What size coal, how wide is the porch and I'm assuming there is access to window under the porch?
If you're getting it delivered via a regular high lift coal truck you may be surprised, one of the benefits of regular coal truck is you get velocity on the coal because you can raise it way up. If it's a short distance under the porch like 4 or 5 feet and they can get even a little pitch on a chute laying flat you can put the main chute coming off the truck onto the chute laying flat if that makes any sense. If you post some pics I could probably tell you if it will work or not.
Another option is to cut a trap door in the porch.
Other than that a tarp is fine.