Low‐carbon economic optimization method for integrated energy systems based on life cycle assessment and carbon capture utilization technologies

Integrated energy system (IES) is one of the ways to realize energy saving and emission reduction. Facing the crisis of fossil energy depletion and the challenge of global warming, low‐carbon technology and market economic guidance mechanism have an important impact on the low‐carbon operation of integrated energy system (IES). In this study, IES with carbon capture and utilization (CCU) technology is established, the carbon emission model based on life cycle theory is built for the input energy and energy system of IES, and the ladder carbon trading mechanism is considered in the market economy to establish the low‐carbon economic optimization model of IESs. A system combining carbon capture technology and power to gas can effectively consume new energy sources and provide energy dispatch flexibility. The ability to capture greenhouse gases generated by energy units reduces the system's carbon emissions. Based on the IES low carbon economy model, the model is analyzed with examples to analyze the correlation between different carbon trading prices and IES low carbon economy transport. We analyze the impact of CCU technology inputs on the low‐carbon nature of IES. The study proposes an IES dispatching model based on life cycle assessment and CCU technology to provide a new strategy for the operation of IESs and a new method for other energy systems to reduce low carbon emissions.

The current rapid socioeconomic development and expanding energy demand are posing a serious threat to sustainable development due to increasing energy shortage and environmental warming. 1,2In response to the current energy crisis and environmental pollution, China has announced in the United Nations General Assembly that "carbon dioxide emissions will strive to peak by 2030 and achieve carbon neutrality by 2060." 3 Achieving efficient utilization and low carbon development of energy systems is a current research hotspot.
Integrated energy system (IES) is composed of electric power system, natural gas system and heating (cooling) system, which satisfies the demand of load through the mutual conversion and storage of multiple energy sources.IES has various types of energy utilization and conversion equipment, gives full play to IES has multiple types of energy utilization and conversion equipment, giving full play to the complementary characteristics and synergy of different energy forms, realizes the optimal allocation of system energy, improves system flexibility, and thus achieves the purpose of energy saving and emission reduction, 4 which has important research significance and application value.
IES has remarkable low-carbon emission reduction capability, and scholars at home and abroad are currently conducting in-depth research on IES, [5][6][7][8] and the development of low-carbon IES has become a research hotspot in the energy field.The current research results, on the one hand, focus on the modeling of systems for multiple energy types of equipment.Combined cooling, heating, and power (CCHP) systems, as a distributed energy system with multiple forms of energy use and conversion, is more often used in IESs.Ghanem et al. proposed an optimal scheduling model combining PV generation and CHP to improve the consumption of renewable energy, 9 Li et al. extends the coupled power-to-gas (PtG) and carbon capture and storage (CCS) technologiesto the hybrid CSP-CHP IES. 10 On the other hand focus on the solution method of system optimal scheduling.Xu et al. proposed a deepening intensity learning based optimal scheduling method for intelligent park energy management. 11Zhu et al. used multilayer deep Q-networks to solve the multi-park IES cooperative control problem. 12ao et al. constructed a PIES two-stage robust optimization model in the face of source-load uncertainty scheduling problem, using column-and-constraint generation algorithm to solve. 13ith global warming, sea level rise and increasingly severe environmental pollution, the issue of greenhouse gas emissions has received widespread attention.The existing research work on low-carbon IESs mainly focuses on the low-carbon dispatch of energy systems.To reduce the carbon emissions of the IES, Yang et al. proposed a low-carbon dispatch model for system operation by introducing the carbon emissions of the system into the objective function. 14On the other hand, the environmental benefits of the system energy use also consider the environmental carbon emissions brought by the energy purchase.For example, Qiu et al. proposed a multi-objective optimization considering the carbon dioxide emissions from power grid purchase and the process of using natural gas to establish a multi-objective optimization with the minimum emissions as the environmental indicator and the cost of energy purchase as the minimum economic indicator. 15uo et al. considered carbon emissions from large grid purchases and equipment such as combined heat, cold power and gas turbines to establish a carbon emission model for a regional IES. 16arbon capture system (CCS) is the most costeffective measure to reduce greenhouse gas emissions and mitigate global warming in the future. 17Carbon capture utilization is the key to reducing system carbon emissions, and with in-depth research, scholars have applied carbon capture devices to IESs.Yin et al. 18 applied CCS technology to IESs to store captured CO 2 in geological layers for reducing system carbon emissions.Ji et al. 19 applied carbon capture technology to power plants and constructed a carbon capture power plant with low carbon economic dispatch.Götz et al. 20 studied electric hydrogen production and subsequent methanation technology, stating that electric hydrogen production from alkaline solutions is the most economical technology available and that a stable CO 2 source is a key to methanation.
Carbon trading provides an effective way to deal with the relationship between economic development and carbon emission reduction.China has established a national carbon emission trading market to control and reduce greenhouse gas emissions by using market mechanisms, and the current research on carbon trading market mainly focuses on carbon trading methods and carbon tax prices.For example, Zhou et al. proposed to construct a stepped carbon trading cost model by introducing carbon trading mechanism in the economic scheduling model. 21Wang et al. proposed a scheduling model that balances low carbon and economy by introducing carbon trading mechanisms to constrain carbon emissions of IES. 22urthermore, few studies have assessed the whole life cycle carbon emissions of IESs.Wang et al. 22 conducted a study and analysis of the greenhouse gas emissions of various energy chains within an IES.Most of the existing research works on IES carbon emission calculations only consider the greenhouse gases generated during the use of single equipment and energy sources, 18 without considering the carbon emissions caused by the upstream and downstream relationships of the process.
The above research results facing the IES lay a good theoretical and modeling foundation for this work.In the face of the serious problem of low carbon, different scheduling methods are proposed to maximize the efficiency of energy use and reduce carbon emissions in the operation of IESs.Although the current research has achieved many research results in the field of IES dispatching in low-carbon, there are still deficiencies in the following aspects: 1.The optimization of the operation of the IES to achieve the low carbon emission target is focused on the target cooperative scheduling scheme and carbon emission is not considered as a dispatchable resource, and the related cooperative scheduling method needs further study.2. In the modeling of the IES, the impact of carbon capture equipment and carbon emission reduction equipment combining electric hydrogen production and carbon capture on the carbon emission of the system is not considered.3. When studying the carbon emission assessment of the IES, the impact of carbon emission of the upstream process of energy and equipment on the IES is not considered.
In this study, a low carbon economy optimization method of the IES considering carbon capture technology is proposed.First, the carbon footprint of the IES is analyzed using the life cycle assessment method, and a carbon emission model is constructed from energy sources and key equipment respectively; on this basis, a model of the IES containing carbon capture and utilization (CCU) technology is established, the operational characteristics of the equipment are considered, a carbon trading method with a reward and penalty ladder carbon price is introduced, and then a comprehensive low-carbon economy IES scheduling model is established; finally, the mixed integer nonlinear programming problem model is solved by segmental linearization.

