Well All fossil fuels contain Carbon so when they burn they react with the oxygen in the air they produce poisonous gases such as Carbon Monoxide (CO) and Carbon Dioxide (CO2).
Also there may be small amount fo sulphur in the fuel which could create Sulphur Dioxide.
Another problem with fossil fuels is that they're getting really expensive because they are set to run out within the next 100 yrs.
As for the benefits of using other fuels it means that you can save money, help the environment and it also means that non-renewable fossil fuels won;t be as scarce.
I think that it's quite a hard question for an essay though because there's only so much you can say about it. Probably you'll have to base your essay on the fact that fossil fuels give out numerous poisonous gases when they burn in oxygen