Performance evaluation of a novel double-reheat boiler with triple-rear passes

Applied Thermal Engineering 159 (2019) 113801

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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng

Research Paper

Performance evaluation of a novel double-reheat boiler with triple-rear passes

T

⁎

Hua Zhua,b,c, Defu Chea, , Ming Liua, Wei Heb,c, Guangzhou Yib,c, Junjie Yana a

State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China Sichuan Province Key Laboratory of Combustion and Flue Gas Cleaning, Chengdu 611731, China c Dongfang Boiler Group Co., Ltd, Zigong 643001, China b

H I GH L IG H T S

novel double reheat boiler with triple rear passes was proposed. • ADynamic with a control system for the novel boiler were developed. • Dynamic models characteristics of the boiler were obtained. • Double reheat steam temperatures were effectively controlled during load cycling. •

A R T I C LE I N FO

A B S T R A C T

Keywords: Coal-fired boiler Double reheat Dynamic simulation Flexibility Off-design conditions

Double-reheat technology effectively increases the efficiency of coal-fired power plants. The bases for a doublereheat boiler design are reliability, efficiency, and controllability. A novel double-reheat boiler with triple-rear passes was proposed in this study to address several issues, such as low reliability and control precision of double-reheat steam temperatures of conventional double-reheat boilers. Dynamic models with a control system were developed with GSE software to examine the performance of the proposed boiler. Simulation results under steady-state working conditions showed that double-reheat steam temperatures can achieve design values in a wide load range. Then, the dynamic characteristics of the boiler were studied. Results further showed that a coordinated regulation of dampers in the triple-rear passes can effectively control double-reheat steam temperatures. Moreover, double-reheat steam temperatures can be controlled within an acceptable range during load cycling processes.

1. Introduction

net efficiency of 50% or higher with live steam parameters of 35 MPa/ 700 °C [6]. However, demonstration projects have been repeatedly postponed due to material problems. Therefore, a double-reheat USC technology is relevant and practical at present. A double-reheat steam cycle may increase power plant efficiency by 1.0–2.0 percentage points compared with a single-reheat steam cycle with the same level of parameters [7]. Thermoeconomic analysis and optimization of double-reheat USC units are becoming major research topics, given the rapid development of these types of machinery. Boiler is an important component of a double-reheat unit, and its performance directly affects the efficiency of an entire power plant. Several studies related to the double-reheat technology have been conducted. Xu et al. [8] proposed a novel partially underground tower-type boiler design that can considerably diminish boiler height and steam pipeline lengths. Zhou et al. [9]

Fossil fuels will play an important role in energy supply until 2030 regardless of the changing global climate [1]. Coal, which currently cover 40.8% of the global electricity production, is crucial for power generation compared with other fossil fuels [2,3]. Therefore, the development of large-capacity, high-parameter, low-pollution, and highly efficient coal-fired power plants is a feasible approach in saving energy and reducing the immediate effect of coal utilization on the environment. The decreasing resources of fossil fuels intensify the need for energy-saving measures [4]. Many researchers and communities have conducted studies to improve the efficiency of coal-fired power plants, by which increasing the temperature and pressure of live and reheat steams are two effective methods [5]. Many countries have also developed advanced ultra-supercritical (USC) technologies to achieve a

⁎

Corresponding author. E-mail address: [email protected] (D. Che).

https://doi.org/10.1016/j.applthermaleng.2019.113801 Received 30 December 2018; Received in revised form 10 April 2019; Accepted 22 May 2019 Available online 23 May 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.

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Dampers

Super- heaters 1st stage reheaters 2nd stage reheaters Economizers

Super- heaters 1st stage reheaters 2nd stage reheaters Economizers

SCR Water Wall Water Wall

SCR Dampers

Air Preheater

Gas Recirculation Fan Air Preheater

(a) Tower-type

(b) Inverted U-type boiler

Fig. 1. General arrangement of conventional double-reheat boilers.

Super- heaters 1st stage reheaters 2nd stage reheaters Economizers

demonstrated that the power generation efficiency of a double-reheat power plant can increase by 0.49% through parametric and process optimizations. Li et al. [10] presented a circulation heat calculation model for a double-reheat steam cycle. Variny et al. [11] quantitatively analyzed double-reheat thermal systems by developing a mathematical model. Feng [12] proposed a new system of double-reheat coal-fired power plant characterized by an elevated and conventional turbine layout, and the net efficiency of this new system is expected to achieve 49.8% under rated conditions on a low heating value (LHV) basis. Fan et al. [13] reviewed the research and design of advanced USC coal-fired power generation in China and found that double-reheat steam is particularly effective in enhancing unit efficiency. Liu et al. [14] studied a flue-gas waste heat recovery system for 1000 MW USC double-reheat coal-fired unit and found that the efficiency of the power plant can be increased by approximately 1.05%. Fan et al. [15] optimized a heat regenerative system for a steam double-reheat power plant. Wang et al. [16] enhanced the parameters of double-reheat coal-fired power plants through mixed integer linear program and differential evolution. Most present studies on steam double-reheat power plants have focused on their performance under design conditions. For instance, coal-fired power plants have shifted to having flexible designs in recent years [17,18] with the rapid development of renewable energy sources, such as wind and solar power [1,19]. Moreover, renewable energy sources—even if initially widely considered—will be abandoned if the power grid has insufficient peak load regulation capacity [20]; thus,

SCR

Water Wall No.1 No.2 Damper Damper

No.3 Damper

Air Preheater

Fig. 2. General arrangement of a triple-rear pass boiler.

