METHOD FOR PREPARING CLEAN INTAKE AIR FOR GAS TURBINE

Abstract: The invention relates to a method for purifying the intake air of a gas turbine by separating from the ambient inlet air the nongaseous components such as suspended solid particles and water droplets. In the method, air is passed into at least one filter duct having a given inner diameter and operating at a first potential, and the air flowing in the filter duct is ionized by means of pointed ion-charging electrodes operating at a second potential and mounted on at least one electrode that is aligned longitudinally in regard to said filter duct. Next, the air is passed out from the filter duct, and the air is treated with electrostatic plate precipitators. The air flow velocity in the filter duct is maintained such that the pressure differential between the ambient pressure and at a point downstream of the electrostatic plate precipitator is not greater than 150 Pa and the air is treated by ionization and in electrostatic precipitators until the total mass of water mist droplets downstream of the electrostatic plate precipitator is not more than 0.5 % of the mass of water mist droplets in the ambient intake air.

Description: Method for preparing clean intake air for gas turbine The invention relates to a method according to the preamble of claim 1 for purifying the intake air of a gas turbine by way of treating incoming ambient air by ionization and electrostatic plate precipitators. The intake air passed to the compressor of a gas turbine, that is, the combustion air of the turbine, must be cleaned prior to taking the air to the compressor. Generally, the intake air is cleaned using mechanical fiber filters. The function of inlet air filtration is to prevent abrasive and contaminating particulate matter from reaching the compressor and the turbine, and hence to prevent degradation of system efficiency, and to reduce wear and need for cleaning and servicing the equipment ofthe system. Since the massive gas turbines used in energy production require extremely large volumes of combustion air, even the smallest amounts of contaminants cause heavy internal build-up of dirt in the equipment during running, which means that the filtration of combustion air should be as effective as possible. On the other hand, as the pressure losses caused by such filtration in the intake air duct degrade the system efficiency, the design value of filtration efficiency will always be a compromise dictated by local circumstances. The intake air passed to the compressor also carries suspended water droplets and moisture. At low temperatures, water droplets freeze into ice on the inlet air duct surfaces so as to form larger ice build-up that upon detaching from the surfaces may travel to the compressor causing there major damage to the rotor blades. In the worst case, the compressor may undergo total destruction. Today, different kinds of mechanical filters are generally used as the intake air filters of compressors. In these filters, the separating medium is a fiber layer forming a maze that adheres particulate matter thereto. The retaining efficiency of a filter is dependent on the properties ofthe filtering medium and the packaging density ofthe medium. For a given type of filtering medium, the filter efficiency may be improved by using a thicker filter fabric, or alternatively, compressing the filtering medium into a tighter form. Both approaches result in a drastically higher pressure loss over the filter, and therefore, the capacity of mechanical filters cannot be pushed above a given limit, whereby modern fabric filters may be expected to perform practically complete separation of particles having a size of 1 to 5 μm and greater. However, prior-art filters invariably fail to remove particulate matter if its particle size is very small. Moreover, mechanical filters are susceptible to wetting by small air-borne water droplets and the pressure loss imposed by the filter on the air flow passed there through. Under these conditions, a mechanical filter becomes plugged and, given a sufficiently low ambient temperature, may even freeze. Then, the filters must be dried by heating the intake air until the combination of lower relative humidity and higher temperature allow the formed moisture and ice to evaporate from the filter and pass along with the intake air flow to the compressor. According to the latest investigations on turbine power plant efficiency, even the fraction ofthe smallest contaminant particles has been found to affect system efficiency and service life. Yet, since the solids ofthe smallest size are a major cause in the contamination of a gas turbine and its compressor, they should be removed entirely. While this fraction of small particles comprises only a minor portion in the overall amount of contaminants, in the long run and at the immense amount of combustion air consumed by gas turbines, these particles may accumulate to a total mass of several tens of kilograms per annum. Such contamination reduces the efficiency of the turbine and available electric power. Reduction of power generation efficiency is related to plural factors including abrasion by the particulate matter, inefficiency of cleaning methods leading to incomplete removal of all accumulated dirt, wear due to cleaning operations and increasing play in seals causing increasing leak rates. Even the slightest decrease in system efficiency causes significant accumulative economic losses over the service life of machinery. Also the efficiency ofthe turbine degrades with the internal dirt build-up, which necessitates cleaning ofthe compressor and the turbine proper at scheduled intervals. Cleaning is carried out by washing with water complemented with different kinds of crushed material. Such washing