WO2004094065A1

WO2004094065A1 - A method for intensifying electrical particle filtering

Info

Publication number: WO2004094065A1
Application number: PCT/FI2004/050043
Authority: WO
Inventors: Kauko Janka; Jorma Keskinen; Ari Laitinen
Original assignee: Kauko Janka; Jorma Keskinen; Ari Laitinen
Priority date: 2003-04-22
Filing date: 2004-04-21
Publication date: 2004-11-04
Also published as: FI20030603A0; FI20030603A; FI119468B

Abstract

A method for charging flue gases in a combustion plant, which comprises at least a furnace (1) for burning fuel, a separating device (3) for cleaning flue gases, a flue gas channel (2), which is arranged to direct the flue gases from the furnace (1) to the separating device (3), a combustion chamber, which comprises at least a furnace (1) and a space of the flue gas channel (2), where the combustion process and the formation of particles substantially takes place, as well as one or more supply means (4, 5, 6, 7, 16) in connection with the combustion chamber (1) for supplying material to the combustion chamber. Electrically charged substance (8) is arranged to the combustion chamber (1) by means of at least one supply means (4, 5, 6, 7, 16). In addition, the invention relates to a combustion plant using the method.

Description

A METHOD FOR INTENSIFYING ELECTRICAL PARTICLE FILTERING

The invention relates to a method for charging flue gases and for intensifying a particle filtering effect in a combustion plant, which comprises at least a furnace for burning fuel, a separating device for cleaning flue gases, a flue gas channel, which is arranged to direct the flue gases from the furnace to the separating device, a combustion chamber, which comprises at least a furnace and a space of the flue gas channel, where the combustion process and the formation of particles substantially takes place, as well as one or more supply means in connection with the combustion chamber for supplying a substance to the combustion chamber. In addition, the invention relates to a combustion plant for charging flue gases and for intensifying the particle filtering effect, which comprises at least a furnace for burning a fuel, a separating device for cleaning flue gases, a flue gas channel, which is arranged to direct the flue gases from the furnace to the separating device, a combustion chamber, which comprises at least a furnace and a space of the flue gas channel, where the combustion process and the formation of particles substantially takes place, as well as one or more supply means in connection with the combustion chamber for supplying material to the combustion chamber.

Combustion plants, such as, for example, in industrial combustion processes, power plants, refuse incineration, or mixed combustion, create particles in connection with combustion. The particles in question are often harmful for the environment and health, which is why the particles are aimed to be removed from flue gases. Particle filtering takes place typically by using fibre filters or electric filters. Of these, the electric filtering is the most commonly used method in power plant applications. With combustion processes, especially in refuse and mixed combustion, the problem is the filtration of fine particles. The fine particles are the most harmful ones for health, and in addition, harmful substances, such as heavy metals, may accumulate in them. By means of conventional electric filters, a good enough separation efficiency for fine particles in refuse and mixed combustion is generally not reached in an economic manner.

In conventional electric filters, the filtration of particles takes place by utilizing the effect of a strong electric field on the movement of charged particles. The particles are charged electrically, typically by means of corona discharge. The charged particles are collected on collection plates by means of an electric field. Typically, the charging and collecting of particles is combined to an event in the same device, which is mainly placed in the flow direction of the gases, after the flue gas channel and the heat exchanger structures, in an area where the temperature of the flue gases has decreased low enough from the point of view of the charging process.

The charging of fine particles (whose size is typically below approximately 1 μm) is mainly based on diffusion charging. In diffusion charging, the charging efficiency of the particles depends primarily on the number of the ions transferring the charging, the delay time, as well as the temperature. A long delay time on an area of a large charge density is required for the efficient charging of fine particles.

