The right one helps Dane County, WI biogas producers gain access to the pipeline.
By Ben Peotter and Michael Wyrick
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. Here we highlight lessons learned from integrating all the mechanical, electrical, and control systems, as well as arriving at the critical information needed to develop an accurate financial model. We also describe the series of events that culminated in the construction of this unique project.
Projects like these are becoming more common, so these post-project lessons may help others identify and avoid some of the issues addressed in the Dane County project.
Photos courtesy of Tetra Tech.
County Seeks to Offset GHG Emissions and Generate Revenue
Dane County, with a population of about 562,000, is the second most populous county in Wisconsin. The landfill takes in about 250,000 tons of waste per year. The County had implemented several biogas projects in the past 20+ years, beginning with electrical generation. Today, the biogas from the landfill is converted to natural gas pipeline quality, offsetting greenhouse gas emissions and generating revenue and renewable energy credits for the County.
In 2019, Dane County’s extremely favorable power purchase agreement was expiring. The County had previously been able to take advantage of a green energy initiative with a sustainability process that required a percentage of renewable energy on the grid. The excellent power purchase rate was also based on a spike in natural gas prices that allowed some negotiating leverage when deciding what to do with the landfill biogas. As the power purchase agreement was nearing its expiration date, other green energy sources, including wind and solar, were becoming more available to meet renewable energy production requirements. From the initial price of 10.5 cents per/kilowatt hour (kW/hr), the subsequent agreement was reduced to a possible 3.5 cents per kW/hr, making LFG to electricity infeasible due to operations and maintenance costs.
Due to these market conditions, the County began to develop financial models comparing other available options for using the gas. Options evaluated included routing the gas to a Native American casino, vehicle fuel, pipeline injection, and electrical generation from engines. The financial analysis revealed that pipeline injection was the best option for making the project viable, due to the availability of Renewable Identification Numbers (RINs) under the Renewable Fuel Standard Program that would help the County get carbon offset credits for vehicle fueling.
However, to make it work, the County wanted to develop a system that could add gas from other sources to mix with the cleaned and processed landfill gas. The idea was to co-mingle LFG with biogas from brewery waste, food digesters, pig and cow manure digesters, and other agricultural sources.
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, located in the upper left of Figure 1, page 54.. 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.
Raw biogas from the landfill is processed independently through a two-stage pressure swing absorption (PSA) system and associated treatment equipment capable of treating the LFG to TC Energy ANR requirements. Biogas generated at the remote facilities is treated onsite to the required quality level prior to being compressed into storage trailers and then transported to the County facility for unloading. The owner of the natural gas pipeline uses a gas chromatograph to measure gas quality at both points. Design and controls are in place to ensure the gas is of pipeline quality. The gas is then put into a booster station or transmission skid owned by the utility. Controls communicate between the different processing and off-loading operating systems to guarantee gas quality for redundancy.
Pipeline injection began in 2019, and the offloading facility began operations in spring of 2020. RNG produced by the landfill represents about 1,000 dekatherms per day (Dth/d) and the offload facility handles about 600 Dth/d (for a total injection of 1,600 Dth/d upon initial operation of the landfill treatment and offload station). The offload facility alone has sufficient RNG to fuel a fleet of 300 large vehicles such as refuse trucks, based on 40 gallons of diesel per day per vehicle. 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 use the facility each day.
Design, Construction, and Commissioning Challenges
Since offloading is part of a larger integrated system, coordination among vendors at the design phase is critical. It is cheaper and faster to address integration and control issues during the design phase versus fixing them in the field. Equipment quality is important—even at a premium cost—because downtime costs money.
The County faced design, construction, and commissioning challenges that had to be addressed and managed differently than originally anticipated. Challenges included having to design the facility within a small site footprint while still optimizing site access, a compressed design to commission timeline, wide pressure and temperature ranges, the integration of controls across vendor-supplied equipment packages, and other complex elements coordinated with municipal staff acting as the project coordinator and project owner. A central challenge to this project was navigating the extreme temperature drop realized when a trailer is unloaded at a rapid rate. A portion of the offloading station infrastructure had to be designed to handle temperatures as low as -100°F, which presented unique design challenges.
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 (one to two 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 to 36 week leadtimes 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 trailer unloading and processing of raw landfill gas. Ultimately, the County managed these technical and operational constraints and operates a successful system that could be an implementation model for other regional injection sites. | WA
Ben Peotter, PE, is a Client Manager Solid Waste and Renewable Services, at Tetra Tech. He has been a professional environmental engineer and innovator in the solid waste and renewable energy space for more than 23 years and has been involved with nationwide biogas resources development projects since 2005. Ben has served as client liaison and project manager and successfully completed project elements for solid waste, energy, biogas, environmental, and Brownfield Redevelopment projects around the country. He has presented at SWANA’s Landfill Gas Symposium, Global Waste Symposium, American Planning Association state and regional conferences, Parks and Recreational Association state conferences, EPA’s national Brownfield Conference, and other local, state, and regional industry events. Ben can be reached at [email protected].
Michael Wyrick is a Biogas Engineer, Dane County Department of Waste and Renewables. He has broad experience delivering innovative energy and alternative fuel projects and a lifelong passion for the wise use of resources. He also has a deep background in compressed natural gas fueling and virtual pipeline projects. He began his career as the founder of a biofuels cooperative in Chicago and in the 18 years since, Michael has had roles in engineering design, operations, and R&D at two utilities and a vertically integrated alternative fuel provider. Michael can be reached at [email protected].