An innovative waste-to-energy system integrated with a coal-fired power plant

Energy 194 (2020) 116893

Contents lists available at ScienceDirect

Energy journal homepage: www.elsevier.com/locate/energy

An innovative waste-to-energy system integrated with a coal-fired power plant Heng Chen a, Meiyan Zhang a, Kai Xue a, Gang Xu a, *, Yongping Yang a, **, Zepeng Wang b, Wenyi Liu a, Tong Liu a a b

National Thermal Power Engineering and Technology Research Center, North China Electric Power University, Beijing, 102206, China Northeast Electric Power Design Institute Co., Ltd. of China Power Engineering Consulting Group, Changchun, 130021, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 September 2019 Received in revised form 15 December 2019 Accepted 31 December 2019 Available online xxx

An advanced waste-to-energy system integrated with a coal-fired power plant has been proposed to improve the energy utilization of municipal solid waste. In the new design, the energy gained from the waste-to-energy boiler is employed to heat the feedwater and partial cold reheat steam of the coal power plant, and the feedwater of the waste-to-energy boiler is provided by the heat regeneration system of the coal power plant. Consequently, the energy obtained from the waste incineration products is injected into the steam cycle of the coal power plant, and the waste-to-electricity efficiency can be significantly boosted. Based on a 500 t/day waste-to-energy plant and a 630 MW coal power plant, the proposed hybrid scheme was evaluated compared with the conventional separate one. The results show that the waste-to-electricity efficiency is promoted by 9.16% points with an additional net power output of 3.71 MW, attributed to the suggested integration. Furthermore, the energy-saving mechanism of the novel concept was revealed by energy and exergy analyses. Finally, the new design was economically examined, which indicates that the dynamic payback period of the proposed waste-to-energy plant is only 3.55 years, which is 5.87 years shorter than that of the conventional one. © 2020 Elsevier Ltd. All rights reserved.

Keywords: Waste incineration power plant Coal-fired power plant Hybrid power generation system Energy cascade utilization Waste-to-electricity efficiency

1. Introduction The rapid industrialization and urbanization are boosting the production of municipal solid waste (MSW) worldwide. In China, the annual amount of MSW has exceeded 215 million tons since 2017 [1], which is still swiftly increasing and will probably attain 323 million tons in 2020 [2]. MSW management is a momentous issue, not only in terms of affecting human health but also from the perspectives of environment, society and economics [3]. By the end of 2020, all the MSW should undergo harmless treatment in the large cities of China, and more than 95% of the MSW will be innocuously disposed of in the medium-sized cities, according to the 13th Five-Year Plan of the Chinese government [4]. Therefore, how to deal with MSW in an energy-efficient, environmentally friendly and economically affordable way has drawn particular attention for the past few years in China.

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (G. Xu), [email protected] (Y. Yang). https://doi.org/10.1016/j.energy.2019.116893 0360-5442/© 2020 Elsevier Ltd. All rights reserved.

Although plenty of laws/regulations and efforts have been implemented to enhance waste management around the world, further modification on MSW management is still indispensable. As so far, only a limited number of technologies for MSW disposal have been widely applied, and landfilling and incineration are the dominating and preferred approaches in most countries [5]. Landfills are the facilities devised for the safe settlement of MSW with different liners and finally with earth covers [6], however, they clearly have negative impacts on the environment, including the deterioration of landscape, the production of dust and leachate, and the emissions of polluting gases [7]. Hence, there is growing opposition to landfilling because of its disadvantages. Since MSW is actually a resource with huge potential in the recovery of material and energy, waste-to-energy (WtE) can bring the benefits of resource generation and minimization of waste [8]. Incineration is a direct combustion method that converts the feedstock into energy in so-called WtE plants, where the energy production is fulfilled by partially recovering the heat of the combustion products, typically by steam generators [9]. In a WtE plant, saturated steam or hot water is normally generated when there is only thermal demand, whereas, for electricity production or combined heat and power

2

H. Chen et al. / Energy 194 (2020) 116893

Nomenclature

Abbreviations AFWH additional feedwater heater APH air preheater BF bag filter BFPT boiler feedwater pump turbine CFPP coal-fired power plant CON condenser CP condensate pump DEA deaerator ECO economizer EG electric generator ESP electrostatic precipitator EP extra pump EVA evaporator FGD flue gas desulfurization Symbols A AI b C C c DDP EX LHV H h i IC k

heat transfer area (m2) annual income ($) year number in economic period carbon content cash flow ($) price ($/t or $/kWh) dynamic payback period (year) exergy (kW) lower heating value (kJ/kg) hydrogen content specific enthalpy (kJ/kg) rate investment cost ($) loan term (year)

subscripts 0 B c des dis e en ex f

environmental state boiler coal destruction discount electricity energy exergy fuel

production, superheated steam is necessary [10]. Compared with landfilling, incineration can achieve more than 90% volume reduction and needs no further decomposition, thereby the primary option of waste disposal has gradually changed from landfilling to incineration in China [11]. Waste incineration power generation is contemporarily the prime WtE approach of the country [2]. There are several other alternatives for WtE, such as anaerobic digestion, composting, refuses derived fuel and gasification, but their large-scale applications in engineering are still restricted in consideration of costs and technical maturity. In the grand plan of the 13th Five-Year Period (2016e2020), China has set up ambitious goals for waste management, one of which is that incineration is expected to account for over 50% of the national MSW disposal capacity in 2020 [12]. Nevertheless, the current WtE plants are usually characterized by low efficiency (ranging from 18% to 25% for most cases),

fs fw gro HX in FWP HPT IPT LMTD LPT MSW PAH RH RHR SAH SDS SH WtE m N N n NPV O P r s T W y

h int it l lr new nom out o&m P T tf tot w

fluid stream feedwater gross heat exchanger inlet feedwater pump high-pressure turbine intermediate-pressure turbine logarithmic mean temperature difference low-pressure turbine municipal solid waste primary air heater regenerative heater reheater secondary air heater semi-dry scrubber superheater waste-to-energy flow rate (kg/s) nitrogen content annual operating time (h) plant lifetime (year) net present value ($) oxygen content power (kW) ratio specific entropy (kJ/kgK) temperature (K) work (kW) year number in plant lifetime efficiency interest income tax loan loan repayment new scheme nominal outlet operation and maintenance pump turbine tipping fee total waste

especially compared with conventional steam power plants using fossil fuels [13]. The poor performances of WtE plants are mainly induced by the synthetical effects of technical and economic constraints, and the vital points can be identified: a) small scale; b) restrained steam parameters; c) high condensing pressure; d) simple steam cycle (no steam reheating and only few feedwater heaters); e) dramatical waste heat loss; f) overmuch auxiliary power [10]. In consequence, there is enormous room for improving the efficiencies of WtE plants, which is crucial for the sufficient utilization of MSW energy. A volume of work has been devoted to raising the WtE efficiency, mainly focusing on promoting steam parameters, reducing the energy loss of exhaust gas and integration with other thermal systems. Generally, the live steam parameters of a WtE plant are around 4.0 MPa and 400
C, which are much lower than those of fossil-fueled power plants [14]. The WtE efficiency will augment

