Abstract: The ultra-low emission technology can effectively reduce pollutant emissions from coal-fired units and improve the air quality. In order to explore the mechanism of fouling in the tail flue of ultra-low emission units, this paper takes the fouling ash on the induced draft fan and the second stage low temperature economizer (LTE) of a 1000-MW unit with ultra-low emission as the research object. The ash samples are examined by using the elemental analyzer, the X-ray fluorescence spectrometer, the X-ray diffraction and the scanning electron microscopy. Then, the major factors contributing to the formation of the crystalline mineral phase of the ash and the ash deposition occurrence at different locations are analyzed and discussed in details. The results show that the ashes on the blades of the induced draft fan and the second stage LTE are mainly comprised of (NH₄)Al(SO₄)₂·12H₂O, (NH₄)₃H(SO₄)₂, CaSO₄ and SiO₂. Among them (NH₄)₃H(SO₄)₂ covering on the surface of the ash particles is the immediate cause of the severe ash deposition on the induced draft fan and LTE, while, the root cause is determined as the ammonia escape from the SCR. At last, this paper suggests that the temperature drop of the flue gas at the first and second stage economizers should be properly distributed considering the sulfur contents of the coal, the ammonia emissions and the ash concentrations, so as to prevent the ESP being operated out of low-low temperature conditions and avoid drastic flue gas temperature drop at the second stage LTE which may cause the rapid condensing of sulfuric acid mist.

Key words: ultra-supercritical unit, ultra-low emission, induced draft fan, low temperature economizer, ash deposition, ammonium sulfate