The coarse surface is not easy to ignite because of its small contact area with air, while the fine surface is easy to ignite because of its large contact area.
Just like it is easier to make a fire after cutting a large piece of wood.
Similarly, flour burns more easily.
Usually, a pile of flour is not easy to ignite, but if you blow it away or scatter it from a height and let it spread in the air like dust, it is very easy to ignite. Therefore, the fire prevention of flour mill is very important, and it will explode if you are not careful.
You can do an experiment: light a candle, take a pinch of flour and put it on paper, and gently blow it from above the candle. Flour will be ignited when it falls on the flame of a candle. (Pay attention to fire prevention during the experiment, and adults must be present.)