Corrosion Analysis of Air Pre-Heater Tubes of CFBC Boiler and Improvement in Operation
Abstract—
This paper presents an approach for the optimization of air pre-heater design with inline & staggered tube arrangement. The tube failure of an air pre-heater in Gujarat Industries Power Company Limited (SLPP) is one of the main reasons for higher unit heat rate & is responsible for deterioration in boiler efficiency. The main problem of air pre-heater is the leakage of air to the flue gas side & thereby resulting in poor thermal performance. The higher ash content in Indian coal also adds to the problems associated with tubular air pre-heater.
Air pre-heaters are designed to meet performance requirements with consideration of highly influencing parameters viz. heat transfer, leakage and pressure drop. In the present work the performance of tubular air pre-heater is minimized tube failure by using different material of steel or by reducing H2SO4 formation from the flue gas while power plant taking start-up. In the flue gas system of a boiler plant, gaseous acidic substances (such as SOX and HCl) included in high-temperature exhaust gas condense into dew, depending on the moisture content of the gas and the surface temperature of a steel material to which it comes into contact, and corrode the material
I. INTRODUCTION
Surat Lignite Power Plant (SLPP) , a power plant of Gujarat Industries Power Company Ltd promoted by Gujarat public sector undertakings viz. Gujarat Electricity Board (presently Gujarat Urja Vikas Nigam Ltd.), Gujarat State Fertilizers and Chemicals Ltd, Gujarat Alkalis and Chemicals Ltd. and Govt.of Gujarat. Initially the Company has set up a 145 MW gas based Power Plant at Vadodara basically to cater to the power requirement of promoting companies and subsequently ventured into IPPs of 160 MW Naphtha based power plant at Vadodara besides the existing unit and a 250 MW lignite based Power Plant at village Nani-Naroli, taluka Mangrol, Dist.Surat. The lignite based Power plant at Surat. Comprises of 2 x 125 MW units with boilers of 390 Tonne/hr capacities. Boilers of SLPP are of CFBC technology.
II. TECHNOLOGY
CFBC is a class of Fluidized Bed Combustion (FBC) A.technology. FBC initially used in the chemical and process industries was applied to the electricity industry because of its perceived advantages over competing combustion technologies. The project of GIPCL at Mangrol, Surat Lignite Power Project (SLPP) has two 390 t/h CFBC boilers. These are the biggest CFBC boilers in India as well as Asia. These are the first CFBC boilers used in India in a commercial power utility.
FBC technology can effectively use a wide range and B.quality of coals improving combustion efficiency and environmental emissions. Fuel for combustion is admitted as
particles up to 10 mm size in CFBC boilers. Combustion takes place at lower temperatures of 800 – 900 deg C resulting in reduced Nox formation compared with Pulverized Coal Combustion (PCC). SO2 emissions can be reduced by the use of lime as sorbent. A fluidized bed that is operated at velocities in the range of 4 to 6 m/s is referred to as a Circulating Fluidized Bed (CFB).The CFB system is a highly efficient gas-solids process initially developed and patented by M/s. Lurgi of Germany for the calcinations of aluminium trihydrate. This technology has been successfully used in a number of applications to carry out endothermic and heterogeneous gas-solids reactions.
Problems In Operations C.
When sulphur (S) –containing fuels, including heavy oil, LNG, and coal, are burnt, sulphur oxides (Sox) are produced, part of which is turned to SO3. When exhaust gas temperature drops to below a dew point or when the gas is in contact with a low-temperature wall, SO3 and H2O, contained in the gas, are combined to form highly-concentrated sulphuric acid that corrodes steel. This is sulphuric acid dew corrosion, which severely corrodes not only carbon steel but also stainless steel unlike normal atmospheric corrosion.
