Progress and prospects of innovative coal-fired power plants within the energy internet

1 Introduction

In recent years,increasing concern over global energy shortages and environmental problems has spurred active research worldwide on the next energy system,the energy internet,which features multiple,complementary energy sources and real-time,bidirectional communication [1-3].The aim of the energy internet is to construct a convenient platform for energy supply and consumption.Electricity,owing to its high-efficiency in transmission and widespread utilization in modern society,is destined to be the crucial component in the energy internet.The energy internet system,with electricity as its core,presents two different modes that vary in both local and wide energy networks[4].The former is represented by the integrated energy system that is oriented toward terminal energy consumers.This system integrates various regional energy resources from the perspective of overall planning and complementary utilization,achieving high-efficiency energy cascade utilization for the purpose of satisfying multiple electricity,heating,cooling,and gas energy demands.The latter network aggregates large-scale energy bases,consisting of coalfired power plants,wind energy,solar energy,and hydraulic energy,with smart grids and ultra-high voltage transmissions as backup [5].The aims of these networks are the realization of optimal configurations of various natural energy resources across large geographic areas and the accelerated development of distributed renewable energy [6-8].

In the context of the energy internet,revolutionary developments in energy systems and renewable energy impose great challenges on traditional coal-fired power generation.The increasing capacity of renewable energy power plants built in recent years has resulted in exaggerated redundancy within power grids.Moreover,considering conventional coal-fired power generation technologies,resource consumption and environmental pollution put these technologies into a disadvantage position in power system.Using China as an example,the emissions of SO2,NOx,and dust of thermal power plants in 2017 were 1.2 × 106 t,1.14 × 106 t,and 2.6 × 105 t,respectively,which respectively contribute 13.7%,9.1%,and 3.3% of the total discharge volume for China [9,10].Furthermore,these pollutants resulted in a series of environmental problems,such as haze,acid rain,photochemical smog,etc.,and generated negative effects on human health.Therefore,the coal-fired power generation industry is facing a unique predicament,a diminishing development market and rigorous environmental emissions standards.

Traditional coal-fired power plants must be upgraded with innovative technologies to become more efficient,flexible,and environmentally friendly.Therefore,various advanced coal-fired power generation emerged and developed gradually.With the consideration of fuel utilization features and mechanisms of energy conservation,innovative technologies can be divided into three categories:(1) large-capacity and high-parameter power generation technologies,(2) system innovation and specific highefficiency thermal cycles,and (3) coal-fired power generation combined with poly-generation technologies.The first category contains high-efficiency supercritical/ultra-supercritical (SC/USC) and double reheat SC/USC power generation technologies.Thermodynamic analyses and techno-economic analyses are widely employed to demonstrate the performance of both single and double SC/USC power generation systems [11,12].In waterdeficient areas,large-scale air-cooling power generation technologies dominate,and these have been studied and improved through field investigations,model experiments,and numerical modeling [13-15].Considering the second category of technologies,the integration of renewable energy with conventional power plants and boiler-turbine coupled retrofitting are outstanding representatives of innovation.Many scholars have developed methodologies for the standardized modeling of sub-systems,optimal configuration of renewable energy,and technical and economic feasibility analyses for synergetic systems [16,17].The innovative supercritical CO2 Brayton cycle for coal-fired power plants exhibits obvious advantages in terms of its high efficiency and economical and convenient structure [18].The third category of technologies is mainly composed of integrated gasification combined cycle (IGCC) and integrated gasification fuel cell (IGFC) technologies.These advanced technologies for clean and efficient coal-fired power generation are vital for future energy conservation in the electric power industry,as well as a major developmental direction for coal-fired power conservation [19].

Concerning the multiple resources in the energy internet,renewable energy supplies are greatly restricted by the availability of natural resources and fluctuate stochastically,which results in a negative effect on the stability of the power grid.From the perspective of the structural optimization of the energy supply system,there are two approaches to tame the load fluctuations and reinforce the security of the power grid.One option is the building of matched energy storage devices for renewable energy power plants during the initial planning period,such as the large-scale Li-ion battery series used for wind power plants [20].Additionally,molten salt energy storage systems for concentrating solar power (CSP) [21] decrease the range of fluctuations caused by the intermittent injection of renewable energy.Another option is to improve the flexibility of existing power generation units to satisfy the dynamic load of the power grid,which imposes unexpected challenges on active coal-fired power plants.Generally,power generation devices can attain high efficiencies under the design conditions,whereas performance may be drastically reduced under extremely low load,off-design conditions.Therefore,quick response thermal power generation technologies and technologies that continue stable and economical operations under low load rates are crucial developmental direction for coal-fired power under low load rate operating conditions.With respect to the first approach,many scholars have studied the characteristics of the uncertain renewable energy with fuzzy methods,scenario-based methods,information gap decision theorybased methods,and robust analyses [22,23].On the basis of clearly understanding the uncertainties,many studies have been conducted to explore the optimal configuration and operation of energy systems using appropriate physical and chemical energy storage technologies,with the goal of requiring the overall lowest investment,highest revenue,and most reliable energy supply [22-25].Considering the second approach,researchers have investigated the offdesign performance of the flexible transformation units via computer simulations and field tests [26],and proposed innovative control mechanisms and adjustment strategies[27-29],as well as analyzed the economic feasibility and optimization of peak-shaving [30,31].

The energy internet is defined by the innovation of energy infrastructures,with openness and peer-to-peer features borrowed from internet technology [32].The energy internet is designed from the perspective of the cyber-physical system (CPS) to achieve energy sharing and supply-demand matching.The technologies of electrical engineering and electronics,communication,and the internet are applied to control energy flows and the realtime sharing of information.The coal-fired power plant,as a terminal energy supply unit,involves the fusion of energy and information.Smart thermal power plants,which have been discussed frequently in recent years,present an anticipated mode of contemporary and next-generation advanced coal-fired power generation.

As mentioned previously,the coal-fired power generation industry faces inevitable challenges,as well as unprecedented opportunities,from the revolution of energy systems and the application of the energy internet.This review introduces the main developmental directions for coal-fired power plants and provides an overall outlook for relevant technologies.Additionally,the current statuses of energy systems and coal-fired power plants are analyzed,and the advanced technologies of coal-fired power generation are introduced.Finally,the strategies for addressing the challenges of flexibility enhancement and off-design operation,and the prospects of smart plants,including the necessary technological developments and analysis of benefits,are discussed.

