The external sacrificial altar represents the Avodah of refining our animalistic side, like the physical animal which would be elevated through the fires of the altar. We elevate our selfish impulses and behaviours and our material desires through the consuming fires of love of the G-dly soul, aroused by meditation on Hashem’s greatness.
Like the wood on the altar, what is left after this rectification are the glowing coals. In a glowing coal, the coarse wood has been consumed by the fire. The fire permeates the coal so that the fire and the coal become one.
The incense altar, as discussed previously, represents the essence of the soul and its desire for and oneness with Hashem. This is a far more lofty level than the conscious emotions of the Neshama and its involvement with our animalistic soul.
However the coals on which the Ketores was burned on the inner altar had to come from the outer altar.
Refining our animalistic side is a prerequisite to attaining the deeper oneness of the Ketores. It is specifically through the Avodah of elevating our animalistic soul that our G-dly soul can then achieve its own elevation.