carbon cenospheres

https://doi.org/10.1351/goldbook.C00826
Porous or hollow carbonaceous sphere-like particles (frequently in the size range of a few to several hundreds of µ m diameter) formed during @P04961@, also in the course of combustion, of carbonaceous liquid droplets (e.g. heavy fuel) or solid particles (e.g. coal).
Source:
PAC, 1995, 67, 473. (Recommended terminology for the description of carbon as a solid (IUPAC Recommendations 1995)) on page 480 [Terms] [Paper]