Application of chemical looping air separation for MILD oxy‐combustion in the supercritical power plant with CO2 capture

Chemical looping air separation (CLAS) is a novel and promising technology for oxygen production. This paper presents the application of CLAS to the supercritical power plant for MILD oxy‐combustion. Compared with the reference conventional supercritical power plant, the power generation efficiency of the CLAS integrated MILD oxy‐combustion plant is only reduced by about ~1.37% points at the baseline case. CO2 compression process imposes additional ~3.97% points efficiency penalty, which is inevitable to all of the CO2 capture technologies. The net power efficiency of the CLAS integrated MILD oxy‐combustion plant is ~37.37%. Even though a higher reduction reactor temperature could boost the power efficiency and a higher oxidization reactor temperature reversely decreases the power efficiency, the influence of reactor temperature is marginal. The performance of CLAS integrated MILD oxy‐combustion plant is not sensitive to excess CO2 and O2 ratio. Different oxygen carriers have different suitable operating region, but possess similar power efficiency. The carbon capture rate of the CLAS integrated MILD oxy‐combustion plant is up to ~100%, resulting in a virtually carbon‐free fossil power plant.

(CLAS) is an alternative oxygen production method. It is principally similar to chemical looping combustion (CLC). CLAS includes two reactors, one is reduction reactor and the other is the oxidization reactor. The solid particles circulating between the two reactors are oxygen carriers, based on transitional metal oxides, such as CuO-Cu 2 O, MnO 2 -Mn 2 O 3 , Mn 2 O 3 -Mn 3 O 4 , and Co 3 O 4 -CoO. 8,9 In the reduction reactor, the oxygen carrier in a higher oxidization state Me x O y decouples and releases gaseous oxygen O 2 in the atmosphere of inertial purge gas, such as steam or CO 2 , as shown in Equation (1). The decoupled oxygen carrier Me x O y-2 then enters the oxidization reactor, where the oxygen carrier is regenerated to its higher oxidization state Me x O y with incoming air, as shown in Equation (2). Through the continuous circulation of oxygen carrier between the two reactors, oxygen is separated from air.
In CLAS, the purge gas applied could be any gas agent that doesn't react with the oxygen carrier. The most common gas is steam and CO 2 . An oxy-fuel combustion power plant could exactly provide such CO 2 stream needed by CLAS. The produced O 2 and CO 2 stream is then sent to furnace for fuel combustion. From the point of view of power efficiency, CLAS integrated oxy-combustion power plant could have higher efficiency because the conventional cryogenic air separation unit (ASU) is avoided and CLAS process separates oxygen in the air without severe energy penalty.
Moghtaderi et al 8 conducted thermodynamic analysis of metal oxides and identified Cu-, Mn-, Co-oxides were suitable oxygen carriers for CLAS. Li et al 10 prepared Cobased oxygen carrier and produced an O 2 -CO 2 stream in a fixed bed reactor. Song et al [11][12][13] prepared Al 2 O 3 -and SiO 2supported Cu-, Mn-, and Co-oxygen carriers and found the bimetallic oxygen carrier could have higher stability and reactivity. Wang et al [14][15][16] evaluated the Cu-based oxygen carrier for CLAS in a fixed bed reactor and TGA. Steam was used as a carrier gas. It showed that the reduction and oxidation complied with the nucleation and nuclei growth model. The oxygen carrier could remain stability after multicycles.
Moderate or intense, low-oxygen dilution (MILD) combustion is a novel combustion concept. 17 The features of MILD combustion are the preheating and dilution of reactants. The preheated reactants are at a temperature higher than its autoignition temperature. 18 The oxygen fraction of MILD combustion is supposed to be 2-9 vol. % and distributes uniformly in the premixed combustion of fuels. 19 The preheating and dilution of reactants is usually achieved by the in-furnace flue gas recirculation. 20 MILD combustion is flameless, and it has the potential to offer ultralow pollutant emission, high thermal efficiency, enhanced combustion stability and fuel flexibility. [21][22][23] MILD combustion has been successfully applied for gaseous fuels [24][25][26][27] as well as pulverized fuels (with size 50-120 μm). [28][29][30][31] Kiga et al 32 successfully implemented the MILD combustion of high volatile pulverized coal using a drop tube furnace. Suda et al 33 investigated the behavior of pulverized coal in high-temperature air combustion using a burner of 250 kW. It indicated that it was possible to form a stable flame even for low-volatile coals like anthracite. Mao et al 34 conducted experimental investigation of coal-fueled MILD oxy-combustion integrated with flue gas recirculation at a 0.3 MW th furnace. At a jet velocity of 100 m/s, MILD combustion was achieved. Meanwhile the experiment was operated under oxy-fuel atmosphere that was established by the flue gas recirculation. Swirl oxy-fuel combustion was also tested as a comparison. The CO 2 concentration in flue gas could maintain at a high level in the combustion. Li et al 35 investigated the MILD oxy-combustion characteristics of light oil and pulverized coal in a pilot scale furnace. Different burner configurations were compared. The experimental tests proved that the coal-fueled MILD oxy-combustion was technologically feasible. MILD combustion requires a relatively low O 2 concentration feed gas but a high preheating temperature oxidizer. CLAS could exactly provide such a high temperature and low O 2 fraction stream. The general benefits of CLAS and MILD combustions have also stimulated the combination into MILD oxy-combustion. 36 In this paper, CLAS is integrated with an on-site MILD oxy-combustion power plant. CLAS is operated under a region where heat balance is achieved and the CLAS integrated MILD oxy-combustion power plant is in full heat balance range without extra heat sources from outside. The comparison of a conventional supercritical power plant and the CLAS integrated oxy-combustion power plant is analyzed and discussed. A sensitivity analysis of the main process parameters is also performed and their effects on the power plant performance are discussed.

