Techno-economic analysis of integrated carbon capture and utilisation compared with carbon capture and utilisation with syngas production

on comparing ICCU and conventional CCU processes based on Aspen simulations covering mass balance (i.e., CaCO 3 consumption, purge production, annual CO production), energy balance, the total annual cost and the CO cost, etc. Analysis shows that the ICCU process can produce more CO (1.20 Mt year (cid:0) 1 ), less purge (0.21 Mt year (cid:0) 1 ), and less consumption of CaCO 3 (0.62 Mt year (cid:0) 1 ) with higher energy efficiency (37.1 %) than the CCU process. The results also show that the total annual cost of ICCU is $867.07 million, corresponding to a total cost of CO of $720.25 per tonne. In contrast, CCU has higher costs, with a total annual cost of $1027.61 million and a total cost of CO of $1004.53 per tonne. The Cost of CO 2 Avoided of ICCU (317.11$/ton) is much lower than that CCU (1230.27 $/ton). Therefore, ICCU was confirmed as a better choice for further industrial applications. In addition, H 2 is shown to have a significant influence on economic performance, which remains a challenge for further application.


Introduction
Carbon Capture and Storage (CCS) is the technology that removes CO 2 from carbon sources, compresses and transports it to a storage site (e.g., underground or ocean bedrock) without releasing it back into the atmosphere [1][2][3][4].It has been researched intensively for its potential to reduce CO 2 emissions in the atmosphere [5].CCS capacity by 2050 is estimated to be approximately 700 million tons annually, corresponding to 10 % of what is required [6].However, CCS entails high initial capital investment and would drastically reduce power plant efficiencies, with potential negative environmental impacts and risk of accidental leakage during long-term storage [7,8].
Carbon Capture and Utilisation (CCU) is complementary to CCS as it can be used to reduce CO 2 emissions from fossil resources by converting industrially emitted CO 2 into chemicals and fuels [9][10][11].In various CCU processes reported, CO 2 is commonly absorbed in solvents such as amines, KOH, or methanol, followed by the regeneration of sorbents in a desorption step to obtain concentrated CO 2 [12][13][14].The captured CO 2 can then be converted to chemicals such as carbonates [15], poly (carbonates) [16], carbamate derivatives [17], carboxylic acids [18,19] using a range of catalysts that include main-group metal complexes (e.g., Mg, Al, Ca, and In), transition-metal complexes (e.g., Zn, Fe, Cr, and Co), and organo-catalysts [20].However, the CO 2 purification process increases CO 2 -supply cost and greenhouse gas emissions [21].Therefore, it is desirable to decrease the cost of the CO 2 desorption process along with increasing the environmental and economic benefits of CCU [22].
Aspen Plus® is widely used to design a new process, optimise operations of a full process, and predict the behavior of a process using basic engineering relationships (i.e.mass and energy balances) [23,24].For example, researchers have reported simulations for the carbonatelooping steam cycle for a large coal-fired plant [25], a coal-fired power plant with CCS [26], CCUS for a coal-based power plant with the production of urea, methanol or sulfur [27].Syngas is a mixture of H 2 and carbon monoxide (CO).It can be utilised to produce valuable fuels and chemicals via the Fischer-Tropsch process, especially in coal--to-liquid and gas-to-liquid processes [28,29].The reverse water-gas shift (RWGS) reaction is one of the most established reactions to convert CO 2 into syngas [30,31].Besides, the global CO market size was estimated to increase from $11.3 billion in 2021 to $13.15 billion in 2022, reaching $23.19 billion in 2026 [32].Steven et al. reported the technoeconomic for the power-to-syngas (PtS) technology that sustainably utilises CO 2 from syngas into syngas [33].By comparing a series of cases in this research to a referred syngas plant (the levelized syngas production cost of 6.94 $/GJ), the levelized syngas production costs of PtS scenarios in this research range from 8.56 to 13.64 $/GJ.Jeehoon et al. compared the CCU process (monoethanolamine-based chemical absorption and utilisation into syngas) with CO 2 emission from power plants and ironmaking/steelmaking plants, which showed that the minimum selling price of syngas from power plants was 19.31 $/GJ and ironmaking/steelmaking plant was 16.02 $/GJ with its market prices ranging from 7.82 $/GJ to 23.25 $/GJ [34].Although these efforts have been made, the competitive costs of CO 2 to syngas are still an obvious challenge for further application.
