Considering that traditional coal-fired power plants generate high carbon emissions at the same time, carbon capture equipment is introduced into traditional coal-fired power plants and transformed into carbon capture power plants. The power balance relation of carbon capture power plant is: P t = P t N + P F + P t C (1) Where P t indicates the total power of the system; P t N indicates net output of carbon capture power plants in t period; P F indicates fixed energy consumption, independent of running state, small value and fixed value; P t C indicates the capture energy consumption, including the loss of electricity needed to compress and process CO₂, which is proportional to the CO₂ capture amount in period t W t C O 2 (Peng et al., 2021), namely P t C = λ C W t C O 2 (2) Where λ C indicates the energy consumption of capturing CO₂ per unit mass.

Based on the carbon capture system, the liquid storage carbon capture system decouples the absorption and regeneration of the carbon capture system by introducing a set of lean and rich liquid memory between the absorption tower and the analytical tower. The schematic diagram of the liquid storage carbon capture system is shown in Figure 1. Where m t R − i n indicates the CO₂ inflow of the liquid-rich memory in t period, m t R − o u t indicates the CO₂ inflow of the liquid-rich memory in t period, m t A T indicates the amount of CO₂ treated by the absorption tower, m t R T indicates the amount of CO₂ treated by regeneration tower, the model of liquid-rich memory is as follows: m t A T = γ t ξ i P t (3) m t R − i n = γ t m t A T = γ t ξ i θ C P t (4) m t R T = m t R − o u t = W t C O 2 (5) Where γ t indicates the flue gas split ratio; ξ i indicates the carbon emission intensity factor for generating unit i; θ C indicates CO₂ capture rate;

CO₂ extracted from solution memory exists in the form of compounds in ethanolamine solution, the relationship between CO₂ capture W t C O 2 and the volume of ethanolamine solution at t period V M E A is: V M E A = ∑ t = 1 T W t C O 2 M M E A M C O 2 θ μ R δ R (6) Where M M E A indicates MEA molar mass; M C O 2 indicates molar mass of CO₂; θ indicates the capacity of regeneration tower; δ R indicates solution density.

In the Figure 2, the flexibility of absorbing CO₂ is limited by conventional carbon capture power plants. At peak load times, thermal power plants require higher net output power and produce more CO₂. If they need to be fully absorbed for capture treatment, larger capture rates are required. Energy consumption and load conflict. If a solution storage is added, CO₂ can be absorbed during this period, but no capture is required, that is, the capture energy consumption can be reduced during high-power operation, but carbon emissions can still be reduced. When the load is low and the thermal power plant is running at low power, increase the capture energy consumption to deal with excess CO₂.

The liquid-storage carbon capture system is used to capture the CO₂ generated in the power generation process of the unit to supply P2G, P2G uses chemical reactions to synthesize CO₂ and H₂ to supply CH₄ to the natural gas network, forming a LSCCS-P2G joint operation system. The block diagram of LSCCS-P2G combined operation system is shown in Figure 3.

In Figure 3, P2G technology constitutes a bidirectional coupled power—gas integrated energy system in the power system, which is of great significance. The P2G equipment uses the CO₂ captured by the LSCCS as the production raw material, which reduces the CO₂ raw material cost of P2G, decreases the carbon emissions of coal-fired units and environmental pollution. In addition, when the wind power is abundant, P2G converts the wind power with zero marginal cost into natural gas to supply gas turbines and gas loads, which reduces the impact of renewable energy on the power grid and improves the stability of power system input. LSCCS uses lean and rich liquid storage to store part of CO₂ in peak load period and increases CO₂ processing capacity in low load period, which flexibly adjusts carbon capture energy consumption and changes unit net output. Under the demand of system peak shaving, the coordinated operation of the two significantly reduces the carbon emissions of the system and improves the flexibility of the system operation.

As an important coupling equipment in IEGS, P2G uses chemical reaction to synthesize natural gas from hydrogen and carbon dioxide. The specific reaction equation is as follows: 2 H 2 O → o p e n   t e l e g r a m 2 H 2 + O 2 (7) C O 2 + 4 H 2 → h i g h   t e m p e r a t u r e h i g h − p r e s s u r e C H 4 + 2 H 2 O (8)

In this process, the relationship between the amount of CO₂ that P2G can use in period t W t P 2 G and the generated natural gas Q P 2 G , t is: W t P 2 G = 3.6 ρ C O 2 Q P 2 G , t (9) Q P 2 G , t = φ P 2 G P P 2 G , t H G V (10) Where ρ C O 2 indicates the density of CO₂; φ P 2 G indicates P2G conversion efficiency, with 0.6; P P 2 G , t indicates the power consumed by P2G during t period.

In order to achieve low carbon power, the economic operation of electric-gas integrated energy system needs to consider carbon emissions. Compared with the high carbon emissions of coal-fired units, the CO₂ emissions of gas turbines are less and negligible. In order to mobilize the enthusiasm of power generation enterprises to reduce emissions, the cost of carbon emissions should be considered into total cost.

