6. Estimation of Flue Gases Velocity and Temperature

Transcription

1 6. Estimation of Flue Gases Velocity and Temperature Simulated Cold Air Velocity Test results are useful for estimation of velocity at different cross sections of pressure parts. Simulation results also help to locate the places where there is possibility of increase in velocity. In the boiler, flue gas passes over pressure parts. Thermo-physical properties and composition of flue gases are quite different than air. Hence CAVT estimation results cannot be directly applied to flue gases. Thermo-physical properties of flue gases are temperature and coal quality dependent. Constituents of coal and ash percentage define quality of coal. Hence accuracy of estimation of flue gas velocity and temperature depends on estimation of flue gases properties. This chapter deals with estimation of flue gases properties as a function of flue gas temperature and quality of coal. These estimated properties are used as input values for simulation. Inertial resistance for flue gases is re-estimated and given as input. Finally simulation is run for and is subsequently presented. The boundary conditions are changed to the operating conditions of the boiler that are obtained from NTPS plant data. In addition to existing solver settings, energy equation is activated for the heat transfer prediction. These properties are specific volume, density, specific heat, thermal conductivity and viscosity of flue gases. These properties are estimated as function of temperature. 6.1 Properties of flue gas as a Function of temperature Flue gas temperature at various sections across the pressure parts is obtained from the plant. Base values of various properties were used for iterations and the data is processed in excel to get the polynomial relationship of property in terms of temperature. Table: 6.1: Properties of Flue Gases at various pressure parts. Flue Gas Property AH Inlet ECO Inlet LTSH Inlet FSH Inlet CRH Inlet SH Platen Inlet Temperature (K) * Pressure (Pa) vacuum Specific volume (m 3 /kg) Density(kg/m 3 ) Cp(J/kg-k) Thermal Viscosity(kg/m-s) 2.98E- 3.25E- 3.91E- 4.02E- 4.61E- 4.85E- * Annexure VII. Plates showing operational Temperature and Pressure across pressure Parts 104

2 Using these values polynomial relationships were obtained. Following are the relationships. Further for these relations values of constants were determined iteratively. These are shown in the Table: 6.2 Table No. 6.2 : Polynomial Relations of thermo-physical properties: Property Relationship in terms of flue gas temperature, T, K Property = C1 + C2 T + C3 T + C4 T + C5 T + C6 T Density(ρ) 7 2 = ( ( x ( x10 ( x10 + ( x10 Specific Heat(Cp) Thermal Conductivity (k) Dynamic Viscosity (µ) = ( ( ) T ( x10 + ( x10 ( 1.38x ( ( 2x ( x10 ( x10 + ( 2.25x x10 + (4.85x10 ( 1.5x ( x10 + ( 4.17x10 ( 1.1x10 = = Excel sheets of iterations are attached in Annexure VIII Values obtained are given as constants input while deciding the material properties. 6.2 Adiabatic Flame Temperature Estimation: Power Station authorities have provided the operating condition data. Instead of using the data directly it is tried to theoretically calculate the Flame temperature at the core of fire ball in tangentially fired boiler. Actual coal quality is quite different than the expected quality. For theoretical estimation of velocity and temperature of flue gases, adiabatic flame temperature estimation is necessary. These calculations are based on the theoretical combustion calculations. The adiabatic flame temperature [10] is the maximum theoretical temperature that can be reached by the products of combustion of a specific fuel and air (or oxygen) combination, assuming no loss of heat to the surroundings and no dissociation. The heat of combustion fuel is the major factor in the flame temperature, but increasing the temperature of the air or the fuel also raises the flame temperature. This adiabatic temperature is a maximum with zero excess air (only enough air chemically required to combine with the fuel). Excess air is not involved in the combustion process; it only acts as a diluting agent and reduces the average temperature of the products of combustion

