Having a large expansion tank prevents the pressure release valve from releasing. The design criteria for the tank size is commonly set by a pressure rise no larger that 5 psi below the safety. Yes the water in the boiler will be very hot, but it's stationary. The very hot water is a result of an over-fire condition, e.g. the combustion blower was on stoking the coal fire and then just as the aquastat high set point is reached the circulator turns off because the room thermostat is satisfied. With coal, the boiler water will continue to get hotter. It has no where to go because the circulator is not running. You have two choices active a dump zone or let the expanding water go into a large expansion tank. If you store it in your boiler, it's available minus the boiler's radiation losses, for the next time the thermostat calls for heat. Yes, the initial water circulated from the boiler will be very hot but it will quickly cool as it reaches the distribution piping and/or radiation system. If you are using PEX for distribution piping you need to assure it doesn't exceed the pipe temperature rating. Many ways to do this, injection pump mixing at the boiler, mix some of the boiler return water to cool it, etc. Normally with metal supply piping you don't need to do anything special. Especially if you are using primary secondary piping with coal and some other conventional fueled boiler. Just make the primary loop all metal pipe. The primary circulator will mix the return water and cool it down to normal design conditions.