EP1355722A1

EP1355722A1 - Method and installation for purifying cement plant fumes

Info

Publication number: EP1355722A1
Application number: EP01999416A
Authority: EP
Inventors: Gilles Vicard
Original assignee: Neptune Technologies
Current assignee: Neptune Technologies
Priority date: 2000-12-04
Filing date: 2001-12-03
Publication date: 2003-10-29
Also published as: CA2436315A1; AU2002216158A1; WO2002045824A1; FR2817487B1; FR2817487A1; US20040042946A1; PL362055A1

Abstract

The invention concerns a method which consists: in extracting (through 18) fumes to be purified outside the furnace (2), at a temperature ranging between 250 and 400 DEG C; directing the extracted fumes towards a cyclone-type dust suppressing unit (24); evacuating (through 26) through a first outlet of the dust-suppressing unit, the fumes cleaned of dust, which are denitrified (in 28); evacuating (through 30) through a second outlet of the dust-suppressing unit, a marginal fraction of fumes, ranging between 1 and 6 % of the fumes input into the dust-suppressing unit, said marginal fraction being collected simultaneously with the dust; and mixing (in 36) the denitrified fumes and said marginal fraction of fumes.

Description

PROCESS AND PLANT FOR PURIFYING CEMENT SMOKE

The present invention relates to a process and an installation for purifying cement fumes.

Cement, in particular Portland cement, is widely used in sectors as varied as bridges and pavements, the construction or realization of large pipes. Several varieties of cements and several processes aimed at their manufacture are used, each with its own specificities.

The cement manufacturing process begins with the preparation of a mixture made from raw materials such as clay, limestone, lime. These elements provide silicon, calcium, aluminum and iron which are the essential elements in a cement.

A grinding of these various elements is then carried out, either wet or dry. This grinding can generate a huge amount of dust and it is necessary to use smoke treatment devices, such as bag filters, cyclones or sometimes electrostatic precipitators.

In a subsequent key step, the ground material, dry or wet, is calcined in ovens, where the product is brought to a maximum temperature of about 1450 ° C. Several types of ovens exist and one can quote in particular the ovens of the type "LEPOL", as well as the long rotary ovens. Preheaters are often used.

During this process, the supplied mixture undergoes chemical transformation and gives rise to the clinker. But the attrition and continuous mixing of these solids generate a significant amount of dust. Furthermore, the effluent gases leaving the ovens are usually also very loaded with such dust. Certain intermediate streams can therefore contain concentrations of dust exceeding 100g / Nm ³ of smoke.

Several methods and approaches are used, with a view to .dusting of these fumes. Cyclones and electrostatic precipitators can recover this dust, which is either returned to the furnace, or sometimes purely and simply eliminated. Bag filters are also used. However, bringing to a temperature above 1000 ° C a product in an atmosphere containing oxygen and nitrogen also generates, as in a combustion process, nitrogen oxides. The pressure inherent in the emission standards in force, or anticipated, leads to treatment to reduce the total quantity of these pollutants released during the implementation of cement manufacturing processes.

Several solutions are potentially available and some of them are commercially used. A selective, non-catalytic process for the reduction of nitrogen oxides (SNCR) can be used. In such a process, a reducing agent, such as ammonia NH ₃ or urea NH ₂ -C0-NH ₂ , is injected and reacts with the oxides to transform them into nitrogen. This process, if it makes it possible to reduce nitrogen oxides by 50 to 70%, imposes particular constraints and cannot always be put into service.

Indeed, to be effective, the reagent, namely ammonia or urea, must be introduced in a narrow window of temperatures. This, which is between 950 and 1150 ° C, is not always accessible. In addition, these methods generate a certain amount of nitrous oxide N ₂ 0, a pronounced greenhouse gas. US Pat. No. 5,137,704 presents an improved variant of this technology. Another known method consists in using burners of a particular type, called low NO _x burners, or "Low NO _x ". However, the reduction of the NO and NO ₂ contents is limited, and the quality of the clinker obtained can be reduced by the use of these burners, due to the alteration of the speciation of sulfur. '

Methods using electron beams have been proposed in other industries, but these devices are too expensive for the cement industry.

Finally, a so-called SCR (Selective Catalytical Reduction) process has already been used. This process allows a reduction of more than 90% of nitrogen oxides and consists in passing over a catalytic bed, for example containing oxides of vanadium and tungsten, the gases to be treated to which a reagent such as ammonia has been added.

However, the catalyst used is sensitive to dust and must be protected. If a filter is used, the temperature should generally drop below 200 ° C. However, most catalysts must work above 250 ° C. The exchangers that are used represent additional equipment and therefore corresponding investments. The cyclones, which are also used, have a too low yield to be able to guarantee a dust content compatible both with the input contents imposed by the process, and those necessary for a reliable and economical use of the SCR catalysts. These must have a lifespan of several years, and therefore be protected.

The present invention aims to propose a process for purifying cement fumes, making it possible to overcome the various drawbacks explained above.

