A METHOD AND ARRANGEMENT FOR CLEANING INTAKE AIR TO A GAS TURBINE

Abstract: The invention relates to a method and an arrangement for cleaning the intake air to a compressor of a gas turbine employed in energy generation. Conventionally, mechanical fabric filters are used as the intake air filters of gas turbines. As known, fabric filters are capable of separating particulates of relatively large size only from the air, whereby a portion of the contaminating particles suspended in the air may enter the equipment causing dirt build-up therein. By virtue of the present invention, the intake air can be cleaned at a higher degree of efficiency using a method according to which the mechanical filtering stage of the intake air is combined with at least one electrostatic filtering stage and a complementary stage in which the electrical properties of particulates suspended in the air are improved electrically or chemically.

Description: λ method and arrangement for cleaning intake air to a gas turbine The present invention relates to a method according to the preamble of claim 1 for cleaning the intake air to gas turbine equipment employed in electric power genera¬ tion. The method also concerns an arrangement suited for imple- menting the method. The intake air to the compressor section of a gas tur¬ bine, that is, the combustion air of the turbine is cleaned with the help of mechanical fiber filters. The purpose of intake air filtration is to obstruct the entry of abrasive and contaminating particles into the com¬ pressor and the turbine, thus preventing the wear of equipment and reducing the need for cleaning and mainte¬ nance. Because large gas turbines employed in electric power generation also require extremely large amounts of combustion air, very small concentrations of contaminat¬ ing particulates result in heavy internal build-up of dirt in the equipment during running, which means that the filtration of the combustion air should be as effec- tive as possible. On the other hand, as the pressure losses caused by such filtration in the intake channel degrade the efficiency of the equipment, the design value of filtration efficiency will always be a compromise dictated by local circumstances. Today, different kinds of mechanical filters as a rule are used as the intake air filters of compressors. In these filters the separating medium is a fabric pleated so that impurities can readily adhere thereto. The fil- tration efficiency of a filter is dependent on the pro¬ perties of the filtering medium and the packaging density of the medium. For a given type of filtering medium, the filtration efficiency may be improved by making the filter fabric thicker, or alternatively, compressing the filtering medium into a tighter form. Both approaches result in a drastically higher pressure loss over the filter, and furthermore, the separation efficiency of mechanical filters may be improved only so much, and in fact, the particle size cut-off of complete filtration by conventional fabric filters has been limited to 2 - 5 urn. Particles with a sufficiently small size can therefore always pass air filters presently in use. While the use of multiple and effective filtration sys¬ tems have not been considered necassery, latest investi¬ gations on turbine equipment efficiency have revealed that also the particle fraction comprising the smallest- size contaminating particles affect the efficiency and service life of the equipment. In fact, as the fraction formed by the smallest particles is the dominating cause to the dirt build-up in gas turbine and compressor equip- ment, this fraction should be carefully separated from the intake air. While the proportion of small particles is only as low as approx. 1 % of the total contaminating solids, they accumulate over time, and with the large volumes of gas turbine intake air, may amount to a total mass of several tens of kilograms annually. Dirt build-up degrades the efficiency of the turbine and reduces its output power capacity. Such output power reduction is caused by a number of different factors including wear due to the particles, inefficiency of methods used for internal cleaning of equipment leading to gradual accu¬ mulation of dirt and extra wear due to such cleaning as well as increase of tolerances in seals resulting in higher leak rates. Even the smallest degradation of output power over the service life of the equipment causes significant economic losses. Also the efficiency of the turbine degrades with the internal dirt build-up, which necessitates cleaning of the compressor and the turbine at scheduled intervals. Cleaning is carried out by washing with water complemented with different kinds of crushed material. Such water wash in particular is hampered by freezing under low-temperature conditions. Extra operating costs associated with dirt build-up in equipment are traceable to increased fuel consumption, loss of output power capacity and compressor cleaning costs. The extent and impact of dirt build-up are naturally dependent on the operating environment and the degree of contamination in the intake air. Because, despite of washing of the equipment, all adhering dirt cannot be removed, gradual dirt build-up in equipment will remain one of factors that cause an output power degradation amounting to as much as tens of percents already in a few years. Accumulation of dirt in filters results in pressure losses due to clogged filters necessitating the replace¬ ment of filters at scheduled intervals. The need for filter replacement is the more frequent the finer the filter, and the replacement of filters invokes costs at a level dependent on filter prices and duration of shut¬ downs. Conventionally, attempts to improve the efficiency of filtration has been sought through research into better materials for fine and coarse filters