Carbon capture technologies include pre-combustion, oxygen-enriched combustion, and post-combustion. Among them, post-combustion capture has been widely used due to its low retrofitting cost and broad applicability and is thus adopted in this study.

Figure 1 shows the brief scheme of CCS. The cooled flue gas enters the absorber tower from the bottom and reacts chemically with the absorbent sprayed from the top. CO₂ is captured by the chemical absorbent inside the tower, and the flue gas is led out of the absorbent tower and exhausted to the atmosphere through a cooler unit. The CO₂-rich absorber solution, which is called rich liquid, is pumped to the regeneration tower to separate CO₂. The discharged CO₂ gas is liquefied, pressurized, and sealed. The separated solvent is re-boiled and pressurized for recycling. The energy supply of the reboiler comes from the extracted steam at the outlet of the immediate pressure cylinder. The exothermic steam is returned to the regenerative heating system of the CPP unit.

The CCS system can consume and store the CO₂ exhausted by the power plants. Before studying the energy consumption of the CCS system, the total amount of CO₂ must be analyzed. The CO₂ exhausted from the units consists of two main components: one is generated through coal combustion, and the other is through the desulfurization process. The flow rate of CO₂ generated by the coal combustion process ( D i , CO 2 , 1 , t C C P P ) can be calculated as follows: D i , CO 2 , 1 , t CCPP = M CO 2 M C δ C B i , t CCPP (1) where M is the relative molecular mass, δ C is the percentage of carbon element in the coal, B i , t C C P P is the mass flow rate of the ith CCPP unit at time t.

During the desulfurization process, the flow rate of the CO₂ generated by the desulfurization process is related to SO₂. For per mol of SO₂ desulfurization, 1 mol of CO₂ is produced. Therefore, the flow rate of the CO₂ generated by the desulfurization ( D i , CO 2 , 2 , t C C P P ) is calculated as follows: D i , CO 2 , 2 , t CCPP = M r , CO 2 M r , SO 2 D i , SO 2 , t CCPP (2) where D i , SO 2 , t C C P P is the mass flow rate of SO₂ of the ith CCPP unit at time t.

Combined with the above equations, the amount of CO₂ emission ( D i , CO 2 C C P P ) can be expressed as follows: D i , CO 2 , t CCPP = D i , CO 2 , 1 , t CCPP + D i , CO 2 , 2 , t CCPP (3)

The flow of flue gas entering the CCS system can be adjusted by the inlet regulating valve. If the valve’s position is μ i , t C C P P , the flow of CO₂ discharged into the atmosphere ( D i , CO 2 , r , t C C P P ) is obtained as follows: D i , CO 2 , r , t CCPP = ( 1 − η i , CO 2 CCPP μ i , t CCPP ) D i , CO 2 , t CCPP (4) where η i , CO 2 C C P P is the efficiency of the CCS.

The decarburization efficiency and reboiler temperature are controlled in the carbon capture process by adjusting the lean liquid flow rate and the extraction steam flow rate (Wu et al., 2018). Assuming that the decarburization efficiency remains unchanged, the extraction steam flow rate can be expressed by the captured CO₂ flow rate. The influence of the extraction steam flow rate on the electric power output can refer to the static analysis of the power-heat conversion of the CHP unit. Therefore, the electric power consumed by the carbon capture process ( P i , O , t C C P P ) can be expressed as: P i , O , t CCPP = μ i , t CCPP m i , CO 2 CCPP k i , CO 2 CCPP η i , CO 2 CCPP D i , CO 2 , t CCPP (5) where k i , CO 2 C C P P is the extraction steam flow rate required to capture a unit of carbon, m i , CO 2 C C P P is the electric power corresponding to unit extraction steam flow.