3 The adiabatic temperature is determined from the adiabatic enthalpy of the flue gas: HHV Latent Heat of H 2 O + Sensible heat in air Hg = Weight of Wet Gas Where Hg = adiabatic enthalpy, J/kg Knowing the moisture content and enthalpy of the products of combustion, the theoretical flame or gas temperature is obtained from Fig. 3, Enthalpy of flue gas above 77F at 30 in. Hg. Steam: Its Generation and Use, Principles of Combustion, 10-12, (Annexure IX) Adiabatic condition assumption results in the conclusion that, the sum of enthalpy of reactants (H R ) and calorific value of fuel is equal to the total enthalpy of products of combustion (H P ). CV + H R = H P...(68) The terms on the left-hand side of equation 1 are simple to evaluate. CV is the calorific value of fuel used and H R, the enthalpy of the fuel and air (ref C), can easily be calculated from H R = (T i -25) (mc p ) R. (69) where T i is the initial temperature. The specific heats of the fuel, oxygen and T nitrogen can be evaluated at the mean temperature and the enthalpy of the reactants is thus easily evaluated. The right-hand side of equation 1, however, is not as easily evaluated as it is defined by H P = (T f 25) (mc p ) P (70) where T f is the flame temperature. This relationship cannot be solved explicitly for T f as there is considerable difference between T f and the reference temperature 25 0 C, hence the value of T f is required to evaluate the specific heats of the combustion products. Simple ideal gas theory predicts that the specific heat of a gas is not a function of its temperature or pressure. While the latter implication is effectively true in practice, the specific heat of a gas increases with temperature above about C. Empirical equations which allow the calculation of specific heats as a function of temperature have been in use for some time. A polynomial expression is normally used, with either fractional or integer powers. However, an interpolation method is used to determine the specific heat of the components of flue gas based on 106

4 the charts which are available in the range 175 K K.[10] Adiabatic flame temperature is calculated by executing following steps: i) Heat of Reactants is evaluated. Estimated CV or experimentally determined CV is used to determine total heat on Reactant side. i.e. (CV + H R ) ii) Initial value of T f is guessed to find the specific heats of the combustion products at the average between the flame and the reference temperature, i.e. T 0 C iii) The equation is solved for T f iv) The new value of T f is compared with the original estimate. v) If there is a substantial difference, the new value is used to re-evaluate the specific heats, looping back to step 2 until satisfactory convergence is achieved. With regard to step 2 above, taking the average temperature over the interval is only valid if the relationship between specific heat and temperature is linear. Though there are few constraints on the assumption, taking a simple arithmetical average is quite acceptable. For this estimation, the proximate and ultimate analysis of coal given by power plant authorities is used. Proximate analysis of coal (Weighted average of one month data of coal) Fixed carbon % Ash % Volatile Matter % Moisture % Ultimate analysis of coal (from chemical laboratory) %C %H 2.81 %N With excess air as 20%, mass flow rate of coal in the furnace as T/hr (29.75 kg/s), calorific value of coal has been calculated from the weighted average of the one month data of coal from the power plant. It is estimated as kcal/kg ( kj/kg). 107

5 Reactants and products are as follows: Reactants: Carbon C (kg/s) Hydrogen H 2 (kg/s) 0.84 Moisture H 2 O (kg/s) 3.21 Oxygen O 2 (kg/s) Nitrogen N 2 (kg/s) Total (kg/s) Air/Fuel Ratio Products: Carbon Dioxide CO 2 (kg/s) Steam H 2 O (kg/s) Oxygen O 2 (kg/s) 9.40 Nitrogen N 2 (kg/s) Total (kg/s) Mass flow rate of Primary air (T/hr) Mass flow rate of Secondary air (T/hr) Temperature of Primary air ( 0 C) 336 Temperature of Secondary air ( 0 C) 327 Inlet Pressure (Pa) Inlet temperature of Reactants( 0 C) Reference temperature( 0 C) 25 Mean Temp. ( 0 C) Cp C(kJ/kg/ 0 C) Cp H 2 (kj/kg/ 0 C) Cp H 2 O (kj/kg/ 0 C) Cp N 2 (kj/kg/ 0 C) Cp O 2 (kj/kg/ 0 C) Enthalpy of Reactants (kj) Enthalpy of Reactants Hr(kJ/kg of coal) CV of coal (kj/kg) Combustion efficiency 0.97 Total Enthalpy (kj/kg of coal)(hr + CV)

