An improved configuration of lignite pre-drying using a supplementary steam cycle in a lignite fired supercritical power plant

Applied Energy xxx (2015) xxx–xxx

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An improved configuration of lignite pre-drying using a supplementary steam cycle in a lignite fired supercritical power plant q Cheng Xu a,b, Gang Xu a,b, Shifei Zhao a, Luyao Zhou a, Yongping Yang a,⇑, Dongke Zhang b a b

National Thermal Power Engineering and Technology Research Center, North China Electric Power University, Beijing 102206, China Centre for Energy (M473), The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia

h i g h l i g h t s  An improved lignite pre-drying using a supplementary steam cycle is proposed.  Thermodynamic and economic analyses are quantitatively conducted.  Lower exergy destruction and better matched energy level are obtained.  Higher energy efficiency improvement and greater economic benefits are achieved.

a r t i c l e

i n f o

Article history: Received 23 October 2014 Received in revised form 19 January 2015 Accepted 20 January 2015 Available online xxxx Keywords: Lignite pre-drying Super heat Supplementary steam cycle Thermodynamic analysis Economic analysis

a b s t r a c t A novel concept of improved configuration of lignite pre-drying using a supplementary steam cycle incorporated in a lignite fired supercritical power plant was proposed in this study. Differing from the conventional lignite pre-drying power plant configuration, in this lignite pre-drying power plant (LPDPP) concept, the steam bleeds for the dryer and some regenerative heaters (RHs) are redirected from the high pressure turbines and low pressure turbines through a separate turbine named the Regenerative-turbine (R-turbine). With the R-turbine in place, the degree of super-heating of the bleeds for the dryer and for RH3–RH5 is significantly reduced, thus leading to a reduction in the heat transfer temperature difference and exergy destruction rate. The net energy efficiency and the economic benefits of the proposed LPDPP are also enhanced as compared to the conventional configuration. The analysis showed that, for a 600 MW supercritical LPDPP, the exergy destruction of the dryer could be reduced from 14.23 MWth in the conventional configuration to 13.25 MWth in the proposed design. The net energy efficiency could be further improved by 0.3 percentages points with a heat rate reduction of approximately 59.4 kJ/kW h. The net economic benefit of the proposed LPDPP could reach $47.6 M per year, which is $0.9 M greater than that of the conventional lignite pre-drying unit. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction There are large deposits of lignite in countries such as Australia, German, USA and China, and long term future of lignite’s role in power generation and as raw materials for chemicals is predicated to grow [1,2]. Lignite accounts for approximately 40% of coal reserves worldwide and is primarily used to generate electricity, due to its low price and the expanding number of coal-fired power q This article is based on a short proceedings paper in Energy Procedia Volume 161 (2014). It has been substantially modified and extended, and has been subject to the normal peer review and revision process of the journal. This paper is included in the Special Issue of ICAE2014 edited by Prof. J. Yan, Prof. D.J. Lee, Prof. S.K. Chou, and Prof. U. Desideri. ⇑ Corresponding author. Tel./fax: +86 10 61772011. E-mail address: [email protected] (Y. Yang).

plants [3]. As low-rank coal, lignite generally features high water (25–60% by weight) and volatile contents but low net heating value. Thus lignite fired power plants consume more coal as compared to bituminous-coal fired power plants with the same electric power outputs, which translates to a concomitant increase in Green House Gases (GHG) emissions [4–6]. In addition, a lignite fired boiler with a relatively high exhaust flue gas temperature leads to a greater heat loss of the flue gas and further lowers the efficiency and economics of the lignite fired power plant. Therefore, improving the energy utilization efficiency in lignite fired power plants will play an important role in both reducing coal consumption rate and lessening the immediate impact of the GHG emissions on the environment. The lignite pre-drying technology, which is considered as an effective technology to increase the lignite calorific value as well

http://dx.doi.org/10.1016/j.apenergy.2015.01.083 0306-2619/Ó 2015 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Xu C et al. An improved configuration of lignite pre-drying using a supplementary steam cycle in a lignite fired supercritical power plant. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.01.083

