What are the fossil fuels that causes very little pollution?

Answer 1

Modern power plants do not produce any smog or harmful emissions other than Carbon Dioxide, and in vehicles the amount of harmful toxins has much more to do with the types of filters the exhaust system utilises than the type of fuel that is used. So I am assuming that by "pollution" you mean Carbon Dioxide (CO2) emissions.

The real answer to your question then is "none", as they all produce significant amounts of CO2 emissions. A possible exception is Hydrogen, as it produces no CO2 during combustion whatsoever. However, it is not commercially exploited as a fossil fuel due to the limited quantity in fossil deposits and its high volatility.

The least amount of CO2 emission per unit of energy produced would be natural gas, followed by liquid hydrocarbons like petrol, diesel and kerosene. The most CO2 intensive fuel, and one of the most widely used, is brown coal. A relatively newly exploited fuel that also produces immense amounts of CO2 throughout its life cycle is Tar sands (bituminous sands).