Heat Recovery BoilersGas Turbine,diesel exhaust HRSGsHydrogen Plant reformed gas,flue gas boilersSulfuric acid plant applicationsSulfur recovery plants_Claus plantsIncineration heat recovery liquids,solids,fumesFlue gases-cat crackers,fluidized beds,furnacesUnfired/fired boilersBare/finned tube unitsFire tube,water tube,combination,A or D type boilers,modular units with superheaters,economizer,condensate heaters,auxiliariesNOx,CO reduction equipment

Classification of Waste Heat Boilers

Waste Gas Analysis

Composition of Typical Waste Gases - % volume

Gas

temp-C

Pres

N2

NO

H2O

O2

SO2

SO3

CO2

CO

CH4

H2S

H2

NH3

HCL

1

300-1000

1

80

10

10

2

250-500

1

81

11

1

7

3

250-850

3-10

66

9

19

6

4

200-1100

1

70

18

3

9

5

300-1100

30-50

0.5

37

6

8

5.5

43

6

200-500

200-450

20

60

20

7

100-600

1

75

7

15

3

8

175-1000

1

72

10

6

12

traces

9

250-1350

1

76

8

4

7

5

10

150-1000

1

73

20

2

5

11

300-1450

1.5

55

23

6

6

3

3

4

1.Raw sulfur gases 2.SO3 gases after converter 3.Nitrous gases 4.Reformer flue gases 5.Reformed gas 6.Synthesis gas

7.Gas Turbine exhaust 8.MSW incinerator exhaust 9.Chlorinated plastics incineration 10.Fume or VOC incinerator exhaust 11.Sulfur condenser effluent

Water Tube BoilersSuitable for high steam pressures and temperatures and large gas flowsProvision can be made for cleaning within gas stream Superheater can be located anywhere inside the gas pathExpensive if gas pressure is high. May have to be located within a shellMultiple pressure steam generation feasibleLower holdup inside drum and tubes compared to fire tube and hence response to gas flow variations is fasterExtended surfaces may be used(if gas is clean) to make HRSG compact

Fire Tube BoilersLimited to low steam pressures and capacities typically less than 100,000 lb/h gas flowCan handle high gas pressures up to 2000-3000 psig with easeDue to smaller heat transfer coefficients,large in size.Finned tubes also cannot be usedLocation of superheater either at gas inlet or outlet,not an optimum location as in water tube boilersEconomizer and superheater can be added if requiredSluggish response to load changes due to large water holdup

Thickness of Tubes under Internal vs External pressure

Tube Thickness vs Pressure ASME sec 1

Tube thickness, in

External pres, psig

Internal pres, psig

0.105

575

1147

0.120

686

1339

0.135

800

1533

0.150

921

1730

0.180

1172

2137

[2 in OD,sa 178a and sa 192 carbon steel tubes at 700 F]

Fire Tube Boiler DesignsGas flow=110,000 lb/h.inlet gas=1450F.exit=500 F.steam pr=300 psigsteam =28,950 lb/h.boiler duty=29.4 MM Btu/h

size1.75x1.5211.75x1.5211.75x1.5212x1.772x1.772x1.772.5x2.2382.5x2.2382.5x2.238Vel,ft/s109141166110140165109140166Tubes1100850725800630535510395335Length,ft19.020.021.022.524.025.029.531.533.Surf,ft2831867666059835170156205881172866474Ui9.7411.7813.259.611.4312.899.1511.021243P,in wc2.504.46.32.64.46.22.54.36.2

Fire Tube and Water Tube Boilers

Effect of Scale on Boiler Heat TransferTable of k values of scale(Btu/ft2hF/in)Analcite 8.8Calcium phospate 25Calcium sulfate16Magnesium phospate15Magnetic iron oxide 20Silicate scale0.6Boiler steel310Fire brick 7Insulating brick0.7ffo=k/L. heat flux q=U(tg-ts)Scale temperature drop =q x ffo .A 0.03 in silicate scale results in ffo=0.03/0.6=0.05 ft2hF/Btu.Fire tube boiler. Gas flow=100,000 lb/h. gas inlet=1500 F.exit=500 F(clean). Steam pres=150 psig sat, feed water=220 F, 5 % blow down. Tubes:2x1.77,600 tubes,20 ft long.surface=6280 ft2

