A carbon tax imposed by a US county is headed to court.

Montgomery County, Maryland imposed a tax of $5 a ton on carbon emissions from stationary sources — for example, from power plants as opposed to cars. The tax only applies to entities owning sources that emit more than 1 million tons of carbon a year.

Mirant, a large US independent power company, filed suit in federal district court in June charging that the tax only applies to it and, as such, violates three provisions of the US constitution. The company owns the only power plant in the county, the 835-megawatt Dickerson power plant that has three coal-fired units and two units that run on natural gas and oil. The company said the tax is an unconstitutional “bill of attainder,” or seizing of property by legislative act aimed at a single person, and that it also violates a guarantee in the 14th amendment to the US constitution to equal protection under the laws and in the 8th amendment against “excessive fines.”

The county estimates the tax will raise $11.1 million a year. The first payment is due on July 30.