The fat of dead pigs, cattle and chickens is being used to make greener jet fuel, but a new study warns it will end up being worse for the planet. Animal fats are considered waste, so aviation fuel made from the material has a much lower carbon footprint. Demand for fuel made from animal by-products is expected to triple by 2030, with airlines leading the charge. But experts fear scarcity will force other industries to use more palm oil - a huge generator of carbon emissions. | 