| IES STRUCTURE WITH CCU TECHNOLOGY
The IES combines a variety of flexible and dispatchable energy devices to effectively meet a wide range of load demands. 23The structure of the IES model established for the carbon-containing capture and utilization technology is shown in Figure 1.The IES consists CCHP, photovoltaic generation, gas boiler (GB), battery, CCS, PtG, methanation, air condition system, air source heat pump, heat storage, nature gas storage, and so forth.
The combination of carbon capture and methaneization provides a potential pathway for reducing CO 2 emissions while simultaneously producing energy.Postcombustion flue gas shunt carbon capture for high carbon emission units in IESs to avoid CO 2 entering the atmosphere and reduce CO 2 emissions.By capturing CO 2 emissions by converting them into methane, this method contributes to achieving carbon neutrality. 24The working principles and process flow of the CCU technology are illustrated in Figure 2.
The carbon capture system consists of a precooling tower, an absorption tower, and a desorption tower.The carbon capture equipment captures carbon-containing flue gas generated by the CCHP units and GB units.First, the flue gas emitted by the CCHP and GB units is diverted for capture.The carbon-rich high-temperature flue gas is sent through the precooling tower by the ventilation facility and then enters the absorption tower.In the absorption tower, the carbon dioxide flue gas is absorbed by ethanolamine solution to form a rich solvent.The rich solvent is sent to the desorption tower, where the high temperature separates the carbon dioxide from the ethanolamine solution, transforming the rich solvent into a poor solvent.Finally, the released highconcentration carbon dioxide is compressed stored using an air compressor.
PtG refers to the process of using an alkaline electrolysis cell to electrolyze water and produce hydrogen gas. 25This hydrogen gas can then be combined with a suitable carbon source through catalysis to form methane.
The process of methanation involves the reaction of CO 2 with H 2 in the presence of a catalyst to produce CH 4 and H 2 O.The combination of CCS and PtG methanation process offers some advantages.The captured carbon dioxide has a high concentration ranging from 80 to 98 vol%, providing abundant feedstock for the methanation reaction.The required hydrogen gas for the reaction is obtained through electrolysis of an alkaline solution, ensuring a controlled and sufficient supply of H 2 to facilitate the complete reaction.CCU technology reduces the system's carbon emissions and synthesizes CH 4 feed systems.Low-carbon operation and improved the economy of the system.

