Pipeline injection vs. a closed-loop system: which RNG route is right for your facility?

Northern Colorado Clean Cities recently interviewed CGRS and City of Longmont, Colorado, representatives about the ins and outs of constructing a system that converts biogas at a wastewater treatment plant to Renewable Natural Gas (RNG). The goal: provide useful information about the decision-making and construction processes so other jurisdictions can determine if a similar project is right for their facilities and their communities. The following article is based on the content of those interviews.

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Cory Kahler
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The City of Longmont found all the pieces were in place to complete its biogas system puzzle:

  • its Wastewater Treatment Plant (WWTP) produced biogas;
  • it had or could attain the funds for a Renewable Natural Gas (RNG) project, which was part of its sustainability plan; and
  • a bulk of its Waste Services diesel-fueled fleet was up for replacement

The solution to build a closed-loop system, in which the RNG produced at its WWTP would be piped to a CNG facility and used to fuel its Waste Services fleet, was a no brainer.

But that solution may not work for every jurisdiction or WWTP.

For some, like the City of Boulder, the answer may be injecting the RNG produced at its Water Resources Recovery Facility into a utility pipeline to which other end users have access. In this case, Western Disposal in Boulder is purchasing all of the City’s RNG to fuel up to 38 commercial vehicles in its fleet of 56. CGRS also constructed that biogas system.

For others, the best use of the RNG would be for heat and electricity.

Doing the math to determine how much biogas the WWTP produced and whether it would fuel enough vehicles wasn’t too difficult for Longmont officials, said John Gage P.E., Public Works and Natural Resources Civil Engineer for the City of Longmont.

With a population of approximately 100,000, Longmont’s average wastewater flow rate is 7 million to 8 million gallons a day, which produces 100 cubic feet of biogas per minute, or 100,000 to 150,000 cubic feet a day. Biogas systems usually have packages that treat certain flow rates, so Longmont “fell in that nice threshold … and system sizing worked out well for us,” Gage said.

If a WWTP has an anaerobic digester and, hence, biogas, it’s not a complicated process to convert methane from flaring off to a biogas treatment system, he said. Generally, biogas treatment systems are a modular, bolt-on type, where contaminants are removed in a “black box” and the biogas is treated. The system can bolt into your existing plant without much trouble.

For a heat-and-power unit that generates electricity and heat, officials would need to consider how the WWTP hot water loop works and how to incorporate the RNG system into it, Gage said. Pipeline-injection quality RNG likely requires other treatment units in the RNG package.

That also requires a bigger capital investment, said Cory Kahler, CGRS Water/Wastewater Operations Manager and an expert in RNG/Compressed Natural Gas (CNG) systems.

Renewable Natural Gas RNG - Environmental Consulting Company - CGRS

Longmont city administrators made the decision to not only turn the biogas into RNG, but also to use it for its own purposes based on a number of factors that other jurisdictions can apply to their situation, determining if and what kind of a biogas project is right for them.

  • Do you have sufficient fleet demand to justify your own CNG system? Do you have enough consumption that would actually use up most of if not all of your renewable natural gas? Because Longmont has a fuel master system that tracks all of its fuel consumption, officials knew their Waste Services fleet had stable operations all year long. “That was a big check for us that said we have internally the ability to fuel one portion of our fleet,” Gage said.
  • Do you have diesel trucks in your fleet or CNG trucks, and how do you maintain them? If need you to convert, what are the implications? The City of Longmont was needing to replace 11 waste services trucks out of 19, “so we pumped the brakes there and didn’t replace them with diesel trucks yet. We used them longer and decided to jump to CNG,” Gage said.
  • Do you have a fleet maintenance building set up to maintain CNG vehicles? If not, it would need to retrofit its current one or build a new one. Longmont administrators had considered that CNG vehicles would enter the picture years ago, so its vehicle maintenance building was built with the capability to maintain them, Gage said.
  • Whether the jurisdiction injects its RNG into the pipeline or uses it for its own purposes, administrators need to understand what is their natural gas utility’s stance on pipeline injection is, Gage said. “This is not something done everywhere, but it is becoming more common. We’re really fortunate here on the Front Range to have had the City of Boulder and South Platte Valley Water Renewal Partners both move forward with pipeline injection projects and both get approval.” Boulder is now injecting into the pipeline, setting a great precedence for negotiating and making sure the infrastructure is in place for other utilities to follow, he noted.

Gage stressed that determining whether to incorporate an RNG system into WWTP operations and what kind is more about prioritizing and what the community cares about. They should asked themselves these questions:

  • Is quick payback essential?
  • Do they have the staffing and capacity to add a new process to their organization?
  • Do they have their own sustainability goals they need to meet?

RNG systems “really tend to be driven by the organization’s values,” Gage said, adding biogas projects are “outward facing. They’re great opportunities to interface with the public and show that you’re good stewards of resources and you care about the community, air quality and climate-action goals.

“It has provided a lot of benefits to us in ways that we didn’t realize beyond monetary,” he added. “Consider not only financial components but your organizational values, as well.”