operations with water are complicated particularly under low-temperature conditions due to freezing. Extra operating costs associated with dirt build-up in machinery are traceable to increased fuel consumption, loss of output power capacity and compressor cleaning costs. Obviously, the extent and impact of dirt build-up are dependent on the operating environment and the degree of contamination in the intake air. Inasmuch all adhering dirt cannot be removed by washing, dirt forms one ofthe detrimental factors causing an operating efficiency loss of several percent already in a few years. Furthermore, during cold seasons, inlet air passed to mechanical filters must be preheated in order to minimize the risk of filter plugging due to wetting from condensation and ice formation from water droplets therein. In an annual scale, such intake air preheating invokes significant energy production losses and increase of operating costs due to reduced operating efficiency. Accumulation of dirt in filters causes high pressure losses due to plugging of filters, whereby the filter replacement must be performed 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 shutdowns. Conventionally, better filtration has been sought from the improvement of materials used in coarse and fine filters rather than from combining different methods of filtration. The reliability of electrostatic precipitators has been considered so low that the risk of using them as the only filter in the system has been found intolerably high. A shortcoming of electrostatic filtration methods has been related to moisture in the air flow and internal short-circuit problems caused therefrom. As dirt accumulates on the insulators of an electrostatic precipitator, the deposited dirt forms a conducting path over which voltage break-over takes place. One particularly complex type of contamination comprises different kinds of fibers that due to their elongated shape may create conducting bridges between the high-potential filter elements thus trig- gering break-over discharges in the precipitator. The amount of various fibrous and other contamination in the intake air is dependent on the ambient conditions ofthe gas turbine operating environment such as the traffic volume, industrial plants, flora and type of soil in the nearby environment. Respectively, the required efficiency of droplet separation varies widely according to the conditions ofthe local climate. Particularly in weather conditions of thick mist in combination with a low temperature, icing may occur on the plates of an electrostatic precipitator. The greatest problem caused by ice formation is the cracking ofthe ice layer deposited on the electrostatic precipitator plates into large ice chunks that, if allowed to enter the compressor, can cause serious damage as well as plugging of he air passageways between the precipitator plates. The risk of ice formation is further aggravated by the risk of cooling the air in the inlet ducts due to reduced pressure to such a low temperature that freezing may occur in the precipitator even with the ambient temperature still being above the freezing point. Admittance of moisture to the plates of an electrostatic precipitator is further objectionable since it may scrub off dirt accumulated on the plates and thus give it free access in relatively large particles to the compressor and the turbine. Due to the above-described reasons, mechanical filters or generally used combinations of filter means cannot provide an economically viable degree of separation in regard to very fine particulate matter, because the increase of pressure loss over the filter system at a higher degree of separation makes the improvement in filtration performance costwise unrealistic. Furthermore, mechanical filters are incapable of removing moisture from the intake air, but rather, these filters suffer from moisture problems. While moisture as such is advantageous in the combustion air as to the operation of a gas turbine, the moisture entering along with the intake of ambient air is difficult to cope with in the process control due to complications caused by freezing, for instance. It is an object ofthe invention to provide a method for purifying the intake air of a gas turbine by way of separating water droplets and contamination from incoming ambient air and simultaneously keeping the pressure differential between the ambient air intake and the gas turbine compressor inlet at a low level. The goal ofthe invention is achieved by passing ambient air into a cleaning process channel, wherein the air is ionized and treated with electrostatic plate precipitators until the total mass of moisture droplets in the treated air is not greater than 0.5 % of the respective total mass of moisture in the ambient air and the air flow velocity in the intake air system is controlled so that the pressure differential between the ambient pressure and the pressure at the inlet ofthe electrostatic plate precipitator does not become greater than 150 Pa. According to a preferred embodiment ofthe invention, the intake air is treated until the concentration of solid particulate matter therein does not exceed 30 % ofthe respective concentration of solid particulate matter in the ambient air. More specifically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1. The invention provides significant benefits. The invention makes it possible to deliver at an extremely low pressure loss very pure and dry air suitable for use as the intake air of a gas turbine. As compared with mechanical filters, the present invention offers superior moisture separation capability and is able to remove water in droplet format at least sufficiently effectively degree prior to the admission ofthe air to an electrostatic plate precipitator used in the air