Circulating Fludize bed combustion (cfbc) boiler


Presentation Description

basic principal of cfbc boiler, basic mechanism of failure , suggested solution for prevent it..


By: jaywin7 (4 month(s) ago)

300mw power plant. Where?

By: SACHIN.PYASI (4 month(s) ago)

Hello Sir, I am Sachin Pyasi Sr. Engineer 300 MW power plant please send copy on my mail

Presentation Transcript

“Water tube leakages in cfbc boiler” : 

“Water tube leakages in cfbc boiler” Prepared By: Jaivin Ghelani Kaushal Dhameliya


CONTENT OF PRESENTATION Introduction (Problem Identification) Objectives Literature Review Methodology Implementation Of Methodology Work Review/ Summary


INTRODUCTION TO CFBC BOILER CFBC boiler work on principle of w hen an evenly distributed air or gas is passed upward through a finely divided bed of solid particles such as sand supported on a fine mesh, the particles are undisturbed at low velocity. As air velocity is gradually increased, a stage is reached when the individual particles are suspended in the air stream – the bed is called “fluidized”. In circulating fluidized bed combustion boiler, generally two types of air primary as well as secondary are feeded in the boiler. A primary air is introduced through bottom of the bed. The bottom of the bed supported by water cooled membrane walls with specially designed air nozzles which distributes the air uniformly. The fuel and limestones are fed into the lower bed. Carbon particles in the fuel are exposed to the combustion air. The balance of combustion air is introduced at the top of the lower, dense bed. This staged combustion limits the formation of nitrogen oxides.


INTRODUCTION The bed fluidizing air velocity is greater than the terminal velocity of most of the particles in bed and thus fluidizing air elutriates the particles through combustion chamber to u-beam separator at furnace exist. The captured solids including any unburned carbon and unutilized carbon oxide, are re-injected directly back into combustion chamber without passing through an external recirculation. This internal solids circulation provides longer residence time for fuel and limestone, resulting in proper combustion and improved sulphur capture.




OBJECTIVE Objectives of the erosion control scheme are follows: To reduce localized erosion. To reduce tube leakages (breakdown). To reduce replacement of tubes in annual shutdown. To increase life of boiler tubes.

Failure mechanisms: 

Failure mechanisms Primary Failure Mechanisms Stress rapture Water-side corrosion Fire side corrosion Erosion LITERATURE REVIEW

Failure mechanism: 

Failure mechanism Stress rapture: Short term over heating High temperature creep

Failure Mechanism: 

Failure Mechanism Short term over heating : Symptoms: Failure results in a ductile rupture of the tube metal and is normally characterized by the classic “fish mouth” opening in the tube where the fracture surface is a thin edge. Causes: • Blockage of tube internally • Loss of boiler coolant circulation or low water level • Loss of coolant due to an upstream tube failure • Over firing or uneven firing of boiler fuel burners

Failure mechanism: 

Failure mechanism High temperature creep: Symptoms: The failed tube has minimal swelling and a longitudinal split that is narrow when compared to short-term overheat. Tube metal often has heavy external scale build-up and secondary cracking. Causes: • Partial blockage by debris, scale, or deposits • Exposure to radiant heat • Before the change to a higher grade material • Just above the final outlet header • Exposure to high gas temperature due to blockage of gas passages or lining • Have incorrect grade of steel material • Have higher stresses due to welded attachments

Failure mechanism: 

Failure mechanism Water-Side Corrosion Caustic Corrosion Hydrogen Damage Pitting (Localized Corrosion) Stress Corrosion Cracking

Failure mechanism: 

Failure mechanism Caustic corrosion: Symptoms: Localized wall loss on the inside diameter (ID) surface of the tube, resulting in increased stress and strain in the tube wall. Causes: • Selective deposition of feedwater system or preboiler corrosion products at locations of high heat flux • Concentration of sodium hydroxide from boiler water chemicals or from upsets in the water chemistry

Failure mechanism: 

Failure mechanism Pitting (Localized corrosion) Symptoms: Aggressive localized corrosion and loss of tube wall, most prevalent near economizer feedwater inlet on operating boilers. Flooded or non-drainable surfaces are most susceptible during outage periods. Causes: • Exposure of the tube to water with high acidic or oxygen concentrations • Existence of close-fitting surfaces and deposits where differences in oxygen concentration can be produced

Failure mechanism: 

Failure mechanism Hydrogen damage Symptoms: Intergranular micro-cracking. Loss of ductility or embrittlement of the tube material leading to brittle catastrophic rupture. Causes: Hydrogen damage is most commonly associated with excessive deposition on ID tube surfaces, coupled with a boiler water low pH excursion. Water chemistry is upset, such as what can occur from condenser leaks, particularly with salt water cooling medium, and leads to acidic (low pH) contaminants that can be concentrated in the deposit. Under-deposit corrosion releases atomic hydrogen which migrates into the tube wall metal, reacts with carbon in the steel (decarburization) and causes intergranular separation.

