Description:‘DISCHARGE ELECTRODE MATERIAL AND METHOD OF MANUFACTURING THEREOF FOR DRY ELECTRO-STATIC PRECIPITATORS IN POWER GENERATION SECTOR’

FIELD OF THE INVENTION:
[001] This invention pertains to the selection of electrically conducting, high temperature strength and corrosion resistant and available stainless steel 317 LMN material in wire form for discharge electrode fabrication in electrostatic precipitators. Such electrostatic precipitators used in power generation sector which is large in size and are typically of the dry-type. These discharge electrodes are useful for inducing a specific charge to the flue gas dust particles to attract and separate to avoid the releasing into atmosphere.

BACKGROUND/PRIOR ART OF THE INVENTION:
[002] Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.

[003] In power generation sector, specifically, in thermal power generation, coal is major fossil fuel used for generating heat in boilers by combustion which in turn converts water to steam and work done by this steam is converted to electricity. A lot of flue gas is emanated by combustion of coal in boilers. This flue gas consists of sub-micron sized particles which are harmful to the environment. Hence, electrostatic precipitators are commonly used in power generation sector for scrubbing particulate matter from flue gas coming out of boilers. This helps in cleaning the exhaust flue gas from the boilers, thus, reducing the pollution in the atmosphere.

[004] As the flue gas flows through the electrostatic precipitators, it is channelled towards negatively charged discharge electrodes. The high negative charge applied to the discharge electrodes causes electric corona discharge in the air passing around these electrodes, thus, ionizing various particles present within them. Subsequently, these charged particles are further channelled towards collector plates that are electrically grounded, leading to their deposition over the plates. These collector plates are either mechanically vibrated (in dry type) or cleaned with a stream of water or mist (in wet type), thus, dislodging the deposits.

[005] The discharge electrodes are typically wires, wires having fins, wires having helical screw fins, wire with attached plates and sharp ends, rods, wires with secondary and tertiary wire branches, semi-circular wires with plates attached and sharp end, fibres with specific or random orientations, membranes made with such fibres, rods with rotating blades or any combinations of the aforementioned components. The material of construction is ambiguously described as electrically conducting materials but include rolled carbon steel, stainless steel, metals, composites with carbon fibres, carbon nanotubes and thermosetting resins with dopants, ceramics, semiconductors or a combination of them. They are usually located at the exit path of the channel or pathways from which flue gas enters the electrostatic precipitator.

[006] In power generation sector, flue gas consists of high sulphur content which can cause stress-corrosion cracking, soot and fly ash content which is erosive in nature and organic content. None of the prior art describes in detail a specific material of selection used nor the reason for selection other than providing broad-spectrum of criteria such as electrical conductivity and high corrosion resistance. Therefore, a detailed and meticulous material selection procedure was conducted in this work to address the selection of a specific material, i.e., 317 LMN, for application in discharge electrodes of dry type electrostatic precipitators for power generation sector.

[007] Now, reference may be made to the following known arts:
[008] “Electric precipitator module desulfurization equipment including the same”, KR102079796B1, 20 Feb. 2020. This patent describes a novel design of discharge electrode assembly in an electrostatic precipitator. This novel design involves a collection of discharge electrodes assembled in an inclined manner in the gas flow path and are also capable of being washed with liquid in case of wet collection module. No information on the material of construction is available from this patent description or claims.

[009] “Discharge electrode or electrostatic precipitator and electrostatic precipitator using the same”, JP2019177365A, 17 Oct. 2019. This patent claims utilizing a novel design assembly of discharge electrode wires for a dry electrostatic precipitator. This design assembly prevents failure of single long discharge electrode by reducing the load on each member electrode. The material of construction for the electrode was mentioned in the description as stainless steel material or but is not specific.

[0010] “Wet electrostatic precipitator”, EP2411154B1, 09 Aug. 2017. This patent describes utilization of a composite material comprising of a blend of carbon fibres/ fibre-glass and thermosetting resin for discharge electrode applications in wet electrostatic precipitator construction, thereby, providing electrical conduction, corrosion resistance, resistance to sparks, erosion resistance, arcing resistance in dry and saturated mist environments at elevated temperatures. Inventors also claim that this material can reduce dead weight of the electrostatic precipitators in comparison with stainless steels or titanium alloys. This is not addressing the dry electrodes of ESP.