| Reference plant
The software Aspen Plus is applied in the simulation work. The reference power plant used in this work is a 1000 MW e supercritical power plant, 37 in which approximately 2855 tonne/h steam generated in the boiler. The steam turbine conditions correspond to 26.0 MPa/600°C as main steam with 600°C at the reheater. Major subsections of the plant include coal milling, boiler, steam cycle with regenerative feed water heating train, and ash and dust removal units. The plant operates with a designed efficiency of 42.71% (LHV). In the boiler, heat exchangers are arranged in the order of high-temperature superheater, high-temperature reheater, low-temperature superheater, low-temperature reheater, economizer, and air preheater. For simplicity, the chemical processes of flue gas desulfurization and denitration are not considered in the simulation. The regenerative feed water heating train includes three high-pressure and four lowpressure closed heat exchangers, and one open feed water heat exchanger, that is, deaerator. The diagram of the steam cycle is shown in Figure 1. Primary parameters of the reference plant are described in Table 1. A bituminous coal is adopted in the simulation and the coal analysis is shown in Table 2.

| CLAS integrated MILD oxycombustion plant
The schematic diagram of the CLAS integrated MILD oxycombustion plant, including CLAS, boiler, steam cycle, and CO 2 compressors, is presented in Figure 2.

| CLAS process
The CLAS process is modeled using two separate reactors. Air is preheated in a heat exchanger and then fed into the oxidization reactor. The reduced oxygen carrier from the reduction reactor is oxidized in the oxidization reactor. The exhaust gas from the oxidization reactor is oxygen-depleted air. The heat of the oxygen-depleted air is recovered by preheating the inlet fresh air. The exhausted oxygendepleted air temperature is fixed constantly at 50°C. The oxygen carrier in a higher oxidization state leaves the oxidization reactor and enters the reduction reactor. CO 2  gas is bled from the furnace flue gas channel. The required mass flow rate of CO 2 -rich gas is determined by the reduction temperature. The flow rate of air and CO 2 stream is as close to stoichiometric as possible in order to minimize the flow rates. In the reduction reactor, gaseous oxygen is released from the oxygen carrier at the atmosphere of CO 2 . The O 2 and CO 2 mixture stream is directly fed into the furnace for coal combustion. The reduced oxygen carrier then circulates back to the oxidization reactor for next cycle.
The heat from oxidization reactor is used to compensate heat required by reduction via inert solid circulation. The temperature of reduction is therefore lower than that of oxidization reactor. Only a small amount of heat is required by reduction reactor from outside, which comes from the inlet CO 2 -rich gas. The CO 2 -rich gas is bled from two ports of the boiler flue gas channel to mix into a stream with the target temperature demanded by reduction. The management of CLAS is as follows, and the detailed operating region of CLAS can be referred in our former work 38 : 1. Select the oxidization reactor temperature; 2. Select the reduction reactor temperature (lower than oxidization temperature); 3. Temperature difference of the two reactors determines the inert solid flow rate (the inert solids take away all of the heat released by oxidization to reduction); 4. Reduction temperature determines the CO 2 flow rate (based on chemical equilibrium);  5. Heat deficiency of reduction reactor determines the inlet CO 2 temperature; 6. CO 2 flow rate and temperature determine the flue gas extraction flow ratio from two ports of the flue gas channel.

