How does sulfuric acid dehydrate sugars?
$\ce{H2SO4}$ protonates the alcohol to make $\ce{R-OH2+}$ and $\ce{HSO4-}$, the $\ce{HSO4-}$ then takes a hydrogen from the carbon atom adjacent to the alcohol, yielding an alkene and water.  Alcohol dehydration reactions using acid are just elimination reactions.

Additionally, at temperatures above 340 degrees Celsius, sulfur trioxide exists in equilibrium with sulfuric acid in significant concentration.  The presence of sulfur trioxide explains the presence of sulfur dioxide  (via dissociation), or some redox chemistry occurring with the sugar.