A smokestack that has stood along the St. Paul riverfront for more than 90 years came crashing down in a cloud of dust Sunday morning. The 289-foot-tall smokestack was part of the Island Station power plant, which has been vacant for more than 40 years. The facility, which overlooks the Mississippi River near Shepard Road and Randolph Avenue, is being demolished to make way for new development. Explosives were used to bring down the structure. The charges sounded three quick booms and crumbled the building attached to the smokestack, causing it to fall. The property’s former owners, Breckner River Development of Burnsville, had been trying to sell it for years. Last summer they successfully fought efforts to have it designated locally as a historic property. The plant had repeatedly failed to win federal historic status. It was purchased last fall by St. Paul River Walk of Scottsdale, Ariz., for $4.05 million. Developers haven’t yet mentioned plans for the site. The full demolition work is expected to be finished by late spring, city officials have said.