New Chinese ramjet uses magnesium-burning afterburner.

"Chinese researchers have reportedly developed a new type of afterburner for scramjet engines that could achieve Mach 6 at altitudes of 98,425 feet (30 km). According to the team behind it, this was achieved by cleverly incorporating magnesium powder into the hot exhaust gases produced by burning conventional jet fuel." (Interesting Engineering, China’s magnesium-powered scramjet breakthrough nearly doubles thrust at Mach 6)

Chinese use magnesium in the engine's afterburner there the exhaust gas ignites it. The magnesium can also be delivered into the engine as the magnesium tape. That tape increases temperature similar way as the electric arcs increase temperature in electric jet engines. 

There is the possibility that the electric arcs along with the microwave systems can inject magnesium powder. That makes the system capable of using magnesium as its fuel in the entire engine. 

During the WWII. German researcher Dr. Alexander Lippisch invented a supersonic aircraft that used carbon as fuel. The idea was that the ramjet engine burn carbon powder as fuel. The problem with that Lippisch P.13a plane was that the particle size of the carbon was too big. It is sometimes suggested that gunpowder with very small particle sizes can also be an effective fuel for ramjet engines. 

"Model of Lippisch P13a at the Technik Museum Speyer"

"The solid-fuel powered P.13 was one of several distinct Lippisch design studies to be so designated and became identified as the P/13a. It underwent much the same variations of form as the P.12, being presented in a brochure with the large fin and integral raised cockpit, and with an articulated, double-hinged landing skid. The wing trailing edge is angled slightly forwards and the downturned tip surfaces have been discarded. The outer wing sections could be folded upwards for transportation by rail" The engine was successfully tested in Vienna.  (Wikipedia, Lippisch P.13a)

In hydrogen-powered jet planes, the hydrogen-producing unit or electrolytic system can be in the airplane.

The aircraft will fill its tanks with water. And then the electrolytic system will break those water molecules. The system can get that electricity from any electric source. So the operators must only connect the electric wire to the airplane. Or if those people have time they can cover their aircraft using solar panels that create needed electricity. 

Lippisch got that idea from the dust explosions where the dust with a large fire surface will detonate. Those dust explosions destroyed many mills. So the ramjet engines can theoretically use any powder as fuel. The carbon or molecular-size carbon powder can explode giving thrust. In fact, even wheat flour to give thrust if the size of the powder particles is small enough.  If we want to make a ramjet or scramjet engine that is carbon-free, we must use some fuels that don't involve carbon. 

So, the answer to that problem can be in metals like magnesium. Or metal compounds like electrons. If the particle size of the metal- or metal-iron oxide compound is small enough the system can spray that metal powder to the ramjet engine. The electron is a compound of magnesium and iron oxide. And that chemical compound can give very high thrust to the engine. Because fuel itself involves oxygen that system allows the ramjet to operate also outside the atmosphere. Magnesium powder can be a good alternative to normal hydrocarbon. Or, at least Chinese research that thing as fuel. 

https://interestingengineering.com/innovation/chinese-team-develop-new-magnesium-afterburner?group=test_b

https://en.wikipedia.org/wiki/Lippisch_P.13a