Enhancing the operational flexibility of thermal power plants by coupling high-temperature power-to-gas

Applied Energy 263 (2020) 114608

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Enhancing the operational flexibility of thermal power plants by coupling high-temperature power-to-gas

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Yang Suna,b,1, Ligang Wangb,c,1, , Cheng Xua, Jan Van herleb, François Maréchalc, Yongping Yanga a

Key Laboratory of Power Station Energy Transfer Conversion and System (North China Electric Power University), Ministry of Education, Beijing, China Group of Energy Materials, École Polytechnique Fédérale de Lausanne (EPFL), Sion, Switzerland c Industrial Process and Energy Systems Engineering, École Polytechnique Fédérale de Lausanne (EPFL), Sion, Switzerland b

H I GH L IG H T S

coupled with coal power plants for renewable power accommodation. • Power-to-gas of coal power plant enhanced while storing excess renewable power in hydrogen • Flexibility load rate of coal power plant significantly reduced by 18% points. • Minimum limited penalty on power generation efficiency caused by the coupling. • AOptimal operating strategies identified considering efficiency and CO emission. •

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A R T I C LE I N FO

A B S T R A C T

Keywords: Coal power plant Operational flexibility Minimum load reduction Renewable power accommodation Power-to-gas Solid-oxide electrolyzer

The increasing penetration of intermittent renewable power challenges the stability of the electrical grid, thus coal power plants are usually required to extend the operation range by reducing the minimum load. This work proposes a concept of coupling solid-oxide cell stack based power-to-gas with coal power plants to allow for dual functions of (1) storing excess renewable electricity and (2) reducing the minimum load of coal power plants by combustion stabilization with oxygen-rich air from power-to-gas. The performance and operating strategy of such an integrated plant are evaluated with detailed off-design characteristics of the considered coal power plants. The results show that the integration of power-to-gas affects the distribution of the heat absorbed by radiative and convective heat exchangers in the boiler, stabilizes coal combustion, and reduces the superheat degree of live/reheated steam. It allows the power plant for operating at a significantly low load of down to 22% of the nominal load, compared with 40% before the coupling; meanwhile, a very limited penalty is caused with the plant efficiency reduced from 34.4% down to 34.1% (with 13% of the normalized power-to-gas capacity). Minimizing the power-to-gas contribution to the accommodated renewable power is advantageous for a minimal CO2 emission; nevertheless, maximizing the power-to-gas contribution with the coal power plant at high load allows for a maximal system efficiency.

1. Introduction The increased penetration of intermittent renewable energy challenges the grid stability and requires an enhanced grid operating flexibility [1]. When electricity storage is not sufficient for grid balancing, fossil-fueled power plants are frequently regulated from high loads as the leading base-load suppliers to very low loads as the auxiliary suppliers. For example, the average load of coal-fired power plants (CFPPs)

owned by China Energy Investment Corporation (one of the major Chinese corporations on coal-based power generation) was 70% in 2018 and, particularly, the plants in North and Northwest China have operated with a load below 50% for a long run and participated heavily in deep peak shaving with their loads reduced even below 40% [2]. However, when the loads of fossil-fueled power plants cannot be further reduced due to technical (e.g., unstable combustion [3]) or economic (e.g., high operating costs of shut-down and start-up [4,5]) limitations,

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Corresponding author at: Group of Energy Materials, École Polytechnique Fédérale de Lausanne, 1951 Sion, Switzerland. E-mail address: ligang.wang@epfl.ch (L. Wang). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.apenergy.2020.114608 Received 27 September 2019; Received in revised form 27 January 2020; Accepted 2 February 2020 0306-2619/ © 2020 Elsevier Ltd. All rights reserved.

