Boiler design for ultra-supercritical coal power plants

5 Boiler design for ultra-supercritical coal power plants K. ZHANG, Y. ZHANG, and Y. GUAN, North China Electrical Power University, Beijing, China and D. ZHANG, University of Science and Technology Liaoning and The University of Western Australia, Australia

DOI: 10.1533/9780857097514.1.104 Abstract: Coal-fired power plants operating at supercritical steam temperatures and pressures have been widely available and their operational capabilities broadly demonstrated. The transition to even higher steam temperatures of above 600°C is a major new stage of development significant for boiler design. Successful transition to high steam temperature and pressure is dependent on appropriate design of air heaters, economizers, superheaters and reheaters and also on the choice of the water-cooled walls in the furnace. Types of ultra-supercritical boilers and the radiation and convection heat transfer calculations relating to coal-fired boilers are discussed. The characteristics of heat exchangers, including water-cooled walls, superheaters, reheaters, economizers and air heaters are illustrated, concentrating on difference between subcritical and (ultra-)supercritical boilers. Key words: ultra-supercritical, boiler type, heat transfer, furnace water-cooled wall, superheater, reheater, economizer, air heater.

5.1

Introduction

The increasing worldwide demand for cheap electricity and public concerns regarding environmental issues are leading to an increase in the efficiency and operating flexibility of coal-fired power plants (Bugge et al., 2006). A boiler, or steam generator as it is often called, is an essential part of any power plant. Research efforts in engineering have aimed to improve thermal efficiency for economic gain for over 250 years: the goal is to achieve higher efficiencies and is usually obtained by increasing the steam pressure and temperature. To date, supercritical coal-fired boilers have been successfully put into commercial operation with a maximum capacity of 1100 MW (Feng, 2008). 104 © Woodhead Publishing Limited, 2013

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In order to further improve thermal efficiency and to reduce carbon dioxide emissions, research programs are underway in Europe, the USA, China, Australia, and other countries to develop advanced stainless materials that are suitable for use at temperatures of up to 760°C (Feng, 2008, 2011; Liu, 2007). These studies have shown that a higher heat transfer rate and a larger surface for heat exchange are necessary for the design and operation of (ultra-)supercritical coal-fired boilers. With regard to heat transfer processes in a boiler, the feedwater is preheated, evaporated and then further superheated through the absorption of heat from the high-temperature flue gas. In the boiler, the principal heat exchange components are the water-cooled wall, superheater, reheater, economizer and air heater. The heat transfer process on the flue gas side of an ultra-supercritical boiler is almost the same as that in the subcritical unit; however, the mechanism of heat transfer in the working medium within the water-cooled wall changes substantially when the two-phase mixture of steam and water is turned into a single-phase fluid above the critical point of water at 22.12 MPa and 374°C. In addition, the heat transfer processes in the superheater, reheater, economizer and air heater in an ultra-supercritical boiler should be taken into account as their operational temperatures are higher than those in the subcritical unit. In the past 20 years, the net efficiency of coal-fired power plants can be increased by about 7%; around 2% of this improvement can be attributed to the increase in steam parameters, and around 0.7% to the increase in feedwater temperature. It is clear, then, that the heat transfer calculation is of significant importance in the design and optimization of ultra-supercritical boilers. This chapter begins with a brief review of boiler types in Section 5.2. The fundamental heat transfer calculations and the differences between supercritical and subcritical boilers are then explained in Sections 5.3 and 5.4, respectively. The second half of the chapter is dedicated to a description of the main elements of heat exchange in an ultra-supercritical pulverized coal (PC) boiler, with a focus on the water-cooled wall in Section 5.5, and discussion of the superheater, reheater, economizer and air heater in Sections 5.6 and 5.7.

5.2

Boiler types and structures

Although steam parameters such as pressure and temperature have been steadily increased over the last few decades, the function of the boiler remains the same, that is, it converts water into steam to drive the turbine for generating electricity. Modern coal-fired boilers used in power plants can be classified in various ways, according to their steam parameters, steam-water circulation, combustion method, or design configuration (Malek, 2005).

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5.2.1 Classification on the basis of steam parameters Coal-fired boilers are mainly operated at either subcritical or supercritical conditions. From a thermo-physics perspective, two phase mixtures of water and steam are replaced by a single supercritical fluid when the boiler pressure increases above the critical pressure of 22.12 MPa and the corresponding saturation temperature of 374°C (Shen and Cheng, 2004). This eliminates the need for steam-water separation in the drums during operation, and allows a simpler separator to be employed during start-up. Figure 5.1 shows that the heat rate increases with the steam conditions in a single reheat process as an example. The operating efficiency of subcritical plants is about 37% lower heating value (LHV), while that of the supercritical plants is about 40%. For ultra-supercritical units, the operating efficiency ranges from 45% to 48%, or even higher in some cases. To date, no unified standard has been drawn up for the parameters required for ultra-supercritical plants. Ultra-supercritical and advanced ultra-supercritical commercial units require higher operational steam pressures and temperatures than supercritical units. Current ultra-supercritical boilers are usually defined as those with a main steam pressure of greater than 27 MPa and a steam temperature of above 580°C; this should increase to 40 MPa and 700°C /720°C in the near future (Fan, 2010).

Relative heat rate improvement

10%

600/620°C 600/600°C

5%

580/600°C 565/565°C 540/540°C 0% 18

24

30

Main steam pressure (MPa)

5.1 Heat rate improvement depending on steam conditions.

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Boiler design for ultra-supercritical coal power plants (a)

3

(b)

3

4 1

107

2

2 5 1

1. Economizer; 2. Furnace; 3. Superheater; 4. Drum; 5. Circulating pump

5.2 Schematic diagram of different steam-water circulation systems in coal-fired boilers. (a) Drum type and (b) once-through type.

