A gas turbine comprises three main sections: a compressor, a combustion chamber and a turbine. The air required for combustion is initially led to the compressor section of the gas turbine, in which the pressure of the air is increased. After the compressor section, the combustion air is led to the combustion chamber, where it is mixed with fuel, after which the fuel-air mixture is burned. Finally, the hot combustion gases are led through the turbine section and out of the gas turbine.

In gas turbines used in industrial applications, solid particles are separated as carefully as possible from the mass of the combustion air, before the air is led to the compressor section. Usually, the particles are removed using a so-called barrier filter, in which the flow of air being purified is led through the material used as a filter, when the particles are separated from the flow of air and remain on the surface of the material. Dirty filters must be cleaned or replaced frequently, which can cause problems in the operation of the gas turbine. In addition, a dirty filter causes a quite large pressure drop in the flow of air being purified. Particularly in gas turbine applications, a pressure drop in the combustion air is detrimental. Because a gas turbine has a large air coefficient, i. e. the quantity of combustion air led to a gas turbine is considerably greater than the theoretical quantity of air required for the combustion of the fuel, so that even a small pressure drop will significantly reduce the effective power of the gas turbine. If the air to be purified is moist, under suitable conditions water may condense on the surface of the barrier filter and detach the dirt accumulated on the filter. In that case, devices, such as the gas turbine's compressor section, located downstream from the filter will be dirtied. Condensation of moisture is probable, if the temperature of the air being filtered is above 7°C. If the temperature of the air being filtered is about-1°C... 7°C, the moisture in the air may freeze on the surface of the filter and on surfaces downstream from the filter, due to the reduction in the static air pressure caused by the barrier filter. To prevent freezing, air heated in the gas turbine's compressor section can be recirculated in

front of the filter, though this will reduce the efficiency of the compressor section. In addition, the power of the gas turbine will drop as the temperature of the air led to the compressor rises. Though the problems described above are caused mainly by purifying particles from the combustion air led to a gas turbine, barrier filters cause similar problems in many other applications.

Numerous different solutions have been developed to separate particles from the mass of a flow of gas, including the so-called ion-blast technique, in which, in one embodiment the flow of gas to be purified is led to a duct with a annular cross- section. The flow develops strong vortices as it moves down the duct, while the gas is simultaneously ionized with the aid of electrodes located in the duct. The mechanical and electrical forces acting on the particles in the mass of the flow of gas being purified and the centrifugal forces within the flow cause the particles to move to the surface of the duct wall. One weakness in the above technique is the large pressure drop caused by the vortices in the gas being purified.

The present invention is intended to eliminate the defects in the above cleaning techniques and to create a new kind of arrangement and method for applying the ion- blast technique to purify a flow of gas.

The invention is based on leading the flow of gas to be purified to one or several flow ducts, inside each of which there is an electrode running parallel to the flow duct, with ion-producing tips attached to the electrode. The electrode is a closed piece with a volume, in which case the gas to be purified does not flow through it and the flow of gas in the duct has a ring-shaped cross-sectional surface. A voltage is connected to the electrode, so that electrically-charged ion beams are induced in the ends of the ion-producing tips. The walls of the flow duct are earthed, so that the difference in potential between the electrode and the walls directs the ion beams towards the wall. The ion beams meet the gas to be purified flowing through the flow duct, when substances in the mass of the gas other than those in a gas phase, such as particles and drops of water, are moved by the mechanical and electrical forces produced by the ion beam onto the surface of the inner wall of the flow duct. The cleaning effect of an apparatus according to the invention is mainly due to the

mechanical force directed on the gas to be purified, and not so much to electrostatic filtering, as, for example, in an electrostatic precipitator.

More specifically, the arrangement according to the invention is characterized by what is stated in the characterizing part of claim 1.

Furthermore, the method according to the invention is characterized by what is stated in the characterizing part of claim 1.

The invention provides essential benefits.