6 The estimated flame temperature after 10 iterations is obtained as C with the error of E-05. The flame temperature given by power plant authorities at the core of fire ball is 1396 K. The difference in the theoretical value and the practical is quite obvious and may attribute to certain assumptions i.e. adiabatic conditions, calculations of specific heats of the components of flue gas, variation in coal quality, temperature of combustion air, leakage of air in the flue gas space from outside and evaporation of water from water seals at the bottom. Detailed iteration is given in the excel sheet in the attached CD. Table No. 6.3 : Iterations for calculating flame temperature Iteration T f initial assumption( o C) 1125 Tref( 0 C) Tf ( 0 C) Tmean ( 0 C) Cp N 2 (kj/kg/ 0 C) Cp O 2 (kj/kg/ 0 C) Cp H 2 O (kj/kg/ 0 C) Cp CO 2 (kj/kg/ 0 C) Mass flow rate coal (kg/s) Enthalpy of Products Hp (kj/kg of coal) T f ( 0 C) calculated Residual E-05 Table 6.4. Estimation of inlet flue gas temperature. Parameter Quantity Source Volume of furnace, m NTPS Data Width, m NTPS Data Depth, m NTPS Data Height, m Calculated Projected area of furnace (m 2 ) Calculated Mass flow rate of coal (T/hr) NTPS Data CV of coal (kcal/kg) NTPS Data CV of coal (kj/kg) Calculated Mass flow rate of coal (kg/s) Calculated Heat release rate (kw) Calculated Heat release rate per unit area of furnace (kw/m 2 ) Calculated Heat release rate (BTU/hr) Calculated Projected area of furnace (ft 2 ) Calculated Heat release rate per unit area of furnace (BTU/hr/ft 2 ) Calculated 109

7 Average temperature in F (Curve fit in Annexure. IX) Calculated * Average temperature in o C Calculated Average temperature in K Calculated Input to fluent inlet temperature for simulation (K) 1357 Input Temperature as given by Plant authority, K 1396 * Ref: Steam: Its Generation and Use, Edition 41, Babcock & Wilcox., Chapter-4, Fig. 34. The adiabatic flame temperature is a misleading high value and cannot exist. Actual flame temperatures are lower due to two main reasons: [43] a. Combustion is non-instantaneous and there are heat losses. Heat losses are less if combustion is fast and incomplete combustion is the result of slow combustion leading to some unburned fuel. b. At temperatures above 1650 o C, some of the CO 2 and H 2 O in the flue gases absorbs heat in the process and dissociates. At 1950 o C, about 10% of the CO 2 in a typical flue gas dissociates to CO and O kj heat is absorbed per kg of CO formed, and about 3% of the H 2 O dissociates to H 2 and O 2, with heat absorption of kj/kg of H 2 formed. The dissociated CO and H 2 recombine with the O 2 with cooling of gas and the heat absorbed in dissociation is liberated. Thus the heat is not lost; however, the overall effect is to lower the maximum actual flame temperature and it is approximately 70 to 75 % of adiabatic flame temperature. 6.3 Estimation of Flue gas Inlet Temperature: For estimation of flue gas temperature across the pressure parts, it is necessary to give input flue gas temperature as input to Fluent. Flue gas temperature, at the inlet domain can be estimated if heat release rate is known. Heat available is a combined, single variable that includes the heat energy supplied by the fuel and by the preheated combustion air, corrected for unburned combustible loss, radiation loss, and moisture from the fuel. The heat available divided by the equivalent flat projected furnace enclosure plus furnace platen area is called the furnace heat release rate. The heat input from fuel divided by the furnace volume is called the furnace liberation rate. The furnace exit plane defines the boundary of the furnace volume and flat projected furnace enclosure area. [43, Chapter 4, 25] Furnace exit gas temperature (FEGT) is primarily a function of heat release rate rather than liberation rate. The furnace exit is defined as the plane located at the face of the first tube bank having a tube spacing of less than 38.1 cm side centers where convection conductance typically becomes the predominant heat transfer mode 110