5.2.2 Classification on the basis of steam-water circulation As shown in Fig. 5.2, the steam-water circulation systems can be categorized as either ‘drum’ or ‘once-through’ types. In the former, steam-water separation occurs, and water is recycled back to the water-cooled walls in the furnace. In once-through type systems, however, the water and steam generated in the water-cooled walls passes through only once. Unlike the drum-type boilers, the once-through units do not have a fixed evaporation point: the final evaporation occurs in the water-cooled walls, with steam that is leaving the furnace wall being superheated to a certain level designated by the design and operation target. The once-through supercritical boiler was first installed in the United States in the 1950s (Richardson et al., 2004) and has been used ever since. Some of the earliest constructed units experienced various problems related to operation and reliability. The original supercritical units were designed to be operated at a constant pressure, that is, the boiler was intended to operate at full load pressure from start-up and across the entire load range. The start-up valve on a constant pressure boiler such as this experiences a greater pressure differential during bypass operation, which leads to increased erosion damage and hence to a need for more frequent valve maintenance. Severe slagging on the water-cooled walls and superheater loops was one of the major issues in traditional coal-fired once-through boilers constructed during the 1960s and 1970s, mainly due to the relatively small size of the furnaces. Subsequently, the furnace size was subject to continuous review to achieve better performance, and recently constructed units have had much larger furnaces. Appropriate furnace dimensions including the intended area, height and volume must be provided to reduce slagging potential, regardless of whether the boiler is intended to operate as a subcritical or ultra-supercritical unit.

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Unlike sliding pressure boilers, boilers designed to operate at constant pressure require a start-up bypass system, which is complex in configuration and operation. A constant pressure boiler also requires a longer start-up time, and the minimum load must be higher than that required for sliding pressure units (Chen, 2005). Once-through ultra-supercritical units are therefore usually operated as sliding pressure boilers.

5.2.3 Classification on the basis of combustion method The main types of the firing system used in coal-fired boilers are entrained flow pulverized fuel (PF) combustion and circulating fluidized-bed (CFB) combustion. In PF combustion, very fine coal particles quickly burn at temperatures of above 1300°C as they mix with air and pass through the furnace. The PF firing units are also usually called PC boilers for this reason. Different types of PC boilers are currently available for use in ultra-supercritical power plants; these are described in detail in the following section. CFB boilers, on the other hand, use relatively coarse coal particles that are 5–13 mm in size, and a relatively low temperature of around 850°C. Solid fuels have to re-circulate for the purposes of improving combustion efficiency. The advantages of this type of boiler are a uniform temperature profile and low SO2 and NOx emissions due to vigorous circulation of solids, low furnace temperature, staged combustion and the addition of limestone into the furnace. Figure 5.3 shows the process: coal and a SO2 sorbent are dropped into the lower zone of the CFB furnace. The primary air to be used in the combustion process is then passed through a grid of nozzles on the distributor that also serves as the furnace floor. The nozzles uniformly distribute the combustion air into the combustion zone. The air/gas velocity in the furnace must be high enough to entrain the coal and sorbent, as well as any residual ash. The finer particles entrained are lifted into the freeboard, the upper portion of the furnace, before being collected by a cyclone as the gas–solid separator; they then return to the lower section of the furnace through the standpipe. Coarser particles, on the other hand, cannot be completely entrained in the up-flowing gases, and consequently fall back into the lower furnace, where they mix with the finer particles that are circulated. Circulating the particles in this way ensures that the temperature for combustion remains very uniform; it also provides a long residence time for complete burning and for emission control. This in turn makes it possible for one unit to operate with a wide range of solid fuels, including low-grade fuels, which cannot be effectively fired in a PC furnace. Despite the advantages of CFB boilers listed above, the majority of coal-fired power plants still use PC boilers (Goidich et al., 2005).

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Flue gas

Air

l

Fue

Air

5.3 Schematic diagram of the CFB boiler.

5.2.4 Classification on the basis of design configuration Modern coal-fired boilers are a complex configuration of thermal-hydraulic (steam and water) sections, which preheat and evaporate water and superheat steam. Figure 5.4 shows the Π type and tower type, which are the most common configurations in modern subcritical and ultra-supercritical boilers (Xie et al., 2010). A brief summary of their features, advantages and disadvantages is given below. The majority of coal-fired boilers worldwide have the Π-type configuration, also known as a two-pass boiler. The mechanism of this type of boiler is described in Weitzel (2011), using an ultra-supercritical boiler from Babcock & Wilcox Power Generation Group, Inc. (B&W PGG) as an example, as follows: Spiral-wound furnace circuitry is normally utilized in the lower furnace of a Benson boiler to accommodate variable pressure. Smooth or ribbed tubes may

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(b)

5.4 Two types of the PC boilers at modern power plants. (a) ∏ type and (b) tower type.

be used in a spiral furnace enclosure. Fewer and slightly larger tubes are used, increasing the mass flux and routing the pipes around the furnace periphery to pass through the varying heat flux zones providing more even fluid outlet temperatures. Near the elevation of the furnace arch, transition headers and piping are used for the conversion from spiral wound tubes to vertical smooth tubes in the upper furnace and arch. When the furnace is larger so the steam flow per meter (foot) of perimeter is increased, vertical tubes with optimized rib profile may be used alternatively. The superheater, reheater and economizer heating surface arrangement are a combination of radiant and convective heat transfer surface. Pendant radiant platens are supported from the roof. At the rear of the boiler, the gas turns down into the horizontal convection pass area where the pipe surface is stringer- or end-supported at the enclosure walls. The longer length banks are stringer-supported by economizer tube legs and are usually primary superheater and horizontal economizer surfaces. Double reheat cycles would have portions of the second (low-pressure) reheater in both down passes and some final outlet pendant surface. (Weitzel, 2011)

Weitzel (2011) also discuss the advantages that the Π-type configuration offers: (1) the steel structure is shorter in this configuration than in a tower design, (2) the parallel construction sequence provides considerable time savings, (3) the support mechanism for the high-temperature pipe sections is less complex, (4) it is cheaper to construct that the tower design, and (5) the pendant surfaces require less soot-blowing than the high-temperature horizontal surfaces used in the tower design. The main disadvantages, on