| Boiler
In the CLAS integrated MILD oxy-combustion plant, O 2 is produced from CLAS process and sent to the furnace. The oxygen concentration produced from CLAS process is at a relatively low level, <12 vol. %. This level is far less than what is available for conventional oxy-fuel combustion, which requires an oxygen concentration around 28-35 vol. %. 39 Therefore, conventional oxy-fuel combustion cannot be directly adopted in existing boilers and novel combustion technology is needed. Here, MILD combustion is applicable and employed for such a low-oxygen concentration stream. The original boiler of the reference power plant is thereby not adopted due to the different characteristics of air firing and MILD combustion, but the general structures of the boilers are similar. For a fair comparison, critical parameters such as main steam temperature (600°C/600°C), steam pressure (26.0 MPa), stack temperature are not changed as possible in the MILD oxycombustion boiler. In the MILD oxy-combustion boiler, the air preheater is removed, and a feed water preheater substitutes the air preheater at the end of the flue gas channel. After one regenerative feed water heater, a portion of the feed water to the boiler is bypassed to the feed water preheater in the flue gas channel instead of the feed water heating train. This arrangement diminishes steam bleeding from the steam turbine. CO 2 -rich gas stream is bled from the flue gas channel of the boiler. The CO 2 -rich gas stream bled is at high temperature, conventional compressors or blowers are not feasible for gas bleeding, and so an ejector is employed. The ejector operates as a compressor, but with no moving parts. The ejector utilizes a high-pressure fluid stream to drive a low-pressure stream. The primary fluid passes through a nozzle where the pressure energy is converted into kinetic energy. The high-velocity jet entrains the secondary fluid. The two streams mix in a confluence pipe. The ejector is widely used in SOFC system for fuel gas circulation. [40][41][42][43] There are two bleeding ports in the flue gas channel to easily adjust the CO 2 -rich gas temperature and flow rate. The first port is after the first superheater, that is, at the outlet of the furnace; the second port is after the economizer, that is, before the newly added feed water preheater. The bleeding ratio at the two ports is adjusted to meet the target CO 2 temperature and flow rate, which is determined by the CLAS process. At the second port, the flue gas at a lower temperature is bled and pressured as the primary flow of the ejector, and the flue gas at higher temperature at the first port is the secondary flow for the ejector. The two CO 2 streams are mixed together. The bled CO 2 stream is particulate-removed before entering CLAS reactors. For simplicity, the treatment of flue gas with a series of chemical process and scrubbers to remove sulfur dioxide and nitrogen oxide is not considered here.