In addition, the start-up of the CCS system requires a power supply ( P C C S ), which is independent of the unit’s operating conditions and can be considered a constant. Thus, the net power output of CCPP unit ( P i , E , t C C P P ) can be obtained as follows: P i , E , t CCPP = P i , t CCPP − P i , O , t CCPP − P CCS (6) where P i , t C C P P is the amount of electric power of the CCPP unit.

The relationship between the net electric power and flow rate of the captured CO₂, called power-carbon characteristic, can be obtained in Figure 2. In this figure, the area A ^ B ^ C ^ D ^ indicates the electric power adjustment range of the unit during the operation of the CCS system. A ^ indicates the maximum power output of the boiler in CCPP. As the flow rate of captured CO₂ increases, the net power output of CPPP will decrease, which is represented by the line A ^ B ^ in Figure 2. At the point B ^ , all the flue gas will enter the carbon capture system for carbon absorption and removal, and the flow rate of CO₂ produced by combustion at this time is D i , CO 2 , max C C P P . Consequently, the expression of line A ^ B ^ is: P i , E , t CCPP = − m i , CO 2 CCPP k i , CO 2 CCPP ⋅ η i , CO 2 CCPP D i , CO 2 , t CCPP + P i , max CCPP − P CCS (7) where P i , max C C P P is the maximum power output of the ith CCPP.

Similarly, the line D ^ C ^ means that the boiler operates in the minimum load and the flow rate of CO₂ in the flue gas is D i , CO 2 , min C C P P . At this time, the relationship between the net power output of the unit and CO₂ flow rate can be expressed as: P i , E , t CCPP = − m i , CO 2 CCPP k i , CO 2 CCPP ⋅ μ i , t CCPP η i , CO 2 CCPP D i , CO 2 , t CCPP + P i , min CCPP − P CCS (8) where P i , min C C P P is the minimum power output of the ith CCPP.

It is noted that the CCS system has the maximum capture ratio at the line B ^ C ^ , that is, the regulating valve of the flue gas entered CCS system has the largest opening. As the capture ratio decreases, the line B ^ C ^ moves to the left and coincides with A ^ D ^ when the capture ratio is zero. At any intermediate point, when the capture rate is μ i , t C C P P , the abscissas of the upper and lower boundary points of the CCPP are B ^ 1 and C ^ 1 , whose abscissas are μ i , t C C P P ⋅ η i , CO 2 C C P P ⋅ D i , CO 2 , max C C P P and μ i , t C C P P ⋅ η i , CO 2 C C P P ⋅ D i , CO 2 , min C C P P , respectively.

Based on the above analysis, with the condition that the carbon capture rate is maintained constant during the entire power system dispatch, the spinning reserve capacities of CPP and CCPP at each carbon capture ratio can be obtained, which is shown in Figure 3.

It can be seen from Figure 3 that when the carbon capture system is not added, the CPP unit has no additional power consumption and the adjustable capacity has been maintained at P i , max C C P P − P i , min C C P P . After introducing the carbon capture system, the regulation capacity of the CCPP unit ( Δ P i , t C C P P ) will also change under different capture rate conditions, which can be expressed as: Δ P i , t CCPP = − m i , CO 2 CCPP k i , CO 2 CCPP η i , CO 2 CCPP ( D i , CO 2 , max CCPP − D i , CO 2 , min CCPP ) ⋅ μ i , t CCPP + ( P i , max CCPP − P i , min CCPP ) (9)

From Eq. 9, it can be seen that if the carbon capture rate cannot be dispatched, the adjustment range of the unit will decrease as the capture rate increases. However, by adjusting the boiler load and the inlet flue gas regulating valve of the carbon capture system, the CO₂ emission flow rate can be reduced, and the net power connected to the power grid can be adjusted flexibly. Figure 4 shows the maximum regulation capacities of CPP ( Δ P i , max C P P ) and CCPP ( Δ P i , max C C P P ). Through calculation, we can get: Δ P i , max CPP = P i , max CCPP − P i , min CCPP (10) Δ P i , max CCPP = P i , max CCPP − P i , min CCPP + m i , CO 2 CCPP k i , CO 2 CCPP μ i , t CCPP η i , CO 2 CCPP D i , CO 2 , min CCPP (11)

Therefore, compared with CPP, CCPP has a broader power regulation range after introducing the carbon capture system, which provides more flexibility for the operation of the power system and is conducive to consuming more renewable energy.