ItemCleanFouledFoul factor0.0010.05Overall htc U9.66.52Heat flux,Btu/ft2h60864133Drop- fouled layer,F6207Tube wall temp,F377577Duty,MM Btu/h28.1324.47Exit gas temp,F500630Annual loss,$Basis-87,800Steam flow,lb/h27,71024,100

Elevated Drum Fire Tube BoilerThis design also has a superheater. Due to its location,the steam temperature will be low. Locating at the gas inlet leads to corrosion/overheating concerns.

Hcl CorrosionHcl also has a low acid dew point in the range of 130-150 Fand one must consider this while designing the economizer.Hcl also is responsible for high temperature corrosion.

Hcl Corrosion-2Many metals and alloys are susceptible to severe corrosion attack when exposed to halogen gases at elevated temperatures. Halogen reacts with many metals and forms metal halides,many of which exhibit high vapor pressures and or low melting points. In Cl2,Hcl environments,corrosion is dependent on whether the environment is oxidizing or reducing. In Cl2 ,iron and steel are very susceptible to chlorination attack. Adding chromium and or nickel improves corrosion resistance.Thus ferritic and austenitic stainless steels can resist chlorination attack at higher temperatures than cast iron or carbon steel. Ni and Ni-bearing alloys are significantly more resistant to chlorination attack than iron,carbon steel,stainless steel and 800 type steels. In oxidizing environments containing both Cl2 and O2,MO and W are detrimental to alloys resistance to chlorination attack. In reducing environments containing Hcl,Ni and Ni based alloys are better than iron and austenitic stainless steels. Carbon and stainless steels are susceptible to fluorine attack at 570 F or even lower.Ni has the best resistance to fluorine attack. HF is less corrosive than F for most metals. Hot corrosion proceeds in two stages. An incubation period exhibiting low corrosion rates(formation of protective oxide layer) followed by accelerated corrosion attack(breakdown of protective layer)

Melting Point of CompoundsUse radiant furnace to cool gasesDouble spaced screen to avoid bridging of molten productsFGR to cool the incoming gases below melting pointLow superheat temperature to minimize Hcl corrosionUse of retractable soot blowers to clean depositsLow gas velocity to minimize erosion

CompoundMelting point,F50NaCl-26Na2So4-24Na2Co3113465Na2So4-35Nacl1153NaCl1474Na2So41623Ca2O2257Fe2O32664NaCl-ZnCl2504CaCl2-PbCl2884

Materials & Corrosion Cr:improves oxidation resistance provided temperature does not exceed 950C for long periods. Volatility of Cr2O3: 1.Improves sulfidation resistance: 2.High Cr beneficial to oil ash corrosion and attack by molten glass 3.Decreases carbon ingress-helps carburization resistance 4.Detrimental to fluorine environments at high temperatures 5.Detrimental to nitriding resistance6.Increases high temperature strengthSi improves oxidation,nitriding,sulfidation and carburizing resistanceMO, W:Improves high temperature strength,good in reducing chlorination resistance.Improves creep strength,detrimental for oxidation resistance at higher temperaturesNi:Improves carburization,nitriding and chlorination resistance.Detrimental to sulfidation resistanceC:improves strength,helps nitriding, carburization resistance.oxidation resistance adversely affectedAl:improves oxidation,sulfidation resistance. Detrimental to nitriding resistanceTi:Detrimental to nitriding resistanceNb:Increases short term creep strength. Detrimental to nitriding resistanceMn:Slight positive effect on high temperature strength and creep.Detrimental to oxidation resistance.Increases solubility of nitrogenCo:reduces rate of sulfur diffusion.helps with sulfidation resistance Source: VDM Technologies Corp

Water temperature determines tube wall temperature Acid vapor condenses if the tube wall temperature falls below the acid vapor dewpoint Tube wall temperature tw=0.5(tf+tg-U(tg-tf)(1/hg -1/hi)Typically in an economizer,hg=15,hi=1000 and U=14.77If tg=750 F,ti=250 F,tw=0.5(250+750-14.77(750-250)(0.066-0.001)=260 Fif tg=350 F,ti=250 F,tw=0.5(250+350-14.77(350-250)(0.066-0.001)=252 F Thus for a difference of 400 F,the tube wall temperature changes by only 8 F. Hence the fluid temperature governs tube wall temperature and not the gas temperature.