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as the energy utilization efficiency, has recently attracted a great deal of attention amongst the R&D and industry communities [7–11]. The performance analyses and comparisons of various drying technologies, e.g., hot air drying, combustion gases drying, or superheated steam drying, have also been comprehensively executed. Atsonios et al. [7] conducted a comparison of flue gas recirculation drying method and air pre-drying technology. Kakaras et al. [8] performed a computer simulation on plant thermal efficiency with steam-heated lignite dryers and concluded that the improvement in overall plant efficiency varied between 2% and 5%, depending on the water content of the raw coal and on the degree of pre-drying. Liu et al. [9] developed a theoretical model based on the principle that pre-drying can increase the power plant efficiency and compared the thermodynamic performance of flue gas pre-drying and the steam pre-drying methods. A steam fluidized bed drying plant has operated several years at Loy Yang in Australia, supplying dried coal over 3 km to Loy Yang B power plant [10]. Another alternative followed by an American corporation in a 550 MW power plant is the utilization of low grade heat from the cooling water, to evaporate part of the lignite moisture in an air fluidized bed dryer from 38% to 29%. [11]. Recently, some non-evaporative drying technologies, such as the Mechanical and thermal dewatering process and hydrothermal dewatering have also been proposed [12]. However, these non-evaporative approaches are still not commercially available even though they feature low energy consumption. The steam drying technology, which helps reduce the risk of spontaneous combustion and effectively utilises the latent heat of steam, has been widely considered and attracts wide interest in lignite pre-drying power plants [13]. Guo et al. [14] analysed the plant efficiency improvement of a 600 MW pre-dried lignite-fired power system with and without waste heat recovery of the steam dryer. Xu et al. [15] proposed a steam lignite pre-drying system with low-grade heat integration and found that the net power efficiency can be further enhanced by the integration of lignite drying and air heating processes. Many previous studies about the lignite pre-drying technology have focused on the comparisons of different types of dryers integrated with the power plant [7–9]. The analyses of the power plant efficiency improvements through using the steam lignite pre-drying have also been conducted [14,15]. However, the energy matching between the steam bleed and the raw lignite in the dryer and performance of the steam cycle incorporated a steam-heated dryer from the viewpoint of the exergy analysis have not been comprehensively involved. It should be noted that, with the deployment of the supercritical technology, an increasing number of lignite fired power plants are using supercritical or even ultra-supercritical steam parameters. In German, the 1000 MW BoA unit located at Niederaussem power station is an example, with BoA being the German abbreviation for lignite-based power generation using optimized plant engineering. This optimized led to steam parameters of 580 and 600 °C that helped to achieve an overall efficiency in excess of 43% [10]. The 858 MW lignite-fired supercritical Belchatow unit in Poland commissioned in 2011 is another example, which is the largest unit and most efficient lignite plant in Poland [16]. In these high parameter units, the steam bleeds from the turbines will be highly super-heated and the temperature difference between the bleeds and lignite is amplified. The same condition also occurs in some regenerative heaters (RHs) downstream of the reheater, i.e., RH3–RH5. Thus, large exergy destruction occurs and presents a thermodynamic disadvantage in lignite dryers and RH3–RH5. Therefore, this paper presents an improved configuration of lignite pre-drying using a supplementary steam cycle, to achieve the following objectives: (1) to significantly minimize the degree of

super-heating of the steam bleeds for the lignite dryer and RH3– RH5 and to further increase the net energy efficiency of the lignite pre-drying power plant (LPDPP) through using the supplementary steam cycle; (2) to conduct the comprehensive thermodynamic analysis and determine the energy efficiency improvements in the proposed LPDPP; and (3) to assess economic performance of the proposed LPDPP as compared to the conventional one. 2. The lignite pre-drying system integrated with the steam cycle 2.1. The application of the steam lignite pre-drying The lignite pre-drying process, which is decoupled from the lignite firing in the furnace, is carried out at a relative low temperature level and has the potential to significantly increase the calorific value of the lignite. In addition, after the pre-drying process, the evaporated coal-inherent water of flue gas is reduced, and the corresponding acid dew point temperature decreases. As a result, the temperature of the exhaust flue gas temperature of LPDPP could be lower and the heat loss of the boiler exhaust flue gas can be reduced as well. Accordingly, the energy efficiency of the power plant firing lignite could be improved. One of the most promising lignite drying technologies is using the steam as the dryer heat source and the commonly used steam dryer is the parallel flow, indirect steam rotary-tube dryer (Fig. 1), which can reduce the chances of the spontaneous combustion as well the loss of volatiles. As shown in Fig. 1, the raw lignite is fed into the dryer and transported to the exits by the rotary motion. The steam, which is normally ranging from 0.15 to 0.50 MPa, releases heat in the dryer and then produces water condensate in the tube, and commonly returns back to the next stage RH. After being cleaned in a filter, the dryer exhaust, i.e., the waste moisture evaporated from the raw lignite mixed with air, is dumped into the atmosphere. 2.2. Reference power plant description The reference power plant here is a widely used 600 MW supercritical power plant, which is firing lignite directly. The live steam is produced at 24.2 MPa, and 566 °C. A single reheat operates at 3.63 MPa and 566.0 °C. The schematic of the thermal cycle of the reference power plant is shown in Fig. 2. The live steam generated

Raw Lignite

Steam

Vapor mixed with air

Dried Lignite

Water condensate

Fig. 1. Schematic of the steam rotary-tube dryer.

Please cite this article in press as: Xu C et al. An improved configuration of lignite pre-drying using a supplementary steam cycle in a lignite fired supercritical power plant. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.01.083

C. Xu et al. / Applied Energy xxx (2015) xxx–xxx

EG HPT

LPT

IPT FWP

ST

Boiler

RH1

To COND

RH3

RH2

COND

RH6

RH5

FWP

RH7

RH8

FWP Feedwater Pump LPT

EG CP

Electric Generator Condensate Pump

IPT

Inermediate-Pressure Turbine

Low-Pressure Turbine

COND Condenser RH ST

Regenerative Heater Secondary Turbine

Fig. 2. Schematic of the reference power plant.

in the boiler is expanded in the high-pressure turbine (HPT), and then is reheated in the boiler and successively expanded through the intermediate-pressure turbines (IPT) and the low-pressure turbines (LPT). A classical regenerative system is employed to heat the

Table 1 Thermodynamic performance of the reference 600 MW lignite fired power plant. Item Coal input rate Lower heating value of coal (LHV) Total energy of coal input (LHV) Live steam mass flow rate Reheat steam mass flow rate Gross output power Total auxiliary power Net output power Net efficiency

kg/s MJ/kg,ar MWth kg/s kg/s MWe MWe MWe %

119.5 11.51 1375.8 463.4 401.0 600.0 33.0 567.0 41.2

Table 2 Steam parameters of the regenerative system of the reference power plant. Items Pressure Temperature Saturated (bar) (°C) Temperature (°C)

