Patent Publication Number: US-2015083165-A1

Title: Suspensions of inorganic cleaning agents

Description:
PRIORITY INFORMATION 
     The present application claims priority to French Patent Application FR1302240 filed on Sep. 26, 2013. 
     FIELD OF THE INVENTION 
     The present invention is directed to a group of cleaning agents that includes solid inorganic particles suspended in liquids and methods of their use. In particular, the cleaning agents are configured to be sprayed on the walls of fouled mechanical parts, such as those in combustion equipment, in order to remove the deposits thereof without causing any erosion effect. 
     BACKGROUND OF THE INVENTION 
     A turbine mainly comprises of: an air compressor, a combustion system, and an expansion turbine. In a turbine, the hot parts of the turbine are those parts that are in contact with the combustion gases, and mainly include: the parts forming the combustion system (fuel nozzles; liners; transition pieces etc.) and the components of the expansion turbine: “vanes” (fixed blades) and “buckets” (rotating blades). These hot parts are made of metal superalloys (e.g., nickel-based superalloys) and can have ceramic coatings (e.g., anti-corrosion coatings serving as thermal barriers). 
     When a combustion equipment burns ash-forming fuels, the ash particles are transported by the combustion gases along the hot gas path and partly deposit on the hot parts, causing their progressive fouling. Correlatively, the performance of the turbine decays following the gradual alteration of the superficial cleanliness of the turbine components and, in case of marked fouling, due to a possible narrowing of the hot gas passage. Liquid petroleum oils containing traces of metals (i.e., “contaminated fuels” and “heavy fuel oils”) are likely to generate ashes in combustion equipment. For example, heavy oils often contain calcium that forms deposits of calcium sulphate (CaSO 4 ) that is considered difficult to remove, as well as vanadium, an element which, once treated with magnesium to inhibit its corrosive effects, forms magnesium vanadate and potentially some magnesium oxide (MgO) that has also a fouling effect. Some primary biomass fuels (“biofuels” and “biogas”) are also likely to generate ashes in combustion equipment. Some process gases also likely to generate ashes in combustion equipment, such as coke-oven gases, blast-furnace gases, or syngas fuels resulting from the gasification of a large variety of solids (e.g., coals; lignite; diverse biomasses; heavy fuels; residue of sewage treatment plants; etc.). 
     In order to clean the hot gas path parts of a turbine having undergone such fouling without manual intervention, one can perform either a “water wash”, which is an off-line operation since it requires a shutdown of the turbine. Alternatively, one can perform a “dry cleaning” operation, which is an on-line operation since it proceeds while the turbine runs. In a dry cleaning operation, solid particles are injected into the hot gas path of the turbine. From a general viewpoint, solid particles used for cleaning have a descaling effect depending on their speed and geometric, physical, mechanical and thermal properties. These properties must be selected so that they are efficient for the removal of deposits but do not erode the structural materials. That is, these particles must be able to remove the ash deposits without eroding the hot parts to be cleaned. 
     To ensure a uniform cleaning, this injection must be performed through a certain number (“N i ”) of points of the turbine that must be judiciously distributed in a portion upstream of the hot gas path. These points are generally located around the combustion system in such a way that the injected cleaning agent reaches the highest possible fraction of surface of the hot parts. In practice, to propel the particles of the cleaning agent, one uses compressed air that has a pressure higher than that in the turbine, according to a transport mode similar to a “pneumatic transfer.” For example, in the case of a combustion system of the can-annular type that has “N c ” combustion chambers, the particles of the cleaning agent are forwarded to these N c  chambers: one has in this case: N i =N c . Once introduced into the hot gas path, these particles rapidly reach the speed of the combustion gases before hitting, in their race, the hot parts walls from which they unstick, during these collisions, all or a fraction of the deposits. The ash particles and the fragments of the cleaning material resulting from these shocks are entrained by the combustion gases and discharged through the turbine exhaust. 
     The types of solids or cleaning agents that had been historically identified and traditionally used to carry out these dry cleaning operations are fragments of ligneous materials (nut shells; cores of fruits; wood from some tree species) whose dimensions are of a few millimeters. However, these classic ligneous materials present two main disadvantages. First, due to their flammable character, an important fraction thereof gets burnt before having hit the hot parts to be cleaned. Second, since these materials are light and very soft (the density of wood lying between 0.45 to 0.85 g/cm 3  and its hardness being much less than the rating 1 of the Mohs hardness scale), their shocks against deposits have limited descaling effects. 
     While these classic ligneous materials proved to be effective in former turbine models having firing temperatures in the range of 900° C. to 950° C., these classic ligneous materials have a rather poor (if not null) effectiveness when they are used in modern turbines having firing temperatures of 1000° C. or more. That is, when exposed to increasing temperatures, any ash deposit that is formed on hot parts in the combustion equipment undergoes a sintering process, which is a phenomenon that affects any powdery, crystalline solid exposed to high temperatures over a long period. During sintering, the grains of this solid tend to lose their porosity, to stick to each other, harden, and to adhere to walls. Further, the kinetics of these transformations increase substantially with the temperature. Consequently, the classic ligneous materials can no longer destabilize such hardened deposits in modern turbines having firing temperatures of 1000° C. or more, but on the contrary they disintegrate during their impacts. 
