Power generation based on chemical looping combustion: Will it

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Power generation based on chemical looping combustion: Will it qualify to reduce greenhouse gas emissions from life cycle assessment? Junming Fan, Hui Hong, and Hongguang Jin ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00519 • Publication Date (Web): 20 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018

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ACS Sustainable Chemistry & Engineering

Power generation based on chemical looping combustion: Will it qualify to reduce greenhouse gas emissions from life cycle assessment? Junming Fan 1, 2, Hui Hong 1,2*, Hongguang Jin 1,2 1 Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, PR China 2 University of Chinese Academy of Sciences, Beijing 100049, PR China

Corresponding author: Hui Hung; E-mail address: [email protected]. Fax: +86 10 82543158. Phone: +86 10 82543158. Mailing address: 11th west road of north fourth ring, Beijing 100190, People's Republic of China

Abstract:

The aim of this study is to disclose relationship between global warming impact (GWI) of chemical looping combustion (CLC) and four essential factors to investigate the environmental sustainability of this technology, namely the types of oxygen carrier (OC), the lifetime of OC, the global warming potential (GWP) of OC and thermodynamic performances of CLC power facility. At designed conditions, the GWI of CLC power plant is expected to be 63.4 kg CO2 eq. /MW h by using Ni-based OC. The lifetime of OC has a major influence on GWI before it reaches to 4000 hours, and further increment of OC lifetime presents tender influence on reducing GWI, whereas at such condition, the GWP of OC presents obvious influences. Higher CLC system efficiency contributes to lower GWI, thus integrating CLC with more-efficiency combined cycle rather than steam cycle and screening high-temperature resistance OCs should be focused on for future research needs. Key words: chemical looping combustion, carbon capture, life cycle assessment, system performance, oxygen carrier

1

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Introduction

According to the IPCC, “warming of the climate system is unequivocal and human influence on the climate system is clear” 1. At the end of 2015, nearly 200 nations organized as a global community agreed in Pairs to put forward a more ambitious temperature target of “well below 2 oC” even to pursuit efforts towards 1.5 oC for offering sustainability for our descendants 2. The climate change is most likely as a consequence of anthropogenic greenhouse gas emissions. The fossil fuel combustion represents by far the largest source of emissions among the human activities related emissions, increasing from zero to over 32 GtCO2 in 2014 since the industrial revolution 3. Given the large flow of combustion flue gas in conventional power plant, a significant issue for CO2 capture is the CO2 diluted by N2 from air, resulting in large energy penalty to capture CO2 from flue gas by means of conventional chemical or physical methods (e.g. chemical wet scrubbing). It would reduce power output by 20% to 30% for a typical coal-fired power plant 4. Significant R&D efforts worldwide are underway for greenhouse gas emissions reduction, of which carbon capture and storage (CCS) is expected to be essential for delivering significant emission reductions 5. Nevertheless the pace of CCS development has betrayed its initial anticipations as more than 20 large-scale projects have been cancelled in 2010-2016 owing to the sharply fluctuated policy and financial support 2. The reasons behind are huge capital investment associated with capture equipment and energy penalty for capturing carbon, making currently CCS-equipped plants insufficient to reach large-scale commercial maturity. Its cost has been increased from 63 USD/ MW h to current 150 USD/ MW h depending on different capture technologies 6. Nevertheless under the International Energy Agency 2 oC scenario (2DS), 94 GtCO2 is posed to be captured and stored via CCS in the period 2013-2050, with the highest capture rate of 50 GtCO2 from 2041 to 2050; by 2050 approximately 850 GW of electricity generation should be equipped with CCS, accounting for 12% of global power 7. As another option, chemical looping combustion (CLC), with its ability for inherent separation of CO2 has made it very attractive option considering as a promising and long-term implement for carbon capture 5. With regards to CLC, the conventional combustion reaction (oxidation of fuel by air) is decomposed into two subreactions which are typically happened in a fuel reactor (FR) and an air reactor (AR), and one kind of intermediate oxygen carrier (OC) circulates between FR and AR to obtain redox reactions. In the FR, the fuel is oxidized with OC to generate a gaseous stream mainly composed by CO2 and H2O, with simultaneously reduction of oxidized OC to reduced one. In order to regenerate reduced OC, it is re-oxidized in the AR. To obtain pure CO2 stream, water vapour condensation is adopted in CLC process. To assess the environmental impacts of CLC power plant, and to provide potential solutions to sustain environmental sustainability, life cycle assessment (LCA) is one possible approach. LCA is a decision-support tool that aims at quantification of the environmental impact throughout a product’s life cycle by accounting all emissions 2


