Propane and natural gas run on different pressures. They can be converted back and forth, but it takes some know how.
There should be a test date for the tank, stamped onto the handle. It looks something like 5-02. Month and year. The pressure tests are only good for a couple years. It used to be 12 years. I think? I'm likely mistaken.
The Fed now requires all filling stations to fill only "OPD" tanks, which have a triangular valve handle, and an "overfill protection device". Been required for a couple years.
You can get infared heaters, cook stove, gas grilles, weed burners, etc. Camping supplies, and some gear at
Your biggest challenge is that curbside tanks are usually the old style, and no one wants to refil them.
If you have a waterfront property, you can tie a weight and a long rope, paint the tanks, and use them for water marker buoys. Let the propane out, and then close the valve. Paint them bright colors. You might also be able to use them for flotation to keep your dock afloat.
Probably worth $10 a ton for scrapsteel. Wish I was more optomistic. Sometimes things are in the trash cause they are not worth anthing.