Enthalpy (kJ/kg)

Mass flow rate (kg/s)

RH1 RH2 RH3 RH4 RH5 RH6 RH7 RH8

3045.2 2790.5 3427.4 3222.3 2998.5 2758.2 2645.9 2511.3

25.5 36.9 20.6 20.5 24.2 17.6 15.1 10.2

60.1 42.7 21.5 10.6 4.3 1.3 0.6 0.2

350.6 306.3 482.3 380.7 267.0 141.6 85.9 62.4

275.7 254.3 216.1 182.4 146.2 107.1 85.9 62.4

Table 3 Comparisons of the proximate and ultimate analysis data of the raw and pre-dried lignite. Items

Mar (wt%)

Raw Lignite 39.5 Dried Lignite 15.0

feed/condensate water. The steam for the RH1–RH2, RH3–RH4, and RH5–RH8 are bled from the HPT, IPT and LPT, respectively. A secondary steam turbine (ST) is equipped to satisfy the power requirement of the feed water pump (FWP). The parameters of the regenerative system and overall thermodynamic performance are listed in Tables 1 and 2, respectively. Moreover, the composition of the used lignite (on an as-received basis) is shown in Table 3. The boiler efficiency is 91.7% with the exhaust flue gas temperature at 148 °C. 2.3. Brief analysis of a conventional LPDPP

CP

Deaerator (RH4)

HPT High-Pressure Turbine

3

Aar (wt%)

Car (wt%)

Har (wt%)

Oar (wt%)

Nar (wt%)

Sar (wt%)

12.09 16.99

34.59 48.60

2.03 2.85

11.30 15.87

0.35 0.49

0.14 0.20

The schematic of a conventional LPDPP with an indirect steam rotary-tube dryer is shown in Fig. 3. The steam parameters and the used raw lignite are similar with those of the reference plant. While, in the LPDPP, the moisture content of the dried lignite fed into the boiler is reduced to 15.0% (wet basis). The temperature of the exhaust flue gas decreases to 130 °C and the boiler efficiency can reach 93.7%. The details of the components of the dried lignite are shown in Table 3. According to the parameters of the reference steam cycle, the appropriate steam required of the dryer is provided by the bleeding from RH5, which is in the downstream of the reheater. Generally speaking, the parameters of the steam leaving from the reheater in a supercritical unit normally ranges 35– 42 bar/550–600 °C, and the corresponding saturated temperature ranges from 240 to 255 °C. Thus, the degree of super-heating, i.e. the temperature difference between the exiting steam bleed and its saturated temperature, is rather large. Fig. 4 shows the detailed temperature distributions of steam bleeds for the dryer and RH3– RH5 downstream of the reheater in the conventional LPDPP. As can be seen in Fig. 4, the degree of super-heating is approximately 122 °C in the dryer and is even larger in RH3– RH5, which reaches 268 °C in RH3. Moreover, for RHs and the dryer, most of the required energy comes from the latent heat of the steam bleeds and the sensible heat released only accounts for approximately 15–20% of the total required heat. As a result, the sensible heat released by the superheated steam can be considered as an ‘‘auxiliary heat source’’, which leads to larger temperature difference and exergy destructions, especially for the dryer with great heat transfer capacity. In addition, the steam bleed with high temperature at the inlet of the dryer also endangers the operation safety of the lignite dryer. Therefore, to appropriately decrease the degree of super-heating of the steam bleeds not only can lower the temperature difference and the exergy destruction, but also enhance the operation safety of the lignite dryer. 2.4. Proposal of an improved lignite pre-drying using a supplementary steam cycle Fig. 5 shows a schematic of the proposed LPDPP incorporating the supplementary steam cycle. Compared with the conventional LPDPP, the major change is the redirection of the steam bleeds for the dryer and RH3–RH5 from the IPT and LPT through a separate tuning turbine named the Regenerative-turbine (R-turbine). This improved cycle application is termed the supplementary steam cycle. To be specific, the R-turbine is a separated turbine bleeding on the cold reheating line downstream of the HPT. Part of the steam exhaust from the HPT directly flows into the R-turbine and the steam required for the RH3 and RH4 is obtained from the bleedings of the R-turbine. After the R-turbine, the steam exhausts into the dryer and RH5, so no R-turbine condensate pipe is needed. Moreover, the R-turbine drives the feedwater pump and a separated electric generator (termed R-turbine electric generator, abbr. REG), thus the secondary turbine (ST) could be removed

Please cite this article in press as: Xu C et al. An improved configuration of lignite pre-drying using a supplementary steam cycle in a lignite fired supercritical power plant. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.01.083

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C. Xu et al. / Applied Energy xxx (2015) xxx–xxx

7

EG HPT

LPT

IPT 6

1

2

3

4

FWP

ST

5

9 8

Boiler

10

11

12

13

To COND

COND RH1

RH2

RH3

RH6

RH7

RH8

14 CP

15

HPT EG ST IPT RH FWP LPT COND CP

RH5

FWP Deaerator (RH4)

High-Pressure Turbine Electric Generator Secondary Turbine Inermediate-Pressure Turbine Regenerative Heater Feedwater Pump Low-Pressure Turbine Condenser Condensate Pump

18 16 Raw Lignite Crusher

Dryer

17

19

18 Water

Pre-dried Condensate Lignite

Fig. 3. Schematic of the conventional LPDPP.