8 at this side spacing. The furnace exit plane is used for the accurate calculation of overall heat transfer, is set at the face of the first tube bank having a tube spacing of 91.4 cm side centers or less in order to include the convection conductance in the heat transfer calculations. The approximate relation of FEGT to heat release rate at the furnace exit plane for a typical pulverized bituminous coal is given in Fig. 34 of Chapter 4 of Steam: Generation and its Use. Heat release rate estimation and further flue gas inlet temperature estimation is done as shown in Table Furnace volume and projected surface area is calculated from the furnace drawing and orientation of tubes in the furnace. Using coal consumption rate and heating value of coal obtained from plant, heat release rate per square meter is calculated. Steam: Its Generation and Use, Edition 41, Babcock & Wilcox., Chapter-4, Fig. 34 gives variation of Furnace exit gas temperature as function of heat release rate. The fig is reproduced in the Annexure IX. To suit to the fig, the heat release rate is further converted to (BTU/hr/ft 2 ). From the fig, thus furnace exit gas temperature is calculated. It is 1353 K. It is now assumed that flame temperature is the gas inlet temperature and hence FEGT is used as 1396 o C. It is found that heat release rate depends on quality of coal. Hence further it is explored for estimation of flue gas inlet temperature for other quality coals from various quarries and estimated flame temperature with correction factor is used as FEGT. Input flue gas temperature value given by plant authorities are in good agreement with the estimated value. Hence it is used as an input to fluent. Annexure X gives the estimation for different qualities. Using this input temperature a comparative simulation for different coal qualities is possible. Estimated flue gas exit temperature for different coal qualities is given in Table No Table No. 6.5: Estimated Input Temperature for Various Coal Coal Input Temperature, K NTPS 1396 WANI 1463 UMRER 1463 CHARGAON 1452 KOREA 1530 DAMAN 1575 NCPH

9 6.4 Estimation of Inertial Resistance and Porosity: Inertial resistance and porosity is explained in Chapter 5. Now to give these values as input to Fluent, inertial values and porosity of all pressure parts is calculated for flue gases. This calculation for FSH is as per following steps. Similar steps are repeated for all other pressure parts and calculated. The results are given in Table No.6.6. Water wall is not modeled as porous media, but as a tube bank. Hence it will not have any inertial resistance and porosity. Initially the inertial resistance is calculated with reference to recommended velocity of flue gases in the pressure parts. This calculated resistance then fed to fluent as an input. The simulation is run and mass flow rate of flue gas is monitored. It is observed that required mass flow rate of flue gases is not obtained across the pressure parts. Hence the inertial resistance is manually varied till the desired mass flow rate is obtained. Thus the inertial resistance is determined and used as input and further all simulations are run. Table No. 6.6: Estimation of Inertial Resistance of PSH Inlet Temperature ( 0 C) 1396 Outlet Temperature ( 0 C) 1285 Mean Temperature ( 0 C) Density (kg/m 3 ) Pressure drop (mmwc) 1 Velocity of flue gases (m/s) 8 Thickness of porous block (m) Inertial Resistance per meter Table No. 6.7: Inertial resistance and Porosity of Pressure Parts ZONE P. D. Velocity* Inertial Porosity Porous Block Pressure (mmwc)* m/s Resistance Thickness*, m Part (1/m) PSH RH FSH LTSH Strip LTSH Main ECO * Power Plant Data 112

10 6.5 Estimation of Source Term Boiler is a heat exchanger in which hot flue gases passes over the tube coils. Feed water/steam pass through the tube coils. Feed water receives heat from flue gases which used for heating of feed water, steam generation, reheating of steam or superheating depending upon the nature of the pressure part. Thus as the flue gas passes over the pressure parts they are cooled by heating water/steam. For correct estimation of flue gas temperature over these pressure parts it is necessary to know the heat contained by flue gas, heat absorbed by water and efficiency of this heat transfer. In fluent this heat transfer is to be given as Source Term. In simulation this term is given as (-ve) to the zones (viz. PSH, FSH etc.) as heat is received by them. The source term is estimated for all pressure parts. Procedure followed for estimation of source term is explained for PSH and similar method is used for other pressure parts. For all calculations it is assumed that heat given by flue gases is absorbed by water. Thus performance factor is assumed as 1. Heating value of coal and coal consumption rate for 100% capacity is known from the plant data. Using air consumption data for one kg, mass of flue gas generated is calculated. Then total energy contained by flue gases per kg of coal is calculated. From these results heat content of total flue gases is determined. Temperature of flue gases before and after each pressure parts is given by the plant authorities. It is used to determine the heat rejected by the flue gases. Water-wall is not modeled as porous media. These are four platens. It is assumed that source term is equally divided in all four water-walls. Estimation of source term on Flue Gases basis and on Feed Water basis for pressure part is given in Annexure XI. Sample source term estimation is given in Table No Table No. 6.8: Estimation of Source Term for PSH Flue Gases Total energy content of Flue gases (MJ/kg of coal) Mass flow rate of coal (T/hr) Mass flow rate of coal (kg/s) Heat content of flue gases (MW) Heat rejected in PSH Mass flow rate of flue gas (T/hr) Mass flow rate of flue gas (kg/s) Inlet pressure (bar)