| LIFE CYCLE CARBON EMISSIONS ASSESSMENT OF THE IES
The IES play a crucial role in achieving sustainable development and reducing environmental impacts.Conducting a comprehensive LCA of these systems is essential to fully understand their environmental effects.LCA is a method for quantifying the environmental performance of a system throughout its entire life cycle, from cradle to grave. 26The life cycle assessment evaluates the environmental impact of the system, considering all resource inputs, such as energy, natural resources, and raw materials, as well as their environmental consequences, including greenhouse gas emissions, solid waste, and liquid waste. 27The objective of conducting a life cycle assessment is to identify the F I G U R E 2 Methanation system for electric hydrogen production and carbon capture.| 4241 factors that affect the environment and to seek better solutions for reducing environmental impacts.
Research based on the ISO 14040 standard for LCA.The life cycle assessment process is illustrated in Figure 3.Following the LCA method involves establishing the objectives and scope defining the IES boundaries including the process boundaries of PV, electricity, natural gas, and CCU, identifying the relevant processes encompassing extraction, assembly, transportation, operation, recycling, and abandonment.Then inventory analysis, where data on unit processes are collected.For major energy and material processes, data from the GaBi database were supplemented.Life cycle modeling of systems using GaBi.The impact assessment analysis is conducted, LCA results are calculated by multiplying the life cycle inventory results by characterization factors, which represent the environmental impacts caused by various processes throughout the life cycle.This research used the global warming potential impact to represent the environmental impact of the IES.

| Life cycle carbon emission assessment of natural gas
In the IES energy system, the fossil fuels for CCHP, GB, and gas loads are primarily natural gas, which produces greenhouse gases during use and leaks during extraction and transportation.The full life cycle of natural gas consists of three main phases: extraction, transportation, and use.Carbon emissions from the extraction phase include carbon emissions from the use of other energy sources, materials, and methane leaks during extraction.Carbon emissions from the transportation phase include emissions from transportation methods, distances traveled, and the pressurization of gas pipelines or the transportation of liquefied natural gas.Carbon emissions in the use phase are caused by the use of CCHP, GB, and gas loads.The carbon emissions from these three segments are combined to obtain the full life cycle carbon emissions of natural gas in the IES, expressed shown in Equations ( 1) and ( 2).
= , where C gas is the life cycle carbon emissions of natural gas.C gas,Ex , C gas,Tr , and C gas,Op are carbon emissions from the extraction, transportation, and operation of natural gas, and so forth.η pw,c is the carbon emission factor for the type of energy used in extraction.E pw,c is the amount of various energy used in extraction.η escape,gas is the carbon emission factor of natural gas leaked during natural gas extraction.Q escape,gas is the amount of leaked natural gas extracted; η escape,gas is the carbon emission factor of leakage during transportation.Q tsp,gas is the amount of natural gas leaked during transportation.U g,i is the carbon emission factor of the transportation process using different modes of transportation.D g is the gas transportation distance.η use,gas is the carbon emission factor for natural gas use.Q use−gas is the amount of natural gas used.

| Life cycle carbon emission assessment grid purchased electricity
China's power generation system has a complex structure.There are many different types of power generation facilities and energy sources, mainly including thermal, hydro, nuclear, renewable and other energy generation.
According to the report released by the state, for the CO 2 emissions generated by the purchased use of electricity, calculated by Equation (3).
where C e is the emissions from the use of purchased electricity carbon, P e−buy is the use of electricity purchased from the grid, EF e is the grid carbon emission factor.