Fig. 4. Temperature distribution curves of the steam bleeds for the dryer and RH3– RH5 in the conventional LPDPP.

adiabatic turbine efficiency. The efficiency of the R-turbine could also be designed higher than the ST, due to its larger volume and greater steam mass flow rate. Fig. 6 shows the temperature distributions of steam bleeds for the dryer and RH3–RH5 of the proposed LPDPP. The following observations can be made when the conventional LPDPP is compared with the current proposal: (1) The degree of the super-heating of the steam bleeds for the dryer, RH3–RH5 decreases dramatically and the steam bleeds for the RH4, RH5 and dryer are even slightly wet; (2) The temperature of the steam bleeds at the inlet of the dryer and RH3–RH5 significantly decreases by 122–254 °C, as compared to the conventional design; (3) The heat transfer temperature difference of the dryer and RH3–RH5 can be minimized when the temperature range of the feed/condensate water and the lignite remains unchanged; and (4) For the dryer in the proposed LPDPP, the temperature of the inlet steam bleed decreases to 146 °C, which lowers the risk of the spontaneous combustion of the lignite. 3. Performance evaluation methodology

completely and the electric power generated by the REG could be added to the gross electric output of the power plant. Clearly, in the supplementary steam cycle, the inlet steam of the R-turbine is bled from the exhaust of the high-pressure turbine, which is located upstream of the reheater. As a result, the temperature and corresponding super-heating of the steam at the inlet of the R-turbine is relative low. This characteristic also denotes that steam bleeds from the R-turbine are relatively in cold condition and that some bleeds from the R-turbine steam, i.e., the steam bleeds for the dryer and RH5, might even be slightly wet. In addition, the removal of steam bleeds from the IPT and LPT to the R-turbine also means that the ratio of the steam flowing through the reheater of the supplementary steam cycle is reduced, subsequently making it possible to create savings of reheat steam lines and reheater surface in the boiler of the newly-designed power plant. Moreover, the design of IPT of the proposed system becomes easier and all steam bleeds disappear, leading to slightly higher

3.1. Models of the system simulation The thermal cycle and energy equilibrium of these two LPDPPs configurations are based on the same reference power plant and are simulated by EBSILON Professional software, which is widely used for the design, evaluation and optimization of different types of power plants [17,18]. The software includes an important database on the basis of the energy conversion that can help to directly calculate the thermodynamic state of a power generation system and therefore can be used for simulating plant with a high degree of accuracy. The model details of the main components selected in this study are shown in Table 4. 3.2. Thermodynamic evaluation criteria The net energy efficiency and heat consumption rate are commonly used in the electric power industry to evaluate the thermal

Please cite this article in press as: Xu C et al. An improved configuration of lignite pre-drying using a supplementary steam cycle in a lignite fired supercritical power plant. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.01.083

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7

1

Boiler

EG

LPT

IPT

HPT

REG FWP

2 3 6

10

11

13

12

R-Turbine 4

5

RH2

RH3

15

RH6

RH5

FWP

COND

8

9 RH1

14

RH8

RH7

CP

Deaerator 18

(RH4)

HPT ESP EG CP IPT RH FWP LPT COND REG

High-Pressure Turbine Electrostatic Precipitator Electric Generator Condensate Pump Inermediate-Pressure Turbine Regenerative Heater Feedwater Pump Low-Pressure Turbine Condenser Regenerative-Turbine Electric Generator

16

Raw Lignite Crusher 19

Dryer

18

17

Pre-dried Water Lignite Condensate

Fig. 5. Schematic of the proposed LPDPP incorporating a supplementary steam cycle.

Table 4 Details of the main components of the model. Components

Models

Boiler

Fig. 6. Temperature distribution curves of steam bleeds for the dryer and RH3–RH5 in. the proposed LPDPP.

performance of power generation units. The calculation of the heat consumption rate is given as [19,20]:

q¼

_ net _ coal  LHV W 3600m ¼ 3600= ¼ 3600=gnet _ net _ coal  LHV m W

ð1Þ

_ net and m _ coal denote the net electric power output (kW) and where W the mass flow rate of raw lignite consumed (kg/s), respectively. gnet is the net power generation efficiency (LHV basis). To be mentioned, in the proposed LPDPP, the electric output not only includes the output of the original steam turbine but also the electric output of the R-turbine in the supplementary steam cycle. 3.3. Economic evaluation criteria 3.3.1. Fixed capital investment (FCI) estimation The fixed capital investment (FCI) of the project is calculated as the sum of the purchased equipment costs (PEC), the installation

Dry ash extraction and single reheat is modelled as a black box Lignite dryer The sub-cooled of the water condensate = 20 °C, Heat loss = 0% Steam turbines Isentropic efficiency HPH/IPT/LPT/ST/R-Turbine = 0.890/ 0.920 (0.925 in the proposed design)/0.900/0.840/0.865. Exhaust pressure = 0.0058 MPa. Mechanical efficiency = 0.998 RHs Upper terminal temperature difference, and the lower terminal temperature difference of the after-cooler, are to be specified. Pressure loss = 3.3–5% in steam extraction of different RHs, Heat loss = 0% Condenser Inlet temperature (20 °C) and pressure (0.1 MPa) of cooling medium is specified, upper temperature difference = 5 °C, pressure loss = 0.005 MPa Pumps Isentropic efficiency = 0.80, mechanical efficiency = 0.998, discharge pressure: condenser pump = 1.18 MPa, feed water pump = 32.60 MPa Electric generator Generator efficiency = 0.991