| Steam cycle
The steam cycle arrangement is only marginally changed with the CLAS integration. The steam turbine is divided into three parts: a high-pressure part fed with main steam, an intermediate-pressure part fed with reheated steam and a low-pressure part. As part of efficiency improvement in the plant, regenerative feed water heating is still necessarily implemented, using steam bled at different ports of the turbine to heat the feed water as shown in Figure 2. The heating train includes three high-pressure, four low-pressure closed feed water heat exchangers and a deaerator. The boiler feed water pump is also driven by steam which is finally led to the condenser.

| CO 2 recovery and compression
CO 2 -rich gas exhausted from the boiler is cooled to 30°C in a heat exchanger. The resulting CO 2 stream is compressed using a four stage intercooled compressor for pipeline transport and geological sequestration. Cooling water at environmental temperature is used to cool the compressed stream in each stage. The majority of water remaining in the CO 2 stream is removed in the flash drums of each compressor stage. The CO 2 is finally pressurized to 150 bar with a purity of 99.1%. In a practical facility, a flue gas desulfurization unit (FGD) needs to implement in order to control SO 2 in the CO 2 -rich gas stream. A low temperature partial condensation process integrated with a distillation column may also be applied to remove other impurities from the CO 2 , 44 but this is not the scope of this work.

| ANALYSIS METHODOLOGY
The net power efficiency in the oxy-fuel combustion power plant is determined using the following equation: Here, e is the plant net power efficiency, %; m coal is the input coal mass flow, kg/s; LHV coal is the coal lower heating value, kJ/kg.
The net power output of the oxy-fuel combustion power plant W e is calculated as follows: Here, W st is the steam turbine power, kW; W aux is the auxiliary power, such as the condensate pump, water recirculation pump, blowers, etc., kW, W CO 2 ⋅comp is the CO 2 compression power, kW.
To evaluate the penalty imposed by CO 2 compression, the power generation efficiency excluding CO 2 compression is defined as following: Here, g is the power generation efficiency, %; W g is the power output excluding CO 2 compression, kW; For reference power plant, the net power efficiency is also the power generation efficiency because there is no CO 2 compression. CO 2 is bled from the flue gas channel. The CO 2 bled ratio is defined as: i,CO 2 is the CO 2 bled ratio at Port i, %; M i,CO 2 is the mass flow rate of the CO 2 -rich gas bled at Port i, kg/h; M t,CO 2 is the total mass flow rate of the CO 2 -rich gas leaving the furnace, kg/h. CO 2 capture efficiency is defined as: CO 2 is the CO 2 capture efficiency, %; m CO 2 is the CO 2 flow after compression for storage, kg/s; and m CO 2 ,total is the total CO 2 generated in the plant, kg/s.