Node Pressure Boundary

Flowrate Boundary Heat Slab

Branch Heat flux line Fig. 3. Diagram of a heater development. 2

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To SHP

To IP

To HP

Seperator

FRH 2 Water tank

SH 3

SRH 2

FRH 3

SH 2

SH 1 From HP exhaust FRH 1

SRH 1

From SHP exhaust

Furnace and water fall

From feed-water heater

To the condenser

SH1: Primary superheater

Economizer

SH2: Platen superheater

SH3:Final superheater

FRH1: Low temperature 1st stage reheater FRH2: Medium 1st stage reheater FRH3:Final 1st stage reheater SRH1: Low temperature 2nd stage reheater SRH2: Medium 2nd stage reheater SRH3: Final 2nd stage reheater SHP: Super high pressure turbine HP: High pressure turbine IP: Intermediate pressure turbine Fig. 4. Diagram of the boiler system model.

language. Starklof et al. [27] controlled the dynamics of an oxy-fuel coal-fired boiler during main fuel trip and blackout. However, many of the previous studies on power plant flexibility have concentrated on the subcritical boiler or HRSG but not on the double-reheat USC boiler. Moreover, the flexibility of coal-fired power plants that utilize double reheat has not been studied comprehensively. Coal-fired power plants with double reheat also require frequent cycles. The difficulties in handling double-reheat boilers are more apparent than those of single-reheat boilers, especially in satisfying the requirements of flexible operation because three steam temperatures (i.e., main, primary reheat, and secondary reheat temperatures) must be adjusted during the load cycling process. The current double-reheat boiler has many problems, including its unsuitability for fast loadchange operation because all three steam temperatures must be matched to the rated values. The current regulating methods for conventional double-reheat boilers can also lead to a highly complicated control system, and multiple-valued results may occur during the regulation process, thus failing to satisfy grid requirements. In overcoming the challenges of insufficient operational flexibility and operational efficiency under different work conditions, the structure of the boiler and the arrangement of heating surfaces need to be

conventional thermal power plants have been required to participate in peak shaving to balance the fluctuation of generated electricity [21]. The flexible design of coal-fired power plants indicates that these power plants can be operated in a wide load range, in which the load can be frequently changed. Numerous investigations have been conducted to improve the flexibility of power plants. Many scholars have developed simulation models to study the dynamic behavior and evaluate the flexibility of power plants. Studies on the dynamic model of a boiler, as well as that of a heater, is a basic and effective method for exploring transient performances. Lin and Jiong [22] used a fuzzy-based approach for the dynamic modeling of a subcritical coal-fired power plant. Alobaid et al. [23] proposed the conduct of numerical and experimental studies of a heat recovery steam generator (HRSG) during the start-up procedure by using Aspen Plus Dynamics, an advanced processing simulation software. Alobaid et al. [24] presented a comprehensive review paper on the dynamic simulation of thermal power plants that has become highly meritorious in simulation research. Oko et al. [25] presented a dynamic model of a subcritical coal-fired power plant that can capture key dynamic behavior over a wide operating range (70%–100% load). Casella and Pretolani [26] developed a model of a natural circulation HRSG rated at 130 MW by using the Modelica 3

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Turbine System

Boiler System

Coordinate Control System

Boiler Exhaust

Ambient Air

Air Pre-Heater Module LP Module

Economizer Module IP Module

Air Supply Control Module

Coal Supply Control Module

2nd Stage Reheater Module HP Module

1st Stage Reheater Module Temperature Control Module SHP Module Superheater Module Feed Water Control Module

Flue Gas Furnace Module

Fig. 5. Diagram of the control system model.

arrangement of a tower-type boiler in an advanced double-reheat power plant is shown in Fig. 1(a). The feedwater in the boiler is heated in binary-stage economizers, water wall, and superheaters to produce a live steam. The exhaust steam of a super high-pressure turbine is heated in the first-stage reheat system to produce the first-stage reheat steam. After the first stage, the steam is led again to the boiler for second-stage reheat system operation to produce the second reheat steam, which is drawn to the middle-pressure turbines by connecting pipes. The temperature of the live steam is controlled by regulating the water–fuel ratio and binary-stage spray desuperheaters, whereas the temperatures of the first and second reheat steams are controlled by regulating the damper opening in the flue gas duct and the height of burners. The use of the spray desuperheater is considered an emergency regulation method for the temperatures of the reheat steams. The use of tilting burners as a regulation method is constrained by the problem in which the units must all be functional to increase the heat absorption for reheat systems during partial loads, which will evidently change the combustion condition. Moreover, the reheat steam temperatures cannot sufficiently reach the rated values of less than the 75% load ratio because reheat steam temperatures are insensitive to burner tilting. The general arrangement of inverted U-type boilers in advanced

