Biodiesel
Most biodiesel comes from plants.  This means a lot of farmland is used to grow plants for fuel instead of for feeding animals and people.  There are scientists, chemical engineers and agricultural engineers who are studying ways to use algae and saltwater plants to make oil.  This would not take away farmland that is used to grow food.  Also, some biodiesel comes from waste oil and waste animal fat, that would otherwise be thrown away.  NASA is testing biodiesel made from chicken fat to fuel their airplanes.