1-Why Boiler is considered as explosive equipment / system?
Boiler involves the generation of steam at higher pressure & temperature upon release of this into atmosphere could cause disasters.
2-How do you specify the Boilers?
Boilers are specified by their operating pressure, temperature, steam generating capacity & even by heating surface.
3-What are the various circuits involved in Boilers?
Water circuit, Steam circuit, air circuit & flue gas circuit
4-Write down the various energy used / generated in power plants?
Chemical energy (fuel)
Thermal energy (Heat present in flue gases & steam)
Kinetic energy (rotation of Turbine rotor),
Mechanical energy (shaft power on Turbine)
Electrical energy (In generators).
5-Why steam, is generated at high pressure & temperature?
High pressure & temperatures steam is having more heat content & also has more potential to do work.
6-At what pressure do you carryout Hydraulic test on Boilers?
For new boilers it is 1.5 times the design pressure & for old boilers it is 1.25 times the design pressure (also depends on Boiler life & its physical conditions)
7-What is the standard method or procedure followed for boiler pressure rising during hydraulic tests?
Pressure rise is 3.5 kg/cm2 per minute
8-What will happen if drum safety valves have been set at higher blow down rates?
Super heater coils may over heat due to insufficient flow of steam
8a-What are the reasons for failure of super heater coils?
Overheating due to insufficient fuel
Erosion due to high flue gas
Internal scaling
Priming
9-What is the MOC of steam drum?
Carbon steel: SA 516 Gr.70
9a-What is the MOC of water wall tubes & super heater coils?
Carbon steels 210 Gr.A
Super heater coils: SA 213 Gr.T11, T22, T 91 & T92
10-Why do you carryout pre heating & post heating before welding works?
Pre heating: For expelling out the moisture present in the materials
Post heating: It is done after the welding is done for relieving residual stress due to welding
11-Why the pressure gauges installed at boilers firing floor show more reading as compared that of installed at lines?
Firing floor gauges show actual line pressure & pressure head due to height from line to firing floor (around 30 meters)
12-Why it is been recommended to operate boiler safety valves at least once in a shift or day or week?
To avoid seat stuck up due to rusting
13-What is the allowable reduction of tubes thickness?
It is around 20% of original thickness. For example a SH coil of original thickness is 5mm, then it should be replaced if its thickness at straight portion reduces up to 5-5X 20% = 4 mm
14-What are the reasons for overheating Boilers pressure parts?
Overheating is due to
Wrongly set burners/spreaders
High velocity of flue gases
More firing rate
Internal scaling
15-On what factors Boilers thermal expansion depends?
It depends on
Boiler operating temperature
Boiler tube materials composition
Length of tubes
16-What are the reasons for fish mouth failure of boiler tubes/coils?
Over heating
Erosion & corrosion
16a-What do you mean by priming in Boilers?
It is the carryover of water particles into steam
17-What could be the probable causes of priming?
Drum level fluctuation
Boiler load fluctuation
Boiler parameters fluctuation
18-What is the reasons for monitoring of Oxygen level in flue gases?
To control combustion & to achieve maximum efficiency of combustion by optimising excess air
19-What are the functions of steam drum?
Steam drum acts as water storage device
Separates steam & water
Provides space for internal chemical treatment
Removes sludge from boilers through blow down
Accommodates safety valves to relieve excess pressure during abnormal operating conditions
20-What is the function of start-up vents in boilers?
To control steam pressure during abnormal operating conditions
To provide minimum flow through super heater coils
To increase main steam temperature during low loads
21-Why steam drum dish end thickness is lesser than that of cylindrical portion
Because: Stress produced in dish ends are circumferential & are less as compared to stress developed in longitudinal portion.
22-What do you mean by an equivalent evaporation?
Quantity of water evaporated at 100 deg C to dry saturated steam at 100 deg C
23-What are the reasons for more main steam temperature?
More excess air
High moisture fuel
More convective heat transfer at super heater zone due to loss of turbulence
Failure of attemperator control valve
Operating the Boiler at lower feed water temperature at economiser inlet.
24-Why it is been not recommended to give blow down during high steaming rates?
Sudden opening of IBD valves or water wall bottom header valves will affect circulation rate & may lead to failure of water wall tubes.
25-Why Deaerators are placed at higher elevation?
To give NPSH to boiler feed pumps
26-What are the functions of Deaerator?
To remove dissolved oxygen
To store the feed water
To increase the feed water temperature
To give NPSH to boiler feed pumps
Allows space for LP dosing (Oxygen scavenger dosing)
27-More height of Chimney creates what?
Creates more natural draught & helps to reduce load on ID fans
28-What is the function of draught system?
To force air for combustion
To expel out products of combustion from Boiler
29-What are the different types of draughts used in Boilers?
Forced draught, induced draught & Balanced draught?
30-On what factors chimney construction depends?
Quantity of flue gas generated per hour
Draught to be produced
Sulphur content in fuel
Environment clearance
31-Why the ID fans are not situated between APH & ESP?
If placed between ESP & APH, Fan size increases due to higher specific volume of gases. As flue gas volume decreases as its temperature decreases.
32-Where do you use steel Chimneys?
For economy at lower flue gas flow steel chimneys are more preferred
33-What is the velocity of flue gas inside the chimney?
It is 10 to 14 m/sec
34-What is the velocity of flue gas inside the ESP?
It is around 0.75 to 1 m/sec
35-What factors affect the draught produced in Chimneys?
Flue gas temperature & air temperature
35a-What is the velocity of wind considered for Boiler Construction?
Around 39 m/sec
36-What do you mean by cold air inversions in chimney?
It happens when outside air pressure is more than flue gas pressure inside the chimney
37-When can cold air inversions happen?
This phenomenon happens when number of boilers operating at various loads are connected to a single Chimney
38-What is the disadvantages of concrete chimneys over metal chimneys?
High susceptible for thermal shock
High cost of construction
39-What does it mean that Chimney is producing good draught?
If chimney is producing good draught means, flue gas temperature is more & hence Boiler efficiency is less
40-How do you say that high pressure & temperature power plants have greater efficiency?
High pressure & temperature steam will have more heat content & high enthalpy drop will be available in turbine expansion.
41-What do you mean by circulation ration in boilers?
It is the inverse of mass steam content
Circulation ratio = Mass flow rate of steam & water mixture / Mass flow of steam through tubes
42-What do you mean by a load control range of a Boiler?
Boiler operating generation range in which boilers auto controls work.
43-In which cases do you take emergency shutdown of Boilers?
During tubes leakage
Overshooting of steam temperature
Boiler furnace explosion
Boiler brick work damage
Furnace draught fluctuation
Failure of Boiler feed pumps
Failure of drum level gauges (local & remote)
44-What do you mean by MCR & ECR in Boilers?
MCR: Maximum continuous rating
ECR: Economic continuous rating
45-In what load Boiler will have higher efficiency?
In ECR
46-What is the lowest load range of boiler to operate it comfortably?