     To improve the efficiency of the dry cleaning processes, two inventions (U.S. Pat. No. 4,065,322 of Langford (1977) and U.S. Pat. No. 7,185,663B2 of Koch et al. (2007)) propose the use, as a cleaning agent, of powdery graphite (or “coke”) which can be introduced through one of the fluids injected into the turbine, particularly through the combustion air. As a cleaning agent, graphite actually offers two major advantages as compared with the classic ligneous materials: they are harder and practically non-flammable. In the patent by Koch, a mixture of two qualities of powdery graphite, are used, of which one features a high thermal expansion coefficient (“expandable graphite”) and the other a classic thermal behavior (“non-expandable graphite”). 
     However, graphite remains a very soft and relatively light solid, with a hardness that is relatively low (about 1.5 on the Mohs scale). Though its hardness exceeds that of ligneous materials, it hardly exceeds that of talc, the softest inorganic material (rated 1 in the Mohs scale). Additionally, graphite&#39;s hardness is much lower than that of the inorganic phases found in typical ash deposits, such as magnesium oxide (about 7 on the Mohs scale) and calcium sulphate as gypsum (about 3 on the Mohs scale). Moreover, graphite&#39;s density (in the order of 2.15) is much lower than that of most ordinary inorganics such as iron oxides (approximately 5.5); titanium oxide (approximately 5.5), or aluminosilicates (approximately 6). 
     Consequently, if one wishes to limit the quantity of graphite to be injected, in order to limit the costs and the emission of dust in the atmosphere, one is bound to use sufficiently large particles. This is consistent with the fact that the patent by Langford mentions a size of particles ranging preferably up to 3.4 mm (value corresponding to the sieve No. 6 of the ASTM E11 standard), whereas the patent by Koch mentions a distribution ranging up to 1 mm. Such particle size distributions ranging from about 1 mm to several millimeters will be referred to below as “peri-millimeter” particle size distributions. 
     Another important aspect of the dry cleaning processes lies in the way the cleaning agent is used and particularly in its mode of transfer to the turbine. As mentioned above, when a ligneous material is used, it is usually transferred to “N i ” entry points of the turbine by propelling it with compressed air. However, experience shows that this mode of pneumatic transfer is not very reliable. Since it is a diphasic particles/air flow and due to the high counter-pressure that exists in the turbine, it is not simple to control the instant flow of particles and to ensure their uniform distribution between the N i  injection points. Now the performance of a cleaning operation strongly depends on these two aspects. The two abovementioned patents certainly mention alternate ways for introducing the graphite, namely through the liquid circuits (fuel or water) sent to the turbine. However, they do not specify the manner of meeting this objective which appears to be in fact rather delicate to meet. In fact, on the one hand, the “peri-millimeters” size of the graphite particles is too large to enable creating sufficiently stable suspensions. On the other hand, the introduction of such particles in a fuel or water circuit would immediately cause serious “functional problems” such as fouling, mechanical wear, even seizing of the mechanical devices of that circuit. Finally, the graphite particles are friable and will be fragmented and will thus, as mentioned above, lose in descaling effect. 
     To be viable, such suspensions must have stability durations substantially higher than their residence time in the circuit via which they are injected (e.g., residence time on the order of one minute). As a matter of illustration and without restricting the reach of the present invention, a minimum stability period of 30 minutes will be taken as a conservative criteria. Such objective of stability assumes that the particles do not exceed a maximum size which, in the current state of technology of suspensions, will not exceed a few hundred micrometers. 
     Considering the state of the art, it is desirable to have a new type of cleaning agent and an associated injection method which, without eroding the hot parts, will secure a cleaning efficiency at least equivalent to that of powdery graphite and will lend itself to a satisfactory injection mode of the descaling material, i.e. enable, on the one hand, easy and satisfactory control of the instant injection flow and, on another hand, uniform distribution at different injection points. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Aspects and advantages of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention. 
     Methods are generally provided for on-line cleaning hot parts of a gas turbine during its operation by injecting a cleaning agent composition into a hot gas path of the gas turbine. In one embodiment, the cleaning agent composition comprises a liquid carrier and a descaling material suspending in the liquid carrier. The descaling material comprises, in one particular embodiment, at least one oxide, in an anhydrous or a hydrated form, that is derived from calcium, magnesium, titanium, iron, aluminium, silicon in the form of silicates having non-fibrous structures, or phosphorus in the form of alkaline-earth phosphates. 
     Cleaning agents are also generally provided for use in a gas turbine. In one embodiment, the cleaning agent comprises a liquid carrier, and a descaling material suspending in the liquid carrier. The descaling material can include at least one oxide, in an anhydrous or a hydrated form, that is derived from calcium, magnesium, titanium, iron, aluminium, silicon in the form of silicates having non-fibrous structures, or phosphorus in the form of alkaline-earth phosphates. The descaling material has, in certain embodiments, a sub-millimeter particle size that is about 5 μm to about 315 μm. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which: 
         FIG. 1  is a schematic cross sectional side view of an exemplary gas turbine suitable for use with the cleaning agent described herein. 