purification system, thus overcoming the humidity and ice formation problems that in the prior art have curtailed the use of an electrostatic precipitator. Moisture occurring in droplet form may now be separated so efficiently that it even becomes possible to contemplate the use of water mist spraying at high ambient temperature in order to thereby intercool the intake air for enhanced turbine efficiency. Inasmuch the mechanical construction and design ofthe filter system used for implementing the method according to the invention may be varied in plural ways, the system may be readily retrofitted in lieu of existing intake filter equipment of gas turbines. The retaining capacity of the novel system can be modified by varying the particulate matter ionization and/or collection voltages ofthe system, thus making it possible to tune the filtration efficiency according to the degree of contamination in the ambient inlet air. Due to the extremely efficient charging of solid particles and liquid droplets in the filter, their collection in the cells of an electrostatic plate precipitator is exhaustive. As the area of collecting plates in an electrostatic precipitator is very large, the plates do not need frequent cleaning, since the amount of contamination accumulating on a collecting plate is small in regard to the interplate spacing in the filter cell. Obviously, the need for cleaning of plates is dictated by the degree of contamination in the ambient air and uniformity of air flow patterns in the cells. Next, the invention will be examined closer with the help of exemplary embodiments and by making reference to the appended drawings in which: FIG. 1 shows a filter system suitable for implementing the method according to the invention; and FIG. 2 shows a preferred embodiment of an apparatus for charging particulate matter in the turbine intake air. In the context ofthe following description, the terms "particles" and "particulate matter" must be understood to refer to all the nongaseous components of air, such as solid particles, liquid droplets and fibers. In FIG. 1 is shown a filter apparatus comprising an intake air duct 1 having adapted at its ingoing end a plurality of ionization chamber units 2 via which all the incoming air passed to the intake air duct 1 must flow. The flow direction of intake air is indicated by arrows in the diagrams. Intake air duct 1 forms a flow channel for the ionized air and has adapted at the end thereof a unit comprising conventional electric particulate matter collecting means formed by electrostatic plate precipitators 3. Since the particles carried over by the air to be purified are effectively charged in the ionization chamber units 2, the electrostatic plate precipitators 3 do not need to be equipped with corona wires for charging the particles, but rather, the particles pre- charged in ionization chamber units 2 maybe collected directly on the plates ofthe electrostatic precipitator. The purified air is passed along a duct 4 to the compressor ofthe gas turbine. Particle charging may be carried out using any known electrode construction suited to handle a corona discharge voltage of 50 to 250 kV applied over a large air gap. Herein a problem arises in providing a uniform potential field in ionization chamber unit 2. Planar or wire electrodes typically create corona discharges only at given areas offering a discharge path of lowest energy. Another problem is posed by voltage break-over that tends to occur frequently. Hence, it is advantageous to perform charging of intake air, more specifically of air-borne particles therein, by means of electrodes equipped with sharp barbs that induce corona discharge to occur only at the electrode barb tips. International patent application PCT/FI99/00315 discloses some appropriate electrode structures for this purpose. FIG. 2 shows an embodiment of a ionization chamber unit described in cited patent application. While such ionization chamber units 2 are generally adapted to operate in an upright position as illustrated in FIG. 1, they may as well be mounted in any other position such that the air flow to be cleaned is passed to the substantially vertical ionization chamber units 2 at the bottom thereof. Each one ofthe ionization chamber units 2 incorporates a longitudinal center electrode 5 equipped with ion- charging barbs 6. The ion-charging barbs 6 may be made from a metal wire, for example. The center electrode 5 is taken to a high- voltage potential that creates a corona discharge at the tips of ion-charging barbs 6 and thus invokes a continuous stream of electrons from the barbs to the counterelectrode. The shape ofthe ion/electron discharge patterns is determined by the number of ion-charging barbs 6 and their distance from the counterelectrode. To obtain a maximal charging efficiency, the number of ion-charging barbs should be high and the potential field patterns created by the barbs should advantageously overlap, whereby particles cannot find intermediate areas in which they could avoid becoming charged. While electrode 5 is generally taken to a high- voltage potential of 50 to 250 kV, the arrangement according to the invention may be implemented using a higher or lower corona discharge voltage. The inner wall 7 of ionization chamber unit 2 is either taken to ground potential or a potential opposite to the ion-charging electrodes, whereby the potential difference between the ion-charging barbs 6 and the wall 7 causes the ion streams to be urged toward the wall. Hence, the chamber wall 7 acts as the second eletrode also called the counterelectrode in this text. While this electrode can be taken to a potential different from the ground potential, grounding ofthe counterelectrode is the simplest arrangement. The gas composition being cleaned flows