Integrated Carbon Capture and Utilisation (ICCU) has been reported recently to improve CCU by utilising the captured CO 2 directly.Gassner and Leitner integrated conversion with capture through the hydrogenation of CO 2 to formic acid in aqueous amine solutions [35].Lu et al. captured and converted CO 2 into formic acid using a photoelectrochemical system assisted by an aqueous-ionic liquid (1-aminopropyl-3-methylimidazolium bromide) solution [36].Scott et al. demonstrated the conversion of saturated aqueous solutions (CO 2 overpressure 5-10 bar) of monoethanolamine (MEA) into the corresponding formate adducts [37].In the ICCU processes, CO 2 was converted into methanol [38][39][40][41][42], oxazolidinones [43,44], urea [45][46][47], methyl formate [48], organic acids [49], syngas [50].Iyer et al. reported an integrated progress wherein CO 2 desorption and dry reforming of the CO 2 -methane mixture simultaneously occurred in one reactor to produce syngas [50].This was a partial integration process, which involved one separation column and one reactor.Luis et al. also reported an ICCU strategy that combined CO 2 capture and conversion in two reactors with unsteady-state operation under isothermal conditions to produce syngas [51], which also required two reaction systems.
Calcium looping (CaL) using CaO-based sorbents is a promising alternative to both oxy-combustion and chemical solvent absorption [52,53].The mechanism of the CaL process includes carbonation of CaO to give CaCO 3 in a carbonator operating normally at 650 • C under atmospheric pressure, and calcination of CaCO 3 to give CaO at typically over 930 • C under a highly concentrated CO 2 environment (70-90 vol%) [54,55].Compared to an amine-based post-combustion capture system where the heat of absorption cannot be recovered, the heat of the carbonation reaction in the CaL process can be recovered at high temperature by steam evaporation, superheating, or reheating [53].It has already been demonstrated on a scale of up to 1.9 MWth [54,56].
The integration of the CaL process and RWGS reaction at the same temperature in a single reactor to achieve the capture and conversion of CO 2 by using CaO-based materials has been reported by our group [57,58].The ICCU process with syngas production using only CaO as both sorbent and catalyst has been demonstrated with over 75 % CO 2 conversion efficiency at 600-700 • C [58].This process can eliminate the energy requirement, corrosion, and transportation issues associated with CCS and CCU.Bin et al. have also reported combining the CaL process and RWGS, where CO 2 conversion efficiency reached nearly %, and CO selectivity was close to 100 % by introducing transition metals (Co and Fe) into CaO [59].These studies mainly focused on materials development and optimisation of process conditions, there is an urgent need for technical and economic performance analysis.
Currently, the techno-economic analysis of ICCU has been reported based on the capture with Zeolite 13X [50], MEA/K 2 CO 3 /ABS/aminebased resin [60], methanol [22], ionic liquids [61], MDEA and PZ solvent [62], etc.In addition, it is noticeable that ICCU is not always better than CCU in cost when ICCU is compared with CCU [22,59].For example, it was estimated that ICCU based on CaL costed $165/t CO (only operational cost was considered) which was less than the simply combined process of CaL and RWGS conversion ($393/t CO 2 ) [59].In contrast, Jens et al. compared CCU and ICCU (using methanol to capture CO 2 from raw natural gas to produce methyl formate) to conclude that only if CO 2 in the input was 30 mol%, ICCU could be cheaper than CCU [22].When CO 2 concentration was lower, ICCU required a higher cost than that of CCU, due to the higher heating demand of the separation of methyl formate and byproduct water from methanol.However, in this CaL and RWGS-based ICCU technology, the syngas product was shown high purity without purification [58].
The primary aim of this study is to compare the CCU and ICCU processes based on the CaL process and RWGS reaction using the same reference, namely a coal-fired power plant (CFPP) [63].In the CCU process, as shown in Fig. 1a, CO 2 is captured and concentrated by the CaL technology; then, the released CO 2 is used as a feedstock for the RWGS reaction reactor to produce syngas.On the other hand, in the ICCU process, as shown in Fig. 1b, the CaL reactor and the RWGS reactor are integrated, where the CO 2 capture, desorption, and utilisation occur isothermally in one reactor by switching the inlet gas between CO sources (e.g., flue gas) and H 2 .Therefore, this work is the first to present techno-economic analysis of ICCU with syngas production, by comparing CaCO 3 consumption, purge production, annual CO production, energy efficiency, the total annual cost and the CO cost as well as the Cost of CO 2 Avoided.