H. Chen et al. / Energy 194 (2020) 116893

with the increases of the live steam parameters, but the heat transfer surfaces are subjected to severe high-temperature corrosion under high steam parameters, attributed to the large concentrations of chlorine, sulphur, etc. in MSW [15]. Thus, increasing the steam parameters of a WtE plant may be strongly associated with new steel/alloy/coating that is capable of functioning at extreme risk of high-temperature corrosion, which is still under developing and relatively costly [16]. Three techniques to achieve higher steam parameters in a WTE plant have been reviewed and examined by Ref. [17], involving dividing the flue gas into two fractions at the grate, reheating partial steam using the saturated steam from the boiler drum and further superheating the steam through the hot exhaust gas of a gas turbine, and these options have been adopted in several WtE plants of Europe. Xu et al. [18] proposed an approach that encapsulates aluminum alloy-based phase change materials in ceramic bricks like traditional refractory bricks in the combustion chamber, by which the superheated steam of over 600
C could be gained. Assembling radiant superheaters in the sections of a WTE boiler where the flue gas temperatures are very high can contribute to higher steam temperatures as well [19]. Via decreasing excess air [20] and using flue gas recirculation [21], the exhaust gas flow rate can be reduced in a WtE plant and the relative energy loss will decline. Another candidate to diminish the energy loss of flue gas is waste heat utilization, which recovers the energy in the exhaust gas to heat feedwater [22], provide district heat [23], generate power by an organic Rankine cycle [24] and so on. The exhaust gas could even be cooled to below the dew point temperature for recycling the latent heat [25]. Whereas, cooling the exhaust gas is probably confined considering the low-temperature corrosion [26] and the gas cleaning process [27]. A number of studies have been done regarding the incorporation of a WtE system and another thermal system. Consonni [28] and Poma [29] have explored the hybrid design containing a WtE system and a natural gas-fired combined cycle, where the saturated steam produced in a WtE boiler is exported to the heat recovery steam generator of the combined cycle for being superheated, and then fed into the steam turbine serving both the combined cycle and the WtE system. The compositive dual-fuel cycle can reach a much higher electrical efficiency than the separate production, without high-temperature corrosion limitation owing to the external superheating of the heat recovery steam generator. Carneiro et al. [30] developed a comprehensive method to praise a hybrid WtEgas turbine system, which combines four classic procedures (energy, exergy, economic and environmental analyses) to synergistically inspect the availability of such a system. The possibility of converting waste into syngas by gasification and being burned in internal combustion engines for power generation has been investigated by scholars [31, 32]. Also, the co-combustion of MSW and coal/biomass has been studied, but which is still difficult to be applied in practice [33, 34]. Ismail et al. [35] explored the hydrothermal decomposition process that converts the organic constituent of waste into solid fuel, which can be commercially utilized for co-firing with coal in coal-fired power plants. In addition, Mendecka et al. [36] introduced an integrated solar-WtE system that superheats the saturated steam of the WtE boiler in an external heat exchanger receiving heat from the solar tower. Above all, much research has been performed on the advancement of WtE, while little literature has been published concerning the integration of waste incineration power generation and coal-fired power generation based on steam cycles. If a WtE system can be organically incorporated with a coal-fired power plant, the useful energy acquired from MSW incineration may be utilized in a more efficient way and there may be huge potential in the performance improvement of WtE.

3

Coal provides about 40% of the world’s electricity, more than any other sources [37]. Coal-fired power plants (CFPP) possess the advantages of reliability, affordability, abundance, known technologies, safety and efficiency [38]. Coal power dominates the electric supply of China, where the installed capacity of CFPPs surpassed 60% of the total generation capacity and CFPPs satisfied over 64% of the national electric demand in 2017 [39]. It is predicted that coal will still contribute to beyond 50% of the country’s power supply in 2030 [40]. According to the power statistics of China (2017), the average net efficiency of CFPPs attains 39.81%, which is well above that of WtE plants [39]. On condition that a WtE system can be organically integrated with a CFPP, there may be immense potential in enhancing the overall efficiency. Against this backdrop, a novel WtE system merging with a largescale CFPP on the basis of multiple couplings has been put forward. In the proposed design, integration is chiefly accomplished by using the saturated steam of the WtE boiler to warm the feedwater of the CFPP and heating the partial cold reheat steam of the CFPP in the superheater (SH) of the WtE boiler. Besides, the feedwater of the WtE boiler is taken from the heat regeneration system of the CFPP. As consequences, more electricity will be converted from MSW and the waste-to-electricity efficiency can be significantly improved. Moreover, several prime components of the WtE plant (turbine, generator, stack, etc.) can be saved in the new configuration, which reduces the investment of the WtE system. The performance of the hybrid scheme was assessed in the views of thermodynamics and economics, based on a 500 t/day WtE plant and a 630 MW CFPP. The benefits of the option of adopting the novel concept were determined as compared to the conventional separate option, under a power-boosting mode. The root cause of energy conservation due to the innovative design was revealed via energy and exergy analyses. This work may contribute to facilitating the energy utilization of MSW and providing a science and technology foundation for advancing the WtE technology. 2. Reference plants and concept proposal 2.1. Reference WtE plant To introduce and evaluate the proposed hybrid power generation system, a typical WtE plant and a large-scale CFPP have been picked for case study. The reference WtE plant that locates in Eastern China can dispose 500 tons of MSW per day, and its flowchart is depicted in Fig. 1. The pretreated MSW is incinerated in a reciprocating grate boiler, which can handle large volumes of MSW without previous sorting or shredding and accommodate big variations in waste composition and calorific value with excellent operational stability [41]. The flue gas generated during the combustion passes through the evaporator (EVA), SH and economizer (ECO), and delivers energy to the feedwater/steam. Then the exhaust flue gas will be treated by the emission control equipment, including the semi-dry scrubber (SDS) and bag filter (BF). Furthermore, the flue gas temperature needs to be maintained above 850
C for more than 2 s, to prevent the dioxin formation [42]. The superheated steam out of the SH is exploited to drive the turbine for power production. The MSW composition and the basic data of the reference WtE plant are listed in Table 1 and Table 2, respectively. The lower heating value of the MSW is low and its contents of moisture and ash are high, which negatively affects the combustion in the furnace. For the regular stainless-steel tubes constructing the SH, the flue gas temperature is supposed to dwindle below 600
C prior to the SH [43], and the steam temperature usually cannot exceed 400
C [44], in case of high-temperature corrosion. Hence, the parameters of the superheated steam out of the SH are confined,

4

H. Chen et al. / Energy 194 (2020) 116893

SAH

Turbine

Drum

EG EVA CON

RH2

SH

PAH 3

CP

DEA (RH1)

ECO FWP

2

BF

WtE boiler

Stack

SDS 1

Steam

Feedwater

Condensate

Air

Flue gas

Fig. 1. Diagram of the reference WtE plant.

Table 1 MSW composition of the reference WtE plant (as received basis). Item

Value

Proximate analysis (wt%)

Moisture Ash Carbon Hydrogen Oxygen Nitrogen Sulphur Chlorine

Ultimate analysis (wt%)

20.59 41.75 21.97 1.91 12.78 0.50 0.20 0.30 7000

Lower heating value (kJ/kg)

Table 2 Basic parameters of the reference WtE plant. Item MSW consumption rate SH

Drum

ECO

Inlet/outlet flue gas temperature Steam flow rate Inlet/outlet steam pressure Inlet/outlet steam temperature Outlet steam flow rate Outlet steam pressure Outlet steam temperature Inlet/outlet flue gas temperature Feedwater flow rate Inlet feedwater pressure Inlet/outlet feedwater temperature

Boiler efficiency Turbine efficiency Gross power output Net power output Net waste-to-electricity efficiency

which are merely 4.10 MPa and 400.0
C (the parameters of the live steam into the turbine are 3.90 MPa and 395
C). The exhaust gas

Unit

Value

kg/s
C kg/s MPa
C kg/s MPa
C
C kg/s MPa
C % % MW MW %

5.79 599.0/431.0 13.50 4.54/4.10 258.0/400.0 14.12 4.54 258.0 389.0/190.0 14.19 5.20 130.7/258.0 80.74 27.91 9.77 8.30 20.49

temperature should not get too low for protecting the ECO from low-temperature corrosion and satisfying the temperature

H. Chen et al. / Energy 194 (2020) 116893

b) The SH of the WtE boiler is exploited to heat the partial reheat steam of the coal-fired boiler. A portion of the cold reheat steam from the HPT is sent to the SH of the WtE boiler for absorbing energy, and then the warmed reheat steam returns to the main stream at the RHR2 inlet. Hence, the reheat steam will request less heat in the coal-fired boiler, and the saved energy can be utilized to yield more live steam. c) The saturated steam exported from the WtE boiler delivers energy to the feedwater of the CFPP in the additional feedwater heater (AFWH), and the drain water of the AFWH is destined for the RH2. The feedwater out of the RH2 flows into the AFWH and acquires heat from the saturated steam. Thus, plenty of the extraction steam into the RH1 can be conserved, which will in turn raise the work output of the steam turbine. d) The feedwater of the WtE boiler comes from the DEA of the CFPP. A part of the feedwater out of the DEA of the CFPP is pumped into the ECO of the WtE boiler. Afterwards, the feedwater is divided into two streams at the ECO outlet, one heads for the drum and the other one is conveyed into the RH1. e) The combustion air of the WtE boiler is preheated by the feedwater of the CFPP. Some feedwater out of the RH5 is drawn to warm the primary air in the PAH1 and the secondary air in the SAH1. Subsequently, the primary air and the secondary air receive heat from the feedwater out of the RH2 and the feedwater out of the RH3, respectively. f) The exhaust flue gas of the WtE boiler is finally discharged through the stack of the CFPP, after the treatment of the SDS and BF, thereby the stack of the WtE plant is unwanted.

requirement of the gas cleaning process. As a consequence, the exhaust gas temperature is as high as 190.0
C, which is a squandering of energy and a major factor causing the low boiler efficiency. Two regenerative heaters (RH) are arranged in the heat regeneration system of the studied WtE plant, and their parameters are collected in Table 3. From the foregoing, because of the poor fuel quality, low live steam parameters, high exhaust gas temperature and simple heat regeneration, the cycle efficiency of the WtE plant is restricted and the net waste-to-electricity efficiency can only attain 20.49%. To facilitate the MSW burning in the boiler, the combustion air is preheated before its access to the furnace, which can raise the theoretical combustion temperature of the fuel and maintain the necessary furnace temperature [45]. Table 4 displays the parameters of the combustion air preheating system in the reference WtE plant. The primary air is heated from 15.0
C to 220.0
C via the three-stage primary air heater (PAH), absorbing energy from the saturated steam and the extraction steam. The secondary air temperature is increased to 166.0
C through the warming of the extraction steam in the secondary air heater (SAH). Note that the energy utilization during the heat exchange between steam and air is not sufficient and massive exergy may be destructed owing to the relatively large temperature gaps.

