STORY: On paper, the planned revamp of this Rodeo, California, plant seems promising. Instead of processing crude oil into traditional gasoline and diesel, the facility will begin turning things such as used cooking oil, soybean oil, and beef fat into renewable diesel, which can fuel heavy-duty trucks.

And Phillips 66 has bragged that the change will slash greenhouse gas emissions from the site in half.

But skeptics say the plan, called Rodeo Renewed, might instead see little change in emissions and swap out one set of risks to the local air and environment for another package of possible problems.

The Phillips 66 facility, across the bay from San Francisco, is a test case for similar efforts efforts worldwide, and it's yet to be seen whether it's a model, or a cautionary tale.

Jolie Rhinehart is the general manager of the refinery.

“After we complete our Rodeo Renewed facility conversion and convert our facility into renewables, we will no longer process crude oil. And so all of our historical operations again have been based on processing and refining crude oil into transportation fuels. After our facility conversion, we will no longer process crude oil.”

The company says the changeover will cut the number of regulated pollutants emitted from the facility, and the switch to renewable diesel will mean more trucks burning cleaner fuel across the country.

"Heavy-haul trucking is a vital aspect to our way of life in this country and in this world. And renewable diesel is the lowest emission way to fuel that energy that we need to keep our trucks moving.”

But the company's claim that it will cut greenhouse gases in half doesn't match emissions estimates published by county regulators, which shows a one percent reduction, according to a Reuters analysis of the data.

The county data encompassed the overall facility, including pollution from increased hydrogen use and changes in transportation to the plant.

And in changing to renewable diesel, the facility will need to bring in more fuel stocks by rail and boat, potentially raising the risk of leaks or spills. And some researchers say that the emissions produced in raising and harvesting the soy and beef needed to create renewable diesel offset those saved by moving away from petroleum.

Mark Jacobson is a professor of civil and environmental engineering at Stanford University.

“I expect to see no improvement whatsoever. You'll just get a different set of chemicals coming out of the refineries compared with the traditional refineries of diesel and gasoline.”

The results will be closely watched. Several dozen new U.S. renewable diesel plants are planned and production capacity could triple by 2026 according to biofuels consultancy Stratas Advisors.

The community here has long lived with the fallout from the refinery operations. Maureen Brennan is a resident and a member of Rodeo's air monitoring committee.

“I've been a clinical lab scientist for 45 years and in the public health realm a lot. And I think that's partly why I'm fascinated by what's going on here and saddened as well. It's a toxic soup that we live in every day. And no one's... We're basically a disposable community here.”

Whether a switch to renewable diesel will mean cleaner air or less emissions is left to be seen, but those who will feel it first are likely to be those living here.

"So there's a lot of unknowns here because they have not been open with us about what the feedstock will be. So unknown feedstock, unknown effects. So we're in a place where we just don't know what's going to happen here.”