It may sound crazy, but yes, there really were plans for a coal-powered jet fighter in Nazi Germany towards the end of the Second World War. What's more, the engine was test-flown aboard a Dornier Do-17 bomber - and it worked!
The aircraft and its unconventional propulsion system were the brainchild of Dr. Alexander Lippisch. He'd helped design the Messerschmitt Me 163 Komet rocket-propelled fighter, and was convinced that the delta wing held great promise for high-speed aircraft. He proposed a delta-wing fighter known as the Lippisch P.13a, a drawing of which (courtesy of Wikipedia) is shown below.
However, aircraft fuel was in very short supply at the time, and production facilities for jet engines were already overstrained. The project therefore envisaged the use of a coal-fueled ramjet engine, described as follows.
Initially, it was proposed that a wire-mesh basket holding coal be mounted behind a nose air intake, protruding slightly into the airflow and ignited by a gas burner.
Following wind-tunnel testing of the ramjet and the coal basket, modifications were incorporated to provide more efficient combustion. The coal was to take the form of small granules instead of irregular lumps, to produce a controlled and even burn, and the basket was altered to a mesh drum revolving on a vertical axis at 60 rpm. A jet of flame from tanks of bottled gas would fire into the basket once the P.13a had reached operating speed (above 199 mph), whether by using a RATO unit or being towed.
The air passing through the ramjet would take the fumes from the burning coal towards the rear where they would mix under high pressure with clean air taken from a separate intake. The resulting mixture of gas would then be directed out through a rear nozzle to provide thrust. A burner and drum were built and tested successfully in Vienna by the design team before the end of the war.
There's more at the link.
Photographs exist of the Kronach Lorin test engine being flown aboard a Dornier Do 17 bomber.
They're included in this brief video clip about the P.13a project.
A glider prototype of the Lippisch P.13a was designed, but not completed before the war ended. American occupying forces ordered the Lippisch team to complete the glider, which was then brought to the USA and successfully test-flown (without an engine, of course). Lippisch himself also came to the USA under Operation Paperclip, and assisted with the development of delta wing technology for the USAF. His influence can be seen in Convair's F-102 Delta Dagger and F-106 Delta Dart fighter aircraft, as well as the B-58 Hustler supersonic bomber.
However, the coal-fired ramjet idea died at the end of World War 2, and has never been resurrected. I daresay a lot of pilots were relieved about that!