and engineering fees (IEF), and the process and project contingency (PPC) and is estimated at $490 M for the reference 600 MW power plant in China [21], which is much lower than that of the most developed countries, due to its relative low steel prices and labor costs [1]. For the LPDPP, auxiliary equipment such as coal crushers and dryers are required, which leads to an increase in the fixed capital investment, i.e., DFCILDPP ¼ FCIdryer þ FCIcrusher . In the proposed LPDPP, the secondary turbine (ST) in the reference power plant is omitted, but the R-turbine and the REG will be added, as well as the dryers and crushers, i.e., DFCI0LDPP ¼ FCIcrusher þ FCIdryer þ FCIRTurbine þ FCIREG  FCIST . In this study, the scaling up method is adopted to calculate the FCI of equipment, which is expressed as [22]:

 FCI ¼ n  FCI0 

S n  Sr

f ð2Þ

where S and f denote the scale parameter and scale factor in the present scale, respectively. Sr is the scale parameter of a single train

Please cite this article in press as: Xu C et al. An improved configuration of lignite pre-drying using a supplementary steam cycle in a lignite fired supercritical power plant. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.01.083

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of a reference component of size. n is the number of equally sized trains operating at capacity of 100%/n.

Table 5 Main thermodynamic parameters of the conventional and proposed LPDPPs. Stream ID Conventional LPDPP

3.3.2. Annual economic benefits calculation Annual economic benefits of a LPDPP are represented by the annual cash flow (Ct), which is commonly used as an economic criterion for evaluating the feasibility of a project [23,24].

C t ¼ C p  ðFCI  ðCRF  ð1 þ aÞ þ O&MÞ þ C c Þ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

ð3Þ

where Cp, and Cc denote the annual electricity sales income, coal cost, respectively. FCI represents the fixed capital investment, and O&M denotes the ratio of annual operating and management cost to FCI. a is the interest rate during construction. The capital recovery factor (CRF) means the ratio of annual average investment, which is a function of the discount rate (i) and the expected plant lifetime (n), and is shown as the following equation [18]: n

CRF ¼ i=½1  ð1 þ iÞ 

ð4Þ

Cc can be calculated according to the following:

_ coal  ccoal  LHV  N  w C c ¼ 3:6m

ð5Þ

a

Proposed LPDPP

M (kg/s) P (bar) T (°C)

M (kg/s) P (bar) T (°C)

27.6 434.0 40.0 22.3 22.2 32.6 356.8 26.3 47.1 9.2 16.1 7.2 250.9 389.4 501.5 119.5 85.1 47.1 37.7

29.4 314.9 42.7 30.7 28.7 148.6 314.9 32.7 56.5 8.7 16.7 6.5 283.1 404.1 535.7 119.5 85.1 56.5 37.7

60.12 42.66 42.66 21.49 10.64 10.64 10.64 4.30 4.30 1.22 0.60 0.22 0.06 0.06 324.00 1.00 1.00 4.09 1.00

350.6 306.3 306.3 482.3 380.7 380.7 380.7 267.0 267.0 141.1 85.9 (0.997)a 62.4(0.955) 35.5(0.907) 35.4 275.2 25.0 108.0 124.4 108.0

60.12 42.66 42.66 21.49 10.64 42.66 10.64 4.30 4.30 1.22 0.60 0.22 0.06 0.06 324.00 1.00 1.00 4.09 1.00

350.6 306.3 306.3 228.0 182.6(0.967) 306.3 380.7 146.2(0.919) 146.2(0.919) 141.1 85.9 (0.997) 62.4(0.955) 35.5(0.907) 35.4 275.2 25.0 108.0 124.4(0.928) 108.0

The value in the bracket represents the dryness degree of the steam.

_ coal and ccoal denote the coal flow rate (kg/s) and coal price where m ($/MJ) on a LHV basis, respectively. The annual electricity sales income Cp can be shown in Eq. (6):

_ net  N  w  ce Cp ¼ W

ð6Þ

N is the operation hours per year and w indicate the average capacity factor. ce denotes the grid feed-in tariff of electric power ($/kW h). 4. Results and discussion 4.1. Thermodynamic analysis 4.1.1. Energy analysis of the conventional and proposed LPDPPs In this study, a constant raw lignite input is chosen for the reference power plant and the two LPDPPs, while the mass flow rate of dried lignite fed into the boiler in the LPDPPs decreases. Thus, the flow rate of the flue gas is reduced, leading to a reduction in the power consumption of the mill and induced fan. Whereas, the rotary dryer and coal crusher, which are driven by the electromotor, will partly increases the power plant auxiliaries. In sum, the power plant auxiliaries of the LPDPPs are estimated increased by approximately 4% as compared to the reference power plant in this study according to Refs. [25,26]. The net efficiency is used to evaluate the thermodynamic performance of the LPDPP. The parameters of the pressure, temperature, mass flow of main streams in the conventional LPDPP (as shown in Fig. 3) and the proposed LPDPP (as shown in Fig. 5) are listed in Table 5. The thermal performance and the net efficiency of the two LPDPPs are presented in Table 6. The following observations are derived from Tables 5 and 6: (1) The temperature of the drying steam decreases from 267.0 °C to 146.2 °C (a slight wetness of 0.081) in the proposed LPDPP, whereas the steam pressure is unchanged. The same condition occurs in RH3–RH5 as well. Numerically, the temperature of the steam bleeds for RH3–RH5 significantly decreases by approximately 122–254 °C; (2) The mass flow rate of the bleeds for the dryer only increases by 8.4 kg/s in the proposed LPDPP. This is because that most of the required energy for dryer comes from the latent heat of the steam bleeds; (3) Part of the HPT exhaust steam directly put through the R-turbine rather than flowing into the reheater, thus the ratio of the steam flowing through the reheater is significantly reduced from 0.87 [(434.0 kg/s)/(501.5 kg/s)] in the conventional LPDPP to 0.59 [(314.9 kg/s)/(535.7 kg/s)] in the proposed

