Chapter 5 Gas to Liquid Heat Recovery ( )

10
120 Chapter 5 Gas to Liquid Heat Recovery Waste Heat Recovery Boilers (WHRB) 5.1 Introduction Consider the enthalpy of flue gas escaping from the boiler. This enthalpy is higher than that of combustion air entering the boiler. So, there is a net heat loss. This loss is given by ( ) % 100 100 100 100 Q Q 4 0 2 2 Q q h h q a ah g - - = = α (5.1) where 2 q is percentage heat loss with waste gas 2 Q is heat loss through the stack gas Q is the available heat /heat input g h is the flue gas enthalpy at the exit of air heater, KJ/kg fuel 0 a h is the theoretical cold air enthalpy entering boiler, kJ/kg fuel ah α is the excess air coefficient at the exit of the air heater ( ) 100 100 4 q - is the correction factor owing to the difference between calculated and actual fuel consumption [ 4 q refers to the heat loss due to unburnt carbon] From Eq. (5.1), it is observed that there is a significant loss of heat with the exhaust. Oxygen furnace, cement kiln, open-hearth steel furnace, and petroleum refinery, etc emit flue gases at temperatures between 400 0 C and 1900 0 C [ ref:Ganapaty,1991]. Similarly, the exhaust of an ordinary boiler is at 140 0 C and for a Diesel engine power plant, the emitted gas is at around 350-400 0 C. Enthalpy of flue gases emitted by various plants can be economically recovered by generating steam in a waste heat recovery boiler. The steam generated may be used either for process heating or for generation of electricity depending upon the availability. The waste heat boilers do not usually fire any fuel. They are essentially counter- current heat exchangers. In case steam is required at all the times, even when hot gas is not available supplemental gas or oil firing may be added in this type of boiler. The gas side pressure drop across the boiler is generally kept within 2.5- 3.7 kPa and the minimum temperature difference between gas and steam water (pinch point) is kept at 11-28 0 C. A high resistance through the boiler will adversely affect the efficiency of the gas turbine.

Transcript of Chapter 5 Gas to Liquid Heat Recovery ( )

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Chapter 5 Gas to Liquid Heat Recovery

Waste Heat Recovery Boilers (WHRB)

5.1 Introduction Consider the enthalpy of flue gas escaping from the boiler. This enthalpy is higher than that of combustion air entering the boiler. So, there is a net heat loss. This loss is given by

( )%100 100

100

100 Q

Q

40

22 Q

qhh

qaahg

−−

==α

(5.1)

where 2q is percentage heat loss with waste gas 2Q is heat loss through the stack gas Q is the available heat /heat input

gh is the flue gas enthalpy at the exit of air heater, KJ/kg fuel 0ah is the theoretical cold air enthalpy entering boiler, kJ/kg fuel

ahα is the excess air coefficient at the exit of the air heater

( )100

100 4q− is the correction factor owing to the difference between calculated and

actual fuel consumption [ 4q refers to the heat loss due to unburnt carbon] From Eq. (5.1), it is observed that there is a significant loss of heat with the exhaust. Oxygen furnace, cement kiln, open-hearth steel furnace, and petroleum refinery, etc emit flue gases at temperatures between 4000 C and 19000 C [ ref:Ganapaty,1991]. Similarly, the exhaust of an ordinary boiler is at 1400 C and for a Diesel engine power plant, the emitted gas is at around 350-4000 C. Enthalpy of flue gases emitted by various plants can be economically recovered by generating steam in a waste heat recovery boiler. The steam generated may be used either for process heating or for generation of electricity depending upon the availability. The waste heat boilers do not usually fire any fuel. They are essentially counter-current heat exchangers. In case steam is required at all the times, even when hot gas is not available supplemental gas or oil firing may be added in this type of boiler. The gas side pressure drop across the boiler is generally kept within 2.5-3.7 kPa and the minimum temperature difference between gas and steam water (pinch point) is kept at 11-280 C. A high resistance through the boiler will adversely affect the efficiency of the gas turbine.

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Usual problems encountered with WHRB are

� Waste gas is corrosive rendering fouling of the heat transfer tubes. � A dust load with the waste gas contaminates the heat transfer surfaces,

thus reducing the heat transfer rate. For highly dust laden flue gases such as those from open-hearth furnaces and cement kilns, a three-drum design is suitable.

The advantage of WHRB over other heat recovery devices are as follows:

1. WHRB involves boiling of water, thus process involves very high heat transfer rate. It is a compact unit requiring less surface area.

2. Cost of installation is usually lower than other waste heat recovery units. 3. The WHRB can withstand high exhaust temperature 4. Response rate is high and load can be adjusted by varying the steam

pressure with the help of the safety valve. 5. The temperature of the tube wall is close to the boiling liquid temperature.