11 Outlet pressure (bar) Mean pressure (bar) Inlet temperature (K) 1396 Outlet temperature (K) 1285 Mean temperature (K) Cp of flue gas (J/kg-K) Heat rejected by flue gas (W) Heat rejected by flue gas (MW) Surface Area 1010 Heat flux, W/m Volume (porous block) Source Term, Energy, W/m For simulation, Heat Source term, i.e. Energy, W/m 3, on water basis, higher value is given as cell zone condition to Fluent. Table No. 6.9 shows the source term for different zones. Water side source term is given as an input because heat given by flue gases is received by water. While giving the input it is used with ve sign. Table No: 6.9 Source Term, Energy, W/m 3 Zone Source Term, Energy, W/m 3 On Water Basis On Flue gas Basis PSH RH FSH LTSH Strip LTSH Main ECO WWP (Combined) Boundary Conditions and simulation: Pre-processing: Geometrical model of boiler which is used for CAVT is proved to be fairly correct after validating the simulated CAVT results with experimental CAVT results. Hence the same model with same meshing scheme is used for estimation of velocity and temperature of flue gases. Processing: Problem Setup: Following table shows the boundary conditions given for simulation. 114

12 Models Model Settings Space 3D Time Steady Viscous Standard k-epsilon, Standard Wall Function Heat Transfer Enabled Radiation None Coupled Dispersed Phase Disabled Material Properties Material: flue-gas (fluid) Property Property Unit Method Values, C1, C2, C3, C4, C5, and C6 respectively Density Kg/m 3 Polynomial , , e-07, e-09, e-12, e-16 Sp. Heat J/kg K Polynomial , , , e-06, e-10, e-13 Thermal W/m-K Polynomial , , e-06, Conductivity e-09, e-12, e-16 Viscosity Kg/m-s Polynomial e-06, e-08, e-11, e-15, e-18, e-21 Material: Steel (solid) Property Unit Method Values, Density kg/m3 constant 7753 Cp (Specific Heat) J/kg-K constant 485 Thermal Conductivity W/m-K constant Cell Zone Conditions Zones Name ID Type FLUE-GAS 2 Fluid ECO 3 Fluid LTSH_PART 4 Fluid 115

13 LTSH_STRIP 5 Fluid FSH 6 Fluid RH 7 Fluid PSH 8 Fluid WWP 9 Fluid Power Law Model C 0 = and C 1 = 0.96 Boundary Conditions: Condition Value Inlet Gauge total pressure (Pascal) Direction specification method Normal to the boundary Turbulence Specification Method Intensity and Hydraulic diameter Turbulent intensity (%) Hydraulic Diameter (m) Total Temperature (K) 1369 Outlet Gauge pressure (Pascal) Backflow Direction Specification Normal to Boundary Method Backflow Turbulent Intensity (%) Backflow Hydraulic Diameter (m) Backflow Total Temperature (K) 641 Wall Stationary with No slip condition Operating Condition Operating Pressure (Pascal) Solution Method Pressure Velocity Coupling Scheme Special Discretization Gradient Pressure Momentum Turbulent kinetic Energy Specific dissipation Rate Energy SIMPLE Green Gause Cell Based Standard Quick Quick Quick Quick 116

14 Solution Controls Under Relaxation Factor Pressure 0.3 Density 1 Body Forces 0.9 Momentum 0.6 Turbulent kinetic Energy 0.7 Specific dissipation Rate 0.7 Energy 1 Turbulent Viscosity 0.9 Solution Limits Minimum Absolute Pressure 1 Maximum Absolute Pressure 1e+20 Minimum Temperature 641 Maximum Temperature 1396 Min Turbulent kinetic Energy 1 Max Specific dissipation Rate e-21 Min Turbulent Viscosity Ratio 1e+10 The solution is initialized and then simulation is run for 2000 iterations for various coal qualities used in the plant. Converged solution data is further used for post processing. 6.7 Post Processing After getting the converged solution, ANSYS CFX POST is used for post processing. CAVT counters highlighted the locations and areas prone for increase in velocity. Virtual Location Planes were drawn at these locations and at few more places where there was past history of tube failures. List of these planes is given in the adjoining table. Table No Cloud points (Table No. 6.11) are taken at locations where the area of passage abruptly changes. Contours of pressure, velocity, and temperature are obtained along and across different planes, cloud points, Lines as given in the table. Velocity and temperature of flue gases is predicted at these locations. Inferences from these contours are discussed in detail. The flame temperature of various coals used in Indian Power Plants and Flue gas inlet temperature corresponding to that flame temperature is estimated in previous sections of this report is used to estimate flue gas temperature and velocity for different coal. Velocity and Temperature of flue gases are estimated and the consolidated result is given in the Table No: 6.12 are given in the adjoining table. 117