2 Current status and development potential of coal-fired power generation

An adequate and reliable energy supply is a prerequisite for the sustained economic development of industrialized society as a whole [19].Along with the rapid development of society,the overall demand for energy is continually increasing.Renewable sources of energy,like hydroelectric power,wind power,and solar power,together with nuclear energy,have seen rapid development in both their capacities and proportions within the energy system.However,for many years in the future,we will still be reliant upon thermal power to provide for a growing economy.According to the 2018 Energy Outlook of British Petroleum(BP) [33],coal will remain the largest fuel source for power generation through 2040 (Fig.1).

Fig.1 Shares of total power generation over time [33]

From the perspectives of the structure of natural primary energy sources,the cost of energy production,and national energy supply security,it is clear that coal-fired power generation will remain an inevitable power supply method in the near future.Considering the multiple factors mentioned in section 1,many developed and developing countries have proposed their own energy development plans,corresponding to their respective energy conservation and emissions reduction targets.For developed countries and areas,the European Commission and the United States(U.S.) are typical representative examples.The European Commission has issued energy policy plans,such as the 2020 climate and energy package [34],the 2030 framework for climate and energy policies [35],and the roadmap for moving to a low-carbon economy in 2050 [36],aimed at following sustainable development rules and leading to the mitigation of emissions and a resource-efficient Europe.These power generation and distribution plans are focused on the electricity produced from renewable sources,which are widely distributed across Europe,but hard to completely replace fossil fuels.Using “Energiewende,”the German energy transition plan as an example,continuous increases in the capacity of installed,renewable energy sources have been observed in Germany,in which the share of renewable sources of electricity production reached 25.8% in 2014 [37],whereas the lignite consumption has stayed relatively stable since coal-fired power units undertook the responsibility of peak-shaving.

The U.S.has traditionally relied on coal-fired power plants as its largest source of power generation,although the share of coal has declined substantially in recent years and is expected to decrease further over the next two decades.However,coal-fired power plants currently cannot be replaced by renewable energy due to their obvious advantages in terms of cost and stability.The Trump administration is planning to support struggling coal-fired power plants by having grid operators purchase electricity from them,citing national security reasons [38].Meanwhile,China is representative of a developing country,with rapid economic development and enormous energy demands.Coal has held the dominant position in energy supply system for many years due to Chinese primary resources endowment,which also promoted existing large capacity coal-fired power plant for China's power.Recently,large stores of shale gas and shale oil are explored in China,especially,the reserve of technically recoverable shale gas is largest in the world (1115 trillion cubic feet,14.7% of world technically recoverable resources [39]).Shale gas is cleaner than coal,and can be used as transitional resources from coal to renewables.The discovery brings new challenge for coal-fired power plants apart from the renewable energy and environmental protection policies.However,comprehensive considering the stable energy supply,asset utilization,social employment rate,and national security,coal-fired power plants will continue to occupy a certain proportion in the energy supply system.The 13th five-year plan for national economic and social development emphasized that the clean and efficient utilization of the coal as the key technology for adjusting the energy structure [40].Sources of electricity generation in Organisation for Economic Co-operation and Development (OECD) Europe,the U.S.,and China are presented in Fig.2 [41,42].

Fig.2 Five-year sources of electricity generation in Organisation for Economic Co-operation and Development(OECD) Europe,China,and the United States

3 Current status and prospects of advanced coal-fired power generation technologies

With the continued development of technologies,many advanced methods of coal-fired power generation have emerged.These can be summarized into three categories:large-capacity and high-parameter power generation units,system innovation and specific high-efficiency thermal cycles,and coal-fired power generation,combined with poly-generation technologies.

3.1 Large-capacity and high-parameter power generation technologies

Currently,600-1200 MW-class SC/USC power generation technologies are the typical units of largecapacity and high-parameter thermal power plants,and have reached a stage of commercial operation [19].Higher parameters USC thermal (620/650/700 ºC),double reheat USC,and large-scale air-cooling coal-fired power generation technologies,as well as ultra-low emission circulating fluidized bed coal-fired power generation technologies are promising for achieving cleaner,higher efficiency,and low-carbon goals.

3.1.1 620/650/700 ºC ultra-supercritical thermal power generation technologies

According to thermodynamic analyses,increasing the average temperature of an endothermic process or decreasing that of an exothermic process can improve the efficiency of the Rankine cycle.It is clear that SC/USC power plants are the directions for the development of thermal power generation.Since the first USC power plant with parameter settings of 31 MPa and 621/566 ºC began operating in the U.S.,supercritical/ultra-supercritical units have experienced 60 years of development both in theoretical and practical applications,accumulating quite a foundation for their design,manufacture,construction,and operation [43].Using China as an example,which has witnessed great progress,the number of 1000 MW ultrasupercritical thermal power plants reached 103 in 2017,and the percentage of SC/USC thermal power generation units reached 46% capacity in operating coal-fired power units[44,45].In turn,the standard coal consumption of 6 MW and above electric power plants decreased to 309 g/kWh in 2017 [46].

According to relative studies on the parameter determination and structure selection for USC units conducted by the Xi'an Thermal Research Institute,the benefits caused by improving the initial parameters of thermal power plants can be summarized by the following equations [47]:

where q is the specific heat consumption of a unit [kJ/(kW·h)],pmain is the pressure of the main steam (MPa),Tmain is the temperature of the main steam (ºC),and Treheat is the temperature of the reheat steam.Equation (1) shows that,in the parameter scope of ultra-supercritical units,for pressure higher than 31 MPa and temperature higher than 600 ºC,each 1 MPa increase can result in a corresponding 0.13-0.15% decrease in the specific heat consumption of the unit.Equations (2) and (3) indicate that each 10ºC increase in the main steam or reheat steam temperature results in a corresponding 0.25-0.3% or 0.15-0.2% decrease in the specific heat consumption of the unit.The efficiency of the current power supply is normally 41-42% for supercritical units and above 43% for ultra-supercritical units,and the power supply efficiency for next-generation advanced supercritical pulverized coal-fired power plant technology with operation temperature of 700ºC (AD700) is expected to reach 46-48% or higher [19].Therefore,many countries and regions have invested in research to develop higherparameter USC thermal power generation technologies(600/700 ºC),and the European Union (E.U.),U.S.,Japan,and China are developing their own AD700 programs.The technological development of coal-fired power generation in these countries is shown in Fig.3.

Fig.3 Five-year development of ferritic (blue),austenite (red),and Ni-based (green) coal-fired power generation technologies by country

The E.U.began an advanced 700ºC pulverized coal-fired power plant project in January of 1998 [48].It comprises～40 companies,ranging from utilities and manufacturers to research organizations and laboratories.The overall technical objective of the project is the phased development and demonstration of an economically viable,pulverized coal-fired power plant technology,with a net efficiency of more than 50%.The project consists of the following four stages:(1) research of high-temperature materials,(2) research on processing performance,(3) real furnace experiments on key components,(4) the construction and operation of a 700ºC USC demonstration plant.The former three phases have been fundamentally completed,while the project has been delayed by European energy policies and shortages of research funding [49].

