During my fall egg drying/freezing marathon I accumulate a ton of egg shells. I toss them into a pie pan lined with foil and into the oven @350F for ten minutes or whenever I remember to go take them out. If Facebook is involved, that could be several hours. Baking them will make them very brittle, and much easier to crush.
Trust me, you want the foil. The little bit of egg goop that remains on the shells will burn onto your pan otherwise, and it's the dickens to get off. Once they've been baked they go into the food processor to be pulverized as close to dust as I can get them.
Then into a mason jar for later use.
Your chickens need calcium to form egg shells. Eggshells are calcium. Add it to their feed mixture, or better still, make a small PVC feeder and dump it into that so they have free access to it. Chickens are pretty good about eating what they need, so if they need it, they'll eat it.
The crushed shells are also great in the garden, sprinkled around tomato plants or just added to the compost pile.