Table 6 Thermodynamic performance of the conventional and proposed LPDPPs. Item

Conventional LPDPP

Proposed LPDPP

Total energy input (raw lignite, LHV, MWth) Heat injection into the boiler (MWth) Live steam mass flow rate (kg/s) Gross electric power output (MWe) Power plant auxiliaries (MWe) Net electric power output (MWe) Net electric power output increment (MWe) Net heat rate (q, kJ/kW h) Net heat rate reduction (kJ/kW h) Net efficiency (gnet, %) Net efficiency increase (%-points)

1375.8 1461.7 501.5 621.5 34.3 587.2 20.2 8435.3 300.6 42.7 +1.5

1375.8 1461.7 535.7 625.6a 34.3 591.3 24.4 8375.9 360.0 43.0 +1.8

a The electric power output of the original steam turbine and the R-turbine is 607.5 MWe and 18.1 MWe, respectively.

LPDPP; (4) The mass flow rate of live steam (feed water) in these two LPDPPs is larger than that of the reference plant. That is due to the heat released in the furnace increases through firing the dried lignite with the higher net heating value and then is absorbed by the increased live steam mass flow. For the conventional LPDPP, the mass flow rate of the live steam increases to 501.5 kg/s, which is 38.1 kg/s greater than that of the reference power plant. For the proposed LPDPP, the flow rate of the live steam can further increase to 535.7 kg/s. This finding can be attributed to the reduced heat rate of the steam turbines through the application of the supplementary steam cycle; and (5) The net electric power outputs of these two LPDPPs are both greater than that of the reference power plant under the constant raw lignite input, although the plant auxiliaries slightly increase. The net electric power output of the conventional LPDPP is 587.2 MWe. In the proposed design with a complementary steam cycle, the net electric power output could increase to 591.3 MWe, which is 4.1 MWe greater than that of the conventional LPDPP. To sum up, in the proposed LPDPP using a supplementary steam cycle, the net power plant efficiency could further increase by 0.3 percentage points with the heat rate reduction of 59.4 kJ/kW h relative to the conventional LPDPP.

Please cite this article in press as: Xu C et al. An improved configuration of lignite pre-drying using a supplementary steam cycle in a lignite fired supercritical power plant. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.01.083

7

C. Xu et al. / Applied Energy xxx (2015) xxx–xxx

4.1.2. Key processes exergy analysis based on Energy-Utilization Diagram To investigate the distributions of the exergy destruction and to further disclose the efficiency improvement mechanism brought by the supplementary steam cycle, we assess the heating exchanger process within the dryer and RH3–RH5 through EUD (Energy-Utilization Diagram) methodology. This method may provide the internal exergy destruction mechanisms of the heat exchangers more specific information beyond that of the exergy difference between the output and the input than the commonly-used black box exergy analysis method [27,28]. In a typical energy conversion process, energy is released by the energy donor and received by the energy acceptor. The energy level A ¼ De=DH is used in EUD, where De and DH represent the exergy change and energy change during the thermodynamic process, respectively. Thus, inefficiency or exergy destruction is represented by the area between the energy donor Aed and the energy acceptor curve Aea. The detailed distributions of exergy destruction of dryer and RH3–RH5 in the conventional and proposed LPDPPs are presented in Fig. 7. The steam bleeds play the role of heat donor and the

feed/condensate water and raw lignite being the energy acceptor are heated in the RHs and dryer, respectively. Apparently, the energy level of the steam bleeds of the dryer and RHs are significantly reduced in the proposed LPDPP. To be specific, in Fig. 7(a), the steam bleeds for the dryer release heat from A = 0.45 to A = 0.25 to heat the raw lignite from A = 0 to A = 0.22, which leads to 14.23 MWth exergy destruction. In the proposed design, as shown in Fig. 7(b), the energy level of inlet steam of the dryer decreases to A = 0.29, and the steam releases large amount of latent heat as it cools from A = 0.29 to A = 0.28, thus, the energy level difference between the steam bleeds and the raw lignite is less, which in turn minimizes the exergy destruction to 13.25 MWth. Likewise, the energy level of the inlet steam bleeds for the RH3–RH5 has also been aggressively reduced, which leads to the less energy level difference between the steam bleeds and the feed/condensate water. Although the heat flow rate increases in RH3–RH5 of the proposed LPDPP due to the increased feed/condensate water mass flow, the exergy destruction of these heaters still decrease by 1.41 MWth, 0.90 MWth, and 0.47 MWth, respectively. In summary, the narrow energy level gap between the steam bleeds and raw lignite/water in dryer and RHs causes lower exergy destruction, which reveals the essential reason of the better thermodynamic performance and saved energy of the lignite power plant with the improved steam cycle. 4.2. Economic analysis The values and units for all the parameters used for calculating FCI of the related equipment are illustrated in Table 7 and main assumptions of the net cash flow calculation are listed in Table 8. Table 9 shows the results of the economic evaluation of the two LPDPPs. In LPDPPs, one coal crusher and four rotary steam dryers are required and subsequently increase the power plant FCI by $1.8 M. In the proposed LPDPP, the R-turbine and REG are added, whereas the ST is removed, thus resulting in a further $3.6 M increase in the total investment, as compared to the conventional LPDPP. Interestingly, though the FCI of the proposed LPDPP increases by $5.4 M as compared to the reference power plant, the FCI of the proposed LPDPP only increases by 1.1%, from $490.0 M to $495.4 M. This result is obtained for the following reasons: (1) The added R-turbine of the proposed LPDPP is operated under the relatively low temperature range with commonly-used steel material and the coal crusher, rotary steam dryer and REG are also quite commercially mature; and (2) The overall FCI of the 600 MWe power plant is extremely large as compared to the cost of auxiliary equipment in the proposed LPDPP, such as the coal crusher, steam rotary dryer, R-turbine and REG. Thus, the FCI increase of the LPDPP is not quite large and within an acceptable range.

