Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With regeneration – reclamation – reuse – recycling or...
Patent
1995-04-13
1997-02-18
Czaja, Donald E.
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With regeneration, reclamation, reuse, recycling or...
162 47, 162375, 110238, 122 7R, D21C 1112
Patent
active
056038038
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 recovering heat in a spent liquor recovery boiler, such as a soda recovery boiler, whereby the boiler walls are formed of water-cooled tubes 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 the pulp manufacturing processes. Organic substances dissolved in the spent liquor are combusted, thereby generating heat which is utilized in steam generation. 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. 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 super-heated 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. C., 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. Therefore, in the furnace structures 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 to corrosion.
The lower section of the furnace and the bottom of the soda recovery boiler are made of water cooled tubes, which constitute part of the pressurized section of the boiiler. Due to the structure of the bottom and the lower section of the furnace, there is natural circulation of water in the bottom tubes, i.e., the circulation is effected by the difference of density. The boiler water is led at a high pressure and, e.g., over 300.degree. C. temperature below the bottom of the furnace, to the distributing pipe, wherefrom the water is then distributed into the the bottom and wall tubes. In the lower section of the furnace, water first flows nearly horizontally or obliquely upwardly in the bottom tubes and then further upwardly through wall tubes to the upper section of the boiler.
The natural circulation sets limits to the structure of the furnace and bottom of the boiler because all tubes should be either rising or vertical. Furthermore, the natural circulation structure complicates the free layout and shaping of the melt discharge openings and air ports.
Proper water circulation is of 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 may cause disturbances in the water circulation. Especially in the bottom tubes, where water flows nearly horizontally, even a relatively small 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. 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 contact with the water-cooled bottom tubes of the furnace, is normally in a solid state due to the cooling effect of the tubes. This solid layer protects the water tubes.
Formation of the solid layer onto the bottom of the furnace may be disturbed by tempe
REFERENCES:
patent: 3915654 (1975-10-01), Oni
Ahlstrom Machinery Corporation
Czaja Donald E.
Nguyen Dean T.
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