Before you burn any coal in your furnace you should read up on how a hand fed stove works... you have to have grates and a way to shake them to get the ash to fall into an ash pan below. You also need a way to regulate air coming in under the caol bed. IMO your idea to burn blocks of coal will probably have some insurmountable challenges.
But then again where there is a wil and plenty of time there is a way.
A stoker uses a catch pan under the stoker mechanism or a pit to remove the ash. Burning coal is way different than anything else and needs a proper design to work. My suggestion would be to read up on as many designs as you can before attempting to design your set up for burning coal.
Have fun, your boiler looks like a well thought out efficient design for burning wood.