The efficient charging of large particles (approximately 1 to 100 μm) requires a strong electric field. In conventional large particle electric filters, the aim is as strong as possible field intensity and as small as possible charge density, inter alia by using different pulse techniques, because a strong electric field combined with a great charge density increases the energy consumption and device requirements of the system greatly. Thus, with conventional electric filters there is a conflict between the requirements set by large particles and small particles. The long delay time required for charging small particles on an area of great charge density is a problem, which in conventional electric filters leads into an increase in the size and energy consumption of the filter, which significantly increases the investment and usage costs. Different kinds of solutions have been aimed to be developed in order to increase the filtering efficiency of different size particles, and one manner is to pre-charge the particles before the filtration. From patent publication US 4,778,493 is known a method for pre- charging particles for increasing the filtering efficiency of electric filtering. In the method in question, the particles coming from the combustion process are charged before the filter by utilizing diffusion charging, field charging and/or electron charging. In the pre-charger solutions is used a charger part placed before the electric filter, such as, for example, a corona charger part, by means of which it is possible to implement the great ion density required for charging fine particles. Voltages can still be kept low, because charging large particles and collecting particles takes place in a conventional electric filter after the pre-charger. Reaching an adequate delay time for fine particles, however, requires a large-sized pre-charger.

The main purpose of the present invention is to provide a method, by means of which particles can be charged without a large-size pre- charger.

In order to attain this purpose, the method according to the invention is primarily characterized in that electrically charged substance is provided to the combustion chamber by at least one supply means in order to charge the particles in the combustion chamber. The combustion plant according to the invention is, in turn, primarily characterized in that in connection with the supply means there is a charger part for charging the substance supplied to the combustion chamber, and said supply means is arranged to supply said charged substance to the combustion chamber in order to charge the particles in the combustion chamber.

The other, dependent claims present some preferred embodiments of the invention.

The basic idea of the invention is to supply electrically charger substance to the combustion chamber of a boiler, i.e. the furnace and/or the flue gas channel. Preferably this takes place by charging the gas otherwise supplied to the combustion chamber, which takes part in the particle formation process and/or their detachment. The pre- charging of particles is performed in the combustion chamber advantageously already in the creation phase of the flue gas particles. The formation of fine particles is typically a result of the oxidation of the basic materials of the evaporated particles, which takes place during the combustion process. When charge carriers are brought to the combustion chamber during the combustion process, the charging process of the created fine particles can be started directly at the creation of the particles. Since the free charge carriers are carried along with the flue gas flow, the charging process of particles continues advantageously even after the furnace in the heat exchanger area of the boiler and in the flue gas channels. The long delay time required by the efficient pre-charging of the fine particles is thus reached in the present invention by using the capacity of the furnace of the boiler and the flue gas channels as a charging area.

By means of the method according to the invention, it is possible to intensify the charging of fine particles and their filtration in conventional filters, such as electric filters, cloth filters and wet scrubbers. The efficient charging of the particles already in the formation phase of the particles also achieves filtration of the particles forming via a space charge phenomenon already in the combustion chamber itself. The space charging filtration is based on the repulsion force of the particles charged of the same sign. The cloud of particles charged of the same sign aims to expand, in which case a part of the particles drifts with great probability to the walls of the combustion chamber. The particles adhered on the walls loosen from the walls as larger sized pieces. The smaller pieces of these drift along with flue gas to the filter, where they are filtered efficiently because of their large size. The largest pieces loosening from the walls, i.e. dust cakes, fall to the lower part of the boiler, from where they are removed as bottom ash.

In an embodiment of the present invention, the charging is brought to the combustion chamber via combustion air nozzles. Chargers are arranged to said combustion air nozzles, such as, for example, corona chargers, by means of which the combustion air is charged. Producing a charge to a hot space by conventional means is very difficult, because the desired electric discharge is difficult to create in hot gas. In addition, material problems in a corrosive and high-temperature environment are difficult to solve. When placing chargers in the combustion air nozzles according to the embodiment in question, these problems do not occur. In a combustion air nozzle the flow rate of the combustion air is great and the rate can be further increased by means of suitable design of the nozzle, to, for example, the value of approximately 100 m/s. A great flow rate of air at the corona point is very advantageous from the point of view of charging, because thus inter alia the created ions drift away quickly from the corona. This expulsion of the space charging caused by ions decreases the electric field attenuating the discharge forming around the corona electrode, and thus further the required corona voltage. The diminished voltage requirement in turn diminishes the electric power required for maintaining the discharge.

From the point of view of the operation of the corona charger, the temperatures of the combustion air, which are typically below 200 °C are trouble-free, while in temperatures above 600 °C charging is more difficult. In addition, when the charger is located in clean air according to an embodiment of the invention, fouling of the charger is avoided, in which case the maintenance need for the charger is small.

