The Ijen volcano is a group of composite volcanoes in the Banyuwangi Regency of East Java, Indonesia.
It is surrounded by a one-kilometer-wide turquoise lake that identified as the "largest highly acidic crater lake in the world.

The Ijen Volcano of Java Island brightens the night with its electric blue flames and lava,
The burning continues twenty-four hours a day,
but it's most visible in darnkess. The flames rise up to five meters high. Locals call the caldera surrounding Ijen "Gunung Merapi" which means Mountain of Fire.

Why does it happen??

Many scientists and photographers went to the Kawah ijen volcano to find the answer.When experimented it was found to be sulphur.

There are many elements which when burned gives different coloured flames. For example, copper produces a green flame, lithium and strontium a red flame, calcium an orange flame, sodium a yellow flame, and barium a pale green flame.

Grunewald and filmmaker Regis Etienne produced a documentary about the volcano called Kawah Ijen, the Mystery of the Blue Flames.

References-
https://interestingengineering.com/blue-lava-largest-sulfuric-acid-lake-ijen-volcano/
https://en.wikipedia.org/wiki/Ijen