Bed material agglomeration during fluidized bed combustion. Technical progress report, April 1, 1993--June 30, 1993

Abstract

A survey of agglomeration and deposit formation in industrial fluidized bed boilers indicate that several facilities are experiencing some form of bed material agglomeration. Deposit formation was reported at nine sites with these deposits being, found most commonly at coal feed locations and in cyclones. Deposit and agglomerate samples have been received from four units. Our analyses of the cyclone deposits indicate they are primarily composed of an iron-aluminosilicate material. The bulk of the deposit is about 30 wt % SiO{sub 2}), 18 wt % Al{sub 2}O{sub 3}, and 30 to 33 wt % Fe{sub 2}O{sub 3}. Titanium is also present between 3 and 4 wt %. All the iron in the deposit is now in the Fe+3 state, but there is evidence that at the time of deposit formation it was in the Fe+2 state. The atomic structure of the bulk of the deposit is that of the spinel mineral maghemite. In nature, this is a rare mineral which forms by the oxidation of the mineral magnetite. Magnetite contains iron in both the +2 and +3 states and can incorporate titanium into its structure at low oxygen fugacities. The relatively high titanium content of the deposit suggests that it may have originally been magnetite (formed at a low oxygen fugacity), but was later oxidized to maghemite under higher oxygen fugacities. We have hypothesized that the cause of deposit formation was most likely interaction of iron, as a flux (in a reduced form), with aluminosilicate materials (clays). The iron probably originated as pyrite or iron sulfides in the feed. It also appears the quench water plays a role in enhancing, the deposit formation in the cyclones. We have hypothesized that the high iron content of solids in the sour quench water probably did not play a role in deposit formation; although, the high water vapor pressure may have had an effect on the viscosity of the material. The alkalic elements, Na and K, probably played only a minor role in deposit formation.