Process simulation and methodology
A coal-fired power plant with a net electrical power of 600 MW and a net efficiency of 40.6 % was used as a reference plant [63].Software Aspen Plus® was used to develop the processes of CCU and ICCU to estimate material balances, energy and utility requirements.The reactors were simulated using the property method of the Peng-Robinson with Boston-Mathias modifications (PR-BM), which is suitable for gas processing and refinery applications and provides accurate results for hydrocarbon mixtures and light gases, such as H 2 and CO 2 .The reactors were modeled as stoichiometric reactor blocks.The outlet flue gas in this power plant had a flow rate of 540.1 kg s − 1 and its composition is summarised in Table 1 [64].

CCU model description and assumption
The CCU process shown in Fig. 2 was designed ideally without adding CaO, which came instead from the calcination of CaCO 3 .The specific assumptions are shown in Table 2.The following reaction was modelled in the carbonator: CO 2 capture efficiency was assumed to be 85 % when the mole ratio of CaO to CO 2 was ca. 5 [63][64][65][66].A steam generator extracted the excess heat (Q1) from the exothermic CO 2 capture reaction [63,64,66].
A cyclone (CYCLON1) was used to separate the solids from the decarbonised flue gas, leaving the carbonator with a separation efficiency of 100 %.The flue gas stream was then cooled down to approximately 279 • C with the extracted heat Q2 [63], and the solid stream separated from the cyclone was transferred to the calciner.In the calciner, the temperature was maintained at ca. 900 • C yielding a 90 % conversion of CaCO 3 .In order to make up for activity decay, the purge rate was assumed to be 4.6 %.It was reported that 5 % was set to keep the activity of the sorbent [63,65].The heat for the endothermic calcination reaction (Q3) was provided by the combustion of natural gas using air.Furthermore, a makeup stream (MAKEUP) of fresh limestone consisting of 100 % CaCO 3 was constantly fed into the calciner.The stream out of the calciner (CALCOUT) entered another cyclone (CYCLON2) with a separation efficiency of 100 %.The gas stream (CO 2 ) was separated from the solids (CaO) that were carried back to the carbonator.In the RWGS reactor, CO 2 was transformed into CO and H 2 O, as shown in R2.The temperature was kept at ~650 • C with an excess of hydrogen (H 2 : CO 2 = 3: 1 M ratio) and 55 % conversion of CO 2 into CO [67].After the reaction, the gas stream was cooled down to approximately 100 • C, providing heat output Q5.

ICCU model description and assumption
In practice, the ICCU process is conducted in one reactor.However, for illustration, the process in Aspen was simulated in two reactors with the same conditions (Fig. 3).CO 2 capture efficiency was assumed to be the same as the capture process in CCU, with 85 % CO 2 capture efficiency [63][64][65][66].Heat Q1 was also obtained from the exothermic CO 2 capture reaction, which was set at 650 • C. A cyclone (CYCLON1)separated the solids from the decarbonised flue gas, leaving the carbonator with a separation efficiency of 100 %.After that, the flue gas stream was cooled down to around 279 • C with the extracted heat Q2.The solid stream separated from the cyclone was transferred to the reactor (CU), where CO 2 was converted into CO at 650 • C with the addition of excess hydrogen (H 2 :CO 2 = 3:1 M ratio).The conversion is assumed to involve two steps, CaCO 3 into CaO and CO 2 into CO, with efficiencies of 55 % and 75 %, respectively [57].Heat Q3 was generated in CU.After CU, the gas stream was separated by the CYCLON2 with a gas-solid separation efficiency of 100 %, and then cooled down to approximately 100 • C with heat Q5 and the solids (CaO) were carried back to the carbonator with a purge ratio of 1 %.The purge of ICCU is lower than CCU because in non-  isothermal CCU the CaCO 3 /CaO stability is much less than that in isothermal ICCU [58].
All the operating conditions of the ICCU process are shown in Table 2.

