Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With regeneration – reclamation – reuse – recycling or...
Patent
1996-04-01
1997-09-30
Czaja, Donald E.
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With regeneration, reclamation, reuse, recycling or...
162 301, 110203, 110238, D21C 1112, F22B 3302
Patent
active
056722460
DESCRIPTION:
BRIEF SUMMARY
CROSS REFERENCE TO RELATED APPLICATION
This application is a U.S. national phase of PCT/FI95/00424 filed Aug. 10, 1995.
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to a method of and an apparatus for recovering energy and chemicals from waste liquor in pulp processes. The invention especially relates to an increase of the capacity of a recovery boiler, i.e. kraft recovery boiler, or the like.
When combusting waste liquor 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 a 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 the subsequent stages of processing, be converted into a suitable form to be reused in the cooking process.
The kraft recovery boiler has, until now, proved to be superior for the recovery of heat and chemicals from waste liquors. The recovery boiler prior to the main heat absorbing sections can be considered as consisting of three distinct zones: a reduction zone at the bottom of the boiler, a drying zone, where liquor drops dry when falling into the reduction zone, and an oxidation zone in the upper section. The waste liquor is sprayed through liquor nozzles in the form of small drops into a furnace of the boiler. Combustion air is also introduced into the recovery boiler. Air is usually introduced at three different levels: primary air at the lower part of the furnace, secondary air above the primary air level but below the liquor nozzles, and tertiary air above the liquor nozzles to ensure complete combustion. These three air levels are conventional basic air levels in a modern recovery boiler, but also other air levels may be provided in said recovery 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 liquor drops. The gases inflame, thereby delivering heat to heat surfaces, a superheater and a boiler bank, 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 brought through various stages of processing conveyed back to the cooking process.
The flue gases from the kraft recovery boiler contain a large amount 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 (Na.sub.2 SO.sub.4, Na.sub.2 CO.sub.3, NaCl) 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 risk of clogging 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 shutdowns required for washing, inspection and maintenance.
A high temperature of the boiler tubes accelerates the formation of deposits and thereby the corrosion risk of the heat surfaces. Thus, the deposits affect particularly the heat surfaces of the superheater. Usually, the corrosion of the materials is reduced by controlling the temperature of the superheater surfaces.
The principal way of avoiding corrosion is, at least at the present, 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 production of electric power in steam turbine plants.
Attempts have been made to reduce fouling of surfaces and clogging of flue gas channels by dimensioning the convection section large enough and increasing the distance between the superheater surfaces. Larger clearances facilit
REFERENCES:
patent: 1719130 (1929-07-01), Richter et al.
patent: 4738835 (1988-04-01), Kiiskila
patent: 4739729 (1988-04-01), Rettemeier et al.
patent: 5032143 (1991-07-01), Ritakallio
Ahlstrom Machinery Oy
Czaja Donald E.
Nguyen Dean T.
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