The carbon trading system treats carbon emissions as a freely traded good. The regulatory authorities divide up the overall amount of carbon emissions among the many carbon sources in order to limit it. If real carbon emissions from carbon sources exceed the allotted amount, the surplus must be purchased. On the other hand, the surplus can be sold in the market for carbon trading when the actual carbon emissions of carbon sources are lower than the carbon allotment (Wei et al., 2022). This carbon trading mechanism not only penalizes businesses with excessive carbon emissions, but it also inspires power producing businesses to conserve energy and cut emissions.

Under the carbon trading mechanism, economic penalties are imposed on carbon sources exceeding quotas in power plants. The cost of carbon emissions in power plants C _T can be expressed as: C T = K C ( Q C − Q C C S − Q C I N I ) (11) Where K C indicates carbon trading price coefficient; Q C indicates all-day carbon emissions of coal-fired units; Q C C S indicates carbon capture in carbon capture systems; Q C I N I indicates free carbon emission quota for coal-fired units. Q C = ∑ t = 1 T ∑ i = 1 N λ i P i , t Δ t (12) Q C C S = ∑ t = 1 T ∑ j = 1 N C C S W j , t C O 2 = ∑ t = 1 T ∑ j = 1 N C C S P j , t C / λ C (13) Q C I N I = ∑ t = 1 T ∑ i = 1 N λ h P i , t Δ t (14) Where λ i indicates the carbon emission intensity coefficient of the ith coal-fired unit, λ h indicates the initial quota coefficient of carbon emission per unit power of coal—fired units, W j , t C O 2 indicates the CO₂ captured by the jth carbon capture unit at time t, P j , t C indicates the capture energy consumption of the jth carbon capture unit.

In the carbon capture system, the carbon storage cost C s t o r e is: C s t o r e = ∑ t = 1 T c s t o r e ( Q c , t − Q u , t ) (15) Where c s t o r e indicates carbon storage factor, Q c , t indicates CO₂ capture by carbon capture equipment at time t, Q u , t indicates CO₂ utilization for P2G equipment.

Comprehensive carbon cost C 2 is expressed as: C 2 = C T + C s t o r e (16)

In this paper, the minimum operating cost and carbon emission cost of electric-gas integrated energy system are taken as the optimization objectives. min ⁡ F = min ( C 1 + C 2 ) (17) Where C 1 indicates system operating costs, including the operation cost of thermal power plants C O P , depreciation cost of carbon capture equipment C Z , abandoned wind cost C Q , gas source point purchase cost C g a s and CO₂ raw material cost of P2G C P 2 G ; C 2 indicates carbon emission cost, mainly including carbon transaction cost and carbon storage cost.

The power grid and the natural gas network are connected by P2G and gas turbine in this study, which adopts the enhanced IEEE 30 - bus power grid and the natural gas network with a single gas source point. Figure 4 depicts the combined electric-gas energy system. In this illustration, the thermal power plant G1 is converted into a liquid storage carbon capture power plant with a maximum energy consumption of 200 MW, a solution memory volume of 22,000 * 2 m³, an initial storage capacity of 11,000 m³, and a total cost of 11.7971 million dollars for the carbon capture equipment. The document contains the liquid storage carbon capture system’s parameters (Cui et al., 2021a; Chen et al., 2021). G2, a gas turbine, while G3, G4, and G5 are regular coal-fired units. Table 1 lists the necessary generator set specifications. Node 9 links 300 MW wind farms and 200 MW P2G units. The gas source point quotation is 4$/kcf, and the CO₂ raw material cost factor for P2G is 20 $/MW; Maximum output of a gas source point is 15,000 m³/h, with a minimum production of 1,000 m³/h (Cui et al., 2021b); CO₂ storage costs 4.89 ($/t), and the penalty cost of wind curtailment is 100/($/MWh). In order to run IEGS as efficiently as possible over a 24-h day, this study uses a 1-h scheduling window. Figure 5 displays the system’s predictions of the electric load, gas load, and wind power. The model is optimized by CPLEX.

In order to verify the effectiveness of the established model, four scenarios are set to calculate and analyze the economic operation cost and carbon emissions of the system.

Scenario 1: IEGS does not contain carbon capture devices, the objective function contains only the operating cost of the system;

Scenario 2: IEGS contains carbon capture devices, the objective function contains only the operating cost of the system;

Scenario 3: IEGS contains liquid storage carbon capture equipment, the objective function contains only the operating cost of the system;

Scenario 4: IEGS contains liquid storage carbon capture equipment and introduces carbon trading mechanism, the objective function contains the operating cost and carbon cost of the system;

In Scenario 1, the changes of system abandonment air volume and total system cost under different P2G capacities are analyzed, as shown in Figure 6.

From Figure 6, it is clear that as the P2G capacity of the system is increased, the system’s total operation cost gradually lowers. However, when the P2G capacity in the system reaches a certain value, the system’s total operational cost reduces, because the P2G operating cost gradually rises as P2G capacity increases. Therefore, the P2G capacity setting should not be too high because the system’s wind power consumption and operational economy are limited by the choice of P2G physical capacity. To address this issue, a carbon capture technology is presented in this work. Although the operation of the carbon capture equipment literally consumes the thermal power units’ output, it can indirectly use the system’s wind power to reduce the amount of wind curtailed. The operation cost of P2G is significantly decreased by using the captured CO₂ as raw material.