     
    
    
     DEFINITIONS 
     The “hot parts” of a combustion equipment are those of its components which are in contact with the combustion gases. In the present document, gas turbines or “turbines” will be taken as paradigms of such combustion equipment. 
     For the simplicity of the expression, the term “powder” and the adjective “powdery” refer to any solid being in a divided state irrespectively to its particle size distribution. 
     In terms of terminology, the expression “oxide combinations” refers to any chemical association—binary, ternary, or whichever—of metal oxides or metalloids. For example, perovskite (CaTiO 3 ) is a binary combination of the oxides CaO and TiO 2 , while diopside (Ca 2 MgSi 2 O 6 ) is a ternary combination of the oxides CaO, MgO and SiO 2 . Moreover, in a larger way, the expression “oxides or combination of oxides” will also cover mixtures of oxides or “associations of oxides” as they have just been defined, as, for example, a mixture of perovskite and diopside. 
     The expression “hot gas path” designates the volume within which the combustion gases flow and which is limited by hot parts walls. The upstream portion of the hot gas path is the combustion system which, in modern turbines, has an “annular” or “can-annular” geometry. According to the usage, the “firing temperature” of a turbine is the temperature of the combustion gases at their entry in the expansion turbine, not the temperature which develops in the flames. The efficiency of a turbine increases with its firing temperature which, in contemporaneous models, exceeds 1000° C. 
     It is to be understood that the use of “comprising” in conjunction with the cleaning compositions described herein specifically discloses and includes the embodiments wherein the cleaning compositions “consist essentially of” the named components (i.e., contain the named components and no other components that significantly adversely affect the basic and novel features disclosed), and embodiments wherein the cleaning compositions “consist of” the named components (i.e., contain only the named components except for contaminants which are naturally and inevitably present in each of the named components). 
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
     A group of solid, refractory, and non-combustible materials are generally provided. Such materials, whose combined physical and mechanical characteristics, enable their use as powdery descaling agents as well as their use within cleaning agent compositions, i.e. which enable their putting in suspension, at sub-millimeter scale, in liquids. 
     The cleaning agents are particular suitable for use in a method of on-line cleaning hot parts of a gas turbine. These “cleaning agents” can particularly be used “on-line”, in any combustion equipment that burns fuels that generate ash particles likely to deposit on the hot parts of the said equipment. They are more particularly used when the combustion gas feature temperature and speed levels that exceed 1000° C. and 10 m/s respectively, as a matter of illustration. 
       FIG. 1  illustrates an example of a gas turbine  10  that may be cleaned utilizing the cleaning agent described herein in at least one component, particularly in the hot gas pas. As shown, the gas turbine  10  generally includes a compressor section  12 . The compressor section  12  includes a compressor  14  having a plurality of compressor blades  15  and stator vanes  17 , with the compressor blades  15  attached to the shaft  24 . The compressor includes an inlet  16  that is disposed at an upstream end of the gas turbine  10 . The gas turbine  10  further includes a combustion section  18  having one or more combustors  20  disposed downstream from the compressor section  12 . The gas turbine further includes a turbine section  22  that is downstream from the combustion section  18 . A shaft  24  extends generally axially through the gas turbine  10 . The turbine section  22  generally includes alternating stages of stationary nozzles  26  and turbine rotor blades  28  positioned within the turbine section  22  along an axial centerline  30  of the shaft  24 . An outer casing  32  circumferentially surrounds the alternating stages of stationary nozzles  26  and the turbine rotor blades  28 . An exhaust diffuser  34  is positioned downstream from the turbine section  22 . 
     Generally, each compressor blade  15  and rotor blade  28  has a leading edge, a trailing edge, a tip, and a blade root, such as a dovetailed root that is adapted for detachable attachment to a turbine disk. The span of a blade extends from the tip edge to the blade root. The surface of the blade comprehended within the span constitutes the airfoil surface of the turbine airfoil. The airfoil surface is that portion of the turbine airfoil that is exposed to the flow path of air from the turbine inlet through the compressor section of the turbine into the combustion chamber and other portions of the turbine. 
     In operation, ambient air  36  or other working fluid is drawn into the inlet  16  of the compressor  14  and is progressively compressed to provide a compressed air  38  to the combustion section  18 . The compressed air  38  flows into the combustion section  18  and is mixed with fuel to form a combustible mixture which is burned in a combustion chamber  40  defined within each combustor  20 , thereby generating a hot gas  42  that flows from the combustion chamber  40  into the turbine section  22 . The hot gas  42  rapidly expands as it flows through the alternating stages of stationary nozzles  26  and turbine rotor blades  28  of the turbine section  22 . 
     Thermal and/or kinetic energy is transferred from the hot gas  42  to each stage of the turbine rotor blades  28 , thereby causing the shaft  24  to rotate and produce mechanical work. The hot gas  42  exits the turbine section  22  and flows through the exhaust diffuser  34  and across a plurality of generally airfoil shaped diffuser struts  44  that are disposed within the exhaust diffuser  34 . During various operating conditions of the gas turbine such as during part-load operation, the hot gas  42  flowing into the exhaust diffuser  34  from the turbine section  22  has a high level of swirl that is caused by the rotating turbine rotor blades  28 . As a result of the swirling hot gas  42  exiting the turbine section  22 , flow separation of the hot gas  42  from the exhaust diffuser struts occurs which compromises the aerodynamic performance of the gas turbine  10 , thereby impacting overall engine output and heat rate. As shown in  FIG. 1 , the diffuser struts  44  are positioned relative to a direction of flow  60  of the hot gas  42  flowing from the turbine section  22  of the gas turbine  10 . 
