ITMI20120408A1 - PROCEDURE FOR COMBINED AND SIMULTANEOUS FILLING FROM COMBUSTION FUMES OF VOLATILE ORGANIC COMPOUNDS, OXIDES OF NITROGEN AND PARTICULATE OF COMBUSTION, AND PLANT TO REALIZE THIS PROCEDURE - Google Patents

Description

PROCEDURE FOR COMBINED AND SIMULTANEOUS ABATEMENT OF COMBUSTION FUMES OF VOLATILE ORGANIC COMPOUNDS, NITROGEN OXIDES AND COMBUSTION PARTICULATE, AND PLANT TO CARRY OUT THIS PROCEDURE

The present invention relates to a process for the combined and simultaneous abatement of volatile organic compounds, nitrogen oxides and combustion particulates from the combustion fumes.

The invention also extends to the plant for carrying out this process.

Most of the fume treatment systems of coal-fired thermoelectric power plants perform denitrification by means of SCR-DeNOx processes, while de-pulverization by installing electrostatic precipitators or bag filters. This last technology, even if only recently included among the Best Available Technologies (BAT), is finding an ever wider application thanks to its greater efficiency in the abatement of the finest fly ash.

The post-combustion method that has become widely established for the elimination of nitrogen oxides and volatile organic compounds (VOCs), in particular dioxins, from combustion gases still containing oxygen, is the selective catalytic reduction (SCR) with ammonia. . The SCR system uses a catalyst and a reducing agent (ammonia) to dissociate NOxin nitrogen and water vapor.

With SCR systems, abatement efficiencies of over 90% can be obtained with a concentration of unreacted (slip) ammonia in the stack below 5 ppm (v / v). Conventional SCR systems use catalysts, which generally operate in a temperature window between 320 and 380 ° C and with a pressure drop between 7 and 10 mbar relative to a flue gas flow rate typically of the order of 2 million m < 3> / h at standard conditions of temperature and pressure (STP) of 25 ° C and 1 atm. The fumes deriving from the combustion of coal contain ashes in an amount ranging from a few tens of milligrams to a few grams per m <3>. It is possible to prevent the accumulation of combustion particles and the occlusion of the catalytic bed through the use of parallel flow type catalysts. There are two types of parallel flow catalysts:

- plate-type monoliths, in which catalyst plates with various shapes are assembled into elements having dimensions 500x500x500 mm;

- honeycomb monoliths obtained by depositing a thin layer of catalytic material on a ceramic structure or by extrusion of the catalytic material to obtain an element with dimensions of approximately 150x150x600 mm with square section channels.

On the market there are catalysts with a separation gap between the plates or with a channel width varying between 4 and 10 mm. The variation of this dimension determines a difference in the specific geometric area of the catalyst. A higher plate separation or channel size is employed in the treatment of coal combustion fumes which have a higher ash concentration.

Different locations of the DeNOx reactor are possible with respect to the other elements that make up the complex of a thermoelectric power plant.

The DeNOx catalytic reactor, in the "high dust" configuration of the plant, is placed between the coal combustion boiler and the combustion particulate separation precipitatone carried by the fumes coming from the boiler. This known system has the advantage that the fumes arriving at DeNOx are already at the temperature necessary for denitrification. However, it has the disadvantage of feeding the DeNOxcon fumes having their highest combustion particulate load, which causes the degradation of the denitrification catalyst, both from the mechanical point of view (due to erosion and the like), and from the chemical point of view (for catalyst poisoning).

Other known solutions provide for the displacement of DeNOx downstream of the precipitatone of combustion ash ("low dust" systems), or even after the final desulphurizer ("tail end" systems) which, compared to the "high dust" system, however suffer from The first drawback of requiring the use of an electrostatic precipitatone capable of operating hot and, the second, of requiring additional heating of the fumes entering the DeNOx reactor. Installation in the "tail end" configuration is by far the most advantageous from the point of view of the average life of the catalyst as:

in the desulphurizer the main sources of deactivation and formation of ammonium sulphates and bisulphates are eliminated: HC1, HF, SO2, SO3; - allows the DeNOx reactor to be operated in the absence of fly ash (further source of poisoning) which are retained by the electrofilters;

- for the same conversion of N0X, a smaller quantity of catalyst is required compared to the "high dust" system (since the combustion particulate is absent, there are no occlusion phenomena of the channels of the monoliths and it is possible to reduce their passage section, consequently increasing the development of the geometric contact surface).