Failure mechanism: 

Failure mechanism Stress corrosion cracking Symptoms : Failures from SCC are characterized by a thick wall, brittle-type crack. May be found at locations of higher external stresses, such as near attachments. Causes: SCC most commonly is associated with austenitic (stainless steel) superheater materials and can lead to either transgranular or intergranular crack propagation in the tube wall. It occurs where a combination of high-tensile stresses and a corrosive fluid are present. The damage results from cracks that propagate from the ID. The source of corrosive fluid may be carryover into the superheater from the steam drum or from contamination during boiler acid cleaning if the superheater is not properly protected.

Failure mechanism: 

Failure mechanism Erosion Fly Ash Coal particles

Failure mechanism: 

Failure mechanism Fly ash: Factors influencing fly ash erosion in coal fired boilers are The velocity of flue gas: For low ash coals, the weight loss in pressure parts due to erosion is proportional to flue gas velocity to the power of 1.99. However for high ash Gondwana coals the erosion rate is velocity to the power of 3 to 5. The power depends upon the percentage of ash in coal, the percentage of silica in coal ash, the percentage of quartz in this silica, the percentage of alpha quartz in this quartz, and the structure of alpha quartz. The temperature of flue gas: Higher temperature softens the minerals in the ash as well as reduces the strength properties of the material of pressure parts; due to this ash erosion is not predominant in high temperature zones like furnaces, final superheaters , exit reheaters , etc. The ash erosion mainly starts in the conventional two-pass boilers from the area where gas temperature is around 700 – 750 deg.C . The low temperature superheater (LTSH) and economizer are the areas where ash erosion is severe in a conventional two-pass boiler. The temperature of flue gas entry to LTSH can be around 650 to 700 degree C and leaving, the economizer can be around 350 – 300 degree C. The minerals, which mainly constitute the ash in flue gas at these temperatures, become hard and attain its full abrasiveness

Failure mechanism: 

Failure mechanism Fire side corrosion: Water wall high temperature corrosion Super heater fireside ash corrosion Low temperature corrosion

Failure mechanism: 

Failure mechanism Waterwall Fireside Corrosion Symptoms : External tube metal loss (wastage) leading to thinning and increasing tube strain. Causes: Corrosion occurs on external surfaces of waterwall tubes when the combustion process produces a reducing atmosphere (sub stoichiometric ). This is common in the lower furnace of process recovery boilers in the pulp and paper industry. For conventional fossil fuel boilers, corrosion in the burner zone usually is associated with coal firing. Boilers having maladjusted burners or operating with staged air zones to control combustion can be more susceptible to larger local regions possessing a reducing atmosphere, resulting in increased corrosion rates.

Failure mechanism: 

Failure mechanism Super heater fireside ash corrosion Symptoms: External tube wall loss and increasing tube strain. Tubes commonly have a pock-marked appearance when scale and corrosion products are removed. Causes: Fireside ash corrosion is a function of the ash characteristics of the fuel and boiler design. It usually is associated with coal firing, but also occur for certain types of oil firing. Ash characteristics are considered in the boiler design when establishing the size, geometry and materials used in the boiler. Combustion gas and metal temperatures in the convection passes are important considerations. Damage occurs when certain coal ash constituents remain in a molten state on the superheater tube surfaces. This molten ash can be highly corrosive.

Failure mechanism: 

Failure mechanism Low temperature corrosion & cold end corrosion: Causes : Sulfur in fuel. Excess oxygen. Low Economizes inlet temperature. Low gas / air temperature at APH inlet. Low-temperature corrosion appears in the boiler as well as on other surfaces where the temperature is under approx. 135°C. It is caused by condensation of the acidic sulphurous and chlorine-containing gases. This type of corrosion is temperature-dependent. New plants are being designed differently in order to avoid low-temperature corrosion.


METHODOLOGY Boiler tube failure analysis. Find out main causes of tube failure. Suggested solution and check feasibility Optimum solution and implementation


IMPLEMENTATION OF METHODOLOGY Boiler Tube Failure Analysis.: Generally in cfbc boiler erosion caused in second pass of coal fired boilers due to impingement of high velocity ash particles on boiler tubes near sidewalls and rear / front wall. Boiler Tube Failures (BTF’s) represent the cause of these failures is due to fly ash erosion. Wear damage results from the combined effects of impact velocity, solids loading, fly ash and target material properties. In addition to BTF’s, erosion damage to non-pressure part components can result in costly maintenance and repair to ductwork, structural members, heat exchangers and ash separation equipment.