| Life cycle carbon emission assessment CCU
The carbon capture system consists of three main components: the absorber, the stripper, and the compressor.The carbonaceous flue gas combines with the ethanolamine (MEA) solution in the absorber tower, where over 90% of the carbon dioxide in the flue gas is removed, resulting in a solvent rich in carbon dioxide.The solvent, after passing through a heat exchanger, enters the stripper tower where high temperature and pressure cause the carbon dioxide to be released.After compression and purification, carbon dioxide with a purity of over 99.5% is obtained for storage.In the stripper tower, the lean solvent containing a small amount of carbon dioxide returns to the absorber tower through a heat exchanger, completing one capture cycle. 28he carbon capture system generates certain pollutant emissions and resource consumption during equipment manufacturing, the capture process, and decommissioning.The boundaries of the carbon capture system are defined, primarily including the construction, operation, and decommissioning stages.The construction stage involves the construction of the capture equipment and compression devices.The operation stage includes the energy and material losses in the capture and storage processes.The decommissioning stage mainly considers the resource recovery process of the equipment.
The carbon emissions of the CCS system include the energy consumption for electricity and steam inputs and the material inputs such as the consumption of ethanolamine solvent and circulating cooling water.GaBi software is used to assess the environmental impact of CCS technology throughout its life cycle, with GaBi Databases as the basic database.The energy and material inventory for a system using a 40 wt% MEA aqueous solution to remove 90% of CO 2 from the flue gas, as described in various studies, 28,29 is referenced to establish the life cycle carbon emission model for the CCS system.
The carbon dioxide captured from carbon capture is catalytically converted to methane along with hydrogen produced from water electrolysis in the methanation unit.The environmental benefits of the methanation system throughout its life cycle are studied, with process boundaries including equipment construction, operation, and decommissioning.The research examines the environmental impacts of the material and energy inputs and outputs at each stage of the methanation system's life cycle.The material and energy consumption inventory for the PtG equipment is referenced from the article 30 to establish the life cycle carbon emission model for the carbon capture methanation system.CCS captures 10 kt/a of CO 2 per year from the flue gas on the net stack flue after ultraclean emission removal, with a CO 2 capture rate of 92.24% and a product gas CO2 mass fraction of 99.5%, and the associated consumption per ton of CO2 product gas capture unit is shown in Tables 1 and A4.

| Life cycle carbon emission assessment photovoltaic system
The photovoltaic system is mainly composed of PV solar panels.The life cycle stages of a photovoltaic system include the extraction, assemble, transport, operation and end-of-life disposal or recycling. 31The production and construction, transportation and installation, recycling and abandonment aspects of PV solar panels are the main sources of carbon emissions for the whole life cycle of PV systems.Some carbon emissions are also generated during the operation of the equipment, which is very small. 32In this research PV power generation is regarded as zero carbon emissions during the operation phase.
In this research, a 210 Wp polycrystalline 60-cell module is analyzed for full life-cycle carbon emissions and the material distribution of PV solar panels is shown in Table 2 to calculate the carbon emissions generated from the life-cycle of the PV system.Using GaBi software to model and calculate the PV panel, the model is shown in Figure 4.
The LCA process for PV systems in Figure 4. establishes the carbon footprint of the energy and materials used in the production and construction process according to the Table 2 material information.The distance and mode of transportation during the transportation process are taken into account in the LCA.The additional material consumption required during installation and use is represented in the model.Although, during the operation of the PV system there is a certain amount of manual maintenance and additional energy and material inputs, this contribution is relatively small, so the operation process is considered as zero carbon emission.At the end of the life cycle of the photovoltaic system part of the material recycling.In the process of life cycle assessment of the research object, | 4243 there are some small and difficult to accurately count the data, and when the data account for less than 0.1% of the total carbon emissions, the cut-off principle will be followed to exclude them, so as to ensure that the life cycle assessment is comprehensive and accurate, and to reduce the unnecessary complexity.
A life cycle carbon emissions model for photovoltaic power generation is established and represented by Equations ( 4) and (5).
where η E p,N,i p,N,i and η E m,N,j m,N,j represents the environmental impact of the primary energy and material input required in stage N, respectively, Ex, As, Tr, Ab, Cy in the "extraction," "assemble," "transport," "abandon," "cycle," U D p,Tr,i Tr,i are the environmental impacts of transport modes and transport distances in the transport phase.

| Life cycle carbon emissions of the IES
According to the models of carbon emissions for each input energy source and the system, the LCA carbon emissions for natural gas, grid electricity, photovoltaic system and CCU are calculated and modeled using GaBi.The LCA carbon emissions for each of these sources are presented in Table 3.

