I suggest you look into photoelectric (optical) smoke detectors. There are 2 main types of these, simple IR/LED units that look like a typical ionization type detector found in homes and beam type detectors that have either a separate emitter/collector or combine both in one unit with a mirror at the other side of the monitored area. I imagine you could design one to alarm when the beam is partially obscured by smoke (so as not to trigger on a solid object) but a can't conceive a way to distinguish thick smoke from a solid object optically other than perhaps using multiple beams.