upward in flow channels 2 where the gas stream meets the ion streams emitted from ion-charging barbs 6, whereupon the mechanical and electrical forces resulting from this encounter urge the non-gaseous phase components, such as particles and water droplets, suspended in the gas stream to separate from the stream so as to travel to the walls 7 ofthe discharge unit chambers. Subsequently, the water droplets separated from the gas stream flush the particulate matter deposited on the walls into, e.g., a dump located at the bottom ofthe ionization chamber 2. Finally, the partially cleaned and ionized gas stream is passed out from the ionization chambers 2 via their top portion and further therefrom to electrostatic plate precipitators, wherein the remainder ofthe charged particles are collected. In the latter kind of equipment, the electrode gaps are made substantially large, in the order of 100 - 1000 mm. Inasmuch the flow resistance and pressure loss imposed on the flow ofthe gas stream being cleaned should preferably be as low as possible, the diameter ofthe ionization chambers 2 must be made large in regard to the volumetric flow rate ofthe passing gas and, simultaneously, the flow pattern in ionization chambers 2 must be designed to be maximally smooth and nonturbulent. The flow velocity in ionization chamber 2 is fastest at the center ofthe flow channel formed by the ionization chamber and slowest in the vicinity ofthe inner wall 7. Hence, any flow path possibly formed by the inner bore ofthe center electrode 5 in the ionization chamber must be plugged, because otherwise a major portion ofthe gas stream entering the chamber would pass through the bore of electrode 5. Due to such a design of electrode 5, its center bore cannot be utilized for establishing ion streams therein and thus obviously cannot serve as an ionizing/cleaning space. Hence, the gas stream being cleaned passes through an annular cross section in ionization chamber 2, whereby the gas stream sweeps along the inner wall ofthe flow channel but not through the electrode 5 located in the center ofthe ionization chamber 2. These design requirements are met by making electrode 5 into, e.g., a tubular element closed by at least one end in the interior space of ionization chamber 2 or, alternatively, into a solid rod that cannot pass a gas stream. The shape of ionization chamber 2 dictates the location of electrode 2 in the interior ofthe flow channel. In a flow channel of circular cross section, for instance, electrode 2 is advantageously located in the center of ionization chamber 2, thus making the distance of electrode 5 to wall 7 equal over the entire length ofthe flow channel. A more detailed description ofthe ionization chamber discussed above is disclosed in international patent application PCT/FI99/00315. The system according to the invention functions as follows. The ambient air is first passed to a ionization chamber 2 equipped with an electrode 5 ofthe above-described type. The volumetric flow rate of incoming air is set such that the pressure differential between the ambient pressure and the inlet manifold pressure ofthe electrostatic plate precipitator is not greater than 150 Pa. A reasonably low flow rate can be attained by designing the overall cross section ofthe inlet ducts in the apparatus sufficiently large. This can be accomplished without difficulty, because the collection efficiency in the ionization chamber and the electrostatic plate precipitator is not impaired by larger inlet duct dimensions and, furthermore, these units themselves do not cause a major flow resistance as compared with the ducts. Obviously, such apparatus parameters as the plate gap and length of discharge channel in the electrostatic plate precipitator must be selected to meet the general design criteria of these filter types. An electric field is established from the tips ofthe electrode barbs 6 toward the grounded wall 7 ofthe ionization chamber unit, and particles in the incoming air flow when meeting this field become strongly charged, whereby they migrate toward chamber wall 7 and finally adhere to the wall. In addition to solid particles, also water and other liquid droplets become vigorously charged, whereby they are substantially separated from the intake air flow already at this stage ofthe cleaning process. Moisture is herein separated very effectively. Due to the high ionization voltage that may be 50 to 250 kV, for example, the electric field formed in the chamber is very strong and, thus, gives good ionization efficiency. The strong electric field also acts as a powerful carrier of ionized particles and liquid droplets. Next, the air is passed to electrostatic plate precipitators 3 that are displaced at a distance from the ionization chambers 2 and perform the collection of the strongly charged particles are collected to the plates ofthe electrostatic precipitator 3. Under certain conditions it is possible that all or at least a major portion of suspended particles are separated from the intake air already in the ionization chamber 2, whereby the electrostatic plate precipitator will not need to handle any substantial amount of suspended particulate matter. It is an object ofthe invention to provide a maximally effective system for charging suspended particulate matter and then collect all thus charged particles. Herein, an ionization chamber functions as an effective charger of particulate matter and, especially, of large particles such as fiber and water droplets. When leaving the ionization chamber, any particles still being suspended in the intake air flow retain their strong charge, whereby the charged particles can be collected on the cells of an electrostatic plate precipitator operating at a lower voltage serving for collection only. Typically, the