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Nomenclatures

T ṁ β ex LHV η h P

Abbreviations CFPP PtG SOE MLD OEC EP RH-SG RH-G RSG PCM PCV FPSH RPSH MRH HRH HSH LSH ECO

coal-fired power plant power-to-gas solid-oxide electrolysis/electrolyzer minimum load of design oxygen-enhanced combustion electrostatic precipitator regenerative heaters for sweep gas regenerative heaters for the gas mixture of steam and premixed H2 regenerative steam generator pressure control module pressure control valve front plate superheater rear plate superheater middle-temperature reheater high-temperature reheater high-temperature superheater low-temperature superheater economizer

temperature (°C) mass flow rate (kg/s) a sharing coefficient of energy for power generation specific exergy (kJ/kg) lower heating value (kJ/kg) efficiency enthalpy (kJ/kg) normalized power

Subscripts and superscripts

ERP c ad es st mw SA ORA b t gen g

excess renewable power combustion adiabatic exhaust steam steam extraction make-up water sweep air O2-rich air boiler turbine generation generator

Mathematical symbols

P

power (MW)

and high-accuracy combustion monitoring system [12]). To avoid the cost-intensive retrofit, thermal energy storage on the turbine side has been proposed [9,10]; however, the minimum load can only be reduced in a limited range (< 30 MW for a 300 MW CFPP with MLD below 50% of nominal load [9]). From the viewpoint of combustion, furnace temperature can be alternatively maintained by (1) increasing air temperature and (2) employing oxygen-enhanced combustion (OEC) [13,14]. The first solution is frequently employed to maintain combustion stability in the lowoxygen-concentration environment (e.g., flameless combustion with the air temperature above 800 °C) [13]. For conventional CFPPs, increasing air temperature (especially temperature of the secondary air) is also

a vast amount of renewable power still has to be curtailed [6] during low-demand periods [7,8]. To maximize the accommodation of renewable power for a given electricity infrastructure, CFPPs, the dominating contributors of power generation in many countries, need to be more flexible for load variation, particularly to be capable of operating at extremely low loads (even below the minimum load of design, MLD) [3,4,9,10]. The major challenge for further reducing the MLD is the unstable ignition and combustion in the furnace due to the insufficiently-high temperature at a low fuel flow rate [3]. The conventional solution is auxiliary ignition with oil/gas (effective but expensive) and pulverized coal (requiring plant retrofits, such as flexible fuel-feeding system [11]

Fig. 1. The schemes of the capability of renewable power accommodation by the integrated plant. Note that the current load of the CFPP is considered as 60–70%, corresponding to the situation in China in 2018 [2,22]. 2

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operating strategy of the integrated system are described in Section 2. The CFPP and SOE stack for the case study are introduced in Section 3.1, followed by the detailed system design and modeling in Sections 3.2 and 3.3. The methodology for system evaluation is provided in Section 4. Then, minimum load reduction and performance enhancement attributed to the integration are investigated in Section 5 with a sensitivity analysis for identifying the optimal operating strategies. Finally, the conclusion is drawn in Section 6.

helpful; however, it is limited by the heat transfer area of air preheater and insufficiently-high temperature of flue gas at a low load. The second solution introduces less inert gases for combustion thus increasing the combustion temperature. The OEC can employ either pure O2 [15] or O2-rich air, which leads to the so-called low-concentration OEC. If the OEC is used not for CO2 capture, then low-concentration OEC is advantageous in terms of costs related to the oxygen production and plant retrofits [14,16,17]. The CFPPs and power-to-gas (PtG), which stores excess renewable power in form of hydrogen or methane and produces pure oxygen or oxygen-rich air as the by-product [18,19], therefore, can be integrated on site to accommodate renewable power (Fig. 1(c)) more than that handled by standalone PtG (Fig. 1(b)). This is achieved by the OEC using the oxygen produced from PtG to reduce the load of CFPPs further below MLD (Fig. 1(c)). In addition, for the PtG based on high-temperature electrolysis, particularly solid-oxide electrolysis (SOE) [20,21], additional high-quality heat from the PtG can be introduced into the furnace to further enhance the combustion stability. In this context, this work proposed a scheme of enhancing CFPPs’ operational flexibility by coupling SOE based PtG, in which the PtG stores surplus renewable power with the high-temperature O2-rich air from PtG employed to stabilize coal combustion. The thermal integration of SOE-based PtG has already been studied with nuclear power plant [23,24], geothermal plants [25,26], biomass boilers [27], ammonia production [28], solar thermal/photovoltaic units [29–32], etc.; however, previous works focused on the efficiency and product cost of energy storage/gas production, and did not study the potential of using the PtG for reducing their minimum operating load. Thus, it is considered that this paper presents a novel way of the PtG application, which can potentially enhance the capability of accommodating variable renewable power, compared with standalone PtG plants or coal power plants. A case study of a typical CFPP is investigated with calibrated off-design characteristics of the key components based on the data from the original equipment manufacturer. Three critical and fundamental issues of such coupling are investigated, for a given capacity and design of the CFPP,