Low temperature CorrosionIndustrial experience shows that the corrosion rate is higher at 20-30 C below the dew point and hence a few boiler suppliers lower the feed water temperature in order to lower the stack gas temperature. Hot casing is another method often used to minimize casing corrosion.Amb temp=70 F ,emissivity=0.9,wind vel=100 fpm

Thick,inTemp,FName ThickTemp,F0183Casing o1990.5550Min fib31596Cbm 3155641800Ks4 41800

Minimizing CorrosionOne approach is to preheat the incoming cold feed water to near dew point temperature using water from exit of economizer.Also startup and shut down of boilers/turbines fired on oil or waste heat units firing dirty fuels should be on clean gas so that acid residues may not deposit on surfaces. Steam may also be used to preheat the water.However the cost of the system is more due to handling of condensate and energy wastage.

An inexpensive way to minimize corrosion is to increase the deaerator pressure slightly so that the feed water temperature increases.At 5 psig,the water temperature is 228 F,while at 15 psig,it is 250 F.In oil fired gas turbines,under shut down conditions,moisture from air reacts with acid residues on the tubes,making it dilute and corrosive. This is more so with cyclic operation.Hence tubes should be maintained above acid dew point conditions. Duplex steel,which is a hybrid of austenitic and ferritic steel, is sometimes used in condensate heaters to avid corrosion.This is however very expensive.This may cost 9 times that of carbon steel. Sometimes,it may be worth building the coil in two parts,one which operates at low tube wall temperatures and then replace it as required.

Boiler Tube Materials

MaterialCompositionTemperature,FSa 178aCarbon steel950Sa192Carbon steel950sa210A1Carbon steel950sa210CCarbon steel950sa213T111.25Cr-0.5Mo-si1050sa213T222.25Cr-1Mo1125sa213T919Cr-1Mo-V1200sa213TP304H18Cr-8Ni1400sa213TP347H18Cr-10Ni-Cb1400Sa213-TP321H18Cr-10Ni-Ti1400sb407800H33Ni-21Cr-42Fe1500

Waste Heat Boiler for MSW

D type Waste Heat Boiler

A Type Boiler

Waste Heat Boiler for FCC unit

Water tube boiler for FCC unit

HRSG for FCC application

Bare Tube HRSG Moderately dirty gasesMulti-pass bare tube evaporator and economizerSoot blowers usedSuitable for small gas flows

Waste Heat Boiler with Radiant FurnaceA radiant furnace is used if the gas stream contains slagging constituents or salts with low-melting points such as compounds of sodium,potassium,non-ferrous metals

Crossflow Waste Heat BoilerCrossflow finned tube boilerExternal downcomers

Vertical Fire Tube Boiler

Heat recovery in Claus Plants

Claus plant waste heat boilerSulfur is present in natural gas as H2S. It is removed in an exothermic reaction by combustion of acid gas with air in a series of exchangers. H2S+ 0.5O2 S +H2O . Many other complex reactions occur and in each stage some cooling is done using waste heat boilers. The gas stream contains CO2,H2S,SO2,H2,CH4 and H2O.Reaction furnace operates at about 1800-2800 F.Hence this boiler design is done considering the high gas temperature and casing corrosion.Sat steam at 600 psig is typically generated. Gas is cooled to about 1200 F in first pass and to about 650 F in second pass. Common steam drum is used as shown.

A two-pass Fire Tube Boiler

HRSG for Incineration ApplicationIdeal for clean gases. Note the bare tube screen section in parallel witha finned tube evaporator with buried/shielded superheater

Combination Fire and Water Tube Boilers Widely used in small hydrogen plantsProcess gas boiler is a fire tube unitFlue gas boiler is a bare/finned tube sectionThe boilers operate in parallel with common steam drum

Steam Reforming process

Heat Recovery System in a RefineryHeat recovery boiler in a refinery consisting of a feed gas heater,evaporator and economizer. The evaporator is a natural circulation coil,connected to a steam drum located above by downcomers and risers.