Energy efficiency
Energy efficiency for the power plant with CCU/ICCU was defined as the ratio of the total output to input energy: where ∑ E out is the sum of the electricity from the coal-fired power plant (600 MWe) and the heat from CCU/ICCU.∑ E in is the sum of the energy required for the coal-fired power plant (1478.33MW) and the CCU/ ICCU.To convert the heat into electricity, herein the efficiency of generating electricity was about 35.3 % [73].

Assumptions and method of economic evaluation
In these two cases, total annual capital cost (ACC) and operation and maintenance (O&M) costs were considered.The sum of ACC and O&M was the total annual cost (TAC), as shown in Eq. (2).
The O&M costs mainly constituted two parts, namely, fixed and variable O&M costs.Fixed costs included four contributions: annual maintenance cost, direct labor cost, property taxes and insurance as well as administrative, support and overhead cost.The variable costs were associated with the cost of catalyst, H 2 , natural gas, and CaCO 3 .
ACC was a combination of capital recovery factor (CRF) and total capital investment costs (C total ), which is expressed in equation Eq. (3).
The equipment costs were assumed equal to the overnight costs, without considering any scaling effect, due to the modularity and the simplicity of installation of both these technologies.And C total was representative of the total capital investment cost, involving the total cost of equipment, owner's cost and contingency and land permitting and surveying costs during plant construction.Total costs of equipment are listed in Table 3.
The capital cost of each component (C m ) was empirically estimated using the scaling factor exponent, through Eq. ( 4) [74,[76][77][78].The sum of the equipment cost was defined as C.
where the C r and f represent the reference cost (with reference size S r ) and equipment scaling factor exponent, respectively.The plant was assumed to have 8,000 operating hours annually, a 25year plant lifetime (n in Eq. ( 5)), and an interest rate of 8 % (i in Eq (5)).The CRF was defined as the ratio of constant annuity to the present value at a period with a certain interest, as shown in Eq. ( 5) [79].
To evaluate the technology of ICCU and CCU systems, the cost of CO ($/t CO) was also calculated according to Eq. ( 6).
Cost of CO = TAC AnnualCOproduction (6) Parameters for capital cost estimation with their reference values are summarised in Table 4.The costs of other components such as water pumps, splitters, mixers and separators were not included.
In addition, according to Eq. ( 7), a cost index was used to estimate the cost from year m to year n.Herein, n was selected as 2020; thus, the cost in the previously built or current period (year m, C m ) was used to calculate C n .The ratio of cost index value in year m (I m ) and year n (I n ) multiplied by C m gives C n (Chemical Engineering Plant Cost Index, CEPCI: 650) [83][84][85][86].Costs were calculated in US dollars at an exchange rate €/$ of 1.126 and A$/$ 0.770.
The cost of CO 2 avoidance (CAC) was obtained by comparing the TAC and the CO 2 emission rate to the reference plant with and without CCU/ICCU, shown in Eq. ( 8).

System boundaries
System boundaries define the elements included in economic analysis, which influence the cost of CO 2 avoided and production [87].The value chain of CCU/ICCU involves the following stages: the capture of CO 2 , the conversion of CO 2 into the syngas, the energy to drive the capture and conversion process, and heat recovery during the capture and utilisation process (Fig. 4).In this research, it did not include the transport or distribution of the final product and the H 2 source.In addition, as CO was the main product and the key element of syngas, the final indicator to compare CCU and ICCU was set as CO cost.

Sensitivity analysis
Sensitivity analysis (SA) is acknowledged as a standardized way to evaluate the influence of a certain parameter on the TEA result [88].It is essential to conduct a sensitivity analysis as there are both inevitability and uncertainty surrounding the final results [89,90].According to The DOE/NETL Quality Guidelines for Energy System Studies [91] and the European Best Practice Guidelines for Assessment of CO2 Capture Technologies [92], sensitivity analysis can be researched based on the variations in the primary data elements from the input data, financial assumptions, and state of technology development, which are diverse timely or spatially in different cases [93,94].The parameters tested in this paper were the H 2 cost, interest rate and plant life.However, similar to other techno-economic analysis for CCS/CCU, the sensitivity analysis here is relatively superficial, which could facilitate more detailed investigations and research in the future [24,93].

