SAN FRANCISCO – An explosive three-alarm structure fire burned in a downtown industrial area Monday afternoon, sending plumes of black smoke over the city. It took firefighters about an hour to extinguish the blaze, as the smell of burning rubber tires pervaded the site close to the city’s Mission District. “There were probably at least 15 explosions,” said David Grace, 50, who said he was about a block away when the blaze broke out. “One of them was so big that we felt the heat on our face.” The fire started around 3 p.m., when heavy smoke was reported in the back of a Hertz Equipment Rental lot, said Pete Howes, a San Francisco Fire Department spokesman. Propane tanks were stored at the business, which has a propane filling station and rents out construction equipment. Howes said the blaze spread to an abandoned house that was adjacent to the back of the lot. 160Want local news?Sign up for the Localist and stay informed Something went wrong. Please try again.subscribeCongratulations! You’re all set!