     According to the presently disclosed method, the cleaning agent can be injected into the hot gas path at any point or points in the compressor section  12 , the combustion section  18 , and/or the turbine section  22  of the turbine  10 . For example, the cleaning agent can be injected into the hot gas path through the fuel nozzles  21 , such as a mixture with the fuel. Alternatively or additionally, cleaning agent injectors  39  can be located within the gas turbine  10  for the injection of the cleaning agent into the hot gas path. Although shown located within the combustion section  18 , the cleaning agent injectors  39  can be positioned in any location within the compressor section  12 , the combustion section  18 , and/or the turbine section  22  to inject the cleaning agent into the hot gas path. 
     To transfer the cleaning agent from the tank (not shown) in which it is stored and/or prepared towards the turbine, a pumping device able to accommodate solid suspensions can be utilized. For example, a centrifugal pump can be utilized that creates sufficient pressure to overcome the pressure in the turbine, which is around 10 to 20 bar. Either a single-stage pump (for example, model TH632A890® of Brinkman Pumps) or a two-stage one (for example, the association, in series, of two models TH632A890® and FH632A89® from the same manufacturer) can be utilized, which provides a discharge pressure of 35 to 48 bars according to the flow. The injection pressure must be great enough in order to spray the cleaning agent, which is introduced through injectors, sharply penetrates the flow of combustion gas which has a strong kinetic energy. Moreover, to ensure perfectly uniform flows between the N i  injection points, the N i  lines that connect the pump discharge to the N i  injection points can be equipped with devices creating high pressure drops, these pressure drops being identical in the N i  lines, so that the slight pressure differences which may exist between these points do not cause differences in the N i  injection flows. In this case, the abovementioned configuration relying on two pumps in series is recommended to obtain the important discharge pressure required by these devices. These devices can be the injectors themselves. 
     I. Descaling Material 
     According to the experimental study discussed below, the present inventors have discovered the empiric conditions which allow the use of sub-millimeter particles (“the size criterion”) to obtain good cleaning efficiency without eroding the substrates. These conditions, which apply to the hardness/density properties of descaling material, are as follows: in order to ensure a good cleaning performance (“the efficiency criterion”), it is appropriate that the product of the density (“D”) of the descaling material by its Mohs hardness (“H”) be greater than 12; this product will be called the “efficiency factor” and noted “F”, i.e.: 
         F=D*H≧ 12  (formula 1)
 
     in order not to erode the hot parts, with a sufficient safety margin during a repeated application of the cleaning process (“the non-erosion criterion”), the Mohs hardness of the descaling material must be equal to or less than 7 and the efficiency factor F must itself be less than 35 and, i.e.: 
         F=D*H≦ 35  (formula 2)
 
         H≦ 7.  (formula 3)
 
     By associating in a same formulation two or several sub-millimeter size inorganics satisfying equations (1) to (3), one obtains a “composite material” which also satisfies the efficiency and non-erosion criteria. To satisfy the “full specification” of the present industrial application, it is also appropriate that these materials must induce no corrosion phenomenon, particularly and mainly, no high temperature corrosion of the hot parts (“the non-corrosion criterion”). Additionally, they must not pose any safety issue (flammability/explosiveness risk), and they must not be harmful or have any negative impact on the environment (“the EHS criteria”). Incidentally, it will be noted that the safety clause is fulfilled beforehand as a result of the intrinsic chemical stability of the selected refractory materials. 
     In one embodiment, the oxide has a Mohs hardness that is less than or equal to 7 and an efficiency factor, defined as the product of its density by its Mohs hardness, that is between 12 and 35. 
     It is noteworthy that the composition of the descaling material can include, in minor contents (i.e. less than a few %), other phases than those listed in Table 1, provided that they do not contain elements that are potentially detrimental in terms of corrosiveness and EHS. Such potentially detrimental elements to be excluded include: alkaline metals; halogens, vanadium, sulphur, lead, phosphorus if not associated with alkaline-earth elements; silicon if in the form of fibrous materials, chromium nickel, selenium, arsenic, antimony, cobalt, barium, cadmium, mercury and the elements having atomic numbers superior to that of mercury, as well as, by way of precaution, the elements having doubtful effects on the environment: manganese, copper, zinc. 
     The substances listed in appended Table 1 particularly suitable descaling materials, and can meet the five criteria that have been defined (i.e., particle size; efficiency; non-erosion; non-corrosiveness, and EHS criteria). Generally, the descaling materials have been grouped in six chemical classes: iron oxides; titanium oxides; titanates; silicates; aluminosilicates, and phosphates. It is reminded that, by associating, in a same formulation, inorganics contained in Table 1, one obtains “composite materials” which also meet the criteria. 