Consider a riser tube of the boiler. The hot liquid with bubbles flow through the tube, the heat for boiling is obtained from the waste gas flowing over the pipe. The tube wall temperature is calculated by

21

21

hh

ththt fg

w ++

= (5.2)

The gas side heat transfer coefficient 1h is very less compared to the liquid side heat transfer coefficient whereas fg tt ≥ . Considering order of magnitude of the

parameters, it is evident that fw tt ≅ . 5.2 Classification of WHRB WHRB are classified in 2 (two) major categories

1. Fire Tube boiler 2. Water tube boiler

� Natural circulation boiler � Forced circulation boiler

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5.2.1 Fire Tube Boiler Fire tube boilers were used extensively for industrial power generation till late 18th century. They are no longer used for large utility power plants. However, they are in still use in some industries for producing saturated steam with upper limit of pressure to 18 bar and capacity of 6.3 kg/s. Interestingly, general design of fire tube boiler has not changed much for last three decades. In fire tube boilers the flue gas products of combustion flow through boiler tubes surrounded by water (Fig.1). The heat transferred through the walls of the tubes to the surrounding water generates steam.

Fig.5.1 A fire tube boiler The flue gases are cooled as they flow through the tubes, transferring their heat to water. Therefore, the cooler the flue gases, the greater the amount of heat transferred. Cooling the flue gas is a function of heat conductivity of the tube material, the temperature difference between the flue gases and water in the boiler, the heat transfer area, the time of contact of the flue gases with tube wall etc. Fire tube boilers used today evolved from the earlier designs of a spherical or cylindrical pressure vessel mounted over the fire with flame and hot gases around the boiler shell. Installing longitudinal tubes in the pressure vessel and passing flue gas through the tubes have improved this obsolete approach. This improves the heat transfer coefficient and increases the heat transfer area. The results are the two variations of the horizontal return tubular boiler (HRT) as shown in Figs. 5.2-5.3

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Figs. 5.2-5.3 HRT boilers A parallel evaluation of fire tube boiler is the locomotive boiler with the furnace surrounded by a heat transfer area and a heat transfer area added by using horizontal tubes (Fig. 5.3).

Fig. 5.4 Locomotive boiler The Scotch Marine boiler design, as shown in Fig. 5.4, with the furnace a large metal tube, combined the feature of the English Cornish type of boiler of the 1800s and the smaller horizontal tubes of the HRT boiler. This boiler originally was developed to fit the need for compact shipboard boilers. As the furnace is cooled completely by water, no refractory furnace is required. The radiant heat from the combustion/waste gas is transferred directly through the metal wall of the furnace chamber to the water. This allows the furnace walls to become a heat

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transfer surface - a surface particularly effective because of high temperature differential between the flame/high temperature exhaust and boiler water.

Fig. 5.5 Scotch Marine Boiler Advantages:

1. Low first cost 2. Reliability in operation 3. No need for skilled labour 4. Less draught is necessary 5. Quick response to load change: fire tube boiler is having large water

storage and thus can meet sudden load demands with only small pressure change

Salient Features:

� Good for small steam requirement � Good for community hot water/domestic hot water generation � It must be used when gas pressure is high � Useful even when the gas is corrosive, the shell side is then free from

corrosion � All the tubes are immerged in water � Limited applications- In industrial plants it is used to produce saturated

steam. Scotch Marine boilers have been using for waste heat recovery. � Recommended to use at a pressure below 18 bar and steam flow rate of

6.2 kg/s.

Major Shortcomings: Size and pressure limitations: The tensile stress of boiler drum is given by the equation

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tDp 2

=σ (5.3)

where, σ is the tensile stress, N/m2

p is the gauge pressure, N/m2 D is the internal diameter of boiler shell, m t is the thickness of shell, m

From Eq. 5.3 it is observed the following:

(a) If the gauge pressure p is high, the tensile stress becomes high proportionately which results in increase in thickness t of boiler shell.

(b) If the boiler storage capacity is high, the shell diameter D becomes more, which results in high stress demanding for more thickness.

It is obvious from above analysis that for both the cases mentioned above lead for high cost of boiler. Thus there is limitation on size and pressure on boiler design.

5.2.2 Water Tube boilers 5.2.2.1 Natural and controlled circulation boiler The flow of water and steam within the boiler circuit is called circulation. Adequate circulation must be maintained to carry away the heat from the flue gas. If circulation is caused by density difference, the boiler is said to have natural circulation. If a pump causes it, it has forced or controlled circulation.

Fig.5.6: Natural circulation boiler Fig. 5.7: A forced circulation boiler

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� Circulation should be adequate to maintain heat balance of waste gas and fluid. Adequate circulation prevents any hot spot on boiler tube.

� Circulation ratio (CR) is the ratio of flow rate of saturated water in down comer to the flow rate of steam released from the drum

� For maintaining the natural circulation, 6 ≥CR for the riser. This means that 6 kg of saturated water should be circulated through the down comer for each kg of steam produced.

� Too much steaming is undesirable as it creates blanket (film) retarding heat transfer. Inserts like, steel ribbons, twisted tapes, wire coils can reduce the film formation on riser tube. However, boiler used for producing hot water need not contain the inserts.