Mass balances
Table 5 presents the mass balance for all four CCU and ICCU processes evaluated in this study.With the same input of flue gas (540 kg s − 1 ) and H 2 (13.85 kg s − 1 ), the CaCO 3 makeup of CCU (4.6 %) was higher than that of ICCU (1 %).The CaCO 3 makeup of CCU was 1.52 Mt year − 1 , while that of ICCU was 0.62 Mt year − 1 .As a result, more CO (1.20 Mt year − 1 ) was produced in the ICCU process than in the CCU process (1.02 Mt year − 1 of CO produced).In addition, the lower purge ratio of ICCU led to less purge for ICCU (0.21 Mt year − 1 ) than CCU (0.86 Mt year − 1 ) due to the improved stability of sorbents in ICCU compared to CCU.

Energy balances
In the CCU process (Fig. 2), the energy was produced from the carbonator CC (Q1) and the two heat exchangers (Q2 and Q5), while being consumed by the calcination reactor CAL (Q3) and the RWGS reactor (Q4).As shown in Table 6, the net heat requirement was 742.65 MW in the CCU process, while it was 575.05 MW in the ICCU process.On the other hand, 525.90 MW of heat was output in the CCU process and for the ICCU process, 455.89MW was produced.Electricity production from CCU and ICCU was 185.65 MWe and 160.93 MWe, respectively.Considering the electricity production and requirement of the power plant, the energy efficiency of ICCU (37.1 %) was higher than that of CCU (35.4 %).ICCU process is isothermal with extremely excellent cyclic and stable performance, which means less energy demand.Besides, the CCU requires the regeneration and transportation of sorbent, which is energy intensive and with heat loss.

Process performance analysis
To find optimum process parameters in CCU and ICCU, the CaO/CO 2 ratio, conversion of CaCO 3 , and conversion of RWGS were investigated with regard to CO production and energy efficiency.
As shown in Fig. 4a & b, the increase of CaO/CO 2 ratio leads to an increase in annual CO production and a decrease in energy efficiency.When the CaO/CO 2 ratio increased from 2 to 10, the annual CO production of CCU changed from 0.87 Mt year − 1 to 1.15 Mt year − 1 , and that of ICCU increased from 1.12 Mt year − 1 to 1.28 Mt year − 1 .In terms of energy efficiency, the ICCU process decreased from 37.1 % to 35.9 %, while for the CCU process it declined from 36.9 % to 34.3 %.It is indicated that ICCU produced more CO with higher energy efficiency than CCU when the same CaO/CO2 ratio was applied.The CaO/CO2 ratio means less CaCO3 input.Based on the experiment in the lab, ICCU happens with a longer cycle and better stability of sorbents, so the fewer sorbents input can also operate well [58].

