YULIN, China -- The streets of this northwestern Chinese city were once packed with Ferraris, BMWs and other luxury cars. Now, they are all gone. Driving across the city's downtown, you see vacant buildings and empty restaurants. The changes in Yulin, a former coal boomtown, are a clear indication of China's changing attitude toward coal. The country has relied on this resource for years to fuel its economy, making coal-rich Yulin one of the fastest-growing Chinese cities. But as China has started restructuring its economy and favoring cleaner energy sources, coal companies here have begun to feel the pain of falling demand.