| Performance analysis
A typical operating condition of the CLAS integrated MILD oxy-combustion plant is selected as a baseline case here, using the assumptions in Table 3. Mn 2 O 3 -Mn 3 O 4 is used as the oxygen carrier as a result of its natural abundance, low cost, and non-toxic character. 45 The essential simulation assumption of CLAS integrated MILD oxycombustion plant is the same as the reference plant except for the CLAS process. The reduction reactor temperature is 830°C, and the oxidization reactor temperature is 840°C. CO 2 -rich gas stream bled from the boiler flue channel into the reduction reactor is at an excess ratio 1.1 to decouple all of the gaseous oxygen. The excess O 2 ratio here is less than that of air firing plant. The excess O 2 ratio in air firing plant is typically 1.2 to minimize carbon burning loss. In this work the excess O 2 ratio is 1.05 for a full coal conversion. This value is commonly adopted in oxy-fuel combustion to minimize the oxygen content in the flue gas. 46 temperature (>800°C) of O 2 inlet gas may trade off this effect. CLAS is completely in a region of heat balance. It must be indicated that this case may not be the optimal one in power efficiency. The performance of the CLAS integrated MILD oxy-combustion plant and the reference plant is listed in Table 4, and the important selected flows within the CLAS integrated MILD oxy-combustion plant is shown in Table 5.  It is found that, the net power efficiency of the MILD oxycombustion power plant decreases after adding CLAS process. The plant net power efficiency (LHV) for the reference plant is 42.71%, whereas the net power efficiency of the CLAS integrated MILD oxy-combustion plant is 37.37%. The energy penalty imposed by CCS generally comes from two aspects: CO 2 capture and CO 2 compression. The efficiency penalty typically associated with CO 2 in this CLAS integrated MILD oxycombustion power plant results from CO 2 compression. If CO 2 compression process is not considered, the power generation efficiency drop is only 1.37% points including CO 2 capture. The CO 2 compression imposes additional 3.97% points efficiency penalty. The total efficiency decrement including CO 2 capture and CO 2 compression is ~5.34% points. This efficiency decrement is low and less than other coal-fueled counterparts with post-combustion CO 2 capture or cryogenic air separation oxy-fuel combustion, which commonly impose 8-12% penalty with final power efficiency about ~30-33%. [48][49][50][51] Therefore, the efficiency drop in the proposed CLAS integrated MILD oxycombustion plant is acceptable and satisfactory.
The bleeding of high-temperature CO 2 is used for oxygen carrier decoupling to release gaseous O 2 . The produced O 2 and CO 2 mixture stream at a lower temperature level turns back to the furnace. It is a circle. The energy for the production of O 2 comes from the thermal energy of flue gas, which means that less steam is produced for stream turbine and less power generation. This is the "capture penalty" of the entire CLAS integrated MILD oxy-combustion plant. For the bled flue gas, only a small portion of heat is consumed for oxygen carrier decoupling and a large portion of heat in the flue gas is recovered as the flue gas is recycled to the furnace. For cryogenic ASU, the cryogenic ASU requires electricity for compression and refrigeration. 52 However, the electricity production requires more thermal energy. That is one reason of low efficiency penalty of CLAS. Moreover, the pure oxygen separated from air in the cryogenic ASU is then mixed with CO 2 . From the point of view of the second law of thermodynamics, the mixing process imposes enormous exergy loss. The exergy loss here means that the electricity for production of pure O 2 is wasted. The CLAS process exactly provides a mixture stream of O 2 and CO 2 required by the furnace, which avoids pure oxygen production process and the later mixing loss. This is also an explanation why the CLAS integrated oxy-combustion power plant owns higher net power efficiency.
For CLC, there are commonly two ways to process solid fuels such as coal. The first is: coal is gasified in a gasifier, producing syngas. The resulting syngas reacts with the oxygen carrier in a fuel reactor. The disadvantage is the requirement of a gasifier and an ASU, which impose energy penalties. The second way for coal-fueled CLC is coal is in situ gasified in the fuel reactor with oxygen carrier. The coal pyrolysis, char gasification and oxygen carrier reduction occur in the same reactor. This eliminates the gasifier and ASU. However, the coal, char, ash, and oxygen carrier are mixed together, and the separation of unconverted char, ash, and oxygen carrier is a challenge. Li et al 53,54 experimentally investigated the separation of char and oxygen carrier. The separation efficiency of char was around 80-90%, depending on operation condition, and the unconverted char will enter the air reactor and results in CO 2 emission. For CLAS, it highlights gaseous O 2 production in an individual reactor, that is, the reduction reactor, the produced O 2 then reacts with the coal in the furnace of the boiler. The oxygen production and combustion processes are separated. Coal is burned in an O 2 /CO 2 atmosphere and never mixed with the oxygen carrier. Char and volatile matter conversion are better in gaseous O 2 than in CLC. 55 The separation of unconverted char, ash from oxygen carrier is also avoided. This is the advantage of CLAS over CLC.

