Furnaces – Process – Incinerating refuse
Patent
1994-04-18
1995-09-19
Gromada, Denise L.
Furnaces
Process
Incinerating refuse
110238, 110297, 122 7C, F23G 704
Patent
active
054508037
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention relates to a method for the combustion of waste liquids in connection with recovering of chemicals from waste liquid preferably black liquor from pulp production for recovering of chemicals and energy. Most often the combustion is in the latter case carried out in a specially designed boiler which is called a soda furnace. The invention is related to the way of supplying combustion air into such a boiler.
PRIOR ART
With the aim to better clarify the invention below it is only referred to combustion of waste liquids from pulp production which, however, shall not be regarded as limiting the invention. By the production of chemical cellulose pulp from wood or other cellulose containing raw materials a rest product in the shape of spent liquor is obtained which after evaporation for reducing the water content suitably is burnt for recovering of the chemicals used at the pulp production and to transfer the organic compounds in the spent liquor to useful energy.
Most often the chemicals used at the pulp production consist of sodium salts and in these cases the main cation content of the waste liquor consists of sodium. When after evaporation sodium containing waste liquor is burnt, the main part of the inorganic substances are liberated in the form of ash in melt form which primarily consists of sodium carbonate, so called soda (Na.sub.2 CO.sub.3).
The evaporated waste liquor which still contains some water is added to the soda furnace at some level above the bottom of the furnace usually by means of one or more injection nozzles. The accordingly finely distributed waste liquor is in the soda furnace subjected to three stages which can be called the drying stage, the pyrolysing stage and the carbon combustion stage. The last stage occurs primarily in the melt bed (char bed), which is made up of the residue from the pyrolysing on the bottom of the soda furnace. This bed with a high content of carbon is a condition that the main part of the content of sulphur in the ash shall be tapped in reduced form together with the melt, that is as sulphide (Na.sub.2 S). To have the reduction reaction carried out and to tap the ash in melt form it is required that the temperature of the melt bed shall be kept above a certain level. This is possible due to the fact that the combustion of carbon is an exotherm reaction, i.e. energy in the form of heat is developed by the reaction. This is kept alive by the addition of combustion air through the walls of the soda furnace at a low level. Combustion air which is supplied at the lowermost level in a soda furnace is usually called primary air.
Both drying and pyrolysing are endotherm processes, i.e. energy has to be supplied from the environment. If a too large part of these processes should occur at the melting bed the temperature of the bed would sink under the required level and all reactions should cease (so called black bed). Due to this reason one has to make sure that the main part of the drying and pyrolysing stages have been fulfilled when the solid material reaches the melting bed on the bottom of the soda furnace.
Two principally different methods to bring about the necessary drying and pyrolysing is today practised industrially. The one is to let the injected waste liquor droplets after some initial drying hit some of the walls of the soda furnace. The waste liquor will then due to convective heat supply from the walls of the furnace, radiation from the melt bed and radiation from the above the bed burning pyrolysing gas initially be dried to a finishing stage and then the pyrolysing stage starts. In connection with the pyrolysing a swelling of the material occurs which then is liberated and falls down on the melt bed.
The second method is that by finely distributing of the injected waste liquor and/or violent gas turbulence in the fire make sure that a sufficiently great part of the drying and pyrolysing reactions occur when the waste liquor still is suspended in the furnace gases and before the waste liquor particles reach the melt bed.
REFERENCES:
patent: 2911284 (1959-11-01), Hochmuth
patent: 3703919 (1972-11-01), Owens et al.
patent: 5007354 (1991-04-01), Uppstu
patent: 5022331 (1991-06-01), Simonex
Svensk Lennart
Trusler Winston
Van Rensburg Salmon J.
Gotaverken Energy AB
Gromada Denise L.
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