It is around 30 to 40%
47-In all most all cogeneration & Thermal power plants lowest range of super heater temperature is 485 to 500 deg C, Why?
This is for protecting Turbine last stage blades from water particles erosion damage
48-What adverse effect do you observe in an overloaded Boiler?
It affects circulation velocity
Flue gas temperature increases
Steam temperature increases may result into SH coil failure
49-Why do you measure O2 & CO2 Percentages in flue gas?
To know about combustion, whether it is proper or not.
50-How do you differentiate hot, warm & cold start-up of Boilers?
Hot start up: Boiler is restarted within 6 to 10 hours shutdown (Within a shift)
Warm start up: Boiler is restarted within 10 to 70 hours shutdown (Within 3 days)
Cold start up: If boiler is started after 70 to 90 hours of shutdown (more than 3 days)
51-What are the various methods of Boilers preservation during shutdown?
Wet preservation method
Dry preservation method
52-Where do you use wet preservation method?
Used for standby boilers & they should be available for immediate use
53-Where do you use Dry preservation method?
Used for Boilers which are kept shutdown for long periods & they are not required for immediate use
54-What is the concentration of Hydrazine for wet preserved Boilers?
It is >200 ppm
55-What is the alternative chemical used for wet preservation
It is Sodium sulphite & concentration maintained is 350-400 ppm
56-What is the significance of Slag screens?
These are the staggered tubes installed in flue gas path to avoid clogging of tubes as a result of cooling & adhering of molten slags.
57-What are the drum internals?
These are the internal parts of steam drums utilized for separation of steam water mixture & for chemical dosing
Drum internals are
1. Cyclone separators
2. Chevrons
3. Demisters
4. HP dosing connection
5. Feed water connection
6. CBD line
58-What do you mean by steam separation?
It is the process of separating bulk water particles from steam
59-What are the various methods used for separation of water particles from steam?
Centrifugal separation method
Use of baffles
Abrupt change of steam water mixture direction
By gravity method
Direct hitting on plates
60-What is dry pipe in Boilers?
Dry pipe is the perforated pipe fitted at the most height level of boiler to provide dry steam (around 98%)
61-What are the Boiler auxiliaries?
These are the system or equipments used in Boilers to boost of the Boiler efficiency & performance
These are
Economiser
Super heaters
Air preheaters
Boiler fans & pumps
Soot blowers
Water storage tank
Chemical dosing system
PRVS
62-What are boiler mountings?
These are the devices used for safe operation of Boilers
Mountings are:
Safety valve
Boiler level gauge
Steam stop valve & NRV
Start-up vent valve
63-Why do ID fans & FD fans are not installed before ESP & after APH respectively?
This is for reducing fan size & load on it .As flue gas & air volume is more at higher temperatures, in order to drag & push these high temperature flue gas & air need to install bigger size fans
64-What type of Impellers used in ID & FD fans & why?
Backward curved fans are used, as these offer very less resistance to rotate
65-Why do you use super-heated steam in power plants?
More heat is with SH steam & more enthalpy drop available & hence more efficiency. And also SH steam is having no water particles due to this equipments life increases
66-What is the necessity of forced circulation in high pressure boilers?
As the pressure increases beyond 150 kg/cm2 up to 220 kg/cm2 density of steam & water becomes same & hence there will not be any chance for natural circulation due to density difference. Hence there need arises for forced circulation
67-Do you prefer soot blowing after shutdown of Boilers?
Generally NO, because soot blowing may result into explosion due to leftover unburnt fuel & hot ash
68-What do you mean by wire drawing?
It is the process of steam expansion & no any work done or loss. It happens in orifice, flow meters & throttled valves.
In PRV & PCVs stations does not contribute in work done or loss & hence upon reducing pressure of normal (saturated steam) steam, it becomes super-heated.
69-What will happen if economiser inlet feed water temperature drops?
Lower feed water temperature at economiser inlet lead to more absorption of heat from flue gases. This causes drop in flue gas temperature, may lead to acid corrosion.
70-What will happen if economiser outlet feed water temperature increases more than design?
More feed water temperature at economiser outlet may lead to steaming of water, which could ultimately cause overheating & failure of economiser tubes. This happens in low load operation.
71-What is the recommended flue gas temperature at APH & ESPs?
It is more than acid due point temperature to avoid corrosion of materials. For coal & biomass fired boilers it is always >140 Deg C
72-What will happen to the enthalpy of steam if its pressure increases & temperature remains constant?
Its enthalpy decreases
73-What will happen to the enthalpy of steam if its temperature increases & pressure remains constant?
Its enthalpy increases
74-What will happen to the enthalpy of steam if its temperature & pressure increase?
Enthalpy increases
75-What will happen to the enthalpy of evaporation of saturated fluid if its pressure increases?
Enthalpy of evaporation decreases if pressure is increased
76-What will happen to the enthalpy of evaporation of saturated fluid if its temperature increases?
Enthalpy of evaporation decreases if temperature is increased
77-What will happen to the density of water if its temperature increases?
Density decreases
78-What will happen to the density of water if its pressure increases?
Density decreases
79-What happens to density of steam if pressure & temperature increase?
Density also increases
80-What is the function of bypass valve arranged for main steam stop valve?
It is mainly for pressure equalizing to open the main stop valve & also for initial charging of steam line
81-When do you use single & 3-element drum level controllers?
Single element is used when the Boiler load is less than 30% of MCR
3-element controller is used when boiler load is >30% of MCR
82-A 85 TPH boiler is operating on 80% load & its blow down is 0.8%.Calculate the makeup water required
Boiler operating load = 85 X 80 / 100 =68 TPH
Blow down water quantity = 68 X 0.8 / 100 =0.544 TPH
So make up water quantity is 0.544 TPH
83-What will happen to the boiler tubes if pH is more or less than recommended?
If pH is less it leads to corrosion of tubes & if more it leads to scaling
84-What are the scale forming contents of water?
Salts of calcium & magnesium, sodium carbonates, nitrates & silica
85-A feed water sample is not showing the residual hydrazine, then what does it mean?
It means that there is no treatment for oxygen scavenging, or chemical dosed is insufficient. This could lead to corrosion of pressure parts.
86-Which chemicals are used in Boilers HP & LP dosing systems?
HP dosing: Tri-sodium phosphates
LP dosing: pH booster & Oxygen scavengers
87-When your chemist asks you for giving IBD?
IBD is given, when following parameters in Boiler water exceed?
Silica, Phosphate, pH & conductivity
88-What is the major loss in coal fired & bagasse fired boilers?
In coal fired Boilers major loss is heat loss due to dry flue gas. And in Bagasse fired boilers major loss is heat loss due to moisture present in fuel
89-What do you mean by air to fuel ratio?
It is the amount of air required to burn 1 kg of fuel
90-What do you mean by steam to fuel ratio?
It is the amount of steam generated on burning 1 kg of fuel
91-What is the velocity of flue gas at various zones of Boilers?
Furnace: 1 to 2 m/sec
Super heaters: 2 to 4 m/sec
Economiser: 6 to 7 m/sec
APH: 8 to 12 m/sec
ESP (Inside): 0.75 to 1 m/sec
ID fan inlet: 13 to 16 m/sec
92-What is the function of SCAPH?
SCAPH (Steam coil air pre heater) is used to heat FD air which is entering into APH or some type SCAPH is used an alternative of APH.
93-Which contents of fuels are responsible for more excess air?
Bagasse moisture, hydrogen & carbon contents cause more requirement of combustion air
94-Which content of the fuel has very least on combustion air requirement?
Sulfur
95-Which content of the fuel upon increase in % causes less combustion air?
Oxygen
96-What is the excess air required for combustion of Bagasse, Coal, Gas, Oil & Wood?
Bagasse: 25-35%
Coal: 20-25%
Gas: 5-10%
Oil: 10-15%
Wood: 35-45%
97-Which fuels require more & least excess air for complete combustion?
Wood needs more excess air & Gas needs less excess air among all fuels
98-What can cause if Boiler line safety valve is set at higher blow down?
Results into
Drum level fluctuation
More load on Super heater coils
Steam loss
99-What is the function of over fired Air (OFA)?
OFA is used to create turbulence & to increase residence time of fuel particles in furnace
100-What are the reasons for incomplete combustion?
Reasons are;
Less excess air
More ash content in the fuel
More moisture in the fuel
Not achieving the 3Ts (Temperature, Time & Turbulence) in furnace
Unbalanced draught in furnace
101-What happens to the velocity & pressure of steam in steam line if flow is increased more than design?
Velocity of steam increases & pressure drop occurs
102-How do you reduce excess load from ID fans?
By maintaining optimum excess air
By arresting all cold air ingress points
By arresting APH tubes leakages
By conducting regular maintenance of fans
103-How do you reduce excess load from FD fans?
By maintaining optimum excess air
By arresting all air & flue gas leakages
By controlling moisture level in the fuel
Boiler design considerations:
Factors considered for Boiler engineering/Boiler Design
1-Boiler Design code: IBR 1950 with latest all amendments
2. Ambient air temperature: Depends on the location of the plant, in India Generally 30 to 45 Degree C is considered.
3. Relative humidity for performance: (Normal: 60% to 70%, Maximum: 80% & Minimum: 20%).Generally it depends on site condition
4. Wind Velocity & direction for structure design: as per IS 875 (39 m/sec)
5. Seismic coefficient As per Is 1893 Zone-I
6. Wind velocity for insulation: generally 1 m/sec
7-Maximum Continuous Rating (MCR) required for particular process or use: It is the capacity of the Boiler in TPH or Kg/hour required.
8-Operating pressure & temperature: Operating pressure & temperature of the Boilers depends on the requirement of process of prime mover. Higher the operating parameters higher will be the Boiler efficiency & lower will be the fuel consumption. For high pressure Boilers water quality required is
9-Type of fuel: It depends on the easily availability of fuel, for example: Sugar based cogeneration Boilers are designed for bagasse fuel. Utility Boilers are designed for Briquettes & Indian/ Indonesian coal. Thermal power plant Boilers are designed for coal.
Gujarat state area Boilers are designed for natural gas fuel. Middle yeast country Boilers are designed for Oil fuel.
While designing the Boilers & combustion system fuel GCV, ash & moisture contents are considered.
10-Peek Capacity of the Boiler: It is the maximum allowable load on Boiler for short period. Generally it is kept 110% of MCR.Peak capacity is allowed 30 minutes in 8 hours.
11-Range of load variation/Steam temperature control range: It is the range of load (as % of MCR) during which Main steam temperature is maintained constant. Generally it is around 60-100%.
12-Percentage of excess air or Excess air coefficient at Boiler furnace & Boiler outlet
13-Circulation ratio
14-Feed water temperature at economiser inlet: In order to achieve desired efficiency & fuel consumption, feed water at economiser inlet is very must. Higher the feed water temperature, lower will be the fuel consumption & Vice versa.
14a-Feed water quality like pH, conductivity, TDS, Silica, hardness, Alkalinity etc
15-Steam temperature drop at Super heater coils: In order to get the desired main steam temperature at super heater outlet, need to maintain the ∆T across super heaters.
16-Flue gas temperature drop: While engineering Boilers, flue gas temperature drop at every zone (Super heaters, Bank zone, Economiser, APH etc) should be considered.
17-Combustion air temperature: FD & SA air temperatures after APH should be optimum. More air temperature leads to formation of clinker & less air temperature leads to less combustion efficiency.
18-Air & Flue gas ducts sizing: In order to avoid pressure drops in ducting proper sized ducts have to be considered. Flue gas pressure drop is very important for considering heat transfer at APH, Economiser, Bank zone & Super heaters.
Material of MS material (IS 2062) for air ducting is of 5 mm & for flue gas ducting is 6 mm
19. Fuel feeding system: Considerations of No. of fuel feeding system & their individual capacity is necessary for operating the Boilers at MCR & Peak loads at varying fuel parameters. If the fuel GCV, moisture & ash content vary then fuel consumption will also vary. So while engineering the Boilers fuel feeding capacity is considered 25 to 30% more.
Turn down ratio of fuel feeding system. Maximum moisture handling capacity of the system
20. Heat released in Furnace: This important factor for designing the furnace height, width & breadth
21. Heat released on Grate/Bed: This important factor for designing the furnace & combustion compartments, grate materials etc
22. Flue gas velocity at various zones of Boilers: This is very important factor to be considered while designing the Boilers. Higher velocity of flue gas leads to erosion of pressure parts & ducting, lower flue gas velocity leads to poor heat transfer.
Generally for AFBC, Travelling grate, WHRB, oil fired Boilers ducting are designed for 18 m/sec velocity.
23. Percentage of combustion air flow through FD & SA. Generally from FD 60 to 70% and from SA 30 to 40%
24. Steam pressure drop in all stages of Super heater coils
25. Velocity of feed water & saturated water at each pressure part
26. Overall heat transfer co-efficient & LMTD in super heater, economiser, Bank zones
27. APH: Overall heat transfer co-efficient & LMTD .Pitching of tubes along & across the air flow. Heating surface area
28. Pitching of super heater coils along & across the flue gas flow: Wide pitching of super heater to minimize fouling.
29. Desuperheating spray water quantity at MCR & other various loads
30. Maximum steam temperature at each super heaters
31. Maximum allowable metal temperature at each Super heaters
32. Pressure parts thermal expansion calculations & Maximum/minimum tolerances
33. Grate or Bed area loading in mm Kcal/M2/Hr
34. Dust concentration at Boiler outlet in gram/NM3
35. Quantity of flue gas produced
36.No.of Safety valves installed
Reasons for increase in fuel consumption in Boilers
1. Decreased economiser inlet feed water temperature:
On every 6-8 deg C decrease in Economiser inlet feed water temperature causes the rise in Boiler fuel consumption by 1%.
2. Increased Boiler outlet flue gas temperature:
On every 22 deg C increase in flue gas temperature causes the reduction in Boiler efficiency by 1% & hence boiler fuel consumption increases for generating same steam.
3. Increased moisture content in the fuel
Boilers fuel consumption increases as the moisture in the fuel increases. As it requires more excess air & reduces combustion efficiency leading to unburnt losses
4. Increased excess air
Increase in excess air causes dry flue gas loss & hence more fuel consumption. And also leads to more auxiliary power consumption.
5. Increased unburnt loss
Unburnt fuel or incomplete combustion of fuel leads to increased consumption of fuel. Unburnt is due to improper air fuel mixture or unbalanced draught or variation in the fuel quality
6. Higher blow down
Blow down water carries saturated water/steam through it, so leads into more fuel consumption. Maximum acceptable blow down rate for normal Boiler operation is 0.5 to 1%.
7. Operating the Boiler at lower or partial loads
Operating the Boilers on partial load requires more excess air & leads to incomplete combustion forming unburnts.
8. Operating the boiler at non-standard operating parameters
Operating the Boilers at non standard parameters like pressure, temperatures, flow etc will lead to the higher fuel consumption
9. Leakage into & out of the Boilers
Air & flue gas leakages into the Boiler & out of the Boiler will reduce the working efficiency there by increasing the boiler fuel consumption
10. Steam leakage
It is a direct cause for higher steam consumption. Generally steam leakage is from vent & drains valves, welding & flange joints
11. Boiler heating surfaces internal scaling
Pressure parts namely water wall tubes, economiser tubes & super heater coils internal & external scaling will result into poor heat transfer, which increases fuel input to produce required amount of steam.
Pressure parts internal scaling can damage the pressure parts by over heating
12. Radiation & Convection losses
Radiation & convection losses in the boiler causes increased fuel consumption. These losses may be due to uninsulated or unlagged surfaces of boiler
13. Lower combustion air temperature
On every 20 Deg C decrease in combustion air temperature leads to Boiler efficiency reduction by 1%
14. More ash content in the fuel
More ash content in the fuel takes away heat associated with it during discharging through hoppers. Especially bed ash is having more temperature. Also higher ash fuel are having lower GCV
15. More Volatile matters (VM) in the fuel
Calorific value of the fuel reduces as the VM increases. Boilers using Lower GCV fuel consume more fuel
16. Other potential reasons for increased fuel consumption of a Boiler are;
Boiler design related issuers
Wrong selection of auxiliaries like fans & fuel feeding system
Low quality of Bed materials
Defects in fuel burners & fuel spreaders
Over refractory on internal heating surfaces
Faulty field instruments
Tips to reduce LOIs in Boilers
1-Optimization of fuel moisture in the fuel:
Higher moisture in the fuel leads to unbalanced draft in the furnace or combustion chamber. Which ultimately results into poor mixture of air & fuel, higher moisture fuel demands more excess air. So optimization of fuel moisture will help to reduction in LOI.
2-Maintaining balanced draft in furnace:
Unbalanced draft is nothing but more FD air/less ID draft or less FD air /More ID Draft.
More FD & less ID causes back fire & improper mixing of air & fuel.
More ID & less FD causes escape of fuel particles from furnace without proper combustion
3-Maintaining 3Ts of combustion:
3Ts of combustions are: Temperature, Time & Turbulence
Temperature: For proper combustion temperature of the furnace must be sufficient enough to burn the fuel completely.
Time: There must be sufficient time for combustion
Turbulence: There should be proper turbulence in furnace for proper mixing of air & fuel
4-Increasing the secondary air quantity & pressure:
This will help to increase the residence time of fuel in furnace resulting into complete combustion
5-Maintaining the required excess air:
Lesser excess air than required will lead to incomplete combustion that is conversion of carbon into carbon monoxide instead of carbon di-oxide. So sufficient air is required to achieve complete combustion.
6-Using correct or designed GCV of a fuel:
Lesser GCV fuel requires more air for combustion, even may not achieve the designed parameters of Boilers like flow, pressure & temperature.
7-Ultimate & Proximate analysis of fuel:
In order to achieve proper combustion, we must know the contents of fuel properly. Need to operate the Boilers at designed parameters of fuel. For example if FC of the fuel increases, then need to increase air & combustion time and need to reduce turbulence.
So it is very important to know the fuel contents.
8-Operating the Boilers at stable loads:
Stable loaded Boilers will not lead into much LOI, as air & fuel mixture is constant & will not vary frequently. LOI cannot be reduced in variable load boilers.
9-Operating the Boiler at little positive draught:
Operation of Boiler at positive draught will help for complete combustion. If boiler is operated at more negative draught, then there will be more chances of escaping of unburnt fuel particles from the furnace & causing secondary combustion at super heaters & other convective zones.
10-Reusing unburnt or cinder:
Generally unburnt/cinder from Boiler Bank & economiser zones are re-injected into furnace by using “Cinder re-injection” system. This will help in re-burning of cinder & reduction in LOI
Boiler calculations
1-Oxygen percentage in Boiler outlet flue gas is 4.9%, then what will be the percentage of excess air?
We have excess air EA = O2 X 100 / (21-O2)
EA = 4.7 X 100 / (20-4.7)
EA = 30.71%
2-Calculate the Oxygen level (O2) in flue gas, if excess air is 25%
We have Excess air EA = O2 X 100 / (21-O2)
25 = O2 X 100 / (21-O2)
O2 = 4.2%
3-A Boiler’s combustion system requires 5.5 kg of air for burning 1 kg of fuel, then calculate the total air required for complete combustion if its flue gas has 4.1% of O2
We have,
Total air = (1 + EA/100) X Theoretical air
EA = O2 X 100 / (21-O2)
EA = 4.1 X 100 / (20-4.1) = 25.78%
Therefore Total air = (1 + 25.78/100) X 5.5 = 6.92 kg of air per kg of fuel burnt
4-A Coal fired boiler having total heating surface area 5200 M2 produces 18 kg of steam per square meter per hour of heating surface, then calculate the Boiler capacity in TPH
Boiler Capacity = (Heating g surface area X Steam generation per square meter)
Boiler Capacity = 5200 X 18 = 93600 kg/hr =93600 / 1000 = 93.6 TPH
5-Calculate the Theoretical air & Excess air required to burn 10 MT of coal having carbon (C) 48%, Hydrogen (H2) 3.8%, Oxygen (O2) 8.2% & Sulphur (S) 0.6% in it. For complete combustion operator is maintaining 4% of O2 in flue gas.
We have Theoretical air, Th = (11.6 X %C + 34.8 X (H2-O2/8) + 4.35 X S) / 100
Th = (11.6 X 48 + 34.8 X (3.8-8.2/100) + 4.35 X 4) / 100
Th = 5.84 kg/kg of fuel
We have excess air EA = O2 X 100 / (21-O2)
= 4 X 100 /(21-4) = 23.52%
Total air = (1 + EA/100) X Theoretical air
Total air = (1 + 23.52/100) X 5.84 = 7.21 kg of air per kg of fuel burnt
6-Calculate the Oxygen required for complete combustion of 1 kg of Carbon
On complete combustion, Carbon becomes carbon dioxide
C + O2 = CO2 + Heat (8084 kcal/kg)
Write down the molecular weights of carbon, oxygen & carbon dioxide
12 + 32 = 44
Divide by 12
1 + 2.67 = 3.67
So, 2.67 kg of oxygen is required for complete combustion of 1 kg of Carbon
7-Calculate the amount of Oxygen required for complete combustion of 1 kg of Methane
On complete combustion, Methane becomes carbon dioxide & water
CH4 + 2O2 = CO2 + 2H2O Heat
Write down the molecular weights of Methane, oxygen & carbon dioxide
16 + 64 = 44 + 36
Divide by 16
1 + 4 = 2.75 + 2.25
So, 4 kg of Oxygen is required for complete combustion of 1 kg of Methane
8-A Coal sample having Carbon, Hydrogen, Oxygen & Sulphur percentages 50%,3.5%,8.6% & 1% respectively, then calculate the GCV of coal.
GCV of coal = (8080 X C + 34500 X (H2-O2/8) + 2440 X S)) / 100
GCV of coal = (8080 X 50 + 34500 X (3.5-8.6/8) + 2440 X 1)) / 100 = 4901 Kcal/kg
READ Power plant O&M books
9-A Boiler uses imported coal to generate 100 TPH of steam, the O2 & CO2 in flue gases are 5% & 14.5% respectively. Calculate the mass of flue gas generated if following is the ultimate analysis of fuel.
Carbon C = 52%
Hydrogen H2 = 3.25%
Oxygen O2 = 8.3%
Sulphur S = 0.3%
Nitrogen N2 = 1.1%
We have Theoretical air, Th = (11.6 X %C + 34.8 X (H2-O2/8) + 4.35 X S) / 100
Th = (11.6 X 52 + 34.8 X (3.25-8.3/100) + 4.35 X 0.3) / 100
Th = 6.82 kg/kg of fuel
We have excess air EA = O2 X 100 / (21-O2)
= 5 X 100 /(21-5) = 31.25%
Total air = (1 + EA/100) X Theoretical air
Total air = (1 + 31.25/100) X 6.82 = 8.95 kg of air per kg of fuel burnt
Mass of flue gas generated Mfg = Mass of CO2 in flue gas + Mass of N2 in fuel + Mass of N2 in air + Mass of O2 in the flue gas + Mass of SO2 in the flue gas
Mass of flue gas generated Mfg = (Carbon percentage in fuel X Mol.weight of CO2) / Mol.weight of Carbon + 0.011 + (8.95 X 77 / 100) + ((8.95-6.82) X 23 / 100) + (0.003 X Mol.weight of SO2) / Molecular weight of sulphur
Mass of flue gas generated Mfg = (0.52 X 44 / 12) + 0.011 + 6.89 + 0.49 + (0.003 X 64) / 32 =9.3 kg of flue gas per kg of fuel burnt.
10-Calculate the Sulphur dioxide generated per day in a 150 TPH boiler, where coal burned is having 0.5% of sulphur. Consider steam to fuel ratio 5.5 & Boiler operates on full load for 24 hours.
We have S + O2 = SO2
32 + 32 = 64
1 + 1 = 2
That is 1 kg of sulphur generates 2 kg of Sulphur dioxide on complete combustion.
Total coal consumed in a day = Steam generated in 24 hours / Steam to coal ratio
Total coal consumed in a day = 150 X 24 / 5.5
Total coal consumed in a day = 654.54 Tones/day
Therefore total SO2 generated = 654.54 X 2 =1309.08 Tones
11-A 100 TPH coal fired boiler generating 8.5 kg of flue gas (Mfg) per kg of fuel burnt at 150 Deg (Tfg) Calculate the heat loss due to dry flue gas loss. Consider coal GCV 5000 kcal/kg & ambient air temperature 25 Deg C (Ta)
Heat loss due to dry flue gas = Mass of flue gas (Mfg) X Specific heat of flue gas (Cp) X (Tfg-Ta)
Heat loss due to dry flue gas = 8.5 X 0.24 X (150-25) =255 kcal/kg (Specific heat of flue gas = 0.24 kcal/kg)
% of heat loss = 255 X 100 / Coal GCV = 25500 / 5000 =5.1%
12-Calculate the heat loss due to formation of water from 3.22% hydrogen present in coal of GCV 4500 kcal/kg. Consider Boiler outlet flue gas temperature is 145 deg C & ambient temperature 30 deg C
Heat loss due to formation of water from hydrogen in fuel = 9 X H2 X (584 + Specific heat of moisture (Cp) X (Tfg-Ta)
= 9 X 0.0322 X (584 + 0.45 X (145-30)) = 184.24 kcl/kg
% of heat loss = 184.24 X 100 / Coal GCV = 18424 / 4500 =4.09%
13-Calculate the heat loss due to 20% (M) moisture present in coal of GCV 3900 kcal/kg. Consider Boiler outlet flue gas temperature is 145 deg C & ambient temperature 30 deg C
Heat loss due to moisture in fuel = M% X (584 + Specific heat of moisture (Cp) X (Tfg-Ta)
= 0.2 X (584 + 0.45 X (145-30)) = 127.15 kcl/kg
% of heat loss = 127.15 X 100 / Coal GCV = 18424 / 3900 =3.26%
14-A coal fired Boiler’s fly ash collected at APH & analyzed for unburnt. Report shows 22% of unburnt & 750 kcal/kg GCV. Calculate the heat loss due to this unburnt. Consider coal GCV 4700 kcal/kg & ash Percentage in coal 6%
Total ash present in 1 kg of coal = 1 X 6% = 0.06 kg
Total unburnt present in ash = 0.06 X 25% = 0.0015 kg
Heat loss due to unburnt = 0.0015 X 750 = 1.125 kcal/kg
Percentage of heat loss = 1.125 X 100 / 4700 = 0.024%
15-A Boiler losses have been analyzed & found as below
Heat loss due to dry flue gas (L1) = 5.2%
Heat loss due to formation of water from hydrogen (L2) = 3.3%
Heat loss due to moisture in fuel (L3) = 6%
Heat loss due to unburnt fuel in ash (L4) = 0.02%
Heat loss due to incomplete combustion (L5) = 0.8%
Heat loss due to moisture present in air (L6) = 0.6%
Then calculate the Boiler efficiency by indirect method
Boiler efficiency = 100- Total losses = 100-(Sum of L1 to L6)
Boiler efficiency = 100 – (5.2 + 3.3 + 6 + 0.02 + 0.8 + 0.6) = 84.08%
16-A coal fuel with GCV 5500 kcal/kg & having moisture 12% & Hydrogen 3.1% in it is burnt in a Boiler with air fuel ratio 8:1.Neglecting ash, calculate the maximum possible temperature (Tfg) attained in the furnace.Assume whole heat of combustion is given to the products of combustion. Take specific heat of gases generated 0.24 kcal/kg & ambieant air temperature 28 deg C (Ta)
For calculation, need LCV
We have LCV = HCV-(9 X H2% X 586) = 5500-(9 X 0.031 X 586) =5336.5 kcal/kg
Mass of flue gas generated per kg of coal burnt, Mfg = mass of air per kg of coal burnt + 1
=8+1 =9 kg
Heat released by combustion = heat absorbed by gases
5336.5 = Mfg X Cp X (Tfg-Ta)
5336.5 = 9 X 0.24 X (Tfg-28)= 2498.6 deg C
So maximum temperature attained is 2498.6 deg C
17-A 200 TPH boiler generates 190 TPH (Q1) steam at pressure 121 kg/cm2 and temperature 550 deg C & consumes 31 TPH of coal having GCV 5200 kcal/kg. Calculate the Boiler efficiency if it requires 25 TPH (Q3) attemperator water at temperature 125 deg C.
Consider feed water temperature at economizer inlet is 210 deg C & ignore blow down loss.
Boiler efficiency = (Steam generation X Enthalpy –(Feed water at flow X Enthalpy + Attemperator water X Enthalpy)) X 100 / (Coal consumed X Coal GCV)
Ignoring blow down water loss,
Feed water flow Q2= Total steam generated – Attemperator water flow = 190-25 =165 TPH
Refer steam table for enthalpy values
Enthalpy of steam Hg = 830 kcal/kg
Enthalpy of feed water Hf1 = 214.34 kcal/kg
Enthalpy of attemperator water Hf2 = 125.4 kcal/kg
Boiler efficiency = (Q1 X Hg – (Q2 X Hf1 + Q3 X Hf2)) X 100 / (31 X 5200)
Boiler efficiency = (190 X 830 – (165 X 214.34 + 25 X 125.4)) X 100 / (31 X 5200) = 73.94%
18-Calculate the oil consumption of a 75 TPH (Q1) oil fired Boiler having efficiency 88% & generates steam at 65 kg/cm2 pressure & 485 deg C temperature. Consider feed water temperature at economizer inlet 160 deg C & oil GCV 10000 kcal/kg
Boiler efficiency = (Q1 X Hg – Q2 X Hf) / (Mf X GCV)
Here feed water quantity is not given, so assume feed water flow = Steam generation =Q1=Q2
Ignore blow down losses
Now, refer steam tables for enthalpy
Enthalpy of steam Hg =807 kcal/kg
Enthalpy of feed water Hf =161.3 kcal/kg
0.88 = (75 X 807 –75 X 161.3) / (Mf X 10000)
Oil consumption (Mf) = 5.5 TPH
19-A biomass fired boiler of efficiency 60% operates 285 days in a year , it generates 25 TPH (Q1) steam for a process at pressure 21 kg/cm2 & 360 deg C temperature. Calculate the cost of fuel to operate the boiler. Assume fuel GCV 2500 kcal/kg, feed water temperature 105 deg C & cost of fuel per ton is Rs 2800.
First calculate the fuel consumption
Boiler efficiency = (Q1 X Hg – Q2 X Hf) / (Mf X GCV)
Here feed water quantity is not given, so assume feed water flow = Steam generation =Q1=Q2
Ignore blow down losses
Now, refer steam tables for enthalpy
Enthalpy of steam at pressure 21 kg/cm2 & temperature 360 deg C,Hg =754 kcal/kg
Enthalpy of feed water at temperature 105 deg C, Hf =105.3 kcal/kg
0.60 = (25 X 754 –75 X 105.3) / (Mf X 2800)
Fuel consumption (Mf) = 9.65 TPH
Cost of fuel for operating the Boiler for 285 days = 9.65 X 24 X 285 X 2800 =18.16 crore
20-Calculate the specific fuel consumption of a power plant. If steam Turbine consumes 4.2 Tons of steam to generate 1 MW of power. Assume steam to fuel ratio of Boiler is 4.8.
We have Specific fuel consumption (SFC) = Specific steam consumption (SSC) / Steam to fuel ratio of Boiler (SFR)
SFC = 4.2 / 4.8 =0.875 kg of fuel for generating 1 kw of power
21-Calculate the GCV of a bagasse sample having moisture 51% & pole 1.5%.
We have GCV of bagasse = 4600-46 X Moisture -12 X pole
= 4600-46 X 51-12 X 1.5 =2236 kcal/kg
22-A coal sample contains Carbon 40%, Oxygen 8.3%, Hydrogen 3.5% and Sulphur 0.5%, Nitrogen 1.0%, then calculate its GCV/HCV, LCV and NCV if its total moisture content is 12%.
We have the Theoretical formula for GCV,
GCV/HCV = (8084 X C% + 28922 X (H2% 2 O2%/8) + 2224 X S %)/100
= (8084 X 40 + 28922 X (3.5 – 8.3/8) + 2224 X 0.5)/100
= 3968 kcal/kg
LCV = HCV - (9 X H2 X 586)
= 3968 - (9 X 0.035 X 586)
= 3783.41 kcal/kg.
NCV = (GCV - 10.02 X Total moisture)
= (3968 - 10.02 X 4.4) =3923.9 kcal/kg
23-A 200 TPH coal fired boiler is loaded up to 90% of its MCR thought the day. The steam fuel ratio (SFR) of this Boiler is 4.8 & ash percentage in coal is 6.5%.Calculate the revenue generated in a month by selling quantity of ash generated. Consider 30 days in a month & cost of ash per MT Rs 100.
Total steam generated in a day = (200 X 90 /100) X 24 =4320 tones
Total consumed = Steam generated / SFR = 4320 / 4.8 =900 Tones
Total ash generated in a day = Coal consumption in day X Ash % in coal
= (900 X 6.5 / 100) =58.5 MT
Total revenue generated in a month = 58.5 X 30 X 100 = Rs 175500
24-A 150 TPH (Ms) boiler generates steam at pressure 88 kg/cm2 & temperature 520 deg C. A feed water at temperature 105 deg C is being used to reduce steam temperature from 450 deg C to 380 deg C to maintain constant main steam temperature at SH oiutlet.Calculate the water required for desuperheating.
Attemperator inlet steam enthalpy at pressure 88 kg/cm2 & temperature 450 deg C, Hg1 =770 kcal/kg
Attemperator inlet steam enthalpy at pressure 88 kg/cm2 & temperature 380 deg C, Hg2 =750 kcal/kg
Attemperator inlet steam enthalpy at temperature 105 deg C, Hf =105.11 kcal/kg
Heat lost by steam = Heat gained by desuper heating water
Ms X (Hg1-Hg2) = MW X (Hg2-Hf)
Mw =150 X (770-750) / (750-105.11) = 4.88 TPH
25-A boiler steam drum safety valve lifts at 115 kg/cm2 and reseats at 110 kg/cm2, then calculate its
Blow down percentage?
Blow down of safety valve = (Set pressure – Reseat pressure) X 100/Set pressure
= (115 – 110) X 100/115 = 4.5%
Viva Questions & answers for preparation of BOE exam & interview
26-A boiler’s SH steam line safety valve is set at 72 kg/cm2 & blow down rate kept 2.5%, calculate the pressure at which safety valve reseats
Blow down of safety valve = (Set pressure – Reseat pressure) X 100/Set pressure
2.5 = (75-Reseat pressure) X 100 / 75
Reseat pressure =73.13 kg/cm2
27-A air Pre heater (APH) Flue gas inlet/out let & Air inlet/outlet temperatures are 240 deg C/150 deg C & 35 deg C/185 deg C, calculate the effectiveness of APH from gas side & air side
APH gas side efficiency
ηAPHg = (Flue gas inlet temp.-Flue gas outlet temp.) X 100 / (Flue gas inlet temperature -Air inlet temperature)
ηAPHg =(240-150) X 100 / (240-35) =43.9%
APH air side efficiency
ηAPHa = (Air outlet temp.Tao-Air inlet temp.)) X 100 / (Flue gas inlet temperature -Air inlet temperature)
ηAPHa =(185-35) X 100 / (240-35) =73.17%
28-Calculate the economiser effectiveness, whose feed water inlet & outlet temperatures are 165 Deg C & 245 Deg C respectively & flue gas inlet & outlet temperatures 385 deg C & 215 deg c respectively.
ηEco. = (Economizer outlet feed water temperature -Economizer inlet feed water temperature) X 100 / (Economizer inlet flue gas temperature - Economizer inlet feed water temperature)
ηEco = (245-165 ) X 100 / (385-215)
ηEco = 47.05%
29-A HP heater is been used to raise the feed water temperature from 105 deg C to 150 deg C by using Turbine bleed steam at inlet temperature 280 deg C, calculate the HP heater effectiveness. Consider the HP heater condensate out let temperature is 140 deg C
HP heater effectiveness = It is calculated as temperature range of steam X 100/ Temperature range of feed water
HP heater effectiveness = (280-140/ (150-105) = 3.1
30-A HP heater is used to heat 100 TPH feed water from 110 deg C to 145 deg C by using bleed steam at pressure 15 kg/cm2 and temperature 320 deg C, calculate the quantity of steam required if condensate outlet temperature is 155 deg Consider specific heat of water =1 kcal/kg
Enthalpy of steam used at pressure 15kg/cm2 & temperature 320 deg C by referring steam table =735.29 kcal/kg
Enthalpy of condensate leaving HP heater =156.12 kcal/kg
Heat lost by steam = Heat gained by feed water
Mass of steam X (Enthalpy of steam-Enthalpy of condensate water) = Mass of water X Cp X Rise in feed water temperature
Mass of steam = 100 X 1 X (145-110) / (735.29-156.12)
Mas of steam Ms =6.04 TPH
31-A spray cum tray Deaerator inlet & outlet water oxygen concentration is 15 & 0.007 ppm respectively, calculate the Deaerator efficiency
η D/A = (Concentration of Oxygen in inlet water-Concentration of oxygen in outlet water) X 100 /(Concentration of Oxygen in inlet water)
η D/A =(15-0.007) X 100 /15
η D/A = 99.53%
32-In a coal based Thermal power plant, a 10” steam line is left uninsulated around 2 meters of its length. Because of this there is a loss of heat around 17500 kcal/hr .Calculate the extra fuel consumption in a day to compensate this loss. Consider coal GCV & boiler efficiency 4500 kcal/kg & 90% respectively
We have,
Heat loss in terms of fuel = Heat loss / (Fuel GCV X Boiler efficiency) = 17500 / (4500 X 0.9) =4.32 kg/hr
So extra coal consumption to compensate the heat loss = 4.32 X 24 =103.7 kg/day
33-A boiler of generates 85 TPH of steam at pressure 65 kg/cm2 & temperature 490 deg C, calculate the velocity of this steam if it passes through 150 NB steam line.
Velocity of steam inside the pipe line = Flow in steam line (m3/sec) / Area of steam line (m2)
Convert steam flow 85 TPH into m3/sec
That is 85000 kg/hr / Density of steam
Refer steam table & find out density of steam at above pressure & temperature
Density of steam = 20.13 kg/m3
Area of pipe line =∏XD2 / 4 = 3.142 X (0.150)2/4 =0.018 M2
Steam flow = 85000 / (20.13 X 3600) = 1.17 m3/sec
Then,
Velocity of steam inside the pipe line = 1.17 / 0.018 =65.16 m/sec
34-A air preheater of heating surface 2800 M2 need to design for a Boiler of 120 TPH, the total area required for flue gas flow is around 8.5 M2.Calculate the number of tubes & their length. Select tube size OD 63.5mm X 2.34mm thickness.
For finding out number of tubes, calculate the area of one tube
A==∏XD2 / 4
Where D is inside diameter of tube (considered flue gas passes through tubes & air outside the tubes)
Inside diameter of tube = OD -2 X Thickness = 63.5-2 X 2.34 =58.82 mm =0.05882 meters
Area, A = 3.142 X (0.05882)2 /4 =0.0027 M2
Total required area for flue gas flow is 8.5 meter
Then, Number of tubes = 8.5 / 0.0027 =3148.1 nos
Take round figure =3148 Nos
Given that, total heating surface area of the tubes = 2800 M2
Length of the tube,
(2∏RL) X Number of tubes= 2800
R = Outside diameter of tube =63.5/2 =31.75 mm = 0.03175 meter
Length of tubes, L=2800 / (2 X ∏ X 0.03175 X 3148) =4.46 meter or 4460 mm
35-A boiler of operating pressure 66 kg/cm2 has LHS water wall panel total length of 25 meters from bottom header to top header, calculate the maximum thermal expansion of panel. Consider the MOC of tube material SA210 Gr.A (Carbon steel) & ambient air temperature 30 deg C
We have,
Total thermal expansion =Length of the panel X Coefficient of expansion carbon steel pipe X Operating temperature of metal
As per boiler operating temperature & carbon steel material maximum metal temperature will be 390 deg C
Coefficient of expansion carbon steel pipe=11.6 X 10-6 m/m oC
Maximum possible thermal expansion of LHS water wall panel=25 X 11.6 X 10-6 X 390 =0.113 meter =113.1 mm
36-The maximum permissible limit of a 75 TPH boiler’s TDS is 120 mg/l. If the TDS of feed water is 5 mg/l and percentage of makeup water is 7%. Then calculate the percentage of blow down and blow down water quantity.
% of blow down = (Feed water TDS X % of makeup water) X 100/(Boiler permissible TDS - Feed water TDS)
= (5 X 0.07) X 100 / (120 - 5)
= 0.3%
Quantity of blow down water = (0.3 X 75 / 100) = 0.225 TPH
37-What quantity of flash steam is produced when steam drum water operating at 70 kg/cm2 is released in CBD tank at atmospheric pressure at the rate of 1.1 TPH
Sensible heat at high pressure in drum water S1 = 305 kcal/kg…… Refer steam table
Sensible heat of steam at atmospheric pressure in CBD tank S2 = 100 kcal/kg…… Refer steam table
Latent heat of flash steam L = 539 kcal/kg
CBD water flow rate Q = 1.1 TPH
Percentage of flash steam produced % = (S1 - S2) X 100 / L
= (305 – 100) X 100 / 539
= 38%
Total quantity of flash steam produced per hour = (1.1 X 38 / 100) = 0.42 TPH
38-What is the COC of a Boiler, if boiler water has chloride 95 ppm & feed water 21 ppm.
COC = Boiler water chloride / Feed water chloride
COC = 95 / 21 = 4.5
39-Convert 100 TPH boiler capacity into BHP capacity
We have, 1 BHP = 15.65 kg/hr
There fore total BHP = 100 X 1000 / 15.65 = 6389.8
40-A Boiler of capacity 75 TPH operating at 65 kg/cm2 & temperature 490 deg C,calaculate boiler BHP.Assume feed water temperature 150 deg C
Total heat content in out let steam = 75 X 1000 X (Enthalpy of outlet steam- Enthalpy of feed water)
=75000 X (811-151) =49500000 kcal/hr
We have 1 BHP =8436 kcal/hr
So Boiler BHP = 49500000 / 8436 =5867.7
41-Calculate the condensate formed, if 25 TPH steam at pressure 7 kg/cm2 & temperature 180 deg C is supplied to a process plant situated at a distance of 500 meter from the generation plant. The pressure & temperature at the end user point are 6.2 kg/cm2 & 170 deg C respectively.
Enthalpy of steam generating end = 666.74 kcal/kg
Enthalpy of steam at end user = 662.41 kcal/kg
Enthalpy of evaporation at average pressure (7 + 6.2) / 2 =6.6 kg/cm2 = 489.95 kcal/kg
Condensate formed = 25 X (666.74-662.41) / 489.95 =0.22 TPH
Significance of combustion air in Boilers
1. What do you mean by combustion air?
The amount of air required for complete combustion of fuel in furnace is called as combustion air. The efficiency of the Boiler or furnace depends on efficiency of combustion system.
2. On what parameters the requirement of combustion air depends?
Combustion air requirement depends on;
Type of fuel burnt
Type & quantity of its elemental constituents
Type of Boiler and furnace
Amount of moisture content in it
3. What is the relation between moisture content in the fuel & combustion air required?
Combustion air requirement increases as the moisture content in the fuel increases and vice versa
4. What is the relation between carbon & Hydrogen content in the fuel & combustion air required?
Combustion air requirement increases as the % of carbon & Hydrogen content in the fuel increase and vice versa.
5. What is the relation between oxygen content in the fuel & combustion air required?
Combustion air requirement decreases as the % of oxygen content in the fuel increases and vice versa.
6. Do content of sulphur & Nitrogen in the fuel affect combustion air requirement?
Increase and decrease in sulphur & Nitrogen content in the fuel does not affect much on combustion air requirement.
7. What is meant by total air of combustion?
The total air supplied to the Boiler combustion chamber is divided into two parts Primary air and secondary air.
Primary air supports the flame and takes part in the initial combustion process. The second part is called as secondary air. Secondary air is admitted into the furnace from top to create turbulence in furnace and to ensure complete combustion of the fuel.
8. What are the functions of Primary and secondary air in Travelling grate, pulverized coal fired and FBC Boilers?
In case of travelling grate Boilers Primary air is supplied below the grate to support flame & combustion stabilisation. And secondary air from top of the furnace as over fired air to create turbulence for complete combustion. And also secondary air is used to spread the fuel in furnace
In case of Pulverized coal fired Boilers, Primary air is used to carry the pulverized coal into the furnace.
In case of FBC Boilers Primary air is used to carry fuel and fluidisation. Secondary air is supplied above the bed to ensure complete combustion
9-What is meant by theoretical air & excess air in combustion?
Theoretical air: Amount of air required to burn the fuel. It is stoichiometric air, it does not ensure complete combustion.
Excess air: Amount of extra air given for complete combustion
10-Calculate the Theoretical air required to burn imported coal having carbon 55%, Oxygen 8.2%, Hydrogen 3.3% and sulphur 0.32% in it
Theoretical air is calculated by using below formula
Thair = (11.6 X %C + 34.8 X (%H2-%O2/8) + 4.35 X %S)) / 100
Thair = (11.6 X 55 + 34.8 X (3.3-8.2/8) + 4.35 X 0.32)) / 100
Thair = 7.18 kg/kg of fuel burnt
In the above formula, you can vary the % of Carbon, Hydrogen & Sulphur to observe changes in air requirement
11-How do you measure % of excess air supplied?
Excess air is generally measured from Oxygen analyser installed at the out let of Boiler (Economiser)
It is to be noted that, excess air & excess oxygen are not same. Air has around 21% of oxygen in it by volume. So, 100% excess air is roughly equals to 10.5% of oxygen.
12-What is the significance of excess air?
For combustion, if less air is supplied it leads to incomplete combustion forming CO instead of Co2. And if more excess air is supplied it leads to reduction of combustion efficiency by cooling the furnace & carrying the heat through flue gas.
So, it is important to adjust the air supply in such a way that complete combustion will take place without much extra air.
13-Calculate the % of excess air required if oxygen measured in flue gas at economiser outlet is 5.5%.
Excess air = O2% X 100 / (21-O2%)
Eair = 5.5 X 100 / (21-5.5)
Eair = 35.48%
14-Calculate the total air required for complete combustion of coal if Theoretical air supplied is 7.1 kg/kg of coal and O2 measured in flue gas is 6.4%
Actual or total mass of air supplied = (1 + Excess air / 100) X theoretical air
We have
Excess air = O2% X 100 / (21-O2%)
Eair = 6.4 X 100 / (21-6.4)
Eair = 43.84%
Actual or total mass of air supplied = (1 + 43.84 / 100) X 7.1
Actual or total mass of air supplied = 10.21 kg/kg of coal
15-How do you control the excess air?
Excess air is controlled by;
Optimizing the moisture content in the fuel
Improving combustion chamber performance
Auto control of fuel feeding
Continuously monitoring O2 content in flue gas
Incorporating auto combustion control
Incorporating VFD drives to ID, FD, PA & SA fans
16-Among Bagasse, coal and Natural gas, which fuel needs more excess air?
Bagasse, since it has more moisture content
17-A boiler has supplied 27% excess air, calculate % of O2 in flue gas
Excess air = O2% X 100 / (21-O2%)
27 = O2% X 100 / (21-O2%)
27 X 21 -27 X %O2 = 100 X O2%
567= 127 O2%
O2% = 4.46
For more >>>>>read Powerplant and calculations
Read >>>>Top-100 steam turbine interview questions and answers
