Those are the L4D1 cvars. The L4D2 cvars are slightly different.
PHP Code:
tank_burn_duration : 75 : , "sv", "cheat" : Number of seconds a burning Tank takes to die in easy, normal, versus and survival
tank_burn_duration_expert : 85 : , "sv", "cheat" : Number of seconds a burning Tank takes to die in expert
tank_burn_duration_hard : 80 : , "sv", "cheat" : Number of seconds a burning Tank takes to die in hard