Design Interface

BYS.S. KELKAR

Managing Director , PRESSURE VESSEL, PIPING and POWER PLANT CONSULTANTS

VADODARA

INTERFACE BETWEEN BOILER DESIGN & OPERATION

KNOW HOW AND WHY

I. General considerations for industrial and Utility power plants II. Boiler Design features III. Efficiency Related Issues IV. Operation Related Issues andV. Auxiliary power economy related to power plants and boiler plants.

This interactive presentation is divided in five parts related to understanding of:

Your management has decided to purchase a new boiler of larger capacity to meet additional steam requirement and it is now your responsibility to select and formulate the purchase specification of the boiler.

A very common Situation:

How should you go about it?

I. General considerations for industrial and Utility power plants

Requirement of steam parameters to decide boiler type

Considerations:

Steam is required for process or power generation Fuel option

Boiler design features

Why does cogeneration become an attractive proposition?

With new electricity rule, your company wants to start cogeneration. You have large quantity of residual waste fuel/ waste heat available at your plant. You need to check viability of combined cycle, cogeneration/ -topping or bottoming cycle. How will you proceed?

What is meant by combined cycle-open and closed cycle operation and cogeneration?

Why have combined cycle plants gained wider acceptance?

3.0 What is the approximate quantity of 3.0 What is the approximate quantity of steam required with high pressure steam required with high pressure condensingcondensing turbine and for cogeneration turbine and for cogeneration pass out turbine per MW Generation for pass out turbine per MW Generation for various inlet/outlet steam pressure and various inlet/outlet steam pressure and temperature conditions? temperature conditions?

3 to 4 TPH for condensing high 3 to 4 TPH for condensing high pressure cyclespressure cycles and and 5 to 15 TPH/MW 5 to 15 TPH/MW generation with pass out or back generation with pass out or back pressure turbinepressure turbine

II. Design parameters and Design related issues

Factors required to be considered and Factors required to be considered and evaluated carefully byevaluated carefully by plant user or his plant user or his consultant to determine design consultant to determine design parameters of the boiler that cannot be parameters of the boiler that cannot be varied by boiler designer?varied by boiler designer? Number of boilers/ turbine, Max. Number of boilers/ turbine, Max.

continuous evaporation TPH/SOP/SOT/Feed continuous evaporation TPH/SOP/SOT/Feed temp./ Control/Range/ RH Flow TPH/Rh temp./ Control/Range/ RH Flow TPH/Rh Inlet PR & Temp. Fuel- Design Fuel, Best Inlet PR & Temp. Fuel- Design Fuel, Best Fuel, Worst Fuel Properties, Ambient Fuel, Worst Fuel Properties, Ambient Temp., Site Altitude, Seismic Coefficient.Temp., Site Altitude, Seismic Coefficient.

What factors are considered to be theWhat factors are considered to be the outside the control of customer and outside the control of customer and boiler designer dictated by government boiler designer dictated by government agencies and market forces?agencies and market forces?

Statutory IBR requirementsStatutory IBR requirements Environmental requirements of Environmental requirements of particulate emission level / SOX/NOX/ particulate emission level / SOX/NOX/ Noise emissionNoise emission, , safety valve and safety valve and feeding equipmentfeeding equipment and and economic economic factors like availability and costs of factors like availability and costs of fuel, Power and water.fuel, Power and water.

What are the technical functionalWhat are the technical functional requirements of the boiler process that requirements of the boiler process that take precedence over the other design take precedence over the other design possibilities?possibilities?

Combustion, discharge of ash, heat Combustion, discharge of ash, heat transfer and method of keeping transfer and method of keeping heating surface cleanheating surface clean

What are the most important commercial What are the most important commercial requirements of a boiler plant?requirements of a boiler plant?

Reliability, low initial cost, low Reliability, low initial cost, low working cost, ease of working cost, ease of maintenance, early delivery maintenance, early delivery period to reduce over all costs period to reduce over all costs and early capacity addition, low and early capacity addition, low auxiliary power consumption, auxiliary power consumption, optimized over all costs optimized over all costs evaluated over the envisaged life evaluated over the envisaged life of the plant.of the plant.

