Oxyfuel Combustion Makes Carbon Capture More Efficient

Fossil energy carriers cannot be totally replaced, especially if nuclear power stations are stopped and renewable energy is not available. To fulfill emission regulations, however, points such as emission sources should be addressed. Besides desulfurization, carbon capture and utilization have become increasingly important engineering activities. Oxyfuel technologies offer new options to reduce greenhouse gas emissions; however, the use of clean oxygen instead of air can be dangerous in the case of certain existing technologies. To replace the inert effect of nitrogen, carbon dioxide is mixed with oxygen gas in the case of such air combustion processes. In this work, the features of carbon capture in five different flue gases of air combustion and such oxyfuel combustion where additional carbon dioxide is mixed with clean oxygen are studied and compared. The five different flue gases originate from the gas-fired power plant, coal-fired power plant, coal-fired combined heat and power plant, the aluminum production industry, and the cement manufacturing industry. Monoethanolamine, which is an industrially preferred solvent for carbon dioxide capture from gas streams at low pressures, is selected as an absorbent, and the same amount of carbon dioxide is captured; that is, always that amount of carbon dioxide is captured, which is the result of the fossil combustion process. ASPEN Plus is used for mathematical modeling. The results show that the oxyfuel combustion cases need significantly less energy, especially at high carbon dioxide removal rates, e.g., higher than 90%, than that of the air combustion cases. The savings can even be as high as 84%. Moreover, 100% carbon capture was also be completed. This finding can be due to the fact that in the oxyfuel combustion cases, the carbon dioxide concentration is much higher than that of the air combustion cases because of the inert carbon dioxide and that higher carbon dioxide concentration results in a higher driving force for the mass transfer. The oxyfuel combustion processes also show another advantage over the air combustion processes since no nitrogen oxides are produced in the combustion process.


