Furnaces – Refuse incinerator – Refuse suspended in or supported by a fluid medium
Patent
1995-09-07
1997-07-08
Bennett, Henry A.
Furnaces
Refuse incinerator
Refuse suspended in or supported by a fluid medium
110346, 422168, F23G 500
Patent
active
056449968
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method for reburning or calcining lime sludge in a fluidising bed, whereby the calcium carbonate which is formed when preparing pulping liquor for the production of paper pulp according to the sulphate method is calcined to calcium oxide. The invention also comprises an improved fluidising reactor for the performance of the method.
STATE OF THE ART
Reburning or calcining of so-called lime sludge which has been formed during the preparation of pulping liquor for the production of paper pulp according the sulphate method is necessary for recovering the lime or the calcium oxide which is intended to be reused when preparing new batches of pulping liquor. This lime sludge reburning means that the lime sludge is heated to about 900.degree. C. whereby carbon dioxide is released and calcium oxide is formed. The heating usually occurs by direct contact with fumes from burning of liquid, solid or gaseous combustibles.
The lime sludge reburning is normally carried out according to one of three technical methods, namely in a rotating tube-type furnace, in a fluidising bed or as suspension calcining.
In a fluidising bed, the lime sludge intended to be reburnt which is present as a very fine powder having a mean size of 20 .mu.m, is fed into the hot bed where the calcining occurs, whereupon the thus formed calcium oxide particles bake into granules having a size of approximately 2-20 mm and sink down in the bed wherefrom they are taken out at the bottom level in a suitable way. The exiting hot calcium oxide granules are fed concurrently to the incoming combustion air to preheat the air to 300.degree. C., for example.
THE PROBLEM
A part of the fine material which is fed into the fluidising bed will not, however be baked with other particles and will therefore follow the hot fumes upwards above the bed. There, some of these particles will come into contact with the walls of the reactor, which is somewhat cooler than the heating zone itself, and the particles will therefore bake to the walls. The temperature of these walls can be within the range 600.degree.-800.degree. C., which is the critical baking range for calcium oxide which is the dominant part of the particle mass. This covering is very hard and must of course be removed from time to time. In the wall areas of the reactor bed itself, however, no mentionable baking occurs on the walls due inter alia to the fact the granules which have been formed and move along the walls tend to keep these clean. There has, therefore, long been a strong desire to be able to perform the lime sludge reburning in a fluidised bed in such a way that these hard coverings above the bed itself are avoided.
THE SOLUTION
According to the present invention, the above problem has been solved by bringing about a method for lime sludge reburning in a fluidising bed using hot gases as a fluidising medium which is characterized in that a cooling medium is injected immediately above the fluidising bed.
According to the invention, it is suitable that the fluidising gas consists of fumes.
The cooling medium which can suitably be used in accordance with the invention is lime sludge that is to be reburnt. It is forseeable that the whole amount of lime sludge which is to be reburnt is added as cooling medium.
According to the invention, it is possible by means of the cooling medium to control the temperature in the area above the fluidising bed and keep this at a temperature below about 600.degree. C., preferably at about 500.degree. C.
According to the invention, the temperature in the fluidising bed should be below 1000.degree. C., preferably about 875.degree. C.
The solid material in the gases above the fluidising bed should, according to the invention, be separated, for instance, in a cyclone and thereafter fed into the fluidising bed.
The invention also comprises an arrangement for performing the above method, which arrangement comprises a fluidising reactor for lime sludge reburning which is known per se and which is characterized in th
REFERENCES:
patent: 5335609 (1994-08-01), Nelson et al.
patent: 5345884 (1994-09-01), Vandycke et al.
Hanson Charlotta
Wimby Martin
Bennett Henry A.
Kananen Ronald P.
Kvaerner Pulping Technologies AB
O'Connor Pamela A.
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