In another embodiment according to the present invention, the charge is brought to the combustion chamber via nozzles, through which some other substance besides combustion air is supplied to the combustion chamber. The substance supplied with the nozzle can be, for example, fuel or some other substance taking part in the formation of particles. It is also possible to supply the charge along with such a substance that does not take part in the formation of the particles, but by means of which the remaining combustion process and/or the emissions created in it are affected. This kind of substances are, for example, different auxiliary chemicals, such as ammonia. The substance to be charged is advantageously gaseous, but it is also possible to charge and supply liquid or particle substances. The method and equipment according to the invention is better in its efficiency, as well as remarkably more economic in its investment and usage costs, than the known pre-charger solutions. Savings are formed in the investment costs inter alia in that the already existing furnace and flue gas channels are used as charging space. The usage costs are decreased inter alia because according to an advantageous embodiment of the invention the entire corona flow is directed to the charge space, as well as because it is possible to use a lower corona voltage.

In the following, the invention will be described in more detail with reference to the appended principle drawings, in which

Fig. 1 shows an embodiment of the invention,

Fig. 2 shows an embodiment of the charger,

Fig. 3 shows another embodiment of the invention, and

Fig. 4a shows an embodiment of the nozzle in a cross-section,

Fig. 4b shows the nozzle of Fig. 4a in front view,

Fig. 5a shows an embodiment of the nozzle in a cross-section,

Fig. 5b shows the nozzle of Fig. 5a in front view, and

Fig. 6 shows a third embodiment of the invention,

For the sake of clarity, the figures only show the details necessary for understanding the invention. The structures and details which are not necessary for understanding the invention and which are obvious for anyone skilled in the art have been omitted from the figures in order to emphasize the characteristics of the invention. In this kind of figures the invisible structures include, inter alia the superheaters in the boiler and flow controllers.

Figure 1 shows a boiler arrangement of a power plant, which comprises an embodiment of a pre-charging arrangement according to the invention. The boiler arrangement in question comprises at least a furnace 1 and a flue gas channel 2 in connection with it, as well as a filtering equipment 3. Fuel supply means 4 are arranged on the walls of the furnace 1 , such as, for example, nozzles and air supply means 5, 6, 7. The figure shows three separate air supply levels, of which the primary air level 7 is located below the fuel supply means 4, and the secondary air level 6 and the tertiary air level 5 are located above the fuel supply means. The supply of substances taking place from different levels can be carried out from one or more walls of the furnace 1 by maintaining the basic idea of the present invention. For simplicity, the figure only presents the substance supply taking place on one and the same wall of the furnace 1.

The substance supply equipped with the charger 9 according to the invention is placed in the example in question on the tertiary air level 5.

The substance supply implemented in the manner in question can also be located on other air levels, such as the primary, secondary and/or quartiary level. On the level in question, at least a part of the supplied combustion air is charged electrically, in which case a charged substance flow 8 forms from the nozzle to the furnace. The charge of the substance flow 8 may be either positive, such as in the example, or negative. In an advantageous embodiment the charging of the substance flow 8 is performed by means of the charger 9, which is a corona charger integral with the nozzle. An advantageous embodiment of the corona charger is shown more in detail in Fig. 2.

Figure 2 shows an advantageous embodiment of the corona charger. The charger comprises at least a corona electrode 10. The counter- electrode 11 of the corona electrode 10 is placed on the walls of the nozzle to circulate the corona electrode. Preferably the corona electrode 10 is on the symmetry axis 12 of the nozzle. In an advantageous embodiment the flow of the nozzle can be controlled in such a manner that the mouth of the nozzle can be reduced. The reduction is preferably carried out in such a manner that when the flow of the nozzle is constricted, the corona electrode 10 remains on the symmetry axis 12 of the nozzle and the flow.

Also, the advantageous flow constricting form of the nozzle can be seen in Fig. 2, by means of which design the flow rate rises momentarily at the corona electrode 10. With this high rate, an even more efficient expulsion of the space charging caused by ions can be reached, which in turn decreases the discharge attenuating electric field, which forms around the corona electrode 10, and thus further the required corona voltage. The diminished voltage requirement in turn diminishes the electric power required for maintaining the discharge.