2. The concept and operating strategy 2.1. The integrated concept In the integrated system (Fig. 2), the PtG island consists of the SOE stack and the balance of plant (mainly the heat exchanger network). Water and sweep air are heated to the desired temperature (700 °C) by the stack outlet streams and electrical heating (if needed). The SOE stack electrochemically splits H2O into H2 and O2 using excess renewable electricity. The cathode outlet (H2 and H2O mixture) is cooled down with water knock-out. The sensible heat of the anode outlet (O2rich air) is partially recovered in the PtG island, and thereafter, mixed with the primary or secondary air and fed into the furnace of the CFPP. The CFPP, consisting of the boiler and steam turbine islands, can provide steam/air for the PtG (as reactant/sweep air) at different conditions. If the heat recovered in the PtG island is not enough to generate the amount of steam required by the SOE, the remaining steam can be extracted from the steam turbine island as a supplement. 2.2. Operating strategy For the integrated system, renewable power accommodation can be contributed by the load reduction of the CFPP (the CFPP contribution) and the power storage by the PtG (the PtG contribution), as shown in Fig. 1 (b) and (c). To accommodate the same amount of excess renewable power (ΔPERP , MW), there can be multiple ways of operating the integrated system by varying the CFPP and PtG contributions (ΔPCFPP and ΔPPtG , MW), as shown in Fig. 3. The dispatch to CFPP depends on the current and minimum loads of the CFPP, while the PtG contribution relies on the installed capacity and the current load of the PtG. Therefore, the dispatch between the CFPP/PtG contributions, i.e., the operating strategy of the integrated system, needs to be optimized in terms of different indicators.

• how much the load of CFPP can be reduced below the MLD by the PtG installed with different capacities? (Answered in Section 5.1) • what is the effect of the coupling of CFPP and PtG in terms of •

thermal and mass interactions with different PtG capacities? (Answered in Section 5.2) what are the optimal operating strategies of cooperating CFPP and PtG for accommodating excess renewable power with different PtG capacities? (Answered in Section 5.3)

2.3. Benchmarking For comparison purpose, standalone CFPP and PtG will be studied, in which there are no physical interactions between the CFPP and PtG.

The remaining paper is organized as follows: The concept and

Fig. 2. The concept of the integrated CFPP and PtG system. 3

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Fig. 3. Dispatch of the power-to-gas and coal-fired power plant for power accommodation with ΔPERP = ΔPCFPP + ΔPPtG .

Section S1 of the Supplement Materials. The MLD of this plant (120 MW) is reached when the boiler operates at its minimum load with stable combustion with the adiabatic combustion temperature being 1741 °C at an air-fuel ratio of 1.5. At partial load, the air-fuel ratio should be adjusted following Fig. 5 to avoid the increase in the unburnt carbon and soot formation [33]. At a very low load, the flow rate of the exhaust steam of the turbine becomes small; therefore, to avoid the risk of blade damage caused by temperature excursion [34,35], the flow rate is constrained by its minimum value of design [36,37] (200 t/h in this case).

