maize?
Uranium is the fuel that is used.
Coal
Bioethanol is a liquid fuel derived from plant materials such as corn, sugarcane, or other biomass. It is commonly used as a renewable energy source in a blend with gasoline to fuel vehicles.
Uranium is used as nuclear fuel.
Bradwell, near Essex, was a Magnox plant, using natural uranium as its fuel.
Ephedra is a 7 letter medicinal plant that's used to treat allergies and asthma, but I don't know of a 6 letter plant.
You can start an avocado plant from seeds. A glass of water or moist, porous soil mixture can be used to start the plants.
Uranium 235
To convert the heat of combustion to steam which can be used in an engine.
Abacus is an instrument used for calculating. It begins with the letter a.
The Chernobyl nuclear plant used enriched uranium as fuel in its reactor. The accident at the plant resulted in a nuclear chain reaction, leading to a catastrophic explosion and release of radioactive materials.
Some plant oils are used as food, fossil fuel oil is used for generating power, and either kind of fuel can be used to lubricate moving parts where friction may result.