Life cycle study of coal-based dimethyl ether as vehicle fuel for urban bus in China

Life cycle study of coal-based dimethyl ether as vehicle fuel for urban bus in China

ARTICLE IN PRESS Energy 32 (2007) 1896–1904 www.elsevier.com/locate/energy Life cycle study of coal-based dimethyl ether as vehicle fuel for urban b...

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ARTICLE IN PRESS

Energy 32 (2007) 1896–1904 www.elsevier.com/locate/energy

Life cycle study of coal-based dimethyl ether as vehicle fuel for urban bus in China Liang Zhang, Zhen Huang Research Center for Combustion and Environmental Technology, School of Mechanical and Power Engineering, Shanghai Jiao Tong University, No. 951, Panyu Road, Xuhui District, 200030 Shanghai, PR China Received 16 July 2006

Abstract With life cycle assessment (LCA) methodology, a life cycle model of coal-based dimethyl ether (CBDME) as a vehicle fuel is established for China. Its life cycle from well to wheel are divided into three phases. They are feedstock extraction, fuel production and fuel consumption in vehicle. The primary energy consumption (PEC) and global warming potential (GWP) of CBDME pathway are analyzed and compared with coal-based diesel (CBD) as a latent rival to replace conventional petroleum-based diesel (CPBD). This study demonstrates that the LCA methodology is very suitable and effective for the choice of vehicle fuels. One result is that the greenhouse gases (GHGs) emission of coal-based vehicle fuel pathways is usually concentrated on fuel production stage. The percentages of CBDME and CBD pathways both exceed 60%. The application of carbon capture and storage (CCS) is helpful for coal-based vehicle fuel pathways to improve their global warming effect dramatically. Compared with CBD pathway, CBDME pathway consumes less PEC and emits less GHGs emission as well. Even though the CCS and CH4-fired generation are used, the advantages of CBDME are still kept. For saving petroleum energy and reducing global warming effect, CBDME has greater potential than CBD to substitute CPBD under current fuel synthesis technologies. If the hurdles such as the maturity of engine and vehicle technologies, corresponding regulations and standards and infrastructures are reliably solved, CBDME will have better prospect in China. r 2007 Elsevier Ltd. All rights reserved. Keywords: Coal-based dimethyl ether; Vehicle fuel; Life cycle; Primary energy consumption; Global warming potential

1. Introduction The vehicle population of China has exceeded 30 millions since 2005. This is an important reason for China to become a net oil importer from 1993. As predicted, the vehicle population will reach 150 millions at 2020. This would likely bring great pressure on energy supply, especially on oil import, if the current energy structure were still kept in future. As known, China is a country with abundant coal meanwhile a little natural gas (NG) and poor oil. Coal is always occupying large part of Chinese total domestic energy production, and the average percentage during Corresponding author. Tel.: +86 21 64074085; fax: +86 21 64078095.

E-mail addresses: [email protected] (L. Zhang), [email protected] (Z. Huang). 0360-5442/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2007.01.009

recent 12 years is 73% [1]. So, it is a key direction to develop clean coal-based liquid fuels, which is an effective method for the transportation sector away from today’s heavy reliance on oil products. Among coal-based alternative vehicle fuels for conventional petroleum-based diesel (CPBD), dimethyl ether (DME) has prominent characteristics. Compared with CPBD, DME has a shorter molecular formula (CH3–O–CH3) containing oxygen without C–C bond, higher cetane number (455) and lower auto-ignition point (235 1C). These features make DME engine burning cleaner and faster, but are not enough to prove coal-based dimethyl ether (CBDME) as a better choice to save petroleum, because it requires some improvement and redesign of conventional diesel engine, especially on fuel storage, supply and injection systems. From the viewpoints of these aspects, another alternative vehicle

