and place them on top or to the side of your wood stove.(or in your oven on a low temp for 25 min.)
I do this daily, because we use a lot of eggs.
We feed them to our chickens and use them in the garden.
Here are a few links on the subject;
30 ways to use Egg shells!
6 Reasons to start using Egg shells in your garden!
How to Make Calcium using Egg Shells