A burner, process and furnace are provided for regenerating a spent acid stream or other sulfur-containing stream by decomposing the spent sulfuric acid stream and/or other sulfur-containing stream to recover sulfur dioxide from the stream. The burner includes a burner body, and at least one spent acid feed passage positioned at least partially within the burner body or other sulfur-containing feed passage positioned at least partially within the burner body. At least one fuel feed passage may be positioned at least partially within the burner body. A related process and furnace are also provided.