ARTICLE IN PRESS L. Zhang, Z. Huang / Energy 32 (2007) 1896–1904

fuel—coal-based diesel (CBD) is a latent rival. CBD has similar properties to CPBD and needs less engine alterations than CBDME. So, it is difficult to decide which one is better and more adaptable for China only according to engine performance. In order to roundly understand CBDME and evaluate its prospect in China, a life cycle model to analyze primary energy consumption (PEC) and global warming potential (GWP) of CBDME pathway is established, and a comparison with CBD is also carried out. The following is a short review of DME engine and vehicle developments and their life cycle studies. Over past years, DME has been proved as a suitable fuel for compressed ignition (CI) engine to replace CPBD [2–4]. And some research suggests that DME engine has great potential to meet the severe US emission standards in 2010 with simplified after treatment system [5]. In China, Huang [6] first brought forward the idea to use DME as vehicle fuel. After this, many researches have been carried out to study DME combustion process and mechanism in internal combustion engine [7,8]. In recent years, some DME vehicles have been developed all over the world [9–13]. In China, Shanghai Jiao Tong University first developed a DME urban bus at 2005, and Xi’an Jiao Tong University also developed a DME middle bus [14]. These progresses indicate that DME has been gradually accepted as a CPBD substitute in China. In 1997, Verbeek and Van Der Weide [15] researched energy efficiency and CO2 emission of DME from NG and compared them with diesel, gasoline, liquid petroleum gas (LPG), compressed natural gas (CNG), liquid natural gas (LNG), methanol and bio-ethanol. The life cycle energy efficiency of NG-based DME varied from 19.0% to 22.5%, which was the same level as CNG, higher than gasoline and methanol, but lower than diesel. Among fossil fuels, DME had the least CO2 emission. This was the same level as CNG and diesel. Atrax Energi AB [16] studied bio DME as a vehicle fuel and compared it with gasoline, diesel, NG, ethanol and rapeseed oil methyl ester (RME). This study showed that CO2 emission was one of the leading advancements of bio DME pathway, and it was much less than diesel pathway. Ofner and Gill [17] applied sustainable process index (SPI) method to evaluate ecological impacts of DME as fuel for heavy vehicle. In that research, NG-based and bio-based DME, NG-based and bio-based methanol, EUROa and EUROb diesel, CNG and LPG were included. Results indicated that the impact of CO2 emitted by NG-based DME pathway is basically equal to other fuels from fossil energy. Among bio fuels, SPI of DME was the same as methanol. Compared with CNG, DME was easier to be stored and transported, so it was the first choice as diesel substitute. In China, Wu et al. [18] studied the petroleum and total fossil energy consumption of NG-based DME pathway with a background of Chongqing, a city locating at southwest of China. The comparison between DME and other vehicle fuels including gasoline, CNG, methanol–gasoline

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(mixed by conventional gasoline and NG-based methanol), NG-based diesel was carried out. One result was that the life cycle energy consumption of NG-based DME was basically equal to methanol–gasoline, less than NG-based diesel and more than gasoline and CNG. Another result was that the life cycle petroleum consumption of NG-based DME pathway was of the same level as CNG and NGbased diesel, and obviously less than gasoline and methanol–gasoline. 2. Methodology The attention to life cycle assessment (LCA) comes from the increasing awareness of local and global energy and environmental protection. According to the Society of Environmental Toxicology and Chemistry (SETAC) definition [19], LCA is a methodology ‘to evaluate the environmental burdens associated with a product, process or activity by identifying and quantifying energy and materials used and wastes released to the environment; to assess the impact of those energy and material uses and releases to the environment and to identify and evaluate opportunities to affect environmental improvements’. All energy resources have impact on the environment during their life cycle from extraction to end use. LCA is a useful methodology to evaluate their impacts. A LCA is made up of following stages: (1) definition of objectives and boundaries of the assessment; (2) inventory analysis involving identification and measurement of process impacts; (3) impact assessment to evaluate the significance of potential environmental impacts using the results of the inventory analysis; and (4) interpretation and improvement analysis to identify the opportunity to improve the environmental performances of each activity.