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the other hand, are the relatively long high-temperature and large-diameter lines, and thus the thicker tube walls which push up the investment and maintenance costs. In the tower-type boiler, the heating surfaces of the superheater, reheater and economizer are all part of the same support system comprising of a series of vertical pipes. Through the use of this system, the boiler is able to react to dynamic processes such as load changes. The erosion rate in this type of boiler is lower, since there is no change of direction when the solid particle containing flue gas travels through the superheater, reheater, and economizer. All the horizontal heat transfer sections of the superheater, reheater and economizer are designed to be drainable (Sathyanathan, 2010). According to Weitzel (2011), the main advantages of the tower design are: (1) the gas flow is better distributed, leading to lower pipe metal upset temperatures, (2) the pipes are more widely spaced in this design, which allows high fuel ash removal to a single furnace hopper, (3) it provides a means of successfully firing low rank coals, and (4) the heating surface can be drained. One more challenging aspect of this design is that all the horizontal heat transfer surfaces must penetrate the furnace water-cooled walls in the upper section of the boiler, which demands the careful design and construction of the sealing arrangement. Boiler manufacturers make use of different furnace configurations depending on the technology that they traditionally use: for example, the UP-type, CE-type and FW-type ultra-supercritical boilers developed by American companies typically use the Π-type configuration, SIEMENS ultra-supercritical boilers are both Π-type and tower-type, while Japanese-produced boilers are mainly Π-type (Fan, 2010). Both types have been successful, and each is more suitable for some applications than others. Finally, a T type or modified tower type is provided by some manufacturers and/or developers. The downward flue pass is designed as two convection passes of the same size in a T type boiler, arranged symmetrically on both sides of the furnace. Consequently, the arrangement of the down-stream heat transfer surfaces in T type boilers is not as difficult as that in Π type boilers. T type boilers can also reduce the flue stack height and the thermal deviation of flue gas along the height. The disadvantages of this type are that the heat transfer areas are larger than with a Π type boiler, the steam pipe connection system is complex, and a large amount of metal is required for construction. Recently, the modified tower designed was put forward for use in the conceptual 800 MW advanced ultra-supercritical boiler (Weitzel, 2011), which adopts the features of the Π type. As a result, the structure is shorter than the tower design, and steam leads are shorter and closer to the steam turbine.

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5.3

Heat transfer calculations for boilers

A boiler is a closed vessel for generating steam under pressure, comprising of a feedwater system, steam system and fuel system. The feedwater system provides water to the boiler and regulates it automatically to meet the steam demand. The steam system collects and controls the steam produced in the boiler, in which steam pressure is regulated using valves and checked with steam pressure gauges. The fuel system includes all equipment used to provide the coal required to generate the necessary heat. In the furnace, high-temperature flue gas (or flame) is generated through the pulverized coal combustion, and is cooled when the heat is transferred to the working fluid. The economizer, superheater and reheater have the same functions in the subcritical and (ultra-)supercritical boilers: the economizer heats the feedwater before it enters the furnace, and the superheater and reheater heat the steam. It should be noted that the water-cooled wall in the furnace is used to convert the feedwater into saturated steam in subcritical and (ultra-)supercritical boilers, but this is not its only function in the latter, where it is also used to increase the steam temperature (Ge and Feng, 2012). As shown in Fig. 5.5, all three kinds of heat transfer (conduction, convection, and radiation) should be taken into account for heat transfer calculations. Successful experience with existing commercial ultra-supercritical boilers has established that the radiation zone decreases as the steam temperature and pressure increases. In the rear region of the boiler, typically in the economizer or air heater, only convection heat is relevant, since the concentrations of triatomic gases and solid particulates are very low (Che, 2008).

5.3.1 Heat transfer mechanisms The flue gas in the furnace is usually composed of diatomic gases (N2, O2, CO), triatomic gases (CO2, H2O, SO2) and suspended solid particulates (soot, ash and char). N2 and O2 are generally considered as a transparent medium because they absorb or emit only an insignificant amount of radiation. In

n tio

ia

ad R

Flame

Outside wall of Conduction heating surface Flue gases

Inside wall of heating surface

Convection

Working fluid (steam/water)

Radiation + Convection

5.5 Schematic diagram of the heat transfer process in boilers.

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most cases the CO concentration in the furnace is very low; consequently, it is principally the triatomic gases and solid particulates in suspension that contribute to the heat transfer by radiation. The heat transfer process in the furnace (radiant heat transfer) region of a coal-fired boiler is clearly very different to that in the convective heat transfer region further downstream. Radiation is usually the principal mode of heat transfer in the lower section of the furnace, but in the upper section of most modern furnaces heat transfer takes place by a combination of radiation and convection. Based on the theory of heat transfer, energy is always transferred from a higher temperature to a lower temperature region. As noted above, heat transfer in a boiler takes place through all three means of transfer: conduction, convection, and radiation (Rohsenow et al., 1998; Yunus and Cenge, 2003): Conduction: Conduction is the transfer of heat from one part of a body at higher temperature to another part of a body at lower temperature, or from one body at a higher temperature to another body in physical contact with it at a lower temperature (Eckert and Drake, 1972). The heat is transferred from the higher temperature regions with more energetic molecules to the lower temperature regions with less energetic molecules. For the pipe walls within the boiler, the heat is conducted by the motion of free electrons. Fourier’s law is applied to calculate the rate of conductive heat transfer, which is defined as the heat flux, i.e., the heat transfer rate per unit area normal to the direction of heat flow is proportional to the temperature gradient. Convection: Heat is transferred by a moving fluid. The mechanisms of convection heat transfer are the random motion of molecules and the macroscopic motion of the fluid, i.e. by a combination of conduction and convection. In convection heat transfer from a boundary layer (in which heat is transferred by conduction and then carried away by the moving fluid) exposed to a moving fluid stream, the rate of heat transfer is observed to be proportional to the temperature difference, which is conveniently expressed by Newton’s law of cooling (Cao, 2009). The convection heat transfer coefficient is not a property of the fluid, but an experimentally determined parameter that is largely dependent on the physical properties of the fluid, pipe arrangement, pipe wall temperature and fluid velocity. Radiation: Unlike conduction and convection, radiation does not require the presence of a material medium, and is instead carried out by electromagnetic waves. Based on the Stefan–Boltzmann law, the radiation heat exchange between two surfaces is proportional to the difference in absolute temperatures to the fourth power (Baehr and Stephan, 2006). Radiation

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heat transfer plays a dominant role in the furnace when the temperature of the flame is much higher than that of the water-cooled wall.

