Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Including recovery of organic by-product
Patent
1994-05-12
1996-04-23
Lacey, David L.
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Including recovery of organic by-product
110238, 162 31, D21C 1112, D21C 1106
Patent
active
055099971
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method of recovering energy from waste liquors from pulp processes. The invention relates especially to a method wherein waste liquor is burned in a waste liquor recovery boiler, e.g. in a soda recovery boiler, and wherein heat is recovered from the resulting flue gases by producing saturated and/or partially superheated steam in the recovery boiler.
BACKGROUND ART
When combusting waste liquors in pulp processes, the aim is to separate the organic and the inorganic parts of the dry substance of the waste liquor from each other. The heat from the organic part of the dry substance is recovered and the largest possible amount of steam is produced by means of this heat. Pulping chemicals are recovered from the inorganic part of the dry substance in such a form that they can, in subsequent stages of processing, be converted into a suitable form to be reused in the cooking process.
The soda recovery boiler has, until now, proved to be superior for the recovery of heat and chemicals from waste liquors. The waste liquor is sprayed in the form of small drops into the boiler. In the hot combustion chamber water vapor, volatile parts of the dry substance, and eventually gasifiable parts of the dry substance evaporate from the drops. The gases inflame, thereby delivering heat to the heat surfaces disposed in the boiler and are discharged from the upper end of the boiler. The ash from the waste liquor drops, i.e. the inorganic substances of the waste liquor, accumulate on the bottom of the boiler, from which they are removed and through various stages of processing are conveyed back to the cooking process.
The flue gases from the soda recovery boiler contain a great deal of ash, mainly sodium sulfate, a portion of which flows along with the flue gases upwards in the boiler in the form of fine dust or molten drops. The salts contained in the ash melt at a relatively low temperature and become, when melting, easily adhesive and corrosive. The deposits formed by the molten ash cause the risk of clogging of the flue gas channels and, furthermore, cause corrosion and erosion of the heat surfaces of the boiler. The risk of clogging and corrosion increases considerably the number of the shutdowns for inspection and maintenance.
Salt corrodes metal, particularly if the salt is molten or partly molten. A high temperature of the boiler tubes speeds up the formation of deposits and thereby the corrosion of the heat surfaces. Thus, the deposits affect particularly the heat surfaces for the superheated steam. Usually the corrosion of the materials is reduced by controlling the temperature of the superheater surfaces.
In the places of the superheater, in which the temperature tends to rise especially high or in which there is a great deal of liquid-phase chemicals, in other words, where the corrosion and the erosion are a problem, special-alloy steels have to be used. Special-alloy steels are, however, remarkably more expensive than carbon steels or pressure vessel steels, which are commonly used, such as chrome/molybdenum-alloy steels. Even special-alloy steels have their maximum operating temperatures, above which they behave in the same way as the cheaper pressure vessel steels. This temperature is substantially lower in soda recovery boilers than in, for instance, oil-fired boilers. Further, connecting special-alloy steel by welding to carbon steel requires special circumstances, e.g. shielding gas, superalloyed filler metals and a demanding welding technique.
If the endurance of the Superheater can be improved material costs are saved and the utilization rate of the pulp mill is improved due to the reduced need for shutdowns for maintenance.
Today, the principal way of avoiding corrosion is to choose a sufficiently low temperature and pressure for the produced steam, whereby the detrimental effects of the molten salt decrease. This means that the steam cannot be superheated to as high a temperature as desired for the maximal production of electric power in steam turbine pla
REFERENCES:
patent: 2606103 (1952-08-01), Hamm
patent: 2840454 (1958-06-01), Tomlinson et al.
patent: 4135968 (1979-01-01), Dehaas
patent: 4710269 (1987-12-01), Santen et al.
patent: 4953607 (1990-09-01), Erkki et al.
patent: 5201172 (1993-04-01), Hakulin et al.
Kuusio Marjo
Nikkanen Samuli
A. Ahlstrom Corporation
Lacey David L.
Nguyen Dean T.
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