Most coal burners also have straight stacks. Most steam locomotives, including oil burners have some form of spark arrestor in an extended smokebox if they have a straight stack. Quite often there is a slanted metal sheet (or screen) in the smokebox which deflects all the smoke/cinders/et all down almost to the base of the blast pipe. this evens out the draft across the tubes and has the added effect of crushing out almost all sparks.