Boiler Tube Failure Analysis: 

Boiler Tube Failure Analysis Probable Causes Normal Condition Observation Remarks / Action Taken Deviation from operating parameter Inlet water temp. 130°C Outlet water temp. 197°C Inlet feed water press. <81 Kg/Cm2 Inlet flue gas temp. 425 ° C Outlet flue gas temp. 177 °C Inlet water temp. 125 °C Outlet water temp. 185 ° C Inlet feed water press. 68 Kg/Cm2 Inlet flue gas temp. 406 °C Outlet flue gas temp. 178 °C No abnormal deviation found - No action Corrosion internal No corrosion on surface. Could not be seen inside Corrosion inside is ruled out. - No action

Boiler Tube Failure Analysis: 

Boiler Tube Failure Analysis Corrosion external No corrosion on surface - No corrosion -No action Tube’s erosion No erosion on surface External flue gas erosion marks were present on the leaking tube and four other tubes nearby. Check for localised high flue gas velocity. Localised high flue gas velocity Tube pitch – 88mm Spacing between skin plate and tube – 60 mm Tube pitch – found ok. Spacing between skin plate and tube approx 40 mm Less spacing between skin plate and the tube caused localised high flue gas velocity lead to corrosion. Baffle to be provided.

Find out the main cause in boiler: 

Find out the main cause in boiler MAIN CAUSE: Erosion, is the process by which materials are removed from the surface and transported to another location. AIM: Aim of the activity is to control erosion to prevent the boiler tube erosion due to impingement of high velocity ash particles.


SUGGESTED SOLUTION E.C.D (erosion control device) Expanded metal screens Providing Baffles High pressure welding technique New Generation of Metallic Coating


EROSION CONTROL DEVICES (ECD): ECD’s are designed to protect the boiler tube from erosion due to impingement of high velocity ash particles. Erosion control is made up of metal screen of various sizes and Configuration to reduce occurrence of highly localized erosion in the second pass of boiler. It is normally observed, erosion is concentrated in some area of boiler. ECDs distribute the erosion from localized area. Shielding is used as a supporting for ECD. The life of ECD is normally 4 to 5 years. Screens are designed to take the erosion & reduce erosion of tubes. In the project of CFBC boiler, which was suffering heavy tube failure rate due to tube erosion, C.A.V.T. was performed and erosion control scheme recommended.


TESTING C.A.V.T. (Cold Air Velocity Test): C.A.V.T. is to predict the flow profile of flue gas by manually measuring the velocity of cold air inside the boiler at pre defined locations( Second pass of boiler). C.A.V.T. is performed twice: • Before installation of erosion control devices. • After installation of erosion control devices.

PowerPoint Presentation: 

Various types of test Gas velocity test: To ensure equiv. distribution in various ducts, velocity and flow is measured in each duct. Diverter plates with designed shapes are provided in duct at corrected prices to ensure equiv. distributions. Stack emission test: This test is done to find out ash burden at ESP outlet. Thickness survey: To predict the residual life of boiler tubes at various locations. Air leakage test: O2 measurement is carried out at inlet and outlet of duct. O2 percentage will increase if there is air ingress. To pinpoint the air ingress locations surface temperature measurements will reveal the ingress prints.


EXPANDED METAL SCREEN Expanded metal screens are manufactured from fine metallic foils, which are cut out and later expanded in the traverse sense until reaching the metal width programmed. The foil doesn't possess any anti-corrosive protection, and the screen is painted usually in the black color. With the help of carefully designed E.M.S., resistance is introduced in the gaps to reduce velocity in the problem area. The E.M.S. not only add resistance to flow through gap but also take the erosion and protect the boiler tubes from erosion. E.M.S.s are easily repairable and replaceable.


BAFFLES Baffle is a flow-directing vane or panel in some vessels. To prevent high speed flue gas to escape in between the skin plate and outer most tube surface, baffle plates were provided on the top of each bank so that chances of erosion in the outer most tube will get reduce. Similar baffle plates will be provided in other Economizer during its shutdown. It is replaceable and repairable . Production cost and maintenance cost is vey economical. It is easy method for reducing erosion in boiler component.




HIGH PRESSURE WELDING TECHNIQUE The forming of joints and connections ,which is the process of reconditioning damaged or worn engineering components by the application of weld metal or the protection of components against corrosion or wear by the application of an armoring layer of more resistant metal (hard surfacing). As to the nature of the welding process itself, a distinction may be made between pressure welding. Pressure welding usually involves heating the surfaces to a plastic state and then forcing the metal together. The heating can be by electric current of by friction resulting from moving one surface relative to the other. It is used as maintenance technique.


