Its possible but harder to do in the way described then many realize.
The key is concentration percentage.
https://en.wikipedia.org/wiki/Lower_flammable_limit
However I couldn't find the flashpoint fo flour and grains, probably varies by type but maybe not enough to matter.
IIRC from a very long time ago the ration had to be between 25 and 35% but I don't recall or have access to those sources anymore.
Why not use catapults for delivery by the way?
Edit according to the
link above grain dust has a LFL of 125
A Dijinn spreading it around could help but even if no explosion if its concentrated enough to all burn it will look spectacular and scary.