What I have done is "coke" some coal in my stationary forge and save it for demos. After a forging session, while the fire is still hot, pile on fresh coal and let it smoulder with the blower off. After an hour or two, rake apart and let the fire die out. Before firing the forge the next time, pick out the chunks of coke and save for demos.
If you can find it, you could buy coke ready made. There is a lot less smoke either way. I don't demo much so it easier to make it.