Nelsons ChartThe chart gives an idea of the materials to be used based on partial pressure of hydrogen in the gas stream and temperature

Reformed Gas BoilerGas bypass system is used to control exit gas temperature at low loads.As gas flow decreases ,the exit gas temperature decreases,which is not acceptable as performance of catalysts downstream is affected. So more flow is sent through the bypass pipe and less through the tubes using bypass dampers. Tube size varies from 1 in to 2 in.T11 or T22 materials are commonly used considering presence of hydrogen.Heat flux is quite high,on the order of 100,000 Btu/ft2h due to presence of hydrogen and water vapor at high gas pressure. Tube sheet is protected by refractory and ferrules. GC 94 refracory,which is a high alumina refractory is used.

Boiler performance at 50 and 100% load

Tube Sheet DetailsThe tube sheet temperature as well as the thermal stresses across it are significantly reduced by using refractory and ferrules

Effect of Gas properties on Design

Design of Fire Tube Boiler

Item

Reform

Flue gas

Gas flow,lb/h

100,000

100,000

Gas inlet temp, F

1650

1650

Gas exit temp, F

650

650

Gas pressure, psia

315

15

Duty, MM Btu/h

70.00

28.85

Steam, lb/h

69,310

28,570

Gas pr drop, in wc

9

5

Heat flux, Btu/ft2h

92,200

12,300

Surface area, ft2

1566

4266

No of tubes

350

1300

Length,ft

15

11

heat tr coefficient

87

13.4

Max gas velocity, ft/s

68

165

Tube wall temp, F

653

498

Elevated Drum Water Tube BoilerFor large capacity units an elevated drum with external downcomersand risers is used.

Fire Tube Boilers with common steam drumSteam is generated by two different gas streams or in two different passes and connected to the same steam drum system

Fired Waste Heat BoilerHandles waste flue gases and also uses a burner to augmentsteam generation. Performance to be evaluated in different modes

Heat Recovery systems

Typical Steam Drum InternalsSteam purity of 50 ppb or less can be attained depending on boiler water solids and proper selection of drum internals.

Flow Accelerated CorrosionFAC is a process whereby normally protective magnetite (Fe3O4) layer on carbon or low alloy steel dissolves into a stream of flowing water or water/steam mixture. Both the PH and temperature and level of dissolved oxygen in the stream influence the stability and solubility of magnetite oxide layer.Past industry water chemistry practices believe that all of dissolved oxygen must be eliminated from feed water to control corrosion. To deoxygenate the feed water,oxygen was mechanically removed by the condensaer/deaerator with supplemental additions of an oxygen scavenger.(eg hydrazine) being applied to maintain a 40-100 ppb hydrazine residual. Maintaining an oxygen scavenger residual causes the feed water to become more and more reducing and has for all ferrous systems produced the opposite desired effect of producing a protective oxide film to one where erosion-corrosion of iron based materials is increased. This mechanism is active in condenser shells,feed water and wet steam piping,feed water heaters,economizers.FAC occurs in many materials but more in carbon steel piping in 212-482 F range.In VGB,maximum O2 in feed water has been increased from 0.02 to 0.1 mg/kg.Flow velocity also has to be lower to reduce erosion of protective magnetite layer.

Flow accelerated corrosion-2While dissolved oxygen in water can induce serious corrosion especially during shut down,the complete removal of oxygen during normal operation can be very problematic and has led to FAC,which develops at elbows,flow disturbances,reducers in strongly reducing atmospheres.Magnetite is a mixture of Feo(ferrous) and Fe2O3(ferric).The ferrous ions are those that are susceptible to FAC and these ions migrate out of the magnetite matrix and weakens the tube.Many once through systems now use oxygenated treatment programs in which oxygen is injected into the condensate and feed water to produce ferric oxide hydrate.A dissolved oxygen of 30-50 ppb is common in US.Lowers corrosion rate. Copper carryover to the turbine is an issue if feed water heaters are present.ORP(oxygen reduction potential) monitoring system, which checks the electrochemical potential and adjusts it by injecting hydrazine to maintain a low electrochenical potential(-100mv).HRSGs have sharp bends in economizers,evaporators and prone to FAC. Combined cycle plants do not have feed water heaters and hence no copper. Oxygenated systems with ORP monitioring are done in a few plants.Bends of evaporators, economizers,sometimes made with chromium alloys to minimize FAC.