3. Life cycle model of CBDME and CBD 3.1. Definition of system boundary, input and output The life cycle of CBDME can be divided into three phases. They are feedstock phase, fuel phase and consumption phase. The first phase includes the extraction and transportation of raw coal. The second phase involves DME production and transportation. The third phase is the consumption of DME in vehicle. For convenience, the former two phases are combined into upstream and the last phase is called as downstream. During the upstream of CBDME pathway, some kinds of energy are required to maintain the normal energy conversion processes. In this model, they are called as process fuels including raw coal, crude oil, fuel oil, CPBD and electricity. It is obvious that

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the production and conversion of process fuels also need themselves. So, these process fuels and relative processes are integrated into a sub-system and combined with the upstream of CBDME pathway to build a comparable closed system. As described above, Fig. 1 shows the system boundary of CBDME life cycle model. The system input includes raw coal and crude oil, the output are greenhouse gases (GHGs) emission. The real arrows are used to indicate the directions of energy and mass flow and the dashed arrows are used to demonstrate inner relations between process fuels in sub-system. 3.2. Functional unit In this paper, an urban bus fueled with DME as shown in Fig. 2 is selected as functional unit. This DME urban bus is a prototype developed by Shanghai Jiao Tong University in 2005. It aims at forming a demonstration fleet in Shanghai at the end of 2006. In order to achieve its energy consumption under a unit driving distance, a typical urban bus test cycle in China [20] is selected as shown in Fig. 3, and a vehicle simulation method based on engine rig experiment [21] is applied. The simulation indicates that the functional unit consumes 1.05 GJ of DME, equal to 29.6 liters of CPBD, and emits 73,157 g CO2 according to carbon conservation during 100 km driving distance. Input

3.3. Assessment index In this paper, raw coal consumption (RCC), crude oil consumption (COC), total PEC and GWP are used to evaluate energy consumption and global warming effect of CBDME pathway. 4. Upstream analysis 4.1. Coal extraction According to statistics [1], the raw coal yield of China in 2002 is 1.38  109 t and the process fuel consumption during coal mining and dressing is listed in Table 1. The direct coal extraction efficiency is 98.0% and majority of process fuels consumption is concentrated on raw coal (66%) and electricity (30%). For GHGs emission, one part is CO2 from the combustion of process fuels, and another important part is the CH4 emitted into atmosphere along with coal extraction. The combustion CO2 emission can be calculated based on the quantities of consumed process fuels according to carbon conservation. The non-combustion CH4 emission can be obtained from some references. Yang et al. [22] estimated the non-combustion CH4 emission of China in 1990 is 472.5 g/GJ coal output. Ma [23] thought that this number in 1996 was 5.62 g/kg standard coal equivalent (SCE) output, which is equal to 192.0 g/GJ coal output. Out put

Life cycle of CBDMEas a vehicle fuel Downstream

Upstream Feedstock phase Raw coal

Raw coal extraction

Fuel phase

Raw coal transportation

DME production

Consumption phase

DME transportation

DME Consumption in vehicle

GHGs CH4 N2O CO2

Process fuels

Raw coal Crude oil

Fuel oil transportation

CPBD transportation

Electricity transportation

Fuel oil production

CPBD production

Electricity generation

Crude oil transportation

Raw coal transportation

Raw coal transportation

Crude oil transportation

Crude oil transportation

Raw coal extraction

Crude oil extraction

Crude oil extraction

Crude oil extraction

Sub-system of process fuels

Fig. 1. Life cycle of CBDME as a vehicle fuel.

