all over the worlddd.
There is also a great tutorial video on http://sageenergy.ca/learning-centre-tools/online-operator-training/ which explains the process in detail.
Internal combustion engines can be made to work on natural gas as on gasoline or diesel. Then it is just connected to a compressor.
You change the orifice in the burner. Propane and natural gas require different orifice sizes. You should check with your gas or propane supplier, or contact the manufacturer for more detailed information. If you have good information and are a pretty handy individual, you might be able to do the conversion yourself. But, be warned, if you make a mistake, if could be tragic.
buy a new fireplace.
Burning coal and natural gas releases long held carbon dioxide which is extra to the greenhouse gases in the atmosphere. The carbon cycle can't cope with the extra carbon dioxide and it stays in the atmosphere trapping the sun's heat, causing global warming.