[0011] “Discharge electrode wire, and wet type dust collector”, JP2016043311A, 04 Apr. 2016. This patent describes application of an electrode material for utilization as a core wire or a lead member of the discharge electrodes or electrode assembly for wet electrostatic precipitator applications only. A generic claim of utilization of metallic material as a core material having a tensile strength of 400 MPa or more is made. This patent also mentions about wet type not a dry type.

[0012] However none of the above fulfils requirements of the present invention, for which it has been devised.

OBJECTS OF THE INVENTION:
[0013] Primary object of the invention is to provide Discharge electrode material and method of manufacturing thereof for dry electro-static precipitators in power generation sector.

[0014] Another object of the present invention is to provide method of manufacture 317 LMN stainless steel to fabricate discharge electrodes for electrostatic precipitator applications, which is simple.

[0015] This is achieved by understanding material degradation in electrostatic precipitators and applying alloy design fundamentals for selection of an appropriate electrically conducting, high temperature corrosion resistant stainless steel. By utilizing this material for fabrication of discharge electrodes in electrostatic precipitators, a combination of high strength, high pitting corrosion resistance, high stress corrosion cracking resistance and high resistance to sulphur environment is achieved.

[0016] These and other objects and advantages of the present invention will be apparent to those skilled in the art after a consideration of the following detailed description taken in conjunction with the accompanying drawings in which a preferred form of the present invention is illustrated.

SUMMARY OF THE INVENTION:
[0017] One or more drawbacks of conventional systems and process are overcome, and additional advantages are provided through the apparatus/composition and a method as claimed in the present disclosure. Additional features and advantages are realized through the technicalities of the present disclosure. Other embodiments and aspects of the disclosure are described in detail herein and are considered to be part of the claimed disclosure.

[0018] According to this invention, there is provided a discharge electrode made of 317 LMN material for application in dry type electrostatic precipitators comprising of the following:

Carbon ( C ) max. 0.020%
Chromium (Cr) 17 to 19%
Manganese (Mn) 1.5 to 2.1%
Molybdenum (Mo) 4 to 4.5%
Nitrogen atomic form (N) 0-0.3%
Nickel (Ni) 12-14%
Phosphorus (P) max. 0.03%
Sulphur (S) max. 0.005%
Silicon (Si) 0.35-0.5 %

[0019] The discharge electrode is manufactured using cold-drawn method with soap lubrication and final de-coating for maintaining surface imperfections less than 20 microns and for improving tensile strength to 1050 +/- 75 MPa.

[0020] The present invention involves the following:
- With an application in power generation sector;
- With shape of the electrode in wire, wire with any further attachments in any form such as rod, plate, helical pitch and further wire branches;
- With shape of a mesh or a membrane;
- With shape of a spring;
- Any component of electrostatic precipitator discharge electrode containing 317 LMN material;
- Any component of electrostatic precipitator discharge electrode made up of a composite material containing 317 LMN material;
- Any component of electrostatic precipitator discharge electrode assembly containing 317 LMN material;

[0021] The discharge electrode material of invention is resistant to flue gas corrosion, erosion and other corrosive processes that are imminent in electrostatic precipitators.

[0022] The discharge electrode material of invention is used in wire form due to the ability to provide torsion, tension and for making it in the form of individual or assembly of (i) wires in as-drawn form or (ii) springs which are typical shapes used for discharge electrode assembly.

DETAILED DESCRIPTION OF THE INVENTION:
[0023] The present invention should be understood, however, that it is not intended to limit the disclosure to the particular forms disclosed, but on the contrary, the disclosure is to cover all modifications, equivalents, and alternative falling within the scope of the disclosure.

[0024] Electrostatic precipitator components mainly consist of inlet filtration equipment, flow guiding components, discharge electrodes, collector electrodes, mechanical vibration system and power supply system. The inlet filtration equipment, filters the inlet exhaust flue gas from boiler and the flow guiding components direct exhaust flue gas from the boiler towards the discharge electrodes. The high voltage system is responsible for charging the discharge electrodes and collector electrodes. The discharge electrodes possess high negative charge and are responsible for ionizing their surrounding dust particles. This effect of charging or ionizing the particles surrounding the discharge electrodes is termed as Corona effect. These dust particles then move towards either electrically ground or positively charges collector electrodes and get deposited over them. The mechanical vibration system, provides a series of vibrations to the collector electrodes leading to dislodging of the collected particles which subsequently fall into the collection pit.

[0025] The material selection philosophy in the present case considers the corrosiveness of the flue gas environment, material operating temperature (300o C approximately) and differential expansion of the material in relation to the assembly components which hold electrodes made up of the intended material in place. Thermal fatigue is also an important property considering thermal fluctuations these electrodes encounter. Material should also be free of slag inclusions. Considering corrosiveness of the environment, resistance to pitting, stress-corrosion cracking and erosion resistance to fly ash are required.