Compared with the integrated system, the main features of the separated system are (1) no combustion enhancement; (2) electrical power as the only energy source of the PtG. 3. Case study 3.1. Case description 3.1.1. Coal-fired power plant The reference plant (module (a) in Fig. 4) is a typical 300 MW CFPP, of which the plant layout and the detailed information are described in

Fig. 4. The integrated system of the CFPP and SOE-based PtG. 4

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Fig. 5. Variation of the air-fuel ratio with load rate.

Table 1 Key specifications of the stack at the selected design point [18]. Item

Value

Operating pressure (bar) Inlet temperature of the stack (°C) Outlet temperature of the stack (°C) Cell voltage (V) Current density (A/cm2) Flow rate of cathode inlet stream (sccm/cm2) Flow rate of anode inlet stream (sccm/cm2) Steam utilization factor Power consumption rate of a single SOE stack (kW)

1.12 700 819 1.36 0.86 10.8 7.83 62% 5.98

Fig. 7. The minimum load (rate) vs. the normalized PtG capacity (ṁ esmin : minimum mass flow rate of turbine’s exhaust steam).

Fig. 8. Boiler efficiency vs. load rate and PtG capacity (ṁ esmin : the minimum mass flow rate of turbine’s exhaust steam).

turbine of the CFPP. 3.2. Layout of the integrated system

Fig. 6. The adiabatic combustion temperature at reduced loads with different PtG capacities. Notes: (1) 13% is the minimum normalized PtG capacity, with which the load reduction of CFPP is no longer limited by combustion; (2) The black dot lines represent the borderlines of the CFPP’s minimum load; (3)ṁ esmin : the minimum mass flow rate of turbine’s exhaust steam.

Detailed layout of the integrated system is shown in Fig. 4 considering heat and mass interactions related to air and steam. Part of the preheated air from the air preheater outlet is fed into the PtG unit as the sweep gas after an electrostatic precipitator (EP). To avoid electrical heating and recover sensible heat of the cathode outlet, the regenerative steam generator (RSG) and regenerative heater for the cathode outlet gas (RH-G) are employed. The sensible heat of anode outlet is partially recovered by the regenerative heater for the sweep gas (RH-SG), and the remaining is introduced into the furnace. The terminal temperature differences of these heat exchangers are set as 62 K (lower) for the RSG and 112 K (upper) for the RH-SG and the RHG. The steam for the PtG unit is extracted from the outlet of the IPST, which is close to atmospheric pressure. The steam pressure can be controlled by the pressure control module (PCM) [36,40] in the CFPP unit and pressure control valve (PCV) in the PtG unit. Compared with standalone PtG, the following are highlighted for the hardware of the integrated plants:

3.1.2. Solid-oxide electrolysis stack A stack is comprised of a number of cells in series and an interconnector between each two neighboring cells. Each cell includes a cathode for H2O split, an electrolyte for O2− transportation, and an anode for O2 reformation. The stack has been modeled in detail with the calibration at various operating conditions and the performances of relevant power-to-fuel systems have been investigated comprehensively at different operating points of the stack [18,21,38,39]. In this paper, the operating point of the stack (with 64 plate cells × 80 cm2 active area) is fixed and the PtG capacity can be scaled by varying the stack number. Technical specifications of the stack at the selected operating point are presented in Table 1. The stream data of the stack inlets/ outlets can be found in Section S2 of the Supplement Materials. The bottleneck of SOE-based PtG is the significant heat requirement for steam generation. In a standalone PtG system, additional heat required is usually provided by the electrical heating; while in the integrated system, part of the steam can be supplied by the steam extraction of the

(1) The PCM, PCV, and EP, have already been widely used in the onservice CFPP unit; 5

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Fig. 10. Distribution of heat absorption in the boiler for CFPP load rate 40% with different PtG capacity between 0 and 20%: (a) portion of heat for evaporation, superheating, and preheating, (b) heat transferred in the convective heat surfaces. The heat transferred by the convective heating surfaces is mainly determined by the logarithmic mean temperature difference, as shown in Fig. S7. For interpreting the abbreviations of the heat surfaces, kindly refer to Fig. 4.

