Rice coal packs tightly. You might get smoke down the short pipe with any fuel, but with rice coal, rarely down the long pipe. If your fuel doesn't pack like coal, the fan forces the smoke through the alternate fuel. You can try reducing the air setting a little.
We mixed 50/50 coal and wood pellets, maybe 30% corn/70% coal. For the price of alternatives, coal is cheap. You MUST keep an eye on these blends when burning. If they burn too deeply in the pot, discontinue use. It is possible for wood pellets to burn backwards down the coal tube when the unit is resting. Be safe. If the pipe feels very hot, stop burning the mixture.