2.2. Reference CFPP The schematic diagram of the selected 630 MW subcritical CFPP is illustrated in Fig. 2. The reference power plant serves in Eastern China, which is composed of a pulverized coal-fired boiler, a reheat steam turbine (including the high-pressure turbine, HPT; intermediate-pressure turbine, IPT; and low-pressure turbine, LPT) and an electric generator (EG). The coal composition and basic data of the reference CFPP are presented in Table 5 and Table 6, respectively. A heat regeneration system with eight RHs has been adopted to warm the feedwater, and its parameters are given in Table 7. Since the parameters of the live and reheated steam are relatively high and the steam cycle is comparatively perfect, the net thermal efficiency of this CFPP reaches as high as 40.75%, which is well beyond that of the WtE plant.

2.3. Proposed concept To enhance the energy usage of MSW, a conceptual WtE design incorporated with a CFPP has been developed, as introduced in Fig. 3. In the hybrid scheme, the waste incineration power generation process is highly integrated with the coal-fired power generation process, and the overall power output can be remarkably improved. There are several connections built between the WtE system and the CFPP, which are described as follows. a) The WtE plant and the CFPP employ the same steam turbine and EG that originally belong to the CFPP, for synergistically producing electricity. As a result, a few chief components of the WtE system, such as the turbine, CON and EG, are not necessary anymore.

5

Above all, by systematically combining the WtE system and the CFPP, the useful energy of the waste combustion products is injected into the steam cycle of the CFPP, contributing to a dramatic increment in the global power output. Furthermore, a lot of the equipment in the WtE plant is no longer needed, which can save massive capital and operating costs. 3. Methodology 3.1. System simulation Software EBSILON Professional was implemented to conduct the simulations of the studied power systems. EBSILON Professional is an expert in the design, analysis and optimization of thermodynamic cycles, which takes advantage of a matrix solution method and requires the linearization of all dependencies [46]. The models of the WtE plant, coal-fired power plant and integrated power system (the modeling details of the main components as modules are given in Table 8) were built based on the design data of the reference WtE plant and coal power plant. The reference WtE plant and coal power plant are real, which have been in service in Eastern China. This WtE plant and coal power plant work stably, and their operational data are similar to their design data. Their design data has been exploited for the simulation and evaluation, which was obtained from the companies (owners of these two plants). Besides, their design data was originally

Table 3 Parameters of the heat regeneration system of the reference WtE plant. Heater

DEA RH2

Extraction steam

Drain water

Feedwater

Pressure (MPa)

Temperature (
C)

Flow rate (kg/s)

Temperature (
C)

Flow rate (kg/s)

Inlet/outlet temperature (
C)

Outlet flow rate (kg/s)

0.27 0.08

195.4 92.8

0.97 1.04

e 92.8

e 1.04

88.0/130.0 38.5/88.0

14.19 11.04

6

H. Chen et al. / Energy 194 (2020) 116893

Table 4 Parameters of the combustion air preheating system in the reference WtE plant. Item

PAH1

PAH2

PAH3

SAH

Substance of inlet/outlet hot fluid

Condensate/condensate

Steam/condensate

Steam/condensate

Steam/condensate

Hot fluid flow rate (kg/s) Inlet hot fluid pressure (MPa) Inlet/outlet hot fluid temperature (
C) Air flow rate (kg/s) Inlet/outlet air temperature (
C) LMTD (
C) Heat duty (MW)

0.62 4.54 225.3/104.3 20.51 15.0/31.0 135.1 0.33

1.08 1.31 287.3/98.3 20.51 31.0/166.0 80.6 2.80

0.62 4.54 258.0/225.3 20.51 166.0/220.0 60.3 1.13

0.49 1.31 287.3/98.5 8.38 15.0/166.0 86.6 1.28

IPT

HPT

LPT EG

SH2

SH3

RHR2

BFPT

CON

SH1 RHR1 RH1

RH2

DEA (RH4)

RH3

RH5

RH6

RH7

RH8

CP

ECO

FWP

Stack

FGD

ESP

APH

Coal-fired boiler

Steam

Feedwater

Condensate

Air

Flue gas

Fig. 2. Diagram of the reference CFPP.

Table 5 Coal composition of the reference CFPP (as received basis). Item Proximate analysis (wt%) Ultimate analysis (wt%)

Lower heating value (kJ/kg)

provided by their constructors, which was also validated by the responsible design institutes. Hence, their design data is reliable and useable. The built models were examined and verified by comparing the simulation results with the design data of the reference WtE plant and coal power plant under different design conditions, and some

Value Moisture Ash Carbon Hydrogen Oxygen Nitrogen Sulphur

7.40 15.18 65.20 4.21 6.43 0.88 0.70 25080

of the comparations are displayed in Table 9 and Table 10. It appears that the simulation models are accurate and excellent. Moreover, the parameters of the heat exchangers inside the boilers were determined in accordance with energy and mass balances. Then the parameters of the proposed hybrid scheme were derived via simulation and calculation, as presented in Section 4.1.

H. Chen et al. / Energy 194 (2020) 116893

7

Table 6 Basic parameters of the reference CFPP. Item

Unit

Value

Coal consumption rate SH

kg/s kg/s MPa
C kg/s MPa
C MPa
C kg/s MPa
C
C % % MW MW %

58.16 520.14 17.45 541.0 435.91 3.62/3.53 317.4/430.0 3.53/3.45 430.0/538.6 520.14 19.60 276.4 120.0 94.11 46.85 629.11 594.51 40.75

Steam flow rate Outlet steam pressure Outlet steam temperature Steam flow rate Inlet/outlet steam pressure of RHR1 Inlet/outlet steam temperature of RHR1 Inlet/outlet steam pressure of RHR2 Inlet/outlet steam temperature of RHR2 Feedwater flow rate Inlet feedwater pressure Inlet feedwater temperature

RHR

ECO

Exhaust flue gas temperature Boiler efficiency Turbine efficiency Gross power output Net power output Net thermal efficiency

Table 7 Parameters of the heat regeneration system of the reference CFPP. Heater

RH1 RH2 RH3 DEA RH5 RH6 RH7 RH8

Extraction steam

Drain water

Feedwater

Pressure (MPa)

Temperature (
C)

Flow rate (kg/s)

Temperature (
C)

Flow rate (kg/s)

Inlet/outlet temperature (
C)

Outlet flow rate (kg/s)

5.92 3.58 1.62 0.73 0.30 0.12 0.05 0.02

383.3 316.6 442.6 328.3 227.7 135.5 82.4 57.9

39.40 42.55 23.24 23.28 19.58 14.29 15.56 15.63

249.5 207.5 175.6 e 107.1 85.2 60.7 35.2

39.40 81.95 105.19 e 19.58 33.87 49.44 65.06

243.9/276.4 201.9/243.9 170.0/201.9 130.7/170.0 101.5/130.7 79.6/101.5 55.1/79.6 29.6/55.1

520.14 520.14 520.14 520.14 391.67 391.67 391.67 391.67

3.2. Evaluation criteria 3.2.1. Essential assumptions To examine the feasibility of the novel concept, the proposed hybrid scheme was assessed comparing with the conventional separate scheme. A few necessary assumptions have been made for the contrastive study. a) The coal consumption rate and MSW consumption rate are kept the same. b) The exhaust flue gas temperature and efficiency of the coalfired boiler remain identical. c) The thermal efficiency and auxiliary power of the CFPP are maintained constant. The power output produced from the coal is regarded as fixed. d) The exhaust flue gas temperature and efficiency of the WtE boiler stay unchanged. e) The influence of the surrounding environment is not considered.