The last two characteristics are also common to the "low dust" arrangement, but in this case there is the disadvantage of having to operate with a hot electrostatic precipitatone which, from an economic point of view, is much more expensive than a cold one.

The great disadvantage that made the "tail end" arrangement not very widespread is the considerable energy consumption required to heat the flue gases (after the desolorizer the fumes have a temperature of 50-60 ° C) at the operating temperature of the catalyst (300 -400 ° C). On the other hand, the notable diffusion of the "high dust" configuration derives from the fact that the gas leaving the economizer is already at the ideal temperature for the SCR process. The following table summarizes the advantages and disadvantages of the three configurations:

Table - Comparison between the three SCR provisions

(<*>) arbitrary units

The main purpose of the present invention is to provide a process and the relative plant for carrying out the combined and simultaneous abatement, from combustion fumes, of volatile organic compounds, nitrogen oxides and combustion particulates which, compared to traditional systems used in the sector, allow to obtain a greater compactness of the system and an easier installation in existing systems.

A further object of the invention is to provide a process of the aforesaid type which, unlike the prior art, allows it to be carried out in the described "high dust" configuration, advantageous because it does not require additional heating of the fumes to be denitrified and yet capable of have all the advantages of the "tail end" configuration, in which the catalyst never comes into contact with the combustion ashes.

These and other purposes are achieved with the method, the bag filter, the plant and the catalyst of claims 1, 3, 6 and 10 respectively. The remaining claims result from the preferred ways of carrying out the invention. In relation to the traditional systems for the abatement of dust, nitrogen oxides and VOC, the one of the invention offers the advantage of allowing a greater compactness of the system and its easier installation on the already existing ones.

A further advantage of the invention is represented by the fact that the combined smoke treatment unit is installed in the "high dust" configuration, however offering all the advantages of the "tail end" configuration, since the catalyst does not come into contact with the ashes. combustion.

Furthermore, the catalyst in the form of pellets has, compared to the one in the form of monolith, a considerably lower cost and has a much higher abatement efficiency, thus allowing a considerable reduction in the volume of the plant and in the investment costs.

The objective of the present invention is the development of a combined process for the simultaneous abatement, in a single treatment unit, of both combustion particulates and nitrogen oxides (N0X), i.e. volatile organic compounds, in particular dioxins, contained in the fumes of the thermoelectric power plants fueled by coal dust. For this purpose, a heterogeneous fixed-bed gas-solid catalyst is used consisting of granules with a shape and size suitable for ensuring low pressure drops. The catalyst is placed inside suitable containment baskets, which are in turn positioned inside the filter bags.

These and other objects, advantages and characteristics result from the following description of a preferred way of realizing the system of the invention illustrated, by way of non-limiting example, in the figures of the attached drawings. In them:

Figure 1 illustrates a diagram of a known plant for the abatement of dust and nitrogen oxides from combustion fumes;

Figure 2 illustrates a bag filter according to the invention, used for the simultaneous removal of combustion particles and nitrogen oxides;

Figure 3 illustrates the detail of a filter bag of the filter of Figure 2, equipped with a catalyst;

Figure 4 is an exploded view of the filter bag of Figure 3;

Figure 5 illustrates a diagram of the plant of the invention provided with the bag filter of the previous figures; And

Figure 6 illustrates the graph of the conversion of N0X as a function of the reaction temperature of the plant of the invention.

The system of the previous figures, described below for the combined and simultaneous abatement of nitrogen oxides and particulates from the combustion fumes, is actually equally usable to reduce volatile organic compounds (VOC), in particular the dioxins.

The system illustrated in Figure 1, of the "high dust" type, comprises a boiler 1 from which a stream 2 of combustion fumes containing particulate matter (combustion ash) and nitrogen oxides N0X comes out. This stream 2 of fumes, at a temperature of 320-380 ° C, is then sent into the SCR DeNOx3 reactor for denitrification of the fumes, by passing over a suitable bed of catalyst 12. The denitrified fumes 4 leaving the SCR reactor DeNOx3 exchange heat in the device 5 for preheating the air 6 sent to the inlet to the boiler 1. The cooled flue gas stream 7 is then sent to an electrostatic precipitatone or bag filter 8, in which the combustion ashes are separated. The stream 9 of denitrified and dedusted fumes thus obtained, after having transferred heat in the gas-gas exchanger 10, is finally sent to the desulphurization unit 11, from which a stream 13 of denitrified, dedusted and desulphurized fumes comes out.