� The downcomers are fewer in number but bigger in diameter. They are meant to make the saturated water to fall by gravity. Bigger the diameter, less the pressure drops due to friction, as pressure drop is inversely proportional to the tube diameter.

dVlf

p 2

2ρ=∆ (5.4)

where p ∆ = Pressure drop in the downcomers

f = Friction factor ρ = Density of saturated water, kg/m3

V =Average velocity of water in the downcomers, m/s d = Internal diameter of downcomers, m

Generally, diameter d of a down comer is chosen in the range of 150-200 mm or even higher and average velocity of flow V is in the range of 0.4-1.4 m/s2. The mass flow rate of saturated water in the downcomers is

Vdnm 4

2f ρπ

��

���

�= (5.5)

where fm = Mass flow rate of saturated water in the downcomes, kg/s

n = Number of downcomers tube d = Internal diameter of down comers, m

ρ = Density of saturated water, kg/m3

V = Average velocity of water in the downcomers, m/s Using Eqs. 5.4-5.5 , the number of tubes in downcomers can be found out.

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Riser Tubes:

� Nucleate boiling should occur in the riser tube and film boiling is to be avoided. Too much steaming and less circulation ratio in a riser may cause a departure from nucleate boiling (DNB) and an onset of film boiling.

� Internal twisters and springs at the inner surface of the riser tube are provided which break the vapor film and retard the onset of DNB. Another method is to rib the inside of the riser tube. The ribbing creates a centrifugal action and directs the water droplets to the vapor film clinging to the surface and to wash it away.

� Adequate circulation should be maintained to prevent formation of hot spot � For the same total cross sectional area, the smaller the diameter, the

larger surface exposed to hot gas. Hence, risers are smaller in diameter and more in number.

� Generally riser tube diameter is in the range of 62.5-76.5 mm Natural circulation can be maintained upto 30 bar. Both the downcomers and riser tubes are placed inside the furnace. However, for pressure above 30 bar, downcomers are placed outside the furnace. It is used among others for heat recovery in iron, steel, copper, zinc, glass, ceramic and cement industry. Also it is used behind coke ovens, gas turbines, Diesel engines, sulfur burning plants and refuse incineration. The waste gases of wood, bagasse or black liquor are utilized. Due to the amount of gas and the space limitations, all boilers are tailor-made. The peculiarities of the different gases have to be taken into account: 1) the melting point of withdrawn dust or ash has to be considered to avoid excessive fouling of the tubes. They not only reduce the efficiency of the heating surfaces, they can also cause corrosions. 2) Before entering the convection banks the gas has to be cooled down to a temperature quite below the melting point. In several cases, empty passes have to be provided. 3) The gas velocity in convection banks depends on the gas and dust. The velocity should not be too high to avoid erosion of the tubes. On the other hand the velocity in horizontal passes should not fall below a certain limit. If the velocity is too low, the dust separates into coarse and fine particles. The coarse particle fall down in the lower part and the fine particle gather on the upper side of the tubes leading to a reduced efficiency. 4) At the back end of the boiler the saturation temperature or water inlet

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temperature in economizer has to be higher than the dew point of the gases to avoid low temperature corrosion 5) The best cleaning method has to be chosen. Typical methods are: steam or air soot blowers, shot cleaning, rapping or knocking, washing and sonic cleaning. Soot blowers cannot be used in any case. If the waste gas is used in a chemical process the air or steam changes the composition. 6) The mass flow in the tubes has to be checked to assure a sufficient cooling of the tubes in all operation conditions. Figures 8-10 present straight tube boiler. Bent tube boiler and bi-drum bent tube boiler respectively. 5.2.2.2 Straight tube boiler: This is the earliest design of boiler. Straight tubes are rolled into header at each end, since straight tubes could be made, installed and replaced easily. The tubes were 75-100 mm O.D., inclined upward at about 150 to the horizontal and staggered. Nearly saturated water leaving the drum flowed through one header, called the downcomer, into the tubes. While flowing upward in the tubes some of this water on being heated by flue gases flowing outside get transformed to vapor and the two-phase water-steam mixture went back to the drum through the other header, called the riser. The density of nearly saturated water in the downcomer is larger than the density of the two-phase mixture in the riser. This density difference causes natural circulation in the system.

Fig.5.8 Straight Tube Boiler

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5.2.2.3 The Bent Tube Boiler

Figs. 5.9 Bent Tube Boilers Straight tube boilers have many disadvantages, such as 1. Less accessibility and poorer inspection capability 2. Considerable time, labor and money is required to open or close the bolts

in the headers, to replace or remove the gaskets 3. Leakage past hand hole caps as design is inadequate and fabrication is

imperfect 4. Sluggish circulation due to low head. The disadvantages of straight tube boiler have been overcome by introducing the bent tube boilers. Tubes enter and leave the boiler drum radially in these boilers. They have the following advantages:

1. Accessibility for inspection, cleaning, maintenance 2. Ability to operate at higher steaming rates 3. Drier steam is delivered.