The aim is to create a flow, in the flow duct of an apparatus according to the invention, of the gas to be purified that is as steady and undisturbed as possible, so that the pressure drop in the flow will be substantially less than in other purifying apparatuses with a corresponding efficiency. For example, when filtering air led to a gas turbine's compressor, the pressure drop in the flow caused by an apparatus according to the invention may be only one-tenth of the pressure drop caused by a barrier filter, which increases the efficiency of the gas turbine and reduces freezing of moisture in the air caused by the static pressure drop. In addition, a apparatus according to the invention will not only remove particles, but also drops of water from the gas being filtered, which also reduces freezing of the gas being purified. In gas turbine applications, the invention can be used to reduce the recirculation of warm air, thus reducing the temperature of the air led to the compressor section and increasing the efficiency of the gas turbine. The pressure drop in an apparatus according to the invention remains practically constant and does not vary with dirtiness as in a barrier filter. The drops of water removed from the gas flow flush away the particles collected on the walls of the flow duct, which considerably reduces the need to clean the surfaces and eliminates the additional shutdowns needed for the replacement of barrier filters. An apparatus according to the invention is simple to construct and economical to implement, and can be used in many other applications besides cleaning the air led to the compressor section of a gas turbine.

The invention is next explained in detail with references to the enclosed drawings.

Figure 1 shows one arrangement according to the invention.

Figure 2 shows one example of a cross-section of the flow ducts in the arrangement according to the invention.

Figure 3 shows another example of a cross-section of the flow ducts in the arrangement according to the invention.

Figure 4 shows third example of a cross-section of the flow ducts in the arrangement according to the invention.

Figure 5 shows the flow damping plate located in the lower part of the flow duct.

In Figure 1, the flow of gas to be cleaned is led into the lower part of vertical flow ducts 1. Each flow duct 1 has an electrode 2, with ion-producing tips 3 attached, set longitudinally in it. The ion-producing tips 3 can be made of e. g. metal wire. An electrical current is connected to electrode 2, whereupon electrically-charged ion beams are produced at the ends of ion-producing tips 3. The shape of the ion beams is dependent on, among other things, the shape of the ends of the ion-producing tips 3. The voltage of the electrical current connected to electrode 2 is typically 100-250 kV. The wall of the flow duct 1 is earthed, so that the difference in potential between the ion-producing tips 3 and the wall directs the ion beams towards the wall. The gas being cleaned flows upwards in flow ducts 1, so that the gas meets the ion beam produced by the ion-producing tips 3, the mechanical and electrical forces of which cause substances other than those in a gas phase, such as solid particles and drops of water, to separate from the flow and to move to the walls of the flow ducts 1.

Simultaneously, the drops of water separated from the flow of gas flush the particles collected on the walls into, for example, a drain set under the flow ducts 1. Finally, the purified gas is led out of the upper ends of the flow ducts 1.

To minimize the flow resistance and pressure drops in the flow of gas being purified, the diameter of flow ducts 1 is large relative to the amount of gas used, the flow of

gas in flow ducts 1 being made as steady and undisturbed as possible. The velocity of the flow arising in duct 1 is greatest in the centre of flow duct 1 and least close to the inner walls. Therefore, air must be prevented from flowing through the interior of electrode 2 in the centre of duct 1, as otherwise a considerable part of the gas to be purified would flow through electrode 2. The shape of electrode 2 is such that ion beams cannot be formed in its centre, and a cleaning effect cannot be created. The cross-sectional surface of the flow of gas being purified in flow duct 1 thus has a ring-shaped shape, so that the gas flows close to the inner walls of flow duct 1, but not in the centre of duct 1 at electrode 2.

Electrode 2 may be, for example, a tube closed at least at one end in relation to the volume of flow duct 1 or a solid bar, through which the gas cannot flow. The shape of flow duct 1 affects the location of electrode 2 inside flow duct 1. For example, in a flow duct 1 with a annular cross-section, it is preferable to locate electrode 2 in the centre of flow duct 1, so that the distance between electrode 2 and the wall of flow duct 1 is equal in all directions. Figures 2,3, and 4 show the preferred cross-sectional shapes of flow ducts 1. To minimize the pressure drop in the gas, the flow of the gas being purified is made as steady as possible and is distributed uniformly between the various flow ducts 1. Flow damping plates 4, which can be moved vertically, are located in the lower parts of the flow ducts 1, allowing control of the flow rates between the different flow ducts 1. Figure 5 shows an example of the construction of a flow damping plate 4.

In Figure 2, the earthed flow ducts 5 have a annular cross-section, making it preferable to locate electrode 2 and the attached ion-producing tips 3 in the centre of flow duct 5. This distributes ion beams produced by ion-producing tips 3 as evenly as possible over the entire cross-section of flow duct 1. The gas to be cleaned is prevented from rising between flow ducts 1 by, for example, plates set between flow ducts 1.