| Initial allocation of carbon emission quotas
China adopts a policy of free allocation of carbon emission allowances and establishes an allowance allocation mechanism based on carbon emission intensity control, which sets corresponding carbon emission benchmark values according to different energy unit types and uses the historical method and industry benchmark method 33 to determine the method of carbon emission allowances.For the power sectors with relatively complete databases, the industry benchmark method is used to allocate carbon emission allowances.
The objects with free initial carbon emission allowances are: purchased energy, CCHP, GB, PV, and the calculation method is shown in Equations ( 6) and (7).
where E t IES is the free carbon credits allocated by the IES; IES−e and E t IES−g are the carbon credits allocated to the electricity and heat supply units in the IES; E t PIES−CCHP and E t IES−GB are the carbon credits allocated by the IES's power supply and heating units; λ e is the carbon emission allowance per unit of electricity supply; P e−buy,t is the power purchased from the superior grid; η g−e and η g−h is the gas- electricity conversion coefficient and gas-heat conversion coefficient of the CCHP unit; η g−gb is the energy conversion coefficient of the GB; λ g is the carbon credits per unit of natural gas; P g−buy,t is the nature gas purchased from the superior gas grid; E t PV is the carbon credits allocated to the PV system, and P PV,t is the amount of PV electricity generated; t is the dispatch cycle.

| Carbon emissions based on life cycle carbon footprint assessment
The actual carbon emissions are calculated using a life cycle carbon footprint model, as described in Chapter 3. The total carbon emissions can be found in Equation (8).
where E is the carbon emissions of the life cycle of IES.

| Carbon trading cost model
In this research tiered carbon pricing trading system, the IES participates in the carbon market with a trading share equal to the difference between the total carbon emissions and the carbon emission quota.The tiered carbon trading mechanism divides the purchase of | 4245 carbon emission quotas into multiple intervals, the purchasing price increases as the IES requires more carbon emission quotas.The tiered carbon trading model is shown in Equation (9).Equation (10).
(1 + 3 )( − 3 ) (1 + 4 )( − 4 ) where C carbon is the cost of carbon trading; λ is the basic price of carbon trading; d is the length of carbon emission interval; a is the increase of carbon trading price.

| Objective function
During the operation of the IES, the goal is to respond to low-carbon energy-saving and emission reduction.To achieve the optimal balance between economic and environmental benefits, this research establishes a lowcarbon economic model for the IES with two dispatching objectives: minimizing operating costs and minimizing carbon emissions.The operating costs include energy procurement costs and operation and maintenance costs, while carbon emissions are represented by a carbon trading cost model.The objective functions for the model are to minimize the system's energy procurement cost, operation and maintenance cost, and carbon trading cost, as shown in Equations ( 11)- (13).
where C IES is the target low carbon economic operating cost; C buy is the cost of energy purchase; C carbon is the cost of participating in carbon trading; C operation is the operation and maintenance cost; β e is the power price; P e−buy is the purchased power; β g is the natural gas price; P g−buy is the purchased gas power; C k is the operation and maintenance cost factor of the equipment; P k is the power output of the equipment.
where P e,cchp,t is the power supply of the CCHP unit; P h,cchp,t is the heating power of the CCHP unit; P c,cchp,t is the cooling power of the CCHP unit; η CCHP_e is the power generation efficiency of the CCHP unit, where P h,gb,t is the heating power of the GB unit; η gb is the heat production efficiency of the GB, P g,gb ∆ is the climbing rate of the GB.C gb,t operation is the operating cost of the GB.

| Energy storage units
where x is the type of energy storage equipment, in the constructed IES there are battery (electric storage, ES), gas storage (GS), heat storage (HS), S x is the equipment energy storage capacity; a x,cha and a x,dis are the energy storage equipment storage mark; η x,cha and η x,dis are the energy storage equipment storage efficiency; P x,cha and P x,dis are the energy storage equipment storage power.