The U.S.proposed an advanced USC (A-USC) project to improve main steam parameters to 35 MPa and 760ºC.The project was executed in 2001 and jointly sponsored by the U.S.Department of Energy (DOE) and the Ohio Coal Development Office [49].However,with consideration of the fact that the U.S.has no working USC power plants at 600 MW and above,combined with the shocking impact of the shale gas revolution,many uncertain factors still remain influencing the development of the A-USC plans in the U.S.

Japan investigated the technical feasibility of 700ºC power generation in 2000,and performed the feasibility study for the replacement of existing units with 700ºC technology in 2006.In August of 2008,Japan officially launched its A-USC technology project,aiming at achieving 700ºC and above for main steams and attaining 46-48%net thermal efficiency (higher heating value,HHV) [50].For the purpose of identifying the material performance and technological process of turbine rotors and cylinders,Japan constructed an electrical heating test platform for the turbine component.China is also gradually initiating studies on 700ºC technologies via a series of scientific and technological projects.The National Energy Administration has implemented the key project,“Research and development of key equipment of 700ºC ultra-supercritical power generation and demonstration,”which involves an overall schematic design,crucial material and technical boiler and turbine research,the establishment of a test platform for key components,and a feasibility study of the demonstration power plant.The first test platform has been finished and is operating.

After a long period of efforts by various organizations and countries,many key technical problems of 700ºC USC technologies have experienced breakthroughs.However,the core problem of 700ºC technologies - how to develop lowcost,easily processed and reliable materials suitable for the high-temperature surroundings of power plants - remains troubling for researchers.Data show that the operation of 700ºC USC coal-fired power plants may be postponed to 2026 due to the limitations of material and cost performance[51].Considering the feasibility of these systems for practical application,making full use of existing mature materials and improving main steam and reheat steam temperatures offer other outstanding ways of achieving higher efficiency [52].

The E.U.and Japan have improved the reheat temperatures of their coal-fired plants to 620ºC.Increasing the capacity to 1100 MW,based on existing materials and practical demonstrations,such as the German Datteln 4 units,has proven feasible [53].The China Power Engineering Consulting Group (CPECC) proposed 650ºC class high-efficiency USC coal-fired power generation technologies,inspired by the AD700 project [54].The net efficiency of the scheme can achieve 48% with the parameters of 33 MPa and 650/670ºC.

3.1.2 Double reheat ultra-supercritical thermal power generation technologies

As mentioned in the preceding section,the AD700+project is a promising,highly efficient coal-fired power generation system,but it cannot be employed in the near future.The high-parameter double reheat USC unit is a new generation USC units that can be ranked between existing USC units and the E.U.-financed AD700 project.Double reheating can not only fulfill the exhaust steam humidity requirement of a low-pressure cylinder,but also improve the heat efficiency of a unit.The efficiency of double reheat power system can be theoretically 1.0-2.0% higher than that of single reheat unit [55].Compared with the frequently used parameter setting of 25 MPa and 600/600ºC in a USC unit,setting the turbine inlet parameter of a double reheat unit to 30.0 MPa and 600/620/620ºC can improve the heat efficiency of the unit by 2.4-2.6% [56].

Research on double reheat technologies is derived from that on USC technologies.The U.S.built two 325 MW (34.4 MPa and 649/566/566ºC) double reheat units during the 1960s,which were the first USC double reheat units in the world.After that the construction of double reheat thermal power generation plants appeared in the 1980s-1990s.Japan built two 700 MW (31 MPa and 566/566/566ºC)double reheat USC liquefied natural gas units,which began operation in 1989 and 1990.Furthermore,a power plant with a 412 MW (28.4 MPa and 580/580/580ºC) double reheat units was put into operation in 1997 in Denmark.After a year,a 420 MW unit with a steam setting of 29 MPa and 582/580/580ºC was also put into operation.This unit used a deep seawater cooling scheme,in which cooling water was 10ºC,thereby achieving an efficiency of 47%[57].However,compared with a single reheat unit,the complicated configuration of the turbine,boiler,and thermal system of double reheat system resulted in a negative effect on the overall benefits which impeded the development of double reheat technologies.

With continuous increases in the cost of fuel and the serious global environmental situation,double reheat technologies in power generation have attracted considerable attention again in recent years [58].Building double reheat power plants is an effective means for utilizing natural coal resources which is suitable for the fundamental reality of China.Since 2009,many research institutions and universities have delved into the design of double reheat USC units in China.The Huaneng Anyuan Power Generation Company's 660 MW (31 MPa and 600/620/620ºC) double reheat USC No.1 unit began operation on June 17 of 2015,becoming the first double reheat USC power unit put into commercial operation in China.After that,the Taizhou Power Plant of China Guodian Corporation and Huaneng Laiwu Power Plant,which also begun construction in 2012,started operating in 2015.The Huadian Jurong Power Plant and the Shenhua Guohua Beihai Power Plant are currently building units to add to the growing list of double reheat USC units in China[57].The double reheat power plants in China that are currently under construction or have been put into operation recently are listed in Table1.

Table1 Construction status of double reheat coal-fired power plants in China.

Main steam temperature(ºC)Anyuan Power Plant 2 × 660 31 600/620/620 Bengbu Power Plant 2 × 660 31 600/620/620 Suqian Power Plant 2 × 660 31 600/620/620 Pingshan Power Plant 1 × 1350 30 600/610/620 Laiwu Power Plant 2 × 1000 31 600/620/620 Leizhou Power Plant 2 × 1000 31 600/620/620 Jurong Power Plant 2 × 1000 31 600/620/620 Laizhou Power Plant 2 × 1000 31 600/620/620 Taizhou Power Plant 2 × 1000 31 600/610/610 Huilai Power Plant 2 × 1000 31 600/610/610 Qingyuan Power Plant 2 × 1000 35 615/630/630 Yuncheng Power Plant 2 × 1000 35 615/630/630 Plant name Capacity(MW)Main steam Pressure(MPa)