Table 7 Basic capital cost data of facilities. Component

Reference FCIa (M$)

Rotary steam 0.32 dryer Coal crusher 0.50

Fig. 7. (a) EUD diagram of the dryer and RH3–RH5 in the conventional LPDPP. (b) EUD diagram of the dryer and RH3–RH5 in the proposed LPDPP.

a b

Reference Scale Scale scale factor 32.0

113.3 0.80

408.7

430.2 0.67

Secondary turbine R-Turbine

5.70

32.5

26.1 0.67

5.70

32.5

45.6 0.67

REG

9.00

300.0

18.1 0.67

Scale unit

n

Ton water evaporate/hour Ton raw lignite/ hour Gross power output MW Gross power output MW Gross electric output MW

4

b

1 1 1 1

The reference FCI is taken from Refs.[21,29]. The scale factor is taken from Refs. [23,30].

Please cite this article in press as: Xu C et al. An improved configuration of lignite pre-drying using a supplementary steam cycle in a lignite fired supercritical power plant. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.01.083

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C. Xu et al. / Applied Energy xxx (2015) xxx–xxx

proposed LPDPP still offers better economic benefits and will perform more competitive to power industry utilizing low rank lignite source.

Table 8 Basic assumptions and symbols for economic analysis. Items

Values a

Coal price (ccoal) LHV Coal flow rate (mcoal) Grid feed-in tariff (ce) Annual discount rateb (i) Plant economic lifeb (n) Interest during constructionc (a) O&Mb Annual operation hours (N) Annual capacity factor

3.05$/GJ LHV 21.13 GJ/kg 119.5 kg/s 0.061$/kW h 8% 30 yr 9.8% of FCI 4% of FCI 6900 h/yr 0.8

4.3. Comparison with the LPDPP using outer steam coolers (OSCs)

a The coal price is based on Ref. [15] according to China’ s market condition, which will increase by 20% in most OECD member countries and decease by 20–50% in some large coal producing countries, such as Australia and Unite States [31]. b Plant economic lifetime, O&M, and discount rate were obtained from Refs. [19,32]. c Taken from literature data [19].

Table 9 Economic performance of the conventional and proposed LPDPPs. Conventional LPDPP

Proposed LPDPP

Total fixed capital cost (FCI, M$) 491.8 Construction interest (a FCI, M$) 48.2 Total plant investment ((1 + a) FCI, M$) 539.8 Annual carrying charges (CRF  (1 + a) FCI, 48.0 M$) Operating and maintenance costs 19.7 (O&M  FCI, M$) 83.4 Annual coal costs (Cc, M$) Annual electricity sales income (Cp, M$) 197.7 Net economic benefits (Ct, M$) 46.7

495.4 48.5 543.6 48.3 19.8 83.4 199.1 47.6

In addition, the proposed LPDPP with greater net electric power output leads to the annual electricity sales income increase from $197.7 M to $199.1 M. Finally, the net economic benefit of the proposed LPDPP reaches $47.6 M, which is $0.9 M greater than that of the conventional one. Thus, although the investment is larger, the

Another alternative approach to reduce the super-heating of the steam bleeds is using the outer steam cooler (OSC) whereby the heat is transferred from the steam bleeds to the feedwater in the thermal cycle. The schematic of the thermal cycle using OSCs to reduce the super-heating of steam bleeds for the dryer and RH3– RH5 in the conventional LPDPP is shown in Fig. 8. In this configuration, three-stage OSCs are installed to heat the outlet feedwater of the RH1, RH3 and RH4, respectively. The terminal temperature difference of the OSC is set to 10 °C. Clearly, through the utilization of the super-heating of steam bleeds in OSCs, the temperature of the outlet feedwater of the RH1, RH3 and RH4 increases. As a result, part of the steam bleeds for the RH2 and RH3 could be saved and the final feedwater temperature could reach 279.2 °C, resulting an improved power plant efficiency. The comparisons of the thermodynamic analysis and economic performance between the proposed LPDPP and LPDPP using OSCs are shown in Table 10. The net power plant efficiency of the LPDPP using OSCs could reach 42.9% and the net economic benefit could reach $47.3 M, which are both greater than those of the conventional LPDPP. Therefore, using OSCs is an effective and economic approach to improve the power plant performance by reducing the super-heating of the steam bleeds. Although the efficiency of the LPDPP using OSCs is 0.2 percentage points higher than that of the conventional one, it is still 0.1 percentage points lower as compared to the proposed LPDPP. This is due to that the degree of super-heating in the LPDPP using OSCs could not be reduced as much as in the proposed LPDPP using an R-turbine, as a result of the restrictions of the thermal cycle configuration and the required terminal temperature difference. As for the economic performance, although the overall FCI of the proposed LPDPP is $2.0 M higher than that of the LPDPP using OSCs, the net economic benefit of the proposed LPDPP is still $0.3 M greater. This is because that the greater electric power output of