5.3.2 Heat transfer calculations Heat transfer and coal combustion occur simultaneously in the lower section of the furnace. All the factors relating to heat transfer and combustion are connected: for instance, the combustion process varies as the fuel is adjusted, resulting in a change in the heat transfer process due to the change in flame temperature distribution and shape. Any change in the heat transfer process will cause a variation in the temperature distribution, which will subsequently lead to a change in combustion status and coal burnout. In the combustion chamber of the furnace radiative heat transfer is predominant and the proportion of heat transfer due to convection is very small. This is mainly due to the high temperature of the flame, which may be around 1500°C, and the low wall temperature of the water-cooled pipes, which is generally below 400°C, as the gas velocity is quite low in the furnace. In a typical PC boiler, the flame temperature rises rapidly to a maximum value and then decreases continuously along the central line of the furnace: this is because the heat generated by the fuel burning is more than that transferred to the furnace enclosure in the root area of the flame, less than that transferred to the furnace enclosure as the flame ascends. The point with the maximum temperature is known as the flame center. Heat absorption in the furnace is performed by volumetric radiation depending on the shape and size of the furnace. A larger furnace volume leads to more radiant heat absorption, which also lowers the temperature of the exit gas from the furnace. Operating conditions can also affect heat transfer in the furnace. For instance, fouling on heat transfer surfaces can result in an increase in the temperature of the heat absorbing surface, with a subsequent reduction of heat transfer within the furnace. The actual flame temperature is lower than the theoretical flame temperature due to the heat exchange between the flame and the water-cooled walls. According to the principle of conservation of energy, the heat absorption from the flue gas in the furnace can be considered to be equal to the enthalpy drop from the adiabatic flame temperature to the temperature at exit (Che, 2008), that is Q

ϕ Bj (Q1

I 1″ )

[5.1]

where ϕ is the heat retention factor, Bj the fuel consumption rate, Q1 the available heat, and I the enthalpy of the flue gas at the furnace exit. Ignoring the radiant heat from the triatomic gases and ash particulates in the flue gas, the overall convective heat transfer can be calculated by the following correlation:

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Boiler design for ultra-supercritical coal power plants

λ3

α2,t2

λ2

t b2

t b3

t b4

δ3

λ1

δ2

115

α1,t1 t b1

δ1 Ash layer Tube wall T Scale layer

5.6 Schematic diagram of the heat transfer process in the double-pipe exchanger.

Qcr = K A Δt

[5.2]

where Qcr, K, A and Δt are the heat transfer quantity, the heat transfer coefficient, the heat transfer area and the temperature difference, respectively. Taking a tune-and-shell heat exchanger as an example, that is, two concentric pipes of different diameters with one fluid flowing through the smaller pipe while the other through the annular space, the heat transfer process, as shown in Fig. 5.6, can therefore be divided into the following three steps: J G outside J G inside e wall conduction i e wall J Hot flue gas convection

f G working fluid

1. The heat transfers from the hot fluid (flue gas) to the high-temperature wall surface. There is usually a layer of ash on the outer surface of the wall, meaning that the heat from the hot fluid transfers firstly to the ash layer and then to the outer surface of the tube wall. 2. The heat transfers through the tube wall from the high-temperature side to the low-temperature side. 3. The heat transfers from the low-temperature side to the cooling fluid (the working medium). There is often a layer of scale on the inner surface of the wall, which causes the heat from the cold wall to transfer firstly to the layer of scale and then to the working fluid (steam and/or water). The heat transfer from the flue gas to the outside surface of the ash layer, Qd, is represented by Newton’s Law of Cooling as below: Qd

d

d0 l (t1 − tb1 )

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[5.3]

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where d0 is the outer diameter of the ash layer, t1 the average temperature of the flue gas, tbl the temperature at the outer surface of the ash layer, αd the convective heat transfer coefficient. As mentioned above, the radiant heat transfer is proportional to the fourth power of the temperature difference between two bodies. For the sake of convenience, the radiant heat transferred from the high-temperature flue gas to the outside surface of the ash layer can be expressed by Newton’s Law of Cooling (Che, 2008) as below: Qf

f

d0 l (t1 − tb1 )

[5.4]

Combining Equations [5.3] and [5.4], the overall heat transfer is obtained as follows: Q

Qf + Qd

(

f

d

) d0 l(t l (t − t b )

d l (t − t b )

[5.5]

where αf is the radiant surface heat transfer coefficient. Thus Q

d0 l(t1 − tb1 )

1

[5.6]

Q

λ1 (tb − tb 2 )π d1l δ1

[5.7]

Q

λ2 (tb2 b − tb 3 )π d2 l δ2

[5.8]

Q

λ2 (tb3 b − tb 4 )π d3 l δ3

[5.9]

(tbb4 − t2 )π d4 l

[5.10]

Q

2

where d1, d2, d3 and d4 are, respectively, the average diameter of the ash layer, the pipe metal diameter, the average scale layer diameter and the inner diameter of the scale layer; tb2, tb3, tb4 and t2 are the outer wall pipe metal temperature, the inner wall pipe metal temperature, the inner wall temperature of the scale layer and the average working fluid temperature, respectively. Rearranging the above gives t1

t2 =

Q ⎡ 1 δ h ⎛ d0 ⎞ δ m ⎛ d0 ⎞ δ g ⎛ d0 ⎞ 1 ⎛ d0 ⎞ ⎤ + + + ⎢ + ⎥ π d0 l ⎣ α 1 λ h ⎜⎝ d1 ⎟⎠ λ m ⎜⎝ d2 ⎟⎠ λ g ⎜⎝ d3 ⎟⎠ α 2 ⎜⎝ d4 ⎟⎠ ⎦