The key technologies of double reheat power generation involve the performance of the material,the adjustment method of the main and reheat steam temperatures,and the stability of the shafting.Setting the parameters to 31 MPa and 600/620/620ºC,the USC units imposed higher requirements of the boiler and turbine than the existing units,far behind those of the AD700 project.Concerning the boiler,existing high-temperature surface materials,such as HR3C (25Cr20NiNbN) and Super304H(18Cr9NiCuNbN),can satisfy the requirement with abundant redundancy.With consideration of the turbine component,FB2 (X12CrMoCoVNbB9-1) and CB2(ZG12Cr9Mo1Co1NiVNbNB),which were developed during the European COST522 project,are able to accommodate the high temperature,operational,and thermal stresses of the double reheat units.Concerning the adjustment methods of the main and reheat temperatures,there are two conventional approaches to adapt the reheat temperature,(1) adjusting the opening of flue gas baffle and (2) a de-superheating spray method.Ma et al.[59]investigated flue gas recirculation technologies in a double reheat power plant,which can solve the problem of reheat steam temperatures being lower than the design value during operation.Furthermore,as double reheat USC units include one more cylinder when compared with single reheat units,the overall length of the shafting will increase over the existing power plants.It is therefore necessary to optimize from the perspective of the structure of the cylinder.The Laiwu Power Plant successfully operated a five-cylinder long-shafting device in a double reheat USC 1000 MW unit.However,Feng [60] proposed an innovative over-under double shafting layout,which configured the high-pressure cylinder and intermediate pressure cylinder at the outlet heads of superheaters and reheaters.The layout decreased the length of the shafting and strengthened the stability of the unit,further reducing the initial investment required by saving the high-temperature material.

3.1.3 Large-scale air-cooling power generation technologies

Large-scale direct and indirect air-cooled power generation technologies present effective solutions for coal-fired power generation in areas lacking water resources.Taking 2 × 600 MW units as an example,water consumption for a wet unit is ～3000 m3/h,whereas that for an air-cooling unit is only 800 m3/h,～80% less than the former [19].Concerning the situation in the regions of Northwest China,which are poor in water but rich in coal,air-cooling power generation technologies have been employed.With consideration of the rapid development of renewable energy resources,the construction speed of newlybuilt coal-fired power plants has declined gradually.However,the construction of large-scale air-cooling power plants are a priority option,as well as large-capacity highparameter double reheat coal-fired power plants.The installed capacity of an air-cooling power plant in China is presented in Fig.4.

Fig.4 Technological development of coal-fired power generation capacities by year for indirect (orange) and direct(blue) air-cooling plants in China

From the negative view,the critical situation of coalfired power plants is also imposing pressures on air-cooling power generation corporations.Many construction plans must be removed from the perspectives of economics and policy.Therefore,the air-cooling technologies applied to large-capacity and high-parameter plants may be an important direction.The Huadian Linwu Power Plant's 2 ×1000 MW USC direct air-cooling unit has been operating since 2010,which was a milestone for air-cooling power generation technologies.Since then,many 1000 MW-class air-cooling coal-fired power units have been built or are currently under construction,such as the Shenhua Guohua Shouguang Power Plant phase I units and the Shenhua Guoneng Fugu Power Plant phase III units.

3.1.4 Ultra-low emission circulating fluidized bedcoal-fired power generation technologies

Circulating fluidized bed (CFB) coal-fired power generation has developed at a rapid speed in recent years,owing to its excellent fuel flexibility,high-efficiency and low-cost desulfurization,and furnace NOx suppression.With the support of the High Technology R&D Program(863) and the Key Project of the National Research Program of China,Chinese scholars have had great breakthroughs,including gas-solid two-phase flow,heat transfer,hydrodynamics of supercritical water,combustion and NOx control,as well as SO2,which are leading globally and widely applied [61].Additionally,a new theoretical framework of the CFB combustion has been constructed in China since the 1990s,and the theory has been applied for redesigning a series of advanced CFB products.In April of 2013,the first 600 MW supercritical CFB units began operation in China,and the installed number of the SC-CFB units reached 13 until February 2017.The 600 MW SC-CFD operations have successfully demonstrated the advantages of CFB combustion and SC steam cycles.However,there still exists development potential for CFB coal-fired power generation technologies by further improving the steam parameters and by combining CFB combustion and USC power generation technologies.Many institutes and universities have launched corresponding research projects,including Tsinghua University,the Huaneng Clean Energy Technology Research Institute (CERI),the Foster Wheeler(FW) Company,Alstom Company and three main boiler manufacturers in China [62].

Considering the most strict emissions standards applied in China (NOx ＜ 50 mg/m3; SO2 ＜ 35 mg/m3),advanced coal-fired power plants can promote the environmentally sustainable performance of thermal power plants by combining with desulfurization and denitrification equipment.The CFB coal-fired power plant is an innovative solution,which can achieve low cost emissions control during the combustion process.Generally,pollutant concentrations of boiler outlet in CFB units (NOx ＜ 200 mg/m3; SO2 ＜ 200 mg/m3) are much lower than that of same part in pulverized coal-fired power plants.The chemical mechanism accompanied by the combustion process can be simply represented by Equations (4) and (5):

However,there are still uncertainties under the current research paradigm.The high-efficiency reduction of emissions in furnaces must be explored more,and the inherent mechanisms of pollutants deprivation should be further investigated.Moreover,considering the practical engineering,related research should be concentrated on improving the recycling efficiency of materials and the average material uniformity,as well as enhancing the flexibility of fuel [63].

3.2 System innovation and specific highefficiency thermal cycle

Apart from large-capacity and high-parameter SC/USC power generation technologies,many institutions and universities have explored innovated power generation approaches at the system level.Many advanced achievements have been made,including renewable energy-aided coal-fired power generation technologies,the supercritical CO2 Brayton cycle for coal-fired power plants,and innovative layouts for waste heat utilization.

3.2.1 Renewable energy-aided coal-fired power generation technologies

A renewable energy-aided coal-fired power generation system (REACPGS) provides a meaningful and promising means of energy conservation and emissions reduction.The mode of the REACPGS is to seek external energy to reform or integrate with traditional coal-fired power systems [64].Compared with the traditional point that improving energy efficiency is one of the most important measures for energy conservation and the reduction of pollutant emissions,REACPGSs provide an innovative approach based on the reasonable aggregation of renewable energy sources and conventional coal-fired power generation technologies.This innovative mode provides an effective way to reduce the cost of renewable energy power generation and to improve the utilization ratio of renewable energy [64].The renewable energy sources that involve a REACPGS include solar energy,geothermal energy,and biomass energy.A schematic of a REACPGS is shown in Fig.5.

