Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With regeneration – reclamation – reuse – recycling or...
Patent
1994-08-12
1995-12-26
Chin, Peter
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With regeneration, reclamation, reuse, recycling or...
162 29, 162 47, 162375, 110238, 122 7R, D21C 1112
Patent
active
054784409
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to a method and apparatus for use in the pulp industry for improving the safety of a spent liquor recovery boiler, such as a soda recovery boiler, whereby the boiler comprises, for recovering chemicals and energy, a furnace cooled by water tubes and connected to the water/steam circulation system of the boiler.
Recovery and regeneration of cooking chemicals effected in the soda recovery boiler are an essential part of sulphate and other Na-based pulp manufacturing processes. Organic substances dissolved in the spent liquor are combusted thereby generating heat which is utilized on one hand, when converting the inorganic compounds contained in the spent liquor back to chemicals to be used in cooking and on the other hand when generating steam. The inorganic matter, i.e., ash, contained in the spent liquor, melts at the high temperature prevailing in the soda recovery boiler and flows as melt downwardly onto the bottom of the furnace and further out of the boiler into a dissolving tank. The soda recovery boiler also serves as a steam boiler, where heat released during combustion is recovered as steam, primarily by water tubes lining the boiler walls, and as high-pressure superheated steam of, e.g. 450.degree. to 480.degree. C./65 to 85 bar, by superheaters disposed in the upper section of the boiler. The temperature rises very high, often to over 1000.degree. inside the furnace of the soda recovery boiler, whereby the conditions in the furnace are highly corroding due to the temperature and the alkali and sulphur compounds contained in the spent liquor.
These special circumstances set constructive demands on the structure of the soda recovery boiler, such demands being different from those set on conventional power plant boilers. In the furnace structures presently used, the whole bottom and walls of the furnace are water-cooled in order to prevent the temperature at the tubes from rising to a level favourable for corrosion.
The bottom and wall tubes nowadays used in soda recovery boilers are usually of gas-tight welded membrane structure, where the tubes are welded together through fins. The fin width is usually 10 to 25 mm. The outer diameter of the tubes is generally 57 to 70 mm. In boilers of another type, the tubes are welded together side by side as a tangential structure with no large spaces therebetween. The furnace has been made gas-tight by welding the tubes together.
The boiler water, which is often highly pressurized and has a temperature of over 300.degree. C., is usually led below the bottom of the furnace, wherefrom it is distributed to the tubes of the bottom and the wall. In the bottom tubes, the water first flows towards the walls and further upwardly therefrom. In the wall tubes, the water normally flows only upwardly.
The water circulation system of the soda recovery boiler functions by-natural circulation. Proper water circulation is of a crucial importance to the operational safety of the boiler. If the water circulation is disturbed, it may result in overheating of the tube material, and consequently corrosion and tube damage. Great changes in the tube temperature as well as in the deposits of chemicals and ashes covering the tubes may cause disturbances in the water circulation. Especially in the bottom tubes, where water flows horizontally or nearly horizontally, a disturbance in the water circulation may be fatal.
A bed is formed on the bottom of the furnace, composed of material remaining in the spent liquor after the evaporable and easily combustible substances have been discharged during the pyrolysis stage. The bed contains, e.g., coal/coke, sodium and sulphur. The amount of the bed mass varies depending on process conditions. On the bottom of the bed, near the bottom tubes of the furnace, there is a layer of melt formed of inorganic chemicals. This layer of melt flows out of the furnace through an opening or openings in the furnace wall.
The lowermost portion of the layer of chemicals, which is in direct con
REFERENCES:
patent: 2161110 (1939-06-01), Tomlinson et al.
patent: 3867251 (1975-02-01), Holme
Pu, "Chemical Recovery Overview in Chinese Pulp and Paper Industry", 1989 Chemical Recovery, pp. 257-262.
A. Ahlstrom Corporation
Chin Peter
Nguyen Dean T.
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