Fig. 9. The temperatures of live/reheated steam vs. load rate and PtG capacity (ṁ esmin : minimum mass flow rate of turbine’s exhaust steam).

4. System evaluation

(2) The heat exchanger network of the integrated PtG is less complex than that of the standalone PtG (referring to Fig. S1) because direct mixing of air eases the heat recovery of the PtG unit; (3) For low concentration oxygen-enhanced combustion, the expensive retrofit of the burner may not be needed.

4.1. Minimum load min The minimum load of the CFPP (PCFPP , MW) is constrained by that (1) the adiabatic combustion temperature (Tc,ad , °C) should be higher MLD than that of the MLD (Tc,ad , 1741 °C in this case), and (2) the exhaust steam flowrate should be over the minimum value of design (ṁ esmin , 200 t/h in this case) [36,37]. The Tc,ad can be calculated referring to Section S4 of the Supplement Materials [47].

Thus, the integration will not lead to significant additional costs. In addition, the necessity of burner retrofit will be verified with the results in Section 5.

3.3. Modeling

4.2. System/subsystem efficiencies

A simulation model of the CFPP has been developed following Refs [41,42] to obtain the partial-load performance [43,44], which is calibrated with the data from the original equipment manufacturer. Details about the model [45,46] and the verification can be found in Section S3 of the Supplement Materials. The standalone CFPP also needs to be simulated for the loads below the MLD, to identify the effect of coupling with the PtG. Such simulations do not consider the combustion stabilization, thus are only theoretical as a comparison basis but not practical. Partial-load operation of the stack is not considered and the operating load of the PtG unit is adjusted by varying the number of the stack in operation.

In the integrated system, the CFPP and PtG are thermally integrated and part of the energy released from the fuel is transferred to the PtG unit; therefore, the efficiency of coal power generation should be calgen , kg/s) by removing the culated with a corrected coal consumption (ṁ coal coal consumption contributed to the PtG due to the steam requirement. This is done by a sharing coefficient based on exergy (β ) as proposed in Section S4 of the Supplement Materials: gen total ṁ coal = β∙ṁ coal

(1)

total where ṁ coal is the total coal consumption (kg/s). Thereby, the corrected gen efficiency of power generation (ηCFPP ) becomes:

6

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Fig. 11. Corrected turbine/plant efficiencies for power generation vs. load rate and PtG capacity (ṁ esmin : minimum mass flow rate of turbine’s exhaust steam). gen ηCFPP =

PCFPP total β∙ṁ coal ∙LHVcoal

(2)

Fig. 12. Sensitivity analysis of the excess renewable power to be accommodated, PtG portion, and current load rates on the corrected power generation efficiency. In the feasible regions, the upper and lower bounds represent the benchmark operating strategies with maximum PtG/CFPP contributions.

where PCFPP denotes the power output of the CFPP (MW); LHVcoal is the specific lower heating value of coal (MJ/kg). The boiler efficiency (ηb ) can be defined as:

ηb =

out in in ∑i ṁ iout ,WAS hi,WAS − ∑j ṁ j,WAS hj,WAS total LHVcoal ∙ṁ coal

+ (ṁ ORA hORA − ṁ SA hSA )

evaluated from 40% load rate with different PtG capacities below 20%. The basic results are summarized below:

(3)

where ṁ and h stand for the mass flow rate (kg/s) and specific enthalpy (kJ/kg) of the sweep air (SA), O2-rich air (ORA) and water/steam (WAS). The corrected turbine efficiency is calculated by:

ηtgen =

(1) The oxygen volumetric concentration of the air into the furnace is within 0.210 – 0.233, which does not need to retrofit the burner [47]. Detailed results about the O2 provided by the PtG are given in Section S5 of the Supplement Materials. (2) The maximum average temperature of the air into the furnace is 309 °C (Fig. S4), within the safe range of typical CFPPs. (3) The minimum sharing coefficient β is over 0.97, indicating a small equivalent coal consumption share of PtG island.

gen ηCFPP

ηb ηg

(4)

where ηg stands for the efficiency of the electricity generator. 5. Results and discussion

5.1. Minimum load (rate) reduction of the coal-fired power plant

In all the discussion below, the PtG capacity (PPtG ), CFPP power output (load rate), and power to be accommodated (PERP ) have been normalized referring to the nominal load of CFPP (300 MW in this case). The effect of the PtG on the minimum load reduction of the CFPP is

By varying the PtG capacity, the operation of the CFPP below the MLD is evaluated with the variation of adiabatic combustion temperature given in Fig. 6. It shows that, as expected, the combustion temperature reduces with a decreasing load. The larger the PtG capacity, the higher the combustion temperature will be, indicating an 7

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(1) Maximizing the PtG contribution lowers the cost of electricity due to higher power generation efficiency (Fig. 12) (up to 3.5 percentage points, referring to Fig. S9(a) in the Supplement Materials) and larger yield of the high-value H2 (up to 1.7 t/h, referring to Fig. S9(b)). Maximizing the CFPP contribution is more environmentalfriendly with lower CO2 emission (Fig. S8) up to 44 t/h (referring to Fig. S9(c)). Optimal dispatch portion between PtG and CFPP can be settled down regarding the compromise between the economical and environmental targets. (2) The current load of the CFPP affects the feasible dispatch between the PtG and CFPP (the PtG portion in Fig. 12). When the current load approaches the MLD, a high PtG portion is required. Under a low PtG portion, a higher current load rate is capable of accommodating more excess power. (3) The load rate of CFPP at which the PtG unit comes into service affects the overall system performance. Putting the PtG into service at higher CFPP load and maximizing the PtG contribution are beneficial for reducing the cost of electricity because of the higher average load of the CFPP. For instance, under the precondition of maximizing PtG contribution, the power generation efficiency with the initial load rate of 60% is always not lower than that of 40% with the maximum efficiency difference of 3.5 percentage points (Fig. S10 in the Supplement Materials). As a contrary, for the scenario of minimizing CO2 emission, the current load of the CFPP is not so important due to the priority of the CFPP contribution for renewable-power accommodation.