As a direct factor affecting the concentration of pollutants, the mass flow rates of SO₂ and NO _x are related to the percentage of sulfur and nitrogen in coal, respectively. D i , SO 2 , t CCPP = M SO 2 M S δ S K SO 2 CCPP B i , t CCPP (12) D i , NO x , t CCPP = M NO x M N δ N K NO x CCPP B i , t CCPP (13) where δ is the percentage of each element in the coal, K SO 2 C C P P and K NO x C C P P are the SO₂ and NO_x conversion rates, respectively.

Generally, the main components of NO _x are NO and NO₂. If the ratio of NO in NO _x is λ , we can obtain: M NO x = λ M NO + ( 1 − λ ) M NO 2 (14)

Another factor that affects the concentrations of pollutants in the flue gas is the volume flow rate of flue gas. It is related to the real-time coal consumption of the unit, which can be expressed as: V i , F G , t CCPP = V FG B i , t CCPP (15) where V F G is the coefficient of the volume flow rate of flue gas and coal consumption, which consists of three aspects: theoretical flue gas volume ( V F G 0 ), excess air volume, and water vapor substituted with excess air. That is: V FG = V FG 0 + ( α − 1 ) V 0 + ρ air , d ρ H 2 O ( α − 1 ) V 0 (16) where α is the coefficient of excess air, V F G 0 is the theoretical flue gas volume, V 0 is the theoretical volume of the air consumed by combustion, ρ a i r , d and ρ H 2 O are the densities of dry air and water steam, respectively.

The theoretical air volume flow required in the coal combustion process depends on the content of each element in the coal, which can be expressed as: V 0 = ( M O 2 M C δ C + M O 2 4 M H δ H + M O 2 M S δ S + M O 2 M N ( 1 − λ ) K NO x δ N + M O 2 2 M N λ K NO x δ N − δ O ) ( 0.21 ρ O 2 ) (17) With the theoretical air supplied, if the coal is entirely burned, the flue gas is composed of CO₂, SO₂, NO _x , N₂, and H₂O, and theoretical flue gas volume is expressed as follows: V FG 0 = V CO 2 + V SO 2 + V NO x + V N 2 0 + V H 2 O 0 (18) where V CO 2 , V SO 2 , V NO x , V N 2 0 , and V H 2 O 0 are the volumes of CO₂, SO₂, NO _x , N₂, and H₂O in the flue gas, respectively, which can be expressed as follows: V CO 2 = M CO 2 M C ρ CO 2 δ C (19) V SO 2 = M SO 2 M S ρ SO 2 δ S (20) V NO x = M NO x M N ρ NO x K NO x δ N (21) V N 2 0 = 0.79 V 0 + 1 ρ N 2 ( 1 − K NO x ) δ N (22) V H 2 O 0 = ( M H 2 O M H δ H + δ H 2 O + ρ air , d dV 0 ) ρ H 2 O (23)

The wet desulfurization method of limestone has been widely used in the desulfurization process of CCPPs. During the desulfurization process, every mol of SO₂ will consume 1 mol of CaCO₃. Thus, the mass flow rates of CaCO₃ in the i th CCPP ( D i , CaCO 3 , t C C P P ) and the j th CHP ( D j , CaCO 3 , t C H P ) are calculated as follows: D i , CaCO 3 , t CCPP = η i , SO 2 CCPP M CaCO 3 M SO 2 D i , SO 2 , t CCPP (24) where η i , SO 2 C C P P is the desulfurization efficiency of the i th CCPP unit.

