The invention concerns a method and apparatus for decreasing the heat, material and impulse exchange in the direct vicinity of the walls of fluidized bed reactors, in which a granulated solid is fluidized by a fluid from below and thereby is given an intensive agitated movement (fluidized bed).
The decrease in the heat, material and impulse exchange is frequently desirable in e.g. chemical reactors or combustion chambers for reducing the loss of heat and also to protect the walls if necessary simultaneously from corrosive and erosive stress.
As a result of the intensive movement of solids the fluidized beds produce e.g. an extraordinary heat transferring capacity. The heat transfer coefficients at the boundary of the fluidized bed are for instance about an order of magnitude higher than with heat exchangers, which only act upon gases (M. Baerns, Chem. Ing. Techn. 40 (1968) 737). This usually thoroughly desired characteristic can in many cases of use or in known phases of operation lead to undesirably high leakage of heat through the reactor walls. Also, the material exchange will be considerably promoted through the agitation of the solids, so that mixing of the solids in fluidized bed reactors can be introduced easily and lead to very homogeneous products.
To decrease undesirably high leakage of heat through the walls and/or corrosive and erosive attack, ceramic linings, e.g., through tamping or lining with bricks or masonry, are customary (Lueger, Encyclopedia of Technology, Fourth Edition (1965) volume 7, p. 266, Stuttgart). The peculiarity of the lining, however, necessitates that the number of passages extending through the wall must be kept as small as possible because of the increased susceptibility of the lining to breakdown where such passages exist. With fluidized bed reactors with higher thermal loading of the reaction space, such as e.g. heavy-duty combustion chambers, they must however be cooled through an immersed heat exchanger, so as to be able to regulate the reaction temperature. In this case a brick lining in the wall is inappropriate on account of the many passages; for this reason water wall panels are used to a greater extent, in which passages are produced easily and they have a sufficient strength. The increased leakage of heat through the cooled walls must, however, be accepted with this construction (E. Wied, Steam Generator with Fluidized Bed Combustion Under Atmospheric and Above Atmospheric Conditions, VGB Kraftwerkstechn. 58 (1978), [8] 554). In particular with heating up of the fluidized bed reactors one obtains such an effect that disproportionately large amounts of start-up heat become necessary, which, usually, must be introduced through extraneous energy.
A tamping of the reactor with a thermally insulating material results only in a comparatively small effectiveness, since through the large number of fastening rivets the heat transfer coefficient will only be reduced insignificantly. In addition, the durability of the tamped lining is, in the region where many passages exist as it is necessary for heat exchange bundles, very limited, and damages are removed only at great expenditure. This obtains also if a lining not for heat insulation purposes but for protection from corrosive and erosive strain must be applied. (W. Gumz, Brief Handbook of Fuel and Heating Technology, Third Edition, pp. 600-603, Berlin, Gottingen, Heidelberg, 1962; Koppers Handbook of Fuel Technology, Third Edition, p. 363, Essen).