What are the usual stand by requirements What are the usual stand by requirements stipulated on a boiler plant?stipulated on a boiler plant?

1.Statutory requirement of adequate 1.Statutory requirement of adequate standby feed equipment capacity and standby feed equipment capacity and independent drive Powered feed pumps.independent drive Powered feed pumps.

2. More than one FD/ID/PA/SA fans, 2. More than one FD/ID/PA/SA fans, rotary air heaters and their drives rotary air heaters and their drives necessary dampers, ESPS, and their necessary dampers, ESPS, and their more than 50% load carrying capacity more than 50% load carrying capacity when only one is in operation.when only one is in operation.

What are its commercial What are its commercial implications?implications?

increases reliabilityincreases reliability probability of probability of two such components failing two such components failing simultaneously will be 0.1x0.1=. 01 simultaneously will be 0.1x0.1=. 01 i.e. only 1%.i.e. only 1%.

How are the first costs of the plant How are the first costs of the plant optimized?optimized?

equating the marginal cost of heat equating the marginal cost of heat absorption by various surfaces absorption by various surfaces such as furnace, super heater, such as furnace, super heater, boiler bank, economizer, air boiler bank, economizer, air heater at various gas heater at various gas temperatures.temperatures.

How is the use of pre-engineered How is the use of pre-engineered components beneficial to the user?components beneficial to the user?

decreases cost of redesign, increases decreases cost of redesign, increases reliability and decreases delivery reliability and decreases delivery period in spite of marginally over period in spite of marginally over design due to steps in capacitydesign due to steps in capacity e.g. e.g. pre-engineered P.F mills of certain pre-engineered P.F mills of certain types & sizes, feeders, fans, soot types & sizes, feeders, fans, soot blower components.blower components.

What features contribute to safety of the What features contribute to safety of the system?system?

Interlocks, sequential operation after Interlocks, sequential operation after ascertaining permissive operation, ascertaining permissive operation, safety valves on SH, RH, HP / LP bypass safety valves on SH, RH, HP / LP bypass system if providedsystem if provided

Why are two flame scanner cooling Why are two flame scanner cooling air fans provided and one driven by air fans provided and one driven by DC drive?DC drive?

To prevent hazard of furnace To prevent hazard of furnace explosionexplosion. .

to keep the costly scanners cool.to keep the costly scanners cool. To save scanner for thermal To save scanner for thermal

deterioration.deterioration.

What are the special design featuresWhat are the special design features introduced on modern boilers to speed up introduced on modern boilers to speed up erection? erection?

Membrane constructionMembrane construction permitting permitting larger components to be lifted,larger components to be lifted, spaced spaced roof tubesroof tubes, , stubbed headers on platen stubbed headers on platen and vertical SHand vertical SH, RH, , RH, shop-assembled shop-assembled componentscomponents, , welded flues and ductswelded flues and ducts maximum transportable sizes of maximum transportable sizes of componentscomponents

(a) what are functional requirements of (a) what are functional requirements of drum?drum?(b) Constructional variations encountered (b) Constructional variations encountered in practice?in practice?  

(a) To accommodate drum internals like (a) To accommodate drum internals like cyclone separators, primary and cyclone separators, primary and secondary scrubberssecondary scrubbers

(b) (b) To decrease its thickness for rapid To decrease its thickness for rapid start upsstart ups

What are the functional requirements What are the functional requirements of furnace?of furnace?

to complete the combustionto complete the combustion, , avoid avoid flame impingement and slaggingflame impingement and slagging, , reduce flue gas temperature below reduce flue gas temperature below Initial Deformation Temperature (IDT) Initial Deformation Temperature (IDT) of the coal ash. of the coal ash.

to obtain uniform temperature to obtain uniform temperature entering SH and shield SH from direct entering SH and shield SH from direct furnace radiation. Wide pitched furnace radiation. Wide pitched platen SH platen SH

to cool gases below IDT and transfer to cool gases below IDT and transfer heat to super heater surface heat to super heater surface

