The fire that killed Nadab and Abihu was the same eternal fire that destroyed Sodom, that burned in the bush Moses saw, that burned the stones and the water of Elijah's offering on Mount Carmel, and that came in tongues of fire to the upper room on Pentecost. It was the fire of God.
As with the burning bush and the tongues of fire above the apostles' heads, this fire did not need to consume anything to be sustained. With or without combustible material, it is an eternal fire. This is because the fire was that of God Himself.
For the LORD thy God is a consuming fire, even a jealous God.
(Deuteronomy 4:24, KJV)
Therefore have I poured out mine indignation upon them; I have
consumed them with the fire of my wrath: their own way have I
recompensed upon their heads, saith the Lord GOD. (Ezekiel 22:31, KJV)
For our God is a consuming fire. (Hebrews 12:29, KJV)
God's presence, as a consuming fire, was already present in the sanctuary. It was this fire which had lit the seven golden candlesticks--a fire which the priests were never to allow to burn out, and from this fire they were to light their censers. It was because Nadab and Abihu had not lit their censers from the fire of God, but had instead brought in "strange fire," that God's fire destroyed them--a fact repeated several times, as a lesson to all of the importance of carefully following God's instructions.
And Nadab and Abihu, the sons of Aaron, took either of them his
censer, and put fire therein, and put incense thereon, and offered
strange fire before the LORD, which he commanded them not. (Leviticus
And Nadab and Abihu died before the LORD, when they offered strange
fire before the LORD, in the wilderness of Sinai, and they had no
children: and Eleazar and Ithamar ministered in the priest's office in
the sight of Aaron their father. (Numbers 3:4, KJV)
And Nadab and Abihu died, when they offered strange fire before the
LORD. (Numbers 26:61, KJV)