| CCUs units
In IES with carbon capture equipment, the carbon capture system captures the carbon dioxide generated by GBs and gas turbines.The amount of captured carbon dioxide and the energy consumption associated with its treatment are directly proportional.The model for this relationship shown in Equations ( 28)-( 30).
where E CCS,t is the amount of CO 2 capture during the dispatch cycle, η CCS is the efficiency of the carbon capture equipment, γ i,t is the fractionation ratio of the inlet flue gas, C cchp is the carbon emission factor of the CCHP unit, P g,cchp,t is the natural gas power consumption of the CCHP unit, C GB is the carbon emission factor of the GB, P g,gb,t is the natural gas power consumption of the GB.E R,t is the amount of CO 2 capture provided by the lean and rich liquids, V R,t F is the solvent volume, M is the molar mass, θ is capture efficiency, ρ is the solvent density.P CCS,t is the energy consumption required to process CO 2 .
The carbon dioxide methanation process requires a stable source of hydrogen, which can be produced through electrolysis. 34The mathematical model for alkaline electrolysis hydrogen production is represented by Equations ( 31) and (32).
where H PtG,t is the power of hydrogen production; η PtG is the efficiency of the electric hydrogen production equipment; U PtG is the start/stop flag of the electric hydrogen production unit, it takes the value of 0 or 1; P PtG,t is the input electric power of electric hydrogen production; H PtG Max and H PtG Min are the power limits of electric hydrogen production.
Carbon dioxide E CCMR and hydrogen H PtG used in the methanation plant are captured by carbon capture systems E CCS,t and produced by electric hydrogen production H PtG,t .The methanation plant constraints are shown in Equations ( 33)-( 35).
where E CCMR is the input power of methanated carbon dioxide; H PtG is the input power of methanated hydrogen; Mr CO 2 is the relative molecular mass of carbon dioxide; Mr H 2 is the relative molecular mass of hydrogen; P M is the power of methane production; η M is the efficiency of methane production; λ gas is the calorific value of natural gas; m CH 4 is the mass of methane per unit volume.

| Energy conversion equipment
Based on the operational characteristics of the equipment, the devices have power limits and ramp rate constraints.The mathematical models for these constraints shown in Equations ( 36)-(38).
where j is the type of energy conversion equipment.In the constructed IES there are air source heat pumps, electric chillers.P j,t is the output power of the equipment; P j ∆ is the equipment climbing rate; C j,t operation is the operating cost.

| Electric power balance
The actual output power of the CCHP unit P e,cchp,t , the power purchased by the superior grid P e−buy,t , the photovoltaic turbine P PV,t and the storage battery P esdis,t satisfy the electric load demand P e−load,t and the electric energy demand of other energy conversion equipment.The electric power balance constraint is shown in Equation (39).
where P P2G,t is the electric hydrogen power; P AS,t is the air source heat pump power; P ML,t is the electric chiller power; P escha,t is the battery charging power; P CCS,t is the carbon capture device CO 2 compressor electric power.

| Thermal power balance
The actual thermal power output of CCHP unit P h,cchp,t , GB output P h,gb,t , air source heat pump output P h,AS,t and heat storage tank heat power P hs,dis,t satisfy the thermal load demand P h−load,t and other equipment thermal energy demand.The thermal power balance constraint is shown in Equation (40).

.8 | Cold power balance
The actual output cold power of the CHP unit P c,cchp,t and the output power of the electric chiller P c,ML,t satisfy the cold load demand P c−load,t .The cold power balance constraint is shown in Equation (41).
5.2.9 | Gas power balance The IES purchases gas power P g−buy,t from the upper gas grid, methane generator output gas power P g,MR,t and storage tank output gas power P gsdis,t to satisfy the gas load demand P g−load,t and other energy equipment gas demand.The gas power balance constraint is shown in Equation (42).
where P g,cchp,t is the natural gas power consumed by the CCHP unit; P g,GB,t is the natural gas power consumed by the GB; P gscha,t is the gas power stored in the gas tank.

| Model solver
The integrated energy optimization method based on the life cycle theory containing carbon capture technology is constructed in this study.IES contains a variety of energy devices as well as coupled devices with dispatchability and numerous model optimization variables.Using segmental linearization, the above mathematical model and its constraints are transformed into a mixed integer linear programming problem, which is based on MATLAB compiler environment and invoked to solve the model stably using CPLEX solver based on the YALMIP platform.

| EXAMPLE ANALYSIS
This research establishes a IES low-carbon economic dispatch model based on the life cycle assessment theory by considering the demand of electricity, heat, cooling, gas loads and the greenhouse gas emissions of the IES system.To verify the effectiveness of the life cycle low carbon economic dispatching method proposed in this research, the integrated energy with CCU system shown in Figure 1

| Typical parameters
The typical operating parameters of each unit within IES are shown in appendix Tables A1-A3.The heat load, electrical load, cooling load, gas load and PV generation data for the same period are shown in Figure 5.
The calculation example implements a valley-peak electricity price and a fixed gas price for the system.The prices are shown in Table 5, with the peak hour tariff at CNY 1.2/kWh, the weekday tariff CNY 0.68/kWh, and the valley tariff at CNY 0.38/kWh.