Solar-aided coal-fired power generation (SACPG) has attracted much research attention in recent years [65].The heat gathered by solar collectors can be integrated into a direct steam generation (DSG) preheating process with heat regenerators,and preheating boiling process with air preheaters.In terms of engineering applications,the national 863 projects,“Solar Tower Thermal Power Generation Technology and System Demonstration Projects,”have been successfully completed.In 2012,the national 863 project,“Solar Complementary Conventional Power Generation Technology,”was undertaken by the Chinese Academy of Sciences,North China Electric Power University,and the China Datang Corporation,and an additional two units have been approved by the national government [65].In 2015,the “Fundamental Research on efficient and clean energy utilization of coal-fired power generating system”project,as the fifth subsidiary subject of the national 973 projects,was jointly undertaken by the North China Electric Power University,Chinese Academy of Sciences,and Huazhong University of Science and Technology.These projects expanded the basic theory of SACPG,and the demonstration power plant with complementary solar energy demonstrated the feasibility of SACPG.

The geothermal-aided coal-fired power generation is similar to SACPG,as geothermal energy is utilized to replace part of the high-quality steam of the thermodynamic system.Biomass-coupled coal-fired power generation(BCCPG) has been applied successfully in Europe [66].At present,the majority of large-scale coal-fired power plants have been revolutionized,away from the conventional pulverized furnace to a blending of combustion with biomass fuels.Generally,the utilization of biomass energy includes direct burning,blended combustion with coal,and the hydrolysis of liquefied biomass.Moreover,BCCPG is based on biomass gasification,and the blended combustion of biomass and coal can greatly reduce the coal consumption rate [67].Therefore,BCCPG is a promising direction for the application of biomass fuels and will be one of the key technologies in future coal-fired power generation systems.

Fig.5 Schematic of a renewable energy-aided coal-fired power generation system (REACPGS)

3.2.2 Supercritical CO2 Brayton cycle for coalfired power plants

As an advanced power cycle,the supercritical CO2 (SCO2) Brayton cycle has been considered as a promising alternative to the conventional steam Rankine cycle for coal-fired power plants.The S-CO2 Brayton cycle has substantial efficiency benefits,especially when the operating temperature is above 600ºC [68].The net efficiency of a coal-fired S-CO2 Brayton cycle is greater than 48% at a turbine inlet temperature of 650ºC,while that of a steam Rankine cycle is only ～45% [69].Compared with the conventional steam Rankine cycle,the smaller physical footprint and simple layout,compact turbo-machinery,and heat exchangers,greatly reduce the initial capital expenses and decrease the difficulty of installation [70].Moreover,the utilization of the S-CO2 Brayton cycle for coal-fired power plants has been discussed by several scholars.Based on previous investigations,it can be concluded that the recompression S-CO2 cycle is an effective power cycle.Most complicated cycle layouts are derived from this cycle,such as the recompression cycle combined with intercooling and the recompression cycle with preheating.Generally,a recuperation process is required to improve cycle efficiency by minimizing the wasted heat.The intercooler and reheating layouts are adapted to minimize or maximize the compression or expansion work.Additionally,the cycle can be categorized depending upon whether the flow is split or not; for example,Ahn et al.[70] analyzed various layouts that combined intercooling,reheating,recuperation,and flow splitting technologies.

With respect to the overall progress of S-CO2 Brayton cycle technologies,many countries,such as the U.S.,Japan,South Korea,and the Czech Republic have systematically designed and conducted experimental studies on the subject.The U.S.leads the world in this domain,with longitudinal research having been performed at many institutes.In the U.S.,Sandia National Laboratory (SNL),along with the DOE,began developing an S-CO2 power system in 2007.The final target of SNL is to develop a 10 MW-class compact S-CO2 Brayton cycle power system.Meanwhile,the Southwest Research Institute (SwRI) designed a 1 MW-class S-CO2 Brayton cycle power system that is supported by natural gas.In Asia,researchers at the Tokyo Institute of Technology (TIT) investigated the closed S-CO2 Brayton cycle and originally proposed a partial pre-cooling direct cycle mode.The Korea Advanced Institute of Science and Technology (KAIST) invented a small-scale nuclear reactor power cycle with a S-CO2 system,and the Korea Atomic Energy Research Institute (KAERI) constructed a 10 kW-class S-CO2 Brayton cycle power system based on the work at KAIST.China has made many efforts in the parameter optimization and equipment design of the S-CO2 Brayton cycle.It can be inferred that China will soon build its own SCO2 Brayton cycle power system,given the solid foundation that exists in the power sector.

3.2.3 Innovative layout for waste heat utilization and better energy cascade utilization

Waste heat utilization is one of the effective ways to improve the efficiency of coal-fired power plants.The amount of the waste heat contained in the exhausted flue gas and exhausted steam is enormous.Generally,there are three ways to recover wasted heat with respect to innovative layouts and levels of system integration:(1) the direct recovery of low-temperature heat from exhausted flue gas or steam via an organic Rankine cycle (ORC) or CO2 Rankine cycle,(2) utilization of ultra-low-temperature heat in the exhausted steam via heat pump high-back-pressure technologies for district heating,and (3) deep coupling of the turbine and boiler subsystems by a low-pressure economizer (LPE) or reengineering of the innovation process [71].

At present,the feasibility of the ORC systems employed in current power plants remains controversial due to the high initial capital investment.The district heating technologies based on heat pumps have attracted much attention in recent years,but have exhibited limited commercial success,with only a few installations and numerous problems [72].However,high-back-pressure heating technologies have been developed for many years and put into practice in many areas throughout China.

Recently,deep-coupling technologies for turbine and boiler subsystems have drawn enthusiastic support from the studies of various institutes and universities in China.Installation of an auxiliary heat exchanger (LPE)downstream of the boiler flue is commonly used to recover waste heat to heat condensed water,which has been demonstrated in the Waigaoqiao No.3 Power Plant and many other power plants.The deep integration of turbine and boiler subsystems breaks the barrier between them through properly reengineering processes,such as the setting of the bypass flue gas duct.The heat exchanger is configured in the bypass to recover the heat from the flue gas to preheat feed-water or condensed water,which can replace the extracted steam and increase the power output of the unit.An extra air preheater is placed at the downstream end of the flue duct to compensate the reduced heat of the bypass flue gas duct.This innovative layout can achieve energy level matching and energy cascade utilization among the flue gas,air,and feed-water/condensed water.The technology has been adopted in the German Schwarze Pumpe Power Plant (2 × 800 MW lignite generation units)[73].Furthermore,the technology has been developed for up-to-date units,and successfully applied in a 1000 MW USC double reheat coal-fired unit at the Laiwu Power Plant of the China Huaneng Group.The operational data showed that the boiler flue gas temperature did not exceed 90ºC,and coal consumption was reduced by using the system [74].Overall,a low-temperature economizer can be employed broadly in existing coal-fired power plants with only small alterations based on the original design,while deep-coupling technologies are suit for new-build units since their specific structure design.

3.3 Coal-fired power generation combined with poly-generation technologies

Since the beginning of this millennium,global warming and extreme weather caused by greenhouse gases,such as CO2 and CH4,have begun to attract more attention to coalbased poly-generation technologies,such as IGCC and IGFC power generation.

