GB1600764A

GB1600764A - Process and installation for extraction of sulphur dioxide from flue gases and/or vapours produced by drying

Info

Publication number: GB1600764A
Application number: GB16964/78A
Authority: GB
Original assignee: Babcock BSH AG
Current assignee: Grenzebach GmbH and Co KG
Priority date: 1977-05-07
Filing date: 1978-04-28
Publication date: 1981-10-21
Also published as: DK175378A; IT1096257B; NL7804891A; IT7822943A0; ATA309978A; FR2389403B1; SE437474B; SE7804986L; FR2389403A1; DE2720686A1; CH643151A5; BE866791A; AT370638B

Abstract

In order to minimise the content of the acid radical of sulphurous acid in flue gases from thermal power stations and/or in the drying vapours from sugar factories, it is proposed to use the calcium carbonate sludge being formed as a waste product in the juice purification for the production of sugar, in particular beet sugar. In this case, the sludge is diluted with water and then dispersed over a large area into the flue gases and/or vapours to initiate a reaction with the acid radical of sulphurous acid. A plant which can be operated in this way is characterised by a reaction tower which is inserted between the boiler house with the drying gas discharge lines and the stack and which is connected to the sludge take-off of the decanter of the first carbonation stage and downstream of which a drip-off housing for separating the flue gas or vapour gas from sludge and water is provided.

Description

(54) PROCESS AND INSTALLATION FOR EXTRACTION OF SULPHUR DIOXIDE FROM FLUE GASES AND/OR VAPOURS PRODUCED BY DRYING (71) We, BAscocK-BSH AKTIENGES ELLSCHAFT, Vormals Biittner-Schilde-Haas AG, a Federal German Republic body Corporate of Parkstrasse 29, D-4150 Krefeld, Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- Industrial plants, for example sugar factories, which also require steam as well as a power supply for heating and driving purposes for their production output, usually operate their own thermal power plants in which large quantities of fuels are fired with formation of flue gas, in order to generate the different types of energy.

More particularly, when the fuel being used is fuel oil S or lignite tar fuel oils, which by nature have a sulphur content higher than 1.7% by weight, when the sulphur burns it combines with oxygen, and thus sulphur dioxide (SO2) is formed, which is supplied with the flue gases through the chimney to the atmosphere.

For reducing the enviromental pollution due to sulphur dioxide, there are now frequently regulations to the effect that only fuel oils may be used which have at most a sulphur content of 1% by weight.

Fuels which may be considered as fuel oils with such a low sulphur content are the fuel oil EL obtained from petroleum oil or the coal tar fuel oils L and M. It is of course also possible to produce fuel oil S with a sulphur content of at most 1% by weight, but the proz ision of a fuel oil S of this type presents considerable difficulties, because the refineries are not in the position to supply sufficient quantities of this type of oil.

Quite apart from this, all the aforementioned fuel oils with a low sulphur content involve a substantially higher production cost than the normal fuel oil S, so that the operators of the thermal power units are only able to comply with the existing regulations for reducing environmental pollution with sulphur dioxide (SO2) by a considerably increased cost of production. By way of example, for the Federal German sugar industry, an annual increase in costs of at least 12 mill. German marks is involved when meeting these conditions.

The present invention seeks to reduce the environmental pollution with sulphur dioxide (SO2), without a substantially increased expenditure, in the operation of thermal power units. Accordingly, the invention seeks to find a way in which the proportion of sulphur dioxide in the flue gases from thermal power units and/or in the vapours resulting from drying operations in sugar factories, is reduced to a minimum.

According to the invention, this problem can be solved in a simple manner by using calcium carbonate (CaCO3) sludge, formed as a waste product during the purification of the juice in the sugar production, and, more especially, during beet sugar production, for the extraction of sulphur dioxide from the flue gas which forms during operation of thermal power units and/or from the vapours resulting from drying operations.

During the course of the production of sugar, the raw juice which is produced in an extraction plant first of all reaches the preliming stage, which can for example be carried out by the apparatus of BRIEGHEL MÜLLER. In this apparatus, the rawjuice is gradually converted with the aid of quicklime into a weakly basic solution with a pH value from 10.8 to 11.0. This weakly basic solution then forms the pre-limitingjuice, which flows into the cold main liming position, in which it is supplied with a relatively large quantity of quicklime (CaO or Ca(OH)2), until the total quantity of quicklime is about 2% of the weight of sugar beet. The main liming juice is thereafter heated up to 90 to 95"C and passes into a temporary stay vessel for the so-called hot main liming. From this vessel, the main liming juice then passes into the carbonation vessel of the first carbonation, in which carbon dioxide (CO2) separated out with the burning of the limestone (CaCO3) is blown into the juice. At the temperature of 90"C, any remaining calcium oxide (CaO) is again combined with the carbon dioxide (CO2) to form calcium carbonate slurry (CaCO3).

