Coffee and Caffeine FAQ

This website has a list of acids found in coffee and relative amounts created during brewing. http://www.coffeeresearch.org/science/sourmain.htm

To answer your question, calcium carbonate from eggs could have the ability to neutralize the acids in coffee. The large particle egg shells don't have alot of surface area. If you grind the shells into a powder, then you have enough surface area to dissolve and react more readily. As an additive to coffee, you would get the neutralization of the acids and still maintain nutritional calcium intake.