Boiler Performance pdf notes thermodynamics

Boiler Performance pdf notes thermodynamics

1. 1

2. Assessment of a Boiler 2 Boiler performance Causes of poor boiler performance - Poor combustion - Heat transfer surface fouling - Poor operation and maintenance - Deteriorating fuel and water quality Boiler efficiency: determine deviation from best efficiency Heat balance: identify heat losses

3. A) Evaporative capacity or Evaporative ratio can be expressed in terms of: kg of steam/hr kg of steam/kg of fuel fired 3

4. But different boilers give out steam at different temperature and pressure, Evaporative capacity expressed in terms of mass of water getting converted to steam doesn’t give a good comparison Equivalent evaporation refers to the quantity of dry saturated steam generated per unit time from feed water at 100 C to steam at 100 C at the saturation pressure corresponding to 100 C. m e = m a (h- h f )/h fg (i.e. bundles of Latent Heat) 4

5. It is defined as the ratio of the heat received by 1 kg of water at the working conditions to that received by 1 kg of water evaporated from and at 100 C F e = (h –h f )/2257 KJ 5

6. Ratio of heat actually utilized in the generator to the heat supplied by the fuel. I S 875 3 : Indian Standard for Boi l e r E f fi c i e ncy T e s t i ng 6

7. Q1) During the boiler trial observations were made. Steam generation: Coal consumed: Rate of steam generation: Feed water temperature: Total heating surface area: Total grate area: Calorific value of coal: for 24 hours following 16 bar, dry saturated. 10,000 kg 2500 kg/hr 27C 3000 m 2 4m 2 28000 kJ/kg Determine: the mass of coal burnt per m 2 of grate per hour the equivalent evaporation from and at 100C per kg of coal the equivalent evaporation from and at 100C per m2 of total heating surface per hour (iv) the boiler efficiency ( Onkar 11/488) 7

8. Equivalent evaporation from and at 100C per kg of coal = Coal burnt per hour=416.67 kg/hr Coal burnt per m 2 of grate per hour = 104.17 kg/m 2 grate surface per hr Rate of steam generated per kg of coal =5.99 = 6 kg steam/kg coal (approx) Heat added to steam per kg of coal = 6 [hg at 16 bar – 4.18 x 27}=16086.84 kJ Latent, heat from and at 100C = 2257 kJ/kg Equivalent evaporation from and at 100C per kg of coal = 7.13 kg Equivalent evaporation from and at 100C per m 2 of total surface per hour = (7.13 x 416.67)/3000 = 0.99 Kg Boiler efficiency = (7.13 x2257)/28000 = 57.47 % 8

9. 9

10. The efficiency test does not account for: 10 Standby losses. Efficiency test is to be carried out, when the boiler is operating under a steady load. Therefore, the combustion efficiency test does not reveal standby losses,which occur between firing intervals . Blow down loss. The amount of energy wasted by blow down varies over a wide range. . Soot blower steam. The amount of steam used by soot blowers is variable that depends on the type of fuel. . Auxiliary equipment energy consumption. The combustion efficiency test does not account for the energy usage by auxiliary equipments, such as burners, fans, and pumps.

11. Assessment of a Boiler Heat Balance An energy flow diagram describes geographically how energy is transformed from fuel into useful energy, heat and losses FUEL INPUT STEAM OUTPUT Stochiometric Excess Air Un burnt Stack Gas 11 Ash and Un- burnt parts of Fuel in Ash Blow Down Convection & Radiation

12. Assessment of a Boiler Heat Balance Balancing total energy entering a boiler against the energy that leaves the boiler in different forms Heat loss due to dry flue gas Heat loss due to steam in fuel gas Heat loss due to moisture in fuel Heat loss due to unburnts in residue Heat loss due to moisture in air Heat loss due to radiation & other unaccounted loss 12.7 % 8.1 % 1.7 % 0.3 % 2.4 % 1.0 % 100.0 % Fuel 73.8 % Heat in Steam 12 BOILER

13. Heat utilized in making steam 13 Heat taken for generation of steam per kg of fuel burnt shall be Q steam = m steam (h – h w ) m steam is mass of steam generated per kg of fuel burnt, h is enthalpy of final steam produced and h w is enthalpy of feed water. Q steam = Equivalent evaporation x 2257, kJ/kg of fuel burnt

14. A) Heat lost to flue gases 14

15. B) Heat lost due to incomplete combustion 15

16. Heat lost in unburnt fuel: Some portion of heat may get lost in unburnt fuel, which could be given as Q unburnt = m ubf x C In case of solid fuels this loss cannot be completely avoided Heat loss due to moisture in fuel Moisture present in fuel shall get evaporated and superheated as fuel is burnt. For this latent and sensible heat requirement shall be met from heat available in boiler due to burning of fuel. Mathematically, for unit mass of fuel burnt, Q moisture = m moist (hs 2 – hf 2 ) = m moist [h g +C p (t g –t sat ) - hf 2 ] = m moist [ 2676 +C p (t g - 100) - hf 2 ] (as flue gases are at atm press) where m moist is mass of moisture per kg of fuel burnt, hs 2 is enthalpy of final (superheated) steam produced and hf 2 is enthalpy of water at boiler furnace temperature. 16

17. E) Heat lost to steam by combustion of Hydrogen per kg of fuel 17

18. F) Heat loss due to convection, radiation and other unaccountable losses Exact quantification of these losses is not possible, therefore these can be estimated by the difference of total heat available and cumulative heat loss described from A to E. Q unaccounted = (m f x C) – (Q for generating steam + Q dry flue +Q steam in flue + Q incomplete + Q unburnt + Q moisture +Q Hydrogen ) 18

19. Heat Balance Sheet 19

20. Boiler trial refers to running the boiler under test conditions for its performance estimation. It gives the steam generation capacity of boiler, thermal efficiency of plant and heat balance sheet of the boiler. Under trial the boiler is run for quite long durations so as to attain steady state. Generally the boilers are run for 4 to 6 hours duration for the boilers of oil fired type and coal fired types. Duration of boiler run for attaining steady state changes from boiler to boiler. Observations are taken after the boiler attains steady state for a duration ranging from 10–15 minutes. Measurements are made for fuel supply, combustion analysis, steam generation rate and its quality/state, flue gas and their analysis, temperature and pressure at salient locations and all other measurements as required for heat balance sheet preparation. 20

21. 21

22. 22

23. 23

24. 24