| Sensitivity analysis
Sensitivity analysis could reveal the potential approaches to optimize the system. The steam cycle is technologically wellestablished and proven, it is thereby not necessary to conduct the sensitivity analysis in those sections. Several parameters in the newly added CLAS which may affect the performance of the entire system need to be considered, such as reduction reactor temperature, oxidization reactor temperature, CO 2rich gas excess ratio, and O 2 excess ratio. reactor temperature. The reduction reactor temperature and the oxidization reactor temperature are varied, whereas other design parameters are kept constant. With the variation in temperature, the required CO 2 -rich gas bleeding rate and temperature are also adjusted accordingly. Figure 3 is the plant net power efficiency e including CO 2 compression. The efficiency has already accounted CO 2 compression, which imposes approximate ~4% points efficiency penalty. This is common and inevitable to all the CO 2 capture technologies that release CO 2 at near atmosphere pressure. It can be seen that with the rise of the oxidization reactor, the F I G U R E 4 Selected indicators in the CLAS integrated MILD oxy-combustion plant with the variation in reduction reactor temperature  net power efficiency decreases. With the rise of the reduction reactor temperature, the net power efficiency increases. This tendency leads the region approaching the diagonal to own higher power efficiency, whereas the region near the bottom right corner is at a lower power efficiency. The highest net power efficiency of 37.51% is achieved at the reduction temperature of 810°C and oxidization temperature of 820°C. It also means that with the decrease in the temperature difference between the reduction reactor and the oxidization reactor, the CLAS integrated MILD oxy-combustion plant is of higher power efficiency. The power efficiency diminishes with the rise of the temperature difference between the two reactors. At the blank region in Figure 3, that is, the oxidization reactor temperature 880-890°C and reduction reactor temperature <790°C, the temperature of CO 2 -rich gas bled from flue gas channel is always lower than the reduction reactor demand and therefore unsuitable for decoupling of the oxygen carrier. Figure 4 illustrates some important indicators of the plant with the variation in reduction reactor temperature. The oxidization reactor temperature is kept constant at 820°C. As the reduction reactor temperature increases, the amount of CO 2 needed to decouple the oxygen carrier decreases due to an increased oxygen partial pressure in equilibrium. Accordingly, the oxygen fraction produced increases. As shown in Figure 4A, the combustion temperature and sequent furnace outlet temperature increases due to a lower amount of gas agent at higher reduction reactor temperature. Here, in the CLAS integrated MILD oxy-combustion plant, the air preheater is replaced by a feed water preheater after the economizer. The higher inlet temperature of the feed water preheater allows more water to bypass the regenerative heating train. It reduces the steam bleeding and augments the steam turbine power output. That's why the power efficiency increases with the rise of the reduction reactor temperature. Meanwhile, the CO 2 -rich flue gas is bled from two ports at different temperatures. One is at the furnace outlet, that is, the high-temperature port; the other is after the economizer, that is, the low-temperature port. The CO 2 -rich gas bleeding portion from the two ports is flexibly adjusted according to the reduction reactor temperature.

| Reduction reactor temperature and oxidization reactor temperature
As shown in Figure 4B, the rise of the reduction reactor temperature, the CO 2 -rich gas bleeding ratio I,CO 2 from Port I decreases, whereas the CO 2 -rich gas bleeding ratio II,CO 2 from Port II increases. This is because with the rise of reduction reactor temperature, the required inlet CO 2 temperature for reduction reactor increases. As shown in Figure 4C, the required inlet CO 2 temperature increases from 756°C to 832°C, by 10.0%, Port I temperature also increases from 777.1°C to 907°C, by 16.8%, and Port II temperature increases from 394.6°C to 561.9°C, by 43.4%. The temperature increment of Port II is much higher than that of Port I, whereas the required CO 2 temperature is less increased than that of Port I and Port II, so the CO 2 -rich gas bleeding ratio from Port I decreases. The drop of bleeding ratio in Port I means at a higher reduction reactor temperature more heat in high quality is used for heat transfer and power conversion rather than purging the oxygen carrier. It also reveals why the increase in reduction reactor temperature benefits the efficiency.
In Figure 3, it is also found that with the rise of the oxidization reactor temperature, the power efficiency decreases. This phenomenon is explained as follows: at a higher oxidization reactor temperature, more air is required for oxygen carrier oxidization due to equilibrium. Thus, the power needed by the air blower increases. Meanwhile, the oxidization reactor temperature has little influence on the reduction reactor performance, that is, the oxygen concentration and temperature do not change with the variation in the oxidization reactor temperature. So the oxidization is nearly independent and does not affect the performance of the boiler and steam cycle. Therefore, the increase in air blower power directly decreases the plant power output.