How does arrangementHow does arrangement of super heater of super heater surfaces on a power boiler differ from that surfaces on a power boiler differ from that on an industrial boiler?on an industrial boiler?

steam cooled roof, wide pitched steam cooled steam cooled roof, wide pitched steam cooled platen between nose and roof, steam cooled platen between nose and roof, steam cooled enclosure wallsenclosure walls, , horizontal primary super horizontal primary super heater in counter flow in second pass heater in counter flow in second pass followed by vertical outlet bank, platen followed by vertical outlet bank, platen mentioned earlier in parallel flow and final mentioned earlier in parallel flow and final super heater bank with one or usually two super heater bank with one or usually two stages of spray type attemperator between stages of spray type attemperator between pHs and platen and platen SH and final SH pHs and platen and platen SH and final SH Platen SH is generally arranged for parallel Platen SH is generally arranged for parallel flow, final SH is arranged for parallel or mixed flow, final SH is arranged for parallel or mixed flow to reduce metal temperatures.flow to reduce metal temperatures.

Industrial boiler Super heaters are Industrial boiler Super heaters are generally in two banks with or generally in two banks with or without inter-stage attemperator without inter-stage attemperator and are usually shielded by the and are usually shielded by the furnace nose to avoid its getting furnace nose to avoid its getting overheated during startup.overheated during startup.

Cont. from previous slideCont. from previous slide

Why is drum/surface type attemperator Why is drum/surface type attemperator andand not spray type preferred on industrial not spray type preferred on industrial boilers? What are water purity boilers? What are water purity requirements of spray type Attemperator?requirements of spray type Attemperator?

Water sprayed through an Water sprayed through an Attemperator should have less than Attemperator should have less than 0.02 ppm and SiO0.02 ppm and SiO22 and less than 1-ppm and less than 1-ppm total dissolved solidstotal dissolved solids, , to avoid to avoid contamination of steam and damage to contamination of steam and damage to SH surfaceSH surface. This is often . This is often difficult to meet difficult to meet when a large portion of steam is lost in when a large portion of steam is lost in process heatingprocess heating

What are four methods of What are four methods of controlling Reheater outlet controlling Reheater outlet steam temperature between steam temperature between control load and MCR load?control load and MCR load?

What is function of What is function of penthouse above furnace penthouse above furnace roof? Does seal air fan roof? Does seal air fan blowing in penthouse of blowing in penthouse of help?help?

What are various types of air heaters?What are various types of air heaters? Vertical tubular air heater – tube axis Vertical tubular air heater – tube axis

vertical, gas through tube and air over it vertical, gas through tube and air over it Horizontal tubular air heaterHorizontal tubular air heater – – gas over gas over

tubes and air through tube.tubes and air through tube. Bisector rotary (regenerative type) A/HBisector rotary (regenerative type) A/H Tri-sector rotary (regenerative type) with Tri-sector rotary (regenerative type) with

gas, PA and SA.gas, PA and SA. Quadra sector – primary air between two Quadra sector – primary air between two

secondary air stream and balance half secondary air stream and balance half sector for gas. sector for gas.

Steam air pre-heaters – with finned tubes Steam air pre-heaters – with finned tubes over which air is passed and condensing over which air is passed and condensing steam is passed through tubes. It is used steam is passed through tubes. It is used to keep cold end temperatures of gas air to keep cold end temperatures of gas air heaters above acid dew point heaters above acid dew point

How are soot blowers useful on other How are soot blowers useful on other boiler plants? Why must they be boiler plants? Why must they be provided on rotary air heaters?provided on rotary air heaters?

For - online cleaningFor - online cleaning To reduce stack losses and To reduce stack losses and

draught loss in operationdraught loss in operation.. To avoid the danger of air heater To avoid the danger of air heater

fire if soot deposited on cold air fire if soot deposited on cold air heater during startup is not heater during startup is not removed.removed.