| Comparative analysis of different cases
This research considers the operation of IES under the above four scenarios and analyzes the advantages of low carbon economic operation of the system under different scenarios, with the specific cost per day as shown in Table 6.
As can be seen from Table 6, Scenario 2 uses a reward and penalty ladder carbon price approach to participate in the carbon trading market.Compared with the fixed carbon price trading in Scenario 1, the total cost of Scenario 2 is increased by 16,084.81CNY, or 20.7%.The carbon emissions decreased by 4908.41 kg or 9.47%, verifying that the carbon trading approach would have an impact on the carbon emissions of the system.To study the impact of carbon trading price on the low carbon operation of IES economy.This work studies the changes of carbon emission cost and carbon emission of IES under different carbon trading base price and penalty factor under the ladder carbon trading method, as shown in Figure 6.When the carbon trading base price is less than 280 CNY/t, the carbon trading cost under the low carbon target is not enough to influence the change of operation mode, and the carbon emission of the system not change.When the carbon trading base price is 280 CNY/t, the carbon emission significantly decreases as the carbon trading base price increases to influence the operation of the system.When the carbon trading base price is greater than 280 CNY/t, the carbon emissions tend to stabilize at this time the penalty factor has a significant effect on the carbon trading model.When the penalty factor is less than 0.25, the carbon emission gradually decreases with the increase of penalty factor.When the penalty factor exceeds 0.25, the system carbon emission tends to be stable and only the carbon trading cost is increased, the system carbon emission cannot be reduced by the penalty factor.It has better carbon reduction effect and economic benefit, when the penalty factor is 0.25.Compared to the conventional IES in Scenario 2, the integrated campus energy system with carbon capture and methanation equipment in Scenario 3 has the ability to capture carbon dioxide and a way to supplement the energy source of the system.
1. From Table 6, the total cost of Scenario 3 is reduced by CNY 20,622.08 or 22% compared to Scenario 2. The amount of carbon dioxide emission is reduced by 6488.52 kg, which is 13.84%.The amount of carbon dioxide fixed by the carbon capture equipment is 8512.86 kg. 2. The results of electric power balance of energy optimization scheduling are shown in Figure 6.In Figure 7, during the 23:00-6:00 h, which is in the low electricity price period, the energy storage battery bank completes the storage task during the low electricity price period.In the period of 11:00-16:00, which is the peak period of electricity price, the storage battery bank in Scenario 3 provides power to the system to reduce the system's power purchase cost.Photovoltaic power generation system is in the peak period, to consume the electricity generated by photovoltaic power generation, the system reduces the purchase of electricity from the large grid.At this time, the system reduces the power output of high electrical load equipment and increases the power output of other energy-using forms of integrated energy equipment.3. The results of output diagram of CCHP and GB unit scheduling are shown in Figure 7. Through Figure 8A,B, the CCHP unit and GB unit in Scenario 3 are in working condition at full time, the emission reduction capacity is enhanced due to the introduction of CCU unit IES with carbon capture capability, and the output of the GB for efficient heating of the low-carbon emission CCHP unit is significantly increased, and the greenhouse gases in its emission tail gas can be captured by the carbon capture unit.
4. The results of nature gas balance of energy optimization scheduling are shown in Figure 9.In Figure 8, in Scenario 3, the CCU equipment installed, PtG provides the hydrogen needed for methanation and most of the carbon emission tail gas in the system is captured and stored.The methanation equipment has sufficient material to produce methane at high power.Under Scenario 3, CO 2 methanation replaces some of the gas purchased from the natural gas grid, and the IES energy purchase cost decreases and the total cost decreases.
In the IES carbon emission calculation process only the carbon emission of the operation process is considered, Scenario 4 evaluates the carbon emission of IES based on the whole life cycle carbon emission.Through Table 6, the total cost of its result is increased by CNY 11,344.32 or 15.36% compared to Scenario 3. Scenario 4 uses the whole life cycle carbon emission assessment method, which not only includes the carbon emissions generated from the operation process, but its CO 2 emissions are increased by 6739.67 kg or 16.6% compared to Scenario 3.