Raw coal extraction

GHGs CH4 N2O CO2

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Liu and Yu [24] divided the CH4 emission at the coal mining site into two parts in China. They are mine ventilation and deflation. The total quantity of first part in 2004 was 140  108 N m3 and the majority entered atmosphere. The second part in 2003 was 15.2  108 N m3

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and 41% was utilized. The raw coal yields in 2003 and 2004 were 17.28  108 t and 19.56  108 t [25]. So, the CH4 emission along with coal extraction can be calculated as 15:2  108  ð1  41%Þ 140  108 þ 8 19:56  108 17:28  10 ¼ 7:68 N m3 =t raw coal output. This number is equal to 262.0 g/GJ coal output, which is located between the analysis of situations in 1990 and 1996 of China. It should be in accordance with the actual situation. Therefore, this number is applied in the following analysis. 4.2. DME production Fig. 4 shows a flow diagram of DME production from coal. There are five main steps. They are coal gasification, water gas shifting, acid gas removing, methanol synthesis and DME synthesis. At the first step, feedstock coal is converted into coarse coal gas with gasification device. Besides coarse coal gas, the output of this step includes some ash and residue. The main components of coarse coal gas are CO, CO2 and H2. Only CO, H2 and a small quantity of CO2 are necessary for subsequent methanol synthesis reactor. And there are some other trace gases containing carbon such as CH4, CmHn, and COS. These trace gases are not useful in methanol and DME synthesis, so they will form GHGs emission after combustion. Because the ratio of CO, CO2 and H2 in coarse coal gas does not meet the requirement of methanol synthesis reactor, a water gas shifting process is needed. In order to obtain enough H2, CO usually react with water steam. The chemical reaction is:

Fig. 2. DME urban bus.

CO þ H2 O2H2 þ CO2 . It is obvious that a lot of CO2 will be produced and they are not needed so much for methanol synthesis. So, the output gas from water gas shifting process will pass an acid gas removing process to get rid of excessive CO2.

Fig. 3. Typical urban bus test cycle in China.

Table 1 Direct energy consumption during coal mining and dressing in China [1] Raw coal

Crude oil 6

18.89  10 t 13.7 MJ/GJ output

Total consumption Normal consumption

Ash &residue

Electricity 6

0.012  10 t 0.0 MJ/GJ output

49.88  109 kW h 6.2 MJ/GJ output

0.55  10 t 0.8 MJ/GJ output

CO2

Coal gasification Feed stock coal

CPBD 6

Water gas shifting

Coarse coal gas

Acid gas removing

Methanol synthesis

Synthesis gas

Fig. 4. Flow diagram of DME production from coal.

DME synthesis

Methanol

DME product

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After synthesis gas shifting, the qualified synthesis gas will convert into methanol with appropriate catalyst. And then, DME product is made from methanol with dehydration reaction. With investigation of coal-to-methanol (CTM) plant and methanol-to-DME (MTD) plant in China, the consumption of feedstock energy and process fuels are listed in Table 2. The direct energy efficiencies are 54.8% from coal to methanol and 98.0% from methanol to DME, so the corresponding efficiency from coal to DME (CTD) is 53.8%. As indicated above, the most GHGs emission at production stage is the CO2 emission from synthesis gas shifting process. Its GWP reaches 75,192 g equivalent CO2/ GJ DME output and occupies 84.0% of total direct GWP in DME production stage. 4.3. CBD production CBD is another important product of coal chemical industry. It can be produced via Fischer–Tropsch (FT) technology. CBD offers possibilities for compliance with stricter environmental rules in the transportation sector [26]. It is another potential way for China to develop coalbased alternative vehicle fuels. So, it is necessary to compare CBD with CBDME. Before this work, there were lot of literatures [26–29] to research FT process and its products. These studies

discussed FT process in detail and showed its good prospect in those countries with abundant coal reserves. Table 3 lists a summary of these systems. The energy efficiencies from these data sources vary from 41.0% to 45.5% when coproduction is not considered. Ref. [35] gave an evaluation on a demo example in China. Its energy efficiency was 43.4%. According to carbon conservation, 163,360 g CO2 would be emitted for 1 GJ CBD output in this example. 5. Results and discussion 5.1. Upstream analysis Fig. 5 compares the life cycle PEC of CBDME and CBD pathway with same energy output during upstream. The majorities of PEC of these two fuel pathways are raw coal and their percentages both exceed 98.0%. The remnant part of PEC is crude oil. It is caused by the use of CPBD as process fuel during fuel transportation. The low proportion of oil consumption indicates that these two fuel pathways can both save petroleum in large scale. Their application is

Table 2 Direct energy consumption from coal to methanol and DME in China (investigation) Process

Unit

CTM

MTD

CTD

Raw coal Electricity

MJ/GJ output MJ/GJ output

1753.7 71.7

0.0 2.6

1785.0 75.6

Fig. 5. Comparison of life cycle PEC between CBDME and CBD pathways during upstream.

