Fuels are the sources which run the world today. Think of your routine from morning. You wake up, glance down in your phone- phone runs on electrical energy, to walk in to have breakfast- food is made on oven using LPG, you set out for work in your car or bus- all vehicles run on fossil fuels, at work you turn on fans, air conditioners, computers- all run on fuels by burning down fuels. 
HENCE we can clearly SEE we don’t run the fuels, but fuels run us. But what are “fuels”? 

Fuels are those energy sources which the help of which various devices of daily use can be run, by using them in the same form or by converting one form of energy into ANOTHER. Fuels can be categorized in two ways:- 
Fossil fuels such as coal, petroleum, crude oil, diesel, LPG (Liquefied Petroleum Gas) form the non renewable energy sources. They had been formed million of years ago when plants and animals and other vegetative matter got suppressed under tremendous pressure and heat and hence some of them fossilized and some turned to fossil fuels.  

Today those are the fuels that run the world. Humans are heavily dependent on these fuels to fulfill all his Knick knacks. And this has endangered the reserves of these fuels. The demand is more than what the earth has. Hence the reserves are depleting fast and a time may come when there will be nothing left for the future generations. 

Not only this fossil fuels also leave behind a huge number of waste products after use. For example- coal leaves behind ash when burned for producing electricity, petrol and diesel leave behind harmful gaseous smoke when used in cars and it is released directly into atmosphere. Even nuclear power plants emit very dangerous radioactive wastes that have to be stored in pits deep below the ground otherwise it MIGHT mess up the human DNA due to radioactivity. 

Only there are two sources which do not emit any harmful waste and they are LPG and CNG (Compressed Natural Gas). These two fuels are now widely used in gas cylinders for cooking purposes and in autorickshaw’s. They are by products of crude oil. But there is a limitation. There reserves are very limited. Their availability is even lesser than petrol reserves hence they are not much dependable sources. 



Renewable energy- fuels that can be replenished after using. 

Non- renewable energy- fuels that cannot be replenished after using. 

So far the use of the second category of fuels is more than the first category. And this the major concern of the ENVIRONMENT activists of today