Yes.
Change out the gas flow orifice size to propane and adjust the gas valve.
There are a variety of gas furnaces that can without much effort be converted from one type of gas to the other, Natural gas and propane gasses work at different pressures; one gas is denser than the other.Converting gas to an electric furnace, you would be gutting almost the entire furnace, and replacing the gas components with electric heating coils, diodes and relays. This will take a lot of man-hours, fabrication, engineering, and parts. while not imposable, the conversion would be very expensive.
Furnace it has to reach much higher temperatures than the fireplace
the propane company says that it can not be converted to natural gas because of the laws and government regulations
In the United States their is only Natural Gas and Propane Gas. If you don't have a Propane Tank in your yard you have Natural Gas. If you have a propane gas furnace and want to change it to a Natural Gas furnace you have to have a heating/ac guy do it or you could burn your house down. The change in pilot size is directly proportional to the btu output of your furnace and is custom sized for each different make and model (1000's of different models in the US)
A supply house such as johnstone. Take all information of the furnace and old orifice with you and show them what you need.
any gasoline engine will run on either natural gas or propane ( LPG )
It can- but propane and natural gas have different heat values- check with your gas company to see if yours can be converted SAFELY.
Any fuel that can be burnt has chemical energy. Once it is burned, that energy will of course be converted into heat energy.
It uses fuel (oil/natural gas/propane) to make a fire, usually for space heating purposes.
how do we get our gas range converted back to natural gas
Maybe, contact the manufacturer of the furnace or fireplace insert and ask what the procedure is and if you can get the proper parts from him.