In German, the word "silo" is not used for coal bunkers. I could not say whether sugar beets, the other example named as bulk, are stored in silos, but I think for most bulk goods, the general idea is to store it as a heap. Perhaps as a heap in an enclosed, lowered area, but that wouldn't be a silo (or not what you'd think of as a Silo in German).
More specifically, a "Silofahrzeug" is for transporting bulk good with the property "Rieselfähig"- which translates to pourable, except it also makes clear that it's solid, not liquid (otherwise it would be "Gießfähig")
If you check your british link, you'll see that they are talking a lot about granular materials - powder is only handled in the last paragraph. The German link is about a "Staubgutwagen", which is indeed for powder.
Both powder and granular materials are "Rieselfähig", transported in "Silofahrzeugen" and stored in silos. I think it could make sense to say that powder is granular, since granular basically means grainy and powder just has a very low grain size, but categorizing something granular - like wheat grain - as a powder would be very strange.
I'm not a native english speaker, so I don't know what's wrong about "Pourable Goods", but if it has to go, I'd suggest "Granular Goods" rather than powder.