| Excess CO 2 ratio
The minimal CO 2 flow rate into the reduction reactor is at the chemical stoichiometric of thermodynamic equilibrium to decouple all oxygen carriers to release gaseous oxygen. But in practical facility, CO 2 will be in excess for a better reduction performance or fluidization. The influence of excess CO 2 on the performance of the plant is shown in Figure 5.
The plant performance is not sensitive to the excess CO 2 ratio, even though a higher CO 2 blower power is required but its effect is marginal. The furnace outlet temperature decreases at a higher excess CO 2 ratio, but the amount of CO 2 increases in the furnace downstream channel. It does not affect the heat exchange for steam cycle, and there is little variation in the power output in the steam turbine.

| Excess O 2 ratio
In a conventional boiler, excess air is commonly adopted for full coal combustion. In the oxy-fuel combustion plant, oxygen could be also excessive to benefit the solid fuel combustion. Figure 6 illustrates the influence of excess O 2 ratio on the performance. It must be mentioned that the excess O 2 primarily provides a better combustion environment, benefitting the reaction kinetics and reducing the penalty from unburnt solid fuel. Some studies have shown that the burnout of coal was reduced under MILD combustion condition. 28,56 The carbon burnout is significantly affected by the particle residence time, 28 particle size, 34 and reaction rate. 35 Due to the lack of specific relationship of operation condition on carbon burnout, this effect is not directly reflected in the simulation. Here, it is still assumed that unconverted char ratio is constant, and the remaining oxygen goes into the furnace flue gas channel and CO 2 compressors. Figure 6 shows that with the rise of oxygen excess ratio, the total power output is nearly not affected, that is, the boiler and the subsequent steam cycle are not influenced by the oxygen excess ratio. There is a slight decrease in the net power output. This is due to that CO 2 compression power is increased because O 2 is mixed into the CO 2 -rich gas stream. The trace O 2 is removed in the flasher as non-condensate gas from the concentrated CO 2 stream after CO 2 is condensed. With the rise of oxygen excess ratio, the power of air blower increases as well, which reduces the net power output of the plant.  oxides offer lower power plant efficiency. This is because the reduction is operated at a relatively low temperature, 420°C. The flue gas temperature at Port II is 465°C, which is still higher than the minimal CO 2 temperature 454°C required by the reduction reactor. Port I bleeding is also eliminated. The temperature of the bled CO 2 -rich gas is higher than that required, and the redundant sensible heat is not utilized in the integration. This results in an efficiency loss. In fact, any heat can be utilized if a more delicate heat network is implemented, but this will complicate the power plant system. For simplicity and comparison, it was not considered.
These CLAS integrated MILD oxy-combustion power plants possess similar power efficiency. CLAS process produces oxygen for oxy-combustion with small energy penalty. CLAS is in heat balance without tight connections to other components in the plant, and just needs a CO 2 -rich gas stream from the furnace flue channel. The bled CO 2 -rich gas is finally recycled back to the furnace. The oxygen production and heat integration of the entire system is not appreciably affected by the oxygen carrier variation. From the perspective of thermodynamics, CLAS is almost an independent block, and all of the oxygen carriers are suitable for this integration with small efficiency deficiency. So, other criteria should be imposed in oxygen carrier selection, for example, the MnO 2 -Mn 2 O 3 is operated at a temperature span of around 380-440°C. At this medium temperature range, the reaction kinetics needs to be considered. CuO-Cu 2 O oxygen carrier particles tend to agglomerate at a temperature higher than 900°C due to the relatively low melting temperature of copper (1084°C). So kinetics, activity in multiple cycles, agglomeration, and other characteristics are points of attention in selection and preparation of oxygen carrier.