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from construction and plant operation to final decommissioning. Though it is clear that the main purpose of CLC power plant is to achieve zero-CO2 emission during operation, a LCA concerning on greenhouse gas emissions is necessary. Very few studies have focused on such aspect expect for a preliminary LCA study with examining three-stage chemical looping process for hydrogen production 8. In this case, environmental sustainability of chemical looping has been demonstrated from life-cycle perspective because of its lower carbon emissions. Whereas, for a systematic LCA assessment, three major aspects should be outlined by questioning whether CLC technology still holds environmental sustainability from life-cycle assessment. First, emissions resulting from OC manufacturing are a key process to determine life-cycle emissions. Focuses should be paid on this aspect by determining the reasonable OC lifetime range thus maintaining CLC attractive. Policy-makers worldwide are today in an awkward situation since there is no systematically environmental assessment of this technology available in the literature to the best of our knowledge. Second, the relation between life-cycle greenhouse gas emissions and thermodynamic performances of CLC power plant is not deeply analysed. Thermal efficiency obtained from different operating conditions, OC types and thermal cycles has a decisive influence on life-cycle emissions. Such a focus is important for technology development by minimising environmental impacts as early as possible in the development phase. Finally, a study for predicting GWI of future CLC power plant is quite essential. Influential parameters such as both lifetime and GWP of future OC are deserved to be analysed in advance to examine trade-offs between their influences as well as thermodynamic performances of CLC power plant and GWI in order to find the potential risks as well as to realise the great potential of this technology. In this study, we therefore conduct life-cycle greenhouse gas emission assessment of CLC power plant with addressing the aforementioned issues to discuss the environmental sustainability of this technology as future implement for carbon capture. We use LCA to address the relations between global warming impact (GWI) of CLC and four aforementioned factors, namely the types of OC, the lifetime of OC, the GWP of OC and thermodynamic performances of CLC power facility. To the best of our knowledge, such a deep and systematic analysis has not been performed in earlier studies. By performing these investigations, we aim to point out future research needs to reach environmental sustainability.

Methodology

Goal and scope definition The LCA method used in this study follows the methodological framework (ISO 14040 to 14044) built by the International Organization for Standardization (ISO) within the context of four main phases, i.e. the goal and scope definition, the 3



inventory analysis, the impact assessment and the interpretation 9. The aim of this study is to examine GWI of the CLC power plant. The obtained results are used to provide some personal recommendations to the community for better understanding the future research needs and can be used as preliminary data source for the policy-makers to design effective carbon mitigation policy. The boundaries of CLC power plant are shown in Figure 1. The functional unit is defined as 1 MW h of electricity output from the power plant. All the major processes including the upstream and downstream emissions and emissions during plant operation have been considered. Specifically, with regards to upstream stream, the emissions from raw material manufacturing used for facility construction, such as steel, concrete, iron and aluminium, should be counted. The emissions resulting from natural gas (NG) exploitation and manufacturing OC from natural ore as well as its transport are involved. To avoid pressure loss, re-compression of NG along the pipeline is necessary. For downstream processes, wastes (mainly spent OC) are disposed of landfilling. Notably further utilization of spent OC is not considered in this study because of reactivity loss and mechanical strength loss (e.g. spent ilmenite cannot be used to produce titanium pigment and alloy-bearing titanium owing to reactivity losses 10) and toxicity of OCs (e.g. nickel 11). The captured CO2 is transported to the storage site, and similarly recompression of captured CO2 is necessary to offset pressure loss. The energy required for CO2 injection to the reservoir is negligible because of the sufficiently high-pressure received CO2 stream. CO2 leakage is not considered to ensure the safe operation12, 13.

