Challenges to Choosing the Best RNG Processing and Offloading Station

The right one helps Dane County, WI biogas producers gain access to the pipeline.

Authors: Ben Peotter, PE, Tetra Tech and Michael Wyrick, Dane County Department of Waste and Renewables

Dane County’s Department of Waste & Renewables operates a state-of-the-art renewable natural gas (RNG) processing facility that converts an adjoining landfill’s biogas into pipeline quality natural gas and injects it into a nearby interstate pipeline. The RNG facility includes a virtual pipeline station that allows agribusinesses, like Wisconsin dairy farm manure digesters, to transport their gas to the Dane County facility for offloading and pipeline interconnect. As the first project in the nation to certify several pathways and feedstocks at the same time, the novel offloading station concept reduces the barriers for biogas producers to gain access to the pipeline, renewable energy markets, and other federal and state credits.

But, like many new ideas, the trail-blazing project was not without challenges.

County develops novel system

In 2017, the County began developing a landfill gas to RNG and compressed RNG offloading facility for agricultural digesters. The overall facility cost was about $28 million, with $5.5 million dedicated to the RNG trailer offloading facility. Treated LFG is comingled with RNG offloaded from area agricultural digesters and then injected into the TC Energy ANR Pipeline Company line via a single interconnect.

Pipeline injection began in 2019, and the offloading facility began operations in spring of 2020. A key feature of the RNG offloading station that makes this project so unique is its ability to accept RNG from multiple offloaders through a single unloading and monitoring point. This feature has opened the door to area farms who had not previously considered converting their agricultural digester gas into RNG. Now that a new connection point has been made available to the public, pending contractual agreements, nearby dairy farms and other biogas generators that do not have the resources or ability to tie into a pipeline can take advantage of this combined system, making more projects economically viable. The facility is designed to minimize the time it takes to unload a trailer, which in turn maximizes the number of customers that can utilize the facility each day.

Lessons learned

Lesson #1 – Know your customer

It may seem obvious, but the first thing anyone hoping to develop a remote offloading system for digester gas should know is who is your customer? RNG owners, operators, gas transporters, gas/credit marketers each have different goals and requirements. Knowing your customer also means identifying and communicating with those who make the decisions for operational, technical, commercial, and regulatory issues. Use this information to develop a scheduling plan and a reporting system. Scheduling and coordination among parties should maximize offloading given customer constraints. Coordination must consider how fast different tanker designs unload, what is the lowest pressure they can get to, and how you will account for overlap between tankers.

Lesson #2 – Know your customer’s trailers

Repeat after us: trailer build matters! Using a trailer outfitted with traditional Type 1 steel storage cylinders versus Type 4 composite cylinders found on the largest trailers will have major impacts on engineering, operation, and economics. It is critical to accurately understand trailer specs upfront for engineering and operations planning. Different temperatures, pressures, and flow limits unique to each trailer type must be respected. There are trailer-specific minimum temperature and pressures as well as trailer-specific operating and emergency plans. Without the proper planning, one trailer type can restrict a system and reduce the number of customers the facility can serve.

Lesson #3 – Pay careful attention to designing for low temperatures

There are important design, operation, and business case considerations for systems developed for use in low temperatures. For example, rapid offloads (1-2 hours) in winter (~30ºF for Dane County) may result in more product gas left on the trailer than planned. When sizing any trailer offloading station, one must thoroughly understand temperature and pressure limitations; otherwise system restrictions may result in trailers offloading at a slower rate than what the system is designed for. As described earlier, the more rapidly a trailer is unloaded, the lower the temperature of the RNG will drop and as a result, equipment may freeze. Cold temperatures during the winter months exacerbate this issue, as all equipment, including the trailer and associated RNG, will be at a much lower temperature than in summer months before the offload process even starts.

There are a few main areas where low temperatures can add significant restrictions to the offloading process:

• Residual moisture in the RNG inside of the trailers can freeze and block flow, requiring a warmup period to remove the restriction.
• Standard stainless and carbon steel pipes and valves are only rated to a certain minimum temperature – specialized low-temperature rated material may be required to safely operate the facility.
• The trailers themselves have a minimum design temperature that they must stay above to avoid damage to or even failure of the storage cylinders.

With proper planning and the inclusion of infrastructure specifically designed to operate at these potentially extremely low temperatures, many of these potential issues can be successfully mitigated.

Lesson #4 – Develop specific low temperature operations plans

Both ambient and process temperatures are critical to operation, but one must expect and plan for failures and challenges during winter. Systems should be rated down to a temperature of at least

–65ºF, at minimum. Though RNG facilities at digesters will be designed to meet the pipeline tariff moisture limit, even the very small amount of residual water in the gas may freeze as a result of the rapid temperature drop during offloading. Stricter moisture limits on the digester agreements, made possible by the inclusion of additional robust drying equipment, may alleviate some of these concerns. While there will always be a small amount left on a trailer, there is a payback point at which it might make sense to invest in a gas dryer on the digester side to maximize the trailer offload rate.

Lesson #5 – Tips on systems and parts

RNG compression and unloading systems need to be capable of operating across a wide range of temperatures, pressures and flow rates, so of course safety is of paramount concern. When operating a system at pressures in excess of 4,000 PSI, any failures have the potential to be catastrophic and cause injury and death, in addition to the damage or destruction of expensive and specialized infrastructure. Incorporation of adequate safety equipment such as pressure relief valves, accurate real-time pressure and temperature monitoring, and gas detection devices is a project necessity.

A good understanding of the supply chain and associated vendor lead times is also important for any RNG project. There are leading suppliers spread around the globe that provide equipment required for these facilities, but anyone looking to develop a project should expect at least 24-36 week lead times for most equipment, with some systems approaching or exceeding one year. Once a facility is operational, it is imperative to maintain a stock of critical spare parts. Remember → downtime = lost $$.

Planning leads to successful implementation

As the Dane County project evolved, the team identified numerous specific technical issues inherent to the trailer rapid offloading, managing such elements as unloading time, connection types, and space constraints, as well as coordination of the variety of stakeholder trailers and associated differing pressure and temperature limits. They also had to continually overcome challenges that arose associated with specific design elements of the two distinct systems – high-pressure RNG successful system that could be an implementation model for other regional injection sites.

A version of this article originally appeared in Waste Advantage Magazine, March 2024.