rather than through combining different methods of filtration. Electrostatic methods have been considered so unreliable that the risk associated with their use as the sole filter has been found intolerably high. A shortcoming of electrostatic methods has been related to humidity and particularly short-circuiting caused thereby. It is an object of the present invention to provide a method capable of exceeding prior-art performance in the efficiency of intake air filtration for gas turbines used in power generation and achieving higher cost-efficiency as well as invoking lower pressure losses. The goal of the invention is achieved by complementing the mechanical filtration of intake air with at least one electrostatic filtering stage an a stage in which the electrical properties of particles suspended in the air are improved electrically or chemically. More specifically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1. Furthermore, the arrangement according to the invention is characterized by what is stated in the characterizing part of claim θ. The invention offers significant benefits. The most important advantages of the invention are to be found in the stabilization of operating efficiency and in slower rate of output power decrease. Electrostatic precipitation methods have not been used in the prior art for cleaning large-volume air flows, because conventional filtering would have required such a high electric power to be expended for charging the suspended particles that the cost-efficiency of filtration would have been lost. However, when combined with prefiltration and convention¬ ally also with fine-filtration, electrostatic precipita¬ tion may advantageously be used, because essentially lower charging voltages can then be used in such a com¬ bination without compromising a good degree of separa- tion. The pressure losses of filtration may be reduced and mechanical filtration can be implemented using a filter of a lower degree of separation, whereby the pressure drop over the filter is smaller and the fouling rate of the filter in use is lowered, which permits less frequent replacement of the filters. Resultingly, process shutdowns and replacement costs of filters are reduced. The filtration arrangement may also be designed so that the system comprises a partially or entirely duplicated parallel filter system, whereby maintenance of the filtration system may be performed off-line, while the power generating system is run continuously. The electro- static stage achieves separation of smaller particles from the intake air than is possible with conventional filtration methods, whereby the total mass of contaminating particles passing through the filter is reduced essentially. Such improvement in filtration effi- ciency is truly significant as if a relative value of 1000 is assigned to the total mass of particles passing a mechanical fine filter, then a relative value as good as 50 is attained by combining mechanical coarse filtration with two stages of filtration based on electrostatic methods. As the intake air consumption of a gas turbine may easily be as high as 120 rnVs, it is obvious that the intake air flow carries along a great quantity of con¬ taminating particles that must be removed. By improving the intake air filtration using the method according to the present invention, essential reduction in the dirt build-up rate is attained resulting in cost savings, whereby the extra energy of electrostatic methods are easy to accept as the costs are minimal compared to the benefits. Maintenance of the equipment will be easier and service costs will be lowered, because the filter elements of the first filtering stages that will be subjected to the heaviest stress may be replaced or washed at a low cost. The short circuit problems of electrostatic precipitators can be overcome by using two different types of electrostatic precipitation or heating of insulation in the filter units. In the following the invention will be examined in greater detail with reference to the attached drawing illustrating an embodiment of the filter unit according to the invention. In the text below the following definitions are used. An electrostatic precipitator is an apparatus comprising planar parallel plates connencted to a voltage source. One type electrostatic fiber filter is a filter comprising electrically charged fibers which have a permanent electric polarization. In an other type of electrostatic fiber filter the fibers or a porous filtering media is not permanently charged but an active electric fied is used to activate the filter. All these types of apparatuses are commonly called electric filters. Referring to the figure, the filter arrangement shown therein comprises a mechanical fiber filter 1, an elec¬ trostatic precipitator 3 having corona wires 2 in the front of the unit and followed by an electrostatic fibre filter 4. Conventionally, the filter arrangement also incorporates a framework 5 to which the filter units are attached. The farmeworm is shown herein only diagrammatically. The mechanical filter 1 comprises a fiber layer, wherein the fibers may be formed to a fabric. The fiber layer is permeable to the air flow, and when and during the passage a portion of the suspended particles passing through the fiber layer adhere thereto. As the mechanical filter 1 is followed by two more filtering stages, its filtering separation capability need not extend down to the smallest particles, whereby a filter 1 of lower separation capability and higher air permeability can be used. Hence, the function of the mechanical filter l is to separate a major portion of the total mass of contaminating particles thus protecting the subsequent filter stages. Following the mechanical filter 1 in the filter arrange¬ ment are adapted corona wires 2. These wires 2 are taken to a