C. F. Keil and Franz Delitzsch on Proverbs 26:21

Pro 26:21 21 Black coal to burning coal, and wood to fire;      And a contentious man to stir up strife.

The Venet. translates פּחם by καρβών, and גּחלת by ἄνθραξ; the former (from פּחם, Arab. faḥuma, to be deep black) is coal in itself; the latter (from גּחל, jaham, to set on fire, and intrans. to burn), coal in a glowing state (e.g., Pro 25:22; Eze 1:13). Black coal is suited to glowing coal, to nourish it; and wood to the fire, to sustain it; and a contentious man is suited for and serves this purpose, to kindle up strife. חרר signifies to be hot, and the Pilpel חרחר, to heat, i.e., to make hot or hotter. The three - coal, wood, and the contentious man - are alike, in that they are a means to an end.