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Renewable Natural Gas as a Sustainable Energy Source

With regulatory and community pressure to use sustainable and cost-effective energy sources, alternative renewable energy options are regularly evaluated. One example with increased interest is the development of renewable natural gas. John Heyman, our oil and gas business development leader, and Brian Bakke, our biogas practice leader, explain how RNG has become a viable energy source.

Q: What is RNG? 

Brian: In simple terms, RNG is sustainable biogas or landfill gas used as a natural gas replacement. Raw biogas is generated at municipal solid waste landfills or through the anaerobic digestion of agricultural wastes or animal manures, yard and crop wastes, food and food processing wastes or biosolids from wastewater treatment plants. It typically has a methane content between 45% and 65% depending on the source, so it’s taken through a series of steps to convert into RNG. Treatments vary based on the end use, but typically include removing moisture, hydrogen sulfide, carbon dioxide and trace-level contaminants as well as reducing the nitrogen and oxygen content. Per the Environmental Protection Agency, once purified the RNG has a methane content of 90% or greater for vehicle fuel use and typically 96% to 98% for pipeline injection.

Q: How is RNG used?  

John: Over time, market drivers have shaped how RNG is used. In 2011, nearly all U.S. RNG projects provided RNG to generate a combination of heat and/or electricity. As the market for renewable transportation fuels emerged as a result of the EPA Renewable Fuel Standard and state rules like California’s Low Carbon Fuel Standard, the overall number of RNG projects grew rapidly, and the end use of RNG shifted. In 2017, 76% of RNG projects converted RNG into transportation fuels per the Coalition for Renewable Natural Gas.

In addition to providing sustainable vehicle fuel, RNG is injected into natural gas pipelines to supplement local distribution companies’ product offerings to consumers. Natural gas utilities that have received or plan to receive RNG in their networks include many of our clients — Black Hills Corporation, Dominion Energy, Duke Energy, Southern Company Gas, National Grid, Puget Sound Energy, Williams Gas Pipeline, and more.

Q: What are the benefits of RNG? 

John: A few benefits of RNG include: 

  1. Diversifying fuel supplies and increasing fuel security. Biogas sources used in RNG are almost continuously available, and using these alternative sources helps diversify domestic energy production and enhance the United States’ energy security. 
  2. Providing economic benefits to communities. RNG produced at municipal wastewater treatment plants and solid waste landfills can generate revenue from a frequently wasted resource. The additional revenue can offset the increasing cost of treatment or disposal. In addition, constructing infrastructure and selling vehicles that can use RNG can benefit local economies by creating jobs.  
  3. New revenue source for American farmers. Selling biogas sources creates an opportunity for dairy, hog and poultry farmers to convert a waste problem into a valuable supplementary revenue source. 
  4. Reducing greenhouse gas emissions. Not only is there less direct natural methane in the atmosphere because the biogas is being processed into RNG, the RNG itself burns cleaner than diesel fuels. According to the American Gas Association, methane from animal waste and other biomass sources that otherwise would have entered directly into the atmosphere is instead combusted as RNG. This results in the release of greenhouse gases that are approximately 21 times less potent than methane released directly into the atmosphere.  
  5. Positive Community Reactions. Members of the public often view local green programs positively, which can build community support, and present great marketing and publicity opportunities. 

Q: How many operational RNG projects are there? 

John: According to the Coalition for Renewable Natural Gas’ RNG Production Facilities Database, as of September 2020, there are 119 operational RNG production facilities in the United States with another 36 under construction and 62 in substantial development.

Q: What technologies are used to produce RNG? 

Brian: Several technologies are used to convert biogas to RNG. Technology selection is project-specific and influenced by the quantity and quality of the raw biogas produced as well as the end use or natural gas pipeline requirements. The most critical and expensive portion of the treatment process is removal of carbon dioxide to increase the heating value of the biogas to natural gas quality. Three main technologies used are:

  1. Membrane treatment. In this approach, porous membranes are used to separate methane from carbon dioxide and other contaminants. Membranes are typically used on small- to medium-capacity applications and require significant biogas pretreatment to remove hydrogen sulfide, siloxanes and volatile organic compounds that could foul the membrane.  
  2. Pressure swing adsorption. PSA is a continuous batch process used to adsorb carbon dioxide and other molecules onto a selective media and allow the pure methane to flow through the process. PSA equipment has the advantage of having fewer pre-treatment and post-treatment requirements.  
  3. Waterwash scrubber. This style of gas conditioning capitalizes on the different solubilities of methane and other contaminants in water to separate RNG from contaminants using a pressure scrubber and flash tank. Waterwash scrubbers are typically used on larger applications and do not require pretreatment. Water is removed from RNG with a dryer and further compressed for pipeline injection.  

Q: How do you evaluate the best technology solution to maximize project value and potential for growth of an RNG project? 

Brian: To determine the most beneficial solution for biogas utilization, a financial feasibility assessment can quantify the value of RNG along with the capital investment required, while addressing ongoing operations and maintenance costs to bring it to market. For example, in 2018, we completed a biogas utilization feasibility study for the City of Lincoln, Nebraska, to determine the best use of its biogas. The city was considering a co-generation plant replacement at the Theresa Street Water Resource Recovery Facility. Study results showed conditioning the biogas to RNG quality for pipeline injection was a more economical option than continued power generation or a dedicated RNG fueling station.  

Many clients have sustainability goals, greenhouse gas initiatives or climate action plans that value environmental outcomes in addition to economics. In these cases, a Sustainable Return on Investment analysis can determine the hidden value of a project and project options by assigning monetary values to a series of costs and benefits: economic, social and environmental. This helps compare value versus cost to make decisions that are transparent, accountable and defensible.  

A key part of the analysis is determining if and where local, state and federal funding is available. In addition to aiding the decision-making process, the results of an SROI analysis help communicate the full value of a project, thus improving funding opportunities and, in many cases, increasing public support.

Q: What barriers are there to RNG project development? 

John: Generally, there are two interrelated types of barriers in RNG project development: economic and technical. Economic barriers are a challenge because of the uncertainty and risk associated with RNG markets in the long term, and current market conditions make it more expensive to produce RNG than to purchase natural gas. On top of that, processing biogas to RNG can be expensive and require large capital investments.  

Technical barriers to RNG apply to both natural gas utilities and RNG producers and can become economic barriers if not addressed. RNG producers deal with the challenge of balancing the high cost of treatment and monitoring equipment to meet utility pipeline specifications. To be injected into pipelines, RNG must meet gas quality specifications that can vary by state and pipeline system.  

Utilities are developing acceptance criteria and guidelines to protect them from risks associated with taking in RNG instead of natural gas. As an advocate for the industry, the Gas Technology Institute is conducting studies that will provide a fact-based, objective study on the quality, analysis, risk and compositional variability of final end-use RNG. GTI is also identifying sensor systems that can detect and analyze trace contaminants that could negatively affect end-use equipment. 

RNG producers also face the challenge of trucking and handling of food wastes, organics and other feedstocks that can create unintended consequences for digestion facilities. Odor generation from anaerobic digestion facilities can erode public support quickly. RNG producers also have to consider how accessible their markets are by considering project locations with access to either a pipeline with enough capacity for maximum production of RNG or a vehicle fleet or industry that could utilize the gas. 

These challenges impact the cost of an RNG project or to its potential revenue, sometimes making other sustainable energy alternatives more attractive. However, with the current status of federal and state incentive programs, RNG can still be beneficial to producers and end users if economic and technical barriers can be overcome.