[0026] Commercially available alternative metallic materials that can be used are carbon steels, stainless steels and chrome molybdenum alloys. Carbon steels are cheapest of all the options available but they are prone to pitting and stress corrosion cracking phenomena. Stainless steels are highly corrosion resistant and hence, are preferred to flue gas environments. Chrome molybdenum alloys offer excellent pitting and stress corrosion cracking resistance but are very expensive.

[0027] A few materials that can be used from the above categories of stainless steels and chrome molybdenum alloys along with their characteristics are briefed here:
1. SS316L, SS316LN are an austenitic stainless steel with iron as base element and major alloying elements as chromium and nickel which provide high oxidation resistance, and molybdenum to provide improved pitting resistance. Nitrogen is added in SS316LN to provide strength increment. However, these are inferior in performance to SS904L.

2. SS316Ti is an austenitic stainless steel with iron as base element and major alloying elements as chromium and nickel which provide high oxidation resistance, in which molybdenum is added to provide improved pitting resistance, and titanium is added to improve intergranular corrosion resistance and also titanium combines with carbon to form titanium carbide which provides strength.

3. SS904L is an austenitic stainless steel with iron as base element and major alloying elements as chromium and nickel which provide high oxidation resistance to the alloy. Molybdenum is added to provide improved pitting resistance and copper is added to provide improved mechanical strength at elevated temperatures through precipitation hardening. Manganese addition in this alloy leads to stabilization of austenitic phase further.

4. AISI 202 is an austenitic stainless steel with iron as base element and major alloying elements as chromium, manganese and nickel. Chromium provides high temperature oxidation resistance, while manganese and nickel stabilize austenitic phase and provides strength. This is a potential alternate material for better performance and cheaper discharge electrodes, but has inferior corrosion resistance compared to SS904L, SS316L, SS316LN and SS316Ti.

5. Hastealloy 2000 is a nickel alloy with nickel as base element and, chromium, molybdenum and cobalt as major alloying elements. This alloy has better corrosion resistance compared to SS904L, SS316L, SS316LN and SS316Ti, due to higher percentage of chromium, molybdenum and cobalt elements. however, it is very expensive.

[0028] Presently, SS904L material which is a stainless steel material is being used in wire form for discharge electrode applications in electrostatic precipitators for power generation sector. The cost of this material is relatively high and hence, a material selection procedure is followed to replace this material with an alternate cheaper material with same or better performance as discharge electrodes.

[0029] From a detailed study of the compositions, mechanical properties and corrosion behaviour of the commercially available metallic materials, the following can be summarized as requirements for selecting alternate material:

1. Similar thermal expansion coefficient in comparison to SS316 grades and SS 904L is required.
2. Chromium percentage of 17 to 20 weight percent in addition to the alloy improves oxidation resistance.
3. Molybdenum percentage of 4 to 5 weight percent in addition to the alloy improves pitting resistance.
4. Nitrogen of 0.1 to 0.3 weight percentage in addition to the alloy improves tensile strength of the material. This can improve the weight bearing ability of the electrodes, thus, leading to usage of smaller cross-sections and significant savings in terms of overall weight is required. Apart from this, nitrogen also improves stress corrosion cracking resistance.
5. Manganese of maximum 2 weight percentage in addition to the alloy prevents grain boundary embrittlement by forming Mn3C carbide phase, improves fatigue limit, improves resistance to sulphur environment, stabilizes austenite degradation temperature and controls inclusion rate in the alloy.

[0030] Hence, 317 LMN stainless steel is selected which meets all the above requirements and possesses all the required properties. 317 LMN stainless steel material is also known by the following standard nomenclature as per various standards: SS 317LMN, Type 317LMN, WNR 1.4439, UNS S31726, AISI 317LMN, Grade 317LMN and ALLOY 317LMN.

