Simultaneous Inner and Outer Thermography of Rotary Kilns for Hazardous Waste Incineration ― Controlled protective slagging results in a considerable prolonging of refractory life

摘要

Rotary kilns are furnished inside with a fire resistant lining of Al_2O_3-rich refractory bricks (250 mm ... 300 mm original thickness) to protect the kiln's supporting outer steel shell against the inner combustion temperatures. The expensive lining (ca. 300.000 (EUD)) should be protected from rapid brick wear, caused ― especially in case of high temperature incineration ("slagging combustion") ― mainly by liquefied molten slag in direct contact with the brick surface and mortar joints. But "slagging combustion" isn't, in principle, worse for a kiln than "dry (ashing) combustion", just the contrary, if the kiln is operated properly between "too hot" and "too cold". The inner kiln temperatures must be high enough for slag melting and at the same time low enough for slag freezing (induced by radial heat transfer through the "multilayer kiln wall") to allow for a protective frozen slag-layer on top of the brick surface. If a frozen slag-layer can be achieved everywhere and permanently, the brick life may be extended considerably, even under the high temperature conditions of "slagging combustion". The addition of slag forming agents for brick protection has become a practice in some plants, while others only tested and then stopped it because of unsatisfying results and operational difficulties; as did BAYER AG in 1970. But these difficulties can be overcome and a considerable prolonging of refractory life can be achieved, when "protective slagging" is precisely controlled. Proper protective slagging should be based on "simultaneous inner and outer thermography", as applied by the authors within BAYER AG for the first time worldwide. The temperatures, measured thermographically at both outer and inner kiln surfaces, must first be aligned exactly, then their local differences must be evaluated, with respect to the corresponding "equivalent brick thickness" (total thickness of residual brick and frozen slag). For this evaluation a suitable PC-based software is needed, besides of the necessary thermographical hardware itself. The applied software, "RotaVos", was developed in 1997/98 and finally brought into a practical form, adaptable to almost any unobstructed kiln without external water cooling. Thus "simultaneous inner and outer thermography" was transferred into a "non-contact wall-thickness measurement", applicable during kiln operation. The initial operational results of the new method were favorable, and therefore the necessary thermographical hardware as well as the special software "RotaVos" were installed in 1998/99 at two neighbouring rotary kilns at the BAYER-waste management center in Leverkusen-Buerrig, and since then successfully used by the kiln operators. The method has been applied for more than three years, confirming that proper "thermographically controlled slagging" is practical in normal plant operations indeed. The rate of brick wear has been reduced from > 200 mm/year to < 50 mm/year. Thus "controlled protective slagging" is allowing for a brick life of at least 4 years (prolongation factor ca. 2 ... 4).