The villainous gremlins are attempting to cause as much damage to the lands of the Empire as possible with as little effort as practical. They have numerous plans to this end. One such plan is to blow up the granaries/silos which store all the recently harvested grain (mostly wheat and barley), depriving the Empire of food and possibly killing people as an added bonus. They don't have explosives, but if conditions are right all they need is a flame.
It is well known in the modern era that granaries and silos are an explosion hazard, due to dust explosions. However, from my investigations, it would appear that such explosions were only of considerable risk after industrialisation. The list of notable dust explosions starts in the 19th century. The smaller scale of pre-industrial grain and flour handling seems to have reduced the risk of explosions. Food storage was typically in the form of whole grain rather than finely powdered flour, which would further reduce the risk of explosions.
In favour of explosions, grain is still combustible (although I don't know how combustible). And our saboteurs can deliberately stir up grain and dust to improve the chances of an explosion. But it is unclear if these factors will be adequate.
The climate of the region is comparable to Great Britain or some other parts of Europe (with cold, wet and sometimes snowy winters). The technology is roughly comparable to the late medieval era. Sufficient grain is produced to warrant bulk storage over winter.
I'm not entirely sure on what layout a practical granary or silo for this climate might look like. Great Britain preferred to have shacks raised above the ground as granaries. The pictures don't show how the grain is stored, but it seems to be at the same level as you enter, which seems to suggest that it is confined to boxes or sacks or something which might inhibit explosions (but if you have information which says otherwise, do share).
Another way of storing grain is in a silo, which is essentially a pit in the ground. This is the method implied in the answers to this question. Having a large pile of grain sounds like a prime candidate for an explosion. However, a source I have found seems to suggest that pre-industrial silos were preferred in dry lands, and some wet lands depending on their soil (such as France) were not suitable for silos. (I don't have full access to that source, so can only read the first page.)
If there was need for additional grain storage, would the people of the Empire build silos or granaries, or could they build either? If these structures can explode with different measures of ease, then the relative abundance of these structures would affect how easy it is for the saboteurs to damage the Empire's food supplies.
In these circumstances, would it be relatively easy to cause a catastrophic dust explosion in a medieval grain storage building? I am aware that it should be possible to contrive a scenario where such an explosion can occur, but if the scenario is too contrived then the saboteurs would look for an easier method to destroy the Empire's food supplies. On the other hand, if blowing up a granary is as simple as kicking up some dust and chucking in a match then they could use the strategy with abandon.
Any examples of pre-industrial grain explosions would be appreciated. Otherwise a solid argument on the plausibility or otherwise of grain explosions in the medieval era will be good. Bonus points for being applicable to my setting's climate.