designed comprehensively. In this study, a novel double-reheat boiler with triple-rear passes was proposed to address the issues encountered by the conventional types. In contrast to the conventional tower-type or two-pass-type boilers, three flue passes are arranged at the back end of the proposed boiler to achieve operational flexibility, high part-load efficiency, and fast load-change capability. Dynamic models were developed by using the GSE software to evaluate the performance of the novel double-reheat boiler. The performance of the proposed boiler under steady working conditions and different load ratios was examined. Then, the dynamic response to the disturbance of a damper opening was analyzed, thereby providing a theoretical basis for designing a double-reheat boiler with triple-rear passes. Subsequently, the control performances of the reheat steam temperatures were simulated by using the developed dynamic models. 2. Proposed double-reheat boiler with triple-rear passes 2.1. Conventional double-reheat boiler Conventional double-reheat boilers mainly include two types (i.e., tower type and inverted U-type), as illustrated in Fig. 1. The general 4

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FMID,PF

FF,AI

MFT

LDM

RB

A A

F(x)

a

b

F1(t)

A F2(t)

A

a

b

Ffwd Fig. 6. Diagram of the feedwater load demand logics. Super- heaters 1st stage reheaters 2nd stage reheaters Economizers

S2

SCR

SHPT

HPT

IPT

LPT

Water Wall

BFPT

A

Condenser

Deaerator

Air Preheater

G

A

FWP

RH.1 RH.2

RH.3

RH.4

RH.6 RH.7 RH.8 RH.9 RH.10

RH: regenerative heater; BFPT: boiler feedwater power turbine; FWP: feedwater pump; SHPT: super high-pressure turbine; HPT: high-pressure turbine; IPT: intermediate pressure turbine; LPT: low-pressure turbine. Fig. 7. Diagram of the reference power plant.

arranged in the front and back ducts, respectively. Dampers can balance the heat absorption of the two-stage reheaters and maintain their deviations. The FGR with a gas recirculation fan (GRF) is used to maintain the total heat absorption of the two-stage reheaters. Compared with the

double-reheat power plants is shown in Fig. 1(b). Another regulation method that uses the flue gas recirculation (FGR) system and dampers for a double-reheat system is applied to avoid the limitations of burner tilting. The low-temperature first- and second-stage reheaters are 5

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Table 1 Plant data under the design condition.

Flue-gas Water/Steam

2000

Items

100% load

1800

Gross power (MWe) Flue gas flow rate (t/h) Excess air coefficient Boiler exhaust temperature (°C) Feedwater temperature (°C) Live steam flow rate (t/h) Live steam temperature (°C) Live steam pressure (bar) Flow rate of the first-stage reheat steam (t/h) Temperature of the SHPT exhaust steam (°C) Temperature of the first-stage reheat steam (°C) Flow rate of the second-stage reheat steam (t/h) Temperature of HPT exhaust steam (°C) Temperature of the second-stage reheat steam (°C)

660 233.48 1.16 123 314 1659.49 605 31.20 1524.65 427 623 1311.52 443 623

1600

Temperature/

1400

FRH2

1200

SRH2

1000

Water Wall

FRH1

800

SH1

600 400

SH2

ECO APH

SH3 FRH3 SRH1

200 0 -200

0

200

400

600

800

1000

1200

1400

1600

1800

Cumulative thermal duty in boiler/MW

burner-tilting regulation mode, the FGR system has the ability to maintain the rate temperature in a wide load range. However, the FGR system also demonstrates high station service power consumption rate. Moreover, the reliability of the GRF must be an area of focus considering its high-temperature and high-dust working conditions. As discussed previously, conventional double-reheat boilers have several disadvantages. A tower-type boiler with burner tilting cannot sufficiently reach the rated temperatures of double-stage reheat steams of less than the 75% load ratio and inflects combustion during burner tilting, which negatively affects the stable operation and economics of power plants during long-term service. The inverted U-type boiler has an additional system (i.e., FGR), which leads to additional GRFs, ducts, and electricity costs. In addition, the reliability of the FGR system has yet to be completely ascertained because the high temperature and high dust of flue gas inflect the working conditions of GRFs.

Fig. 8. Diagram of heat exchanging processes in boiler heat exchangers.

are used to control the flue gas flow rate and adjust the steam temperatures. In this manner, the steam temperature (including the main steam, primary reheat steam, and secondary reheat steam temperatures) can be separately controlled in multiple stages, a scheme that cannot be achieved in conventional two-pass boilers that use adjustment methods, such as the GRF and the burner swirl, each with its own corresponding disadvantages. During the load-change transient processes, the steam temperature in the novel three-pass boiler can be controlled more easily compared with that in the two-pass boiler. This attribute can help contribute to the improved flexible operation of the power system. 3. Development of performance evaluation models

2.2. Triple-rear pass boilers

Dynamic models are developed to evaluate the controllability and flexibility of the novel double-reheat boiler.