| CONCLUSION
In this work, a low-carbon economic model of the IES containing CCU technology is established.The IES composed of photovoltaic, combined cooling, heating and power system, electric hydrogen production, carbon capture, and energy storage meets multiple load demands.The carbon emissions of the whole process of  CCU system, input energy and new energy system have been studied using the life cycle assessment method.The reward and penalty ladder carbon trading is applied to IES to constrain the system carbon emissions.
In summary, this research proposes a low carbon economic dispatch model for IES containing CCU technology based on life cycle theory.LCA is used to study the life cycle carbon emissions of IES, which is used to improve the system carbon emission calculation and introduce the reward and punishment ladder carbon price trading, which is used to improve the utilization rate of low carbon energy equipment and constrain the carbon emissions of energy equipment and load energy use.IES containing CCU technology for capturing CO 2 and converting it to supply system energy.
This study next summarizes the advantages and disadvantages and follow-up research directions.The current, low-carbon economic optimization method for IESs with life cycle carbon emissions has the following benefits: 1.This study establishes an IES containing carbon capture technology, and CCU technology captures and utilizes CO 2 to reduce the energy purchase cost, carbon trading cost and environmental impact of the system.It provides an economic and environmentally friendly solution for the design and operation of future IESs.2. Constructing a low carbon model of the IES including carbon capture technology, and using LCA to study the input energy of IES and the carbon emission of each energy system, which improves the dimensionality and accuracy of the system carbon emission calculation.3. The emission reduction effect of IES is improved through reasonable carbon price guidance and supplementary carbon capture equipment.
The model has the following limitations: The model proposed in this study provides a way to calculate the carbon emissions of IESs, but there are certain simplifications in the LCA methodology, which are mainly due to data limitations.These simplifications may affect the accuracy of carbon footprint calculations.In addition, the model focuses primarily on singleobjective optimization with an emphasis on low carbon emissions and economic efficiency.Future research may explore the incorporation of multi-objective optimization to achieve broader sustainable development goals.
The follow-up work is as follows: (1) Explore different optimization objectives that consider not only carbon emission reductions, but also maximization of energy efficiency and sustainable energy consumption rates.( 2) explore suitable multi-objective optimization algorithms to find trade-off solutions.(3) When future policy decisions incorporate consideration of objectives, twotier optimization can consist of an upper-tier problem (government policy formulation) and a lower-tier problem (optimal energy dispatch) thereby integrating policy and operations.(4) Green energy sources have uncertainties that have a significant impact on the model's scheduling results.
T A B L E A4 Life cycle material energy inventory for power to gas technology to produce 1 kg of methane.| 4255

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I G U R E 7 Electric power balance of Scenario 3. F I G U R E 8 Output diagram of CCHP and GB unit.
List of operating losses of 1000 kg CO 2 capture unit.
T A B L E 1 T A B L E 2 Materials and energy input required to produce 1 m 2 of photovoltaic modules.
F I G U R E 4 Life cycle of one square PV panel building in GaBi.TRADING MODEL Calculation results of LCA carbon emission from different input energy.
T A B L E 3

η
CCHP_h is the heat production efficiency of the CCHP unit, η CCHP_c is the cooling efficiency of the absorption chiller in the CCHP unit, λ gas is the natural gas heat value, P is used as an example for simulation study.The dispatching results of the IES under four different scenarios are compared and analyzed.Four scenarios are shown in Table4.1.Low carbon economy dispatch of the system considering only the carbon emissions of the operation process, without CCU devices, and carbon trading with a fixed carbon price of 51.23 CNY/ton CO 2 .2. Low carbon economy dispatch of the system considering only the carbon emission of the operation process, excluding CCU devices, carbon trading method is reward and punishment ladder carbon price, carbon trading base price is 250 CNY/ton CO 2 .3. Integrated and flexible operation mode of CCU devices, low-carbon economic dispatch of the system using reward and penalty step carbon trading.4. Low carbon economy scheduling based on life cycle carbon emission theory.
T A B L E 4 Scenes of IES in various ways.
F I G U R E 5 Electric load, gas load, thermal load, cooling load, and photovoltaic generation forecast output curve.T A B L E 5 Time-of-use power price.
System scheduling state results.
T A B L E 6 F I G U R E 6 Total carbon expenditure and total carbon emissions under different carbon trading prices and penalty factor.