     It must be highlighted at this point that the accurate quantity of descaling material that one needs to inject to complete a cleaning operation, in given operating conditions of a turbine, can be determined only through an empirical approach. And, this invention is not aimed at, and is not in a position to define a priori this quantity because it depends not only on the size, density and hardness of the descaling selected material, as it has been already set out, but also and strongly on three key operation parameters that can greatly vary, i.e.: (1) the “contamination of the fuel” (the nature and concentrations of contaminants that it contains), which determines the chemical nature and properties of ash deposits, (2) the operating period between two cleaning operations which determines the quantity of ash deposited, and (3) the firing temperature which conditions their degree of sintering and consequently their hardness. 
     For example, the first two parameters being identical, a new generation gas turbine operating at a firing temperature of 1100° C. can require for its cleaning a mass of particles that is twice greater than that required by a former generation gas turbine operating at 950° C. The turbine operator will thus be led to determine the optimum quantity of cleaning agent to be injected, according to these three parameters and relying for example on the monitoring of the classical performance parameters of the turbine (instant power output; specific fuel consumption; pressure at compressor discharge; etc.). 
     Among the substances listed in Table 1, three of them are of particular interest due to their special physical properties and their general availability as well as their moderate costs, as natural inorganic products: the items of the iron oxides class (in anhydrous or in hydrated form); wollastonite, which belongs to the class of silicates; and kyanite, which belongs to the class of aluminosilicates. 
     The class of iron oxides, in anhydrous or hydrated form, is comprised of: iron oxide (III) or ferric oxide (i.e., Fe 2 O 3 ) that has two polymorphs: α (hematite) and γ (maghemite); two hydrated allotropic forms of Fe 2 O 3 : goethite (α-FeOOH) and lepidocrocite (γ-FeOOH); iron oxide (II, III), of formula FeO—Fe 2 O 3  or Fe 3 O 4 , which is called magnetite and sometimes also referred to by the terms of spinel or “ferrous-ferric oxide; and finally, iron oxide (II) or ferrous oxide (FeO), called wustite. 
     The use of such iron oxide based preparations as cleaning agents is interesting for several reasons. From the physical and mechanical standpoint, the iron derived phases of these preparations have densities approximately twice greater than that of graphite (5.2 g/cm 3  for hematite and magnetite; 4.9 for maghemite) as well as a much greater hardness (5.5 to 6.5 for hematite; 5.5 to 6 for magnetite and maghemite, in the Mohs scale). From a thermal standpoint, these phases are refractory components with melting or decomposition temperatures greater than 1500° C. and are totally non-combustible. From an economic standpoint, they are widespread and inexpensive materials that are used for instance as pigments in the paint industry (“red or yellow pigments”) or are available, as good purity by-products from the steel industry. Finally, these particles are totally benign from an environmental standpoint. 
     These iron oxide preparations can be dispersed in a combustible or non-combustible liquid medium, such as shown in the examples discussed below. 
     Wollastonite, which belongs to the class of silicates, is also of interest as a descaling material in spite of its moderate efficiency factor (F=15.5). Wollastonite is a calcium silicate mineral (CaSiO 3 ) that may contain small amounts of iron and/or magnesium substituting for calcium. It is a natural mineral, boasting a good refractoriness (melting point of about 1540° C.), and a good absorption capacity of liquids, which favours the stability of suspensions that one can carry out. Moreover, it is economically interesting as widespread “filler” in the manufacturing of paints. Its use will be illustrated in the third example of application given below. 
     Finally, kyanite which has an average Mohs hardness of 5.5 and an efficiency factor of 19.5 is also interesting as it has a high fracture modulus and a low scaling rate which helps the fragmentation of the impacted deposits. As it is an abundant and widely used inorganic material as a structural ceramic component, it also represents an economically interesting descaling material. 
     In general, a distribution of particles in the cleaning agent can have a lower size value of a few micrometers (e.g., 5 μm) and an upper size value in the order of 315 μm. The value of the lower size value which is not an essential data in the descaling process is merely given for guidance in order to provide a sufficient definition of the object of the invention and is not likely to restrict the field and reach of the invention. The upper size of 315 μm, which corresponds to the sieve No 26 of standard NFX 11.501 (and, approximately, to the sieve No. 50 of the standard ASTM E11 that equals 297 μm), must also be taken as an order of magnitude. That is, the indication of this precise value does not either restrict the reach of the invention. From a statistic standpoint 95%, 98% or 99% of the particles are lower than the upper size value. From a terminology standpoint, to designate this range of particle sizes, one shall use, in this document, the expressions “sub-millimeter size” or, more simply “sub-millimeter particles”, as opposed to the “peri-millimeter size” (equalling or exceeding one millimeter) that is associated with graphite. Moreover, for the sake of clarity, the expression “(powdery) descaling material” will be used to designate the powdery solid which performs the descaling function and that of “liquid carrier” for the liquid in which this descaling material is suspended, the term “cleaning agent” being applied to the complete preparation constituted of the descaling material and the liquid carrier. 