enhanced combustion by the PtG. For a specific PtG capacity, the combustion temperature decreases first and then increases along with the load reduction. The turning point is due to the maximum excess O2 ratio reached. When the PPtG is below 13%, the load reduction is constrained by the adiabatic combustion temperature at MLD but is no longer true when above 13%. The relationship between the PtG capacity and minimum load (rate) is further summarized in Fig. 7, which reflects the flexibility enhancement of the CFPP attributed to the PtG. When the PPtG is below 13%, the minimum load of CFPP island is decreased approximately linearly from 40% (120 MW) to 30% (88 MW). With PtG capacity of 13%, the minimum load can be safely reduced to around 22.6%. A further increase in the PtG capacity will not be beneficial for further load reduction, since the CFPP will be constrained by turbine but not the combustion anymore. 5.2. Performance variation of coal-fired power plant The effects of the PtG coupled to the CFPP performance are further analyzed with the insights at the component level. As shown in Fig. 8, the boiler efficiency is improved with the increase in the PtG capacity, because using O2-rich air reduces the inert gas (mainly N2) in the flue gas, thus reducing the heat loss due to the emitting flue gas. The temperatures of the live/reheated steam are further investigated in Fig. 9 to understand with more details on how the boiler performance is affected. The temperature of the live steam decreases from 541 at MLD down to 506 °C with 13% PtG capacity (Fig. 9(a)). Similarly, the temperature of the reheated steam decrease from 510 at MLD down to 470 °C with 13% PtG capacity (Fig. 9(b)). This indicates that the integration of the PtG changes the distribution of heat absorption in the boiler. As shown in Fig. 10 (a), with the increase in the PtG capacity for a given CFPP load, the portion of heat absorbed for steam generation by the radiative heat exchange is increased with the increased combustion temperature. The total heat absorbed for superheating and preheating is decreased (Fig. 10(a)) with an increased heat absorption by the radiative heat transfer and a significantly reduced heat absorption by the convective heat transfer, particularly LSH1 – 2 (Fig. 10(b)). The reduction of convective heat exchanged is due to (1) the reduction of flue gas flow rate which reduces the convective heat transfer co-efficient, and (2) the slight reduction of the temperature of the furnace outlet flue gas, as shown in Figure S5. The temperature decrease of the reheated steam is resulted from the decrease in the temperature of the live steam (Fig. S6), leading to a reduced temperature after expansion. Although the total heat transferred to the reheated steam is increased (Fig. 10(b) and S6), the mass flow rate of the reheated steam is also increased (Fig. S5), limiting the temperature increase in the reheated steam (Fig. S6). Due to the decrease in the live/reheated-steam temperatures, the corrected turbine efficiency decreases from 36.8% at MLD down to 36.3% with 13% PtG capacity (Fig. 11(a)), and the corrected plant efficiency for power generation decreases from 34.4% at MLD down to 34.1% with 13% PtG capacity (Fig. 11(b)). The efficiency penalty at the turbine side (attributed to PtG integration) is partially compensated by the energy-saving effect at the boiler side. The comparison between the integrated system and the standalone CFPP (neglecting the combustion constraint) shows that the integrated system can reduce the minimum load with a very limited efficiency penalty.

6. Conclusion In this study, a novel concept of integrating power-to-gas to coalfired power plant is proposed to accommodate renewable power at a large scale. With the integration, the operative range of coal-fired power plant is significantly enlarged by using the oxygen produced by the power-to-gas for combustion stabilization. The minimum load reduction, the performance variation of coal power plant caused by the integration, and the performance under different operating strategies are evaluated through a case study. Compared to the standalone powerto-gas, a larger margin for dispatching renewable power can be endorsed by the integration. The following conclusions are drawn: (1) The integration increases the combustion temperature, indicating an enhanced combustion. Increasing power-to-gas capacity below 13% (normalized) can reduce the minimum load from 40% (minimum load of design, 120 MW) down to 22.6% (68 MW). A further increase in the power-to-gas capacity is not beneficial for further load reduction, since the combustion stability is not the main constraint anymore. (2) With the integration of power-to-gas, the plant efficiency for power generation is slightly reduced from 34.4% at minimum load of design down to 34.1% with 13% of the normalized power-to-gas capacity. The efficiency penalty at the turbine side, caused by the superheat degree’s reduction of live/reheated steam, is partially compensated by the energy-saving effect at the boiler side (attributed to the reduction of heat loss by emitting flue gas). (1) The low-load operation of the coal power plant, especially when the load rate is lower than 29.2% with an air-fuel ratio of 1.6, is the main factor leading to the reduction of the plant efficiency and the temperatures of live/reheated steam, which limit the economical and safe operation. (2) The load of coal power plant that power-to-gas comes into service constrains the feasible range of power-to-gas dispatch, thus influencing the control of the integrated system. To achieve a higher system efficiency for renewable power accommodation, keeping a higher power-to-gas contribution and putting it into service at higher load of coal-fired power plant are preferable. However, to minimize the CO2 emission of the integrated system, the operating

5.3. Sensitivity analysis: Identification of optimal operating strategy As mentioned in Section 2.2, the capacity of excess power to be accommodated (ΔPERP , MW), the dispatch between PtG and CFPP, and the current load of CFPP determine the performance of the integrated system. To identify the optimal strategy of coordinating these three factors, sensitivity analysis is performed for the corrected overall efficiency and CO2 emission, as shown in Figs. 12 and S8: 8

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strategy of minimizing the power-to-gas contribution should be adopted.