A new type of boiler with triple-rear passes was proposed in this study to enhance the operation economics of the double-reheat power plant boiler, as shown in Fig. 2. The design concepts are as follows: (1) The inverted U-type configuration is adopted. (2) Other reheaters, except the middle-temperature first-stage reheater, have pure convectionheating surfaces, and low-temperature first- and second-stage reheaters and superheaters are arranged in the front, middle, and back flues. (3) The live steam is controlled by regulating the water–fuel ratio and binary-stage spray desuperheaters, which are the same as those of conventional once-through supercritical boilers. The first and second reheat steam temperatures are adjusted by the connected actions of the three dampers. The electrical cost is diminished because of the absence of FGR systems or tilting burners, and the combustion condition can be kept stable. The proposed triple-rear pass boiler model is designed to enhance the economics and reliability of the unit. The novel three-pass boiler requires more complicated connected action but is more efficient than the two-pass boilers. The reasonable temperature match between hot and cold work media increases the efficiency of the unit in all working conditions. Compared with twopass boiler, heat transfer areas are more suitably arranged in the lowtemperature zones of the three-pass boiler. The energy of low flue gas can therefore be efficiently used, especially in part-load work conditions. The low-temperature superheater and the low-temperature firstand second-stage reheaters are arranged in three passes. Three damps

3.1. Fundamental equations The GSE simulation system [28], which is the platform used for model development, provides a modeling program for the full establishment of thermal plant simulation and real-time testing. Dynamic models are developed on the basis of the general laws of heat, mass, and momentum conservation. The models of mass, momentum, and energy conversion are expressed in Eqs. (1)–(3). 1) Conservation of mass

A=

∂ (αρ)f ∂τ

+

∂Ff ∂Z

=

∑

∂Sf ∂Z

+

∂Γf (1)

∂Z

where A is the cross-section area of the node (m ); α, ρ, Ff, and Sf are the fraction (%), density (kg/m−3), flow rate (kg/s−1), and source term flow rate (kg/s−1) of the fluid, respectively; Гf is the mass flow rate caused by the phase transition (kg/s−1); and Z is the space coordinate in the flow direction. 2

2) Conservation of momentum

Table 2 Compositions and LHVs of the feeding fuel. Items

Mar/%

Aar/%

Car/%

Har/%

Oar/%

Nar/%

Sar/%

LHV/kJ/kg−1

Values

8.25

24.3

54.80

3.88

7.16

0.91

0.7

21,430

6

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1800

1800

Fluegas design values Fluegas simulation values Water/steam design values Water/steam simulation values

1400

1600

1200 1000 800 600

1200 1000 800 600 400

400

200

200 0

Fluegas design values Fluegas simulation values Water/steam design values Water/steam simulation values

1400

Outlet temperatures/°C

Outlet temperatures/°C

1600

0 FUR

FUR

SH2 R1H2 SH3 R2H2 R1H3 SH1 R1H1 R2H1 ECO

SH2 R1H2 SH3 R2H2 R1H3 SH1 R1H1 R2H1 ECO

Heat exchangers

Heat exchangers

(a) 100% load

(b) 75% load

Fig. 9. Comparison between simulation and design temperatures. 1200 600

1000

3.60%

800

500

Simulation values/°C

Simulation values/°C

4.08%

-10.63%

600

400

200

0

-7.30%

400

300

200

100

0

200

400

600

800

1000

0

1200

0

100

200

Design value/°C

300

400

500

600

Design Values/°C

(b) Water/steam temperatures

(a) Flue gas temperature

Fig. 10. Deviations between simulation and design values.

Table 3 Design and simulation dates under off-design conditions. Items

100% load

Feedwater temperature (°C) Live steam temperature (°C) Temperature of the first-stage reheat steam (°C) Temperature of the second-stage reheat steam (°C)

∂Γf ∂τ

∂P →⎞ + = αf A ⎛ + f f − w + f f − f + ρg f ⎠ ⎝ ∂Z

75% load

50% load

30% load

Design

Simulation

Design

Simulation

Design

Simulation

Design

Simulation

314 605 623 623

314 605.01 625.17 625.2

303 605 623 623

303 604.87 628.73 630.1

277 605 623 623

277 605.03 622.82 615.06

247 605 593 583

247 604.84 598.17 581.56

vsrc ∑ δSf →

A=

(2)

where P is the pressure of the node (MPa); ff–w and ff–f are the fluid-towall and fluid-to-fluid friction factors, respectively; gf is the gravity of fluid (N/kg−1); σ is the source flow per unit length (m−1); and νsrc is the velocity of the source flow (m/s−1).