3.2.2. Energy evaluation The thermal performances of the two schemes were compared under a power-boosting mode, where additional power generation is checked with the same fuel consumption [47]. The net power output of the MSW in the hybrid scheme (Pw;new , kW) is formulated as:

Pw;new ¼ Ptot;new  Pc

(1)

where Ptot;new is the net total power output of the new scheme, kW;

Pc is the net power output of the coal, kW, which is unaltered in the two schemes. The net waste-to-electricity efficiency (hen;w ) is defined as:

hen;w ¼

Pw mw  LHVw

(2)

where Pw is the net power output of the MSW, kW; mw is the MSW consumption rate, kg/s; LHVw is the lower heating value of the MSW, kJ/kg. The net total energy efficiency (hen;tot ) is expressed as:

hen;tot ¼

Ptot mc  LHVc þ mw  LHVw

(3)

where Ptot is the net total power output, kW; mc is the coal consumption rate, kg/s; LHVc is the lower heating value of the coal, kJ/ kg.

3.2.3. Exergy evaluation Exergy represents the maximum capacity of a system to perform useful work when it proceeds to a given final state in equilibrium with the environment, and exergy analysis can identify the locations, magnitudes and sources of thermodynamic inefficiencies in a thermal system [48]. The exergy of the fuel (EXf , kW) is estimated as follows [49].

8

H. Chen et al. / Energy 194 (2020) 116893

HPT

LPT

IPT

EG SH3

SH2

RHR2

BFPT

CON

SH1 RHR1 RH1

RH2

DEA (RH4)

RH3

RH5

RH6

RH7

RH8

CP

ECO

FWP1

EP1 EP2

EP3

APH

AFWH Coal-fired boiler Drum 2

FGD

ESP

SAH

EP4

EVA

1

Stack

SH

FWP2

ECO PAH 2

BF WtE boiler

1

SDS

Steam

Feedwater

Condensate

Air

Flue gas

Fig. 3. Diagram of the proposed WtE system integrated with the CFPP.

Table 8 Modeling details of each main component in software EBSILON Professional. Component ID in software

Details

Boilers

Steam generator

Turbines

Steam turbine

RHs

Feedwater preheater Aftercooler Deaerator Condenser Pump Generator Universal heat exchanger

The boilers were modeled as blocks with several inflows and outflows. The efficiencies of the coal-fired boiler and WtE boiler are 94.11% and 80.74%, respectively. The pipeline efficiencies of the coal-fired boiler and WtE boiler are 99.00% and 99.69%, respectively. The inlet pressure of each stage group was defined, and the outlet pressure of each stage group was determined by the inlet pressure of the following stage group. For the last stage group, the outlet pressure was set as the exhaust steam pressure. The isentropic efficiency of each stage group was specified. The mechanical efficiencies are 99.80%. The upper terminal temperature difference of each feedwater preheater and the lower terminal temperature difference of each aftercooler were specified. The pressure losses of the extraction steam for different RHs range from 3% to 5%. The heat losses were ignored.

CONs Pumps EGs AFWH

The upper terminal temperature difference is 5.0
C. The isentropic efficiencies are 80.00%. The mechanical efficiencies are 99.80%. The generator efficiencies are 99.00%. The inlet and outlet temperatures of the hot fluid were specified. The inlet temperature of the cold fluid was also specified. The heat loss was neglected.

H. Chen et al. / Energy 194 (2020) 116893

9

Table 9 Main comparations between the simulation results and the design data of the reference WtE plant under the 100% load. Item

Design value

Simulation value

Relative error (%)

MSW consumption rate (kg/s) Live steam (into turbine)

5.79 3.90 395.0 13.50 6.80 38.5 9.91 190.0 80.74 27.94 8.31 20.51

5.79 3.90 395.0 13.50 6.80 38.5 9.93 190.0 80.74 27.91 8.30 20.49

0.00 0.00 0.00 0.00 0.00 0.00 0.20 0.00 0.00 0.11 0.12 0.10

Exhaust steam (out of turbine)

Pressure (MPa) Temperature (
C) Flow rate (kg/s) Pressure (kPa) Temperature (
C) Flow rate (kg/s)

Exhaust flue gas temperature Boiler efficiency (%) Turbine efficiency (%) Net power output (MW) Net thermal efficiency (%)

Table 10 Main comparations between the simulation results and the design data of the reference coal-fired power plant under the 100% load. Item

Design value

Simulation value

Relative error (%)

Coal consumption rate (kg/s) Live steam (into turbine)

58.16 16.70 538.0 520.14 3.32 538.0 435.91 4.00 29.0 325.94 120.0 94.11 46.92 595.35 40.81

58.16 16.70 538.0 520.14 3.32 538.0 435.91 4.00 29.0 326.07 120.0 94.11 46.85 594.51 40.75

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.04 0.00 0.00 0.15 0.14 0.15

Reheated steam (into turbine)

Exhaust steam (out of turbine)

Pressure (MPa) Temperature (
C) Flow rate (kg/s) Pressure (MPa) Temperature (
C) Flow rate (kg/s) Pressure (kPa) Temperature (
C) Flow rate (kg/s)

Exhaust flue gas temperature Boiler efficiency (%) Turbine efficiency (%) Net power output (MW) Net thermal efficiency (%)

EXf ¼ mf  LHVf 
H O N  1:0064 þ 0:1519  þ 0:0616  þ 0:0429  C C C

hex;w ¼ (4)

where mf is the fuel flow rate, kg/s; LHVf is the lower heating value of the fuel, kJ/kg; H, C, O and N are the mass contents of hydrogen, carbon, oxygen and nitrogen in the fuel. The exergy of a fluid stream (EXfs , kW), such as air, flue gas, steam and water, is expressed as:

EXfs ¼ mfs  ½h  h0  T0  ðs  s0 Þ

(5)

where mfs is the fluid flow rate, kg/s; h and h0 are the specific enthalpies of the fluid at the current state and environmental state, kJ/ kg, T0 is the environmental temperature, K; s and s0 are the specific entropies of the fluid at the current state and environmental state, kJ/kgK. The parameters of the environmental state are 15
C and 1 atm. In general, the exergy balance of a component can be described as Equation (6). The exergy balances of the main components are detailed in Table 11.

EXin þ Win ¼ EXout þ Wout þ EXdes

(6)

where EXin and EXout are the exergy input and exergy output, kW; Win and Wout are the work input and work output, kW; EXdes is the exergy destruction due to irreversibilities, kW. The exergy efficiency of waste-to-electricity (hex;w ) is defined as:

Pw EXw

(7)

where EXw is the exergy of the MSW, kW. The total exergy efficiency (hex;tot ) is formulated as:

hex;tot ¼

Ptot EXw þ EXc

(8)

where EXc is the exergy of the coal, kW. 4. Results and discussion 4.1. Parameters of proposed WtE system The thermal parameters of the hybrid scheme were obtained through simulation and calculation. Then the performances of the new system and conventional system were assessed from the perspective of thermodynamics using the mentioned evaluation criteria. Besides, the root cause of performance enhancement due to the novel concept was discovered by energy flow diagrams and exergy investigation. Eventually, a thorough economic analysis was conducted to demonstrate the financial feasibility of the advanced WtE system. Table 12 shows the parameters of the AFWH, which employs the saturated steam of the WtE boiler to heat the feedwater of the CFPP in the integrated configuration. The feedwater out of the CFPP’s RH2 is delivered to the AFWH and its temperature is prompted by 9.9
C, meanwhile, the saturated steam condenses in the AFWH and leaves to join the drain water into the RH2. As no saturated steam is required to preheat the combustion air of the WtE boiler, all the

10

H. Chen et al. / Energy 194 (2020) 116893 Table 11 Exergy balances of the main components. Component

Schematic view

Exergy balance

Boilers

EX1 þ EX2 þ EX3 þ EX5 ¼ EX4 þ EX6 þ EX7 þ EXdes;B

Turbines

EX1 ¼ EX2 þ EX3 þ W þ EXdes;T

RHs

EX1 þ EX3 þ EX4 ¼ EX2 þ EX5 þ EXdes;RH

CONs

EX1 þ EX3 þ EX5 ¼ EX2 þ EX4 þ EXdes;CON

EGs

W ¼ P þ EXdes;EG

AFWH

EX1 þ EX3 ¼ EX2 þ EX4 þ EXdes;AFWH

Table 12 Parameters of the AFWH in the proposed scheme. Item

Unit

Value

Substance of inlet/outlet hot fluid Hot fluid flow rate Inlet hot fluid pressure Inlet/outlet hot fluid temperature Feedwater flow rate Inlet/outlet feedwater temperature LMTD Heat duty

e kg/s MPa
C kg/s
C
C MW

Steam/condensate 14.12 4.49 257.4/251.0 518.51 245.7/255.6 5.4 24.11

saturated steam (14.12 kg/s) out of the drum can be supplied to the AFWH. In the hybrid scheme, the SH of the WtE boiler is exploited to heat a portion of the cold reheat steam of the CFPP, and some feedwater out of the CFPP’s DEA is sent to the ECO of the WtE boiler. The parameters of the SH and ECO of the WtE boiler in the two schemes are listed in Table 13. The inlet and outlet flue gas temperatures of the SH and ECO are maintained constant, thereby their heat duties do not vary. The reheat steam of 323.3
C is heated to

400.0
C by the SH in the new design, afterwards, it returns to the RHR2 of the coal-fired boiler. The feedwater of 19.55 kg/s from the CFPP’s DEA enters into the ECO of the WtE boiler and absorbs energy, and its temperature is raised from 167.8
C to 258.0
C. At the ECO outlet, 14.19 kg/s of the heated water is poured into the EVA of the WtE boiler and the rest departs to the RH1 of the CFPP. Since the logarithmic mean temperature differences (LMTD) of the SH and ECO dwindle because of the integration, their heat transfer areas may enlarge. After the novel amalgamation, the combustion air of the WtE boiler is preheated by feedwater instead of steam, while the parameters of the air conveyed into the WtE boiler are invariable. The data of the air preheating system for the WtE boiler in the new design are presented in Table 14. The low-pressure feedwater prior to the DEA of the CFPP is taken to PAH1 and SAH1, and the primary air and secondary air are heated from 15.0
C to 110.0
C. Then the primary air gains energy from the feedwater out of the CFPP’s RH2 and its temperature augments to 220.0
C. The warmed secondary air is heated to 166.0
C by the feedwater fetched from the CFPP’s RH3. The LMTDs of the PAHs and SAHs decline sharply in the proposed scheme.

H. Chen et al. / Energy 194 (2020) 116893

11

Table 13 Parameters of the ECO and SH of the WtE boiler in the two schemes. Item

Flue gas flow rate (kg/s) Inlet/outlet flue gas temperature (
C) Substance of cold fluid Cold fluid flow rate (kg/s) Inlet cold fluid pressure (MPa) Inlet/outlet cold fluid temperature (
C) LMTD (
C) Heat duty (MW)

Conventional scheme

Proposed scheme

SH

ECO

SH

ECO

35.61 599.0/431.0 Superheated steam 13.50 4.54 258.0/400.0 185.7 5.60

35.61 389.0/190.0 Feedwater 14.19 5.20 130.7/258.0 90.9 8.12

35.61 599.0/431.0 Reheat steam 29.80 3.73 323.3/400.0 148.7 5.60

35.61 389.0/190.0 Feedwater 19.55 5.20 167.8/258.0 61.3 8.12

Table 14 Parameters of the combustion air preheating system for the WtE boiler in the proposed scheme. Item

PAH1

PAH2

SAH1

SAH2

Feedwater flow rate (kg/s) Inlet feedwater pressure (MPa) Inlet/outlet feedwater temperature (
C) Air flow rate (kg/s) Inlet/outlet air temperature (
C) LMTD (
C) Heat duty (MW)

5.73 0.75 131.7/50.3 20.51 15.0/110.0 28.0 1.96

6.95 19.62 245.7/171.4 20.51 110.0/220.0 41.0 2.30

2.34 0.75 131.7/50.3 8.38 15.0/110.0 28.0 0.80

3.40 19.63 203.5/171.3 8.38 110.0/166.0 48.4 0.48

4.2. Overall performance

4.3. Energy flow diagram

The overall performances of the hybrid scheme and the separate scheme were evaluated, as summarized in Table 15. Since the energy obtained from the MSW incineration products in the WtE boiler is inputted into the steam cycle of the CFPP, both the live steam and reheat steam of the CFPP grow notably. Under the condition that the coal consumption and MSW consumption remain invariable, the gross total power output is improved by 3.64 MW due to the suggested integration. As some equipment (CP, circulating water pump, etc.) of the WtE plant that consumes power in the field has been removed, the total auxiliary power falls by 0.07 MW. Therefore, the net total power output rises by 3.71 MW, resulting in an increment of 0.25% points in the net total energy efficiency. While the power out of the coal is deemed as fixed, the net power output of the MSW is boosted by 3.71 MW and the net waste-to-electricity efficiency is promoted from 20.49% to 29.65%, which implies that the innovative concept brings a significant enhancement in the thermal performance of the WtE system.

To investigate the energy-saving mechanism of the proposed design, the detailed energy flows occurred in the separate and hybrid power systems have been explored, as illustrated in Fig. 4. Induced by the consolidation, the energy flows in the new scheme become distinct from those in the conventional one. The energy gained from the MSW combustion products is poured into the power cycle of the CFPP via water and steam in the incorporative configuration. Some heat is transferred to the WtE boiler from the CFPP through preheating the combustion air. The total energy input of the fuel (including coal and MSW) keeps unchanged for the two schemes, which is regarded as 100%. The net total power output is promoted by 3.71 MW due to the combination, which is almost equal to the total energy loss reduction of the exhaust steam in the CONs (3.69 MW) according to the energy balances. As the total flow rate of the exhaust steam dwindles by 1.14 kg/s in the new scheme as compared to that of the conventional one (shown in Table 13), the overall energy loss of the exhaust steam is cut down by

Table 15 Overall performances of the two schemes. Item

Conventional scheme

Proposed scheme

Difference

CFPP

58.16 520.14 435.91 326.07 94.11 5.79 13.50 9.93 81.00 638.88 36.07 602.81 8.30 20.49 40.20

58.16 523.88 450.29 334.86 94.11 5.79 e e 81.00 642.52 36.00 606.52 12.01 29.65 40.45

0 þ3.74 þ14.38 þ8.79 0 0 13.50 9.93 0 þ3.64 0.07 þ3.71 þ3.71 þ9.16 þ0.25

Coal consumption rate (kg/s) Live steam flow rate (kg/s) Reheated steam flow rate (kg/s) Exhaust steam flow rate (kg/s) Boiler efficiency (%) WtE plant MSW consumption rate (kg/s) Live steam flow rate (kg/s) Exhaust steam flow rate (kg/s) Boiler efficiency (%) Gross total power output (MW) Total auxiliary power (MW) Net total power output (MW) Net power output of MSW (MW) Net waste-to-electricity efficiency (%) Net total energy efficiency (%)

12

H. Chen et al. / Energy 194 (2020) 116893

6.95MW (0.46%) 4.08MW (0.28%)

Fuel

Steam

Water

Work

Power

Loss

0.23MW (0.02%)

Auxiliary Power 1.47 MW (0.10%)

MSW

Boiler (MSW)

40.51 MW (2.70%)

8.70 MW (0.58%)

99.65 MW (6.65%) 1458.76 MW (97.30%)

Boiler (Coal)

Turbine

42.61 MW (2.84%) 21.95 MW (1.46%) 108.35 MW (7.23%)

9.86 MW (0.66%)

8.30 MW (0.55%)

745.60 MW (49.73%)

723.65 MW (48.27%)

1957.62 MW (130.57%)

EG Power grid

602.81 MW (40.20%)

6.44 MW (0.43%)

594.51 MW (39.65%)

635.46 MW (42.38%)

EG

Turbines

Coal

34.60MW (2.31%)

598.51 MW (39.92%)

Auxiliary Power

(a) Conventional scheme 12.64 MW (0.84%) 5.54 MW (0.37%)

MSW

Boiler (MSW)

40.51 MW (2.70%)

8.70 MW (0.58%)

38.70 MW (2.58%)

5.69 MW (0.38%)

741.91 MW (49.48%)

1967.53 MW (131.23%)

Power

Water

Loss

6.49 MW (0.43%)

Power grid

99.65 MW (6.65%)

Boiler (Coal)

Work

5.60 MW (0.37%)

108.35 MW (7.23%)

1458.76 MW (97.30%)

Fuel Steam

606.52 MW (40.45%)

Turbines

EG 649.01 MW (43.29%)

Coal

608.42 MW (40.58%)

Auxiliary Power 36.00 MW (2.41%)

(b) Proposed scheme Fig. 4. Energy flow diagrams of the two schemes.

3.69 MW. In addition, the total energy loss of the boilers stays the same in the two schemes. 4.4. Exergy analysis To further reveal the root cause of energy conservation of the novel concept, the two schemes were comparatively analyzed based on the second law of thermodynamics, and the results are displayed in Table 16. Besides, the exergy loss variations of the main components aroused by the recommended integration are introduced in Fig. 5. A few important points should be noted. a) The exergy loss of the WtE boiler falls by 1.02 MW after the incorporation, which is because the temperature gaps between the hot and cold fluids decline in the SH, ECO, PAHs and SAHs of the WtE boiler. However, the exergy ruined in

b)

c)

d)

e)

the coal-fired boiler goes up and its exergy loss rises by 1.45 MW. Consequently, the total exergy loss of the boilers augments by 0.43 MW in the new configuration. By sharing the large-scale turbine with high efficiency in the hybrid system, the entire exergy destroyed in the turbines is reduced by 3.09 MW. With the installation of the AFWH, the energy utilization in the feedwater heating process of the CFPP seems more rational. Hence, the total exergy loss of the RHs and AFWH in the proposed scheme becomes 0.24 MW less than that in the conventional one. The total exhaust steam flow rate drops obviously in the incorporative design, thereby the global exergy loss in the CONs is brought down by 0.79 MW. The exergy losses of other components have no evident changes.

H. Chen et al. / Energy 194 (2020) 116893

13

Table 16 Exergy analysis results of the two schemes. Item

Conventional scheme

Exergy input of coal Exergy input of MSW Total exergy input Exergy output (electricity) of coal Exergy output (electricity) of MSW Total exergy output (electricity) Exergy loss CFPP Boiler Turbines RHs CON EG Auxiliary power WtE plant Boiler Turbine RHs AFWH CON EG Auxiliary power Total exergy loss Exergy efficiency of waste-to-electricity Total exergy efficiency

0.5

Ratio

MW

Ratio

1492.10 42.80 1534.90 594.51 8.30 602.81

97.21% 2.79% 100% 38.73% 0.54% 39.27%

1492.10 42.80 1534.90 594.51 12.01 606.52

97.21% 2.79% 100% 38.73% 0.79% 39.52%

738.39 67.58 17.25 33.43 6.35 34.60 28.36 2.69 0.24 e 1.64 0.09 1.47 932.09 19.40% 39.27%

48.11% 4.40% 1.12% 2.18% 0.41% 2.25% 1.85% 0.18% 0.01% e 0.11% 0.01% 0.10% 60.73%

739.84 67.18 17.09 34.28 6.49 34.60 27.34 e e 0.16 e e 1.40 928.38 28.07% 39.52%

48.20% 4.38% 1.11% 2.24% 0.42% 2.25% 1.78% e e 0.01% e e 0.09% 60.48%

0.43 0.05

Exergy loss variation (MW)

0.0 -0.07

-0.24

-0.5

-0.79

-1.0 -1.5 -2.0 -2.5 -3.0 -3.09

-3.5 Boilers

Turbines

RHs+AFWH

CONs

EGs

Proposed scheme

MW

Auxiliary power

Main components Fig. 5. Exergy loss variations of the main components caused by the proposed integration.

Generally, the suggested integration diminishes the overall exergy loss by 3.71 MW, and the exergy efficiency of waste-toelectricity is improved by 8.67% points with an increment of 0.25% points in the total exergy efficiency. It is a remarkable fact that the prime contributor to the performance enhancement is the exergy loss decrease in the turbines from the standpoint of the second thermodynamic law, although the dominating energy loss reduction occurs in the CONs according to the first thermodynamic law. 5. Economic assessment For the purpose of identifying the financial feasibility of the innovative concept, the new WtE design was economically evaluated as compared to the traditional one. The cost and earning of the CFPP were regarded as invariable in the two schemes, and the economic performance of the WtE system was individually

examined. The cost of a WtE plant mainly consists of the investment cost and the operation and maintenance cost. The investment cost contains the expenses on facilities, infrastructure, land use, loan interest, etc. [50]. When all the equipment is made in China, the specific investment cost of the reference WtE plant is chosen as 57050 USD/t based on Ref. [50]. The revenue of a WtE plant is principally obtained from electricity sale and MSW disposal, and the feed-in tariff and tipping fee for a WtE system are usually priced by the government in China [51]. The requisite assumptions and data for the economic analysis are presented in Table 17. In contrast with the reference WtE system, some components have been removed, added or modified in the proposed WtE system. The total investment cost of the new WtE plant can be determined by the investment costs of the changed equipment and the total investment cost of the reference WtE plant. The estimation methods for the changed equipment are detailed in Table 18, involving the cost function method and scaling up method. Subsequently, the investment costs of the equipment can be derived, as displayed in Table 19. Since a number of the components (turbine, EG, cooling tower, stack, etc.) are removed in the novel design, the investment cost of 6621.59 thousand USD is conserved. Several extra devices (AFWH, EPs, etc.) are assembled in the new configuration, which leads to an additional investment cost of 331.18 thousand USD. Besides, a few facilities (ECO, SH, etc.) are retrofitted for the integration, and an extra investment cost of 211.98 thousand USD is requested. In general, the total investment cost is diminished by 6078.43 thousand USD caused by the suggested combination, and the total investment cost of the new WtE system falls dramatically to 22.45 million USD. The dynamic payback period (DPP, year) and the net present value (NPV, $) have been applied to measure the economic performance of the WtE system. The dynamic payback period is the time required to recoup all the investment considering the time value of money, which is calculated by the net annual cashflow with a discount rate [61]. The dynamic payback period is defined as Equation (9), and a shorter payback period indicates the better practicability of the project [62].

14

H. Chen et al. / Energy 194 (2020) 116893

Table 17 Basic assumptions and data for the economic assessment. Item

unit

value

Total investment cost of reference WtE plant [50] Annual operation and maintenance cost of WtE plant [50] Loan ratio [50] Loan term [50] Annual interest rate [50] Annual operating time of WtE plant [51] Lifetime of WtE plant

M$ M$ e year e h year year e $/t $/MWh % % %

28.53 10% of total investment cost 70% 15 6.15% 7200 2 30 12% 8.91 96.51 0 12.5 25.0

Discount rate [50] Tipping fee [50] Feed-in tariff [53] Income tax rate during economic period [51]

Construction period [50] Economic period [52]

1st to 3rd year 4th to 6th year 7th to 30th year

Table 18 Investment cost estimation methods of the changed equipment in the proposed and reference WtE systems. Cost function method Component

Function

Source

Turbine

ICT ¼ 6000  ðWnom Þ0:71

[54]

EG

ICEG ¼ 60  ðPnom Þ0:95

[55]

CP, EP1, EP2, EP3, EP4 and circulating water pump

ICP ¼ 3540  ðWnom Þ0:71

[54]

RH2 and AFWH

log10 ðICHX Þ ¼ 4:8306  0:8509  log10 ðAÞ þ 0:3187  ½log10 ðAÞ2

[56]

DEA

ICDEA ¼ 6014  ðmfw Þ0:7

[55]

Scaling up method Component

Basic cost (k$)

Basic scale

Scale unit

Scaling factor

Source

CON Cooling tower Stack SH ECO PAH SAH

4275.92 17009.38 10645.25 45.84 676.13 783.92 783.92

36000 13000 1122 500 13149 8372 8372

m2 m2 kg/s m2 m2 m2 m2

1 1 1 0.68 1 1 1

[57, 58]

[57, 59] [57, 60]

Table 19 Investment costs of the changed equipment in the proposed and reference WtE systems. Component Removed

Modified

Added

Sum

DDP X

Cin  Cout ¼0 ð1 þ idis Þy y¼1

Turbine EG CON Circulating water pump Cooling tower CP RH2 DEA Stack ECO SH PAH SAH AFWH EP1 EP2 EP3 EP4

Reference WtE system (k$)

Proposed WtE system (k$)

difference (k$)

4082.97 370.30 118.78 146.65 1504.68 9.02 19.08 32.30 337.81 147.94 115.51 178.94 42.14 e e e e e 7106.12

e e e e e e e e e 219.69 134.35 262.38 80.09 216.99 1.18 0.42 1.95 110.64 1027.69

4082.97 370.30 118.78 146.65 1504.68 9.02 19.08 32.30 337.81 þ71.75 þ18.84 þ83.44 þ37.95 þ216.99 þ1.18 þ0.42 þ1.95 þ110.64 6078.43

(9)

where y is the year number during the plant lifetime; Cin and Cout

are the cash inflow and cash outflow in year y, $; idis is the discount rate. During the economic period, the cash inflow of the WtE system depends on the revenues of electricity sale and waste management, which is formulated as:

H. Chen et al. / Energy 194 (2020) 116893

Cin ¼ Pw  N  ce þ mw  3:6  N  ctf

(10)

where N is the annual operating time of the WtE plant, h; ce is the feed-in tariff, $/kWh; ctf is the tipping fee for disposing MSW, $/t. In the construction period, the cash outflow of the project is the investment. During the economic period, the cash outflow contains the loan repayment, income tax and operation and maintenance cost, which is expressed as:

Cout ¼ Clr þ Cit þ Co&m

(11)

where Clr and Cit are the loan repayment and paid income tax per year, $, which can be gained by Equation (12) and Equation (13), respectively; Co&m is the annual operation and maintenance cost, $.