Flow accelerated corrosion-3FAC occurs when water or steam water is in contact with the pipe.The rate of FAC depends on temperature,water chemistry,pipe geometry and materials.The bends are prone to failure as the local turbulence is higher. Gas bypassing also increased the gas temperature at the ends and affected the oxide growth formation.

Metal DustingMetal dusting is the term used to describe the catastrophic degradation of metals in carbanaceous gases at elevated temperatures,450 to 900 C.The surface becomes pitted and metal wastage is seen.The term is derived from the appearance of pits which often contains of loose powdery magnetic corrosion product of graphic,metal carbides and oxides.Rapid saturation and supersaturation of carbon in the metal matrix,cementite formation at the surface and at grain boundaries where nucleation is facilitated.Cementite is stable at high carbon activities and can decompose at outer surfaces leading to formation of carbon and metal particles,which act as catalyst for further coke depositionVolume change associated with these transformations generate high internal streeses and results in disintegration of surface into a loose powderThis process of Fe3C formation and decomposition continues till the supersaturated region is consumed. When the reaction products are removed bygas erosion,the process starts again.

Metal Dusting-2Carburization and metal dusting can proceed when environment has CH4,CO,CO2, H2. CO+H2 = C+H2O 2CO=C+CO2 CH4=C+2H2The metal dusting process is valid for low alloy ferritic steels because surface oxide film does not retard the metal dusting process. Alloying with Cr and Si such that (% CR +2%Si)>24 helps.Alumina has proven to resist carbon diffusion into metal matrix. Cr diffusion rates are higher in ferrite than in austemitic and thus protective films are likely to heal at low temperatures. This is reason for success of high Cr 446 ferritic stainless steel in overcoming metal dusting.Many also feel that high Ni based alloys resist metal dusting because of the slower diffusion of carbon in nickel based austentitic matrix. However the same mechanism will also inhibit healing of ruptured surface films by retarding diffusion of Cr at hih Ni contents.Alloy 800 has a high degree of resistance to metal dusting,while Alloy 600 is subject to MD.

HRSG ProblemsDesign basis for cycling: 300 cold starts,1300 warm starts and 9000 hot starts(25 years life).Each startup causes low cycle fatigue.ASME-sec 1 does not give reference to low cycle fatigue..in TRD fatigue caused by thermal and pressure induced stresses is considered at the inner edge of openings in shells. Tubes operate at a different temperature than headers and hence thermal stresses.During warm,hot starts,condensate forms in the superheater/reheater as tubes cool faster than evaporator.Drains have to be installed. In a forced circulation unit,the build up of steam pushes the condensate in the direction of steam as well as gravity is pulling it out. In the natural circulation unit,gravity and steam flow are in opposite directions and water remains in the tubes for a longer time,causing higher temperature differences from one tube to another.In horizontal units,due to the steep inlet duct,the purge air does not touch the upper section of the HRSG where combustible mixtures can be formed as their density is lower than cold air. In vertical gas flow unit,the light gases flow out easily.Carbon steel tubes operate in the elastic range where allowable stresses are based on yield stresses while alloy tubes operate in the creep rupture range where allowable stresses are based on rupture strength.