high DC voltage so that an electric field is created around the wires. The purpose of the corona wires 2 is to charge the passing particles. The corona wires are fol¬ lowed by an electrostatic precipitator 3 comprising planar, parallel oriented plates disposed at a short distance from each other. Also these plates are taken to a DC high-voltage supply, whereby an electric field is created between the plates. When the solids particles passed through the mechanical filter 1 reach the corona wires 2, the particles trigger a corona discharge which gives the particles an electric charge. The charged particles retain their charge while travelling to the electrostatic precipitator 3, where they pass through the gaps between the charged plates. In the electric field between the plates, the particles suspended in the air flow are attaracted to the surfaces of the plates adhering thereto under the attraction caused by electric forces. When properly operated, such a filter is capable of separating at least more than 80 %, under favourable conditions even more than 90 %, of the suspended particles having a size in the range 0.1 - 1 urn. An even better degree of separation can be attained for particul- ates of larger size. The principal operating parameters of such a precipitator are the corona wire current, collecting current applied to the plates, and in particular, the air stream velocity which may not be so high that the adhesion of the particulates to the plates would be impaired. The electrostatic precipitator 3 is followed by an electrostatic fiber filter 4. Such a filter 4 is comprised of fibers which are charged so as to hold a permanent electric charge such that the fibers are polarized. Therefore, this kind of filter requires no external power supply and have an extremely simple structure. In the exemplifying embodiment described herein, the electrostatic fiber filter is given a pleated form so that the mouths of the pleats are oriented to face the impinging air flow. The electrostatic fiber filter 4 is placed as the last filter element along the path of the air flow, because due to the relatively high price of such a filter, its replacement costs represent a substantial investment, and on the other hand, under adverse operating conditions, the efficiency of electrostatic fiber filters may be lost very rapidly. Hence, the optimum use of this filter type is in the separation of the smallest particulates representing a small quantity of contaminating dirt mass. Aside those described above, the present invention may have alternate embodiments. In all of these, the intake air flow is taken first through such a filter that is capable of separating the major portion of particulates from the air, thus protecting the successive filtering stages intended for separating the particulates of smaller size against damage by overload or premature clogging. The first filtering stage is advantageously a coarse mechanical filter or an electrostatic precipitator. One alternative for a filter to be used in the first or successive filtering stage is electrostatic fiber filter based on an active electric field that polarizes both the particles as well as the filter media comprised of fibers or porous material. Such a filter must be provided with corona wires or similar elements such as chemical means or charging plates for charging the particles suspended in the air prior to their passage toward the actual filter element. The combination of corona wire charging with such an electrostatic fiber filter achieves in optimal conditions an almost complete final separation of particulates in the range 0.1 - 1 urn. In a very demanding conditions, e.q. when the air contains a big amount of sand, a cyclone separator may be combined with the arrangement according to the invention, the cyclone is arranged before the mechanical filtering stage and if some kind of electric filtration is performed before the mechanical filtration, before that stage also. The number of filter units and their mutual order may be varied provided that the filter arrangement comprises at least one mechanical filter and, placed after the mechanical filter, an electric filter capable of separating small suspended particles that have passed the mechanical filter. If an electric fiber filter is used, it must be located as the last unit due to its susceptibility to damage. However, the arrangement according to the above-described embodiment has been found the most advantageous combination in practice. In all cases the filter arrangement must be dimensioned so that the pressure loss over it will not exceed that of conventional filter arrangements. In most cases it is even possible to lower the pressure loss below conventional values. Furthermore, the design of the filters and the entire filter arrangement may be varied flexibly. An important accessory in the filter system comprises the different cleaning means of which the most important is the washing apparatus of the electrostatic precipitator. Such means makes it possible to lengthen the maintenance interval of the filter arrangement. Additional improvement to the operating reliability can be imparted by providing heater elements for heating filter insulations of the electrostatic percipitator and by partially duplicating the filter system. For this purpose, one electrostatic stage may receive air selected to pass through one of two parallel mechanical coarse filters, for instance, whereby the replacement of the mechanical filters will be easier. If the system is provided with a plate-type electrostatic precipitator, the filter system achieves a high availability as the washing of the percipitator plates may be readily automated. The system may also comprise two parallal mechanical filters, two parallel electrostatic percipitators and one common electrostatic fiber filter.

