He burned it By ordinary fire, which was used until the fire came down from heaven, (Leviticus 9:24,) though afterward it was forbidden. And if it had not been allowed otherwise, yet this being done by Aaron at the command of Moses, and consequently with God's approbation, it was unquestionably lawful.
Related Commentaries of Leviticus 9:10
Leviticus 9:10
10 But the fat, and the kidneys, and the caul above the liver of the sin offering, he burnt upon the altar; as the LORD commanded Moses.