Economic evaluation results
Table 7 summarises the key economic performances of CCU and ICCU.The total capital costs (C total ) for CCU and ICCU were around 624.65 and 311.26M$, respectively.The capital cost of CCU consists of these main contributions: the carbonator for carbon capture, calcination for regeneration of sorbents and CO 2 desorption, RWGS reactor for syngas production, heater exchangers, and the steam turbine, generator and auxiliaries.Compared to CCU, all the reactions in ICCU occurred in one reactor in the lab as reported [57,58], but two reactors were still included and calculated as one more reactor can be used to increase the efficiency of carbon capture up to 100 % [95].Although the cost of two rectors was considered, the results still implied that the ICCU process was cheaper than CCU (the total capital costs of ICCU were 50 % of that of CCU).
The total O&M cost, including fixed and variable O&M costs, was 969.09M$ year − 1 for CCU, and 837.91 M$ year − 1 for ICCU (Table 7).H 2 cost dominated the O&M cost, occupying 58 % and 67 % of the total O&M cost for CCU and ICCU, respectively.Similar research has indicated that the H 2 price had the most significant influence on the O&M cost [59]; thus, the influence of other factors can be discussed without the H 2 price influence.In terms of fixed O&M costs, CCU (34.04 M$ year − 1 ) was twice that of ICCU (17.06 M$ year − 1 ), as the cost for property taxes, insurance, and annual maintenance of CCU is higher due to their direct relationship to the capital cost.As far as the variable O&M costs are concerned, the cost for natural gas for CCU was higher than that for ICCU, as CCU required more energy due to the separate reaction.Apart from these, both CCU and ICCU shared the same direct labour cost, e ∑ E out is the sum of the electricity produced from the coal-fired plant (600 MWe) and the electricity from CCU or ICCU.The TAC of CCU was higher than that of ICCU, with costs of 1027.61 and 867.07 M$ year − 1 , respectively (Table 7).H 2 cost was the main factor with 54 % and 64 % of the TAC in CCU and ICCU, respectively.For comparison, Steven et al. proposed a CCU process with syngas production in three scenarios [33].When the CO 2 and H 2 feeds were 71.2 kg/s and 3.3 kg/s, respectively, the TAC was 2038.43M$ year − 1 .Therefore, CCU and ICCU can be used to save capital costs without the separation, storage and transportation equipment.
CO cost is another crucial indicator for decision-makers to choose the closely appropriate case for power generation.As shown in Table 7, the CO cost of CCU was 1004.53 $ ton − 1 , while that of ICCU was 720.25 $ ton − 1 .Compared to the market price of CO of $660 ton − 1 and the CO cost of 1394 $ ton − 1 in the reference [60], both CCU and ICCU systems were higher than the market price but lower than the reference cost (Fig. 5).The difference between CO price from CCU and the market price of CO was much higher than for ICCU.The advantage of ICCU in terms of the total cost of CO derives from high CO production and low TAC.
The CO 2 emission intensity between CCU and ICCU is compared in order to determine the more environmental-friendly scenario.There are two sources of CO 2 emissions coming from utilities (indirect emission) and released CO 2 after CO 2 capture in the carbonator (direct emission).
When the conversion rate of natural gas during combustion is assumed as 100 %, the amount of CO 2 indirect emission is 50.40 kg/s and 39.02 kg/s for CCU and ICCU, respectively (Table 8).Direct emission of CO 2 in both CCU and ICCU are the same as they share the same temperature in the process of carbon capture (13.91 kg/s, Table 8).In total, the CCU process release more CO 2 than ICCU, with 64.31 kg/s and 52.93 kg/s respectively (Table 8).Therefore, the cost of CO 2 avoided by CCU (1230.27$/ton) is more than the triple amount of ICCU (317.11$/ton), when they perform the same carbon capture capability.These results indicate that ICCU is a better choice in terms of CO 2 emissions.In addition, the cost of CO 2 avoided of the MEA-based capture reached about 301.73 $/ton, which shows the potential for the application of ICCU (including both capture and utilisation) [96].

Sensitivity analysis
Sensitivity analysis clarifies how uncertainties in the input parameters (i.e.raw materials, fuel price, labour, construction, land, etc.) influence the TEA result/s, which can be used to judge the ability of the project to bear risk.[97][98][99].However, in this case, the price of H 2 was shown to be the most important factor for the economic results; thus, it was used to perform a sensitivity analysis on the total cost of CO for both CCU and ICCU processes.From Fig. 6a, with the H 2 price increasing from 500 to 3500 $ ton − 1 , the total CO cost of CCU increased from 668.62 $ ton − 1 to 1865.26 $ ton − 1 , while that of ICCU climbed from 508.07 $ ton -1 to 1704.71 $ ton − 1 .The gap between CCU and ICCU dropped from 24 % to 9 %, with the H 2 price ranging from 500 to 3500 $ ton − 1 .Thus, the development of the technology to produce H 2 with less cost will affect the development of the carbon economy in this case.From Fig. 6b, when plant life increased from 5 years to 30 years, the CO cost of CCU decreased from 1100.26 to 1001.56 $ ton − 1 while that of ICCU decreased from 760.79 to 719.00 $ ton − 1 .The interest rate and plant life also directly influence CO cost, as shown in Fig. 6c.When the interest rate increased from 0.02 to 0.12, the CO cost of ICCU changed from 709.27 to 729.00 $ ton − 1 , while the CO cost of CCU increased from 978.60 $ ton − 1 to 1025.18 $ ton − 1 .These minor changes indicate a small influence on interest.Overall, in Fig. 6b and c, CCU is shown to have a higher CO cost than ICCU in all scenarios.CO cost climbed with the increasing interest rate but declined with longer plant life.It is reasonable that increasing interest and short plant life lead to higher CO costs.