In an advantageous embodiment, the charge efficiency of the corona charger is automatically controlled according to the load of the combustion plant without specific control means, since the charge efficiency is proportional to the supplied combustion air. The combustion air, in turn, is proportional to the efficiency and the particle formation. When the combustion air flow decreases, also the corona flow decreases by the effect of the space charging cloud forming around the corona electrode.

In such embodiments, where self-control is not optimal in view of the desired result, the charge of the substance can be controlled in other ways as well. One way is to control the corona flow with the control system of the combustion plant, in which case filtering efficiency can be implemented to, for example, depend on the loading of the plant. It is possible to use other parameters as well for controlling the charge, such as, for example, when aiming for a great filtering efficiency, the control can be performed on the basis of the breakdown density taking place in the furnace of the boiler. The breakdowns can be detected by using, for example, a detector monitoring rapid changes in the electric field and placed in an aperture in the furnace (1) wall. However, the invention does not depend on the charging manner of the substance flow 8, but an electric charge can be produced to the substance flow in several different ways and at several different points. In an embodiment, the material flow 8 is charged in the furnace 1 , in which case the charging means 9 are located in the furnace and thus the substance flow supplied to the furnace advantageously functions as a cooling agent for the charging means.

A suitable charger construction 9 and the location of the charger depend on several things, such as, inter alia the type of the boiler and the fuel burned in it. Figure 3 shows another advantageous embodiment of the invention. In the boiler in question, a gaseous fuel is supplied with a fuel nozzle 4 equipped with a charger 9. The fuel flow 8 in question is electrically charged according to the invention, in which case the particles forming when the fuel burns are charged electrically.

The nozzle used for supplying fuel can be implemented in several different ways, and it is possible to bring, for example, the combustion air required in the combustion process via the same nozzle structure along its own channel. In Fig. 3 the combustion air is brought to the combustion chamber via primary air nozzles 7.

The pairs of figures 4a and 4b, and 5a and 5b show some embodiments of the nozzle, where the fuel and the combustion air are supplied from the same nozzle structure. An inner channel 13 is formed in the nozzle for the first substance, and an outer channel 14 circulating the inner channel is formed for the second substance. In the first channel 13, as well as in the second channel 14, it is possible to application-specifically supply either fuel or combustion air or some other substance. There is also a possibility to form several channels in the nozzle, in which case it is possible to supply several substances from the nozzle as well.

In the pairs of figures 4a and 4b there is a charging means arranged in the inner channel 13 of the nozzle, which in the example comprises a corona electrode 10 and its counter electrode 11. Thus, the substance supplied via the inner channel 13 is charged electrically and the charge transfers with the substance flow to the furnace 1 and further to the forming particles.

In the pair of figures 5a and 5b there is in turn a charging means arranged in the outer channel 14 of the nozzle, which in the example comprises several corona electrodes 10 and their counter electrodes 11. In the embodiment in question, several mouths 15 open to the furnace from the outer channel 14, because this has an advantageous effect on the uniform charging of the substance flow. The substance supplied via the outer channel 14 of the nozzle according to the embodiment is thus charged electrically and the charge transfers with the substance flow to the furnace and further to the forming particles.

Fig. 6 in turn presents one embodiment of the invention, where the charged substance flow 8 is brought to the combustion chamber formed by the furnace 1 and the flue gas channel 2 in the end of the actual combustion process or after that via a suitable charger 9 and nozzle 16. This is advantageous when an auxiliary chemical in an gaseous form is added to the combustion gases, for example a gas comprising ammonia. Thus, the charging transfers from the charged substance flow 8 to the particles of the combustion gases in the combustion chamber and especially in the flue gas channel 2.

The charged substance flow according to the invention can also be used in connection with boiler soot blowers. During the soot blowing of the heat exchange surfaces of the boiler, particle emission peaks typically occur, which are caused by, inter alia, the additive particle load of the ash and soot removed by the soot blowers. By charging electrically the substance supplied from the nozzles of the soot blower, which typically is liquid or gas, the released particles can be filtered more effectively. From the charged substance flow supplied by the soot blower, the charge transfers to the particles. The soot blowers in question are not shown in the figures, but typically the soot blowers are located in connection with superheaters and/or heat exchangers. The invention also enables providing a current to the surfaces of the boiler, and especially to its heat exchange surfaces. For example, when the polarity of the charge is positive, the charged particles drifted onto the heat exchange surface create a positive electric current. This current in some cases prevents the corrosion of metal, i.e. the method according to the invention in some situations creates an electric corrosion prevention.

