Crack in fireplace, can i repair with fire clay/cement?
Since you have a non flammable structure, and a solid area to work with I would clean the area out both upper and lower and fill them with fire clay. 
Making sure to use tools to push the clay to the back. At each layer  make sure to mix them , it is impossible or just about impossible to work a bead all the way back and fill the hole.

I roll out a foot or so press it on the crack then use a metal putty knife and a small dowel to push the clay to the back, then add another bead or roll and make sure to punch through to mix the layers. And keep adding until the crack is completely filled and tight so if you push in it pushes out above or below the point you are filling. To make the bead or roll I take a ball of clay and roll it in my hands until it is about 2x the width of the crack, if the crack is longer than the roll no problem make another, on the next layer try not to have the lap in the same place, it won’t matter if properly mixed but is better to have laps in different sections. I usually finish with water and rub the surface smooth, now the important part.

Let the clay dry it may take 3-4 days. Go by the instructions but most fire clays I have worked with have to dry naturally , heat during the drying process can cause a failure, there was 1 brand I used that did require heat but it offgassed a horrid smell and I never used it again, it was quick and did hold , but the several hours of chemical smell was not worth it to me. So follow the instructions on the bag.

Make sure your grate is in good condition and it should be spaced from the back , I don’t know anyone burning coal any more but if it is two close it will cause the cracking, not much of a problem with wood but it can do the same.
Good luck.