Psalms 102:9 - Coke's Commentary on the Holy Bible

For I have eaten ashes— The serpent in Genesis is condemned to go on his belly, and to eat dust, to which his prone posture inevitably subjects him. Casting ashes upon themselves, or rolling themselves in ashes, was a ceremony to express deep distress and sorrow among the Orientals; and if we may suppose that the Psalmist lay prostrate upon the ground in his sorrow, he might be said literally to eat ashes, as well as the serpent is said to eat dust; and his affliction must be highly aggravated in our ideas by such an image as this. See Boch. Hieroz. Psalms 50:4 : Psalms 100:2.