15 Table No Location of planes. Name of Description of Plane Plane 118 Location, m Importance Long vertical Plane ( YX Planes) Z= Plane 1 Between wall near WWP 1 and SH Banks 10.5 Critical Plane 2 Passing through WWP 1 9 Plane 3 Between WWP 1 and WWP2 7.3 Critical Plane 4 Passing through WWP Plane 5 Between WWP 2 and WWP Critical Plane 6 Passing through WWP Plane 7 Between WWP 3 and WWP 4 0 Critical Plane 8 Passing through WWP Plane 9 Between wall near WWP 4 and SH Banks Critical Vertical Cross pane ( YZ Planes) X = Plane 10 Near front wall and WWP banks 0.5 Plane 11 Crossing WWP banks 2.5 Plane 12 Between WWP banks and PSH 6.5 Critical Plane 13 Crossing PSH 7.7 Plane 14 Inlet to CRH 8.5 Critical Plane 15 Cutting CRH 10 Plane 16 Between CRH and HRH 11 Critical Plane 17 Cutting HRH 12 Plane 18 Exit of CRH 13.2 Critical Plane 19 Inlet to FSH 14 Critical Plane 20 Crossing FSH 14.5 Plane 21 Exit of FSH Critical Plane 22 Between the gap of LTSH, ECO coil and wall 16 Critical Plane 23 Cutting LTSH strip 17 Plane 24 Crossing LTSH and ECO 19.5 Plane 25 Plane between eco wall(right) Critical Horizontal Plane ( ZX Planes) Y= Plane 26 Near rooftop 19.3 Plane 27 Below rooftop place 0_crossing WWP, FSH, 17.5 CRH, HRH, PSH Plane 28 Below rooftop place 1_ crossing WWP, FSH, 16 CRH, HRH, PSH Plane 29 Below rooftop place 2_ crossing WWP, FSH, 14.5 CRH, HRH, PSH Plane 30 Below rooftop place 3_ crossing WWP, FSH, 13.2 CRH, HRH, PSH Plane 31 Exit of CRH and inlet of HRH 10.5 Critical

16 Plane 32 At nose bend of CRH_ crossing WWP, FSH, 9.3 CRH, HRH, PSH Critical Plane 33 Lowermost level of PSH 8.2 Critical Plane 34 At crown of bend at bottom of FSH 12 Critical Plane 35 At exit lower of CRH and HRH 11.6 Critical Plane 36 Crossing LTSH Main lower Bank 7.5 Plane 37 Exit of LISH 6.4 Critical Plane 38 Inlet of economizer 5.8 Critical Plane 39 Middle of Economizer 4.65 Plane 40 Exit of Economizer 3.45 Table No. 6.11: Location of Cloud Point Lines Cloud Description of Line Importance No. 1 After PSH Narrow area at Nose Bend 2 First section of contact of High Temp Flue gases Below PSH with Pressure Part 3 Bottom of HRH Narrow area at Furnace Bottom Wall 4 Narrow area at Furnace Bottom Wall and Bottom Exit of HRH Entry to CRH 5 Bottom of CRH and HRH Narrow area between HRH and CRH 6 Bottom of CRH Narrow area at the crown of bottom wall 7 Bottom Inlet of FSH Pressure part after change of direction of flue gases 8 Flue gases entering in the low temperature Exit Bottom of FSH zone 9 Between FW n LTSH UB Entry Narrow area with past failure history 10 Between RW n LTSH UB Entry Narrow area 11 Between FW n LTSH UB and Narrow area with past failure history LB 12 RW n LTSH UB and LB Narrow area 13 FW and LTSH LB Narrow area with past failure history 14 FW n LTSH_LB and ECO Low Temperature zone 15 RW n LTSH LB and ECO Low Temperature zone 16 FW n ECO_ IN Narrow area with past failure history 17 RW and ECO_IN Low Temperature zone 18 FW n ECO Middle Narrow area with past failure history 19 Between RW n ECO Middle Narrow area with past failure history 20 FW n ECO Exit Low Temperature zone 21 RW n ECO Exit Low Temperature zone 119

