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23:00 Dec 13, 2021 

Another possibility is that 'metal plug' refers to the entire recuperative burner. A recuperative burner is what is called a radiant tube. It is made of metal and projects inside a kiln or furnace and might indeed be regarded as a 'plug'. Fuel and air enter the burner from outside the furnace, turn 180° inside the burner, and exit outside the burner. IOW, there is no flame or exhaust gas in contact with whatever is being fired inside the furnace.
Then again, it might be a 'plug-in recuperator' inserted (plugged) into another kind of radiant tube heating system, one consisting of a tube that enters the furnace at one point and exits at another.

18:17 Dec 13, 2021 

@ Johannes : oui, il s'agit bien d'un récupérateur de chaleur. Il s'agit d'une présentation Powerpoint, mais n'ai malheureusement pas accès aux diapos, seulement aux sous-titres de l'intervenant.

@ CadatresToulous : unfortunately, I don't any additional info.

23:32 Dec 12, 2021 

Information on the process involved and its purpose would help narrow things down.
Without it, there is inevitably a lot of guesswork. Reading between the lines, I think of a recuperative burner, i.e. a tube which is sealed at one end (with a 'metal plug'?) and has a fuel injector at the other end. Was the text written by a native speaker?

21:08 Dec 12, 2021 

'recuperation' refers to recuperation of heat. Correct?
Do you can describe or show the diagram, especially the kind of 'plug'?
Est-ce que c'est un « bouchon », une « cheville », un « boulon » ou un « fond de convertisseur » ? Le dernier peut s’adapter au récupérateur.