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CRediT authorship contribution statement Yang Sun performed data curation and formal analysis, and prepared the original draft. Ligang Wang conceptualized the paper, supervised data curation and formal analysis, reviewed and edited the draft. Cheng Xu contributed to the review & editing. Jan Van herle, François Maréchal and Yongping Yang provided valuable supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The research leading to these results has received funding from the Science Fund for Creative Research Groups of the National Natural Science Foundation of China (Grant No. 51821004), Fundamental Research Funds for the Central Universities of China (No. 2019MS014). Yang Sun is also financially supported by the China Scholarship Council. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apenergy.2020.114608. References [1] Al-Habaibeh A, Shakmak B, Fanshawe S. Assessment of a novel technology for a stratified hot water energy storage - The water snake. Appl Energy 2018;222:189–98. [2] https://www.in-en.com/article/html/energy-2279149.shtml, 2019. [3] Prognos Fichtner. Flexibility in thermal power plants: with a focus on existing coalfired power plants. Agora Energiewende 2017. [4] Hentschel J, Babić UA, Spliethoff H. A parametric approach for the valuation of power plant flexibility options. Energy Rep 2016;2:40–7. [5] Henderson C. Increasing the flexibility of coal-fired power plants. IEA Clean Coal Centre 2014. [6] Zhang N, Lu X, McElroy MB, Nielsen CP, Chen X, Deng Y, et al. Reducing curtailment of wind electricity in China by employing electric boilers for heat and pumped hydro for energy storage. Appl Energy 2016;184:987–94. [7] Alizadeh MI, Parsa Moghaddam M, Amjady N, Siano P, Sheikh-El-Eslami MK. Flexibility in future power systems with high renewable penetration: a review. Renew Sustain Energy Rev 2016;57:1186–93. https://doi.org/10.1016/j.rser.2015. 12.200. [8] Saber H, Moeini-Aghtaie M, Ehsan M, Fotuhi-Firuzabad M. A scenario-based planning framework for energy storage systems with the main goal of mitigating wind curtailment issue. Int J Electr Power Energy Syst 2019;104:414–22. [9] Sun Y, Xu C, Xin T, Xu G, Yang Y. A comprehensive analysis of a thermal energy storage concept based on low-rank coal pre-drying for reducing the minimum load of coal-fired power plants. Appl Therm Eng 2019;156:77–90. [10] Xue Y, Ge Z, Yang L, Du X. Peak shaving performance of coal-fired power generating unit integrated with multi-effect distillation seawater desalination. Appl Energy 2019;250:175–84. [11] Heinzel T, Meiser A, Stamatelopoulos GN, Buck P. Einführung Eimühlenbetrieb in den Kraftwerken Bexbach und Heilbronn Block 7. VGB PowerTech 2012:79–84. [12] Fichera A, Pagano A. Application of neural dynamic optimization to combustioninstability control. Appl Energy 2006;83:253–64. [13] Chen L, Yong SZ, Ghoniem AF. Oxy-fuel combustion of pulverized coal: Characterization, fundamentals, stabilization and CFD modeling. Prog. Energy Combust. Sci. 2012;38:156–214. [14] Baukal C. Oxygen-enhanced combustion. CRC Press LLC; 1998. [15] Yan G. Research on combustion-supporting characteristic of oxygen in process of boiler starting-up and combustion-stabilizing. 'Vol.' Doctoral, North China Electric Power University; 2016. [16] Dalton A, Tyndall D. Oxygen enriched air/natural gas burner system development. VA: Springfield; 1989. [17] Zhang J. Study on the high efficiency and low NOx emission technology of local supporting-combustion by membrane oxygen-enrichment for the pulverized coal

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