Who is without parts, without actions, peaceful, blameless, unattached, the supreme bridge to Immortality, like a fire that has consumed all its fuel

Shvetashvara Upanishad VI 19

The Sanskrit/Pali word translated as fuel here is indhana, indhana is something capable of burning, like firewood, cow dung, this is exactly fuel, as the root ind means to kindle.

But Vaccha, suppose they were to ask you: ‘This fire in front of you that is quenched: in what direction did it go—east, south, west, or north?’ How would you answer?
It doesn’t apply, worthy Gotama. The fire depended on grass and logs as fuel. When that runs out, and no more added, the fire is reckoned to have become quenched due to lack of food

MN 72

The Pali word translated as quenched here is nibbuto, that one who reached nibbana (Sanskrit Nirvana). The Sanskrit/Pali word used here and translated regularly as fuel here is not indhana at all, it is upādāna and (although it is commonly translated as such)

Upādāna is translated literally as taking up, what is the relation to fire and fuel then? When fire burns it takes up, consumes logs, grass, it depends on it as its sustenance, so that is why logs are upādāna for the fire, and in this simile when sustenance, the food. The soil and water for a plant are also upādāna, sustenance, food as the food is upādāna for a living being.
The corresponding verb is upādiyati, to take up, to acquire.
When upādāna ends, the fire gets quenched (nibbuto) In this simile it is a wordplay, based on indhana/upādāna, on the fire being the symbol for desire and passion in other Pali suttas and the fire being the symbol of Brahman in Vedas.

And the last word for today upādhi, it is something that is a result of upādiyati, an acquisition, something that was taken up, that is something that has been taken and becomes attached. That attachment limits the one which took up the attachment, upādhi. And it conditions the object, limits it. Like damp fuel, damp is here the upādhi for fuel, it limits it when attached, like fire itself is a general object which may give out smoke or not, but damp fuel give out smoke always. The upādhi does not change the fuel though, but limit and restricts it when being attached to it. Another illustration is that of a red flower which transfers its property of redness to a clear crystal due to their proximity. In this case the red flower is the upādhi of the crystal

So upādhi can be considered as something that disguise the reality, not every fuel gives smoke, that is the reality, but seeing the damp fuel gives smoke disguise fuel as smoky. The crystal is not red but we think it is red because of upādhi

Same with Brahman, upādhi disguise it as God, soul, world, but actually Brahman is like a fire that has consumed all its fuel, no more upādāna, no more upādhi, fire gets quenched (nibbuto), it is nibbana and the true reality shines.