-the-art system that cleans and converts the facility’s biogas into high-quality RNG. The city also laid a 1,440-foot pipeline that transfers this RNG to a new indoor vehicle-fueling station that supplies the city’s trash and recycling trucks with compressed natural gas (CNG). Together, city employees named this the RNG upgrading and vehicle-fueling system.
Until recently, the city’s waste services fleet had consisted of diesel trucks that disturbed residents with greenhouse gas (GHG) emissions, odors, and excessive noise. While “green” vehicles were costlier than their diesel counterparts, the biogas-to-RNG project offered the promise of a sustainable, city-owned supply of RNG and a quieter, cleaner, and odorless neighborhood experience.
In the past two years of operations, the RNG upgrading and vehicle-fueling system has advanced several of the city’s sustainability goals and continues to generate fuel.
By slashing the waste services fleet’s diesel demand by over 100,000 gallons a year, the city saves almost $300,000 in annual fuel costs and has reduced GHG emissions by approximately 1,000 metric tons of carbon dioxide equivalent per year — proportionate to removing 200 cars from the road. Since CNG vehicles do not emit sulfur, heavy metals, or soot, residents now enjoy improved air quality as well as minimized disruptions on trash days.
And, while the city expected and planned to gain some revenue through the sale of renewable identification number (RIN) credits through the EPA’s Renewable Fuel Standard (RFS) program, it hadn’t expected the RIN market nor its payoff to be so robust. Even with high variability across financial quarters, Longmont earned almost $350,000 in 2021 revenue — a value that is only expected to rise with additional run time of the system.
To top it all off, in October 2021, the city won the Water Environment Federation’s Project Excellence Award, a national distinction awarded to efforts showcasing excellence and infrastructure innovation in the water sector.
These boons did not come immediately: Through months of trial and error, careful adjustments, and diligent monitoring, the city navigated many challenges that accompany a complex system with little precedent, especially from the perspective of on-site fueling. None of this could’ve been accomplished without continued collaboration between members of the project team and other city departments.
Evolution of a System
The RNG upgrading and vehicle-fueling system is currently operated nearly continuously to produce approximately 800 diesel-gallon equivalents of RNG vehicle fuel each day. The system converts up to 80 percent of the city WWTP’s biogas into RNG that is then compressed to supply 11 waste services trucks.
However, this output is relatively new for the fueling system. In fact, for the first year of operation, it generated only 50 percent of the waste services fleet’s fuel demands, primarily due to programming difficulties with the CNG trucks and the RNG storage system, which prevented the city from fully utilizing the produced fuel.
Longmont also discovered that the iron-based media used in the biogas-upgrading skid to adsorb hydrogen sulfide (H2S) required replacement every two months, as opposed to its anticipated lifespan of three to four months.
Any unique system that utilizes new technologies, such as decanting panels to maximize use of RNG storage and trucks with new fuel sources, comes with a learning curve. The city’s wastewater and waste services experts took time to familiarize themselves with natural gas practices, understanding how each component of the system works in tandem and how CNG vehicles are operated.
The project’s lead design-builder and local authority on natural gas systems, CGRS Inc. worked with city vendors to adjust the programming of the CNG vehicles as well as across the RNG production and storage systems.
Meanwhile, Unison Solutions and Carollo Engineers, the biogas-upgrading system’s producer and designer, respectively, diagnosed the H2S media’s premature breakthrough as being a result of biogas losing its moisture content while traveling through several hundred feet of stainless-steel piping from the WWTP’s digesters to the biogas-upgrading skid. This was a variable whose impact the city hadn’t anticipated, but quickly pivoted to resolve. After testing three different types of media, an effective alternative designed for dry gas was selected.
Today, the system’s bugs have been resolved and its maintenance is delegated to CGRS, who runs a comprehensive maintenance program with bi-monthly on-site monitoring, system-specific standard operating procedures, and as-needed repairs. Thanks to CGRS’s service, city staff can remain relatively hands-off. They can simply observe the system’s health through the daily reports that are generated automatically using online analyzers and call-out systems.
CGRS also readied contingency measures into the CNG system, such as standby generators and a separate pipeline for purchased natural gas in case the WWTP’s RNG development halts for any reason. Having built numerous waste-to-renewable-gas facilities across Colorado, they understood the criticality of securing redundancy and backup supplies of gas for a fleet that currently has no alternative fueling location.
The city’s RNG costs are currently down to $1.20 a diesel-gallon equivalent, including all expenses associated with the maintenance contract and media replacements. Compared to paying $3 to $4 a gallon for diesel, the city’s reduced fuel expenses prepare it to afford larger maintenance items should they arise down the line.
Smart RIN Management
An enormous financial incentive to construct the RNG upgrading and vehicle-fueling system was its potential to generate RINs to be sold via the RFS program. Though excited by this prospect, the city was completely new to the EPA’s protocols, which raised the question: Who would handle the sale of RINs?
As with other aspects of the project, Longmont brought in an expert; not to turn over its RIN management but instead to guide the city in leading this process itself.
During their training and information workshop for city staff — which included the city Attorney’s Office, accounting, purchasing, and business services group — energy compliance expert Weaver suggested that navigating the EPA’s online portal was a manageable task.
In practice, generating and trading RINs has taken only one to three hours a month and is simple enough to forego hiring a RIN broker who would charge anywhere from 10 to 30 percent of profits. This means that, projecting RIN revenues of $400,000 in 2022, Longmont will save up to $120,000 by managing its credits in-house.
Again, the city arrived at this result by recognizing where consistent collaboration with veterans of their respective fields can derive solutions founded on decades of their firms’ collective experience. By communicating with these partners, the city can focus its spending on areas that are knowledge-intensive and perform its due diligence where it efficiently can.
Sharing and Growing Together
In two years of operation, the RNG upgrading and vehicle-fueling system has earned its place as a regional asset. Longmont serves as an open resource that educates and supports other municipalities considering biogas-to-vehicle fueling projects or other renewable energy efforts.
The city plans to add more CNG vehicles to its waste services fleet while continuing to replace its remaining carbon vehicles with green alternatives. And, as the system continues to produce renewable natural gas with increased consistency, Longmont will consider more options to beneficially reuse any amount that isn’t expended toward its fleets. WW
Published in WaterWorld magazine, February 2022.
