Middle English anelen, from Old English onælan "to set on fire, kindle; inspire, incite," from on- "on" (see an- (1)) + ælan "to burn, bake," from Proto-Germanic *ailan, "probably" [Watkins] from the same PIE root meaning "to burn" that is the source of ash (n.1). It is related to Old English æled "fire, firebrand," Old Norse eldr, Danish ild "fire."
The -n- was doubled after c. 1600 by analogy of Latinate words (annex, etc.; compare accursed, afford, allay). Meaning "to treat by heating and gradually cooling" (of glass, earthenware, metals, etc., to toughen them) was in late Old English. Related: Annealed; annealing.