What are the different types of soot What are the different types of soot blowers?blowers? a) Short retractable soot blowers (with a) Short retractable soot blowers (with

about 250mm travel) have a single large about 250mm travel) have a single large nozzle) and used to clean furnace walls.nozzle) and used to clean furnace walls.

b)b) Long retractable blowers which normally Long retractable blowers which normally remain outside the boiler setting and remain outside the boiler setting and travel in it during blowing. They are travel in it during blowing. They are generally provided in cavities between generally provided in cavities between platen SH, final SH, RH bank and when platen SH, final SH, RH bank and when gas temperature are above 1000 degree gas temperature are above 1000 degree Celsius.Celsius.

c)Non retracting blowers (mass or lane c)Non retracting blowers (mass or lane blowing type) have many small nozzles on blowing type) have many small nozzles on the blower tube and rotate in place while the blower tube and rotate in place while blowing the banks.blowing the banks.

d)Oscillating typed)Oscillating type

WWhy does N.G. fired boiler does not require soot blowers? Why are furnace blowers generally not used on oil fired boilers? N.G. does not have ash. Oil ash is fluid does not interfere with heat transfer will not be effective to remove fluid deposits.   

III. EFFICIENCIES RELATED ISSUES:

What are the major losses in a boiler plant and indicate typical values for a stoker fired boiler firing 3220 GCV coal ? What will be the efficiency on GCV and NCV of 3065 kcal/kg ?

• Dry gas loss (6.3%), moisture loss due to hydrogen(2.95%) moisture in fuel(2.12%) , moisture in air loss(0.41%), loss due to combustibles in refuse and flue gas(6.40%), Radiation loss(0.42%) unaccounted loss(1.0%).Total loss on GCV=19.60%. Efficiency on GCV=80.4%. Efficiency on NCV= Eff. on GCV x GCV/NCV = 80.4x3220/3065=84.46%.

What is meant by optimum value of excess used and what are their typical values for various combustion equipment?

Dry gas loss increases. Excess air that gives the minimum value of the sum of these losses P.F-15%,Circulating /Bubbling Bed-10%, Stoker firing-35-50% depending on ash in coal, Oil/Gas firing-8%.

On what other factors does the dry gas loss depend? What limits reduction in gas outlet temperature at the design stage?Ans. difference between last heat trap gas outlet temperature and the ambient temperature.

Acid corrosion of the heating surface and economic terminal temperature which gives optimum sum of savings of fuel minus cost of capital including depreciation,

O&M minus the auxiliary fan power cost limits the gas temperature aimed at while designing. ETTD will be around 50 C for plain tube pressure parts and about 10-15 C for extended surface pressure parts.

How can you detect mal functioning of economizer and air heater?

Ratio of gas temperature drop/water temperature rise. Higher back end temperature due to variety of reasons like gas baffles passing or left out baffle plates after repairs/initial installation, heating surface fouled up.

What percentage by volume dry O2 levels in flue gas will indicate 10%, 20%, 40%, and 60%excess air in flue gas?

1.9%.3.5%,6.0% and 7.9%.

Why does % CO2 values in flue gas decrease with increase of excess air? For the same excess air, why are the CO2% values obtained with coal firing are more than those with oil and least with Natural Gas?% CO2 in fuel will depend on % C burning to CO2

If fuel has more hydrogen% as in the case of oil or N.G., % C will correspondingly be less and consequently Max CO2% achieved with theoretical air will be less

18.7% and15.3%. Adding excess air dilutes the flue gas by Nitrogen and oxygen from air

What is the significance of CO in flue gas? How can it be determined?

Indicates imperfect combustion,and environmental contamination and loss of heat. Orsat apparatus

Why is information regarding S in fuel important in determining achievable efficiency?

feed water temperature at economizer can not be lower than acid due point and gases can not be cooled below a certain temperature. high S% lowers achievable thermal efficiency.

What is Dulong’s formula for calculating GCV in kcal/kg?

GCV = 81C+339.8(H-O/8)+22.12S where C,H,O,S are in % Wt in fuel. NCV= GCV -( 54xH% +0.06xH2O%) kcal/kg.

