A slurry is transported in one variety of these. The trick is to induce a swirl in the flow, so even though particles are constantly falling out of suspension, they stay suspended. It's mathematically analogous to how putting a twist in magnetic field lines in a tokamak (or stellarator) prevents ExB effects from driving the plasma into the wall, as they would in just a plain toroidal magnetic field.
Lots of water is needed and drying the coal before burning it adds cost.
Acre-feet as a unit just made me realize how many options you get for volume with multiple base units (a foot-mile-inch is ~12m³ in case anyone was wondering). The non-metric system continues to impress (not in a good way)!
You have to physically handle every piece of coal. Extract, load ship, unload.
Natural gas is shooting out of the ground in North Dakota.
You can compare to wind and solar also.
The economics aren't favorable.