■ INTRODUCTION
The use of fossil fuels is still a significant part of the energy mix, and this part is associated with high greenhouse gas emissions. 1,2−5 The CO 2 concentration in the atmosphere is expected to increase to 550 ppm by 2050 if no action is taken. 6,7ccording to the scenario that was reported by the International Energy Agency, 1 Gt CO 2 /year should be captured by 2030, ramping up to 5 Gt CO 2 /year in 2045. 8Therefore, emissions from power plants and high carbon industries should be addressed in the near future. 9Energy intensification is a more efficient tool for emission reduction in different industries; however, there are inevitable carbon emissions that should be treated by other approaches such as Carbon Capture Utilization and/or Storage (CCUS).According to international reports, CCUS could play an important role in reducing CO 2 emissions. 10,11Implementation of this process may vary according to several technological parameters, such as the flue gas flow rate, thermodynamic properties, CO 2 composition, and the origin of flue gas. 12,13here are three main options available for the mitigation of greenhouse gas emissions: oxyfuel combustion 14,15 where the fossil fuel is combusted with oxygen instead of air; air combustion 16,17 where the combustion process happens in the presence of air; and precombustion 18,19 where carbon removal occurs prior to combustion. 20arbon capture, as a kind of end-of-pipe treatment method, is a well-studied alternative where amine-based capture is the dominant technology because of its commercial advantage. 21onoethanolamine (MEA) is a useful solvent in industrial applications due to its low cost, high efficiency, and fast absorption rate.It has a relatively high cyclic capacity, meaning 1 mass unit of MEA is capable of binding 0.45 mass of CO 2 . 22owever, the major inherent drawback of an amine-based CO 2 capture system is the high energy consumption for solvent regeneration, which can result in a significant decrease of 9−13% in the net plant efficiency. 23,24This energy rapidly increases, notably at higher than 90% removal efficiency. 25Oxyfuel combustion is the process of burning fuel in a mixture of pure oxygen and recycled flue gas as the fuel combustion reactant instead of using air. 26In the case of oxyfuel combustion, there can be two options: • Clean oxygen is used; in this case, the flue gas contains only CO 2 and water, and no selective carbon capture is required; 26 • Oxygen with artificially mixed CO 2 as inert is used; this alternative is for existing air-based technologies where the inert effect of nitrogen is replaced with the application of CO 2 . 24e to the replacement of nitrogen with recycled CO 2 , there will be a high CO 2 concentration in the flue gas stream with 90% of CO 2 on a dry base, and the recovery and sequestration of CO 2 become much easier. 27,28Moreover, the substitution of carbon dioxide instead of nitrogen in this technology helps to prevent the emission of NO x as an undesirable product of air combustion technology. 29ushiie et al. 30 conducted experimental and numerical analysis on the effect of NO x formation in the oxyfuel combustion technology, and their results confirmed that NO x emissions under CO 2 −O 2 conditions were lower than those under "air" conditions.−35 The high concentration of CO 2 in oxyfuel combustion is beneficial for separating CO 2 from the flue gas by adsorption or absorption technologies. 36,37In this technology, the combination of recycled flue gas and oxygen is utilized for fuel combustion. 38This stream is used to reduce the flame temperature and compensate for the volume of missing N 2 to ensure that there is an acceptable amount of gas to carry the heat through the combustion process. 39−42 The LCA of the oxyfuel combustion power plant with CO 2 capture presented by Stanger et al. 39 analyzed the current technologies on the development of oxyfuel combustion for different kinds of coal-fired and gas turbine-based power plants.Retrofitting of a coal-fired power plant (CFPP) by a polymeric membrane and cryogenic distillation hybridization for different oxycombustion pathways were evaluated by Garcia et al. 43 They reported that the polymeric membrane system is more efficient than cryogenic distillation regarding power consumption of the former in comparison to that of the latter.
Lim et al. 44 developed a techno-economic analysis for 500 MWe ultrasupercritical steam power generation technology with circulating fluidized bed power plants with air combustion and oxycombustion in the presence of CO 2 capture.Yu et al. 45 investigated the plant performance of two flue gas recirculation modes, wet and dry modes, under different operating conditions for a 600 MW supercritical oxyfuel fluidized bed combustion power plant.Their results show that the dry-mode plant has a net power efficiency of 31.6%, while the wet-mode plant has a net power efficiency of 31.5%.
