Could make a pile of rubble or model charcoal pieces, put them in a group, then a particle system can pick pieces from that group. If you model them, might be easiest to use dynamic topology sculpting so it doesn’t take too long, and a pattern on a burned log could probably be done with a leather texture
Would suggest moving the emitter to another scene layer (M) before making the duplicates real (ctrl+shift+A). Easier to select all the pieces when you copy settings from the active piece to the selected.
Made a quick material for them. Object locations along Z and couple of noise textures are used to make masks for mixing ambient occlusion and the emission. Used ambient occlusion for lighting the pieces quickly but if it needs to be used as a mask, have to bake ambient occlusion out so it can be combined in the material setup.