Bio fules made out of Sugarcane, corn , switch grass, wood residue , soya bean , algae etc are alternate source of petrol which can be used but are they areally healthy and what are limitations when comapred to technology availble to produce them or use them. Here is snapshot which talks about it.
You can click on the image to view the complete details which is linked to the original image source. It is intresting know that though we have natural alternatives but many of them have high greenhouse has emission. Moreover the one which produces very less green house gas doesnt have the etchnology ready.