Aspen Plus software was used to model and simulate a cement plant for integrated oxyfuel combustion and power-to-gas technology. 46In former studies, the kinetic constants, absorption parameters, and energy requirement were commonly adjusted for simulations of MEA-based chemical absorption and fitting data with experiments 47−49 Aspen Plus simulation for CO 2 capture is widely used in research. 50,51Garcia et al. 52 used commercial software for the validation of loading and temperature in a pilot plant.Miquel et al. 53 simulated a model of the desorption column built in Aspen Plus using 30 wt % MEA.The mass transfer correlations and CO 2 solubility were adjusted with experimental data on chemical absorption by Harbou et al. 54 Joel et al. 55 analyzed the intensified absorber for air combustion CO 2 capture using a rotating packed bed with correlations in Aspen Plus rate-based model.They carried out the new correlations in visual Fortran as subroutines and linked them dynamically to the Aspen Plus rate-based model.Zhang et al. 56 focused on the development of a dynamic model for the MEAbased CO 2 capture process for an air combustion pulverized coal plant using the advanced linear model predictive control based on an Aspen Plus steady-state model.Nittaya et al. 57 investigated a mechanistic dynamic model of an air combustion CO 2 capture plant using the MEA absorption process.Bui et al. 58 reviewed the dynamic modeling and optimization of flexible operation in air combustion CO 2 capture plants.Cau et al. 59 carried out the optimization of the process for energy penalization reduction with solvent regeneration through the retrofitting of a power plant with an MEA-based CO 2 capture by full oxyfuel combustion that utilized cryogenic distillation air separation and their hybrid configuration.
Cau et al. 60 assessed the commercial viability of partial oxyfuel combustion and examined simulation models of a carbon capture and storage (CCS)-equipped power plant.Their findings suggest that a greater CO 2 content in flue gas with oxygen enrichment minimizes the energy penalty associated with solvent regeneration.
Cormos 61 evaluated the key technical performances of oxyfuel combustion and postcombustion technologies.An in-depth analysis detected that the carbon capture energy penalties for coal-based supercritical oxycombustion plants are for postcombustion capture using alkanolamines.The cooling water needs of oxycombustion power plants are also reduced when compared with similar systems with postcombustion CO 2 collection utilizing alkanolamines.
Yadav and Mondal 24 provided a comprehensive review of the various novel oxyfuel technology configurations, compared in terms of energy penalty, auxiliary power consumption, CO 2 product purity, and CO 2 capture efficiency.All CCS technologies are associated with a substantial energy penalty.In comparison to other CCS approaches, oxyfuel combustion capture has a lower energy demand in the carbon capture section. 23According to the literature review, oxyfuel combustion is widely considered as a technically promising solution for carbon capture and storage to achieve net-zero emissions, compared to the conventional CCS approaches.
This work proposes a novel process configuration for carbon capture in oxyfuel combustion to deal with the problem related to high specific regeneration duty (SRD) associated with the CO 2 absorption process by MEA solvent.It is compared with carbon capture in the case of air combustion technology, especially at high removal efficiency.In conventional oxyfuel technology, after the burning of fossil fuel in the presence of oxygen, a large proportion of flue gas is recycled into the combustion chamber to control the temperature of the combustion process.In the proposed configuration, the flue gas recycling takes place after the absorber of the carbon capture process.In such a case, the driving force of mass transfer in the absorption process is higher than that of the air combustion process.This phenomenon leads to smaller absorbent requirements, resulting in lower regeneration energy and MEA demands.To obtain a comprehensive technical comparison of air combustion and oxyfuel combustion methods, the carbon capture process with MEA is investigated for five different flue gases.In both methods, the same amount of CO 2 capture is assumed at every capture rate.Then, the SRD and solvent consumption of the two technologies, that is, air and oxyfuel combustion, and the sensitivity analysis of the key operating parameters are compared.