This known system has the advantage of causing a stream 2 of fumes to reach the SCR DeNOx3 reactor which is naturally found, due to its origin from the boiler 1, already at the temperature (320-380 ° C) suitable for the catalytic reaction of denitrification. However, the same known system has the drawback of causing the chemical and physical degradation of the catalyst beds 12 placed inside the reactor 3, due to the latter coming into contact with the combustion ashes.

In order to maintain the advantages of the "high dust" configuration described above, while overcoming the drawbacks linked to the degradation of the catalyst in the SCR DeNOx reactor, the system illustrated in figure 5 has been created. 5 described above, also a unit 14 for the combined and simultaneous removal of the nitrogen oxides and combustion ashes formed in the boiler 1 from the combustion fumes 2.

Said unit 14 is in particular formed by an external metal frame 15, inside which a plurality of bag filters 16 is housed. more external, by a sleeve-shaped filtering fabric 17 and, in the internal part, by a stiffening metal grid 18. The bag filter 16 also has a closed bottom 19 and an upper opening 28, in correspondence with which a basket 21 is housed, also of the reticular type, containing in turn a denitrification catalyst 22 in granules.

This basket 21 is of the extractable type from the opening 28 of the filtering bag 16. In fact, as better shown in figure 4, the basket 21 is simply inserted inside the opening 28 of the filtering bag 16, until it rests with its collar. 20, projecting outwards, above the corresponding housing edge 27 present on the sleeve positioning plate 29 located in the upper part of the bag filter illustrated in figure 2. The holding of the basket 21 in position against the thrust towards the The top produced by the fumes 24 is instead ensured by the fixing of the basket 21 on the housing edge 27 by means of the clamps 30.

It is important to underline that the average life of the fabric sleeves 17 is much lower than that of the granule catalyst 22. During normal operation, the filtering fabric 17 is subject to breakage, so that generally it must be replaced several times before the catalyst 22 reaches the end of its life. The confinement of the catalyst in granules 22 inside the basket 21 offers the advantage that it is not dispersed in the event of the fabric sleeve 17 breaking. Furthermore, the extractability of the baskets 21 from the inside of the filtering bags 16 ensures easy replacement. of the exhausted catalyst 22 without having to remove the sleeves of fabric 17. The fumes 2 to be treated affect the side wall of the sleeve 16 formed by the filtering fabric 17, so as to obtain the separation of the ashes 23. The stream 24 of fumes, thus dedusted, passing through the filter fabric 17, then comes into contact with the catalyst 22 passing through the meshes of the basket 21. In this way, at the exit from the mouth 26 of the basket 21 inserted in the filtering bag 16, a stream 25 of dedusted and denitrified fumes is obtained, subsequently sent to the desulfurizer 11.

As catalyst 22 in granular form to be positioned in the free space available inside the filter bags, two catalysts synthesized in the laboratory were investigated by dispersing the active phase on a commercial support of T1O2 (cylindrical granules 3x3 mm, anatase crystalline phase, specific surface area of 156 m <2> / g, packing density equal to 705 kg / m <3>). The two catalysts differ from each other in the composition of the active phase, for one consisting of 5% by weight of V2O5, suitable for use with combustion fumes with a low sulfur dioxide content (SO2), while for the other 0.7% by weight of V2O5 plus 7% by weight of WO3, on the contrary suitable for a high content of SO2 in the fumes.

The catalytic efficiency tests were carried out in a laboratory reactor consisting of a ceramic fabric filter whose free space inside it was partially filled with 50 g of catalyst in granules, corresponding to a volume of about 0.0401 for both. the samples investigated. The test conditions were as follows:

- Temperature 200-380 ° C - Space Velocity (supply flow rate / filter volume) 8000-14000 h <'1> - Concentration of nitrogen oxides in the mixture 500 ppm v / v - Concentration of ammonia in the mixture 500 ppm v / v - Concentration of oxygen in the mixture 3% v / v - Molar ratio NH3 / NO in the feed 1

- Nitrogen in the mixture to taste at 100% v / v

The Space Velocity (SV) values are calculated referring to the feed rate to the reactor in Nm <3> / h and to the volume actually occupied by the catalyst in granules (40 ml).

