Dust explosion area:
Zone  20: During normal operation, flammable dusts are present or often  present in quantities sufficient to form flammable dust and air mixtures  and / or where it is possible to form uncontrollable and extremely  thick dust layers and inside the container.
Zone 21:  During normal operation, there may be a place where the amount of dust  is sufficient to form a combustible dust and air mixture but not in 20  zones. The area includes: a place adjacent to the point  where the dust is filled or discharged, where the dust layer and the  combustible dust and air mixture, which may produce a flammable  concentration, in the normal operation.
Zone 22: Where  the flammable dust occasionally appears and is only present for a short  period of time or where the flammable dust occasionally appears to  accumulate or may have a dust layer and produce a flammable dust  mixture. If you can not guarantee the elimination of flammable dust accumulation or dust layer, it should be divided into 21 areas.
You said that a class of dust explosion should be suitable for the level of 21 areas, also in the 21.