The passage in Rev 8:2-6 forms a simple chiastic structure as shown below (my translation):
And I saw:
A: …the seven angels who stood before God and there were given to them seven trumpets
B: And another angel came and stood on/at the altar having a golden censer
C: And there was given to him much incense(s) so that he will give [it] with the prayers of the saints
D: all on the golden altar before the throne
C: And the smoke of the incense(s) went up with the prayers of the saints out of the hand of the angel
B: And the angel took the censer and filled it with fire from the altar,
and cast [it] into the earth and there occurred thunders and rumblings and lightings and [an] earthquake
A: And the seven angels having the seven trumpets prepared themselves to trumpet.
The question now remains - what altar did the angel stand at? We have to choices: the bronze altar of burnt offering, or, the golden altar of incense near the inner curtain near the ark of the covenant. In this case it is the latter for the following reasons:
- The sequence of scenes in Revelation appears modelled on the sanctuary service of the OT, and the scenes start outside and progress through the laver, menorah, shew bread, golden altar and finally the ark in Rev 11:18. In Rev 8 we are inside the Holy Place.
- The angel carries a censor while ministering at the altar. Only the altar of incense required a censor - the bronze altar was not associated with a censor.
- The altar is golden - the inner altar of incense was golden while the outer altar was bronze.
- The angel offers incense which is offered on the golden altar of incense. The bronze alar of burnt offering was used to sacrifice animals.
Therefore, the angel (not one of the seven trumpeters) appears to stand at the golden altar of incense in the Holy Place and take fire from this golden altar to place in his censor to throw it on the earth.