■ MODEL DEVELOPMENT
Parameters of Modeling and Configurations.Those oxyfuel technologies are studied where additional carbon dioxide is added as an inert gas to clean oxygen.Carbon capture processes are retrofitted to air combustion and oxyfuel combustion and modeled with Aspen Plus with chemisorption technology, considering 30 wt % aqueous MEA as an absorbent.According to the literature review, 30 wt % aqueous MEA is an industrially preferred solvent for CO 2 removal from gas streams at around atmospheric pressure, and its relatively high loading capability affects the overall efficiency and effectiveness of the carbon capture process. 21,62,63The comparison of the schematic flowsheets of the two alternatives is presented in Figure 1.To compare carbon capture processes as part of air combustion and oxyfuel combustion technologies, only the amount of CO 2 produced during the combustion process is captured.Although oxyfuel combustion modifies conventional chemical processes into more cost-effective, productive, and safer processes, it can also be beneficial as an alternative when there is no place for energy intensification in industrial sectors. 64It should also be mentioned that by using oxyfuel technologies, the formation of NO x compounds can be avoided, providing additional environmental benefits by mitigating greenhouse gas emissions. 65,66ive different flue gases originating from different processes are selected, i.e., (i) gas-fired power plant; 67 (ii) CFPP; 68,69 (iii) coal-fired in a combined heat and power plant (Coal-CHP); 70,71 (iv) aluminum production industry; 72 and (v) cement manufacturing industry. 73,74The compositions and flow rates of the selected flue gases are given in Table 1.For the sake of comparison between air combustion and oxyfuel combustion, the compositions of flue gases are the same, but in the oxyfuel case, the nitrogen content is replaced with CO 2 as an inert gas so that the special effects of pure oxygen can be avoided.The mass flow of the flue gases of oxyfuel combustion origin is higher because carbon dioxide's molecular weight is higher than that of nitrogen.As a result, the density of the intake gas in oxyfuel combustion is substantially higher than in conventional air combustion.However, the amount of CO 2 in the fossil fuel burning process is always the same.
The applied carbon capture process is presented in Figure 2. The accuracy of the model was verified using experimental data by Nagy and Mizsey. 21The flue gas is fed at the bottom of the absorber column and flows upward to have a countercurrent operation with the lean MEA fed at the top of the absorber.The rich MEA to be regenerated is fed into the stripper, which receives heat input from its reboiler.The rate-based model is applied to model the absorber−stripper system because it is more suitable than the equilibrium phase model. 75,76This model  is based on two film theories compared to the equilibrium model, which is based on the theoretical number of equilibrium stages combined with Murphree's efficiency concept.The two modeling methods have been checked with experimental data. 21herefore, it can be concluded that the simulation of the absorber and desorber columns can be accurately and exactly modeled with the rate-based model since it gives a better prediction of the temperature and concentration profiles compared to the results of the equilibrium-stage model. 21,75s a result, for a detailed carbon capture process design, the ratebased model should be applied. 21In the rate-based model, CO 2 , N 2 , and O 2 are selected as Henry components (solutes), where Henry's law is applied.Henry's constants are also specified for these components: water and MEA.
Table 2 shows the main input parameters of the columns and specifications that are used for model development in the absorber and stripper.Most of the specifications are recommended for the rate-based model of the CO 2 capture process by Aspen Tech, 77 and some of them are taken from the literature. 78,79hemistry of the MEA−H 2 O−CO 2 Reaction System.For modeling the MEA−H 2 O−CO 2 system, different thermodynamic assumptions are applied in the liquid and the vapor phases.The presence of dissolved CO 2 in the form of carbonate and carbamate in the liquid phase has an electrolyte behavior; consequently, the electrolyte nonrandom two liquid thermodynamic model is used for the liquid phase. 80As the ions are not assumed to be present in the vapor phase, the SRK EoS model is selected for the description of the thermodynamic behavior of this phase. 81he important chemical reactions taking place in a system are described in this section (eqs 1−5). 82EA protonation: Carbonate formation: Hydrogen carbonate formation: Carbamate formation: Water dissociation: The chemical equilibrium constant K eq is calculated according to eq 6.In this equation, T is the absolute temperature, and A−D are the constants listed in Table 3.
The equilibrium reactions are applied to all of the modules in the process simulation except for the absorber and stripper columns.These two columns are controlled at each stage The rate of kinetic reactions is described by the power-law expression represented in eq 11, where r is the rate of reaction, k is the pre-exponential factor, T is the absolute temperature, R is the gas constant, and E is the activation energy. 84These values are summarized in Table 4.
Key Performance Indicators for Process Analysis.The following key performance indicators are specified in this research to conduct performance evaluation and comparative assessment for air and oxyfuel combustion technologies: Removal Efficiency of the Process.The removal efficiency is defined as the percentage of CO 2 captured by the process.This indicator is calculated according to eq 12.
Where F CO2,inlet and F CO2,outlet represent the inlet and outlet CO 2 flow rates in the absorber unit, respectively.Six different CO 2 removal efficiencies in the range of 80−97% are considered for air and oxyfuel combustion cases.Regarding oxyfuel combustion, 100% removal efficiency is also considered.The removal efficiency indicates only the amount of CO 2 that comes from the fossil fuel combustion process.In the case of oxyfuel combustion, the excess CO 2 is handled as a recycled inert gas.Therefore, the amount of carbon capture is identical for each removal efficiency in the two alternatives.Carbon Dioxide Loading.In conventional carbon capture configurations, carbon dioxide loading indicates the performance of the capture process, which reflects the efficiency of the mass transfer taking place in the absorber.According to eq 13, the CO 2 loading is defined as the ratio of the total mole quantity of apparent CO 2 to the total mole quantity of apparent MEA in the solvent.There are two CO 2 loading definitions in the absorption process: (i) the lean loading, determined for the stream leaving the bottom of the desorber and (ii) the rich loading, calculated for the solvent after the absorption process.
SRD.The regeneration of the absorbent is an energyintensive operation that influences the whole carbon capture process.Therefore, SRD is considered a dominant indicator to evaluate the total performance of the process.SRD is defined in eq 14.
where Q reg notes the regeneration duty in the reboiler and F CO2,inlet and F CO2,outlet is defined as the inlet and outlet CO 2 flow rates in the absorber.Only that amount of CO 2 is considered to come from the fossil combustion process.

