Energy Efficiency

Integrating renewable natural gas

Renewable natural gas (RNG) can be used in any existing infrastructure or process that currently uses conventional natural gas.
By Gas Technology March 26, 2019
This photo shows a wastewater treatment plant and the anaerobic digesters that exist onsite to process the treatment plant’s sludge and turn it into a usable product—biogas—which can be cleaned to become RNG. Courtesy: Enbridge Gas Distribution Inc.

Renewable natural gas (RNG) is carbon-neutral energy created from decomposing organic waste. Methane emissions from municipal landfills, wastewater treatment plants, farms, and industries can be captured, refined and converted into renewable energy.

RNG is not a fossil fuel and does not add carbon to the environment. Even more importantly, RNG captures and converts methane from waste treatment facilities, preventing the release of emissions that are 25 times more harmful than CO2. This significantly reduces the carbon footprint of energy consumption while lowering greenhouse gas emissions.

This drawing shows the renewable natural gas (RNG) process at a high level. The biomethanation stage includes cleaning the resulting biogas from the landfill, wastewater treatment plant, or the anaerobic digester where the organics ended up. Courtesy: Enbridge Gas Distribution Inc.

This drawing shows the renewable natural gas (RNG) process at a high level. The biomethanation stage includes cleaning the resulting biogas from the landfill, wastewater treatment plant, or the anaerobic digester where the organics ended up. Courtesy: Enbridge Gas Distribution Inc.

RNG is fully interchangeable with conventional natural gas for transmission, distribution, and everyday use. It can be used in any existing infrastructure or process that currently uses conventional natural gas.

Integrating RNG

Since RNG is interchangeable with natural gas, that begs the question: Can RNG be integrated into a utility or natural gas system? The answer is, “Yes.”

“Because we are a gas transmission utility company, we do accept RNG into the system,” said Justin Egan, senior advisor RNG, Business Development at Enbridge Gas Distribution Inc. “However, before RNG can be injected into the system, it needs to meet very specific gas quality specifications. Should these specifications be met and there is capacity on the local system, the RNG can be accepted into the system. Because biogas does not meet these specifications, we cannot accept this into our system.”

“We have been accepting RNG in Staten Island since the 1980s from the Fresh Kills Landfill,” said Donald Chahbazpour, director, Gas Utility of the Future at National Grid. “A second RNG facility is expected to come online in Brooklyn later this year (2019) at the Newtown Creek wastewater treatment plant. That project came from a partnership between National Grid and New York City and will upgrade biogas from the wastewater treatment plant that would otherwise be flared. National Grid also is working with seven project developers who have submitted interconnection requests.”

Getting connected to the utility

“Although there are many different sources of natural gas, the process of obtaining an interconnection is similar regardless of the source,” said Jim Lucas, market development manager at Southern California Gas Company (SoCalGas). “The process starts with an interconnection capacity study, which determines the utility’s downstream capacity to take the RNG away from the interconnection point and the associated utility facility enhancement cost.”

Those who wish to integrate RNG into the SoCalGas utility are asked to review and complete the Gas Supplier Interconnection Project Fact Sheet, which SoCalGas will use to evaluate its ability to accept supplies from the potential RNG project. The fact sheet is a multipage application that asks for project profiles, flow and pressure data, and source of gas supply, such as dry gas zone, oil-associated, liquefied natural gas (LNG), or biogas. The section that asks for the anticipated gas quality is extremely detailed. It asks for as many as 20 gas constituents, such as methane, ethane, propane, CO2, several butanes, several types of mercaptans, and much more. That section also asks for other constituents like arsenic, vinyl chloride, toluene, antimony, copper, and lead, as well as their biogas source. These are typically from dairy or publicly operated treatment works (POTW) landfills.

This photo shows an anaerobic digester operation. Courtesy: Enbridge Gas Distribution Inc.

This photo shows an anaerobic digester operation. Courtesy: Enbridge Gas Distribution Inc.

“Interconnectors (those who wish to integrate RNG into the utility) are responsible for the actual costs needed to perform the interconnection capacity study,” Lucas continued. “These costs typically range from $5,000 to $10,000 and requires six weeks to complete. The next step is the preliminary engineering study, which develops the preliminary cost estimates for land acquisition, site development, right-of-way, metering, gas quality, permitting, regulatory, environmental, unusual construction, operating, and maintenance costs. Interconnectors are responsible for the actual costs needed to perform the preliminary engineering study. These costs typically range from $50,000 to $60,000 and requires four to five months to complete. The third step is the detailed engineering study. There are three elements in this study:

  1. Description of all costs of construction.
  2. Development of complete engineering construction drawings.
  3. Preparation of all construction and environmental permit applications and right-of-way acquisition requirements.

“Interconnectors are responsible for the actual costs needed to perform the detailed engineering study as well. These costs typically range from $145,000 to $225,000 and four to six months to complete,” Lucas said.

“If there is an interest in creating/injecting RNG, our group will certainly assist wherever possible,” Egan said. “We contain RNG market knowledge and pipeline system expertise. We will request from you several key aspects of information. We want to know the project location, RNG volume production, project timelines, and type of project (landfill, anaerobic digestion, etc.). Following our analysis, we will provide the market takeaway capacity, injection pressure and location, cost estimate, and a service/purchase agreement. All information is kept confidential.”