Figure 1 Boundaries for the CLC power plant.

Life cycle inventories Data collections are presented in Table 1. The lifetime of the plant is assumed to be 4


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30 years with preliminary 3 years for construction. Concrete, steel, aluminium and iron are the four representative types of building materials to build a power plant. Notably because of the immaturity of CLC power plant, the fundamental material requirements for constructing CLC power plant are taken from an average value of these required in a coal fired plant and a NG fired plant. The greenhouse gas emissions resulting from plant construction only stand for trace percentages of cradle-to-grave emissions in a conventional power plant 14, 15. Based on the obtained results in this study, this percentage is below 1% therefore this assumption is valid for analyse. In terms of NG exploitation and transport, NG resource is assumed to be obtained from Yulin gas field in China and the extracted gas is transported by means of pipeline with a total length of 600 km. The main pipeline accounts for 80% of the total pipeline length, and the diameters for main pipeline and local one are designed to be 711 mm and 219 mm, respectively, following the standard of SY/T 5037-2000 in natural gas industry in China. It should be noted the natural gas requirement for the CLC power plant accounts for a proportion of the amount of the NG in main pipeline. During NG exploitation, through processing and transmission to distribution, a certain amount of NG is lost to atmosphere. NG leakage accounts for 1.4-2.8% of the total NG exploited from a conventional well and we use the lowest value herein 16, 17. The captured CO2 stream from the CLC power plant is compressed to 110 bar and transported via a 300-km pipeline for storage to inject in gas field. The selected distance is a representative length for six pipelines (length range 200-360 km) constructed in North American 18. Within an example located in Bravo Dome to Guymon with the similar capacity, the diameter of pipeline is designed to be 320 mm. Followed by the national standard of GB 5310-2008 in China, the pipeline diameter is considered to be 355.6 mm with a thickness of 24 mm, corresponding to a weight of 196.5 kg/m. The work for CO2 re-compression along the pipeline is 3 kW of electricity per km of CO2 pipeline 16. OC circulates between two reactors (i.e. AR and FR) in CLC, and due to the reactivity loss and attrition, make-up OC is necessary. The GWP for manufacturing metal Ni and Fe is 11.4 kg CO2 eq. /kg 19 and 1.16 kg CO2 eq. /kg 20 respectively. The lifetime of Ni-based OC is within 4500-40000 hours depending on different supports and preparation methods. In terms of Fe-based OC, the lifetime has been sharply reduced to 1315-1600 hours 11, 21. In this study, we choose the lifetime of Ni- and Febased OCs to be 10000 hours and 1315 hours, respectively. We assume the distance for transporting OC to plant site is 100 km within the economic-interest distance 22. The diesel-fuelled trucks are utilized for transport with a dead weight capacity of 10 ton per truck, and the fuel consumption is taken as 17 L/100 km with GWP of 2.68 kg CO2 eq. /L. The carbon emissions from plant operation mainly result from gas leakage between AR and FR in CLC. If fuel gas leaks, it will be oxidized to CO2 and water vapour in the AR, leading to decrease in carbon capture efficiency. The gas leakage has been experimentally found within 0-3% 23, and we use an upper value to present the worst condition for CLC process. 5



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The spent OC is considered to be disposed by landfill through diesel-fuelled truck within the distance of 10 km, following the national standard of China to direct the distance for waste landfill (GB 16889-2008). The plant decommissioning is not considered herein because the emissions are marginal owing to the share of plant construction is even less than 1%. Furthermore recycle of the construction materials is rejected from consideration. Table 1 Data collection for the life cycle assessment inventory.