Clr ¼

ICtot  rl  iint

(12)

1  ð1 þ iint Þk

where ICtot is the total investment cost, $; rl is the loan ratio; iint is the annual interest rate; k is the loan term, year.

i h   ð1þkbÞ   Co&m  iit Cit ¼ AIgro  Clr  1  1 þ iint

(13)

where AIgro is the gross annual income, $, which is equal to Cin during the economic period; iit is the income tax rate; b is the year number during the economic period. The net present value represents the total difference between the present values of cash inflows and the present values of cash outflows for the entire life of the project, calculated as follows [62]. A larger net present value implies the higher profitability of the project.

NPV ¼

n X ðCin  Cout Þ y¼1

(14)

ð1 þ idis Þy

where n is the plant lifetime, year. Using the above formulas, the economic assessments of the proposed and reference WtE systems were contrastively carried out, and the results are shown in Table 20. While the MSW consumption is kept constant, the annual incomes due to waste disposal of the two WtE systems are identical. In the hybrid scheme, the net power output of the MSW is improved by 3.71 MW, 2.92 MW and 1.99 MW under the 100% load, 75% load and 50% load of the CFPP, respectively. In consequence, the annual power output augments by 21.98 GWh and the annual earning owing to electricity sale is promoted by 2.12 million USD. As the total investment cost of the WtE plant is diminished after the amalgamation, the annual operation and maintenance cost declines by 0.61 million

15

USD. Above all, the dynamic payback period is reduced from 9.42 years to 3.55 years caused by the novel concept. In an economic lifespan of 30 years, the net present value of the new WtE system can attain as high as 23.97 million USD, which is 18.76 million USD larger than that of the conventional one. Therefore, the incorporative WtE design is suitable from an economic standpoint. According to thermodynamic and economic evaluations, the proposal is extremely feasible and promising to be implemented in engineering. The novel concept has provided an available option for promoting the WtE technology, which is especially practicable in a country with many high-efficiency coal power plants (such as China). 6. Conclusions An innovative WtE design incorporated with a CFPP has been developed for advancing waste incineration power generation. In the integrated scheme, the saturated steam of the WtE boiler is exploited to heat the feedwater of the CFPP, and the partial cold reheat steam of the CFPP acquires energy in the SH of the WtE boiler. Meanwhile, the feedwater of the WtE boiler is supplied by the CFPP, and the combustion air of the WtE boiler is warmed by the feedwater extracted from the CFPP. As consequences, much more electricity can be produced from the MSW and the waste-toelectricity efficiency can be markedly boosted. Comprehensive thermodynamic and economic analyses were conducted to evaluate the proposed concept, based on a 500 t/day WtE plant and a 630 MW CFPP. Several crucial conclusions can be derived: (1) Attributed to the new configuration, 3.71 MW additional net power output can be generated from the MSW with a wasteto-electricity efficiency improvement of 9.16% points, while the coal consumption and MSW consumption are fixed. According to the energy balances, the dramatic decline in the total energy loss of the exhaust steam is the dominant reason for enhancing the power output in the hybrid scheme. (2) For the novel design, the major exergy loss reductions of 2.92 MW and 0.95 MW occur in the turbines and CONs, induced by sharing the large-scale efficient turbine and decreasing the exhaust steam, respectively. In consequence, the exergy efficiency of waste-to-electricity is promoted by 8.67% points with an increment of 0.25% points in the total exergy efficiency. (3) Because of the recommended consolidation, the dynamic payback period of the WtE system is shortened by 5.87 years and the net present value of the project is improved from 5.21 million USD to 23.97 million USD, which confirms that the innovative WtE design is exceedingly suitable from the perspective of economics.

Table 20 Economic analysis results of the proposed and reference WtE systems. Item

Unit

Reference WtE system

Proposed WtE system

Difference

Total investment cost Annual MSW consumption Annual income due to MSW disposal Net power output of WtE system

M$ t M$ MW MW MW GWh M$ M$ M$ year M$

28.53 150000 1.34 8.30 8.30 8.30 59.76 5.77 7.11 2.85 9.42 5.21

22.45 150000 1.34 12.01 11.22 10.29 81.74 7.89 9.23 2.24 3.55 23.97

6.08 0 0 þ3.71 þ2.92 þ1.99 þ21.98 þ2.12 þ2.12 0.61 5.87 þ18.76

Net annual power supply Annual income due to electricity sale Total annual income Annual operation and maintenance cost Dynamic payback period Net present value

Under full load of CFPP (2400 h/year) Under 75% load of CFPP (3800 h/year) Under 50% load of CFPP (1000 h/year)

16

H. Chen et al. / Energy 194 (2020) 116893

Acknowledgments This work was supported by the National Key R&D Program of China (No. 2017YFB0602104), National Natural Science Foundation of China (No. 51806062) and China Postdoctoral Science Foundation Funded Project (No. 2019M650609). References [1] National Bureau of Statistics. China statistical yearbook (2018): transport and disposal of municipal solid waste in different regions. 2018. Beijing, [In Chinese]. [2] He J, Lin B. Assessment of waste incineration power with considerations of subsidies and emissions in China. Energy Policy 2019;126:190e9.
pez Martínez A, Cuartas Hern
[3] Turcott Cervantes DE, Lo andez M, Lobo García De
zar A. Using indicators as a tool to evaluate municipal solid waste Corta management: a critical review. Waste Manag 2018;80:51e63. [4] National Development and Reform Committee, Ministry of Housing and Urban-Rural Development. Construction plan of harmless treatment facilities for urban domestic garbage in “13th Five-Year”. 2016. Beijing, [In Chinese]. [5] Fei F, Wen Z, Huang S, De Clercq D. Mechanical biological treatment of municipal solid waste: energy efficiency, environmental impact and economic feasibility analysis. J Clean Prod 2018;178:731e9. [6] Gupta N, Yadav KK, Kumar V. A review on current status of municipal solid waste management in India. Journal of Environmental Sciences-China 2015;37:206e17.
ndez-Gonz
[7] Ferna alez JM, Grindlay AL, Serrano-Bernardo F, RodríguezRojas MI, Zamorano M. Economic and environmental review of waste-toenergy systems for municipal solid waste management in medium and small municipalities. Waste Manag 2017;67:360e74. [8] Arafat HA, Jijakli K, Ahsan A. Environmental performance and energy recovery potential of five processes for municipal solid waste treatment. J Clean Prod 2015;105:233e40. [9] Leme MMV, Rocha MH, Lora EES, Venturini OJ, Lopes BM, Ferreira CH. Technoeconomic analysis and environmental impact assessment of energy recovery from Municipal Solid Waste (MSW) in Brazil. Resour Conserv Recycl 2014;87: 8e20. [10] Lombardi L, Carnevale E, Corti A. A review of technologies and performances of thermal treatment systems for energy recovery from waste. Waste Manag 2015;37:26e44. [11] Li Y, Zhao X, Li Y, Li X. Waste incineration industry and development policies in China. Waste Manag 2015;46:234e41. [12] Guo Y, Glad T, Zhong Z, He R, Tian J, Chen L. Environmental life-cycle assessment of municipal solid waste incineration stocks in Chinese industrial parks. Resour Conserv Recycl 2018;139:387e95. [13] Bianchi M, Branchini L, De Pascale A. Combining waste-to-energy steam cycle with gas turbine units. Appl Energy 2014;130:764e73. [14] Main A, Maghon T. Concepts and experiences for higher plant efficiency with modern advanced boiler and incineration technology. In: 18th annual North American waste-to-energy conference; 2010. Orlando. [15] Lee SH. High-temperature corrosion phenomena in waste-to-energy boilers. New York: Columbia University; 2009. €llmer S, Aßbichler D, Murer M, Heuss-Aßbichler S, Rieger K, et al. [16] Müller D, Wo High temperature corrosion studies of a Zirconia coating: implications for waste-to-energy (WTE) plants. Coatings 2016;6:36.  F. A preliminary comparative performance evaluation of [17] Bogale W, Vigano highly efficient waste-to-energy plants. Energy Procedia 2014;45:1315e24. [18] Xu H, Lin WY, Dal Magro F, Li T, Py X, Romagnoli A. Towards higher energy efficiency in future waste-to-energy plants with novel latent heat storagebased thermal buffer system. Renew Sustain Energy Rev 2019;112:324e37. [19] Martin JJE, Koralewska R, Wohlleben A. Advanced solutions in combustionbased WtE technologies. Waste Manag 2015;37:147e56. [20] Strobel R, Waldner MH, Gablinger H. Highly efficient combustion with low excess air in a modern energy-from-waste (EfW) plant. Waste Manag 2018;73:301e6. [21] Liuzzo G, Verdone N, Bravi M. The benefits of flue gas recirculation in waste incineration. Waste Manag 2007;27:106e16. [22] De Greef J, Villani K, Goethals J, Van Belle H, Van Caneghem J, Vandecasteele C. Optimising energy recovery and use of chemicals, resources and materials in modern waste-to-energy plants. Waste Manag 2013;33:2416e24. [23] Hulgaard T. MSc IS. Integrating waste-to-energy in Copenhagen, Denmark. Proceedings of the Institution of Civil Engineers - Civil Engineering 2018;171: 3e10. [24] Behzadi A, Gholamian E, Houshfar E, Habibollahzade A. Multi-objective optimization and exergoeconomic analysis of waste heat recovery from Tehran’s waste-to-energy plant integrated with an ORC unit. Energy 2018;160:1055e68. [25] Eboh FC, Andersson B, Richards T. Economic evaluation of improvements in a waste-to-energy combined heat and power plant. Waste Manag 2019;100: 75e83. [26] Holmes DR. Dewpoint corrosion. Chichester: Ellis Horwood Limited; 1985. [27] Wen Z, Di J, Liu S, Han J, Lee JCK. Evaluation of flue-gas treatment technologies