Table 3 Summary of systems of FT products from coal System

Input

a

Electricity

With coproductiona (%)

Without coproductionb (%)

2.2 GJ

1.0 GJ

0.32 GJ

53.2

45.5

7884 MW 7884 MW 0.075 t SCE 0.076 t SCE 3.25 t SCE

3389 MW 3233 MW 1.0 GJ 1.0 GJ 1.0 t

566 MW 393 MW

46.3 43.2

43.0 41.0 44.2 43.6 43.4

Electricity

18.23 kW h 18.82 kW h 276 kW h

electricity Energy efficiencyðwith coproductionÞ ¼ output FT productþoutput  100%: input coal

FT product Energy efficiencyðwithout coproductionÞ ¼ inputoutput coalþinput electricity  100%: Low-temperature FT process. d High-temperature FT process. b c

Energy efficiency

FT product

Coal

Case 1: Refs. [26,27] (without CO2 capture) Case 2: Ref. [33] (LTFTc) Case 3: Ref. [33] (HTFTd) Case 4: Ref. [34] (HTFT) Case 5: Ref. [34] (LTFT) Case 6: Ref. [35] (Pingdingshan slurry reactor)

Output

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Fig. 6. Comparison of life cycle GWP between CBDME and CBD pathways during upstream.

Fig. 7. Comparison of total life cycle PEC between CBDME and CBD pathways.

affirmatively helpful for China to alleviate the heavy dependence on import oil. Because the energy efficiency of DME production is about 10% higher than CBD production, the upstream PEC of CBDME pathway is 14% less than CBD pathway. This also results in less GHGs emission as illustrated in Fig. 6. The CH4 emission of CBDME pathway during upstream is 548 g/GJ output. This is 16% less than CBD pathway. The main reason is that the feedstock coal at CBDME production plant is less than CBD, so the associated CH4 emission of this part of feedstock coal during coal extraction is also less. Because the direct CO2 emission at CBDME production is less than CBD production, the total life cycle GWP of CBDME pathway during upstream is 31% lower than CBD pathway during upstream.

5.2. Total life cycle analysis Fig. 7 shows the total life cycle PEC of CBDME and CBD pathways. The total life cycle PEC consumed by CBDME pathway is 14% less than CBD pathway.

Fig. 8. Comparison of total life cycle GWP between CBDME and CBD pathways.

The most important reason is that the higher energy conversion efficiency of CBDME pathway at production stage. This also leads to the total life cycle RCC of CBDME pathway decreasing with 14%. Then, less CO2 is emitted and the total life cycle GWP of CBDME pathway reduces to 3/4 of CBD pathway as demonstrated in Fig. 8. Figs. 9 and 10 illustrate the contribution of different phases on the total life cycle PEC and GWP of CBDME and CBD pathways. It indicates that the fuel phase has much more obvious influence than other two phases. The fuel phase’s contribution of these two coal-based fuel pathways to the total life cycle PEC and GWP occupies more than 60%. Therefore, it is very necessary and important to consider upstream for evaluating and comparing coal-based vehicle fuels. From Figs. 9 and 10, it should be noticed that the feedstock phase has higher contribution on total life cycle GWP than on total life cycle PEC, because there emits a quantity of CH4 at coal extraction and its GWP is equal to 21 times of CO2. This implies the utilization of associated CH4 emission at coal extraction is a possible effective method to decrease life cycle GWP. Another thing that

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Fig. 9. Comparison of different phase contribution on total life cycle PEC between CBDME and CBD pathways: (a) CBDME and (b) CBD.

