Coals - Charcoal.

As coals are to burning coals, and wood to fire,.... As brands, wood half burnt, or dead coals put to live and burning ones, soon take fire and become like them, and fit and proper fuel for them, and add to their heat:
so is a contentious man to kindle strife; or "a man of contentions" (k); who is given to it, is full of it; it is agreeable to his natural temper and disposition; he is in his element when at it; such a man is as fuel to the fire, as a dead coal to a living one, which increases the heat of it; so does he, he kindles and spreads the flame of contention and strife.
(k) "vir contentionum", Montanus, Baynus, Piscator, Gejerus.

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., Proverbs 25:22; Ezekiel 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.