NEW GENERATION METALLIC COATING coatings which will help prevent failure of water wall tubes. Starting with a project to determine the cause of circumferential cracking in water wall tubes, they proposed that coatings be used to prevent the initial erosion step in the cracking degradation process. various types of coatings, including Chromizes coatings . Commercial thermal sprays. Weld overlays.


Summary Erosion is mainly produced due to fly ash and coal particle . It is mainly affected due to HIGH FLUE GAS VELOCITY ,system geometry and material. here we are suggest a number of solutions like E.C.D ,expanded metal screen, providing Baffle ,Coating and high pressure metal welding. Some of them are very costly and eliminating erosion drastically and improve the overall efficiency, productivity and generation of power.


COLD AIR VELOCITY TEST The availability of the coal for most of the power stations in India is of “F” & “G” grade with ash percentage varying from 40 to 50 %. Moreover due to low calorific value, extra tonnage of coal needs to be handled by equipment. This high quantity of abrasive ash causes erosion of boiler tubes. NEED OF C.A.V.T: Due to use of very high ash content coal in boiler, the flue gas leaving 1st pass is heavily burdened highly abrasive ash. This ash associated with high velocity, impinges on tubes in 2 nd pass and starts tube erosion. Tube erosion leads to its thinning and ultimately results in tube failure and boiler break down. To reduce the tube erosion, the concept and procedure of (C.A.V.T) is employed, which is explained here after.


COLD AIR VELOCITY TEST It is difficult to physically measure velocities of flue gases inside the boiler when it is in operation. However, it is required to know the velocity flow field in various zones so that its effect on the various failure mechanisms can be predicted. The primary tool to combat Fly Ash Erosion is flow modification in conjunction with a cold air velocity test before and after modification. The cold air velocity test performed to predict the velocities in the respective zones of the boiler In this approach, units are evaluated by the cold air velocity technique (CAVT) to determine local velocity profiles, maximum local velocities of two or more times the nominal velocity have typically found, and these peak velocities usually correspond to the locations of known tube erosion damage. The use of CAVT to identify regions of excessive velocity, followed by the installation of diffusion and distribution screens, may provide utilities with the most optimum solution to the problem. However, the technique has not been adopted by sufficient utilities. The results of this CAVT obtained from the concerned power station are used for validating the model developed for predicting the velocity of flue gases.


EROSION FUNDAMENTALS EQUATION The following simple erosion equation gives the relationship between two factors that can be changed relatively easily in most boiler locations where fly ash erosion is a problem: solids loading and impact velocity. E = C x M” x Vn Eq. 1. where: E - Erosion Rate (mils/hr) C - Correlation constant M” - Solids Flux (lb/hr- sq.ft .) V - Particle Impact Velocity (ft/sec) n - Velocity Exponent This equation indicates that erosion rate is proportional to the solids loading and to the velocity raised to a power - typically in the range of 2.3 - 2.5. From this equation, it can be seen how important it is to keep the velocity under control.

PowerPoint Presentation: 

To estimate the change in Erosion Rate for a change in solids loading and/or impact velocity, the following relationship can be used: E1/E2 = (M”1/M”2) x (V1/V2)n Eq. 2 For a situation where the solids loading and velocity are 20 percent above average levels, then the relative erosion rate would be: E1/ Eavg = (1.2/1.0) x (1.2/1.0)2.4 = 1.86 Note that a 20% increase in ash loading increases the erosion rate proportionately , while a 20% increase in velocity increases the erosion rate by 55%. The combined effect is an 86% increase in erosion rate over locations at average flow conditions (see Figure below). It’s not unusual for some areas to have velocities 1.5 - 2.0 times average conditions!

OXISTOP Coatings—For Boiler Tube Maintenance: 

OXISTOP Coatings—For Boiler Tube Maintenance The ability of Oxistop high temperature metal coatings to reduce both slag and residue buildup, stop corrosion and oxidation and resist fly ash erosion and abrasion has led our client base to evaluate benefit regarding issues in the following areas: BURNERS AND OVER FIRED AIRPORTS — SLAG AND CORROSION. REFRACTORY DOORS AND BURNER REFRACTORY — SLAG ECONOMIZER TUBES – CORROSION AND EROSION TUBE SHIELDS AND DUCT SUPPORTS – CORROSION AND EROSION NOSE AND SLOPE AREAS – CORROSION AND SLAG WATERWALL AREAS – CORROSION, SLAG AND HEAT TRANSFER SUPERHEAT AND REHEAT TUBES – FOULING AND HEAT TRANSFER

OXISTOP Coatings Analysis: 

OXISTOP Coatings Analysis

PowerPoint Presentation: