Steam Generation, Distribution and Utilization

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Steam Generation, Distribution and Utilization

• I.Thanumoorthi
• The Energy and Resources Institute

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Why steam is preferred in process applications
and power generation ?

• Steam is a common heat transport medium used in
  process applications and power generation because
  of its outstanding qualities such as
• Ability to release heat at constant temperature
• High heat content
• Ease of control and distribution
• Fire and flame proof
• Temperature breakdown is very easy by pressure
  reducing valve
• Can be used for direct indirect heating
• Clean, odorless and tasteless

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Boiler System
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Boiler System

• The Fuel system
• The fuel system includes all equipment used to
  provide fuel to generate the necessary heat
• The Water system
• The water system provide water to the boiler and
  regulates it automatically to meet the steam
  demand
• The Steam system
• The steam system collects and controls the steam
  produced in the boiler

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Types of boiler

• Fire tube boiler ( hot gases passes through
  tubes)
• Economical capacity pressure is lower than 25
  tph and 17.5 kg/cm2 respectively.
• Ability to meet wide and sudden load fluctuations
• Low installation maintenance costs
• Water tube boiler ( water passes through tubes)
• Range is available from 4 to 230 tph
• Very suitable for heavy and steady loads
• Normally used for high pressure steam
  requirement.
• High installation and maintenance costs

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Efficiencies related to Steam system

• Combustion Efficiency (Ability to completely
  burn fuel)
• Thermal efficiency (Effectiveness of heat
  transfer in a boiler and not taken into account
  radiation losses)
• Boiler efficiency or Generation efficiency
  (Measure of fuel to steam efficiency)
• Distribution efficiency
• Utilization efficiency
• Overall efficiency

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Steam System overall efficiency
25
20
5
Product
Process
Fuel
100
80
35
Boiler
75
Steam mains
5
10
Condensate return system
15
Generation efficiency 80
Distribution efficiency 83
Utilisation efficiency 47
Overall efficiency 35
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Boiler Efficiency
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Boiler Efficiency Direct method
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Heat Output Heat Input
Boiler Efficiency

Boiler efficiency (?) Where, hg
Enthalpy of saturated steam in kcal/kg of steam
hf - Enthalpy of feed water in kcal/kg of
water

• Parameters to be monitored for the calculation of
  boiler efficiency by direct method are
• Quantity of steam generated per hour (Q) in
  kg/hr.
• Quantity of fuel used per hour (q) in kg/hr.
• The working pressure (in kg/cm2(g)) and superheat
  temperature (oC), if any
• The temperature of feed water (oC)
• Type of fuel and gross calorific value of the
  fuel (GCV) in kcal/kg of fuel

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Boiler Efficiency Indirect Method
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Boiler Efficiency Indirect Method

• The following losses are applicable to
  liquid, gas and solid fired boiler
• L1- Loss due to dry flue gas loss (sensible heat)
• L2- Loss due hydrogen in fuel (H2)
• L3- Loss due to moisture in fuel (H2O)
• L4- Loss due to moisture in air (H2O)
• L5- Loss due to carbon monoxide (CO)
• L6- Loss due to surface radiation and convection
  heat loss
• The following losses are applicable to solid
  fuel fired boiler in addition to above
• L7- Unburnt losses in fly ash (Carbon)
• L8- Unburnt losses in bottom ash (Carbon)
• Boiler Efficiency 100 (L1L2L3L4L5L6L7
  L8)

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Factors Affecting Generation Efficiency
Equipment Design
Operating Parameters Capital and Process constraints
Provision of waste heat recovery Capital and Process constraints
Energy Management Fuel Quality
Energy Management Controls
Load Management
Maintenance/Adjustments


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Steam generation side aspects

• Selection of the boilers capacity utilization
• Burner (or) firing system controls
• Combustion aspects
• Waste heat recovery
• Steam pressure generation
• Insulation

• Fuel handling
• Fuel substitution flexibility
• Blow down
• Flash steam recovery
• Water treatment
• Scales deposits

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Steam distribution utilization - aspects

• Steam distribution
• Piping
• Pressure drop
• Steam traps
• Insulation
• Leakage
• Condensate recovery
• Flash steam recovery

• Steam utilization
• Elimination possibilities
• Optimization
• Low temperature processing
• Application of alternatives
• Control systems

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Auxiliary power equipment

• Auxiliary equipment include
• ID FD fans
• Boiler feed water pumps
• Cooling water pumps
• Cooling towers
• Fuel handling preparation

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What to do in a steam audit

• It starts with preliminary visit initial data
  collection (specifications, energy consumption,
  operating data, etc)
• Review of present energy consumption and
  operating parameters
• Measurements and conducting tests, trials
  surveys
• Boiler efficiency and heat losses evaluation
• Surveys for steam pressure drop, insulation
  aspects, trapping, leakage, etc.
• Steam and condensate balance
• Trials demonstration
• Detailed analysis of observations and various
  parameters
• Identification of energy saving measures
• Evaluation of techno economic viability of the
  measures

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Parameters to be collected

• Flue Gas Analysis
• CO2 in flue gas
• O2 in flue gas
• CO in flue gas
• Flue gas temperature

• Temperature
• Steam
• Makeup water
• Condensate return
• Feed water
• Combustion air
• Fuel
• Wet bulb dry bulb
• Contd..

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Parameters to be collected contd..

• Pressure
• Steam
• Fuel
• Boiler feed pump
• Combustion air
• Primary air
• Secondary air

• Water conditions (TDS)
• Feed water
• Boiler drum /Blow down
• Blowdown quantity
• Blowdown frequency
• Condensate
• Makeup water
• Contd..

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Parameters to be collected contd..

• Flow
• Fuel.
• Steam.
• Feed water.
• Condensate water.
• Combustion air.
• Flue gas.

• Auxiliary power consuming equipment
• Feed water pumps
• ID fans
• FD fans
• Fuel handling, etc

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Instruments required

• Flue gas analyzers
• Temperature probe and indicators
• Infrared pyrometer
• Draft gauges
• Pressure gauges
• Flow meters
• Electrical power analyzer
• TDS meters

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Typical boiler efficiency of coal fired FBC

• Based on Heat loss method
• Figures with under lined are kcal/kg of fuel
• Figures in bracket indicate

Unburnt losses255 (6)
Moisture in fuel air86 (2)
Surface losses42 (1)
Boiler of 18tph (based on per kg of fuel)
Energy in fuel 4283 (100)
Efficiency 3315 (77)
Due to Hydrogen160 (4)
Dry flue gas423 (10)
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Typical boiler efficiency of FO fired boiler

• Based on heat loss method (BS 8753)
• Figures in bracket indicate

Moisture in fuel air (0.30)
Surface losses(0.44)
Energy in fuel (99.58)
FO fired boiler of 5 tph
Efficiency (83.69)
Sensible heat in fuel(0.42)
Dry flue gas(8.65)
Due to Hydrogen(6.93)
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Factors affecting the boiler efficiency

• Loading on the boiler
• Inefficient combustion - excess air or incomplete
  combustion
• Heat loss in dry flue gas losses vary from 8 to
  35 depending upon
• The fuel
• Type of furnace
• Flue gas (temperature, quantity )
• Loading
• Burner, etc.
• Heat loss due to combustible in ash could be
  2-5. Possible causes for high losses are
• Poor air distribution
• To thick fire bed
• Uneven bed thickness
• Less residence time given to coal particle over
  the combustor, etc
• Blow down heat losses

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• Combustible elements is fuel
• C O2 gt CO2 8044 kcal/kg of C
• 2H O2 gt 2H2O 28922 kcal/kg of H
• S O2 gt SO2 2224 kcal/kg of S
• Combustion air requirement
• Element, kg O2, kg Air required, kg
• Carbon 2.67 11.60
• Hydrogen 8.00 34.78
• Sulfur 1.00 4.34
• Air required for one kg of fuel with following
  composition
• Element, wt O2, kg Air required, kg
  Carbon 85.50 2.27 9.87
• Hydrogen 12.00 0.96 4.17
• Sulfur 1.50 0.015 0.07
• Ash 1.00 0.00 0.00
• TOTAL 100 3.25 14.10

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Incomplete combustion

• Incomplete combustion is due to
• Gross shortage of air
• Surplus of fuel
• Bad mixing of fuel and air at the burner
• Local streams of rich and lean mixture (ingress
  of cold air freezing the combustion reaction)
• Inefficient burners
• Improper atomization
• Incomplete combustion/excess air level
• Poor response to the load variation

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• High excess air levels result in
• Dilution of flue gases
• Reduction in flue gas temperature and heat
  transfer
• Increase in flue gas losses and reduce
  combustion efficiency
• Optimum amount of excess air is required in all
  practical cases to
• Assure complete combustion
• Allow the normal variations in the precision of
  combustion
• The optimum excess air level will vary with
• Furnace design Type of burner
• Type of fuel Process variables
• Excess air can be determined by conducting tests
  with different air-fuel ratios.

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Recommended excess air level

• Fuel Excess air O2 in flue gas
• Min Max Min Max
• Natural gas 10 15 2.0 2.7
• Light oil 12.5 20 2.3 3.5
• Heavy oil 20 25 3.3 4.2
• Coal 30 50 4.9 7.0
• Source Energy Efficiency Office, UK . The above
  settings are typical for boilers without low
  excess air combustion equipment
• Per every 1 reduction in excess air there is
  approximately 0.06 raise in efficiency

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Combustion monitoring control system

• A portable O2 /CO2 analyzer and portable draft
  gauge
• Suitable for small boilers having burners with on
  off control
• Boilers with steady loads
• Continuous O2 /CO2 analyzer with manual damper
  control
• Medium capacity boilers with moderate load
  variations
• Continuous O2 /CO2 analyzer with automatic damper
  positioner
• Large boilers with frequent variable loads
• Opt for suitable combustion system based on
  energy and cost saving potential

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Case study Optimization of combustion by
controlling excess air

• Industry One of the leading breweries in India.
• Plant has IEAC make boiler of 5tph to generate
  steam at 100 psig. Fuel is furnace oil
• The following gives the summary of the boiler
• CO2 in the flue gas 9
• Flue gas temperature 240 oC
• Boiler efficiency 79
• Present damper position 40 open
• Excess air level 70
• Observations
• High excess air
• High flue gas temperature
• Low boiler efficiency
• Absence of flue gas monitoring system
• Contd...

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• Action initiated
• Damper adjustment
• Cleaning flue gas paths and de-scaling
• Replacement of temperature gauge
• The results seen are
• CO2 in the flue gas 12.5
• Flue gas temperature 225 oC
• Boiler efficiency 83
• Present damper position 25 open
• Excess air level 25
• Annual FO savings achieved 68 kL (Rs. 5 lakh)
• At present plant is meticulously monitoring the
  flue gas analysis and temperature. For the
  purpose, it had procured a portable gas analyzer

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Optimization of combustion by controlling excess
air

• Industry One of the leading paper producer in
  Southern India
• It has coal fired FBC Boiler of 60 tph for steam
  at 60 kg/cm2
• During the audit boiler efficiency evaluation was
  carried. The summary is
• Boiler operating capacity 34 - 36 tph
• CO2 in the flue gas 7-8 (Optimum is 13.5)
• O2 in the flue gas 12-13 (Optimum is 5)
• Boiler efficiency 77-78 (Optimum is 82)
• Air quantity supplied 19.5 m3/s
• It can be seen that the boiler was not operating
  at desired conditions. In order to achieve the
  efficient combustion (6 of O2) air quantity
  was reduced (about 17.8 m3/s). But the problems
  encountered are
• Contd...

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• The fluidization was poor
• Dull flame
• Frequent formation of Pockets of coal heaps and
  thereby clinker
• All these were encountered due to shortage of air
  supply for the fluidization where the minimum
  required quantity is 18.5 m3/s.
• To achieve the best for the given conditions,
  default minimum air flow rate (18.5 m3/s) is set.
  i.e., for the loads lower than 34 tph. The
  present online O2 analyzer was rectified and put
  into service.
• The Oxygen percentage after the implementation is
  7-8 and the corresponding boiler efficiency is
  about 79-80
• Savings in energy achieved are
• Annual coal savings 120 MT
• Annual cost savings Rs. 30 lakh
• Investment Marginal

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Blow down

• Blow down is given either of
• Instantaneous
• Instantaneous continuous
• This loss varies between 1 to 10 and depends on
  number of factors. Such as
• TDS allowable in the boiler water (100-3000 TDS)
• The quality of makeup water
• The amount of contaminate condensate
• Boiler load variation
• Blow down requirement (S1 X 100)/(S2-S1)
• Where S1 TDS in the feed water in ppm
• S2 Desired TDS in the boiler in ppm

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• Energy conservation measures in the blow down
• Automatic blow down control
• either by timed blow down control
• or by TDS monitoring (Most effective)
• Heat recovery by pre heating the feed water, etc
• Flash steam recovery from blow down

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Installation of Automatic Blowdown system

• The boiler house has a softener for makeup water
  treatment. Every shift blow down (about 1000 kg)
  is given by opening the valve for 30-40 sec.
• The actual feed water TDS 5 ppm.
• The maximum permissible water TDS 3500 ppm
• Presently TDS in the blow down maintained
  lower than 500 ppm
• Equivalent FO consumption for blow down 47
  kg/day
• In view of the good quality of feed water and
  high blow down quantity, installation of a blow
  down controller will help in controlling the
  excess blow down as well as avoiding high TDS
  level.
• Min reduction in blow down quantity 500
  kg/shift
• Saving of Furnace Oil 7.2 kL/year
• Annual value of savings Rs. 0.57 lakh
• Investment Rs. 1.80 lakh
• Simple Payback Period 3 years

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Heat Recovery from Blowdown ( Flash Steam)
Heat exchanger B
Flash team
Feed tank
Feed water to boiler
Steam trap
Condensate
Boiler
Feed purrp
Flash vessel C
Condensate
Blowdown
To waste
Steam trap
Cold feed waler
Heat exchanger A
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Installation of blow down heat recovery system

• Industry Leading Brewery in Nepal.
• The steam requirement 5 tph
• Feed water TDS 300 ppm
• Max. allowable TDS 3500 ppm
• TDS maintained 3000 ppm
• Present blow down quantity 500 kg/h
• Blow down temperature 183 oC
• Fuel used Furnace oil
• Heat loss through blow down 75500 kcal/h
• Contd..

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• It can be seen that significant amount of energy
  is drained. A simple measure was implemented by
  passing blow down in a coil through feed water
  tank.
• Raise in feed water temperature 13 oC. (from 32
  to 45 oC)
• Annual saving in FO 58 kL
• Annual cost saving Rs. 3.21 lakh
• Investment Rs. 1.50 lakh
• Payback period 6 month

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PLATE TYPE HEAT EXCHANGER FOR BLOW DOWN HEAT
RECOVERY

• Type of Industry - Fertiliser
• Capacity of boiler - 5 X 50 TPH
• Steam pressure - 35 kg/cm2
• Fuel used - Coal
• Boiler blowdown - 18 TPH
• Blow down temperature
• (after shell and tube HE) - 65-70oC

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• Proposal
• Replacing shell and tube heat exchanger with a
  plate heat exchanger for boiler blow down heat
  recovery
• Blow down temperature
• (After plate heat exchanger) - 35 - 40oC
• Savings in coal - 1400 MT
• Annual cost savings - Rs. 19 lakh
• Cost of investment - Rs. 5.0 lakh
• Simple payback period - 4 months

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Waste Heat Recovery

• Most commonly used and very economical
• The sensible heat in flue gases can be recovered
  by
• Preheating the feed water or generating hot water
• Preheating the combustion air
• Factors to be considered are
• Flue gas quantity temperature
• Sulfur content in the fuel and sulfur dew point
• Application potential
• For every 1 oC raise in the feed water
  temperature there is an approximately 4 oC drop
  in the flue gas temperature.
• In other way every 6o C raise in feed water
  temperature there is reduction in fuel
  consumption by 1.
• Every 20oC raise in combustion air temperature
  the fuel consumption will reduce by 1

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When to replace the boiler

• Existing plant is old and inefficient
• Boiler is not capable of firing cheaper
  substitution fuel
• Boiler is over/under sized for duty
• Boiler design is not ideal for present loading
  conditions.

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Fluidized Bed Combustors

• Salient features
• FBC technology has ability to burn inferior
  coal having ash as high as 70 with over 75
  efficiently
• Effectively burn low grade fuels such as paddy
  husk, agro-waste, low grade coal, baggasse,
  lignite, etc.
• Improvement in efficiency by 30-40
• Improvement in rating capacity by 30-50 and
  quick response to changes in the load
• Ensured complete combustion
• Less clinker formation
• FBC retrofit is really a conversion of
  conventional chain grate stokers
• Application Any solid fuel fired boiler (This
  measure has successfully implemented in many
  paper mills, textile units, chemical industries)

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Case Study - Fluidized Bed Combustors

• Conversion of chain grate stroker to fluidized
  bed combustor in a boiler
• Type of Industry Chemical
• Type of Boiler capacity Chain grate - 4.0
  tph
• Fuel used Rice husk
• Rice husk consumption 1870 kg/h
• Boiler efficiency 50
• Steam requirement 6 t/h (due addition of
  capacity)
• Auxiliary power 35 kW
• After converting CGS to FBC
• Boiler output 6 t/h
• Rice husk consumption 1100 t/h
• Auxiliary power 50 kW
• Cost savings Rs 32.0 lakh/year
• Investment required Rs.12.0 lakh
• Payback period 5 months

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Steam Distribution

• The major sources of energy losses in the
  distribution network are
• Improper sizing and lengthy steam pipes
• Improper or lack of insulation
• Excessive pressure drop in the system
• Improper selection, incorrect location and
  malfunction of steam traps
• Poor dryness fraction of steam
• Condensate recovery
• steam leakages from valves, joints, etc

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Steam Piping

• Over sizing of steam piping results in
• Greater initial cost
• Greater heat loss
• Greater volume of condensate formed
• Under sizing of steam piping results in-
• High pressure drop and there by lower pressure to
  steam user
• Steam to be generated at high pressure
• Not enough volume of steam to the users due to
  low carrying capacity
• Water hammer and erosion

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Basis for steam piping

• The guidelines for the selection of proper size
  of steam pipes are given below
• Steam velocity ft/s m/s
• Exhaust steam 70-100 20-30
• Saturated steam (for heating) 60-100 18-30
• Saturated steam (for power) 100-130 30-40
• Super heated steam 150-200 45-65
• Alternatively, if the specific volume is known,
  the flow can be calculated as
• W 0.00287 x ( D2 x V ) / U
• where, D dia of pipe, mm V velocity, m/sec
• U specific vol., cum/kg W
  steam flow rate, kg/h

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Insulation

• Surface heat losses forms a large portion of heat
  loss in the distribution network.
• As a thumb rule the surface temperature of steam
  line insulation should not be more than 10-15 oC.
• The commonly employed insulating materials are
• Glass wool (With no binder up to 200 oC, long
  fibre up to 500oC and short fibre up to 700 oC)
• Rock wool- up to 800 oC
• For recommended insulation thickness - please
  refer handouts.
• Now a days it is very common that even flanges,
  valves are also being insulated with pre
  fabricated insulation materials- which is a good
  practice
• Periodic insulation surveys and rectification
  programs are very effective.

