Grain that is stored ina grain elevator is unreactive. If a spark is placed near fine dust in the silos, howevr, an explosive reaction will occur.
I guess you're asking why this is so. Or are you asking is this so? If you are looking for answers you really ought to start by asking unambiguous questions. I'll answer both possibilities. It is so. Big bangs can come from ignited grain dust in silos. Grew up in farm country with lots of soya bean and corn silos around. Occasionally one would go boom and ruin the farmer's day. As to why the dust will go boom, but the grain lying in a pile will not; it's because the dust is surrounded by air with oxygen in it, while the grain is exposed only on the top of the pile. In fact the surface area of dust relative to the volume (mass) of the dust is rho = A/V = 4 pi r^2/4/3 pi r^3 = 3/r This means that as the bit to burn gets smaller and smaller when r gets smaller and smaller, there is relatively more surface area exposed to the air and the oxygen it contains. And the more surface area there is relative to the mass (volume) of the bit, the more rapid it will burn. The very rapid burns are called explosions. So if grain were to ignite, it would burn more slowly than the dust with its higher A/V ratio. We used to have those, too; slow burns in the silos.