What is the formula for calculating theoretical air in kg/kg?

Theoretical air =0.1159C+0.3478(H-O/8)+0.04348 x S; where C,H, O,S are in % by wt.

IV. OPERATIONAL RELATED ISSUES:

How much heat will be lost per kg of unburned carbon in ash?

         It will be 8078 kcal/kg of Carbon.

Reasonable estimate of % unburned loss on a roto-grate stoker fired boiler is given by (1244xAsh% in coal/ GCV or NCV in kcal per kg). 

(A) What features facilitate ease of operation?B) What facilitates improving and maintaining plant efficiency?A(A) Motorized valves, remote manual and local operation of dampers, automatically operated control dampers, feed control valves and spray control valves. But remember that each motorized valve will add to plant costs in lakhs of rupees.

(B) Instrumentation and control equipment, and data acquisition system.

What are the sources of false air infiltration in a balanced draft boiler? What are their ill effects?

Furnace inspection and access doors, idle burners, cracked refractory, roof, expansion joints, leaking air-heaters and seals, leaking dust collectors, and ESP. Air infiltration increases stack loss and aux. Power consumption of fans. It can be detected by O2 measurement/ gas temperature drop.  

V. AUXILIARY POWER RELATED ISSUES

Auxiliary power consumption is reported to be very high and your management wants to control and reduce it. What steps do you steps do you think will be helpful?

List out major power consumers in order of their consumption to undertake effective energy audit.

number of hour in a year the equipment operates under full, ¾,1/2, ¼ and zero load, method of starting/stopping and of control is most efficient and whether running it at slower speed will do. Electric motor uses 20-30 times its first cost in a single year. Pumps and fans with smaller/new efficient impeller and efficient motors will save many times the initial cost of new equipment.

Find from internet latest technical developments. If pay back period is less than 3 to 4 years, include it in your suggestion and obtain sanction for implementation in collaboration with operating section manager. cooling water pumps, Equipment to move air and flue gas FD fans, ID fans, primary air fans, and secondary air fans, ceiling air fans, their drives, flues, ducts, expansion joints, dampers and their drives. Attention to runner efficiency, excess capacity, method of control can reduce power consumption significantly. 

What is pressurized operation of draught plant? How is it beneficial? What are its limitations? FD fans alone are used to overcome the resistance of air and gas path, it is called pressurized operation as the whole setting beyond F.D. fan is above atmospheric pressure. saves auxiliary power considerably (about 30 – 40%). In case poisonous gas firing PF firing with considerable quantity of fly ash preferable to keep setting below atmospheric pressure prevent boiler-furnace surroundings from contamination. balanced draught with ID fan

What is the purpose of PA fan and S.A. Fan in a P.F. Fired boiler and how do their duty differ in the case of a Black liquor fired boiler, stoker fired and bubbling bed/ Circulating bed boilers?

Only 20 to 30% of total combustion air is required to transport pulverized coal out of mill. . Coal mill and PF piping offer very high pressure drop as compared to the burner wind box. It pays to increase the pressure of transporting air to the required pressure. On black liquor fired boiler combustion air is

reduced to generate reducing atmosphere in bottom smelting zone and high pressure S.A. is supplied to complete volatile combustion.

Name different methods of controlling centrifugal fan flow rates (m3 /s) and corresponding pressure (mmWG) and principle involved and their limitations. The output developed by the fan is proportional to (N) 2, whereas the power consumption of the fan is proportional to (N) 3, where N is the fan speed. 5 methods to control fan output: (1) Outlet or inlet damper throttle control. (2) Differential damper control giving spin to incoming gas/air (3) Inlet vane control (RVC control) –

(4) Hydraulic coupling control the driven fan speed. Motor Drive speed is kept constant, (5) Electric drive speed control –         First costs increase from 1to 5.Methods 2 to 5 cannot be effectively used if the resistance of the flow path does not change as square of the flow but remains nearly constant as at full load e.g. Mill differential/ fluidized bed resistance.          Meeting 70% flow load with one fan is possible as power is proportional to the cube of the speed.