Why a Seattle cement plant burns a million tires a year and wants to burn more

One of Seattle’s oldest businesses is also one of its biggest polluters.

For anyone crossing the Duwamish River on the West Seattle bridge, the Ash Grove Cement plant is hard to miss.

Its pair of giant concrete domes and eight-pack of tall silos jut above the low warehouses of Seattle’s industrial Sodo district. Small hills of limestone, gypsum, and other raw materials barged in from Canada line the riverfront. The plant’s 27-story cooling tower, adorned by an illuminated American flag, dwarfs even the silos.

Beneath the cooling tower, a massive cylindrical kiln spins on its side like a giant rolling pin, except this cylinder gets up to 3,000 degrees Fahrenheit inside.

Every day, to heat that kiln and turn limestone into “clinker,” a key ingredient of cement, the plant burns natural gas and thousands of old tires for fuel.

The Ash Grove cement plant has been on the banks of the Duwamish River for nearly a century. It manufactures about one-third of Washington’s cement, used to make concrete for highways, buildings, and other essential infrastructure.

In recent years, community groups in the area have been clashing with the plant over its air pollution.

In November, the Duwamish River Community Coalition protested outside the offices of the Puget Sound Clean Air Agency.

“No more burning, no more fires. Protect our kids from toxic tires,” protesters chanted.

The Duwamish Valley has some of the region’s highest rates of childhood asthma, according to some studies, and some of the worst air pollution.

The activists were hoping to influence the agency’s imminent decision on Ash Grove’s five-year-old request to increase its tire burning beyond 30% of its fuel supply.

Currently, the plant burns 1.2 million tires a year.

“Tires, we know, are bad for the air, the air pollution,” said Mia Ayala-Marshall, clean air program manager for the Duwamish River Community Coalition. “[Burning tires] can emit toxic pollutants like dioxins and furans and mercury, which are really bad for human health. They are cancerous. It can cause respiratory and cardiovascular issues.”

The air agency in December gave the company permission to boost its tire burning to 37% of its fuel supply.

The regional air agency found that burning tires at the very high temperatures found inside a cement kiln would not increase pollution.

Environmentalists appealed that decision in January, calling it arbitrary and capricious.

Puget Sound Clean Air Agency spokesperson Phyllis McElroy declined an interview request, saying the agency isn’t allowed to do media interviews while an agency decision is being appealed.

Before the Ash Grove permit appeal was filed, McElroy issued a written statement.

“The Agency remains committed to protecting air quality and ensuring compliance with all regulations,” McElroy said by email.

“The proposed change is not expected to increase emissions, and may reduce some pollutants by lowering natural gas use,” McElroy said.

The agency’s conclusion relied on studies of cement kilns that burned both coal — the dirtiest of fossil fuels — and natural gas. Ash Grove’s Seattle plant has not used coal in a decade.

The U.S. Environmental Protection Agency also has jurisdiction over the plant’s emissions. Under President Donald Trump, the federal agency emphasizes deregulation and “energy dominance” and declined to weigh in on the decision.

Air pollution is a major concern in the Duwamish Valley, an area that has twice the poverty rate of Seattle and where most residents are people of color.

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A study released Thursday by the Washington Department of Ecology found that South Seattle — the area encompassing the Duwamish Valley — was “highly impacted by air pollution.” The authors linked 57 deaths a year in the area to elevated levels of soot-like fine particulates in the air.

“We estimated the 57 deaths would not have occurred if pollution levels were at baseline,” lead author Kirsten Dodroe said by email.

Ash Grove is one among many pollution sources in South Seattle. It is also one of the largest.

Ash Grove spokesperson Carolina Lucaroni declined repeated requests for a tour of the plant and an interview but did respond to a list of written questions. She said using tires for fuel is “best practice” in the cement industry and supports a circular economy.

“These alternative fuel sources [tires] allow cement plants to benefit from the use of locally available materials that would otherwise end up in landfill, illegally dumped or exported for disposal in developing countries,” Lucaroni said by email.

Using tires as fuel, she said, lets the plant burn less natural gas and contribute less to climate change.

About 300 million worn-out tires were thrown out in the United States in 2023, with 84 million of them being burned as fuel at cement kilns, pulp mills, and power plants, according to the U.S. Tire Manufacturers Association.

Nationally, the cement industry gets about 15% of its energy from tires. Ash Grove in Seattle has been burning tires at twice that rate.

Lucaroni said cement kilns have been safely using tires for fuel for 40 years and the industry plans to get 43% of its fuel supply from such “alternative fuels” as old tires by 2050.

Cement is an essential ingredient of basic infrastructure, yet its manufacture is exceptionally harmful for the climate.

The cement industry causes 7% of worldwide carbon emissions, according to the Global Cement and Concrete Association. In addition to the fossil fuels burned to make cement, the high-temperature chemical transformation of limestone to clinker generates large volumes of planet-heating carbon dioxide.

Ash Grove’s Seattle plant is the largest single source of carbon dioxide emissions in King County, according to the U.S. Environmental Protection Agency, and the largest source of unhealthful nitrogen oxides, carbon monoxide, and ammonia, according to the Washington Department of Ecology.

It is the number-two source of particulates and sulfur dioxide, after the Ardagh Glass Packaging plant, a mile to the south.

“The Duwamish Valley experiences cumulative pollution from both international airports, the railroads, the ports, as well as any number of industries that operate,” Ayala-Marshall said. “Any increase in emissions in a community that's already overburdened by air pollution is unacceptable.”

The Ash Grove plant is currently operating under an air-pollution permit from 20 years ago.

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The Puget Sound Clean Air Agency is proposing to update that permit to meet newer federal standards.

It has also required Ash Grove to test its emissions for particulates, dioxins, and heavy metals including arsenic and lead to make sure its increased tire burning isn’t making Duwamish Valley air dirtier.

“Ash Grove is committed to being a good neighbor and has consistently invested in ensuring compliance with relevant permitting, environmental, health and safety, and other regulatory requirements at its Seattle Plant,” Lucaroni said in an emailed statement.

“Ash Grove is a notoriously bad neighbor in the Duwamish Valley,” Ayala-Marshall said.

She pointed to the company's previous lawsuit with the EPA, an ongoing consent decree, and, by her tally, 51 air-pollution permit violations in a decade.

KUOW could not confirm that number of permit violations, though the Puget Sound Clean Air Agency has issued Ash Grove 10 dust-related permit violations in the past five years after neighbors complained of clinker particles falling on their properties.

A hearing on the environmental groups’ appeal to make Ash Grove burn fewer tires is not expected at the state’s Pollution Control Hearings Board until 2027.