The Greek noun anthrakia means a charcoal fire or a bed of burning coals. Occurring only twice in the NT (John 18:18; 21:9), its rare appearances are at two pivotal moments: Peter's denial (warming himself at a charcoal fire in the high priest's courtyard) and Peter's restoration (Jesus has made a charcoal fire on the beach where He restores Peter).
The two appearances of anthrakia in John's Gospel form a deliberate literary bracket. Peter denied Jesus three times while warming himself at a charcoal fire (18:18). Jesus asked Peter three times 'do you love me?' beside another charcoal fire (21:9). The aroma of the same kind of fire would have triggered Peter's memory. John's detail is theologically intentional: the place of shame becomes the place of restoration. Christ meets Peter at the point of failure, uses the same setting, and replaces denial with three-fold commission. Grace meets us at the exact location of our fall.