As shown in the graph of figure 6, both catalysts proved to be appropriate for use in the proposed methodology, showing conversion efficiencies even greater than 90%.

For an SV of 8000 h- <1> the catalyst V2O5 5% weight / Ti02, in line with the high content of V2O5 which is the most active oxide in the SCR reaction, shows a conversion of NO higher than 80% in throughout the temperature range tested, reaching values of 91.6% at 300 ° C and 94% at 350 ° C. At an SV equal to 14000 h- <1> the conversion efficiency undergoes a decrease, however remaining at high values (88.5% at 300 ° C and 90.5% at 350 and 380 ° C).

The trend of the conversion of NO on the catalyst 0.7% by weight of V205 + 7% by weight of WO3 / T1O2 is very different from that of the sample based on V205 only. Compared to the latter, it shows much lower efficiency values up to a temperature of about 280 ° C, and then quickly recovers at higher temperatures, so much so as to almost equal the values of the first catalyst.

In the light of the experimental results obtained, it was estimated that 150 m of catalyst 22 in granules would be needed to reduce the NOx emitted by a group of 660 MW with an efficiency of 90% (compared to over 650 m of catalyst in the form of honeycomb monoliths. ape 12 of the conventional SCR process). Such a volume would be widely positioned inside the filter bags 16 used in current filtration systems (more than 14,000 with a length of 8 m and a diameter of 10 cm). Assuming its uniform distribution within the latter, each sleeve would be occupied by the catalyst in granules 22 only for 1/15 of the volume and for a height equal to 50 cm. It follows that the integration of the catalyst 22 inside them, contained in the respective basket 21, would not cause excessive additional pressure drops.

Claims (12)

CLAIMS 1. Process for removing volatile organic compounds (VOCs), in particular dioxins, nitrogen oxides (N0X) and combustion ash from combustion fumes, in particular fumes from coal-fired thermoelectric plants, characterized by the fact to provide for the combined and simultaneous abatement of said VOC, N0X and combustion ashes, by passing the said fumes on a denitrification catalyst bed, placed inside respective filter bags. 2. Process according to claim 1, characterized by providing for the passage of said fumes (2) through the filtering fabric (17) of a respective bag filter (16), so as to generate a stream (24) of dedusted fumes, in turn sent towards the catalyst (22) to produce a stream (25) of dedusted and denitrified fumes leaving the upper mouth (26) of said bag filter (16). Bag filter for carrying out the process according to one or more of the preceding claims, characterized in that it comprises a bag-shaped filter fabric (17), in correspondence with the upper mouth (26) of which said denitrification catalyst ( 22). 4. Filter according to claim 3, characterized by the fact that said catalyst (22) is shaped in the form of granules, the mass of which is contained inside a basket (21), in turn housed in a removable way inside the mouth (28) of the bag filter body (16). 5. Filter according to claim 4, characterized in that said catalyst (22) partially fills the internal volume of said bag filter (16). 6. Denitrification catalyst for carrying out the process according to one or more of claims 1 and 2, characterized in that it is a catalyst (22) in granular form, suitable for ensuring low pressure drops. 7. Catalyst according to claim 6, characterized in that said catalyst (22) consists of metal oxides active in the denitrification reaction, supported on granular materials with a high specific surface area and high porosity. 8. Catalyst according to claim 7, characterized in that said metal oxides active in the denitrification reaction are vanadium oxides, tungsten oxides, manganese oxides, iron oxides, copper oxides and other transition metal oxides, supported on granular titanium oxide. 9. Catalyst according to claim 8, characterized in that the composition of the active phase of said metal oxides is compatible for use with combustion fumes, both low and high in sulfur dioxide. 10. Plant for carrying out the process according to one of claims 1 and 2, characterized in that it is provided with the catalyst and the bag filter according to one or more of the preceding claims. 11. Plant according to claim 10, characterized in that it comprises a unit (14) for the combined and simultaneous abatement of combustion ashes (23), NOx and VOC, said unit (14) being provided with a plurality of said bag filters (16). 12. Plant according to claim 11, characterized in that said unit (14) is arranged upstream of a desulphurization unit (11).