I thought it would be helpful to post some info on CNG. CNG is natural gas, but not in liquid form, nor does it produce a flame (hopefully). Below is a quick excerpt from the CNG wiki page that may help. Thanks!
CNG is often confused with liquefied natural gas (LNG). While both are stored forms of natural gas, the key difference is that CNG is gas that is stored (as a gas) at high pressure, while LNG is in uncompressed liquid form. CNG's volumetric energy density is estimated to be 42% of LNG's (because it is not liquefied), and 25% of diesel's.
http://en.wikipedia.org/wiki/Compressed_natural_gas