17 6.8 Velocity and Temperature Estimation: Velocity and temperature contours and plots along all virtual planes are obtained. The contour gives the maximum and minimum values of Velocity and temperature. Location planes of average velocity greater than the critical velocity are marked. These contours are attached in Annexure XII. Velocity and temperature in the entire volume is also attached in Annexure XIII. X-Y plots against the cloud points gives approximate location of zones where there is increase in velocity and/or unanticipated change in temperature. Velocity and Temperature plot at designated planes shows variation in velocity and temperature. On the basis of these plots the critical locations more prone for erosion are highlighted. Table: 6.13 give estimated values at all planes and Table: 6.14 give the velocity and temperature at all cloud points. Locations with increased velocity and temperature are highlighted with shaded region. 120

18 Table:6.12: Results of Estimation of Velocity at various planes Name of Plane NTPS Coal (1396) Velocity, m/s Max. Temp, K Min Temp, K Predicted Velocity and Temperature (Changed IR=IR x 2.5) NCPH Coal (1502) Velocit y, m/s Tem p, K Min Tem p, K DAMAN Coal (1575) Veloci ty, m/s Temp, K 121 Min Velocit Temp y, m/s, K KOREA Coal (1530) Tem p, K Min Tem p, K WANI/UMRER Coal (1463) Veloci ty, m/s Temp, K Min Temp, K CHARGAON Coal (1452) Veloci Temp ty, m/s, K Plane Plane Plane Plane Plane Plane Plane Plane Plane Plane Plane Plane Plane 13 Plane Plane Plane Plane Plane Min Temp, K

19 Plane Plane Plane Plane Plane Plane Plane Plane Plane Plane Plane Plane Plane Plane 32 Plane Plane Plane Plane Plane Plane Plane Plane

20 Table:6.13: Results of Estimation of Velocity at Planes at critical areas Description Recommended Velocity Distance Z, m After PSH Below PSH Bottom of HRH Bottom Exit of HRH Bottom of CRH and HRH Bottom of CRH Bottom Inlet of FSH Exit Bottom of FSH Between FW & LTSH UB Entry Between RW & LTSH UB Entry Between FW & LTSH UB and LB RW & LTSH UB and LB FW and LTSH LB FW & LTSH_LB and ECO RW & LTSH LB and ECO FW & ECO_ IN RW and ECO_IN FW & ECO Middle Between RW n ECO Middle FW & ECO Exit RW & ECO Exit

21 Table: 6.14:Results of Estimation of Temperature at Cloud points at critical areas Description Desired Temperature, K Distance Z, m After PSH to 1285 Below PSH Bottom of HRH to 1104 Bottom Exit of HRH Bottom of CHR and HRH to 1023 Bottom of CHR Bottom Inlet of FSH to 979 Exit Bottom of FSH Between FW n LTSH UB Entry Between RW n LTSH UB Entry Between FW n LTSH UB and LB RW & LTSH UB and LB 979 to FW and LTSH LB FW & LTSH_LB and ECO RW & LTSH LB and ECO FW and ECO IN RW and ECO_IN to 641 FW & ECO Middle Between RW n ECO Middle FW & ECO Exit to 641 RW & ECO Exit

22 6.9 Discussions Temperature of Flue Gases Tables and the contours show the temperature of flue gases along various test planes. It shows that temperature of flue gases is quite higher in the water-wall and platen-wall heater region; i.e. nearly 1340 K that is higher than the estimated. Similarly it is higher in the CHR and HRH zone. Higher temperature of flue gas leads to increased tube surface temperature. Due to this the ductility of the material changes and the material becomes soft. Inner high pressure fluid exerts pressure on the tube and tube bulges out and subsequently burst. Thus increases the possibility of High Temperature Creep failure. The past failure data also highlighted the high temperature creep failure in this region. There is a need to study the reasons for higher temperature in these zones. Reasons for increased tube surface temperature can be attributed to combustion operational parameters. There may not be appropriate cooling of flue gases i.e. heat transfer from flue gases to steam / feed water is not effective. Either mass flow rate through the tubes is less or flue gases flow rate and velocity is high leading to less residual time. Thermal power plant is 35 years old. It is found that metallic screens, baffles, arresters are provided at certain sections to reduce the velocity of flue gases. Effect of these methods on the overall performance of plant is to be studied. Complete effectiveness study of pressure parts is required. Table: 6.15 give the locations of increased velocity in the zones. Flue gases temperature observed on plant is lower in the economizer zone, i.e. 641 K (lower than estimated 770 K). Lower temperature of flue gases, in economizer zone, increases the sharpness factor of eroding material. This makes the tubes more susceptible to erosion failure. This high velocity, sharp eroding particle gets bombarded over the tube surface and cutting wear takes place. It is also necessary to study the causes of reduction in the flue gases temperature in the eco zone. Table: Estimated temperature and relevance with failure Sr. On Site Average Estimated Probable Cause of Region No. Temperature, K Temperature, K Failure 1 WWP H T Creep 2 PSH H T Creep 3 CRH HRH FSH LTSH Creep & Erosion 7 ECO Erosion 125