This invention does not depend on flow fields formed with different kinds of nozzle arrangements, but the substance charge according to the invention can be used with several kinds of nozzle arrangements. With the flow fields formed with the nozzle arrangements, the aim is mainly to affect the mixing of the substance combusted in the furnace, in which case also the mixing of the substances charged according to the invention with other substances is easier with some nozzle arrangements than others.

By combining, in various ways, the modes and structures disclosed in connection with the different embodiments of the invention presented above, it is possible to produce various embodiments of the invention in accordance with the spirit of the invention. Therefore, the above- presented examples must not be interpreted as restrictive to the invention, but the embodiments of the invention can be freely varied within the scope of the inventive features presented in the claims hereinbelow.

Claims

Claims:

1. A method for charging flue gases and for intensifying a particle filtering effect in a combustion plant, which comprises at least - a furnace (1) for burning fuel, a separating device (3) for cleaning flue gases, a flue gas channel (2), which is arranged to direct flue gases from the furnace (1) to the separating device (3), a combustion chamber, which comprises at least the furnace (1) and a space of the flue gas channel (2), where the combustion process and the formation of the particles substantially takes place, and one or more supply means (4, 5, 6, 7, 16) in connection with the combustion chamber (1) for supplying a substance to the combustion chamber, characterized in that in the combustion chamber (1) is arranged an electrically charged substance (8) at least with one supply means (4, 5, 6, 7, 16) for charging the flue gas particles in the combustion chamber

(1 ).

2. The method according to claim 1 , characterized in that the electrically charged substance (8) is one of the following: fuel, combustion air, - oxidizing agent, auxiliary chemical, soot blowing steam, soot blowing air, medium carrying charges.

3. The method according to claim 1 or 2, characterized in that the electrically charged substance (8), which is combustion air or fuel, is arranged substantially at that point of the furnace (1) where the particle formation takes place.

4. The method according to any of the preceding claims 1 to 3, characterized in that the substance (8) to be charged is charged with the charger (9) in connection with the supply means (4, 5, 6, 7, 16), which charger is located: - outside the combustion chamber or in the combustion chamber in such a manner that the flow of the substance (8) to be charged cools the charger (9).

5. The method according to any of the preceding claims 1 to 4, characterized in that the substance (8) to be charged is air and the separating device (3) is an electric filter.

6. A combustion plant for charging flue gases and for intensifying a particle filtering effect, which comprises at least - a furnace (1 ) for burning fuel, a separating device (3) for cleaning flue gases, a flue gas channel (2), which is arranged to direct flue gases from the furnace (1) to the separating device (3), a combustion chamber, which comprises at least the furnace (1) and a space of the flue gas channel (2), where the combustion process and the formation of the particles substantially takes place, and one or more supply means (4, 5, 6, 7, 16) in connection with the combustion chamber (1) for supplying a substance to the combustion chamber, characterized in that in connection with the supply means (4, 5, 6, 7, 15) there is a charger part (9) for charging the substance supplied to the combustion chamber (1), and said supply means is arranged to supply said charged substance to the combustion chamber (1) to charge the flue gas particles in the combustion chamber (1 ).

7. The combustion plant according to claim 6, characterized in that the separating device (3) comprises at least one of the following: an electric filter, a wet scrubber, a textile filter.

8. The combustion plant according to claim 6, characterized in that combustion plant comprises at least one of the following supply means (4, 5, 6, 7, 16) for arranging the charged substance (8) to the combustion chamber: - a nozzle (5, 6) connected to the charger part (9) of the upper air level, a nozzle (7) connected to the charger part (9) of the lower air level, a fuel nozzle (4) connected to the charger part (9), - an auxiliary chemical nozzle (16) connected to the charger part (9).

9. The combustion plant according to claim 6 or 8, characterized in that the charger part (9) is a corona charger.

10. The combustion plant according to any of the preceding claims 6 to 9, characterized in that the separating device (3) is an electric filter and the charged substance (8) is air.