Renewable natural gas represents another phase in the development of biogas as a valuable fuel.


For years there have been two default choices for clean-water plants that produce and use digester methane: Burn it in boilers, or feed it to engines for electricity production (or combined heat and power).

Now, options for use of biogas are expanding to include production of vehicle fuel or pipeline-quality gas suitable for use in homes and business. These new options are developing slowly yet steadily, and in some cases, may prove more beneficial to utilities than the traditional choices.

Consider that for combined heat and power, the utility takes charge of scrubbing impurities from the raw biogas and for operating the engine-generators. That’s to say nothing about the capital cost to install the gas processing, power generation and heat recovery equipment.

Related: Worldwide Biogas Market Thriving, Positioned to Grow

Despite all that, many combined heat and power projects provide excellent return on investment. Still, can a utility get even better returns by simply selling the fuel to a company able to process it for use in vehicles or for pipeline injection? In a time when capital dollars can be hard to come by, it’s a possibility well worth considering.

Here’s a quick sketch of the industry that has been evolving in recent years around extensively processed biogas — under the broad label of renewable natural gas (RNG).

What are the sources?

Related: In My Words: EPA Offers Advice For Combined Heat And Power With Biogas Engines And Turbines

Clean-water plants aren’t the only potential sources of RNG. Others include landfills, animal manure digesters and food waste digesters. The last two facility types have been springing up at multiple locations around the country. In addition, more clean-water plants today are exploring co-digestion of food waste along with wastewater biosolids to increase gas production. Still other sources include organic waste from industries, like meat, vegetable and other food processors.

The potential sources of RNG in the United States are largely untapped. The Water Environment Federation states that of the nation’s 16,000 water resource recovery facilities (the federation’s preferred term for wastewater treatment plants), about 1,500 produce biogas by way of anaerobic digestion. The Coalition for Renewable Natural Gas says that just 42 of 1,000 landfills that could produce biogas have gas-to-energy projects yielding highly purified fuel.

What are the benefits?

Related: Sheboygan WWTP Achieves Net Zero Energy With Biogas

RNG has benefits that cross multiple sectors. Like biogas-based cogeneration, it helps reduce greenhouse gas emissions by consuming methane, which if emitted to the atmosphere has 21 times the climate-changing potency of carbon dioxide. If used as vehicle or home heating fuel, RNG helps reduce reliance on fuel imports. In addition, a growing RNG industry could lead to development of new technologies and generate “green jobs.”

In the vehicle sector, RNG is most often used by commercial and municipal fleets. Clean Energy, a major producer and marketer of RNG, estimates a fleet that uses a million gallons of gasoline per year could cut its greenhouse gas emissions by about 9,700 metric tons by switching to RNG. That’s equivalent to taking 1,940 cars off the road.

What is RNG’s potential?

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How much impact can RNG have on the nation’s energy picture? The answer depends on which source you consult. Studies conducted in recent years by industries and government agencies have determined that RNG from all sources — not just clean-water plants — could satisfy a meaningful share of U.S. demand for natural gas. For example:

  • The National Renewable Energy Laboratory estimated in 2013 that RNG from all feedstock sources “could displace about 5 percent of current natural gas consumption in the electric power sector and 56 percent of natural gas consumption in the transportation sector.”
  • The American Gas Foundation, in a 2011 study, found that if 15 to 75 percent of biomass could be converted to RNG in a concerted national effort, 10 percent of the natural gas demand could be met. This analysis also covered all sources and included 436 clean-water plants.
  • National Grid, a gas supplier that serves Massachusetts, New York, New Hampshire and Rhode Island, estimated in 2011 that RNG had “technical potential” to meet up to 25 percent of natural gas demand in those states.

What exactly is RNG?

Raw biogas, depending on its source, contains 50 to 70 percent methane. It also includes a variety of impurities, including water, oxygen, nitrogen, sulfur and chlorine compounds, and siloxanes (silicon-based compounds in products like cosmetics, shampoos and toothpaste).

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This gas, after relatively minor cleanup, can be fed to engine-generators to produce electricity and heat. However, as fuel vehicles or for injection into natural gas pipelines, the gas must be processed to much higher purity standards to remove the contaminants and increase the methane content to the neighborhood of 90 percent. It is then compatible with vehicle engines and with pipeline natural gas supplies.

What are the barriers?

A variety of factors stand in the way of widespread RNG development. One is the absence of a national standard for RNG quality — the physical and chemical properties the gas must have. At present, utilities and state utility commissions are developing their own standards independently.

This makes it difficult for RNG producers to get approval to inject their product into a pipeline. 

The marketplace also lacks standards for sampling and monitoring of RNG to make sure that it meets quality standards continuously.

Costs for pipeline injection also can be substantial. They include the costs of compression to meet utility requirements, monitoring of fuel heating value, piping to the injection site, the physical connection to the utility and tariffs for transporting the fuel through utility distribution network.

Who are the players?

Some clean-water plants are already in the game with RNG. The Point Loma (California) Wastewater Treatment Plant formed a partnership with BioFuels Energy to purify biogas and feed RNG into the natural gas pipeline system. About five years ago, the plant was flaring some 1.3 million cubic feet of biogas daily.

The clean-water plant in Grand Junction, Colorado, produces RNG as fuel for the city’s fleet of garbage trucks and buses. In New York City, the Newtown Creek Wastewater Treatment Plant is building a purification system to produce RNG for residential and commercial use by way of the National Grid distribution system. The project ultimately could provide enough gas to heat about 5,200 homes, according to the city Department of Environmental Protection.

These are just a few examples of RNG projects at clean-water plants. Other projects are in various stages of discussion, feasibility studies and planning.

RNG has potential to play a meaningful role in diversifying the nation’s energy supply, building a clean-energy economy, creating jobs and reducing emissions of greenhouse gases. It is an alternative clean-water plant teams might do well to explore.


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