The paper mill hulks over a bend in the Potomac River Valley, a castle of pale blue metal between steep green slopes. Clouds of steam billow from its towers. …Even as other factories in this stretch of Western Maryland have closed down, this mill has managed to survive. That’s in part because the 10-story-high boiler deep inside the mill burns a sludge known as black liquor. The substance, a mix of caustic chemicals and wood waste left over from the papermaking process, was once pollution, a byproduct that fouled the rocky banks of the Potomac. Now, Maryland calls it green energy. It’s not a particularly clean form of energy. Burning black liquor releases carbon dioxide, the greenhouse gas that causes climate change. But because it’s a waste product that’s being used to generate power, state legislators declared it a renewable energy source.