∂ (αρh)f ∂τ

+

∂ (Fh)f ∂Z

= A (Γhsat + Q − W )f

∑ δSf hsrc

(3)

where hf is the enthalpy of the fluid (kJ/kg−1); hsat is the saturation enthalpy of the fluid (kJ/kg−1); Qf is the heat input (kJ/s−1); Wf is the shaft work (kJ/s−1); and hsrc is the enthalpy of the source flow (kJ/ s−1). Eqs. (1)–(3) express the calculation bases of the GSE software from which the necessary physical properties of the working fluids are packaged. The modeling process is detailed by previous studies [29,30].

3) Conservation of energy

7

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Table 4 Comparison of three types of double-reheat boilers. Item

Tower-type boiler with burner tilting (Fig. 1(a))

∏-Type boiler with an FGR system (Fig. 1(b))

Triple rear pass boiler (Fig. 2)

Adjustment methods for reheat temperature Electricity cost Load range for maintaining the firststage reheat rate temperature Load range for maintaining the second reheat rate temperature Combustion stability in a load change Effects on the reliability of a unit Investment

Burner tilting + dampers

GRF + dampers

No evident increase 50%–100% BMCR

Evident increase 40%–100% BMCR

United action of the three dampers No evident increase 40%–100% BMCR

75%–100% BMCR

40%–100% BMCR

40%–100% BMCR

Unstable

Stable

Stable

No effect High (originally, the tower-type boiler has a higher investment than the ∏-type boiler because additional cost is consumed in the steel structure given its special height)

Negative effect Middle (based on the ∏-type boiler, additional investment is required from the GRF system)

No effect Low

600 SH3 FRH3 SRH2

625 620

Temperature ( C)

Temperature ( C)

630

615 610

560 520 480 ECO FUR SEP SH1 SH2 SH3

440 400

605

360 600

0

200

400

600

800 1000 1200 1400 1600 1800 2000

0

200 400 600 800 1000 1200 1400 1600 1800 2000

time(s)

time (s)

(a) Live and reheat steams

(b) Heater outlet steam

640

Temperature (°C)

620 600 580 560 540 FRH1 FRH2 FRH3 SRH1 SRH2

520 500 480 0

200 400 600 800 1000 1200 1400 1600 1800 2000

time (s)

(c) Heater outlet steam Fig. 11. Steam temperature dynamics when No. 1 damper is closed.

3.2. Boiler dynamic models

boiler is divided into a furnace, a superheater, double reheaters, an economizer, and an air pre-heater. The dynamic equations for the steam and gas sides are used for the convective heat transfer modeling of the superheater and the double reheaters. The platen superheater and hightemperature part of the second reheater is also accounted for radiative heat transfer. The dynamic models of the boiler equipment are developed and coupled, as shown in Fig. 4.

The boiler equipment includes the combustion system and the heat exchangers. Three layers of burners are arranged in the front and rear walls. Each layer includes six burners. A total of 36 burners are present in the combustion system. A furnace model is developed by using a zero-dimensional model. Along the flow direction of the flue gas, the 8

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600 SH3 FRH3 SRH2

625 620

Temperature (°C)

Temperature (°C)

630

615

560 520 480 ECO FUR SEP SH1 SH2 SH3

440

610 400 605 600

360 0

200 400 600 800 1000 1200 1400 1600 1800 2000

0

200 400 600 800 1000 1200 1400 1600 1800 2000

time (s)

time (s)

Temperature(

)

(a) Live and reheat steams

(b) Heater outlet steam

630 620 610 600 590 580 570 560 550 540 530 520 510 500

FRH1 FRH2 FRH3 SRH1 SRH2

0

200 400 600 800 1000 1200 1400 1600 1800 2000

time (s)

(c) Heater outlet steam Fig. 12. Steam temperature dynamics when No. 2 damper is closed.

3.3. Control system

The dynamic boiler model is an assembly of different heat exchangers. As shown in Fig. 3, a typical recuperative heater can be constructed with nodes, branches, and heat slabs in accordance with the its geometric structure. The inlet boundaries are used to obtain the work medium flow rates and temperatures (enthalpy), whereas the outlet boundary is used to determine the pressure only. A node is used as the control volume for the working fluids. The thermal parameters of the working media, including enthalpy, pressure, and temperature, are set to be uniform for the same node. In a specific node, mass transitions and energy delivery occur with linked nodes (or boundaries), and the quantities of the mass and energy transitions are calculated with the mass and energy conservation equations. According to the momentumgoverning equation, the flowrates of the work medium between different nodes can be obtained from the pressure difference and the flow resistance. Then, according to the structural arrangement of the boiler, different heater models can be linked in series or in parallel to establish an integrated boiler. The flue gas duct is divided into triple-rear passes in which the lowtemperature first- and second-stage reheaters and the superheater series connected with the economizers are laid separately. Total flue gas is distributed into the triple-rear passes through the connected action of the three dampers. Damper action is monitored by the control system, which will be introduced in the next section.

