As far as electric power utilities is concerned, they dispatch load by cost for next MW. That is to say, if the cheapest next MW is from a power gwnerator that doesn't burn coal, they do so. When natural gas prices allow for the next MW be cheaper with gas instead of coal, they use gas.
While I was working coal made the cheapest next MW. We base loaded the coal units, and followed the load with other fuel units. If natural gas is cheaper they would base load the gas unit, and follow the load with coal or oil units (providing the once base load units could ramp up and down fast enough to follow load)
Hence less coal would be used.
Also, some burners were added to the coal units to allow about 10% natural gas. This would also contribute to less coal used.