[0031] The composition of exemplary embodiment of 317 LMN is mentioned herein below:

Carbon ( C ) max. 0.020%
Chromium (Cr) 17 to 19%
Manganese (Mn) 1.5 to 2.1%
Molybdenum (Mo) 4 to 4.5%
Nitrogen atomic form (N) 0-0.3%
Nickel (Ni) 12-14%
Phosphorus (P) max. 0.03%
Sulphur (S) max. 0.005%
Silicon (Si) 0.35-0.5 %

[0032] Though the corrosion resistance of 317 LMN stainless steel wires is much better compared to other materials mentioned above and 317 LMN stainless steel being non-magnetic, the minimum required tensile strength for a material to be applied for discharge electrodes applications is 900 MPa. The commercially available 317 LMN stainless steel possesses much lower tensile strength than the required limit, i.e., 550 MPa approximately, thus, leading to its disqualification for discharge electrode applications. Therefore, in order to apply this material for discharge electrode applications, a novel procedure for manufacturing these electrode materials was developed. The novelty of this work lies in the procedure for manufacturing 317 LMN stainless steel material. A procedure has been adapted to improve the tensile strength of the discharge electrode material by using the cold metal-drawing with soap lubrication. Subsequent to this metal-drawing process, the final drawn material is decoated.
[0033] The added advantage apart from improving tensile strength to > 1050 MPa +/- 75 MPa is that the surface imperfections on the finished/ final material reduce and surface roughness lowers to below 20 microns, thus, making the 317 LMN material manufactured by metal drawing process suitable for discharge electrode applications. Also, Chromium content is maintained to 17.5 weight % (in between 17 to 18 weight %) for reducing the cost of material without compromising on the properties.

ADVANTAGES OF THE INVENTION:

[0034] SS 317 LMN stainless steel is therefore selected as the best available metallic material which can be utilized for discharge electrode applications for electrostatic precipitators after adoption of new manufacturing method to improve tensile properties with superior surface finish requirements. 317 LMN stainless steel when used as a discharge electrode material can lead to savings in material cost upto ~75% in comparison with traditionally used SS904L material. The amount of material used as discharge electrodes is approximately 35 tons. Therefore, utilization of the material being proposed for patent will lead to huge savings on installation costs.

[0035] Each of the appended claims defines a separate invention, which for infringement purposes is recognized as including equivalents to the various elements or limitations specified in the claims. Depending on the context, all references below to the "invention" may in some cases refer to certain specific embodiments only. In other cases, it will be recognized that references to the "invention" will refer to subject matter recited in one or more, but not necessarily all, of the claims.

[0036] Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member can be referred to and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group can be included in, or deleted from, a group for reasons of convenience and/or patentability. When any such inclusion or deletion occurs, the specification is herein deemed to contain the group as modified thus fulfilling the written description of all groups used in the appended claims.

[0037] It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases “at least one” and “one or more” to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles “a” or “an” limits any particulars claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an” (e.g., “a” and/or “an” should typically be interpreted to mean “at least one” or “one or more”); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of “two recitations,” without other modifiers, typically means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogues to “at least one of A, B and C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B and C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). In those instances where a convention analogous to “at least one of A, B, or C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, or C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase “A or B” will be understood to include the possibilities of “A” or “B” or “A and B”.

[0038] The above description does not provide specific details of manufacture or design of the various components. Those of skill in the art are familiar with such details, and unless departures from those techniques are set out, techniques, known, related art or later developed designs and materials should be employed. Those in the art are capable of choosing suitable manufacturing and design details.

[0039] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. It will be appreciated that several of the above-disclosed and other features and functions, or alternatives thereof, may be combined into other systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may subsequently be made by those skilled in the art without departing from the scope of the present disclosure as encompassed by the following claims.

[0040] The claims, as originally presented and as they may be amended, encompass variations, alternatives, modifications, improvements, equivalents, and substantial equivalents of the embodiments and teachings disclosed herein, including those that are presently unforeseen or unappreciated, and that, for example, may arise from applicants/patentees and others.

[0041] While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.
, Claims:WE CLAIM:

1. Discharge electrode material for dry electro-static precipitators used in power generation sector comprising of:

Carbon ( C ) max. 0.020%
Chromium (Cr) 17 to 19%
Manganese (Mn) 1.5 to 2.1%
Molybdenum (Mo) 4 to 4.5%
Nitrogen atomic form (N) 0-0.3%
Nickel (Ni) 12-14%
Phosphorus (P) max. 0.03%
Sulphur (S) max. 0.005%
Silicon (Si) 0.35-0.5 %

2. Method of manufacturing Discharge electrode material for dry electro-static precipitators used in power generation sector comprising steps of:

- cold drawing of 317 LMN stainless steel with soap lubrication to improve the tensile strength, wherein particle size of soap is reduced;
- de-coating the drawn material by washing with solvent including water.

3. The Discharge electrode material and Method for manufacturing thereof as claimed in claim 1 and 2, which maintains surface imperfections less than 20 microns and improves tensile strength to 1050 +/- 75 MPa.