| Plant comparison
Due to the different characteristics of air firing and MILD oxy-combustion, the original boiler is not feasible for the proposed power plant. The MILD oxy-combustion boiler should be redesigned, for instance low O 2 concentration would result in a high flow rates and larger boilers. Compared with the reference supercritical power plant, the sequence of heat exchanger tubes involved in the MILD oxy-combustion boiler is not changed. The regenerative feed water heating train and steam turbine in steam cycle also remains. Heat transfer is definitely changed in the boiler because flue gas is mostly CO 2 rather than air. Table 7 presents the comparison of heat exchangers of the CLAS integrated MILD oxy-combustion plant in the typical operating condition and the reference plant.
Heat exchanger details are calculated by the HeatX block in the Aspen Plus software according to the stream properties, that is, temperature difference, gas components, etc. Radiation contributes a large portion of heat transfer in the first superheater and the water cooling wall. HeatX block cannot reveal the tube size in radiation. So heat duty is listed instead. It can be seen that for most of the heat exchangers, the size needs to be reduced in the CLAS integrated MILD oxy-combustion plant. The air preheater is replaced by a feed water preheater at the end of the flue gas channel. Because the heat transfer coefficient of water-air is higher than air-air, the feed water preheater has a much smaller tube size.

CLAS integrated MILD oxy-combustion plant
Reference plant  Table 8 compares the flue gas flow rate at the outlet of each component within the CLAS integrated MILD oxycombustion plant and the reference supercritical power plant. In the CLAS integrated MILD oxy-combustion plant, the flue gas into the furnace increases due to a recycled CO 2 -rich gas. The flue gas flow rate along the channel is decreasing in the CLAS integrated MILD oxy-combustion plant due to gas bleeding. At the furnace, the flue gas flow rate of the CLAS integrated MILD oxy-combustion plant is about three times than the reference plant. After bleeding Port I, the flue gas flow rate of the CLAS integrated MILD oxy-combustion plant decreases to values lower than the reference plant, except the low-temperature reheater. In the CLAS integrated MILD oxy-combustion plant, the order of components in the flue gas channel are not changed. The size of heat exchangers is technologically reduced compared with the reference power plant. However, for the furnace, the flue gas volume will double the ignition and combustion stability may be affected, and a large size of the furnace is needed. This challenge requires further consideration.

| CONCLUSIONS
The CLAS process to produce a stream of O 2 and CO 2 mixture gas is coupled with the supercritical power plant for MILD oxy-combustion and CO 2 capture. The power generation efficiency of the CLAS integrated MILD oxy-combustion plant is reduced by ~1.37% points compared to the reference supercritical power plant. CO 2 compression imposes about ~3.97% efficiency penalty, which is common and inevitable to all of the CO 2 capture technologies.
Different oxygen carriers have different operating region. It is not reasonable to compare the oxygen carrier at the same operating condition, but different oxygen carriers present similar efficiency because CLAS is in heat balance without tight connections to other components in the plant except CO 2 stream. Using Mn 2 O 3 -Mn 3 O 4 as oxygen carrier, a higher reduction reactor temperature increases the power plant efficiency, whereas a higher oxidization reactor temperature decreases the power plant efficiency. The influence of excess CO 2 and O 2 on the efficiency is marginal. The heat exchanger analysis finds that the size of heat exchanger tubes in the MILD oxy-combustion boiler flue gas channel will decrease, and it is therefore may be possible to retrofit the conventional supercritical power plant with CLAS process for oxy-combustion and CO 2 capture.