     To obtain sub-millimeter descaling materials (i.e. particles not exceeding in size a maximum size taken equal to approximately 315 μm), one can start from a “rough descaling material” having for example a “peri-millimeter size”, to which one can apply one of the following treatments: (a) sieved to retain only the sub-millimeter particles; dry crushed and sieved in order to retain only the sub-millimeter particles; or (c) crushed in mixture with the liquid carrier itself (“in-situ crushing”), followed by filtering the obtained slurry to a sub-millimeter size. 
     In one particular embodiment, the cleaning agent composition comprises at least one descaling material selected from the following inorganics: hematite; maghemite; goethite; lepidocrocite; magnetite; wustite; rutile; anatase; brookite; geikielite; perovskite; ilmenite; wollastonite; larnite; enstatite; akermanite; diopside; merwinite; monticellite; fosterite; fayalite; andradite; andalousite; kyanite; sillimanite; mullite; anorthite; ghelenite; hydroxyapatite; or mixtures thereof. 
     II. Liquid Carrier 
     The cleaning agent composition is formed from a suspension of the descaling material(s) in a liquid carrier. In one embodiment, cleaning agent can include: 
     (a) the descaling material, as discussed above; 
     (b) a liquid carrier which is either hydrophilic (water; alcohols or polyols, polyethylene-glycols, polyethers, mixed or not with water; etc.) or eventually lipophilic (aliphatic hydrocarbons, aromatics; alcohols; ketones; white spirit; etc.); this liquid can be a mixture of liquids; its viscosity can be selected to enhance the stability of the suspension; and 
     (c) one or several dispersing additives, sometimes called “wetting agents,” that are configured to disperse the particles of the descaling material. 
     Such dispersing additives generally have multiple ionic or polar groups, which, by adsorbing on the surface of the particles, prevent them to contact each other due to electrostatic repulsion forces or by steric effect. Thus, such dispersing additives prevent coalescence and subsequent decantation of the descaling material. Suitable dispersing additives include those having ammonium or amine as counter-ion; fatty acid amines; polycarboxylic acids (e.g., ammonium polycarboxylates or amine polycarboxylates); polyamides; polyesters; polyurethanes; sequential copolymers or with “comb polymer structures” based on ether or acrylic groups; etc.; or mixtures thereof. However, suitable dispersing additive exclude polysiloxanes that are likely to release free silica during combustion; anionic dispersants whose counter-ions are metals (alkaline metals). Additionally, polyols, polyethylene-glycols, and polyethers, even though they present intrinsic dispersing properties, are not classified in this document as dispersing additives because they are in general introduced in substantial proportions and are an integral part of the base liquid. 
     Optionally, other processing additives may be included in the cleaning agent composition, such as: viscosity modifier additives (i.e. allowing the increase or reduction of the viscosity and optimize the stability of the suspension or facilitate its pumping); anti-foam additives (non-silicone substances to avoid the release of free silica during combustion); biocide additives; anti-freeze additives, etc., or mixtures thereof. 
     The abovementioned additives will be preferably of the organic type to avoid the generation of ash. All of the components, but for the descaling material, generally constitute the “liquid carrier.” 
     In embodiments where the particles are dry-sieved, the obtained descaling material is mixed with the liquid carrier, which leads to a “finished” cleaning agent. Alternatively, in embodiments where the particles are mixed with the liquid carrier to create a slurry and then filtered, a “semi-finished” cleaning agent is first formed (“semi” meaning a filtration is yet to be carried out), and then a “finished” cleaning agent is formed after filtering at the sub-millimeter level. This “in-situ crushing” operation, which can be preferably carried out in the presence of wetting agents, represents a well-known preparation mode of suspensions (or “slips”) in the ceramic industry. That is, it is known that during the fragmentation of particles, the molecules of the wetting agents are easily absorbed on the surface of the freshly formed fragments, which enhances their dispersing effect. 
     It is noteworthy that, in the case where the liquid carrier is water-based, its vaporization when introduced in the combustion system tends to lower the temperature of the combustion gases which is likely to disturb the combustion process and the control of the turbine operation. To avoid this drawback, polyethylene glycol (PEG) can be added to the water-based liquid. PEG has the double advantage of having a dispersing effect and boasting a moderate combustion heat (23700 kJ/kg). Additionally, PEG can be incorporated in concentration such that this combustion heat compensates approximately the latent heat of water vaporization (2260 kJ/kg). In this manner, the vaporization of the so addivated cleaning agent will be neither endothermal, nor exothermal but “athermal” and will not disturb the combustion process nor the turbine operation. This “athermal” condition can be realized for a PEG content in the order of 10% in mass. 
     EXPERIMENTAL STUDY 
     A systematic study was performed to study the formation and removal of ash deposits by means of a dedicated experimental rig which, based on a “HVOF” (“High Velocity Oxygen Flames”) gun fed with kerosene, allows reproducing the conditions of collision between the flows of combustion gases “targets” or “substrates” constituted of test coupons simulating hot parts within controlled temperature and speed conditions. This device was described in a paper authored by C. Verdy, M. Moliere et al: “Physics and chemistry of ash in gas turbines: the HVOF technique as a powerful simulation tool”, Proceedings of ASME Turbo Expo 2012, article GT2012-68275, Jun. 11-15, 2012, Copenhagen. Using this rig, it is possible to dope combustion gases either (i) with synthetic ash components so that to coat the target with deposits of well-defined quantities and compositions or (ii) with particles of cleaning agents of very diverse nature, so that to simulate the cleaning operations of already formed deposits. One must stress the importance of using a device capable of simulating not only the speed but also the temperature, since the mechanical properties of the particles and the substrate depend on the latter parameter. In particular, the hardness of inorganic material strongly decreases at high temperature as indicated in a paper by G. A Geach entitled: “Hardness and temperature” (International Metallurgical Review, Volume 19, 1974 pp. 255-267). 