Claims: Claims: 1. A method of cleaning the intake air to a compressor of a gas turbine employed in electric power generation, in which method the air is cleaned in at least one stage with a mechanical filter (1), c h a r a c t e r i z e d in treating the air after the mechanical filtering stage for a purpose of conditioning the electrical properties of suspended particulates and subsequently cleaning the air in at least one stage with an electric filter (3) adapted to separate from the air flow particulates that have passed the mechanical filter (1) . 2. A method as defined in claim 1, c h a r a c t e r - i z e d in treating the air with a corona discharge to charge the particulates suspended in the air. 3. A method as defined in claim 1, c h a r a c t e r ¬ i z e d in treating the air chemically to condition the electrical properties of suspended particulates. 4. A method as defined in any of foregoing claims 1 - 3, c h a r a c t e r i z e d in filtering the air first with a mechanical filter (1), then with an electrostatic precipitator (3) and finally with an electrostatic fiber filter (4). 5. A method as defined in any foregoing claim, c h a r a c t e r i z e d in filtering the air with an electrostatic percipitator prior to the mechanical filtering stage. 6. A method as defined in any foregoing claim, c h a r a c t e r i z e d in filtering the air in at least two stages with an electric filter (3) and that at least one of these filtering stages comprises an electrostatically operating fiber media filter. 7. A method as defined in any foregoing claim, c h a r a c t e r i z e d in heating the insulation of the electrostatic filter with the help of an electrical heater element in order to prevent freezing due to precipitated moisture. 8. An arrangement for cleaning the intake air to a compressor of a gas turbine employed in electric power generation, said arrangement comprising a a framework (5) for passing air to the air intake of the compressor and at least one mechanical filter (1), c h a r a c t e r ¬ i z e d by at least one apparatus for conditioning the electrical properties of particulates suspended in the air and by at least one electric filter (3) placed after said mechanical filter (1) on the flow direction of the air, said electric filter having a separation capability for small particulates suspended in the air higher than that of the mechanical filter. 9. An arrangement as defined in claim 8, c h a r a c ¬ t e r i z e d by corona wires (2) for charging parti¬ culates suspended in the air. 10. An arrangement as defined in claim 8, c h a r a c - t e r i z e d by an arrangement for treating the air chemically in order to condition the electrical proper¬ ties of the particulates suspended in the air. 11. An arrangement as defined in any of foregoing claims 8 - 10, c h a r a c t e r i z e d in that said arrange¬ ment comprises on the flow direction of the air first a mechanical filter (l), then an electrostatic percipitator (3) and finally an electrostatic fiber filter (4) . 12. An arrangement as defined in any of foregoing claims 8 - 11, c h a r a c t e r i z e d by an electrostatic percipitator preceding said mechanical filter (1). 13. An arrangement as defined in any of foregoing claims 8 - 12, c h a r a c t e r i z e d by at least two electrostatic filters (3) of which at least one is an electrically operating fiber media filter. 14. An arrangement as defined in any of foregoing claims 8 - 13, c h a r a c t e r i z e d by an electrical heater element adapted to the insulation of said electrostatic filter for heating said insulation.

Inventors: RANTANEN MIKA, RANTANEN, MIKA
Applicants: IMATRAN VOIMA OY, RANTANEN MIKA, RANTANEN, MIKA
Application year: 1996
Main classification: F02C7
IPC codes: B01D50, B03C3, F02C7