[5.11]

The heat transfer coefficient K of the round pipe based on the surface of the ash layer is

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Boiler design for ultra-supercritical coal power plants K=

1 ⎡ 1 δ h ⎛ d0 ⎞ δ m ⎛ d0 ⎞ δ g ⎛ d0 ⎞ 1 ⎛ d0 ⎞ ⎤ + ⎢ ⎜ ⎟+ ⎜ ⎟+ ⎜ ⎟⎥ ⎜ ⎟+ ⎣ α 1 λ h ⎝ d1 ⎠ λ m ⎝ d2 ⎠ λ g ⎝ d3 ⎠ α 2 ⎝ d4 ⎠ ⎦

117

[5.12]

Convection heat transfer in the furnace is often ignored for the purposes of calculation, since radiation is the dominant form of heat transfer. The amount of heat exchange through radiation is generally taken as the overall heat transfer of the furnace for the purposes of traditional boiler design. However, for ultra-supercritical boilers, this assumption can only be made for the lower section of the furnace. The radiation heat transfer can be directly calculated by using the Stephan–Boltzmann law when the flame and the furnace enclosure are considered as two infinite parallel plates, therefore Q

xt

F1σ 0 (Thy4 Tb4 )

where αxt is the system emissivity, which is defined as

[5.13] 1 with 1 / α hy + 1 / α b − 1

αhy and αb standing for the missivities of the flame and the furnace wall respectively. F1 is the area of the furnace enclosure wall. Thy and Tb are the average temperatures of the flame and the furnace wall, respectively. The temperature difference, Δt, is the average temperature difference between the two fluids (the combustion flue gas and the water/stream) involved in the heat transfer. Taking parallel flow as an example, the logarithmic mean temperature difference, Δt, is calculated by: Δt =

Δth − Δtc Δt In h Δtc

[5.14]

where Δth is the temperature difference between the hot fluid and the cold fluid at the inlet position, and Δtc is the temperature difference between the hot fluid and the cold fluid at the outlet position. The heat transfer surfaces are generally arranged in either concurrent or counter-current flow. The counter-current flow arrangement produces the maximum temperature difference, whereas concurrent flow provides the minimum temperature difference. The temperature difference of the other flow arrangements can be calculated by the following equation: Δt = ϕ t Δtn1

[5.15]

where Δtnl is the logarithmic mean temperature difference in terms of the counter-current flow, and ϕt is the temperature correction factor which can be determined for specific arrangements.

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5.4

Comparison of supercritical and subcritical boilers

Supercritical boilers are an extension of the technology used for subcritical boilers: their design starts from the same combustion system, criterion for furnace design, arrangement of the back pass heating surface, and auxiliary equipment. One of the main improvements in supercritical boilers is the steam-water circulation system in the furnace, with a start-up system in both the drum type and the once-through type. The significant difference in heat transfer between the ultra-supercritical and subcritical boilers is mainly due to the fact that the working medium develops into a single-phase fluid from the mixture of steam and water in the furnace water-cooled wall (see Fig. 5.7). In a conventional boiler with a steam drum, feedwater is heated into a mixture of steam and water in the furnace water-cooled wall. The drum stores the saturated steam generated and acts as a phase-separator for the steam-water mixture. The steam is transported to the superheater, while the water is returned to the water-cooled wall. As shown in Fig. 5.8, the feedwater is heated to form saturated steam and then further heated to superheated steam within the water-cooled wall of an ultra-supercritical boiler. The energy absorbed by the steam is usually distributed among different functions: feedwater heating (sensible heat), boiling (latent heat), superheating, and reheating. The distribution ratios are a function of steam pressure. When there is a large amount of latent heat, as in low pressure steam, a large furnace is required for the boiler. As the steam pressure increases, the

T/(°C)

H/(kJ/kg)

Steam and water

22.6 MPa

Steam

Water Subcritical

Supercritical

P/(MPa)

5.7 Comparison of working fluid between the subcritical and supercritical boilers.

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Once-throu gh

Temperature

Pre heat

119

Continuous increase in temper ature Evaporation Superheat

Constant te mp erature

Dru m P rehea at

Evap.

Super Sup e heat

0%

100%

Quality

Enthalpy

5.8 Comparison of heat transfer processes in subcritical and supercritical boilers.

latent heat portion decreases and the superheating and reheating energy absorption increases. The boiler design varies accordingly, with large surface areas required for the superheater(s) and reheater(s) along with a small furnace with a small convective evaporator surface, or no convective evaporator surface at all. The sensible heat absorbed in the economizer is also high at high pressure. The distribution of energy among the various surfaces (the furnace, evaporator, superheater, reheater, and economizer) is somewhat flexible, but the steam pressure plays a significant role in determining the sizes of these surfaces. The change in the proportion of the working medium that absorbs heat under subcritical and supercritical parameters is given in Table 5.1. In a subcritical boiler with a stream drum, the mass flow rate in the pipe increases as the heat absorbed from the flue gas increases, resulting in self compensation. In an ultra-supercritical once-through boiler, however, the heat is used to increase the temperature of the working medium, and the mass flow rate in the pipe decreases as the heat absorbed from the flue gas increases, resulting in the ‘once through’ feature of this type of boiler. It is well-known that the thermodynamic properties of water and steam allow the steam parameters to be improved: the steam pressure and temperature can be increased, and the proportion of preheating heat, evaporation heat and superheat heat of the feedwater can be changed. These changes

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Table 5.1 Heat absorbed by the working medium under different operational parameters Steam parameters and feedwater temperature

The proportion of heat absorption (%)

Steam Steam Feedwater Preheat Evaporation Superheat Reheat pressure temperature temperature (MPa) (°C) (°C) 9.8 13.7 18.2 25.4 27.5