To this end, it relates to a process for purifying cement fumes, in which the fumes to be purified circulate in an oven comprising a drying chamber and a calcination chamber, characterized in that it comprises the following stages:

- the fumes to be purified are extracted from the oven, these extracted fumes having a temperature between 250 and

400 ° C;

- We direct the extracted fumes to a dust collection unit, cyclonic type;

- Evacuated by a first outlet from the dedusting unit, the dedusted fumes, which are denitrified; a marginal fraction of the smoke, including a second outlet from the dust removal unit between 1 and 6% of the fumes admitted to the dedusting unit, this marginal fraction being collected at the same time as dust; and

- The denitrified fumes and said marginal fraction of fumes are mixed.

According to other characteristics of the invention:

- the extracted fumes have a temperature between 280 and 350 ° C;

- the marginal fraction of fumes is between 2 and 4%, in particular 3%, of the fumes admitted to the dedusting unit;

- The denitrified fumes are returned to the drying chamber, these denitrified fumes are extracted from the oven, then the denitrified fumes extracted from the oven are mixed with said marginal fraction;

- The mixture of denitrified fumes and said marginal fraction is directed to an additional dust collector;

- The denitrified fumes and the marginal fraction are mixed, then the mixture of denitrified fumes and of said marginal fraction is directed towards the drying chamber of the oven.

The invention also relates to a plant for purifying fumes from a cement plant, comprising an oven belonging to a cement manufacturing installation, this oven comprising a drying chamber and a calcination chamber in which the fumes to be purified circulate, in service, characterized in that it includes:

means for extracting the fumes to be purified from the oven, these extraction means extending from an intermediate zone of said oven having, in service, a temperature between 250 and 400 ° C., preferably between 280 and 350 ° C;

- a dedusting unit, placed in communication with the extraction means;

- first means of evacuation, making it possible to extract from the dusting unit the dusted fumes; - second exhaust means, making it possible to extract from the dedusting unit a marginal fraction of the fumes admitted into this unit, this marginal fraction being collected at the same time as dust; - a denitrification reactor, put in communication with the first means of evacuation; third means of evacuation, making it possible to extract the denitrified fumes from the denitrification reactor; and a junction zone between the second means for evacuating said marginal fraction and the third means for evacuating denitrified fumes.

According to other characteristics of the invention:

- The dedusting unit is of the cyclonic type, and it is constituted by a cyclone or a battery of cyclones;

- the junction zone is put in communication with an additional dust collector;

- the additional dust collector is a bag filter or an electrostatic precipitator.

The invention will be described below, with reference to appended FIGS. 1 and 2, given only by way of nonlimiting example, which are schematic representations of two plants for purifying fumes from cement works, in accordance with the invention .

Figure 1 shows a furnace 2, belonging to a cement manufacturing installation. This oven, which is of the "LEPOL" type, usually comprises a drying chamber 4, as well as a calcination chamber 6. As is known, a material intended for the manufacture of clinker is admitted by line 8, then passes through the drying chamber 4, as well as the calcination chamber 6, along a conveyor belt 10. The clinker thus obtained is then removed from the furnace 2 at an outlet orifice 12. This furnace is also provided a safety chimney 14.

The fumes generated inside the furnace are collected via cyclones 16, then directed into a line 18, extending from an intermediate zone 20 of the furnace 2. In this zone, the fumes are loaded with dust, at a concentration of between 20 and 300 g / Nm ³ , and have a temperature between 250 and 400 ° C, especially between 280 and 350 ° C.

These fumes are then extracted, outside of the intermediate zone 20 of the oven 2, along the pipe 18. The latter, which is provided with a fan 22, is placed in communication, at its end opposite the oven, with a unit dust removal 24, consisting of a cyclone or a battery of cyclones. A line 26 puts this unit 24 in communication with a denitrification reactor 28. Furthermore, a pipe 30, intended for the outlet of the dust, also extends from the unit 24. In order to stabilize the cyclone (s) forming the dust removal unit 24, a small, or marginal fraction of the fumes initially admitted through the pipe 18 is voluntarily allowed to leave, with the underflow of each cyclone, this fraction mixed with the dust, which is evacuated via the pipe 30 with the solids collected by the dedusting unit 24 is between 1 and 6% of the gas flow admitted into this unit 24, preferably between 2 and 4%. In addition, line 26 carries dusted fumes, which contain between 100 mg / Nm ³ and 5 g / Nm ³ of dust, preferably between 200 mg and 1 g / Nm ³ .

These dust-free fumes are then directed, via line 26, to the denitrification reactor 28, then are evacuated therefrom, via a 'line 32. The latter opens into the drying chamber 4 of oven 2, which allows the denitrified fumes to d be admitted to this room. These fumes are then extracted from chamber 4, via a pipe 34.

The pipes 30 and 34 are then placed in mutual communication, at a junction 36. This ensures mixing between, on the one hand, the fumes collected with the dust, outside of the unit 24 and, on the other hand , the dust and denitrified fumes discharged from the reactor 28, via the drying chamber 4.

