Density discontinuity!

Ilias

When I combust a coal particle (using the standard Fluent laws), at devolatilization's end (as a matter of fact the last value that is calculated with the devolatilization law), a discontinuity in density values occurs, as the law switches to the surface combustion law. Namely density changes like 650->649->648->635!. I tried all the combinations of the standard devolatilization and surface combustion laws and a similar discontinuity arises. Can anybody tell me why?

Thanks Ilias