Power plant construction requirement

Item

Unit

Range

Value

Notes

Concrete

ton/MW

97.7-158.7

128.25

GWP: 0.81 kg/kg 24

Steel

ton/MW

31.0-50.7

40.9

GWP: 1.16 kg/kg 20

Aluminium

ton/MW

0.4

0.4

GWP: 22.4 kg/kg 19

Iron

ton/MW

0.2-0.6

0.4

GWP: 1.16 kg/kg 25

NG losses

%

1.4-2.8

1.4

km

-

600

Assumed

mm

-

711

Assumed

mm

-

219

Assumed

km

200-360

300

-

3

Assumed

-

355.6

Weight: 196.5 kg/m

-

11.4

-

1.16

hour

4500-40000

10000

hour

1315-1600

1315

L/100 km

-

17

0-3

Transport NG exploitation and transport

distance Main pipeline’ s diameter Local pipeline’ s diameter Distance

CO2 transport

OC manufacturing and transport

kW

Work for re-compression

electricity/km CO2 pipeline

Diameter

mm

Metal Ni

kg CO2 eq.

manufacturing

/kg

Metal Fe

kg CO2 eq.

manufacturing

/kg

Lifetime of Ni-based OC Lifetime of Fe-based OC Fuel consumption of truck

Operation

Gas leakage

%

Landfill

Distance

km

GWP: 2.68 kg CO2 eq. /L

3 10

Assumed

Data calculation The GWI (30 years) for CLC power plant (T) are calculated according to the following equation:

T = T + (T + T + T + T ) + ∗ (1 − ) 6


(E1)

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where n denotes year, y; T denotes emissions from plant construction including power plant construction, and pipeline construction, kg; T denotes the emissions during raw material production, namely natural gas exploitation and OC manufacturing, kg/y; T denotes emissions from raw material transport, kg/y; T denotes emissions from re-compressing CO2 and natural gas, kg/y; T denotes emissions from waste transport, kg/y; denotes natural gas requirement, kg/y; denotes the carbon capture efficiency in operation, accounting to be 97%. The total emissions are normalized by the functional unit, to be kg CO2 eq. /MWh electricity generated from the CLC power plant. Life cycle impact assessment (LCIA) LCIA methods include a wide range of categories, such as climate change, freshwater eutrophication, human toxicity, etc. 26. In this study, we focus on climate change indicator (global warming impact, GWI) as a first step to examine the LCIA of CLC power plant. CO2, CH4 and N2O are three representative greenhouse gases, herein only CO2 and CH4 are considered because of the trace emissions of NOx in CLC process 27. The amount of CH4 emissions are transformed into CO2 equivalents with a GWP of 21 over a 100-year horizon 28. Limitation and uncertainties Even though high-quality data are provided at the time of study, several data limitation and uncertainties are identified, mainly attributing to the lack of up-to-date data and use of multiple types of data sources. First due to the nonexistence of CLC demonstration plant, the construction material requirements are not available, and the use of those in conventional plant may result in uncertainties (even the emission share resulting from plant construction is very small). The data sources taken from different regions and countries may results in uncertainties, taking an example of emissions resulting from OC manufacturing, different amount of emissions might be expected in positions with varied manufacturing processes. Notably, the capture efficiency during CLC power plant operation is limited by the thermodynamic equilibrium of OC with CH4 (NG is assumed to be represented by pure CH4). This assumption may lead to small uncertainties because in real CLC plant, the CH4 conversion rate with both Ni-based and Fe-based OCs performs close to be equilibrium in continuous tests 11.

Results

Thermodynamic performance The whole CLC power plant is developed and simulated by application of 7



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commercial Aspen Plus. The PR-BM (Peng-Robinson with Boston-Mathias) function is chosen in this study, recommended by the Aspen Physical guide. Notably the AR temperature is the maximum temperature generated by the oxidation reaction, and at least two approaches have been considered to effectively remove heat, i.e. with combined cycle and with steam cycle, as shown in Figure 2. For integrating with steam cycle, cooling water is generally used for recovering heat through CLC boiler water wall. If CLC is integrated with more effective combined cycle, air cooling is commonly used. When CLC is integrated with combined cycle for power generation, CLC is operated under its optimum pressure at specific turbine inlet temperature (TAR). The FR is operated at adiabatic condition and the heat required for endothermic reaction is supplied by sensible heat of heated OC. All reactions involved in CLC process are restricted to be both phase and chemical equilibrium based on minimizing Gibbs free energy of all components expected to obtain equilibrium 13. The isentropic efficiency and mechanical efficiency for turbines and compressors are set to be 88% and 99%, respectively 29. And the minimum temperature difference (pinch temperature) in the heat recovery steam generation (HRSG) is considered to be 10 oC 30 . The thermodynamic performances of the studied CLC power plant are shown in Table 2. The net power efficiency (η ) is defined by the net power output from this plant ( ) to the necessary energy input ( , LHV basis), as shown in Equation (2). The carbon capture efficiency (η ) is defined by the ratio between captured carbon contents in captured CO2 stream and carbon contents in fuel gas ( !" , molar basis), as presented in Equation (3). η = η =

