Liquid heaters and vaporizers – Miscellaneous
Reexamination Certificate
2000-03-09
2001-07-24
Lu, Jiping (Department: 3749)
Liquid heaters and vaporizers
Miscellaneous
C110S245000, C432S058000
Reexamination Certificate
active
06263837
ABSTRACT:
The present invention relates to a grate construction of a fluidized bed boiler, like a fluidized bed combustor.
Fluidized bed boilers mainly comprise a furnace defined by vertical walls, in which a fluidized bed of solid particles is maintained; a windbox in the lower part of the boiler under the furnace; and a grate mounted between the furnace and the windbox for supporting the fluidized bed in the furnace. In the grate there, are typically means, such as nozzles, for distributing fluidizing air and/or some other gas into the furnace from the windbox. Additionally, means for removing coarse solids, such as bed material, ash or the like from the grate are conventionally connected to the grate. A solids outlet duct is furthermore provided at the bottom of the boiler for withdrawing from the boiler solids removed from the grate.
Thus, the present invention especially relates to the means of the grate construction for withdrawing from the boiler bottom coarse material generated during the fluidized bed combustion.
In a fluidized bed boiler, coarse material will accumulate on the bottom of the furnace, i.e., on the grate when combustibles rich in non-combustibles, such as stones or scrap iron, are used in fluidized bed combustion, or when the combustion process itself produces coarse non-combustibles, e.g., when the ashes agglomerate. The non-combustibles accumulating on the grate should be removed before interacting detrimentally with the operation of the fluidized bed, e.g., by preventing appropriate distribution of fluidizing and combustion air into the furnace. Effective apparatuses are required for the discharge of the coarse material.
Thus, an ash outlet channel or several conventional ash outlet pipes placed at the bottom of a furnace are often not sufficient to effectively remove coarse material, as fluidizing air is not capable of transferring heavy or unfavorably shaped pieces horizontally for long distances. The migration of material along the furnace bottom is especially hampered when the bottom has upwards projecting air nozzles or other irregularities, to which the coarse material might stick.
An attempt to improve the removal of material from the bottom of a fluidized bed is to use directional fluidizing air nozzles which, by blowing, guide the material towards the outlet opening or duct, as disclosed, e.g., U.S. Pat. No. 5,395,596. The bottom could be stepped or slanted downwards towards the outlet, whereby the gravitation assists the horizontal migration of the material. Such solutions have been presented, for example, in the U.S. Pat. No. 4,372,228. The problem with these solutions may, however, be abrasion of the nozzles and their vicinity, caused by the moving material and the transport air fed at a high velocity. It is also difficult to ensure an even air feed over the entire grate area due to the different bed pressures at various nozzles, at least when the bottom has great level differences.
In another proposed method for the discharge of ashes, the furnace bottom is made of hoppers directing non-combustibles from the entire furnace area into common pits for withdrawing the ashes. Here, the furnace grate is made of separate air feed pipes placed above the ash hoppers. Such solutions have been disclosed, e.g., in U.S. Pat. No. 4,757,771 and European patent application No. 289,974. A problem of these solutions is ensuring the working reliability of the separate air feed pipes extending across the boiler. The separate air feed pipes also create pressure losses leading to pressure loss differences between the nozzles along the air feed pipe. Compensation of the pressure loss differences, which depend upon any given location of assembly, by means of the pressure losses of the nozzles, does not work but for a given feed rate of the fluidizing air. Air nozzles, having so great of pressure losses that the effect of the pressure losses of the air feed pipes remains negligible, have therefore been utilized. However, then the total pressure losses and the costs caused by them will become high.
An inconvenience, when using air feed pipes, still to be mentioned is that only such nozzles can be used as air feed pipes wherein counterflow access of the material into the nozzles is prevented, because even a relatively small quantity of material in the air feed pipes might at least partially clog the pipes and further increase the pressure losses therein.
When the grate is made of ash hoppers together essentially covering the entire bottom of the furnace, the problem will also be that the hoppers will, during operation, contain a great quantity of material. As the material content of the hoppers is not fluidized, a substantial amount of uncombusted fuel or other material useful in the combustion process may be removed therewith. The great mass of material accumulating in the hoppers sets requirements on the strength of the hopper construction.
The U.S. Pat. No. 4,263,877 describes among others a construction in which the furnace bottom is made of several windboxes having a slanted upper part. Vertical air nozzles project from the slanted bottom of the furnace and their upper parts are arranged to extend to the same level. Attempts have been made to solve part of the aforementioned problems of this solution, but still the coarse material, such as scrap iron or the like entering the furnace with the fuel, flowing along the slanted furnace bottom, may easily get caught in the long nozzles projecting upwards from the bottom, which nozzles the material has to confront when flowing towards the ash outlet pipe or pipes which have been arranged at a relatively long distance apart.
It is realized from the above description of the prior art that there is a need for new constructions by which coarse ash and other non-combustibles can, efficiently and without deleteriously affecting the operation of the boiler, be transported to ash withdrawal from the entire furnace bottom area.
An object of the present invention is thus to provide an improved grate construction wherein the drawbacks mentioned above have been minimized.
A special object of the present invention is to provide a grate construction with improved coarse ash discharge in which the construction of the means for ash discharge is durable, the load thereon minimized, and wherein unnecessary withdrawal of useful material from the grate has been minimized.
Then, an object of the present invention is also to provide a grate construction, in which the sticking of the material to be removed to the fluidizing air nozzles has been minimized.
A further object of the present invention is to provide a grate construction for avoiding excessive pressure losses in the air feed and air feeding problems caused by pressure differences, and which provides a reliably operating air feed.
Yet another object of the present invention is to provide a grate construction, in which the factors that deleteriously affect the durability of the air feed pipes and the nozzles therebetween have been minimized.
In order to achieve the above-mentioned objects, the present invention provides a grate construction as defined in claim
1
.
The grate according to the invention typically comprises several substantially vertical pockets opening into the furnace and projecting from the grate into the windbox and connected through their bottoms to the outlet channel of the solids for removing coarse bed material from the grate. In a typical grate construction of the invention, the furnace grate of the fluidized bed boiler, i.e., the upper surface of the windbox at the furnace bottom, is thus designed so that the grate will be divided, on one hand, into substantially horizontal primary air feed zones, in which the main part of the air nozzles are and, on the other hand, into ash discharge zones comprising partly vertical and partly downwards inclined surfaces along which the coarse material to be withdrawn runs into one or more ash discharge ducts or shafts.
The pockets beneath the grate are typically formed of two long, parallelly extending vertical walls which are sli
Lehtonen Pekka
Utunen Pekka
Fitzpatrick ,Cella, Harper & Scinto
Foster Wheeler Energia OY
Lu Jiping
LandOfFree
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