Sounds like two problems to me.
If you have a ceiling hung expansion tank, it could be full of water. Opening the drain valve won't empty the tank unless there is another valve to break the vacuum. If there is no second valve, you can stick a hollow tube up through the valve to the top of the tank to break the vacuum. You can EXPECT a bath, so make sure the water is cold.
Changing coal supply isn't going to cause such a high temperature. You could have a failed high limit on the aquastat, not common, or the limit is just set too high. I wouldn't go any higher than 210 on the high and keep the low around 160 in the winter and 130 in the summer.
Your feed could be too high, but if the high limit is properly set and working, the water temp will never get that high.
Shut off the inlet and outlet of your hot water coil and reduce the pressure of your boiler to 15#. If the pressure stops building overnight, it could be a faulty water supply valve to the boiler, or a pinhole leak in the coil.
Increased pressure with increased heat are usually a sign of an expansion tank issue. Increased pressure without increased heat are usually a sign of a bad coil.
You could have a bad water fill valve, a bad coil, a full expansion tank, or a faulty aquastat.