#$%& '() ∑ ( ,-"

(E2) (E3)

where stands for molar flow rate of carbon contents in captured CO2 stream, i.e. CO2, CO and CH4.

Figure 2 CLC system schematic of (a) chemical looping combustion coupled with combined cycle for power generation (CLCCC-P), and (b) chemical looping combustion coupled with steam cycle for power generation (CLCSC-P). Table 2 Thermodynamic performance of the studied CLC power plant (CLCCC-P system) 8


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Variable

Ni-based case (TAR=1200 oC)

Units

Fe-based case (TAR=1000 oC)

Mass balance CH4 flow rate

kmol/s

1

1

OC flow rate (oxides)

kg/s

934.2

5987.9

Captured CO2 flow rate

kg/s

42.2

42.7

Fuel input (LHV)

MW

800

800

Electricity output

MW

402.4

365.3

Net power efficiency

%

50.3

45.7

CO2 capture efficiency

%

96.9

97.0

kg/MWh

11.8

13.0

63.4

69.4

Energy balance

CO2 emission rate (operation) Life-cycle greenhouse

kg CO2

gas emission

eq./MWh

The flow rate of Fe oxides is approximately 6.4 times than that of Ni oxides, mainly assigning by the lower oxygen transport capacity of Fe oxides. The captured CO2 flow rate in case of Ni-based CLC power plant is slightly lower than that in Fe-based case benefited from free of thermodynamic limitation. Clearly, due to the higher AR operating temperature, the net efficiency of the Ni-based case reaches higher value of 50.3% vs. that of Fe-based case (45.7%) with a lower operating temperature (TAR=1000 oC). The capture efficiency remains similar in both cases. Because of the higher net electricity output in Ni-base case, the CO2 emission rate is lower in comparison with that in Fe-based case. Notably, although the GWP for metal Ni manufacturing is even almost ten times as against that for Fe (as shown in Table 1), the GWI of Ni-based case is lower than the latter one. And this observation is associated with several reasons, which will be discussed below. LCA results Figure 3 presents the GWI distribution results for the CLC power plant (taking nickel-based case at designed condition as an example). The GWI is calculated to be 63.41 kg CO2 eq. /MWh, of which the major contributor comes from natural gas exploitation, accounting for 73.51%. While the emissions resulting from OC manufacturing only take percentage of 2.50%, disclosing the intermediate OCs cannot lead to huge emissions at this condition. Notably, the emissions from plant construction are relatively low, with a percentage of 2.26%.

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Life cycle greenhouse emission: 63.41 kg CO2-equivalent/ MW h

2.26%

Plant Construction

73.51%: 67.31% (NG loss ) 6.20% (NG recompression )

Natural gas exploitation and transport

2.50%

OC manufacturing and transport

21.73%

Plant Operation

Figure 3 GWI distribution for the CLC power plant (taken nickel-based case as benchmark).