Generally, the SCR reactor is used for denitrification. NH₃ can reduce NO _x to N₂ under the action of the catalyst. During the reaction process, 1 mol of NH₃, and per mol of NO₂ consume 2 mol of NH₃. Thus, the mass flow rate of NH₃ in i th CCPP ( D i , CaCO 3 , t C C P P ) is expressed as follows: D i , NH 3 , t CCPP = 1 η i , NO x CCPP M NH 3 M NO x ⋅ [ λ + ( 1 − λ ) × 2 ] D i , NO x , t CCPP (25) where η i , NO x C C P P is the denitrification efficiency of the i th CCPP unit.

After desulfurization and denitrification, the concentrations of SO₂ ( C C i , SO 2 , t C C P P ) and NO _x ( C C i , NO x , t C C P P ) at the FGD and SCR outlet of the i th CCPP can be obtained as follows: CC i , SO 2 , t CCPP = ( 1 − η i , SO 2 CCPP ) D i , SO 2 , t CCPP V i , FG , t CCPP (26) CC i , NO x , t CCPP = ( 1 − η i , NO x CCPP ) D i , NO x , t CCPP V i , FG , t CCPP (27)

Similarly, for the j th CHP unit, the concentrations of SO₂ ( C C i , SO 2 , t C C P P ) and NO _x ( C C i , NO x , t C C P P ) at the FGD and SCR can be obtained.

As seen from Eqs 24–27, the ultra-low emission concentration is determined by the desulfurization and denitrification efficiencies. The consumption of limestone and ammonia injection varies with emission requirements.

The integrated energy system containing electricity and heat is required to maintain the energy supply and demand balance in a specific area. Figure 5 shows the structure of a low-pollutant and low-carbon IES, which includes conventional IES and carbon and pollutant reductions systems. In the IES, the distributed power load is supplied by various forms of power generations, such as CCPPs, CHP units, WTs, and PVs. The district thermal load of consumers is provided by the CHP units. In terms of curbing pollutant emissions, CCPPs integrate CPP and CCS to effectively reduce carbon emissions, and the high FGD and SCR efficiencies allow for a remarkably reduction of pollutant emissions. In addition, introducing a BES system stabilizes the fluctuation of RESs, and TES system facilitates the heat–power decoupling of CHP units and maintains a specific peak regulation capacity during the heating supply season.

In the succeeding sections, each subsystem’s operating cost and constraint in an integrated energy system will be analyzed step by step.

The proposed energy management system aims to supply both thermal and electric power with respect to the economic and environmental criteria. The objective function is the operating costs of the IES. It involves the cost of various energy form generations. The objective function (J) can be written as follows: J = min ( C 1 + C 2 + C 3 ) (28) where C ₁ is the operating cost of the CCPP and CHP units, C ₂ is the renewable curtailment penalty cost, and C ₃ is the pollutant treatment cost of the CCPP and CHP units.

The constraints of the objective function are listed as follows.

(1) Power balance

During the operation of IES, the supply and load of electricity and heat need to be balanced separately: P t D = ∑ i = 1 N G P i , E , t CPPP + ∑ j = 1 N C P j , t CHP + ∑ k = 1 N W P R , k , t W + ∑ l = 1 N P V P R , l , t P V − P t ES (36) H t D = ∑ j = 1 N C H j , t CHP − P t TS (37) where P t D and H t D are the electric and thermal power load demand, respectively. P t ES and P t TS are the charge or discharge power of BES and TES, respectively.

(2) Operating region

The power generation units cannot exceed their upper and lower limits during operation. For CCPPs, WT, and PV plants, the limits are listed as follows: P i , min CCPP ≤ P i , t CCPP ≤ P i , max CCPP (38) P k , min W ≤ P R , k , t W ≤ P k , max W (39) P l , min PV ≤ P R , l , t PV ≤ P l , max PV (40) where subscripts min and max are the minimum and maximum powers of various power generations, respectively.

