Leviticus 9:12

"And he slew the burnt-offering; and Aaron's sons delivered unto him the blood, and he sprinkled it upon the altar round about. And they delivered the burnt-offering unto him, piece by piece, and he burnt them upon the altar. And he washed the inwards and the legs, and burnt them upon the burnt-offering on the altar."

Some have supposed that the coming of the "fire from God" in Leviticus 9:24 which "consumed the burnt-offering" contradicts the statements here that Aaron burnt these offerings upon the altar. The explanation is that the altar fire, which was not permitted to go out, was kept smoldering at all times. Thus, when Aaron "burned" these offerings, he merely placed them in a position where they would continue to be burned and eventually consumed. The fire of Leviticus 9:24 instantly consumed the entire offerings.