540 540/540 540/540 543/569 605/603

215 240 278 289 296

18.7 21.2 22.2 30.7 27.4

52.1 33.8 25 0 0

29.2 29.8 36.1 36.1 54.3

0 15.2 16.4 16.4 18.3

can have a significant effect on the arrangement of the heating surface. However, the design of the heating surface of an ultra-supercritical boiler should take into account not only the heat allocation of each heating surface when the boiler operates under the supercritical pressure range, but also its characteristics when it operates under the subcritical pressure range and below. For example, when the working pressure of the ultra-supercritical boiler is 28–30 MPa, with a load of lower than 65–70% boiler maximum continuous rating (BMCR), and the boiler is operated under subcritical pressure, the steam and water are in a particular thermodynamic state, which means that the feedwater is evaporated and then further superheated in the furnace water-cooled walls. When water is close to its critical pressure, the evaporation process requires significantly less heat, but the superheat, reheat and preheat processes require significantly more energy. In the supercritical pressure range, the evaporation and superheating of the feedwater cannot be clearly distinguished within the water-cooled wall. As a result, the superheater and reheater can be arranged in the region where the radiant and convective heat transfers occur. Since the design of the heating surface of the water-cooled wall mainly aims to stabilize coal combustion, control water-steam temperature and avoid film boiling, sufficient radiation heating surfaces must be arranged in the furnace.

5.5

Water-cooled walls

In a supercritical boiler, the water-cooled wall in the furnace is generally also set up as the evaporator. When the steam temperature and pressure are increased, the fraction of the heating surface of the evaporator decreases, meaning that part of the water-cooled wall should be configured as the superheater. The water-cooled wall is therefore a key part of the technology

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used in an (ultra-)supercritical boiler. The laws of thermodynamics dictate that the working medium becomes the supercritical fluid when the pressure is higher than the critical pressure. Under supercritical pressure conditions, the temperature of the working medium in the water-cooled wall increases as the heat absorption of the water-cooled wall increases. The heat transfer of the water-cooled wall is heavily dependent on the thermal and physical properties of the working medium. In order to design a suitable water-cooled wall for use in a supercritical once-through boiler, the heat transfer design should have the following aims: (1) to prevent the fluid deviating from nucleate boiling (departure from nucleate boiling, DNB) and the pipe being burnt dry (see Fig. 5.9); (2) to limit the difference in temperature between the pipes (see Fig. 5.10); and (3) to reduce the peak temperature of the metal and to prevent the fracture of the pipes resulting from the thermal stress. In today’s market, as shown in Fig. 5.11, three kinds of furnace watercooled walls are most commonly used in the design of ultra-supercritical once-through boilers: the multichannel vertical pipe arrangement, the spiral pipe arrangement, and the Benson vertical pipe arrangement. The operational principles of these three arrangements are explained in Fig. 5.12. The multichannel vertical pipe design for the water-cooled wall is used in the first generation of supercritical once-through boilers. It uses internal ribbing in the pipes to improve heat transfer, and can be used in larger units, in which the fluid flow in the pipes is higher due to the lower

DNB Dryout

Bubble layer

Vapour core

Liquid core

5.9 Schematic diagrams of DNB and dryout.

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Ultra-supercritical coal power plants Supercritical once-through T1

T2

T3

Small liquid inventory

T4

Fast response

Subcritical nature circulation Tsat

=

Tsat

Large g liquid q inventoryy

Tsat

Slow response p

5.10 Schematic comparison of temperature difference between the supercritical one-through and subcritical natural circulations.

Smooth tubes

Rifled tubes

Multi-pass

Spiral

BENSON vertical

5.11 Schematic diagrams of the three kinds of water-cooled walls.

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Spiral pipe

123

Benson vertical pipe

Upper furnace Mix header Transition header

Mix header

Two pass lower furnace

Lower furnace

Spiral

Downcomer Lower furnace flow Upper furnace flow

Feed water

Feed water

Feed water

5.12 Operational principle of the three kinds of water-cooled walls.

perimeter-to-furnace ratio. The support system is much simpler, because the vertical pipes are self-supporting within the wall. The heat absorption imbalance can be reduced through the use of multiple vertical pipes, which can be cooled by employing a high mass flow rate of fluid. The main advantages of the multichannel vertical pipe arrangement are: (1) the windbox openings are simpler, (2) the furnace water-cooled wall support system is also simpler, (3) the intermediate furnace wall transition header can be eliminated, (4) the construction costs are lower, (5) pipe leaks are easier to identify and repair, and (6) the pressure drop in the water-cooled wall system is lower, which reduces the required feed power pump pressure. However, a constant pressure is maintained for the supercritical evaporator, and a pressure-relief device is required when the operating load is decreased (ALSTOM, 2007). The spiral pipe design for the water-cooled wall, in which the pipe is wrapped around the unit, relies on a high quality flow rate, which can cool the pipe efficiently and reduce the imbalance of the heat absorption among the pipes, while single upwelling channels can significantly reduce both the pipe and the evaporation surface, and the superheater can operate at the maximum design pressure. In the highly loaded furnace area, a spiral-wound evaporator is usually used, with smooth pipes and high mass fluxes. Since a spiral-wound furnace of this type is not self-supporting, it is reinforced with support straps which are welded to the pipe wall with support blocks. The benefits of this type of system mainly arise from the averaging of the lateral heat absorption variation, that is, each pipe forms a part of each furnace wall. Principal advantages of the spiral system include: (1) the inlet header arrangement is considerably simpler, (2) it benefits from a large number of operating units, (3) smooth bore tubing is used over the whole furnace wall,