Corrosion mechanismsCorrosion fatigue: Corrosion ,fatigue can work together or one can enhance the others effect.Alternating stresses can cause cracks,which allows a path for corroding agent to damage the surface .Or the uneven attack of a corrosive agent would create notches and pits that can become stress enhancers and increase the potential for fatigue cracks. CFC is a form of deterioration that can occur without concentration of a corrosive substance. CCFC refers to cracks propagating through a metal as a result of cyclic stresses operating in a corrosive environment. Even the protective magnetite layer is sufficient to cause this kind of cracking in the presence of sufficient cyclic stresses.The cracks can occur longitudinally and most always propagate in a diretcion perpendicular to the direction of principal stress. They are straight, unbranched and transgranular.If the principal cyclic stress is a result of pressure fluctuations,then the cracks will be longitudinal. If the principal cyclic stress is bending(thermal expn/contraction) then the cracks will be transverse. Cracks seen at physical restraints.Stress Corrosion Cracking:deterioration that occurs with concentration of a corrosive substance(carbon steel- NaOH; stainless steel-NaOH,Chlorides) and sufficient tensile stress(residual or applied).Occurs generally inside tubes.Cracks can be intergranular,continuous or transgranular. Cracks can branch and spread resulting in brittle fracture.SCC below 300F is rare.

Low Cycle Thermal Stress FatigueThermal stress is most intense at the surface since it is caused by the difference between surface temperature and average metal section temperature and caused by transients such as. 1.cold start 2.hot statr 3.daily load change 4.shutdownCOLD START: drum inner surface temperature follows HP evap steam temperature closely and increases rapidly. Drum outer shell lags behind, producing an appreciable temperature difference between outer and inner surface or thermal stresses. These stresses are in the internal drum surface causing it to expand as its temperature increases. It is restrained from expanding by the external surface, which is cold.If startup is aggressive, inner surface yields in compression ,so that residual tensile stresses results when steady load is reached. This residual tensile stress will then release or creep out depending on the operating time and temperature to the next shut down or load change. Repeated yielding will exhaust material cyclic life.HOT START:drum shell outside temperature is higher than inside shell temperature so that the outside shell temperature first decreases rapidly and then as the inside shell temperature increases, the outside shell temperature increases to approximately the inside shell temperature. This reversal in temperature change causes a reversal in thermal stress thus decreasing the pressure part life.SHUTDOWNS:more serious. Magnitude of thermal stress depends on: extent and range of temperature change, surface htc. Thickness. thermal properties. stress concentration factors

HRSG Maintenance Superheater and reheater header joints,feed water heaters,drum to downcomer joints,tube to header joints are prone to failure due to stresses in quick startup and cycling. Low cycle thermal stress fatigue may not have immediate effect.During cold startup,drum inner temperature increases faster than outer.the difference causes thermal stresses.Outer surface prevents inner from expanding causing residual tensile stresses. During hot start,the outer surface is higher now also causing stresses.Magnitude of thermal stress in any location of pressure part is proportional to the difference between the temperature at the location and pressure part average temperature. If there is a soaking or hold,the stresses will relax. Shut downs are more detrimental. Whenever yield strength is exceeded during startups load changes and shut downs,life is expended,which can initiate a crack.For larger sized components,the stresses are more. Casing damage [warping of liners] Low cycle fatigue,creep and corrosion fatigue at hot end[poor drainage,venting]Fatigue,FAC,acid dew point corrosion,stress corrosion cracking at colder endFouling due to ammonium salts[ dry ice cleaning,ammonia slip]Suggestions: maintain eco flow during startups using blow down,bypass lines or recirc linesSplit eco headers and not simply baffling them.Miminimzie oxygen ingress during startupProper design of tube to headers[avoid multiple tubes,combining drains,full penetration welds etc]

HRSG Tube failuresThere are three types of failures in eco inlet header tubes. These are due to thermal corrosion fatigue (due to cycling ), flexibility induced cracking and erosion-corrosion (FAC). Damage caused by the first two will manifest as cracks and the third as wastage and give an orange peel appearance. Erosion-corrosion and thermal fatigue are ID initiated while flexibility cracking is OD initiated. Damage due to corrosion fatigue initiated at stress concentrations associated with bore holes and tube attachments. Pin hole leaks are common. Thermal fatigue will be oriented longitudinally parallel to stub axis,while flexibility cracking is circumferential around toe of the weld. Tube leaks caused by flexibility are generally at header ends,where thermal stresses are higher.

Typical Controls Scheme