Final Audit Utilization of Flue Gas Energy P

Final Audit : Utilization of Flue Gas Energy P M V Subbarao Professor Mechanical Engineering Department Minimize Final Exhaust Gas Temperature…. Properly Utilize Enthalpy of Flue Gas….

3 2 s 2 f 5 4 2 1 6 s

Thermal Structure of A Boiler Furnace DPNL SH Platen SHTR drum R H T R screen tubes LTSH Economiser stack BCW pump Furnace APH ESP ID Fan Bottom ash

Combustion Losses C & R losses Hot Exhaust Gas losses Pendent SH Platen SH CSH Reheater Furnace ater wall absorption

Sequence of Energy Exchange from Flue Gas to Steam FLUE GAS PLATEN SH RH COVECTIVE LTSH PENDENT SH EVAPORATOR Water Wall

Combustion Losses C & R losses Fuel Power Final SH Platen SH Furnace absorption LTSH Reheater Hot Exhaust Gas losses ~4000 C

Details of 500 MW(e) Capacity Indian Power Unit • • • Main steam Flow rate: 425 kg/s. Main Steam Temperature: 5400 C Reheat Steam Flow rate: 38. 7 kg/s. Air Flow Rate: 577. 06 kg/s. Coal Flow Rate: 73. 8 Kg/s.

500 MW

Steam Temperatures Gas Temperatures • • • Platen Super Heater: Inlet Temperature: 1236. 4 0 C Outlet Temperature: 1077 0 C Final Super Heater: Inlet Temperature: 1077 0 C Outlet Temperature: 962. 4 0 C Reheater: Inlet Temperature: 962. 4 0 C Outlet Temperature: 724. 3 0 C Low Temperature Super Heater: • Inlet Temperature: 724. 30 C • Outlet Temperature: 481. 3 0 C • • • Platen Super Heater: Inlet Temperature: 404 0 C Outlet Temperature: 475 0 C Final Super Heater: Inlet Temperature: 475 0 C Outlet Temperature: 540 0 C Reheater: Inlet Temperature: 345 0 C Outlet Temperature: 5400 C Low Temperature Super Heater: • Inlet Temperature: 3590 C • Outlet Temperature: 404 0 C

Flue Gas Temperature At different regions of Furnace: 210 MWe) Design 1 Adiabatic Flame Temp (K) 1957 Calculated 1966 2 FEGT (0 C) 1102 1117 3 Platen SH-I Outlet (0 C) Platen SH-II Outlet-I outlet 4 (0 C) 932 951 859 878 5 RH 3 rd & 2 nd outlet (0 C) 595 604 6 RH 1 st Stage outlet (0 C) 510 531 7 Economiser outlet (0 C) 385 398 8 APH Outlet (0 C) 138 151

Steam and Gas Paths

Suggested Fluid Velocities • Flue gas velocities: 10 – 18 m/s. • Steam in super heaters & reheaters: 10 – 25 m/s. • Water Wall circulation : 0. 35 – 3. 5 m/s.

500 MW

LMTD for various Devices

Surface Area of Heat Exchangers: 500 MW

Economizer • The economizer preheats the feed water by utilizing the residual heat of the flue gas. • It reduces the exhaust gas temperature and saves the fuel. • Modern power plants use steel-tube-type economizers. • Design Configuration: divided into several sections : 0. 6 – 0. 8 m gap

Tube Bank Arrangement

Thermal Structure of Economizer • • • Out side diameter : 25 – 38 mm. Tube thinckness: 3 – 5 mm Transverse spacing : 2. 5 – 3. 0 Longitudinal spacing : 1. 5 – 2. 0 The water flow velocity : 600 – 800 kg/m 2 s The waterside resistance should not exceed 5 – 8 %. Of drum pressure. • Flue gas velocity : 7 – 13 m/s.

Thermal Balance in Economizer. • The energy absorbed by steam • The convective heat lost by flue gas • Overall Coefficient of Heat Transfer, U

Mean Temperature Difference • The average temperature difference for parallel flow and counter flow is expressed as • It is also called log mean temperature difference • When Dtmax / Dtmin > 1. 7, the average temperature may be expressed as: • Generally, the flow direction of the flue gas is perpendicular to the axes of tubes. • If number of bends are more than four, the flow can be treated as counter or parallel flow.

Complex Flow • Parallel flow and counter flow may simultaneously exist in one section of an economizer. • This is called complex flow.

• • • For a given set of inlet and outlet temperatures of the fluids, The temperature difference of parallel flow is the greatest, The temperature difference of counter flow is the lowest And that of complex flow is in between. The average temperature difference in a complex flow can be calculated as: • When • Otherwise, the temperature difference is determined by • The value of Ktd is determined by flow type and thermal parameters.

Steam Temperatures Gas Temperatures • • • • Platen Super Heater: Inlet Temperature: 1236. 4 0 C Outlet Temperature: 1077 0 C Final Super Heater: Inlet Temperature: 1077 0 C Outlet Temperature: 962. 4 0 C Reheater: Inlet Temperature: 962. 4 0 C Outlet Temperature: 724. 3 0 C Low Temperature Super Heater: Inlet Temperature: 724. 30 C Outlet Temperature: 481. 3 0 C Economizer: Inlet Temperature: 481. 3 0 C Outlet Temperature: 328. 5 0 C • • • • Platen Super Heater: Inlet Temperature: 404 0 C Outlet Temperature: 475 0 C Final Super Heater: Inlet Temperature: 475 0 C Outlet Temperature: 540 0 C Reheater: Inlet Temperature: 345 0 C Outlet Temperature: 5400 C Low Temperature Super Heater: Inlet Temperature: 3590 C Outlet Temperature: 404 0 C Economizer: Inlet Temperature: 254 0 C Outlet Temperature: 302 0 C

Combustion Losses Pendent SH Platen SH Furnace absorption C & R losses CSH Reheater Hot Exhaust Gas losses Economizer APH