BOSTON — Utilities are touting renewable natural gas as an alternative to burning fossil fuels as part of their long-term plans to reduce greenhouse gas emissions, but state leaders and environmentalists are raising alarms about the costs, limits and potential harms of relying on the fuel source to meet climate change goals.

One of the state’s largest utilities, National Grid, announced last month that it is seeking to eliminate fossil fuels from its gas networks in Massachusetts and New York by 2050, replacing it with renewable natural gas and green hydrogen.

“Using the existing gas network that connects us to the broader region, this resource can supply fossil free gas to heat our homes and businesses,” the company states in a report outlining its plans. “Our fossil free vision assumes that National Grid eventually procures 10% to 20% of RNG annual supply potential in the eastern U.S. for our customers.”

Renewable natural gas is produced from landfills, cow manure and wastewater treatment plants and can be used interchangeably with regular natural gas that is piped through miles of distribution lines into homes and businesses in the state.

National Grid, which serves more than 900,000 gas customers in Massachusetts, says blending natural gas supplies with RNG will provide a “double benefit” by capturing methane leaking from landfills while reducing reliance on fossil fuels.

“Society will always produce waste, which naturally emits surface level methane as it decomposes,” the company said in its report. “RNG reduces emissions by capturing this potent greenhouse gas before it can escape into the atmosphere and putting it to productive use as an energy resource.”

A law signed by Gov. Charlie Baker requires the state to slash carbon emissions by at least 50% of 1990 levels by 2030 and 75% of 1990 levels by 2040.

But environmental groups are pushing back against claims, pointing out that RNG is five times as expensive as traditional fossil-produced gas, its availability is limited compared to overall gas demand and argue that its role in addressing climate change would be negligible.

“Bio-methane is still methane, and RNG saves nothing really because it’s literally equivalent to methane getting piped into homes and burned,” said Caitlin Peale Sloan, vice president of the Conservation Law Foundation in Massachusetts. “Even if they fix leaks on the distribution system, you’re still venting an enormous amount of methane into the atmosphere.”

CLF and other groups say the state shouldn’t be relying on gas infrastructure as it moves away from use of fossil fuels, saddling ratepayers with the cost.

“The utilities talk about RNG is being beneficial because they’re making huge, unfounded assumptions about the lifecycle of those fuels,” she added.

Cornerstone in plans

In Massachusetts, RNG is being produced is at the Deer Island Wastewater Treatment Plant in Boston Harbor, where methane derived from human-generated waste has been used for years to power steam turbines that produce much of the electricity for the facility.

Federal programs for renewable fuels are providing incentives for projects to convert biogas into renewable natural gas. Congress created the Renewable Fuels Standard program in 2007 to expand the nation’s use of renewable fuels while reducing reliance on imported oil.

Stephen Leahy, vice president of policy at the Northeast Gas Association, said the industry is looking to develop RNG to help reduce its overall carbon output.

“It’s a fledgling industry, supply remains a challenge, and it will take time to develop,” he said. “But it’s proven to work, and it’s fully interchangeable with natural gas.”

In its report, National Grid pointed out that the utility has been injecting RNG into its gas system since 1981 and expects the supply to increase in coming years.

“RNG supply is growing rapidly, increasing by more than six times nationwide since 2015,” the report’s author’s wrote. “Today, there are more than 150 plants in operation in the U.S. and Canada and the thousands of additional sites waiting to be developed.”

Environmentalists said they see a targeted use for RNG in small-scale projects and in reducing harmful methane leaks from regional landfills and other sites.

But utilities envision a large-scale use of the alternative fuel source, and many are making it a cornerstone of their clean energy and sustainability programs.

RNG incentives

Industry officials says the state will continue to need natural gas for a large portion of its energy, even as it turns to more renewable sources.

About half of New England’s energy comes from natural gas, according to ISO New England, which oversees the regional power grid.

Some states are also seeking to expand reliance on renewable natural gas by providing incentives to utilities to expand their reliance on the fuel source.

New Hampshire lawmakers are considering plans to allow natural gas companies to procure up to 5% of their gas from renewable natural gas. Under the plan, the utilities would have to go through a competitive bidding process to purchase the RNG supplies. Those costs could eventually be passed onto ratepayers.

In Massachusetts, environmental policymakers don’t seem to be giving the alternative fuel source much credence, at least not yet.

A major climate change bill approved by the state Senate last month calls for expanding wind and solar power but makes no mention of renewable natural gas.

On Friday, Attorney General Maura Healey released a report calling on the state Department of Public Utilities to reject utilities’ proposed “future of gas” plans that are being reviewed by regulators. Healey’s office specifically cited proposals to expand the use of “clean alternatives” to natural gas “such as hydrogen and other methane-based gases.”

“The AG’s Office urges the DPU to reject this plan because it is too risky and there are too many unknowns around costs, feasibility, and whether this approach would actually reduce greenhouse gas emissions,” it states.

Christian M. Wade covers the Massachusetts Statehouse for North of Boston Media Group’s newspapers and websites. Email him at cwade@northofboston.com.

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