The electric power output of the CHP units is related to the thermal power output. The operating region can be represented as follows: P j , t CHP = ∑ ζ = 1 Ζ ξ j , ζ P j , ζ , e , t CHP (41) H j , t CHP = ∑ ζ = 1 Ζ ξ j , ζ H j , ζ , e , t CHP (42) where ( P j , ζ , e , t C H P , H j , ζ , e , t C H P ) is the ζth extreme point of the j th CHP unit. Ζ is the number of extreme points. ξ j , m is the coefficient of the j th CHP unit’s operating region and its constraint can be expressed as follows: ∑ ζ = 1 Ζ ξ j , ζ = 1,0 ≤ ξ j , ζ ≤ 1 (43)

(3) Power ramp rate

For the coal-fired power plants, the power output ramp rate is limited within a specific range to ensure the safe operation of the unit. − Δ P i , E , max CCPP ≤ P i , E , t CCPP − P i , E , t − 1 CCPP ≤ Δ P i , E , max CCPP (44) − Δ P j , max CHP ≤ P j , t CHP − P j , t − 1 CHP ≤ Δ P j , max CHP (45) where Δ P i , E , max CPP and Δ P j , E , max C H P represent the maximum ramp rates of the i th CCPP and j th CHP units, respectively.

(4) Pollutant ultra-low emission

During the operation of coal-fired units, the concentrations of pollutant emissions must meet the environmental requirements. min ( CC i , SO 2 , t CCPP , CC j , SO 2 , t CHP ) ≤ CC SO 2 (46) min ( CC i , NO x , t CCPP , CC j , NO x , t CHP ) ≤ CC NO x (47) where C C SO 2 and C C NO x are the ultra-low emission standards for SO₂ and NO _x , respectively.

(5) Energy Storage System

Given that ESS can be used as a load device during charging and as a power or thermal source during discharging, it is often used as a flexible regulating system and is widely used in the dispatch of the IES. During the operation of ESS, its capacity has a reasonable boundary to expand the lifetime, which is expressed as follows: E min ESS ≤ E t ESS ≤ E max ESS (48) where E t E S S is the state capacity of ESS at time t . The subscripts min and max represent the minimum and maximum capacities of the ESS, respectively.

The state of ESS is related to the state of the previous moment and the rate of charge and discharge and can be calculated as follows: E t ESS = E t − 1 ESS + η C ESS P t ESS , P t ESS > 0 E t ESS = E t − 1 ESS + P t ESS η D C ESS , P t ESS < 0 (49) where η C E S S and η D C E S S are the charge and discharge efficiencies, respectively. P t E S S is the charge and discharge rate whose reasonable boundary is as follows: − P DC , max ESS ≤ P t ESS ≤ P C , max ESS (50) where P C , max E S S and P D C , max E S S are the charge and discharge maximum rates of the ESS, respectively.

To prepare for the next dispatch, the final state capacity should be equal to the initial capacity of ESS, which is: E 0 ESS = E T ESS (51) where the subscripts 0 and one represent the initial and final moment in the dispatch period.

After obtaining the objective function and operating constraints, the optimal dispatch of IES for economic and environmental benefits can be solved by fmincon solver in MATLAB, which is an effective method to solve nonlinear optimization problems.

The IES in this study has 5 × 600 MW CCPPs, 7 × 90 MW wind farms, 7 × 60 MW PV farms, and 3 × 200 MW extraction CHP units. The proportion of RESs reaches 32.2%. The maximum energy capacities of BES and TES systems are 50 and 150 MWh, with maximum energy conversion rates of 25 and 60 MW, respectively. η C E S S and η D C E S S are set as 95%.

For computational convenience, the CCPP and CHP units have the same characteristics, respectively. The coal consumption rate of the CCPPs at the typical power is shown in Supplementary Table S1. The coal consumption rate coefficient of the CHP units is set based on (Haghrah et al., 2016).