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Steam traps

• Steam traps play vital role in maintaining the
  quality of steam efficiency of heating by
  draining the condensate formed in the steam lines
  and users.
• Most commonly used are (details are given in the
  handout)
• TD traps- intermittent application
• Float traps-continuous/intermittent
• Inverted Bucket traps - intermittent
• Bimetallic traps- semi continuous
• This is one of the very highly neglected areas in
  many plants
• Continuous steam trap management will result is
  sustaining energy saving by avoiding the losses.

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Condensate Recovery

• For every 6 oC raise in feed water temperature
  there could be a saving of 1 in the boiler fuel
  consumption.
• Condensate is pure form of water, if it is not
  contaminated, it can be used as feed water with
  out further treatment. This will results in
• Energy savings due to sensible heat
• Saving in water and chemicals
• Methods of condensate recovery
• Pumping directly to the boiler if it is free from
  contamination
• Direct injection in to feed water tank at the
  bottom by perforated pipe
• If the condensate is contaminated and has
  sensible heat use heat recovery systems

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Flash Steam Recovery

• How flash steam is generated
• Condensate leaving steam trap, is at the same
  pressure of steam (say 5 kg/cm2 , 159oC and ? 159
  kcal/kg). When it comes out of trap the
  condensate pressure is dropped to atmospheric
  pressure. Water at atmosphere will have maximum
  temperature of 100 oC. i.e, 100 kcal/kg.
• The difference in the sensible heat (59 kcal/kg)
  is utilized to evaporate some water in to steam
  (flash)
• flash steam generation (S1-S2)100/L2
• S1 Sensible heat of high pressure condensate
• S2 Sensible heat of low pressure condensate at
  which it is flashed
• L2 Latent heat of flash steam at low pressure
• The recovery of flash steam from high pressure
  condensate is an important energy conservation
  measure.

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Steam Utilization

• Some of the steam saving techniques are
• Exploit alternative modes
• Use of appropriate controls
• Reduction of processing temperature
• Reduction in process time and optimal loading
• Mechanical water removal
• Processing at lower water content
• Pre-heating the products
• Isolating the redundant pipes

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Use of Back Pressure Turbine in place of PRDS

• Captive power plant of a leading petrochemical
  plant supplies power, water to 17 process plant
  located in the same complex.
• Plant has 7 x 125 tph HRSG connected to 7 x 30 MW
  Gas turbine. Normally all are operated at the
  load of 60-70 tph each to generate steam at 115
  kg/cm2.
• Total steam generation is in the range 450-500
  tph.
• Steam requirement at the plants are
• 340 tph of steam at 115 kg/cm2 is supplied
  directly to the process plants
• 62 tph to back pressure cum condensing steam
  turbine (14 tph is tapped at at 4 kg/cm2 and
  remaining is condensed)
• 66 tph of steam at 40 kg/cm2 is obtained from
  passing 115 kg/cm2 steam through PRDS.
• Contd..

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• Proposal Substantial power can be generated by
  installing a back pressure steam turbine in place
  of PRDS (115- 40 kg/cm2).
• Power generation potential 3.5 MW
• Investment required Rs. 500 lakhs
• Annual energy generation 28000 MWh
• Annual cost saving potential Rs. 700 lakh
• Payback period 9 months
• Plant is under the process of identifying the
  suitable vendor since not many manufacturers are
  dealing with this high back pressure steam.

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Thank You