The control system of the double-reheat supercritical unit includes the coordinate and auxiliary control systems, a combustion, a feedwater, and a steam thermometer (see Fig. 5). Different control systems are developed in JControl, a module in GSE for handling logical relationships. In particular, JControl can help establish the relationships of basic logical calculation units through its PID, AND, OR, and NOT functions. The live steam (steam to SHP) temperature in the boiler system is controlled by spray water desuperheaters for mixing the steam with a controlled flow of spray water to achieve the rated temperature of the live steam. The first- and second-stage reheat steam temperatures are controlled by rear gas-pass biasing dampers to control the mass flow rate of the flue gas to the triple-rear passes. Power plant output is influenced by fuel feeding rate and the governor valve of a steam turbine. The target power plant output is directly controlled by the governor valve. Feedwater flow rate demand is one of the most important logics in a boiler, and the detailed logics are presented in Fig. 6. Feedwater demand value (Ffwd), which is decided upon on the basis of boiler load demand (LDM), pre-feedback value from the separator (FMID,PF), and assisted instruction (FF,AI), is used to calculate the basic LDM variation rate. The function of F(x) is to generate the basic feedwater flow rate. F 9

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648

600

640

624

Temperature ( )

Temperature( C)

632 SH3 FRH3 SRH2

616 608 600

560 520 480 ECO FUR SEP SH1 SH2 SH3

440 400

592 360

584 0

200 400 600 800 1000 1200 1400 1600 1800 2000

0

200 400 600 800 1000 1200 1400 1600 1800 2000

time (s)

time (s)

(a) Live and reheat steams

(b) Heater outlet steam

660

Temperature( C)

630 600 FRH1 FRH2 FRH3 SRH1 SRH2

570

540 510 0

200 400 600 800 1000 1200 1400 1600 1800 2000

time (s)

(c) Heater outlet steam Fig. 13. Steam temperature dynamics when No. 3 damper is closed. 50

4. Results and discussions

45

Dampers Opening/%

4.1. Reference power plant 40

The reference case analyzed in this study is a 660 MW power plant. This power plant is a USC coal-fired power plant that uses an inverted U-type boiler with triple-rear passes and double-reheat steam turbines, as presented in Fig. 7. The regenerative system includes five-stage lowpressure and four-stage high-pressure feedwater heaters and a deaerator. The main parameters of the reference power plant are listed in Table 1, while the compositions and LHVs of the feeding fuel are shown in Table 2. The temperatures of the live and first- and secondstage reheat steams are 605 °C and 623 °C. The heat exchanging processes in the boiler heat exchangers are illustrated in Fig. 8.

35 30 25 20

No.1 Damper simulation

15

No.1 Damper design

10

No.2 Damper design

No.2 Damper simulation No.3 Damper simulation

5

No.3 Damper design

0 THA

75%THA

50%THA

30%THA

4.2. Performance evaluation under steady working conditions

Working conditions

4.2.1. Model validation The dynamic model developed in this study is validated by using the design data. The comparison between simulation and design of flue gas temperature and water/steam temperature under 100% and 75% conditions are depicted in Fig. 9. The deviations between simulation versus design values are shown in Fig. 10. The simulation results are consistent with the design results when the boiler operates under different

Fig. 14. Coordinated regulation of the three dampers.

(t) is a second-order inertial element, and the time constant is decided upon on the basis of the operation states of auxiliary devices, such as blower fans or pumps. If a main fuel trip occurs, the inertial will not have any effect on the output value of Ffwd. 10

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110

90

Load(%)

600

620

580

Temperature ( C)

100

100 FRH3 SRH2 Load

80

600

70

580

60

80

50

560

560 0

90

70 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500

Load(%)

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working conditions. For the flue gas and water/steam temperatures, most value deviations are less than 10% and less than 5%, respectively. Validations of the dynamic models (including an experimental heater and a real operation heater) developed with the GSE software are detailed in previous studies [17,29]. Therefore, the models developed with GSE in this study are reliable for simulating boiler performance.

4.3. Performance evaluation during dynamic processes 4.3.1. Dynamic response of damper opening disturbance Under the working 100% load condition, three dampers were closed in a single step. The dynamic performances are shown in Figs. 11–13. The temperature dynamics under the disturbance of No. 1 damper (equipped at the first reheat steam side flue) is shown in Fig. 11. The temperature of the live steam increases from 605 °C to 616 °C, while that of the second-stage reheat steam increases from 623 °C to 631 °C. Meanwhile, the temperature of the first-stage reheat steam decreases constantly. The temperature dynamics under the disturbance of No. 2 damper (at the second reheat steam side) are presented in Fig. 12. The temperature of the live steam increases from 605 °C to 610 °C, while that of the first-stage reheat steam increases from 623 °C to 628 °C. Meanwhile, the temperature of the second-stage reheat steam decreases constantly. The temperature dynamics under the disturbance of No. 3 damper (at the live steam side) are illustrated in Fig. 13. The temperatures of the first- and second-stage reheat steams increase from 623 °C to 641 °C and 623 °C to 630 °C, respectively. The temperature of the live steam increases from 605 °C to 610 °C. The steam temperatures at the other steam heater outlets are shown in Figs. 11–13. The motions of Nos. 1 and 3 dampers can effectively regulate the second- and first-stage reheat steams, respectively. Therefore, No. 1 and 3 dampers are designed as the main regulating methods. The opening of dampers (simulation and design results) under different working

