I'd be very disconfident of coal dust dragged by air. To my opinion, it would be an accident waiting to happen.
You might try to build the whole circuit, including downstream the silo, to withstand the deflagration. I don't like this option because explosions tend to have unexpected consequences.
Can you experiment? On a not too small scale, because some explosions happen at a minimum size. And alas, some materials exploded after having being handled for decades with the same procedure.
To save on nitrogen, you might use CO2 instead. I believe it's easily available at a blast furnace. If you can guarantee it's free of CO, sulphur oxides, phosphorus oxides, nitrogen oxides, then CO2 isn't more dangerous than N2.
I thought recent blast furnaces use oxygen? Then nitrogen should be abundant locally, or?