■ RESULTS AND DISCUSSION
Solvent Consumption of Oxyfuel and Air Combustion.Table 5 presents the absorbent flow rates and the liquid/gas ratio values at different carbon capture rates for the five flue gases selected (Table 1).In the oxyfuel combustion cases, two carbon capture rates are shown: (i) the capture rate of the CO 2 of the combustion origin and (ii) the real capture rate that includes the CO 2 recycled as an inert compound.In the cases of both alternatives, the solvent flow rate increases if a higher carbon capture rate is required.
Since in the case of oxyfuel combustion the nitrogen content of the air is replaced with carbon dioxide, the total carbon dioxide flow rate and its concentration in the absorber tower in this technology are significantly higher than those of air combustion processes.The higher CO 2 content results in a lower absorbent requirement, and the CO 2 removal efficiencies are lower than those of the air combustion cases.The reduction of solvent is calculated to be approximately 24, 20, 13, 10, and 7% for the aluminum industry, gas-fired, coal-fired, coal-CHP, and cement industry, respectively.
In the case of oxyfuel combustion, 100% carbon capture can also be completed, where the real carbon capture rates range between 4 and 26%.The flue gases of the oxyfuel combustion have a much higher CO 2 concentration over the whole capture process, 36 which means a higher mass transfer gradient.Therefore, the absorbent and energy requirement values are much lower than those in the air combustion case, demonstrating a more beneficial capture process.
Comparison of Energy Consumptions.SRD variation with removal efficiency is demonstrated in Figure 3.The change in the energy consumption values is demonstrated by the values of the specific regeneration duties (SRD) for the two alternatives.It is obtained that • The oxyfuel combustion case needs less energy input and a smaller SRD due to the higher driving force for the mass transfer.• If the capture rate is increasing, the SRD is also increasing.
• The increase in SRD is exponential in the cases of air combustion.The result of SRD reduction (calculated in eq 15) in the two processes for all of the simulated flue gases is illustrated in Figure 4. SRD AC and SRD OXY indicate the SRD for air combustion and oxyfuel combustion, respectively.
The results show that higher CO 2 removal efficiency results in higher energy concumption but also that in the case of oxyfuel combustion, the energy requirement is significantly smaller.The reason for the smaller energy consumption is due to the higher driving force of the mass transfer in the oxyfuel combustion than that of the air combustion technologies.The CO 2 mass transfer between gas and liquid in the absorber tower is investigated according to the two-film theory.The mass transfer rate across the gas boundary is calculated by eq 16.K G and A are the binary mass transfer coefficient and mass transfer surface area, respectively, which are fixed for both technologies.So, only the concentration difference can influence the mass transfer.A higher carbon dioxide concentration in the flue gas (y) results in a higher concentration difference and a higher mass transfer rate.
According to the novel configuration proposed in this research, flue gas recycling takes place after the carbon capture process (Figure 1) in the absorption tower.In this case, a higher CO 2 concentration can be carried out in the absorber, and only that amount of CO 2 is captured that originates from the burning process.The excess amount of flue gas is recycled.
To show the differences in the CO 2 concentration (y) in the absorber of the captured process, the ratios of the CO 2 concentrations in the cases of the oxyfuel combustion and the air combustion technologies are calculated and compared (eq 17, Table 6).Table 6 shows the y ratio values for different CO 2 removal efficiencies and every flue gas investigated.The inlet ratios are identical per definition; however, the huge numbers of outlet   ratios show the beneficial feature of this kind of oxyfuel technology.After the y ratio values are seen, it can be explained why the carbon capture process is more attractive in the case of proper oxyfuel technology.Less absorbent can be applied, and the energy requirement is also lower.Effect of Removal Efficiency on CO 2 Lean Loading. Figure 5 shows the corresponding results related to the lean loading variation due to the removal efficiency change in air combustion as well as oxyfuel combustion for the five simulated flue gases.CO 2 lean loading decreases as removal efficiency increases.However, in the case of air combustion, lean loading decreases more quickly for removal efficiencies of >90% (Figure 5a).The reason for this is that guaranteeing the necessary driving force for the CO 2 mass transfer requires a low lean loading.Consequently, the content of CO 2 in the lean solvent stream rapidly decreases.In the oxyfuel combustion case, the lean loading also decreases with removal efficiency; however, there is no rapid reduction in this trend, as the driving force of this technology is relatively high compared to air combustion (Figure 5b).