In order to study the effect of CCS and LSCCS combined with P2G on the total cost of the system, wind power consumption and carbon emissions, the maximum capacity of P2G is set to 200 MW, the CO₂ raw material cost coefficient of P2G in scenario 1 is 20 $/MW, and the CO₂ raw material of P2G in scenario 2,3 and 4 is provided by the carbon capture system. The results of the system optimization are shown in Table 2:

Table 2 shows that Scenario 2’s thermal power operation costs are 1.695% more than Scenario 1’s, while carbon emissions are reduced by 8.64% and the cost of wind curtailment is decreased by 51.56%. This is due to the fact that, in comparison to conventional thermal power plants, carbon capture power plants must supply additional carbon capture energy consumption in addition to carrying electric load, which raises the operating expenses of thermal power plants. A carbon capture system, on the other hand, collects CO₂ to lower system carbon emissions and delivers collected CO₂ to P2G. Currently, the carbon capture system and P2G work together to consume wind energy, which lessens the physical impact of P2G capacity on wind energy consumption and lowers the cost of wind curtailment. On the basis of Scenario 2, Scenario 3 adds the memory for the answer. In comparison to Scenario 2, the cost of wind curtailment is lowered by 55.99%, carbon emissions are reduced by 1.7%, and the overall cost of the system is increased by 0.05%. On the basis of Scenario 3, Scenario 4 introduces the carbon trading mechanism. The advantages of the optimization model suggested in this study for low-carbon economy scheduling are demonstrated by the reduction of carbon emissions by 26.26% and the reduction of the system’s overall cost by 3.03% when compared with Scenario 3.

Four scenarios’ actual carbon emissions are shown in Figure 7 for each time period. Figure 7 demonstrates that the hours of low load and high wind power generation are from 3:00 to 6:00, and that scenario 3 has fewer carbon emissions than scenario 2. For this reason, scenario 3 transfers unprocessed CO₂ from the peak load period to the low load period for processing using solution memory’s “energy time shift” capabilities. This increases the carbon capture equipment’s capacity during the low load period, raises its energy consumption, and lowers the system’s carbon emissions. In order to increase the net output of the system and fulfill load demand during the peak load period of 10:00–12:00, when wind power output is low, the carbon capture unit must lower the capture energy consumption. Scenes 2 and 3 have higher carbon emissions since the liquid storage carbon capture equipment and carbon capture equipment’s current energy usage is quite low. On the basis of scenario 3, scenario 4 introduces a carbon trading mechanism that takes into account the total cost of carbon in the objective function to further increase the capacity of the carbon capture system. This will decrease the net output of the carbon capture unit, open up more space for clean energy wind power, and further reduce the system’s carbon emissions.

The larger the solution storage capacity, the smaller the carbon emissions, but it will bring high investment and carbon capture depreciation costs. If the low carbon and economy are considered at the same time, there is an optimal capacity. In scenario 4, the relationship between different solution storage capacities and total system costs and carbon emissions is shown in Figure 8.

Figure 8 illustrates how the system’s carbon emissions exhibit a trend toward reduction and eventually tend to level off when the solution storage capacity is increased. The capacity of the solution memory is related to the time shift of the carbon capture device. The carbon capture device’s temporal shift increases with the size of the solution memory. Since the system has enough capacity to produce CO₂ transfer, carbon emissions will not decrease when the capacity exceeds 22000 m³. The system’s overall cost initially declines and then rises, reaching a minimum at 14,000 m³. The depreciation expense of the carbon capture equipment can currently be compensated for by the carbon benefit provided by the solution memory capacity. The total cost of the system has increased dramatically as solution memory capacity has increased, showing that this cannot completely offset the expense of carbon capture equipment depreciation.

At present, in China, carbon trading is in the exploratory stage, and the price fluctuates. In order to analyze the impact of carbon trading price changes on the optimal operation of the system, the example sets the carbon trading price from 4–18 $/ton, and the calculation results are shown in Figure 9.

With the increase of carbon trading price, the system carbon emissions gradually decreased, and finally stabilized; the total cost of the system shows a trend of ' increase first and then decrease '. This is because when the carbon trading price is 4–10$/ton, carbon transaction costs account for a small share of total costs. Revenue from reduced carbon emissions cannot offset depreciation costs for carbon capture equipment. Therefore, the total cost of the system shows an increasing trend. When the carbon trading price exceeds 10$/ton, the proportion of carbon capture income in the total cost of the system is increasing, in order to minimize the total cost of the system, increase the output of low carbon units and carbon capture income, so that the system carbon emissions are less than quotas, the system produces carbon trading income, and the total cost is reduced. When the carbon trading price is 10–14$/ton, carbon trading price changes have a great impact on the total cost of the system; When the carbon trading price exceeds 14$/ton, the continuous increase in carbon trading prices will not affect CO₂ emissions from the system, subject to maximum capture capacity.