Uncertainties analysis In this section, uncertainties analysis concerning on key input data is carried out to examine the impact of estimated data. The main parameters considered in such analysis include plant construction material requirement, natural gas losses during exploitation, CO2 pipeline distance and gas leakage in CLC. Notably, effects of GWP of OCs and lifetime of OCs on GWI are discussed below. The varied range for this uncertainties analysis is based on the differences between the upper and/or lower value of these parameters reported in the literature and used in this study (shown in Table 1). The uncertainties analysis results are shown in Figure 4. Clearly, the amount of plant construction material are less sensitive to GWI. Also, the change of CO2 pipeline distance leads to slight sensitivities of GWI. Oppositely, if the NG losses are increased by 100%, the GWI of CLC plant correspondingly rises from 63.41 to 107.31 kg CO2 eq./ MWh. This remains the biggest uncertainties in this assessment, nevertheless such uncertainties exist in any GWI assessments associated with natural gas-fuelled plants. Besides, reduction of GWI has been outlined when the gas leakage in CLC is decreased from 3% to absence, whereas it is not so-much sensitive to GWI compared to other factors, such as NG losses.

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Figure 4 Uncertainties analysis concerning on the input data.

Discussion

GWI is associated with thermodynamic performance

Figure 5 Effect of thermodynamic performances of CLC on GWI.

The GWI of the CLC power plant is associated with thermodynamic performances, mostly relating to the net electricity output. The effects of thermodynamic performance on GWI are shown in Figure 5. Several observations follow from this figure: If CLC process is not pressurized that is operated in the interconnected fluidized bed 31, for the purpose of power generation, CLC is integrated with steam cycle (CLCSC- P, at 850 oC). It inherently hides the thermodynamic benefits of CLC by integrating more efficient combined cycle (CLCCC- P). Therefore the GWI in the case of latter system is much lower than that of CLCSC-P system due to the increment of net power output. GWI is initially decreased with CLC operating pressure at a specific AR temperature, and then is exhibited to be increased. A lowest GWI associated with CLC operating pressure is observed for a specific AR temperature. This observation follows the principle: an optimum pressure ratio is existed for a gas turbine combined cycle power generation system at a specific turbine inlet temperature to obtain the best net power efficiency. Meanwhile, it is noticed that with increase in AR temperature (at the optimum operating pressure), the GWI continuously decreases but the descending rate is weakened with increasing temperature. It is realized by the Carnot efficiency curve; the higher turbine inlet temperature is, the less increment of thermal efficiency obtains. 11



Despite of different thermal cycles, the GWI of Fe-based CLC power plant is a little higher than that of Ni-based case. The reason may be given by larger exergy destruction is observed in the case of Fe-based CLC process, and therefore less power output is obtained.

GWI is heavily dependent on lifetime of OC

Figure 6 Effect of lifetime of OC on GWI.

The lifetime of OC is a crucial parameter, since particle attrition and reactivity losses are apparent in CLC process. The effect of lifetime of OC on GWI is shown in Figure 6. The GWI is heavily dependent on lifetime of nickel-based OC, compared with the less influential Fe-based OC. As the lifetime of OC excesses 4000 hours, the GWI is almost independent with lifetime of OC for both cases, as the decreasing ratios are remained almost unchanged (close to 0%). Therefore we state herein the lifetime of OC at approximately 4000 hours is sufficiently enough to meet environmental benefits, beyond this point, the GWI is decreasing but to be very moderate. As a consequence, further increasing lifetime of OC above 4000 hours may not be worthy from the perspective of mitigating carbon emissions. Interestingly, the less-GWP Fe-based OC exhibits less sensitivity to GWI as a function of its lifetime. When the lifetime of OC is less than 2500 hours, the GWI of Fe-based CLC power plant presents to be competitive with that of Ni-based case.

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Emission share resulting from OC

Figure 7 Emission share resulting from Fe-based OC (left) and Ni-based OC (right).

