Europe is in a bind.

It has ambitious goals for changing its polluting energy system, and plans to source 20% of the EU’s total energy from renewables by 2020. But the main way it’s getting there is a method with murky credentials: biomass.

Biomass can include many things, from quick-growing elephant grass to trees. But no matter how you do it, burning organic material still releases CO2 and other gases. Most biomass sources, while renewable, are still polluting. They’re only “carbon neutral” if enough plants or trees are planted to absorb the carbon produced.

Now, the European Commission is conducting a state-aid investigation into UK-based Drax, a coal power station that’s converted some of its boilers to burn wood.

Investigations are routine—they take place when large power stations are awarded a certain kind of subsidy. What’s unusual is that the Commission mentioned worries that Drax, formerly the largest coal-fired power station in Europe and still an energy behemoth, would require so many wood pellets that the global market could be affected. It would need 2.4 million tonnes of extra wood pellets a year for the project.

“Demand from the Drax conversion project could significantly distort competition in the biomass market,” the Commission says.

Drax argues there’s plenty of wood in the US, from which it sources most pellets. A spokesman for the company told Quartz that US Department of Agriculture data showed an annual surplus of about 50 million tonnes of material from forests in the southeast United States alone.

Nevertheless, the Commission says it’s ”concerned” that the full conversion of a third boiler at Drax (it has two running only on wood already) could have more negative than positive effects on the EU reaching its goals.

Overall, it does want power stations to convert from coal to biomass if possible. The danger is, there won’t be enough to go round.