The smoke mixture thus formed is then directed, via a line 38, to a final dust collector 40, which for example consists of a filter or an electrostatic precipitator. The fumes which are extracted therefrom, via a pipe 42, are then freed from most of their nitrogen oxide, as well as from their dust. These purified fumes are, for example, discharged into the atmosphere by a draft fan, not shown. FIG. 2 illustrates an alternative embodiment of the invention, in which the marginal fraction of fumes mixed with the dust is extracted, via a pipe 31. Furthermore, the dust dedusted, then denitrified, is evacuated from the reactor 28, by the intermediate of a line 33. This pipe 31 and this line 33 are placed in mutual communication, at a junction 37. A line 39, which extends from this junction 37, returns to the drying chamber 4 of the oven 2.

Thus, the denitrified fumes, leaving the reactor 28, and the fumes collected with the dust, outside of the unit 24, are mixed at the junction 37, then returned to the oven 2. This mixture of fumes is then extracted from the oven, via a line 41, and is admitted into the final dust collector 40. The invention takes advantage of the extraction of the fumes, outside the oven, at an intermediate zone of the latter. Such a zone is, on the one hand, of easy access and has, on the other hand, a temperature favorable to the treatment of fumes by means of a denitrification reactor. Such a measure therefore makes it possible to dispense with the use of heat exchangers, which are expensive and of delicate use, taking into account the high dust content of the fumes.

Furthermore, the fact of voluntarily evacuating part of the smoke with the dust, by the underflow of the dedusting unit 24, makes it possible to increase the efficiency of the latter. Such a measure thus has the effect of reducing the dust content in line 26, conveying the dust removed. This therefore guarantees improved protection of the catalyst contained in the denitrification reactor 28. In other words, if the extraction of the marginal fraction of fumes mixed with dust was not planned, it would then be necessary either to replace the catalyst of the reactor 28 at too high frequencies, that is to select a catalyst more resistant to dust, but whose efficiency would be reduced.

Claims

1. A method of purifying cement fumes, in which the fumes to be purified circulate in an oven (2) comprising a drying chamber (4) and a calcination chamber (6), characterized in that it comprises the steps following:

- The fumes to be purified are extracted (by 18) from the oven (2), these extracted fumes having a temperature between 250 and 400 ° C;

- It directs the extracted fumes to a dust removal unit (24), cyclonic type;

- Evacuated (par 26) by a first outlet from the dedusting unit, the dedusted fumes, which are denitrified (at 28);

- a marginal fraction of the flue gases, between 1 and 6% of the fumes admitted to the dedusting unit (24) is removed (by 30; 31) by a second outlet from the dedusting unit, this marginal fraction being collected together with dust; and

- Mixing (in 36; 37) the denitrified fumes and said marginal fraction of fumes.

2. Method according to claim 1, characterized in that the extracted fumes have a temperature between 280 and 350 ° C.

3. Method according to one of claims 1 or 2, characterized in that the marginal fraction of fumes is between 2 and 4%, in particular 3%, of the fumes admitted into the dedusting unit (24).

4. Method according to one of claims 1 to 3, characterized in that one returns (by 32) the denitrified fumes in the drying chamber (4), one extracts (by 34) these denitrified fumes out of the oven, then these denitrified fumes extracted from the oven are mixed (at 36) with said marginal fraction.

5. Method according to claim 4, characterized in that the mixture of denitrified fumes and of said fraction marginal is directed (by 38) to an additional dust collector (40).

6. Method according to one of claims 1 to 3, characterized in that the denitrified fumes and the marginal fraction are mixed (in 37), then the mixture of denitrified fumes and of said fraction is directed (by 39) marginal towards the drying chamber (4) of the oven (2).

7. Installation for purifying cement fumes, comprising an oven (2) belonging to a cement manufacturing installation, this oven comprising a drying chamber (4) and a calcination chamber (6) in which the fumes to be purified circulate, in service, characterized in that it • includes:

- Extraction means (18) of the fumes to be purified from the oven, these extraction means extending from an intermediate zone (20) of said oven having, in service, a temperature between 250 and 400 ° C, preferably between 280 and 350 ° C;

- a dedusting unit (24), placed in communication with the extraction means (18);

- first evacuation means (26), making it possible to extract the dedusted fumes from the dedusting unit;

- second discharge means (30; 31), making it possible to extract from the dedusting unit (24) a marginal fraction of the fumes admitted into this unit, this marginal fraction being collected at the same time as dust;

- a denitrification reactor (28), placed in communication with the first evacuation means (26); - third means (32; 33) of evacuation, making it possible to extract the denitrified fumes from the denitrification reactor; and

- A junction zone (36; 37) between the second means (30; 31) for evacuating said marginal fraction and the third means (32; 33) for evacuating denitrified fumes.

8. Cement fume purification plant according to claim 7, characterized in that the dedusting unit (24) is of the cyclonic type, and in that it is constituted by a cyclone or a battery of cyclones.

9. Installation for purifying cement fumes according to one of claims 7 or 8, characterized in that the junction zone (36; 37) is placed in communication with an additional dust collector (40).

10. Installation for purifying cement fumes according to claim 9, characterized in that the additional dust collector (40) is a bag filter or an electrostatic precipitator.