Fire Department Capt. Joe Zukas said two propane tanks that exploded behind Walmart, 780 Lynnway, on Friday, were being used for heating by homeless individuals living in encampments behind the store. Zukas said a passerby heard two explosions at 4:15 p.m. and called emergency 911. No one was hurt in the explosion but Zukas said homeless encampments along the waterfront have prompted calls to firefighters to respond to fires or other emergencies.