What fuels did Rudolf Diesel first consider for his engine?

Rudolf Diesel initially proposed using either powdered coal or liquid petroleum as fuel for his diesel engine.1

Diesel viewed powdered coal, a by-product of the Saar coal mines, as a readily available fuel source. He intended to use compressed air to inject the coal dust into the engine cylinder; however, this proved difficult to control, and an experimental engine was destroyed by an explosion during testing. Consequently, Diesel shifted his focus to liquid petroleum, which he also introduced into the engine using compressed air.1

Additionally, while petroleum became the standard fuel following the widespread availability of oil, Diesel later conducted extensive research into vegetable oil as a potential fuel source, famously demonstrating an engine running on peanut oil at the 1900 World’s Fair in Paris.2