The thick slurry or sludge then settles in a decanting unit and is separated by means of vacuum rotary filters into filtrate and almost sugar-free sludge. The thick sludge can obviously also be separated out in a concentrating filter or the like. Whereas the filtrate is conveyed for the purpose of further purification to the second carbonation stage, normally the calcium carbonate thick slurry or sludge (CaCO3) comes as a waste product to a point of deposition. According to a preferred form of the invention, however, this calcium carbonate thick sludge, which forms with the first carbonation may further be employed for the extraction of sulphur dioxide from the flue gases occurring during the operation of the thermal power unit and/or the drying vapours which are formed during the sugar production.

Preferably the calcium carbonate, -(CaCO3)-, sludge is diluted with water and, thereafter, for initiating a reaction with the sulphur dioxide (SO2), is widely dispersed into the flue gases and/or vapours.

According to another preferred feature of the invention, the calcium sulphite (CaSO3) sludge forming during the reaction is separated into a concentrate and a less concentrated dispersion and then the concentrate is deposited as waste, while the less concentrated dispersion is used for a fresh reaction on the flue gases and/or vapours.

According to a further preferred feature of the invention, provision may be made for the water to be extracted from the concentrate of the calcium sulphite (CaSO3) sludge and for this water then to be supplied for dilution purposes to the calcium carbonate (CaCO3) sludge.

Finally, it is also proposed, as a preferred feature of the invention, that the calcium cabonate (CaCO3) sludge deposited after a carbonation procedure be used for extracting sulphur dioxide from the flue gas and/or vapours.

The process according to the invention is particularly advantageous for use in the sugar industry, because in the latter, the calcium carbonate (CaCO3) sludge is immediately available in connection with the juice purification. However, it may also be used for the extraction of sulphur from flue gases in connection with other thermal power units, provided there is a possibility of calcium carbonate (CaCO3) sludge being produced as a waste product in sufficient quantities.

When using the process according to the invention, a certain proportion of the calcium carbonate (CaCO3) is converted into calcium sulphite (CaSO3), the harmful sulphur dioxide (SO2) being replaced by harmless carbon dioxide (CO2).

One form of installation according to the invention for carrying the process into effect may comprise - a reaction tower through which flue gases and/or vapours can pass from a boiler house and drying exhaust gas pipes to a chimney, which tower communicates with the sludge outlet from a decanter for collecting waste calcium carbonate from a first carbonation stage in the sugar production, and a drainage housing downstream of the tower for separating the flue gas or vapour from sludge and water before passing to the chimney.

So that a large-area dispersion of the calcium carbonate (cay03) sludge into the flue gases is possible in a simple manner, it is further proposed according to a preferred feature of the invention to arrange, between the sludge outlet of the decanter of the first carbonate stage and the reaction tower, a vacuum rotary filter and a stirrer mechanism, the stirrer mechanism being supplied with water.

It is further proposed according to a further preferred feature of the invention to arrange a decanter downstream of the drainage housing of the reaction tower, which decanter comprises firstly a thick sludge outlet and secondly a dispersion outlet, the dispersion outlet having a line returning to the reaction tower, while the thick sludge outlet has a vacuum rotary filter downstream thereof. Finally, according to a further preferred feature of the invention, this vacuum rotary filter may comprise a waste sludge outlet and a water discharge, the water discharge in its turn being connected to the water supply unit of the stirrer mechanism.

The invention will now be described in detail, by way of example only, with reference to the accompanying flow diagram.

In the production of sugar, the raw juice which is produced in an extraction plant, for example, from slices or chips of sugar beet, is first of all supplied to a so-called pre-liming unit 1, which may, for example, operate with the apparatus according to BRIEGHEL MILLER. In this unit, the raw juice, by supply of a small quantity of calcium oxide (CaO) or calcium hydroxide (Ca(OH)2), is gradually converted into a basic solution with pH value of from 10.8 to 11.0. This preliming juice then flows ta the cold main liming unit 2, to which a relatively large amount of calcium oxide (CaO) or calcium hydroxide (Ca(OH)2) is supplied in the form of quicklime or milk of lime. The calcium oxide is obtained by burning limestone in a lime kiln 3, carbon dioxide gas (CO2) being split off in the latter.

The total quantity of the calcium oxide (CaO) supplied in the pre-liming and the cold main-liming amounts in this case to about 2% of the weight of sugar beet introduced into the extraction plant. From the cold main liming unit, the main liming juice is supplied to a preheater 4 and is heated in the latter to 90 to 95"C. Thereafter, it flows through a short-stay vessel 5 of the hot main liming unit, from whence it passes in turn into the carbonation vessel 6 of the first carbonation stage. The carbon dioxide (CO2) which is released in the lime kiln 3 during the burning of the limestone is blown into the carbonation vessel 6 and into the main liming juice, so that with a temperature in the region of 90"C, the calcium oxide (CaO) which is still free is again combined with the carbon dioxide (CO2) into calcium carbonate sludge (CaCO3). In a following decanting unit 7, the calcium carbonate sludge particles are separated from the clear juice, which is thereafter supplied to the second carbonation stage (not shown).

The deposited thick sludge of calcium carbonate (CaCO3) then passes into a vacuum rotary filter 8, in which takes place a separation into filtrate and practically sugarfree sludge. The filtrate in its turn is supplied to the second carbonation stage, while the calcium carbonate sludge passes into a stirrer mechanism 9. Water (H2O) is supplied to this stirrer mechanism 9, so that a suspension or paste of the calcium carbonate sludge is formed. This diluted calcium carbonate sludge is then supplied to a reaction tower 10, into which is introduced on the other hand the exhaust gases from the boiler house and drying plant. An extremely fine distribution of the diluted calcium carbonate sludge occurs in the reaction tower 10, so that this sludge offers a largest possible surface to the exhaust gases from the boiler house and drying plant. Consequently, there is a reaction between the boiler house and drying exhaust gases with the calcium carbonate sludge, in such a way that the calcium carbonate (CaCO) is converted into calcium sulphite (CaSO3) and, as a consequence, the dangerous sulphur dioxide (SO2) is replaced by harmless carbon dioxide (CO2).

In a drainage housing of a decanter 11, which is arranged following the reaction tower 10, the flue gas or vapour resulting from the drying and freed from sulphur dioxide (SO2) is separated from the calcium sulphite (CaSO3) sludge and from the water and is for example supplied by way of a blower 12 to the chimney 13.

In the decanter 11, the mixture of calcium sulphite (CaSO3) sludge and water is separated in such a way that a less concentrated dispersion and a more strongly concentrated sludge are formed. The less concentrated dispersion is then returned by a pump 14 into the reaction tower 10 and, in the latter, once again participates in the reaction with the flue gas or the vapour resulting from the drying.

The more strongly concentrated proportion of the calcium sulphite (CaSO3) sludge is introduced into a vacuum rotary filter 15 and is separated therein into deposition sludge and water. The water is supplied to the stirrer mechanism 9, for the purpose of diluting the calcium carbonate (CaCO3) sludge, while a pump 16 conveys the deposition sludge to the deposition unit. In the latter, the calcium sulphite sludge is stored in the usual way, together with the calcium carbonate sludge which is not required for extracting the sulphur from the flue gas and/or vapours resulting from the drying.

Arising from the fact that the calcium carbonate sludge is a waste product which accumulates in larger quantities than required, the advantage is obtained that it is possible to work with an excess of CaCO3.

This means that, as regards the deposition sludge, the major proportion consists of CaCO3 sludge and the smaller proportion of CaSO3 sludge.

WHAT WE CLAIM IS: 1. Process for the extraction of sulphur dioxide from flue gases of thermal power units and/or from vapours resulting from drying, wherein the extraction uses calcium carbonate (CaCO3) sludge formed as a waste product in the purification ofjuice in a sugar production plant.

2. Process according to claim 1, wherein the juice is obtained from beet sugar.

3. Process according to claim 1 or 2, wherein the calcium carbonate (Ca CO3) sludge is diluted with water and is then widely dispersed in the flue gases and/or vapours for the initiation of a reaction with the sulphur dioxide present (SO2).

4. Process according to claim 3, wherein the calcium sulphite (CaSO3) sludge formed in the reaction is separated into a less concentrated dispersion, which is recycled for renewed reaction with the flue gases and/or vapours, and a waste concentrate.

5. Process according to claim 4, wherein water is extracted from said waste concentrate and this water is supplied to the calcium carbonate (CaCO3) sludge for dilution purposes.

6. Process according to any one of the preceding claims, wherein the calcium carbonate (CaCO3) sludge used for extracting sulphur dioxide is deposited after a carbonation procedure in the sugar production.

7. Process according to any one of the preceding claims, wherein the flue gases and/or the vapours are from operation of a