4.2.2. Performance evaluation The performances of the double-reheat boiler under off-design loads (i.e., 75%, 50%, and 30%) are simulated. The simulated first and second reheat steam temperatures are close to the design values under different loads, as shown in Table 3. The simulation results of the steady-state working conditions show that double-reheat steam temperatures can achieve the rated values in a wide load range. The comparisons among the three types of double-reheat boilers are listed in Table 4. The maintenance cost of the unit with triple-rear passes is lower than that of the inverted U-type boiler with an FGR system because of the absence of an auxiliary equipment. Combustion stability can be ensured for the triple-rear boiler because burner tilting for load adjustment is unnecessary. Moreover, the first- and second-stage reheat rate temperatures can be kept at the rated value in a wide load range for the triple-rear boiler. Therefore, the proposed triple-rear boiler is a competitive boiler contracture.

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conditions is shown in Fig. 14.

References

4.3.2. Control performance of reheat steam temperatures The control performances of the double-reheat steam temperatures during the load cycling process are shown in Figs. 15 and 16. As shown in Fig. 15(a), when the boiler load decreases from 100% to 75% at 990 s, the first- and second-stage reheat steam temperatures increase to approximately 628 °C and fluctuate at nearly 628 °C. This condition is due to the water spray control that acts when the temperature in the reheat steam is higher by 5 °C than that in the design. When the boiler heat load decreases to 75% at 990 s, the reheat steam temperatures decrease to the design temperature because the stored heat in the boiler is released during the load decreasing process. In general, the reheat steam temperatures can be controlled effectively. As shown in Fig. 15(b), when the boiler load decreases from 75% to 50% at 990 s, the first- and second-stage reheat steam temperatures can also be controlled effectively. As shown in Fig. 16, when the boiler load increases from 75% to 100% at 990 s, the first- and second-stage reheat steam temperatures decrease to approximately 613 °C. When the boiler heat load increases to 100% at 990 s, the reheat steam temperatures gradually increase to the design temperature. The reheat steam temperatures increase to the design values at approximately 2800 s because the stored heat in the boiler increases during the load decreasing process. In general, the findings on the reheat steam temperatures indicate effective control. As shown in Fig. 16(b), when the boiler load increases from 50% to 75% at 990 s, the first- and second-stage reheat steam temperatures can also be controlled effectively.