collection voltage is in the order of a few kilovolts only, e.g., 4 to 6 kV, and anyhow less than 10 kV, which is about one-tenth or one-twentieth part ofthe typical ionization voltage level. In the general sense, the use of a higher ionization voltage, or corona voltage, improves the collection efficiency of electrostatic plate precipitator 3, as well as the use a higher collection voltage in the latter unit. Accordingly, by adjusting these voltage levels the system may be tuned to cope optimally with variations in air quality parameters such as the ambient moisture content and level of contamination or, when so required, with changes in volumetric intake air need. Inasmuch a higher volumetric flow rate degrades the collection capacity, it may be necessary to use higher voltages, whereby the flow rate may not however be allowed to rise so high as to cause an excessive pressure differential. Once the correct flow rate is set, the voltages can be adjusted to attain a desired efficiency in the separation of liquid droplets and solids. The voltages are adjusted such that the mass of mist droplets in the air leaving the electrostatic plate precipitator is not greater than 0.5 % ofthe total droplet mass in an equivalent volume of ambient air. Herein, the conditions of gas temperature and pressure must be taken as they are at the respective points of comparison, whereby the ambient air is assumed to be at the ambient temperature and pressure, while the treated intake air is at the tempera- ture and pressure prevailing at the point of comparison downstream ofthe electrostatic precipitator. Another criterion for voltage adjustment may be selected to be based on the level of contaminants so that the concentration of solid particle contamination is in the cleaned intake air not greater than 30 % ofthe respective concentration of contaminants in the ambient air. Furthermore, the amount of suspended solid particles in the cleaned air with a size greater than 0.4 μm may not be greater than 20 % ofthe amount of solid particle contamination in the ambient air. Inasmuch the polarity ofthe corona discharge is not very critical factor to the degree of separation in the electrostatic plate precipitator operating downstream in the system, the ionization chambers may be operated using a negative or a positive potential. In addition to the above-described exemplary embodiments, different modifications ofthe invention may be contemplated. As mentioned in the foregoing, the construction ofthe flow channels, ionization chamber units and the electrostatic plate precipi- tator may be selected with rather minor constraints. An essential requirement, however, is that ionization shall be carried out in a space isolated from that used for collecting particulate matter, whereby ionization may take place at a sufficiently high voltage without affecting the operation ofthe electrostatic plate precipitator. Hence, the ionization chamber unit must be located at a distance upstream ofthe electrostatic plate precipitator on the path ofthe air flow. A minimum distance between the units is an air gap so wide that no risk of break-over from the electrodes ofthe ionization chamber unit to the collecting plates ofthe electrostatic precipitator can occur. Inasmuch the electrostatic plate precipitators and ionization chamber units due the large volume of air to be cleaned are usually very big and advantageously aligned in a certain position, the requirement of a safe distance is generally fulfilled already by structural length ofthe connecting ducts. The number of electrostatic plate precipitators and ionization chamber units needed in the system is obviously dictated by the volumetric intake air flow rate specified for the gas turbine. In the construction ofthe ionization chamber units, it is essential that the entire gas volume ofthe intake air flowing therethrough is subjected to treatment in the ionizing electric field.

Claims: What is claimed is: 1. A method for purifying the intake air of a gas turbine by separating from the ambient inlet air the nongaseous components such as suspended solid particles and water droplets, the method comprising the steps of - passing air into at least one filter duct having a given inner diameter and operating at a first potential, - ionizing the air flowing in the filter duct by means of pointed ion-charging electrodes operating at a second potential and mounted on at least one electrode that is aligned longitudinally in regard to said filter duct, - passing the air out from said filter duct, and - treating the air with electrostatic plate precipitators, characterized in that - the air flow velocity in said filter duct is maintained such that the pressure differential between the ambient pressure and at a point downstream ofthe electrostatic plate precipitator is not greater than 150 Pa, and - the air is treated by ionization and in electrostatic precipitators until the total mass of water mist droplets downstream ofthe electrostatic plate precipitator is not more than 0.5 % ofthe overall mass of water mist droplets in the ambient intake air. 2. The method of claim 1, characterized in that the air is treated until the concentra- tion of solid particle contamination is not greater than 30 % ofthe mass of solid particle contamination in the ambient intake air and the amount of solid particles having a size greater than 0.4 μm is not more than 20 % ofthe amount of solid particle contamination in the ambient intake air. 3. The method of claim 4, characterized in that the collection voltage used is not higher than 10 kV and ionization voltage used is 50 to 250 kV.

Inventors: ROENNBERG JAN, ROENNBERG, JAN
Applicants: FORTUM OYJ, ROENNBERG JAN, ROENNBERG, JAN
Application year: 2001
Main classification: F02C7
IPC codes: B03C3, F02C7