Sulphuric Acid Dew Corrosion-1
In the flue gas system of a boiler plant, gaseous acidic substances (such as SOX and HCl) included in high-temperature exhaust gas condense into dew, depending on the moisture content of the gas and the surface temperature of a steel material to which it comes into contact, and corrode the material. This phenomenon is called acid dew corrosion. It constitutes a problem in equipment such as the flue gas treatment and heat recovering facilities for heavy oil-fired boilers and heating furnaces of chemical plants: more specifically, economizers, air pre-heaters, dust catchers (electrostatic precipitators and bag filters), gas ducts, the inner ducts of smokestacks and the like 1, 2). As is described below, acid dew corrosion caused by SOX and/or HCl occurs lately also in waste incineration plants and similar
Corrosion Analysis of Air Pre-Heater Tubes of CFBC Boiler (Slpp) and Improvement in Operation
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III. FALLING, PLUGGING AND CORROSION
Deposits in air pre-heaters are insisted by condensation of acid or moisture from flue gas on metal surface operating act temperature below dew point, other things reaming same, degree of falling depends on air pre-heater heating element metal surface. Minimum metal temperature occurs at the cold end, where, as a result, most falling and corrosion occur.
As cold contains less sulphur, corrosion is not normally as much a problem as falling and hence lower exit gas temperature to a level of 120oc is possible. But in the case of oil firing, the corrosion and plugging due to corrosive product of combustion are very common. The gas outlet temperature and/or air inlet temperature has to be raised to restrict the corrosion to the permissible level. Operating the oil fired boiler at very low excess air reduces the acid formation and hence corrosion
.
Corrodes Steel
IV. ACID DEW CORROSION OF FLUE GAS TREATMENT EQUIPMENT
Heavy oil- and coal-burning boilers (sulphuric acid dew corrosion) Dew corrosion occurs in the flue gas treatment equipment (a gas duct, an air pre-heater, a smoke stack, etc.) of thermal power plant sand waste incineration plants as a result of the condensation of corrosive components included in the exhaust gas. In the case of a boiler that burns heavy oil or coal, S in the fuel turns into sulphur oxide (SOX) and, if the inner surface temperature of a steel plate of the smoke duct is lowers than the dew point of sulphuric acid, sulphuric acid condenses into dew, and there occurs sulphuric acid dew corrosion. The condensation is a phenomenon resulting from the thermal and mass transport from the flue gas to the steel plate surface, and the driving force of the condensation reaction is the differences in temperature.
V. WASTE INCINERATION AND RECYCLING PLANTS (SULPHURIC AND HYDROCHLORIC ACID DEW CORROSION)
Schematically illustrates how sulphuric and hydrochloric acid dew corrosion occurs in the flue gas system of a waste incineration plant. The exhaust gas from a waste incineration plant contains SOX (50 to 1,000 ppm), HCl (100 to several thousand ppm) and moisture by a comparatively large percentage (30 to 50%), and thus, when the steel surface temperature of the flue gas treatment equipment is sufficiently lower than the dew points of sulphuric and hydrochloric acids, these acids form.
Fig. 3: Schematic diagram of acid dew corrosion mechanisms (flue gas:SO3: 6ppm, HCl: 300ppm, H2O: 30%)
VI. OBJECTIVE
The corrosion prone area of 1.00mtr length is firmly protected from corrosion due to crevice, galvanic, uniform and progressive corrosion. This not only would prevent unnecessary wastage of material but also would prevent undesired shut down and efficiency loss due to the APH tubes failure. In future only the weaker section of the tube alone could be replaced, Instead of discarding the entire length of the tubes. The actual assessment of benefits would be well measured from plant experience and expertise in the field and art.
Three stage process on 1.00 m length tube for A P H. A.
- Length of the APH tube is 7 m.
- Damage of tube area due to corrosion observed is less than 1m.
- Balance length of the tube is found to be absolutely intact without any reduction of thickness or no severe effective corrosion was observed.
- Further it was also understood that the entire length of the APH tube is quarantined for the damage caused due to corrosion in less than 1mtr. Length of the tubes.
The 1.00 m. tube will undergo the following process. B. - A hard diffused layer of upto10micron is prepared.
- over and above an oxidized layer of up to 3micron is prepared
- Followed by a double coat Thermo bond WCT-E treatment which is baked for inter molecular bonding of the element.
- This treatment is to protect the APH tubes from sulphur chloride condensation and erosion due to flue gas hitting at high velocity.