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. oxide is obtained by burning limestone in a lime kiln 3, carbon dioxide gas (CO2) being split off in the latter. The total quantity of the calcium oxide (CaO) supplied in the pre-liming and the cold main-liming amounts in this case to about 2% of the weight of sugar beet introduced into the extraction plant. From the cold main liming unit, the main liming juice is supplied to a preheater 4 and is heated in the latter to 90 to 95"C. Thereafter, it flows through a short-stay vessel 5 of the hot main liming unit, from whence it passes in turn into the carbonation vessel 6 of the first carbonation stage. The carbon dioxide (CO2) which is released in the lime kiln 3 during the burning of the limestone is blown into the carbonation vessel 6 and into the main liming juice, so that with a temperature in the region of 90"C, the calcium oxide (CaO) which is still free is again combined with the carbon dioxide (CO2) into calcium carbonate sludge (CaCO3). In a following decanting unit 7, the calcium carbonate sludge particles are separated from the clear juice, which is thereafter supplied to the second carbonation stage (not shown). The deposited thick sludge of calcium carbonate (CaCO3) then passes into a vacuum rotary filter 8, in which takes place a separation into filtrate and practically sugarfree sludge. The filtrate in its turn is supplied to the second carbonation stage, while the calcium carbonate sludge passes into a stirrer mechanism 9. Water (H2O) is supplied to this stirrer mechanism 9, so that a suspension or paste of the calcium carbonate sludge is formed. This diluted calcium carbonate sludge is then supplied to a reaction tower 10, into which is introduced on the other hand the exhaust gases from the boiler house and drying plant. An extremely fine distribution of the diluted calcium carbonate sludge occurs in the reaction tower 10, so that this sludge offers a largest possible surface to the exhaust gases from the boiler house and drying plant. Consequently, there is a reaction between the boiler house and drying exhaust gases with the calcium carbonate sludge, in such a way that the calcium carbonate (CaCO) is converted into calcium sulphite (CaSO3) and, as a consequence, the dangerous sulphur dioxide (SO2) is replaced by harmless carbon dioxide (CO2). In a drainage housing of a decanter 11, which is arranged following the reaction tower 10, the flue gas or vapour resulting from the drying and freed from sulphur dioxide (SO2) is separated from the calcium sulphite (CaSO3) sludge and from the water and is for example supplied by way of a blower 12 to the chimney 13. In the decanter 11, the mixture of calcium sulphite (CaSO3) sludge and water is separated in such a way that a less concentrated dispersion and a more strongly concentrated sludge are formed. The less concentrated dispersion is then returned by a pump 14 into the reaction tower 10 and, in the latter, once again participates in the reaction with the flue gas or the vapour resulting from the drying. The more strongly concentrated proportion of the calcium sulphite (CaSO3) sludge is introduced into a vacuum rotary filter 15 and is separated therein into deposition sludge and water. The water is supplied to the stirrer mechanism 9, for the purpose of diluting the calcium carbonate (CaCO3) sludge, while a pump 16 conveys the deposition sludge to the deposition unit. In the latter, the calcium sulphite sludge is stored in the usual way, together with the calcium carbonate sludge which is not required for extracting the sulphur from the flue gas and/or vapours resulting from the drying. Arising from the fact that the calcium carbonate sludge is a waste product which accumulates in larger quantities than required, the advantage is obtained that it is possible to work with an excess of CaCO3. This means that, as regards the deposition sludge, the major proportion consists of CaCO3 sludge and the smaller proportion of CaSO3 sludge. WHAT WE CLAIM IS:

1. Process for the extraction of sulphur dioxide from flue gases of thermal power units and/or from vapours resulting from drying, wherein the extraction uses calcium carbonate (CaCO3) sludge formed as a waste product in the purification ofjuice in a sugar production plant.

2. Process according to claim 1, wherein the juice is obtained from beet sugar.

sugar production plant.

8. Installation for carrying out the process of any one of the preceding claims comprising a reaction tower through which flue gases and/or vapours can pass from a boiler house and drying exhaust gas pipes to a chimney, which tower communicates with the sludge outlet from a decanter for collecting waste calcium carbonate from a first carbonation stage in the sugar production, a drainage housing downstream of the tower for separating the flue gas or vapour from sludge and water before passing to the chimney.

9. Installation according to claim 8, comprising a vacuum rotary filter and a stirrer mechanism downstream of the said filter, arranged between the sludge outlet of said decanter for collecting calcium carbonate from said first carbonation stage and the reaction tower, the stirrer mechanism having a water-supply means.

10. Installation according to claim 8 or 9, wherein the drainage housing of the reaction tower is upstream of a decanter, which comprises firstly a thick sludge outlet and secondly a dispersion outlet, the dispersion outlet having a line returning to the reaction tower, while the thick sludge outlet has a vacuum rotary filter downstream thereof.

11. Installation according to claim 9 and 10, wherein the vacuum rotary filter downstream of the thick sludge outlet comprises a waste sludge outlet and a water discharge arrangement, the latter being connected to the water supply of the stirrer mechanism.

12. Apparatus for the extraction of sulphur dioxide from flue gases of thermal power units and/or from vapours resulting from drying substantially as described herein.

13. An installation for extraction of sulphur dioxide from flue gases of thermal power units and/or from vapours resulting from drying substantially as described herein with reference to the accompanying flow diagram.

14. A process for the extraction of sulphur dioxide from flue gases of thermal power units and/or from vapours resulting from drying substantially as described herein.