John says, "He will baptize you in the Holy Spirit and fire" (3:16). What is the fire?

Luke doesn't say, and John doesn't explain it in these verses. He speaks of fire three times. Of the tree: "Every tree therefore that doesn’t produce good fruit is cut down and thrown into the fire" (3:9). Of the one who comes after him: "He will baptize you in the Holy Spirit and fire" (3:16). And of the chaff: "he will burn up the chaff with unquenchable fire" (3:17). Luke doesn't say whether the fire of 3:16 is the same as the fire of 3:9 or 3:17. Mark and Matthew give John's saying in their own words: "he will baptize you in the Holy Spirit" (Mark 1:8), and "He will baptize you in the Holy Spirit" (Matthew 3:11). Favor's Mark 1 sets the sayings side by side in its go-deeper "He will baptize you in the Holy Spirit". Luke gives no more of what John meant, and these notes don't say what the fire is.