A project to convert one of Europe’s biggest coal plants to biomass may be on the verge of leaving its investors with a stranded asset, according to an assessment by the U.S. Natural Resources Defense Council.

The company behind the retooling of Britain’s Drax power plant “makes rather large claims about the carbon savings it is achieving from converting to biomass from coal, which in fact depend on very particular assumptions,” the Energy and Carbon Blog reports. The most important has to do with the pace at which newly-planted trees will offset the emissions produced by burning organic matter in the Drax facility’s boilers.

“The new report from the NRDC shows that trees will eventually offset the burning of biomass, but this will only happen over the course of many decade,” Wynn writes. The time frame “matters for the climate, which is warming in response to carbon emissions now, regardless of whether these emissions will be offset towards the end of the century. Second, it matters for the economics of biomass-fired power plants, which depend on supportive climate policies, which until now have assumed that biomass is zero carbon.”

Drax describes the plant conversion as “the biggest carbon reduction project in Europe,” contending that it “delivers low-carbon energy at scale.” But in the NRDC analysis, Wynn writes, “biomass would be more carbon-emitting than natural gas, and a little less than coal, until around 2065. Only after 2080 would the growth of replacement trees make the biomass cumulatively burned by then carbon neutral.”