3.3.1 Integrated gasification combined cycle power generation technologies

Integrated gasification combined cycle power generation provides a effective way of improving the efficiency of power generation due to the possibility of co-firing (i.e.,coal with biomass),co-production (electricity,methane,ammonia,etc.),and the ease of carbon capture [75].This process combines two advanced methodologies [76],the first is coal gasification,which transforms coal (and other fuels,e.g.,biomass) to synthesis gas (syngas).The second method is the use of a combined cycle,which is one of the most efficient way of generating electricity.A simplified schematic of an IGCC system is shown in Fig.6.

Fig.6 Simplified schematic of an integrated gasification combined cycle (IGCC) system

In terms of system composition and equipment manufacturing,the IGCC system continuously improves almost all of the technologies in current thermal power systems,integrating and optimizing technologies for air separation,coal gasification,gas purification,gas turbine combined cycles,and steam integration.This system can enhance the cascade utilization of the chemical energy of coal,thus becoming an efficient and eco-friendly power generation technology.

Since the first successful trial run of an IGCC power plant (Cool Water in the U.S.) in May of 1984 validated the feasibility of IGCC technologies [19],IGCCs have experienced great development and come into the commercial stage.There were four 200-300 MW-class coalfired IGCC demonstration power plants designed purely for power generation.All of these plants encountered a number of technical problems in the debugging process and made many improvements through successive modifications to the initial capital investment,overall power efficiency,pollutant emissions,and CO2 reduction.The IGCC was gradually developed from the commercial demonstration phase to the commercial application phase.Nearly 30 IGCC power plants have been put into operation or are currently under construction.With a three-phased schedule,the Huaneng GreenGen 650-MW net power system will become the world's largest IGCC project,and also the first plant explicitly built for carbon capture and storage (CCS).The geographic distribution of IGCC power plants and main research institutes are shown in Fig.7.

With the increasing requirements for power generation efficiency and environmental protection,there have been considerable breakthroughs in the orientation of research on the thermodynamic cycle of IGCC systems,which have resulted in new IGCC technologies,such as the IGFC.The integration of the H/J class gas turbine into the IGCC system in a practical engineering measure that offers another promising direction for the IGCC [77].

Integrated gasification combined cycle power generation technologies will serve as the basis for the future development of the coal industry,which is of great strategic importance in improving the national energy security of China,as well as energy conservation and the reduction of air pollutant emissions.China has proposed smoothly progressing in the development of large-scale IGCC and CO2 sequestration technologies in the “Revolution and Innovation Action Plan for Energy Technology (2016-2030).”The China Huaneng Group has also formulated a plan to construct high-parameter IGCC units in its“Medium- and Long-term Plan for Science and Technology Development (2015-2030).”Moreover,the U.S.,Japan,and the E.U.have also invested in developing IGCC technologies in their future energy plans.

3.3.2 Integrated gasification fuel cell power generation technologies

Solid oxide fuel cells (SOFCs) are promising technologies for electricity generation.Sulfur-free syngas from gas-cleaning units serves as fuel for SOFCs in IGFC power plants [78].The IGFC is an integrated power generation system that combines an IGCC and hightemperature fuel cells,which can achieve a plant-wide power generation efficiency of 60% [19].Fuel cells are high-efficiency electrochemical devices that can convert the chemical energy in fuels into electrical energy.In particular,solid oxide fuel cells are considered suitable for power plant integration as they are fuel flexible,operate at high temperatures,are intrinsically CCS ready and relatively contaminant tolerant.

Fig.7 Distribution of the IGCC power plants and main research institutes; darker color represents countries that developed this technology earlier

System technologies employed in the IGFC power generation concept designs,include a coal gasifier,SOFC technology,gas turbine,and bottom cycle (i.e.,Rankine cycle or ORC) [79].In other words,the IGFC can be regarded as the integration of SOFC and IGCC power generation systems which can attain higher efficiency than the original systems.The IGFC also possesses the advantages of fuel flexibility and production flexibility like the original IGCC.Equation (6) shows possible inputs and outputs of an IGFC system:Coal,coupled with biomass,is a candidate fuel for the gasifier,which can achieve renewable energy-aided coalfired power generation and decrease CO2 emissions [76,80-82].Along with electricity production,IGCC and IGFC are able to co-produce other commercially desirable products that result from the process.The syngas via coal gasification after cleaning is used for the production of chemicals,liquid fuels (e.g.,Fischer-Tropsch (FT) synthetic liquid,methanol,dimethyl ether (DME),etc.) and electrical power.The liquid fuels produced by poly-generation,especially methanol and DME,are suitable alternatives for vehicle fuels,and can greatly mitigate global oil shortages.Furthermore,the IGFC can realize the interconversion between syngas and electricity (i.e.,power-to-gas/hydrogen).The flexibility of the product benefits grid peak regulation by adjusting the ratio between electricity and other co-products.Additionally,national oil security can benefit from polygeneration.

4 Overview and outlook for flexible coal-fired power generation technologies

Concerning large-scale renewable energy connected to the power grid within the structure of the energy internet,fluctuations of the load curve of the energy supply are influenced by the stochastic and intermittent characteristics of renewable energy.Many technological solutions of interest in the frame of the current energy revolutionary prerequisites are presented and discussed here from various viewpoints.Generally,the multi-resource energy system in the context of the energy internet can be expressed by Equation (7):

where ES,Conv represents conventional energy supply technologies,ES,Renew represents intermittent renewable energy supply technologies,and EDemand represents the generic energy demand.

From the view of the energy consumption market,energy demand response technologies provide effective means of minimizing the peak-valley differences of load curves.Currently,the real-time electricity price is a valuable method that has been implemented in many areas,such as Europe,America,and China [83].The adjustable load has attracted much attention as a burgeoning technology for demand response,and has been represented by the building of large-scale electric vehicle charging stations [84].Therefore,Equation (7) can extend to (8):

where ED,conv is the traditional energy demand from society and ED,adjust is the adjustable load.

From the perspective of the energy supply side,the optimization of the structure of the energy supply system is an anticipated way of coping with the current situation.Generally,allocating matching capacity energy storage devices with renewable energy constructions in the planning phase can effectively decrease the negative effects on the power grid.Thus,the real-time energy balance equation can be further extended to Equation (9):

where ES,storage denotes the energy storage devices with respect to stochastically varying renewable energy.

