The fire had no physical effect on Sentry as you think it did. Hammond basically bluffed Sentry into believing it would cause him to overload and release his energy, thus destroying the world as Bob has always believed he was capable of. It was a very convenient way of writing Bob out of the story due to his feeble mind.
Seriously, did you think the fire had a physical effect? Bob's powers arent solar based. Doom, Richards, Stark, and Ultron could never figure it out. Fire had no effect. Hammond even stated that he was well informed about the Sentry when the attack happened, and used the 'Power Of A Thousand Suns' phrase and in turn asked Bob how much fire he could take before overloading him. Being the mental midget that he is, Bob even asked Hammond what he was talking about when he was getting blasted, thus telling the readers that he actually believed Hammonds BS. Like I said, nice way to write Bob out of the picture.