Porous or hollow carbonaceous sphere-like particles (frequently in the size range of a few to several hundreds of ยต m diameter) formed during
pyrolysis, 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