Generation and Control of Vacuum in Furnace P M V Subbarao Professor Mechanical Engineering Department Safe and Efficient Combustion Needs Appropriate Furnace Pressure…

Development of Air & Flow Circuits

Total gas side pressure drop Pa where p1 = total pressure drop from the furnace outlet to the dust collector, Pa p2 = pressure drop after the dust collector, Pa  = ash content in the glue gas, kg/kg pa v = average pressure of the gas, Pa pg o = flue gas density at standard conditions, kg/Nm3

The ash fraction of the flue gas calculated as, where f h = ratio of fly ash in flue gas to total ash in the fuel A = ash content of working mass, % Vg = average volume of gas from furnace to dust collector calculated from the average excess air ratio, Nm3/kg of fuel

prest = pexit + pgas –Dpnd The pressure drop from the balance point of the furnace to the chimney base is prest = pexit + pgas –Dpnd where pexit = pressure drop up to the boiler outlet

Draught Losses Dp Total losses Furnace, SH & RH Losses Economizer Losses Ducts & dampers losses Percent Boiler Rating

ID fan power calculation ID fan power is calculated as:

Air Pressure Losses Dp Total losses Burner Losses APH Losses Ducts & dampers losses Percent Boiler Rating

Modeling of 210 MW Draught System FD Fan Duct APH Furnace Back pass ESP ID Chimney Pressure drop calculation in air & gas path and its comparison with design value. Assessment of ID and FD fan power as a function of furnace pressure.

Important variables along air and gas path

Pressure Variation Duct FD Fan SCAPH APH Duct Wind Box Boiler APH ESP ID Fan Duct

Off Design Pressure Variation Pressure Variation in Air & Gas Path at Part Load -2000 -1500 -1000 -500 500 1000 1500 2000 2500 1 2 3 4 5 6 7 8 9 10 11 12 Path Element Pressure (Pa) Calculated (168 MW) Design (168 MW) ID Fan ESP Boiler APH Wind Box Duct SCAPH FD Fan

Operational Data of 210 MW plant

Effect of Furnace Vacuum on Boiler Efficiency

The net effect is saving in energy of 117 The net effect is saving in energy of 117.32 kW due to increase in furnace vacuum from 58.9 Pa to 230.6 Pa.

New Ideas for Future Research FD Fan Duct APH Furnace Back pass ESP ID Chimney

Analysis of Flue Gas at the ID Fan Inlet Partial pressure of each constituent in flue gas,  pCO2 = 16.366209 kPa  pO2 = 1.138404 kPa  PN2 = 68.142138 kPa  pSO2 = 0.036081 kPa  pH2O = 13.363218 kPa Mass flow rate of each constituent in tons/hour is:  Mass flow rate of O2 in the flue gas =13.2867 tph  Mass flow rate of CO2 in the flue gas = 262.646 tph  Mass flow rate of N2 in the flue gas = 695.893 tph  Mass flow rate of SO2 in the flue gas = 0.84219 tph  Mass flow rate of H20 in the flue gas = 118.33 tph

Energy Audit of Flue Gas Temperature of flue gas = 136 ºC – 150oC Dew point of water is (obtained based on partial pressure of 0.1336 bar) 51.59 ºC Cooling of the exhaust gas below the dew point will lead to continuous condensation of water vapour and reduction of flue gas volume and mass. The temperature of the flue gas in order to remove x% of the available moisture can be obtained using partial pressures of water.

Energy Potential of Flue Gas with 10% water Recovery Flue gas constituents Partial pressure at 136 C in kPa Enthalpy* at 136 C (KJ/kg) Mass flow rate of each constituent at 136 C ( kg/s) Enthalpy*at 49.74 C KJ/kg Mass flow rate of each constituent at 49.74 C ( kg/s) Total thermal power released (MW) CO2 16.37 606.32 3.69075 527.85 3.69 0.2895 O2 1.11 374.43 72.9572 294 72.9 5.8678 N2 68.14 425 193.303 335.09 193.3 17.3797 S02 0.036 487 0.23413 430.55 0.2341 0.0132 H20 13.36 2752 32.8694 2591 30.444 11.576   35.1270

Energy Potential of Flue Gas with 100% water Recovery Flue gas constituents Partial pressure at 136 C in kPa Enthalpy* at 136 C (KJ/kg) Mass flow rate of each constituent at 136 C ( kg/s) Enthalpy at 0 C (kJ/Kg) Mass flow rate at 0 C ( kg/s) Total thermal power released (MW) CO2 16.366209 606.32 3.69075 485.83 3.69 0.444698 O2 1.138404 374.43 72.9572 248.35 72.95 9.198452 N2 68.142138 425 193.303 283.32 193.3 27.38828 S02 0.036081 487 0.23413 399.58 0.2341 0.020468 H20 13.363218 2752 32.8694 2501 90.45671   127.5086

Model Experimentation

Expected Performance of the heat exchanger Cooling capacity of the heat exchanger = 10 kW Cooling load available with the heat exchanger = 115.3 kJ/kg of flue gas Available rate of condensation of the present heat exchanger = 37.85gms/kg of flue gas.

Experimental validation Flue Gas heat exchanger measured data: DATE FLUE GAS I/L JUST OUTSIDE ID DUCT I/L TO HEAT EXCHANGER O/L TO HEAT EXCHANGER WATER I/L TO HEAT EXCHANGER O/L TO HEAT EXCHANGER DP WATER FLOW QTY. OF WATER CONDENSED Temp °C cm WC LPM lt. /Hr. 1.2.10 103 60 30 29 5 12 1.1 105 65 32 31 10 0.9 2.2.10 121 69 82 4.2 1

Calculation of Flue Gas Flow Rate  Dp (cm) Tin 0C  Density (kg/m3)   Flow rate (kg/sec) 5 60 1.051754 0.007159 65 1.036203 0.007106 69 1.024089 0.007065 4.2 82 0.986604 0.006355 Calculation of Condensate Flow rate Gas Flow rate (kg/sec) Mesured condensate kg/hr g/sec Condensate loading (gms/kg of gas) 0.007159 1.1 0.305556 42.67864 0.007106 0.9 0.25 35.17994 0.007065 43.25126 0.006355 1 0.277778 43.70829 Design rate of condensate loading using present heat exchanger = 37.85gms/kg of flue gas.

Combustion and Draught Control The control of combustion in a steam generator is extremely critical. Maximization of operational efficiency requires accurate combustion. Fuel consumption rate should exactly match the demand for steam. The variation of fuel flow rate should be executed safely. The rate of energy release should occur without any risk to the plant, personal or environment.

Furnace Draught

The Control Furnace (draft) pressure control is used in balanced draft furnaces in order to regulate draft pressure. Draft pressure is affected by both the FD and ID fans. The FD fan is regulated by the combustion control loop, and its sole function is to provide combustion air to satisfy the firing rate. The ID fan is regulated by the furnace pressure control loop and its function is to remove combustion gases at a controlled rate such that draft pressure remains constant.

Furnace Draught Control

Windbox Pressure Control

Combustion Prediction & Control

The Model for Combustion Control

Parallel Control of Fuel & Air Flow Rate

Flow Ratio Control : Fuel Lead

Flow Ratio Control : Fuel Lead

Cross-limited Control System

Oxygen Trimming of Fuel/air ratio Control

Combined CO & O2 Trimming of Fuel/Air Ratio Control

Resistance to Air & Gas Flow Through Steam Generator System