The penalty coefficients are the critical parameters, indicating that IES will try to reduce renewable energy curtailment. To promote renewable energy penetration, while considering the appropriate proportion of the curtailment cost renewable energy in the total cost, K W and K P V are set as 80$/MW in this study.

The parameters of decarburization, desulfurization and denitrification are shown in Supplementary Table S2. For the CCS, this study assumes that the carbon capture efficiency remains unchanged, so that the CCS can adjust the capture ratio to consume renewable power generation.

The day-ahead dispatch of the IES is based on the forecasted renewable energy output and load demand for each period of the day in the future. Supplementary Figures S1, S2 show that the forecast load demand and renewable power generations, respectively.

To analyze and verify the impact of CCS on the IES dispatch, two scenarios are set up. Keeping the other elements in IES consistent, the conventional power plants and carbon capture power plants are used for Scenarios 1 and 2, respectively.

Figures 6, 7 show that the power dispatch results in Scenarios 1 and 2, respectively. From these figures, in the morning and evening, the load demand is low, while the wind power output is high at this time, resulting in an imbalance between power demand and supply. In addition, the PV power output reached its maximum at noon, which caused the power grid not to be able to consume it entirely. Due to the power regulation of CCS, the net output of CCPPs can be further reduced, enabling more wind and PV power output to be connected to the power grid.

The high wind power in the morning and evening requires the CHP units to have a low electric power output. Meanwhile, the thermal power output of the CHP units must be sufficient to meet the higher thermal load requirements in the morning and evening. Figure 8 shows the dispatch results of thermal demand and supply. This figure indicates that the heat output of the CHP units is less than the load in the morning and evening, and TES compensates for this difference by exothermic heat. Similarly, in the middle of the day when the heat demand is low, the CHP units generate excess heat to supply the heat storage of TES. Through heat release and storage, TES can achieve heat–power decoupling.

According to the results of electric power dispatch, the addition of carbon capture systems can promote the penetration of renewable energy. Figures 9, 10 show the renewable energy output under Scenarios 1 and 2. From this figure, in Scenario 2, the power acceptance capacity of renewable energy increases, and the curtailment rates of wind and PV power decrease. Compared with the forecast data of renewable power generation, the penetration rates of wind and PV power in the IES can be increased by 10.61 and 3.78%, as shown in Figure 11.

Figure 12 shows the carbon emissions of the IES in a dispatch cycle under Scenarios 1 and 2. From the figure, at each dispatch time, the CO₂ emissions under Scenario 2 are about 300 t less than that under Scenario 1. During the dispatch period, the total CO₂ emissions of IES in Scenario 2 are reduced by 28,812.7 t compared with Scenario 1.

The costs of IES in Scenarios 1 and 2 are shown in Table 1. From this table, after introducing the carbon capture system, the direct operating cost and pollutants treatment cost tend to increase due to the power consumption of CCS. However, the IES in Scenario 1 has a higher penalty cost for renewable energy curtailment, resulting in a higher total cost than Scenario 2.

Pollutant ultra-low emissions is achieved by retrofitting the pollutant treatment system to improve the efficiencies of FGD and SCR. Ignoring the retrofitting cost, we will only analyze the difference in the pollutants treatment cost before and after retrofitting in this study. Table 2 represents the pollutant parameters before and after ultra-low emission retrofitting. It can be seen that, the efficiencies of FGD and SCR, respectively increase by 2.882 and 5.949% after the retrofit. Due to the reduction in pollutant emissions, more SO₂ and NO _x have to be removed by FGD and SCR, resulting in more treatment consumption of 2,899.6 USD.

The pollutant emissions in each dispatch moment before and after ultra-low emission are represented in Figure 13. From this figure, pollutant emissions have been significantly reduced after the renovation. By calculation, SO₂ and NO _x are reduced by 1.87 t and 6.24 t, respectively.