garybial wrote: I don't want to damage my insert by experimenting and if this is not very practicable I will try a plan B with adding equipment elsewhere to my heating system. Thanks
You are going to get very frustrated with trying to burn coal in an insert designed for burning wood. As the guys have told you a coal stove or insert needs a set of shakable grates to facilitate the removal of ash and a dedicated source of under fire draft. A coal fire burns from the bottom up and you control the heat production by how much air you allow to move through the coal bed. Yes, some stoves claim to burn either fuel but they mostly don't. You need to get the appliance designed for the fuel you plan to burn. If it were me I'd look for a coal stove which could be installed using the existing chimney or a coal burning insert which would set right in the fireplace. There are many types of coal appliances available that can either completely replace you current heating system or greatly reduce your heating bills as a supplemental heating source.
Having said that, I remember reading on the forum a posting from someone who placed a handful of coal on his wood fire prior to going to bed. In this way he was able to extend the length of burn of the wood fire. He apparently didn't care about keeping the coal burning but cleaned the ashes out and started over each night. That doesn't take advantage of the benefits of coal but it apparently fit his needs. Good luck, Lisa