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and (4) there are no individual pipe orifices (ALSTOM, 2007). Spiral wall units also contain fewer pipes than a vertical wall unit of equivalent size. For operation at sliding pressure, uniform fluid conditions must be maintained under low load conditions, in order to reduce the potential for pipe damage caused by high temperature. The uniform fluid temperature profile in the spiral water-cooled wall can be achieved in the full range of the loads of the boiler, with no need for a flow adjusting device at the furnace inlet. The spiral pipe water-cooled wall is currently widely used in ultra-supercritical boilers, particularly those operating at sliding pressures (Shen and Chen, 2004). Two types of spiral water-cooled wall are available, the light pipe and the internal thread pipe. The latter can increase heat transfer and make the wall operation safer, but is 10–15% more expensive. In contrast to the high mass flux used by the multichannel vertical pipe and spiral pipe designs, the Benson vertical pipe is a kind of low mass flux design. The low mass flux vertical internal thread water-cooled wall is low quality responding, meaning that the flow can be automatically increased with the heat load. The technology takes advantage of the internal thread pipe to cool the pipes efficiently and to solve the problem of DNB. The main advantages of the Benson vertical pipe arrangement also include: (1) high cooling efficiency with optimized rifled pipes, (2) minimization of the interconnecting pipes with single up-flow pass, (3) simple and standard vertical support of pipes, (4) full variable pressure in the evaporator and superheater, and (5) small power consumption of the auxiliary by low loss of evaporator pressure. To summarize, the main features of the three kinds of water-cooled walls are compared in Table 5.2. Table 5.2 The comparison of the three kinds of water-cooled walls

Reliability

Compensation ability

Multichannel vertical pipe

Spiral pipe

Benson vertical pipe

Big pressure drop of water system, high auxiliary power

Big pressure drop of water system, high auxiliary power

Small pressure drop of water system, low auxiliary power

No

No

Yes

Operation performance

Suitable for basic load, poor adaptability of changing operation

Suitable for all kinds of load

Suitable for all kinds of load

Cost effectiveness

Complicated structure, low cost, and high operating cost

High cost, high operating cost

Simple structure, low cost, low operating cost

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Superheaters and reheaters

Superheaters and reheaters are specially designed to increase the temperature of the saturated steam and to help control the steam outlet temperature. They are simple single-phase heat exchangers with steam flowing inside and the flue gas passing outside, generally in the cross flow. Superheaters and reheaters are often divided into multiple sections to help control steam temperature and optimize heat recovery, and their heating surfaces can be arranged either horizontally or vertically. The physical design and location of the surfaces is dependent on the desired outlet temperature, heat absorption, fuel ash characteristics and cleaning equipment. The main components are usually manufactured from steel alloys, as both the superheater and reheaters are operated at high temperature. As shown in Table 5.3, the net electrical efficiency is heavily dependent on the outlet temperature of both the superheater and reheater. One of the main differences between the superheater and reheater is the steam pressure. The outlet pressure of the superheater in a subcritical drum boiler, for example, is for example at 18.6 MPa, while the outlet pressure of the reheater is only 4.0 MPa. When a supercritical boiler is operated under normal load, the heating surfaces of the reheater and superheater have to increase in order to meet the heat requirement for superheating, which is more than 50% of the total steam heat. The heat absorbed by the superheater is principally in the form of radiation when the temperature of the superheated steam is controlled by the ratio of furnace water flow to fuel input. When the water-cooled wall works under supercritical pressure, however, the temperature of the working medium in the pipes does not increase with heat absorption and there is no clear thermodynamic state boundary. If the temperature of the outlet flue gas is controlled by the absorption of heat by the water-cooled wall, design requirements clearly cannot be met.

Table 5.3 Temperature for main steam/reheated steam vs net electricity efficiency (Cooling water temperature is 25°C) Mode of cycle

Steam parameters

Net efficiency (%)

Subcritical, single reheat Supercritical, single reheat Supercritical, double reheat Ultra-supercritical, double reheat Ultra-supercritical, double reheat

14 MPa/540/540°C

37

24 MPa/538/566°C

40

31 MPa/566/566/566°C

44

31 MPa/600/600/600°C

47

31 MPa/700/720/720°C

49

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To solve the problem of too small a heating surface, the superheater is usually placed above or close to the furnace, creating a superheater system with radiation and convection heat transfers in the supercritical boiler. The heat transfer characteristics of the radiant superheater allow an improvement in the rate of utilization of the radiant heat. At low pressure, the steam has poor cooling ability: to address this, the reheater is usually positioned after the superheater where the temperature of the flue gas is relatively low and principally relies on convection heat transfer. The temperature is gradually reduced from the outlet of the furnace to the tail of the boiler, while the volume of gas is reduced with no air leakage. In order to improve the convective transfer in the convective passes, the superheater and the reheater are placed close to the economizer in the flue gas path of relatively low temperatures.

5.7

Economizers and air heaters

Economizers and air heaters perform a key function in providing high overall thermal efficiency of the boiler by recovering the low level, that is, low temperature heat from the flue gas before it is exhausted to the atmosphere. For every 20°C that the flue gas is cooled by the economizer or air heater, the overall boiler efficiency increases by approximately 1%. The economizers recover the energy by heating the boiler feedwater while the air heaters preheat the combustion air. Coal combustion is enhanced and coal ignition becomes stable when the air is heated before being introduced into the furnace. Economizers and air heaters require a greater heat transfer surface per unit of heat recovered than water-cooled walls, superheaters and reheaters; this is mainly due to the relatively small temperature difference between the flue gas and the feedwater or the combustion air. Both the economizer and the air heater are positioned near the end of the convection pass.

5.7.1 Economizers The term economizer comes from the early use of this type of heat exchanger to reduce operating costs or economize on fuel by recovering extra heat from the flue gas. Economizers are basically tubular heat transfer surfaces used to preheat the feedwater, and are the last water-cooled heat transfer units before the air heaters. The major functions of an economizer are to reduce the temperature of the exhaust gas, raise the overall thermal efficiency of the boiler, and reduce coal consumption. The economizers used in coal-fired power plants can be classified into two types based on their material properties: the cast iron type and the steel pipe type. Steel pipe