[28] [29] [30]

[31]

[32]

[33]

[34]

[35]

[36] [37] [38]

[39] [40]

[41] [42]

[43] [44]

[45]

[46]

[47]

[48] [49] [50] [51] [52]

[53] [54]

[55] [56]

[57] [58]

[59]

for municipal waste incineration: a case study in Changzhou, China. J Clean Prod 2018;184:912e20. Consonni S, Silva P. Off-design performance of integrated waste-to-energy, combined cycle plants. Appl Therm Eng 2007;27:712e21. Poma C, Verda V, Consonni S. Design and performance evaluation of a wasteto-energy plant integrated with a combined cycle. Energy 2010;35:786e93. Carneiro MLNM, Gomes MSP. Energy, exergy, environmental and economic analysis of hybrid waste-to-energy plants. Energy Convers Manag 2019;179: 397e417. Vera D, de Mena B, Jurado F, Schories G. Study of a downdraft gasifier and gas engine fueled with olive oil industry wastes. Appl Therm Eng 2013;51: 119e29.  V, Alexe F. Analysis of biomass and waste gasification Marculescu C, Cenus¸a lean syngases combustion for power generation using spark ignition engines. Waste Manag 2016;47:133e40. Zhang S, Lin X, Chen Z, Li X, Jiang X, Yan J. Influence on gaseous pollutants emissions and fly ash characteristics from co-combustion of municipal solid waste and coal by a drop tube furnace. Waste Manag 2018;81:33e40. Boumanchar I, Chhiti Y, M’Hamdi Alaoui FE, Elkhouakhi M, Sahibed-dine A, Bentiss F, et al. Investigation of (co)-combustion kinetics of biomass, coal and municipal solid wastes. Waste Manag 2019;97:10e8. Ismail TM, Yoshikawa K, Sherif H, Abd El-Salam M. Hydrothermal treatment of municipal solid waste into coal in a commercial Plant: numerical assessment of process parameters. Appl Energy 2019;250:653e64. Mendecka B, Lombardi L, Gładysz P, Stanek W. Exergo-ecological assessment of waste to energy plants supported by solar energy. Energies 2018;11:773. Breeze P. Power generation technologies. Newnes; 2019. Coal fired plants: pros and cons. 2019. https://www.brighthubengineering. com/power-plants/115683-advantages-and-disadvantages-of-coal-for-power-plants/. Department of Industry Development Environment and Resources. Power statistics basic data of China in 2017. 2018. Beijing, [In Chinese]. Zhou L, Xu G, Zhao S, Xu C, Yang Y. Parametric analysis and process optimization of steam cycle in double reheat ultra-supercritical power plants. Appl Therm Eng 2016;99:652e60. Makarichi L, Jutidamrongphan W, Techato K. The evolution of waste-toenergy incineration: a review. Renew Sustain Energy Rev 2018;91:812e21. Azami S, Taheri M, Pourali O, Torabi F. Energy and exergy analyses of a massfired boiler for a proposed waste-to-energy power plant in Tehran. Appl Therm Eng 2018;140:520e30. Branchini L. Waste-to-energy: advanced cycles and new design concepts for efficient power plants. Cham: Springer; 2015. Dal Magro F, Xu H, Nardin G, Romagnoli A. Application of high temperature phase change materials for improved efficiency in waste-to-energy plants. Waste Manag 2018;73:322e31. Zeng M, Du LX, Liao D, Chu WX, Wang QW, Luo Y, et al. Investigation on pressure drop and heat transfer performances of plate-fin iron air preheater unit with experimental and Genetic Algorithm methods. Appl Energy 2012;92:725e32. Chen H, Xiao Y, Xu G, Xu J, Yao X, Yang Y. Energy-saving mechanism and parametric analysis of the high back-pressure heating process in a 300 MW coal-fired combined heat and power unit. Appl Therm Eng 2019;149:829e40. Hou H, Yu Z, Yang Y, Chen S, Luo N, Wu J. Performance evaluation of solar aided feedwater heating of coal-fired power generation (SAFHCPG) system under different operating conditions. Appl Energy 2013;112:710e8. Aljundi IH. Energy and exergy analysis of a steam power plant in Jordan. Appl Therm Eng 2009;29:324e8. Fu Q. Thermodynamic analysis methods for energy systems. Xi’an: Xi’an Jiaotong University Press; 2005 [In Chinese]. Zhao X, Jiang G, Li A, Wang L. Economic analysis of waste-to-energy industry in China. Waste Manag 2016;48:604e18. Zeng X. Cost analysis of waste incineration-power generation projects. Environmental Sanitation Engineering 2014;22:57e60 [In Chinese)]. Guo Z, Wang Q, Fang M, Luo Z, Cen K. Thermodynamic and economic analysis of polygeneration system integrating atmospheric pressure coal pyrolysis technology with circulating fluidized bed power plant. Appl Energy 2014;113: 1301e14. National Development and Reform Commission. The notice on improving price policies of WtE plants. 2012. Beijing, [In Chinese]. Ogorure OJ, Oko COC, Diemuodeke EO, Owebor K. Energy, exergy, environmental and economic analysis of an agricultural waste-to-energy integrated multigeneration thermal power plant. Energy Convers Manag 2018;171: 222e40. Lian ZT, Chua KJ, Chou SK. A thermoeconomic analysis of biomass energy for trigeneration. Appl Energy 2010;87:84e95. Zhang C, Liu C, Wang S, Xu X, Li Q. Thermo-economic comparison of subcritical organic Rankine cycle based on different heat exchanger configurations. Energy 2017;123:728e41. Ye C. Research on the key issue of coal partial gasification technology. Hangzhou: Zhejiang University; 2018 [In Chinese]. Electric Power Planning and Engineering Institute. Quota design reference cost indicators for thermal power projects (2017 level). Beijing: China Electric Power Press; 2018 [In Chinese]. Elsido C, Martelli E, Kreutz T. Heat integration and heat recovery steam cycle optimization for a low-carbon lignite/biomass-to-jet fuel demonstration

H. Chen et al. / Energy 194 (2020) 116893 project. Appl Energy 2019;239:1322e42. [60] Han Y. Collaborative optimization of energy utilization and pollutant emission reduction in cold end of boiler in coal-fired power generation system. Beijing: North China Electric Power University; 2018 [In Chinese)]. [61] Mi X, Liu R, Cui H, Memon SA, Xing F, Lo Y. Energy and economic analysis of

17

building integrated with PCM in different cities of China. Appl Energy 2016;175:324e36. [62] Liu M, Zhang X, Ma Y, Yan J. 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 Manag 2018;161:243e53.