Well, one "classic" synthetic fuels that comes to mind is producer gas, or "town gas". It was produced mostly in the first half of the last century by gasifying coal and was in many ways the precursor of natural gas, by which it was eventually replaced in most parts of the world. Many of the applications that today run with natural gas (e. g. domestic heating and cooking appliances, process heating in industrial furnaces in the metals, glass, ceramics industries, ...) were fueled by producer gas at the time. Even today, there are large industrial furnaces in China which run with producer gas.
Biogas or biomethane (i.e. biogas upgraded to natural gas quality) could also be considered a synthetic fuel.
Hello Dr. Gianluca Argentini, I hope that you have already received the necessary hints about the "Synthetic fuels" from the comments of Dr. Kherris Sahraoui & Dr. Jörg Leicher. However, it seems to me that you are very busy! To obtain the further information about the state-of-art of the field of "synthetic fuel and its utility" in the combustion systems, why don't you perform an extensive survey on this topic? In my understanding, it is only a matter of the allocation of time! For example, If you just use the "ScienceDirect.com" or "google", then you will find a lot of information on your target! The state-of-art of any subject matter is an extensive task, which is difficult to provide here; and this is why, probably, Dr. Kherris Sahraoui has given you the link of the "wiki" related to your keyword. However, I believe that this task should not be very difficult for you. All the best for your research.