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economizers are generally used in ultra-supercritical boilers due to the high steam temperatures and pressures involved. Economizers are usually counter-current flow heat exchangers for recovering energy from the flue gas. The pipe bundle is typically an arrangement of parallel horizontal serpentine pipes with water flowing inside and flue gas flowing outside. The pipes are as tightly spaced as possible to promote heat transfer while still allowing enough of the pipe surface to be cleaned, and limiting the pressure drop on the flue gas side. In order to improve the heat transfer and reduce fouling, the pipes are arranged either in a staggered formation or in a line, depending on the combustion properties of the coal used and the operational conditions of the boiler. In light of the above, the most common and reliable type of economizer is the bare pipe, in-line, cross-flow design. When coal is fired, the fly ash creates an environment with a high level of fouling and erosion. The bare pipe, in-line arrangement minimizes the likelihood of erosion and reduces ash trapping; it offers advantages over the staggered arrangement in this regard. When low-grade coals with high ash content are burned, necessary precautions should be taken to prevent erosion, which should be kept in mind in the design of economizers. To reduce capital costs, most boiler manufacturers have built economizers with a variety of fin types to enhance the heat transfer rate in the flue gas side: the fins are not subject to high pressure and thus inexpensive to install, which can reduce the overall size and cost of economizers. However, successful application of this method is heavily dependent on the flue gas environment, especially for surface cleaning. Moreover, the efficiency of the boiler is directly related to the temperature of the feedwater as listed in Table 5.4 (Editorial department, 2010).

Table 5.4 Effect of steam parameters and feedwater temperatures on boiler efficiency Unit type

Steam parameters

Reheat Feedwater Efficiency number temperature (%) (°C)

Subcritical Supercritical Ultra-supercritical Ultra-supercritical Ultra-supercritical Ultra-supercritical Ultra-supercritical

17 MPa/540°C/540°C 24 MPa/538°C/566°C 25 MPa/600°C/600°C 35 MPa/700°C/700°C 30 MPa/700°C/720°C/720°C 35 MPa/700°C/720°C/720°C 37.5 MPa/700°C/720°C/720°C

1 1 1 1 2 2 2

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275 275 275 275 310 320 335

35 40 45 48.5 51 52.5 53

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5.7.2 Air heaters The air heater utilizes the heat in the flue gas leaving the economizer to preheat the combustion air and enhance the combustion process, with the resulting hot air employed for drying and transporting coal into the furnace. Air heaters can be classified as either recuperative or regenerative. In a recuperative heat exchanger, heat is transferred continuously through stationary solid heat transfer surfaces which separate the hot flow stream from the cold flow stream. Heat transfer surfaces are usually arranged as pipes and parallel plates in recuperative heat exchangers. In a regenerative air heater, however, heat is transferred indirectly as a heat storage medium is alternately exposed to hot and cold flow streams. A variety of materials can be used as the medium. Periodic exposure to hot and cold flow streams can be accomplished by rotary or valve switching devices. In all modern boilers, regenerators are used principally to preheat combustion air. The advantages of regenerative air heaters over recuperative air heaters include: (1) a significant reduction in overall size and weight; (2) ease and low cost of replacing the heating surface with separate cold and hot end packs; (3) low metal weight, allowing the economic use of alloy steel or enamelled elements in the low temperature sections; (4) lower metal temperatures at the cold end; and (5) no adverse effect caused by holing of element plates due to corrosion until the plates actually disintegrate. Nevertheless, the regenerative system does have certain disadvantages, which have in some instances caused significant reductions in unit output: (1) risk of outage of moving parts; (2) leakage of air into gas and gas into air due to entrainment, and air-to-gas leakage through seals due to the air/gas pressure difference; and (3) potential reduction in flow area and increase in fluid pressure losses caused by a relatively thin layer of deposit on the elements.

5.8

Conclusion and future trends

Environmental protection concerns and the need to achieve economical plant operation mean that high efficiency levels and operating flexibility will be a matter of course in future steam power plants. Existing technologies have shown that the PC boiler remains the best option, along with plants with circulating fluidized-bed combustion systems. The Benson vertical arrangement of the water-cooled wall is a good choice for ultra-supercritical boilers operating at sliding pressures. High efficiency and low emission remain a challenge for future coal-fired power plants: successful achievement of these aims is largely dependent on the identification and introduction of new materials for use at high steam

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temperatures and pressures. The ultra-supercritical steam-water cycle will boost maximum steam temperatures from the standard 540–560°C range up to the 700–720°C range and main steam pressure from the current 25 MPa up to 35 and 37.5 MPa. Highly efficient ultra-supercritical units will play an important role in reducing CO2 emissions in the future; for coal-fired plants, however, reducing CO2 emissions always carries a significant cost implication.

5.9

Acknowledgement

This chapter has been written as collaborative work between North China Electric Power University and The University of Western Australia partially supported by the Ministry of Education of The People’s Republic of China under the ‘111’ Program (B12034) and the National Natural Science Foundation of China (51025624 and 51076043).

5.10

References

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Liu, T.L. Supercritical and ultra supercritical technique and development. Guangdong Electric Power, 2007, 20 (1):19–23 (In Chinese). Malek, M.A. Power Boiler Design, Inspection, and Repair: ASME Code Simplified. New York: McGraw-Hill, 2005. Richardson, M., Kidera, Y. and Shimogori, Y. Supercritical Boiler Technology Matures, 2004. http://www.hitachipowersystems.us/supportingdocs/forbus/ hpsa/technical_papers/CG2004.pdf. Rohsenow, W.M., Hartnett, J.R. and Young, I.C. Handbook of Heat Transfer, 3rd Edn. New York: McGraw-Hill, 1998. Sathyanathan, V.T. Difference between Tower Types and Two Pass Boilers, http:// www.brighthubengineering.com/power-plants/59662-difference-between-towe r-types-and-two-pass-boilers/, 2010. Shen, Q.N., Chen J.P. Optimizing analysis of the parameters and thermal system for ultra supercritical unit. Power Engineering, 2004, 24 (3):305–310 (In Chinese). Weitzel, P.S. Steam generator for advanced ultra-supercritical power plants 700 to 760C. ASME 2011 Power Conference, 12–14 July 2011, Denver, Colorado, U.S.A. Xie, G.H., Huang. W., Peng, M. and Zhu, G.M. Design characteristics and type selection analysis of 1000 MW ultra supercritical boiler. Human Electric Power, 2010, 30 (1):30–32 in Chinese. Yunus, A., Cenge L. Heat Transfer: A Practical Approach, 2nd Edn. New York: McGraw-Hill, 2003.

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