23 6.9.2 Velocity of Flue Gases: The predicted velocity profile highlights that velocity of flue gas is maximum in economizer zone. The velocity plot shows that velocity of flue gases is increased in the zone after PSH, but simultaneously there is increase in temperature. Hence the eroding material if any present in the flue gases gets blunt and threat of erosion is reduced. Similarly, in spite of increased velocity in the CHR, HRH and LTSH zone, there is less threat of erosion failure. In economizer zone, there is tremendous increase in velocity and it is bit higher along the walls. There are tube bends of eco-coils near the wall. The flue gas passage is narrowed. Flue gases, when pass through this narrow passage gets accelerated. The safe velocity in this zone is in the range of m/s and in plant and simulation it is nearly m/s. Thus the tubes in this zone are more susceptible for erosion failure. Velocity distribution contours along various planes parallel to vertical, auxiliary vertical and horizontal reference planes are plotted in entire boiler zone to give the velocity distribution in the boiler. Boiler zone starting from Left wall to Right Wall is divided in to 10 parts. Table: 6.17 give the locations of increased velocity in the zones. Corresponding to these zones the probable location of coils is identified and thus coil prone to erosion failure is identified / marked. Table 6.17: Predicted Velocity across various zones and relevance with failure Sr. No. Region Flue gas velocity in flue gas ducts at 100 Predicted, m/s Local maximum Velocity, m/s % MCR 1 WWP PSH CRH HRH FSH LTSH ECO The predicted velocity in different zone against the actual range of flue gas velocity shows that the actual velocity measured at the site is higher for FSH, LTSH and ECO zones and those zones are more prone to failure. Measured velocity is considerably higher than predicted. Past failure data confirms that erosion failures are more in economizer zone. A thought must be given to check whether the economizer design is responsible for rise in velocity. The economizer in the plant under study is a staggered type, straight (longitudinal) finned economizer. Fins are provided to enhance the heat transfer coefficient. But the 126

24 staggered tube and finned arrangement geometry leads to narrowed cross-section between the tubes. It is observed that velocity of flue gases increases in this region too. Hence it is necessary to study the feasibility of In-line un-finned or Staggered tube type un-finned Economizer in place of Staggered tube, straight finned economizer Combined Effect of Increased Velocity and Temperature of Flue gases: In intermediate zone such as FSH and LTSH, velocity is higher as compared to the critical value and temperature is not so higher. Eroding material is bombarded on the tube surface leading to wear of surface and thinning of tube. There is always a critical thickness for a tube up to which tube sustains internal high pressure. Due to thinning of tube and effect of flue gas temperature, surface temperature of tube increases. The tube is subjected to combined creep and erosion failure Development of tube failure predictive tool: The basic equation is deduced for prediction of wear is used to find the wear rate equation for SAE 210 Grade A1; a common boiler tube material is deduced. The constants are used determined from the experimental data. 2 MV 2 2 W = cos α sin 2α sin α total where W is erosion rate, M is mass rate of eroding material; V is velocity of eroding material and α is impact angle of eroding particles. The predicted velocity used in above wear rate equation predicts the quantity of material removed. This wear rate predicts the time in hours to get the tube eroded to reach critical thickness. This gives the expected life of boiler tube installed. The predicted flue gas temperature helps to predict the tube surface temperature and the eroding particle temperature. This helps to take into account the sharpness factor of eroding material. Thus the velocity and temperature prediction leads to development of predictive tool for boiler tube failure. It is discussed in detail in Chapter