     During this test campaign carried out on superalloy substrates, it was observed that the performance of a cleaning operation depends, for a type of defined deposit, on six parameters: (i) the mass of the injected cleaning agent, a parameter under operator&#39;s control; (ii) the hardness of the particles; (iii) their density (defined as a bulk property); (iv) their size; (v) the speed; and (vi) the firing temperature. The hardness, density, and size of the particles are intrinsic properties of the material used, while the speed and firing temperature depend on the turbine model and its operating conditions and are thus imposed by the application. 
     The cleaning efficiency increases as the first five parameters increase but decreases when the sixth one, the firing temperature, increases. It must be noted that relatively low-size particles (i.e., less than 1 μm or a few micrometers) have a negligible descaling effect. 
     Consequently, the mass of cleaning agent that must be injected on-line to descale a defined type of deposit, formed in an application which is itself defined, depends on the hardness, density and size of the particles constituting this cleaning agent. To limit the costs of the cleaning operation and reduce the emission of particles at the turbine exhaust, this quantity must be reduced. 
     EXAMPLES 
     The three examples of embodiment which follow aim at illustrating the characteristics and advantages of this invention in several non-limitative applications; the materials, additives and commercial devices which are mentioned here are taken by way of example. 
     Example No. 1 
     One wishes to carry out the on-line cleaning of a gas turbine that has a thermal input of 300 MW and features a combustion system of the can-annular type with 10 combustors; this turbine has been fouled due to an extended operation, at a firing temperature of 1100° C., with a heavy fuel oil that contains 50 ppm of vanadium and is treated with 150 ppm of magnesium (as vanadic corrosion inhibitor). After 180 hours of full load operation in these conditions, this turbine has lost approximately 8% of its rated electric power output. 
     One formulates a cleaning agent which is an “iron oxide based preparation”, using the following procedure. 
     One selects a rough descaling material which is a magnetite powder constituted by an iron inorganic provided by Rana Gruber (Norway), called “magnetite concentrate” and whose brand name is M-150T®. This inorganic material contains 99% of magnetite and has a density of 5.2 g/dm3 and a Mohs hardness of 5.5 to 6, whereby its efficiency factor is in the order of 30. As its particle size range goes up to 425 μm, one decides to sieve it through a 200 μm mesh sieve (No 24 according to NFX 11.501 or No 70 of the standard ASTM E11 whose mesh is of 210 μm) to obtain “sub-millimeter” particles. 
     In a tank one mixes the following ingredients (percentages are given in mass): 25% of M150-T previously sieved; 22.4% of “polyglycol 200” (PEG 200); 11.2% of Dispersogen FA® (non-ionic dispersing additive; supplier: Clariant); 5.6% of Dispersogen PSM® (cationic, ammonia-based, dispersing additive; supplier: Clariant); 0.3% of Efka 2526® (organic anti-foam additive; supplier: BASF); 0.2% of Myacid S2® (biocide additive; supplier: BASF); and the balance: deionised water (35.3%). 
     These ingredients are mixed for 15 minutes with a paddle stirrer. The suspension obtained is stable for more than one hour. 
     The finished cleaning agent thus obtained contains only minute traces of corrosive elements (alkaline metal content of magnetite M-150T: less than 0.2%) knowing that care has been taken to discard from the formulation anionic dispersants as well as elements that are detrimental to health and environment (bromine being present only as traces). 
     With the pumping system described above (two pumps TH632A890® and FH632A89® installed in series), one injects into the turbine, over a period of 8 minutes, a volume of 110 litres of this cleaning agent which, considering the density of the suspension (1.3 g/dm3), represents an approximate mass of 25/100*99/100*110/1.3=20.9 kg of magnetite. This injection is carried out under a pressure of approximately 32 bars, with a constant, total flow of 13.8 l/mn, through ten identical high-pressure injectors that allow to uniformly distributing the injection over the ten combustion chambers of the turbine, in a manner that the descaling material attains the largest possible surface fraction of the fouled hot parts. 
     This cleaning treatment enables recovering 7.3% of the power output, which corresponds to a recovery rate of approximately 91% of the power loss which was caused by the accumulation of magnesium/vanadium ash on the turbine hot parts. By way of comparison, the injection of 50 kg of a ligneous material based cleaning agent (fragments of apricot cores), provides a recovery of only 1.7% of the power lost by fouling (recovery rate of 21%). 