Conclusion
Process simulation has yielded important outcomes for process flexibility and efficiency improvement.In the ICCU process, more CO (1.20 Mt year − 1 ) can be produced (CCU: 1.02 Mt year − 1 ), while less purge (0.21 Mt year − 1 ) is produced (CCU: 0.86 Mt year − 1 ) with less consumption of CaCO 3 (CCU: 1.52 Mt year − 1 , ICCU: 0.62 Mt year − 1 ).In terms of energy efficiency, ICCU performs better than CCU, at 37.1 % and 35.4 %, respectively.The better performance of ICCU, compared to CCU, can be ascribed to the integrated design of carbon capture and utilisation in one reactor and correspondingly improved stability of sorbents in ICCU compared to CCU.The CaO/CO 2 ratio and conversion   of CaCO 3 into CaO were investigated to evaluate the optimal conditions for high CO production and energy efficiency for the system.In all cases, higher CO production results from larger CaO/CO 2 ratio andhigher conversion of CaCO 3 into CaO.In contrast, the larger CaO/CO 2 ratio and higher conversion of CaCO 3 into CaO lead to lower energy efficiency.It is also shows that ICCU always performs better than CCU.Economic analysis has also been performed according to the indicators such as total annual cost and CO cost.From the results, ICCU performs better than CCU in all cases.The total annual cost and the CO cost of the ICCU process are estimated to be $867 million and $720.25 per tonne, respectively.In contrast, CCU requires a higher total annual cost ($1027.61million) and cost of CO production ($1004.53per tonne).The Cost of CO 2 Avoided by ICCU (317.11$/ton) is much lower than that of CCU (1230.27$/ton).Therefore, ICCU is confirmed to be a better choice for further industrial applications.H 2 cost was the main contributor to the total annual cost (TAC) of CCU and ICCU, at 54 % and 64 %, respectively.For further application, H 2 cost remains a challenge to be addressed.

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.

Fig. 1 .
Fig. 1.Overview of (a) the carbon capture and utilisation (CCU) process and (b) the integrated carbon capture and utilisation (ICCU) process.

Fig. 3 .
Fig. 3. Process model of the CCU process in ASPEN PLUS®.

Fig. 4 .
Fig. 4. The makeup/CO 2 ratio effects on annual CO production (a) and energy efficiency (b).The effects of CaCO 3 to CaO efficiency on annual CO production (c) and energy efficiency (d).

Y
. Qiao et al. administrative, support, & overhead cost, as well as H 2 cost.

Fig. 5 .
Fig. 5. CO cost comparison between CCU, ICCU and reference with the market price.

Fig. 6 .
Fig. 6.Sensitivity analysis: (a) H 2 to the total cost of CO; (b)interest rate i and (c) plant life to the total cost of CO.

Table 1
Composition of the flue gas.
Fig. 2. Process model of the CCU process in ASPEN PLUS®.Y.Qiao et al.

Table 3
Parameters of the scaling function for capital cost estimation.
Y.Qiao et al.
[82]e amount of catalyst is referred to as the reference (each tonne of CO requires 1.06 kg catalyst)[82].

Table 5
Summary of mass balance for the CCU and ICCU.

Table 6
Summary of energy balance for basic models of CCU and ICCU.In ICCU, the calciner and RWGS reactor are combined in the CU reactor.bNetenergy input for CCU is the sum of Q3 and Q4, while net energy input for ICCU is Q3.cNet energy output for CCU is the sum of Q1, Q2 and Q5, while net energy output for ICCU is the sum of Q1, Q2 and Q5.d ∑ E in is the sum of the energy input for the coal-fired plant (1478.33MW) and the required energy for CCU or ICCU. a

Table 7
Economic evaluation summary of CCU and ICCU.

Table 8
CO 2 emissions and Cost of CO 2 avoided.