VII. ADVANTAGE
The corrosion prone area of 1.00mtr length is firmly protected from corrosion due to crevice, galvanic, uniform and progressive corrosion. This not only would prevent unnecessary wastage of material but also would prevent
Corrosion Analysis of Air Pre-Heater Tubes of CFBC Boiler (Slpp) and Improvement in Operation
(IJSRD/Vol. 2/Issue 03/2014/225)
undesired shut down and efficiency loss due to the APH tubes failure.
In future only the weaker section of the tube alone could be replaced, Instead of discarding the entire length of the tubes.
The actual assessment of benefits would be well measured from your experience and expertise in the field and art.
VIII. CONCLUSION
The treatment prescribed on the tubes is in consideration with the condition prevailing at the site and the information provided by you by virtue of experience.
This treatment on tubes could enhance the life of the tubes exorbitantly minimum four times from the prevailing condition; however the actual assessment and quantification could be made on trial.
Further the future handling of the repair if any would become easy to handle
Lump Sum Cost Of Three Stage Treatment On 1.00 M A.Length Tube For Aph
First stage: preparing a diffused hard layer of up to 10 micron/tube up to 1.00 m. = Rs.100/-
Second stage: preparing an oxidized layer up to 3 micron /tube up to 1.00 r. = Rs.30/-
Third stage: application of Thermo bond WCT- E up to 1.00 m/tube = Rs.270/-
Adhesive for fixing the sleeve Rs.20/- per unit Total Rs.420/-per unit
Internal diameter Ф 51 mm to have smooth finish (600 grade emery sheet) wall thickness of 4 mm per side approx.Sleeve to accommodate 50.8 mm of the APH tubes from both the ends.
Moc: Astm A-100 Grade B Seamless Cold Drawn Tube B.
Cost of sleeve made on tailored drawn tubes and machined to the required specification as per the drawing Rs.300/-
Corrosion resistance coating on sleeve to improve the shelf life Rs.25/- Total cost: 325/-
Total Wear Cote Approx. Rs.75000/- C.
By undergoing the three stage treatment the life of APH tube is extended several times from its original condition. The estimated life is expected at least 4times from its original status or replacement. The conclusion is substantiated based on the study and test conducted on salt spray chamber accelerated test which has crossed more than 250hrs nonstop without any trace of brown rust.
The unaffected 6mtr length of the tubes could be left undisturbed along with the sleeves on subsequent replacement.
Further there is a possibility to reuse cold bonded sleeves on subsequent installation.
Laborious removal and replacement of the entire 7mtr length of the APH tubes every two years is eliminated.
Efficiency loss due to the APH tubes failure is eliminated.
Progressive corrosion is avoided by using the cold bonded sleeve in between the APH tube which gives an extended life to the tubes.
Current cost for replacement of damages tubes (pa bottom)
(Rs. Sixteen lacs ninety nine thousand seven hundred fifty six & paisa nine only)
Tube dimension: 50.5odx7070LX2.08&4Thick No. of tubes : 3219 Nos.
MOC: BS6323 Part5, ERW2 GZF D.
By this process we can save approx 10 lacks per year of material damage and we can also able to minimize the losses of energy.
Air Pre-Heater Tube Modification Is Under Process E.
Fig. 4: APH Tubes and S.S Bandeging in APH Tubes Of UNIT #1
Figure 1.4 shows that Metallurgy of the tube improved from carbon steel to Corten steel of material ASTM A423 Gr.1 in cold end of APH blocks of both PA and SA. Provision of higher thickness tubes of bottom 3 rows and 6 columns at both extreme ends. Bandaging the cold ends of APH tubes using SS 304 L sheets of 0.1mm thick and size 300×240 using spring band clamps of SS 304L material.
IX. CONCLUSION
The corrosion prone area of 1.00mtr length is firmly protected from corrosion due to crevice, galvanic, uniform and progressive corrosion.
Corten steel of material ASTM A423 Gr.1 not only would prevent unnecessary wastage of material but also would prevent undesired shut down and efficiency loss due to the APH tubes failure