     Example No. 2 
     Here the cleaning agent is also an “iron oxide based preparation” whose rough descaling material is this time a hematite based inorganic (Fe 2 O 3 ) procured from the same supplier and called “hematite concentrate H-400®”. The mass composition of this inorganic is: 90% hematite, 5% water; the other minor components are SiO 2 , Al 2 O 3  and CaO (in the form of calcium aluminosilicates that are EHS compliant). Hematite has a density of 5.25, a hardness of 5.5-6 and an average efficiency factor of 31.5. As its size particle range goes up to 1 mm, one crushes it dry with for example a disc mill “Premium Disk Mill®” (supplier: Fritsch) and one sieves it through a sieve of mesh 200 or 210 μm, to obtain “sub-millimeter” particles. The particles arrested by the 200 or 210 μm sieve filter represent about 10% of the inorganic mass introduced, i.e. a mass of 2.25 kg which can be kept for a next cleaning operation. 
     The same formulation of liquid carrier is made as in example No 1: one puts and mixes in a tank, using a paddle stirrer, the liquid carrier and the previously crushed H-400 material in a mass ratio of 85/25. The suspension that results from it and which constitutes the finished cleaning agent, contains 25% in weight of the descaling material, i.e. 25/100*90/100=22.5% of Fe 2 O 3 ; it is also stable for more than one hour. The so prepared cleaning agent contains only minute traces of corrosive elements (alkaline metal content of magnetite H-400: less than 0.2%) knowing that care has also been taken to discard from the formulation anionic dispersants as well as elements that are detrimental to health and environment (bromine being present only as traces). 
     With this cleaning agent, one proceeds with the on-line descaling of a turbine identical to that of example No. 1 and which operates in the same conditions: after having operated at full load and for 190 hours with the abovementioned fuel, this turbine had lost approximately 9% of its rated power output. 
     By means of the same pumping system as in example No 1, a volume of 130 litres of cleaning agent is injected into the turbine, over a period of 10 minutes, which, considering the density of the suspension (1.3 g/dm3), represents an approximate mass of 22.5/100*130/1.3=22.5 kg of descaling material. This injection is carried out under a pressure of approximately 28 bars, with a constant total flow of 10 l/mn, through ten identical high-pressure injectors that allow to uniformly distributing the injection to the ten combustion chambers of the turbine, so that the descaling material reaches the largest possible surface fraction of the hot parts. 
     This treatment allows recovering 8.5% of power output, which corresponds to 94% of the power loss which was “lost” by fouling. 
     Example No. 3 
     One formulates a cleaning agent which is based on wollastonite (CaSiO 3 ) that has a density of 3.1 g/dm 3 , a hardness of 5 and an efficiency factor of 15.5, using the following procedure. 
     The rough descaling material is the inorganic material NYCOR R® (supplier: NYCO, USA) containing 98% of wollastonite, the remaining 2% not containing detrimental elements. This inorganic has a particle size range that goes up to 700-800 μm: one decides to proceed with an “in-situ crushing”. 
     The following mixture is prepared (percentages are given in mass): 30% of NYCOR R®; 8% of “polyglycol 200” (PEG 200); 6% of Dispersogen FA® (non-ionic dispersing additive; supplier: Clariant); 4% of Genamin CC 100® (cationic, ammonia-based, dispersing additive; supplier: Clariant); 0.3% of acetic acid (shear-thinning-additive); 0.3% of Efka 2526® (organic anti-foam additive; suppliers: BASF); 0.2% of Myacid S2® (biocide additive; suppliers: BASF); and the balance: deionised water (51.2%). 
     Here also, all the additives used contain only minute traces of potentially corrosive elements (alkaline metal content less than 0.03%) or adverse to health and environment, knowing that care has been taken to separate the anionic dispersants. 
     This mixture is crushed for 5 minutes with an Ultraturrax® crusher/mixer (supplier: IKA) at a rotation speed of 3400 rpm. The resulting suspension that represents the semi-finished cleaning agent, contains 30% in weight of descaling material, i.e. 0.98*0.30=29.4% of CaSiO 3 ; it is stable for approximately 40 minutes. 
     With this cleaning agent, one carries out an on-line descaling of a turbine that is identical to that of example No 1 and which operates in the same conditions: after having run at full load and for 160 hours with the abovementioned fuel, this turbine had lost approximately 7% of its rated power output. 
     One injects into the turbine, over a period of 8 minutes, a volume of 120 litres of this semi-finished cleaning agent through a 297 μm mesh sieve filter (sieve No 50 as per standard ASTM E11), using the same pumping system. Considering the density of the suspension (1.35 g/dm3), this represents an approximate mass of 30/100*98/100*120/1.35=26.1 kg of wollastonite. This injection is carried out under a pressure of approximately 30 bars, with a constant, total flow of 15 l/mn, through ten identical high-pressure injectors that allow to uniformly distributing the injection to the ten combustion chambers of the turbine, so that the descaling material reaches the largest possible surface fraction of the hot parts. The particles arrested by the 315 μm sieve filter represent approximately 7% of the mass of the inorganic material introduced, i.e. 0.8 kg which can be kept for a next cleaning operation. The actual mass of inorganic injected is thus of 25.3 kg. 
     This treatment enables the recovery of 6% of power, which corresponds to the collection of approximately 86% of the power output loss caused by the accumulation of magnesium/vanadium ash on the hot parts of the turbine. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.