# Generation and Control of Vacuum in Furnace

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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 kW due to increase in furnace vacuum from 58.9 Pa to 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 = kPa  pO2 = kPa  PN2 = kPa  pSO2 = kPa  pH2O = kPa Mass flow rate of each constituent in tons/hour is:  Mass flow rate of O2 in the flue gas = tph  Mass flow rate of CO2 in the flue gas = tph  Mass flow rate of N2 in the flue gas = tph  Mass flow rate of SO2 in the flue gas = tph  Mass flow rate of H20 in the flue gas = tph

Energy Audit of Flue Gas
Temperature of flue gas = 136 ºC – 150oC Dew point of water is (obtained based on partial pressure of bar) 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 C KJ/kg Mass flow rate of each constituent at C ( kg/s) Total thermal power released (MW) CO2 16.37 606.32 527.85 3.69 0.2895 O2 1.11 374.43 294 72.9 5.8678 N2 68.14 425 335.09 193.3 S02 0.036 487 430.55 0.2341 0.0132 H20 13.36 2752 2591 30.444 11.576

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 606.32 485.83 3.69 O2 374.43 248.35 72.95 N2 425 283.32 193.3 S02 487 399.58 0.2341 H20 2752 2501

Model Experimentation

Expected Performance of the heat exchanger
Cooling capacity of the heat exchanger = 10 kW Cooling load available with the heat exchanger = 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 65 69 4.2 82 Calculation of Condensate Flow rate Gas Flow rate (kg/sec) Mesured condensate kg/hr g/sec Condensate loading (gms/kg of gas) 1.1 0.9 0.25 1 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