Why the huge fireball anyway? How much contingency fuel does this flight normally carry? Just wondering.
It was reported by RBC (russian business/news TV station) that the company had some financial problems and they were fueling up their AC in DME(Moscow) where they were still allowed to do that.
So, they fuel up tanks in order to carry fuel back to kazan instead of simply fueling AC at Kazan. That might explain long post crash fire..