Impact of Lean Loading on Specific Reboiler Duty.
The lean loading shows the ratio of CO 2 and MEA in the absorbent, leaving the desorber unit.Low lean loading is needed if a high CO 2 removal efficiency is required and/or the CO 2 concentration in the flue gas is low.These two effects are connected.The low lean loading requires higher reboiling, which translates into higher specific reboiler duty.
The effects of lean loading variation on the SRD for the two investigated technologies are listed in Figure 6.The reduction of lean loading needs energy, that is, an SRD increment.This change in air combustion is more extensive than that of oxyfuel combustion due to the significantly different CO 2 concentrations in the flue gases.It can be seen that the oxyfuel combustion is advantageous over air combustion.It can also be seen that the lowest lean loading is required in the case of gasfired combustion and the highest is required in the coal-fired case.
Impact of Oxyfuel Combustion Substitution with Air Combustion on Liquid/Gas Ratios.Figure 7 shows the L/G reduction if air combustion is replaced with oxyfuel combustion.In eq 18, the absorbent gas ratios, that is, (L/G) AC and (L/  G) OXY indicate the L/G ratios in air and oxyfuel combustion, respectively.As a general trend, in each technology, by increasing the solvent flow for the sake of a higher carbon capture rate, the L/G ratios are to be increased.The highest L/G reduction is reported for the flue gas with the lowest CO 2 content (aluminum), and the flue gas from the cement industry has the lowest L/G reduction due to its highest CO 2 content among the case studies.This agrees with the conclusion obtained before that the lower CO 2 composition needs more absorbent and lower lean loading so that the driving force for mass transfer can be guaranteed.
The reduction of L/G by using oxyfuel combustion instead of air combustion shows the potential of this technology to capture the same amount of CO 2 with a smaller amount of solvent, which is inserted into the process as a liquid stream.
■ CONCLUSIONS As mobility will shift to electric vehicles, which require electric power, and electricity is still largely produced on a fossil basis, carbon capture must be seriously considered and possibly promoted.According to this chemical engineering study, oxyfuel technologies can show more chemical attraction for CO 2 capture than air-based ones.The application of clean oxygen with artificially backmixed carbon dioxide as an inert to replace nitrogen is studied, which is usually applied in retrofit cases.Such oxyfuel technology has a higher carbon dioxide composition in the flue gases, resulting in a higher driving force for mass transfer in the absorber.As demonstrated by five different flue gases from oxyfuel technologies and compared to the air combustion alternative, the oxyfuel technology is much better from the carbon capture point of view.Carbon capture requires less energy, especially if more than 90% carbon dioxide removal is designed than that of the air combustion case.Even the total amount, 100%, of carbon dioxide from the combustion process can be easily captured.Besides the lower energy consumption, the carbon capture process can work at a lower absorbent/flue gas ratio and with a smaller amount of absorbent than the air-based processes.The energy savings can be as high as 84%, and, on the other hand, no NO x emissions are produced in the oxyfuel combustion processes, further reducing green-house gas emissions.The application of this oxyfuel process, replacing the inert effect of nitrogen with carbon dioxide, can improve the advantages of carbon capture and facilitate the fulfillment of environmental prescriptions.In this study, only a technical feasibility study of oxyfuel combustion was completed since the economic features are highly country-dependent.In the future, further research is required to evaluate economic, cost, or long-term reliability before practical implementation in the application area.

Figure 1 .
Figure 1.Schematic comparison of CO 2 capture by the absorber−stripper process in air combustion and oxyfuel combustion.

Figure 2 .
Figure 2. Configurations of carbon capture process based on chemical absorption technology.

Figure 3 .
Figure 3. SRD variation with removal efficiency in (a) air combustion and (b) oxyfuel combustion.

Figure 4 .
Figure 4. SRD reduction with the application of oxyfuel combustion.

Figure 5 .
Figure 5. Variation of lean loading with removal efficiency in (a) air combustion as well as (b) oxyfuel combustion.

Figure 6 .
Figure 6.Effect of lean loading variation on the specific regeneration duty in (a) air combustion as well as (b) oxyfuel combustion.

Table 1 .
Flue Gas Compositions and Parameters

Table 2 .
Aspen Plus Model Specifications of Absorber and Stripper Column for Air Combustion and Oxyfuel Combustion

Table 3 .
Reaction Equilibrium Constants Used in This Work

Table 4 .
Rate Constant Values in Absorptive CO 2 Capture 85

Table 5 .
Solvent Flow Rates in Different Flue Gases

Table 6 .
y ratio of the CO 2 Concentration in the Inlet and Outlet Gaseous Streams in the Absorber Tower