Considering the existing and next-generation coal-fired power plants in developing countries,proper strategies should be adopted to improve the flexibility and operation safety of the coal-fired units for power grid load peakshaving.Continuous rated power operations have been planned for current coal-fired power plants,and only a few startups and shutdowns were forecasted during the contracting,design,and construction phases,even for the newest built units [37].In order to guarantee the stability of the power grid,the boiler island,turbine island,and environmental protective facility operations must be more flexible.The corresponding strategies can be summarized by Equation (10),and Equation (11):

In Equation (10),the attributes of s,d,and o belong to the overall available spaces S,D,and O that represent the depth of peak shaving,the rate of load changing,and appropriate environmental devices that can satisfy the part-load operation,respectively.In Equation (11),ECoal represents coal-fired power generation technologies.More specifically,the depth of peak shaving (s) should be enlarged based on the levels of current technologies (30-40% load rate(LR)),or even below a 20% load rate.The load adjustment action (d) must be executed more quickly,and may be improved from 2% LR/min to 5% LR/min.Finally,the desulfurization facility (o) should operate as normal to ensure the catalytic activity under the extra-low load rate.These requirements introduce critical challenges to the efficiency of power plants operations and their ability to adopt to this new context,which is extremely important for their long-term environmental protection and economic sustainability.

In addition to transforming existing equipment in power plants and adjusting operational strategies,many innovative solutions without alternative startup fuels have been proposed.For combined heat and power (CHP)units,the conventional “heat-load-based”mode blocks the renewable curtailment,and thermal-electric decoupling is an effective way to improve the flexibility of CHP plants[85].Generally,decoupling the power from heat production can be achieved by installing additional heat storage (e.g.,water storage tanks or molten salt heat storage systems),depending upon the power used to heat the facility (e.g.,electric heat pump,heat boiler,etc.).

Considering the practical application of the flexibility enhancement project,China will comprehensively upgrade the flexibility of thermal units per the “13th Five-Year-Plan,”and the retrofit capacities will be 133 GW and 82 GW of the condensing units and CHP units,respectively.The ability to adjust the power grid will increase by 46 GW until 2020.The National Energy Administration of China launched a set of demonstration projects for flexibility transformation in 2016,which including 22 typical projects spread across the country in two patches.With the efforts of all participants,the Liaoning East Power Generation Company of State Power Investment Corporation Limited (SPIC) successfully completed the low-pressure cylinder zero-output flexibility transformation of a 350 MW unit.The Yanshan Lake Power Generation Company originally invented the double-back pressure heating retrofit scheme for a 600 MW supercritical air-cooling power plant.The Huaneng Dandong Power Plant and Yingkou Power Plant are flexibility retrofit templates for sub-critical and ultra-critical power plants,respectively.The former can be operated at 25% LR,while the latter can be operated at 30% LR under the automatic generation control (AGC) mode.

Accompanied by the thermal power plant tendency toward flexibility transformation,the power generation incentive policy of deep peak-regulating units and market mechanisms of auxiliary power services have followed and gradually been completed.Meanwhile,increased efforts with respect to the estimation mechanisms of power plants,focusing on the overall revenue and peak-regulating,have attracted much attention recently.Yang et al.[30] conducted economic analyses and developed a compensation mechanism for the peak regulation of coal-fired power plants using big data.However,there are many difficulties involved in building accurate and widely applicable power plant estimation mechanisms upon consideration of more complex factors,such as the gaming between peers and fluctuation of fuel prices.Therefore,it will take a substantial amount of time for developing countries to construct freely bidding electricity markets.

Apart from the flexibility transformation of traditional coal-fired power plants,as mentioned in sections 3.2.1 and 3.3,advanced coal-fired power generation technologies,such as renewable energy-aided coal-fired power generation and IGCC/IGFC technologies,are also anticipated to mitigate the renewable energy curtailment situation and to smooth load change curves.The former extends the fuel scope of coal-fired power plants,which can be seen as a fuel flexibility strategy.Meanwhile,IGCC/IGFC technologies can achieve bidirectional transformation between the electrical energy and chemical energy of fossil fuels.The possibility of shifting between two energy products allows the operation of a plant in a load-following mode,where the reduction in net power output is counterbalanced by increased by-products throughout.

5 Development prospects of smart coal-fired power plants

Real-time bidirectionality is an obvious feature of the energy internet.With the development of advanced information and communication technologies (ICT),an approach for the energy internet is provided by utilizing intelligent control of the electric supply terminal and the electric grid.Coal-fired power generation,as one of the energy supply terminal components,participates in two levels of the energy internet [2]; one is the physical structure of the energy internet network,and the other is the comprehensive operation system of the energy internet,involving advanced sensors,communication,data storage,and data analysis technologies.Therefore,the smart coalfired power plant with a higher level of automation and intelligence is the direction for existing and next-generation coal-fired power units.A smart power plant involves two layers of deep implications:(1) external communication and coordinated operation with other energy supply units within the energy internet and (2) the internal technical improvement of the power plant.

5.1 External communication and coordinated operation of smart power plants

From the perspective of the energy internet framework,smart power plants are large-scale,terminal energy bases,as well as renewable energy bases in energy internet.As shown in Fig.8,smart power plants participate in the power grid and district heating,both in the flow of energy and information.With consideration of stochastic renewable energy supplies and time-dependent power demand,power grid makes scheduling scheme for the overall power system,and sends load instructions to smart power plants.In turn,smart power plants directly influenced by instructions from power grid,status of units,and local surrounding boundaries,and indirectly influenced by both renewable energy and load demand.Moreover,smart power plants involves district heating through local energy management system (EMS),in which considers various energy supply technologies and devices,dispatched loads according to the principle of clean,efficient,stable and sustainable development.

Specifically,smart power plants must accomplish the following four goals:(1) coordinated operation with the sources,interconnections,and loads of next-generation energy systems,(2) complementary optimization in the energy supply side,(3) decentralized communication,and (4) intensive management services.Smart power plants participate in the coordinated operation of the sources,interconnections,and loads of energy systems,guaranteeing the security of the power grid and energy supply by coordinating energy production,transmission,and distribution,and utilizing processes reasonably.Smart power plants allows for complementary optimization in the energy supply side,achieving optimized load distributions among various power plants,as well as providing services of frequency and peak regulation.In the energy trade markets of the future,free electricity trade can facilitate synergy between smart power plants and energy users.Smart power plants accelerate the communication of decentralized information among various energy enterprises,which can contribute to breaking the barriers to the openness and peer-to-peer communication for the next-generation mode.Information and communication technologies for the basis for information sharing among energy enterprises,and the construction of an “internet+”greatly promotes correlative development.Therefore,smart power plants provide an opportunity for innovation the operational modes of enterprises.The construction of the smart power plant is the irresistible trajectory of coal-fired power generation.

5.2 Internal technological improvements of smart power plants

From the perspective of power plant internal technologies,next-generation modes must take into consideration four key goals:(1) real-time safe operation,(2) real-time operation optimization,(3) strict pollutant emissions standards,and (4) effective unit control.The implications of the smart power plant can be further extended to a wider range,which is illustrated in Fig.9.Generally,a smart power plant consists of 3D life cycle visualization modeling,an intelligent equipment level,intelligent control level,and intelligent management.

Fig.8 The layout of Energy Internet

The 3D life cycle visualization modeling begins during the power plant design phase,and provides continuous modifications and updates to 3D models.The 3D models following the principle of a life cycle can simulate the current health status of the equipment and forecast possible malfunctions during the operational period,in which each component can interconnect with other parts.Those jobs that still require human labor can achieve realtime personnel location,and an alarm will sound once an unqualified action has been taken,which can guarantee personal safety and operational security.The use of enormous automatic machines and smart patrol robots in power plants can remove many human laborers from the dangers and difficulties of such jobs (i.e.,achieve unattended operation).For the intelligent control level,which directly benefits from CPS technologies,can achieve the real-time monitoring of conditions,the prediction of trends,intelligent warnings and malfunction diagnoses,and operational optimization.Finally,the intelligent management level can realize intelligent decision support for the power plant,such as load distribution at the plant level,generating adjustable preventive maintenance plans,energy consumption analyses,and intelligent coal yards with optimized coal-mix burning schemes.

Considering the internal technological improvements of smart power plants,many scholars and institutes have explored relevant topics.Yang et al.[86] studied the application of a CPS for thermal power plants from the perspective of data-driven modeling and applied the model to the optimization of an air-cooling unit.Wu et al.[87] designed an intelligent,web-based workstation for operational analysis in smart power plants.There have also been many practical case studies since the concept was first developed.The first e-thermal power plant in China was constructed at the Jingneng Gaoantun gas-fired power plant,which employed 3D visualization modeling in infrastructure management.The Shenzhen Yuhu Power Generation Company developed an intelligent operation and inspection system with robotic data acquisition technologies.The China Datang Corporation has established the Beijing International Electric Power Data Monitoring & Diagnostic Center,which is aimed at achieving automatic diagnoses and data analyses using big data and data mining methods.Shenhua Guohua (Beijing) Gas-fired Cogeneration Company also provides a representative example of a smart power plant under the current conditions.These explorations provide a rich experience for the traditional coal-fired power plant on the way toward becoming a smart power plant,though there is notable potential for improvement.We can infer that smart power plants will become generalized in the near future.

Fig.9 Diagram of the life cycle of smart power plants and its implications.

5.3 The application of CPS in a smart power plant with the Energy Internet

The smart coal-fired power plant is a demonstration of automation and informatization in accordance with many advanced manufacturing plans,such as the German Industry 4.0 and Made in China 2025 strategies,in which the practical application of the CPS is an effective method for handling the four key goals for the power plant.The CPS is a technical framework with clearly systematic characteristics based on multi-source data modeling.The connection,conversion,cyber,cognition,and configuration(5C) structure of CPS is illustrated in Fig.10.

The five-layer structure is comprised of intelligent connection,conversion,cyber,cognition,and configuration levels.The smart connection level provides a selective data collection function from the components and equipment.From the widely distributed sensor network,operational and monitoring parameters can be gathered and transferred to a database,such as a safety instrumented system (SIS) or the distributed control system (DCS).The data-to-information conversion layer is designed to preprocess and clean the continuous data stream to find useful information without any losses,such as the filtering,validation verification and steady-state identification.The cyber layer is aimed at constructing expert model libraries,and a knowledge discovery system based on the information transferred from the former two layers.Methods,such as data mining,big data analyses,information fusion,machine learning,and the use of intelligent algorithms,like neural networks (NNs),support vector machines (SVMs),and genetic algorithms (GA),are widely used and effective for the cyber layer.The cyber layer is the basis for the cognition layer,and the latter provides the corresponding computation and analysis for performance monitoring,malfunction diagnosis and precaution,predictive maintenance and operation optimization,etc..The configuration layer implements the decision-making and execution in the entity space,which in turn controls the correlative equipment and components to improve the equipment performance and the system efficiency.

Fig.10 Connection,conversion,cyber,cognition,and configuration (5C) structure of a cyber-physical system (CPS)

With a 5C structure employed in a smart power plant,the information system (IS) and decision system (DS) can be constructed for optimized decision-making in the CPS.The IS can be defined as:

where U denotes the domain,A denotes the attribute variable sets,and V denotes the range of A.In Equation(13),f1 represents an information function,in which the relationship f (u,a)∈V if u∈U,a∈A always exists.The DS can be defined as:

where C and D denote condition attribute sets and decision attribute sets,respectively,and f2 represents a decision function.

In the context of the energy internet,bidirectional information delivered between energy enterprises and the power grid,local environmental parameters,real-time status of units can be assigned as input sets for the IS or DS for optimized operational strategies.These strategies embedded in the CPS of smart power plants benefit for their clean,efficient,and economic operating.Moreover,they provides stable frequency-regulation and peak-modulation services for overall energy system,and laid a solid foundation for energy internet.

6 Conclusions

In the contexts of the energy revolution and the energy internet,it calls for an inevitable fundamental transition and technological innovation of coal-fired power generation.The state-of-the-art technologies and development trend of coal-fired power generation were overviewed and discussed,based on which the next-generation power system was presented.Considering the adverse effect on the stability and reliability of energy system with the penetration of intermittent renewable energy sources,energy storage and flexibility transformation will enable the quick-response of the conventional coal-fired power plant; REACPGS is another promising approach to reduce the carbon emission from fossil fuel combustion.In the light of CPS technology,the smart power plant is to become reality,with which the coal-fired power units can be connected to and interact with Energy Internet.

Translated into the field of key technologies of innovative coal-fired power generation,the progress and challenges coexist typical as follows:the single and double reheat SC/USC power generation is comparatively mature in technologies and undergoing the engineering demonstration with even higher requirements for the operation flexibility such as peaking depth and rapid response.Thanks to some substantial breakthrough in related material research,AD700 scheme is expected to be applied in the coal-fired power plant to further improve the efficiency.The innovative layout and system integration technology will promote the upgrading of existing coal-fired power units.From the view of the specific thermodynamic cycle,S-CO2 is well believed as a pretty alternative cycle for the conventional Rankine Cycle with high efficiency and compact layout.IGCC and IGFC poly-generation technologies are innovative methods for fossil fuels with carbon capture and by-product utilization.The two technologies can be popularized widely once making progress in cost reduction,as the great breakthrough in High-Temperature Fuel Cell Technology as well.