Tweetin order to burn fat, you should look at this BPM-rule ;
(220-age) * 60 till 90% . So, youll have to stay between 60 (underlimit) and 90 (upperlimit) percent!!!
Basically, youre body has three systems to get your calories burned; phosphate system, lactate system and the oxygen system. You should stay in the last one in order to burn fat. the other systems are more related to shorter body efforts (general bodycondition & musclestrenght)
Good luck!
Luca