The OC is performed as looping material in the CLC process, and should not be regarded as consuming material. However if the lifetime of OC and solid inventory of OC are not reasonable, the emission shares resulting from OC manufacturing and transport (first-load and make-up) may lead to a major contributor to GWI. The emission share resulting from both factors is shown in Figure 7. Several observations are presented in this figure: In terms of Fe-based case, despite of the worst condition that the lifetime of OC is less than 100 hours and the OC inventory reaches 1000 kg/MWth, the emission share resulting from OC is not exceeded 30%. Whereas this share can reach up to 80% in Ni-based case, resulting from the higher GWP of metal Ni. If the lifetime of OC is less than 4000 hours (as reported above), this share is more inclined to be dominated by the lifetime of OC rather than OC inventory. Beyond 4000 hours, it is controlled by the solid inventory, likely as linear relationship. GWI of future CLC power facility As can be expected, GWI should be associated with lifetime of OC, GWP of OC and net power efficiency of CLC plant. For future CLC power plant, improvement of net power efficiency is of importance. As expected before, further increasing TAR (>1200 oC) might be the most potentially feasible approach, however suitable OCs to avoid agglomeration are still not found in CLC field by far. And lifetime of OC might be shortened because of reactivity losses or agglomeration and the GWP for manufacturing high-temperature-resistance OC is still unknown. Therefore it will be interesting to consider trade-off between the GWI reduction resulting from increasing net efficiency (increasing TAR) and GWI enhancement assigning by manufacturing OC. The trade-off results are shown in Figure 8.

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Figure 8 GWI associated with thermodynamic performances, lifetime of OC and GWP of OC.

On the basis of the expected results, some implications follow: GWI is reduced with AR temperature (TAR). At specific lifetime of OC or GWP of OC, with increasing TAR, the net power efficiency is enhanced (as described above), therefore the GWI is decreased. Lifetime of OC plays an important role to determine GWI. Clearly, together with the observations reported above, almost two times of GWI are increased since the lifetime of OC is increased from 100 hour to 4000 hour. Even though the GWI is decreased with the increase in TAR, it is dramatically increased with the reduction of lifetime of OC (e.g. 4000 hour vs. 100 hour). It indicates net power efficiency improvement at the expense of huge reduction of lifetime of OC may not be worthy to reduce GWI. GWP of OC influences the GWI. In the range of 5-20 kg CO2 eq. /kg for OC GWP, the GWI is not significantly varied with the lifetime of OC at 4000 hour. And net power efficiency improvement at the expense of a high GWP of OC at reasonable lifetime range would not lead to obvious increase in GWI. In terms of the relationship between lifetime and GWP of OC, the influences of GWP of OC on GWI are continuously weakened with the increase in lifetime of OC.

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Future research needs

The life-cycle assessment presented in this paper offers an initial attempt at presenting knowledge on the global warming impact of chemical looping combustion systems. Researchers in the community and policy-maker can thus use these findings to guide future research needs and to establish effective carbon capture plans towards more environmental sustainability. Based on our findings, future research needs include following aspects: Efforts associated with improving CLC system efficiency should be focused on. As we stated in this paper, higher CLC system efficiency helps to reduce GWI. Integrating CLC with more-efficiency combined cycle instead of steam cycle can reach such aim. Currently, coupling with steam cycle is the appropriate configuration in terms of CLC system because of the most popular fluidized bed CLC reactor cannot be operated pressurized high-temperature condition, which lacks opportunities for integrating with combined cycle. Therefore pioneering efforts are necessary to find suitable CLC reactor that is suitable for combined cycle coupling, to simultaneously achieve high-efficiency and environmental-friendly goal. Furthermore searching high-temperature resistance OCs is also important since increasing turbine inlet temperature contributes to higher system efficiency. In this study, the nickel-based OC in CLC power plant performs better net power efficiency compared to Fe-based case because of better temperature resistance maximized to 1200 oC, thus leading to lower GWI. Besides, the role of OC lifetime on determining GWI is weakened before it reaches 4000 hours, and further increase in OC lifetime shows trace influence to reduce GWI. Although it is performed by using the recommended LCA method and the most attainable data at the time of the study, limitations and uncertainties (as mentioned above) can be reduced to improve the reliability of the LCA results. We emphasize that using the latest and consistent data sources can improve reliability. Besides, a combination of environmental and economic assessments to cover major aspects of sustainability is necessary before CLC is finally ready for commercialization.

Acknowledgments

The authors gratefully acknowledge the support of the National Key Research and Development Plan (No. 2016YFB0600803) and National Natural Science Foundation of China (No. 51590904).

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For Table of Contents Use Only Synopsis Life-cycle assessment is crucial to examine environmental sustainability of chemical looping combustion power plant.

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Power generation based on chemical looping combustion: Will it

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