In Figure 3, the earthed flow ducts 6 are shaped as regular hexagons, so that the ion beams from ion-producing tips 3 of electrode 2 in the centre of flow duct 6 are also distributed evenly across the cross-section of flow duct 6. If flow duct 6 has a cross-

section in the form of a regular hexagon, the ion beams will be distributed considerably more evenly than they would be, for instance, in a square-shaped duct.

A apparatus according to the invention can also be assembled according to the example in Figure 4, by means of concentrically arranged, annular flow ducts 7,8,9, ducts 7,9 being earthed. Electrode 2 is located inside flow duct 7 and has ion- producing tips 3 attached to it. Flow duct 7 is surrounded by flow duct 8, which has ion-producing tips 3 attached to its external and internal surfaces. An electrical current is connected to the wall of flow duct 8, causing it to act as an electrode. The ion beams of ion-producing tips 3 attached to flow duct 8 are aimed at the surfaces of the earthed flow ducts 7,9. Additional flow ducts to those shown in Figure 4 can be similarly set inside one another.

In Figure 5, a flow damping plate 4 is located in the lower section of flow duct 1.

The quantity of gas flowing through flow duct 1 can be controlled by moving flow damping plate 4 in relation to plate-stop 10 attached to flow duct 1.

A solution according to the invention can also be used in other applications, in which other substances than those in a gas phase, such as particles and drops of water, are separated from the flow of gas. However, the invention is particularly useful in cleaning air led to the compressor section of a gas turbine. The number of the flow ducts 1,5,6,7,8,9, and their diameter and length can be varied in individual applications, for example, according to the flow conditions. For example, in an application where the gas turbine has a fuel power of about 120 MW and the flow of combustion air is about 130 m3/s, there can be 19 flow ducts with a annular cross- section, an internal diameter of 1.7 m and a height of 8 m. Each flow duct contains an electrode, with an external diameter of 0.5 m. The ion-producing tips attached to the electrode are about 5 cm long.

In the embodiment according to the example, the electrode's external diameter is typically at least 38 cm, the ratio of the internal diameter of flow duct 1,5,6,7 to the external diameter of electrode 2 being typically 2-5. Flow duct 1,5,6,7,8,9 can have a cross-section of any shape, besides the aforementioned circle and regular

hexagon. Though electrode 2 can also vary in cross-sectional shape, it is essential that the flow of gas being purified in flow duct 1,5,6,7 has an ring-shaped cross- sectional surface. Here, the ring-shaped means that the gas being cleaned cannot flow through electrode 2, the flow being divided instead in the ring-shaped area between the external surface of electrode 2 and the inner wall of flow duct 1,5,6,7.

The outer circumference of the ring-shaped cross-sectional surface of the flow is thus limited by an optional closed curve and the inner circumference by another optional closed curve. The ring-shaped nature of the cross-sectional surface of the flow of the gas being purified is not, as such, affected by the cross-sectional shape of flow duct 1,5,6,7, or electrode 2.

Flow ducts 1,5,6,7,8,9 need not necessarily be vertical while flow ducts 1,5,6,7, 8,9 and electrode 2 can also be curved, for example, to save space. The flow of gas to be cleaned can enter flow duct 1,5,6,7,8,9 from either end. To reduce pressure drops, it is preferable for the flow of gas to be cleaned to enter one end of flow duct 1,5,6,7,8,9 and to exit from the other, essentially parallel to the longitudinal axis of flow duct 1,5,6,7,8,9. A flow damping plate 4 and plate-stop 10 can be located at either end, or both ends, of flow duct 1,5,6,7,8,9. The number of ion-producing tips 3 attached to electrode 2 must be selected to suit the application, as it is affected by such factors as the dimensions of flow duct 1,5,6,7,8,9, the voltage of electrode 2, and the shape of the ion beams of ion-producing tips 3. Ion-producing tips 3 are preferably attached to electrode 2 in such a way that the ion beams of the ion- producing tips 3 do not overlap, as this will reduce the swirling of the gas being cleaned and pressure drops. If necessary, more than one electrode 2 can be placed in flow duct 1,5,6,7. In addition, instead of earthing flow ducts 1,5,6,7,9, a voltage different to that of electrode 2 can be connected to them. A different voltage to the 100-250 kV referred to can be used in electrode 2, if required, for example, by the dimensions of flow duct 7,8,9.