“National Grid has developed an internal interconnection process supported by gas engineering,” Chahbazpour said. “The process involves a preliminary evaluation, an engineering feasibility analysis, an interconnection agreement, and project commissioning. This process has informed the NY RNG Standard Interconnection Guide. The NY Standard Interconnection Guide is being developed through a collaborative process with all of New York’s gas utilities and RNG industry groups. The guide’s purpose is to maximize the acceptance of RNG into the natural gas network by clarifying the steps to connecting RNG projects and outlining the responsibilities for utilities and project developers. The document bridges both policy and technical concerns of project developers and pipeline operators. The final version is expected to be public soon.”

Ensuring gas quality

“We ensure that the projects that connect to our system produce pipeline-compatible gas,” said Chahbazpour. “In other words, we ensure that the gas will not alter the general composition of gas flowing through the distribution system or impact our customers. We do not use a single gas quality specification because it would be too restrictive. RNG projects are unique in size and feedstock, and gas distribution systems also vary in size and seasonal gas demand. When assessing the ability to interconnect a project, we take into account the pressure and design of the surrounding gas distribution system, we identify which customers will receive the mixture of RNG and traditional natural gas, and we consider the size of the RNG project. Once we understand how the project will interact with our distribution system, we agree on gas-quality specifications with the project developer. By not setting ‘one-size-fits-all’ gas quality specifications, we aim to connect more RNG to our network, while ensuring that our system and our customers are not adversely impacted.”

This photo shows a wastewater treatment plant and the anaerobic digesters that exist onsite to process the treatment plant’s sludge and turn it into a usable product—biogas—which can be cleaned to become RNG. Courtesy: Enbridge Gas Distribution Inc.

This photo shows a wastewater treatment plant and the anaerobic digesters that exist onsite to process the treatment plant’s sludge and turn it into a usable product—biogas—which can be cleaned to become RNG. Courtesy: Enbridge Gas Distribution Inc.

Chahbazpour also said that gas quality and flow rates must be monitored to ensure the gas meets the agreed-upon specifications. Monitoring is performed by online instrumentation for parameters, such as hydrocarbon composition, Wobbe Number, specific gravity and heat content, nonhydrocarbons sulfur compounds, temperature, pressure, and moisture. The Wobbe number is an indicator of the interchangeability of fuel gases such as natural gas, liquefied petroleum gas (LPG), and town gas, and is frequently defined in the specifications of gas supply and transport utilities. “This information is shared in real time with our Gas Control group. Our Gas Control group is capable of isolating the RNG project from our system remotely if gas quality falls out of the agreed-upon specifications,” he said.

According to Lucas, the (SoCalGas) utility point-of-receipt facility has real-time gas monitoring equipment to ensure the gas is compliant with the Rule 30 specifications. Rule 30 provides the general terms and conditions applicable whenever the utility transports customer-owned natural gas over its system. “The supplier or interconnector is responsible for processing the natural gas or biogas as necessary to meet the Rule 30 specifications for pipeline quality natural gas so that it is then capable of being received into our natural gas transportation system for sale throughout Southern California,” he said.

SoCalGas Rule 30 describes the requirements for gas to be injected into the utility pipeline. “These requirements reflect the first and foremost priority of SoCalGas to protect its customers, employees, contractors and pipeline system, as well as the public,” Lucas continued. “The standards cover two major aspects: gas constituent limits (composition-based specifications) and gas interchangeability specifications (performance-based quality specifications). Gas constituent limits restrict the concentration of gas impurities to protect pipeline integrity and ensure safe and proper combustion in end-user equipment. The interchangeability specifications address end-user combustion performance, ensuring safe and proper combustion for customers.”

Gas quality standards for SoCalGas include:

  • Heating value (Btu/scf): minimum = 990; maximum = 1,150
  • Water content (Lb/MMscf): 7
  • Various inerts: CO2 = 3%; O2 = 0.20%; total inerts = 4%
  • Hydrogen sulfide H2S (grain/100scf): 0.25

The utility also does periodic gas sampling for certain gas constituents. SoCalGas Rule 30 requires that gas quality testing on biomethane constituents of concern be done by independent certified third-party laboratories, according to Lucas.

For Enbridge, online quality monitoring is located at the injection station or is obtained via producer signals. The monitoring can be done with several different types of technology. “There are several components that are continuously monitored and measured,” Egan said. “CO2 is always monitored at the injection station. In addition. Water vapor, H2S, and O2 also are measured. RNG also is analyzed for contaminants during commissioning of the RNG facility. After the RNG facility is commissioned, sampling will be performed at a frequency determined by an engineering assessment.”

RNG integration in action

“Currently, the City of Hamilton, Ontario is injecting RNG into the gas grid,” said Egan. “It creates RNG at its wastewater treatment facility. The organics used to make the RNG comes from the city’s wastewater. In addition, the city of Toronto is currently developing an RNG project at its Dufferin solid waste management facility. The project will inject RNG into the natural gas grid. Once in the grid, the city will be able to use the RNG to fuel its collection trucks.

– This article appeared in the Gas Technology supplement.

More info

Enbridge Gas Distribution Inc.

Energy Solutions Center

National Grid

SoCalGas

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