[1] BP, 2018 BP Energy Outlook, 2017. [2] F. Biro, Key world energy statistics 2017, Int. Energy Agency (2017). [3] M. Liu, X.W. Zhang, Y.G. Ma, J.J. Yan, Thermo-economic analyses on a new conceptual system of waste heat recovery integrated with an S-CO2 cycle for coal-fired power plants, Energy Convers. Manage. 161 (2018) 243–253. [4] C. Wang, M. Liu, Y. Zhao, Z. Wang, J. Yan, Thermodynamics analysis on a heat exchanger unit during the transient processes based on the second law, Energy 165 (2018) 622–633. [5] J. Bugge, S. Kjær, R. Blum, High-efficiency coal-fired power plants development and perspectives, Energy 31 (2006) 1437–1445. [6] S. Kjaer, P. Kristensen, F. Klauke, R. Vanstone, A. Zeijseink, G. Weissinger, J. Meier, R. Blum, K. Wieghardt, The advanced supercritical 700 C pulverised coal-fired power plant, VGB PowerTech (2002). [7] Y. Li, L. Zhou, G. Xu, Y. Fang, S. Zhao, Y. Yang, Thermodynamic analysis and optimization of a double reheat system in an ultra-supercritical power plant, Energy 74 (2014) 202–214. [8] G. Xu, C. Xu, Y. Yang, Y. Fang, L. Zhou, Z. Yang, Thermodynamic and economic analysis of a partially-underground tower-type boiler design for advanced double reheat power plants, Appl. Therm. Eng. 78 (2015) 565–575. [9] L. Zhou, G. Xu, S. Zhao, C. Xu, Y. Yang, Parametric analysis and process optimization of steam cycle in double reheat ultra-supercritical power plants, Appl. Therm. Eng. 99 (2016) 652–660. [10] J. Li, T. Ruan, L. Li, X. Shi, Study of circulation heat calculating model for thermal system in supercritical pressure power unit with second reheat cycles, Eng. J. Wuhan Univ. (2006) 80–84 (In Chinese). [11] M. Variny, O. Mierka, Improvement of part load efficiency of a combined cycle power plant provisioning ancillary services, Appl. Energy 86 (2009) 888–894. [12] W. Feng, China’s national demonstration project achieves around 50% net efficiency with 600 °C class materials, Fuel 223 (2018) 344–353. [13] H. Fan, Z. Zhang, J. Dong, W. Xu, China’s R&D of advanced ultra-supercritical coalfired power generation for addressing climate change, Therm. Sci. Eng. Progr. 5 (2018) 364–371. [14] J. Liu, F. Sun, W. Wei, L. Ma, Coupled high-low energy level flue gas heat recovery system and its application in 1000MW ultra-supercritical double reheat, Coal-Fired Unit (2017) V1T. [15] C. Fan, D. Pei, X. He, W. Zhou, Z. Wei, A Modified Master Cycle Off-Design Performance and Heat Rate Improvement Optimization, 2017, pp. V2T–V8T. [16] L. Wang, Y. Yang, C. Dong, T. Morosuk, G. Tsatsaronis, Parametric optimization of supercritical coal-fired power plants by MINLP and differential evolution, Energy Convers. Manage. 85 (2014) 828–838. [17] Y. Zhao, C. Wang, M. Liu, D. Chong, J. Yan, Improving operational flexibility by regulating extraction steam of high-pressure heaters on a 660 MW supercritical coal-fired power plant: a dynamic simulation, Appl. Energy 212 (2018) 1295–1309. [18] C. Wang, Y. Zhao, M. Liu, Y. Qiao, D. Chong, J. Yan, Peak shaving operational optimization of supercritical coal-fired power plants by revising control strategy for water-fuel ratio, Appl. Energy 216 (2018) 212–223. [19] J. Olauson, M.N. Ayob, M. Bergkvist, N. Carpman, V. Castellucci, A. Goude, D. Lingfors, R. Waters, J. Widén, Net load variability in Nordic countries with a highly or fully renewable power system, Nat. Energy 1 (2016) 16175. [20] Y. Zhao, M. Liu, C. Wang, X. Li, D. Chong, J. Yan, Increasing operational flexibility of supercritical coal-fired power plants by regulating thermal system configuration during transient processes, Appl. Energy 228 (2018) 2375–2386. [21] C. Wang, M. Liu, Y. Zhao, Y. Qiao, J. Yan, Entropy generation analysis on a heat exchanger with different design and operation factors during transient processes, Energy 158 (2018) 330–342. [22] J. Lin, J. Shen, Non-linear modelling of drum-boiler-turbine unit using an evolving Takagi-Sugeno fuzzy model, Int. J. Model., Identif. Contr. 12 (2011) 56–65. [23] F. Alobaid, K. Karner, J. Belz, B. Epple, H. Kim, Numerical and experimental study of a heat recovery steam generator during start-up procedure, Energy 64 (2014) 1057–1070. [24] F. Alobaid, N. Mertens, R. Starkloff, T. Lanz, C. Heinze, B. Epple, Progress in dynamic simulation of thermal power plants, Progress Energy Combust. Sci. 59 (2017) 79–162. [25] E. Oko, M. Wang, Dynamic modelling, validation and analysis of coal-fired subcritical power plant, Fuel 135 (2014) 292–300. [26] F. Casella, F. Pretolani, Fast start-up of a combined cycle power plant: a simulation study with modelica, 5th International Modelica Conference, Vienna, Austria, 2006, pp. 4–6. [27] R. Starkloff, R. Postler, W.A.K. Al-Maliki, F. Alobaid, B. Epple, Investigation into gas dynamics in an oxyfuel coal fired boiler during master fuel trip and blackout, J. Process Contr. 41 (2016) 67–75. [28] GSE Systems: high-fidelity simulators, engineering and training, 2005. [29] C. Wang, M. Liu, B. Li, Y. Liu, J. Yan, Thermodynamic analysis on the transient cycling of coal-fired power plants: simulation study of a 660 MW supercritical unit, Energy 122 (2017) 505–527. [30] C.Y. Wang, M. Liu, Y.L. Zhao, Y.Q. Qiao, D.T. Chong, J.J. Yan, Dynamic modeling and operation optimization for the cold end system of thermal power plants during transient processes, Energy 145 (2018) 734–746.

5. Conclusions A novel double-reheat boiler with triple-rear passes was proposed in this study to improve the operational flexibility and performance of double-reheat power plants. The dynamic models used in this study were developed with GSE and validated to evaluate system performance. Then, the dynamic responses of the damper opening disturbance and the load cycling processes were simulated. A preliminary design of the double-reheat boiler with triple-rear passes is introduced in this paper. The proposed model has a simpler structure, higher reliability, and lower cost than the traditional doublereheat boilers. According to the evaluations under steady-state working conditions, the double-reheat steam temperatures can achieve rate values, thereby increasing operational efficiency under off-design conditions. The dynamic response of the damper opening disturbance indicates that Nos. 1 and 3 dampers can effectively regulate the second- and firststage reheat steams, respectively. Therefore, these same dampers should be selected as the main regulating methods for controlling double-reheat steam temperatures. The dynamic simulation of the double-reheat boiler under load cycling processes indicates that double-reheat steam temperatures can be controlled within an acceptable range. Considering that the heat storage changes during load cycling, the double-reheat steam temperatures will be higher and will be lower than the design values when the load increases and decreases, respectively. Acknowledgement This work was supported the National Key Research and Development Program (2017YFB0602101) and Natural Science Foundation of China (Grant Number 51776146).

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