EG HPT

LPT

IPT FWP

ST A

Boiler

E

C D

B

F

To COND

COND 1st OSC

2nd OSC

AB

CD

3rd OSC

E F FWP

RH5

RH6

RH7

RH8 CP

RH1

HPT EG ST IPT RH FWP LPT COND CP OSC

RH2

RH3

High-Pressure Turbine Electric Generator Secondary Turbine Inermediate-Pressure Turbine Regenerative Heater Feedwater Pump Low-Pressure Turbine Condenser Condensate Pump Outer Steam Cooler

Deaerator (RH4) Raw Lignite Crusher

Dryer

Water

Pre-dried Condensate Lignite

Fig. 8. Schematic of the LPDPP using outer steam coolers.

Please cite this article in press as: Xu C et al. An improved configuration of lignite pre-drying using a supplementary steam cycle in a lignite fired supercritical power plant. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.01.083

C. Xu et al. / Applied Energy xxx (2015) xxx–xxx Table 10 Comparisons of thermodynamic analysis and economic performance between the proposed LPDPP and the LPDPP using OSCs. Item Thermodynamic analysis Total energy input (raw lignite, LHV, MWth) Live steam mass flow rate (kg/s) Gross electric power output (MWe) Power plant auxiliaries (MWe) Net electric power output (MWe) Net heat rate (q, kJ/kW h) Net efficiency (gnet, %) Economic performance Total fixed capital costa (FCI, M$) Annual coal costs (Cc, M$) Annual electricity sales income (Cp, M$) Net economic benefit (Ct, M$)

Proposed LPDPP

LPDPP using OSCs

1375.8

1375.8

535.7 625.6 34.3 591.3 8375.9 43.0

504.2 623.9 34.3 589.6 8400.1 42.9

495.4 83.4 199.1 47.6

493.4 83.4 198.5 47.3

9

power plant performance by reducing the super-heating of steam bleeds for the dryer and RHs as compared to the LPDPP using OSCs.

Acknowledgments

The FCI of OSCs is calculated by: FCIOSCs = FCI0  (S/Sr)f; S (28.98 MWth) is the heat capacity of OSCs in the LPDPP and Sr (19.65 MWth) is the heat capacity of the OSC at the reference scale. FCI0 ($1.25 M) is the FCI of the OSC at the reference scale [33]. f = 0.7.

This paper is supported by the National Major Fundamental Research Program of China (No. 2011CB710706), National Nature Science Fund of China (No. 51476053), the 111 Project (B12034). Partial financial support has also been received from the China Scholarship Council (Nos. 201306730017 and 201306735018) and the Australia – China Joint Coordination Group (JCG) for Clean Coal technology for Mr. Cheng Xu and Dr. Gang Xu to visit The University of Western Australia. References

a

the proposed LPDPP leads to larger annual electricity sales income, which finally influences the net economic benefits. Moreover, the added three-stage OSCs and the auxiliary connected pipelines will inevitably make the thermal system more complicated and adversely affect the power plant operation and maintenance. In sum, using an R-turbine and OSCs in LPDPPs are both effective approaches to reduce the super-heating of the steam bleeds for the dryer and RHs, while, the proposed LPDPP using an R-turbine is a better choice due to its lower systematic complexity, higher power plant efficiency and greater economic benefits. 5. Conclusions A novel concept of an improved configuration of lignite pre-drying using a supplementary steam cycle has been proposed. The quantitative analyses from the viewpoints of thermodynamics and economics were comprehensively carried out on a 600 MW supercritical lignite fired power plant. Some meaningful conclusions may be drawn as follows: (1) The thermal efficiency of lignite fired power plant can be increased by lignite pre-drying using steam bleeds from the turbines. However, the super-heating of the steam bleeds in the dryer and RH3–RH5 is rather large, especially for supercritical units. Thus the heat transfer temperature difference in these heaters is quite large, leading to great exergy destruction and thermodynamic disadvantages. (2) In the proposed LPDPP configuration, the super heat of the steam bleeds for the dryer and RH3–RH5 can be reduced aggressively and the net power plant efficiency can be enhanced by using a supplementary steam cycle. Calculations show that the net energy efficiency of the proposed LPDPP can reach 43.0% with a heat reduction of approximately 59.4 kJ/kW h as compared to the conventional one. The exergy destruction of the dryer is reduced from 14.23 MWth in the conventional LPDPP to 13.25 MWth in the proposed design. (3) The net annual economic benefit of the proposed LPDPP reaches $47.6 M, which is $0.9 M greater than that of the conventional one, though the total investment of the proposed LPDPP is slightly greater. Moreover, the proposed LPDPP using an R-turbine is a technically more feasible and economically advantageous approach to improving the

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Please cite this article in press as: Xu C et al. An improved configuration of lignite pre-drying using a supplementary steam cycle in a lignite fired supercritical power plant. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.01.083