Furnace floor

Details Furnace floor Furnace floor

2000-03-30

2001-03-27

Lu, Jiping (Department: 3749)

Metal working

Method of mechanical manufacture

Repairing

C029S402060, C029S402080, C029S402090, C029S426100, C122S00600B, C122S235120

Reexamination Certificate

active

06205634

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to the field of industrial furnaces and boilers and, more particularly, to an improved decanting floor design for kraft recovery boilers.
BACKGROUND OF THE INVENTION
Kraft recovery boilers are used in the pulp and paper industry to recover usable energy from byproducts of the pulp making process. Kraft recovery boilers are similar to conventional fossil-fuel fired boilers. Black liquor fuel is introduced into the furnace along with combustion air. Inside the furnace, residual water is evaporated from the black liquor, and the organic material from the black liquor is combusted. The inorganic portions of the black liquor are recovered as sodium/sulfur compounds.
Gases generated by the black liquor combustion rise out of the furnace and flow across convection heat transfer surfaces. The vertical enclosure walls of the furnace are formed from heat transfer surfaces made of interconnected water tubes. Typically, feedwater enters the recovery boiler at the bottom of a first pass economizer, in which the water is heated as it flows to a steam drum. Saturated water is routed from the steam drum through pipe downcomers to lower furnace enclosure wall and floor inlet headers and a boiler bank. Natural circulation flow in the tubes is induced and driven by heat input to the vertical water cooled enclosure walls of the furnace from the combustion process.
Decanting floors in kraft recovery boilers are known for collecting and directing molten smelt from the black liquor combustion process to discharge openings in the boiler walls. The water tubes forming the floor are cooled by the circulation of water and/or a water/steam mixture through the tubes.
The floors of many known decanting recovery furnaces are essentially flat across the entire surface. Flat floors are subject to minor humping of the tubes, causing domes which form in the upper surfaces of the floor tubes. Steam can become trapped in these humps or domes (steam blanketing) which can cause the tubes to overheat and fail. More particularly, steam blanketing is where steam bubbles are not effectively entrained in the water moving through the tubes. In the flat floor tubes adjacent the furnace sidewalls, the heat input to the tubes may be lower. Since flow through the tubes is by natural circulation, the low heat input to these tubes results in lower fluid velocities and poor circulation in the tubes, which in turn causes steam blanketing.
Currently, the only known method for correcting this defect in flat floor furnaces is to replace the entire tube floor with a new floor. The new floor is sloped to increase the tolerance for heat absorption at lower fluid velocities and to permit venting of the minor humps which would otherwise trap steam and lead to tube overheat and failure. However, replacing the entire furnace floor is both time consuming and expensive, and a cost-effective solution would be welcomed by industry.
SUMMARY OF THE INVENTION
One aspect of the present invention is to provide a method for upgrading a recovery boiler furnace floor without replacing the entire floor.
Another aspect of the present invention is to provide a method and furnace floor for preventing burst tubes which can be implemented relatively inexpensively and efficiently as compared to known solutions.
Accordingly, an improved floor for a recovery boiler furnace having a flat floor has a number of replacement tubes in the floor immediately adjacent each of the sidewalls of the furnace oriented at an oblique angle to horizontal.
In a first embodiment of the invention, immediately adjacent each of the sidewalls, the ends of the replacement tubes at each of the front and back walls are above the remainder of the tubes forming the flat floor and angled to slope downwardly towards a center of the furnace to the level of the tubes forming the flat floor. At the center of the floor, the other ends of the tubes are joined to a central header below the tube floor. The sloped tubes may have a central flat portion adjacent the center of the floor. The radius of curvature of the connection between the flat tube section ends and the central header may be varied to improve fluid flow through the connection, and to join up with the existing flat floor tubes.
In a second embodiment, the ends of the replacement tubes at one of the furnace front and rear walls then are connected to a collection header and are lower than the remainder of the tubes forming the flat floor. The other ends of the replacement tubes at the opposite furnace wall are level with or higher than the remainder of the tubes in the floor. The replacement tubes may be substantially straight along their length. The collection header may be positioned below either the front or back wall of the furnace.
In a method of improving circulation in the floor tubes of a flat floor furnace, several floor tubes immediately adjacent the sidewalls in a flat floor furnace are removed. A series of curved replacement tubes adjacent to the side walls positioned such that the tubes are oriented at an oblique angle to horizontal are connected to the front and back walls of the furnace.
The improved floor design of the invention helps reduce the occurrence of steam bubbles being trapped in humped areas in the furnace floor tubes or forming a steam blanket by improving the fluid flow through the tubes adjacent the sidewalls of the recovery boiler furnace.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific benefits attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.

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