Fig. 10. Comparison of different phase contribution on total life cycle GWP between CBDME and CBD pathways: (a) CBDME and (b) CBD.

should be pointed out is that the majority of CO2 emission in fuel phase is produced from water gas shifting process in fuel production plant. This is a centralized and nonmovable source emission, hence the CO2 sequestration and storage technologies are convenient to be applied to reduce GWP [30,31]. 5.3. GHGs reduction From the viewpoint of global climate change, it seems to be very important to improve global warming impact of coal-based vehicle fuel pathways, if coal is used as the feedstock energy for fuel production. As mentioned above, there are two ways to reduce the GWP of these two pathways. The first is the application of carbon capture and storage system (CCS) at fuel production plant, though it has a number of technical, economical, environmental and public acceptance issues being still resolved [26]. The second is to use associated CH4 emission at coal extraction to power generation. This cannot only reduce GWP via converting CH4 into CO2, but also decrease the consumption of coal-fired power. The following is a simple analysis of these two methods on CBDME and CBD pathways. For a CCS system, it consumes electricity. One part is used to capture CO2; another part is used to transport CO2.

According to Ref. [32], the electricity use for carbon capture in coal-based liquid fuel plants is 350 kWh/t C. This is equal to 95 kWh/t CO2. In Ref. [31], the pressure to transport CO2 efficiently by pipeline needs to be at least 8 MPa. At this pressure the density versus the compression ratio is in many cases optimal. Because of the pressure drop over the pipeline, higher entrance pressures are required. The appropriate target pressure of compression is 12 MPa. On the assumption that the inlet pressure of compressor is 0.1 MPa, the electricity consumption is calculated by [31]:   Poutlet E ¼ 87:85  ln . Pinlet where E is the electricity used (kJ/kg CO2); Poutlet the outlet pressure (MPa) and Pinlet the inlet pressure (MPa). So, it needs 117 kWh of electricity in order to transport 1 t of CO2. Thus, the total electricity consumption for a CCS in coal-based fuel plant is equal to: 95 þ 117 ¼ 212 kW h=t CO2 . Assuming the carbon capture ratio in fuel plant reaches 90%, the impacts of CCS on the total life cycle PEC and GWP of CBDME and CBD pathway can be worked out. Table 4 lists the calculating results when using GHGs reduction methods on CBDME and CBD pathway. One result is that CH4-fired generation has limited effects. For

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Table 4 Impacts of CCS and CH4-fired generation on CBDME and CBD pathway Method

Without GHGs reduction CH4-fired generation CCS CCS+CH4-fired generation

Energy efficiency (%) at fuel production plant

Total life cycle PEC/(GJ/100 km)

Total life cycle GWP/(g equivalent CO2/100 km)

CBDME

CBD

CBDME

CBD

CBDME

CBD

53.8

43.4

2.24

2.61

216,039

287,576

53.8 52.0 52.0

43.4 41.4 41.4

2.21 2.45 2.42

2.58 2.99 2.95

202,351 154,126 139,157

271,593 174,518 156,194

CBDME pathway, it can only reduce 1.3% and 6.3% of total life cycle PEC and GWP. For CBD pathway, these two numbers are 1.1% and 5.6%. On the contrary, CCS is very effective to reduce GWP for coal-based fuel pathways. For CBDME and CBD pathways, their total life cycle GWPs decline to 71.3% and 60.7% of original levels with 9.4% and 14.6% of PEC increments. When these two methods are both applied, the total life cycle GWP of CBDME pathway is still 10.9% less than CBD pathway. 6. Conclusions This paper carries out a life cycle PEC and GWP analysis of CBDME pathway, and compares them with CBD pathway. Some conclusions are achieved as followed. The GHGs emission of coal-based vehicle fuel pathways is usually concentrated on fuel production stage. The percentages of CBDME and CBD pathways both exceed 60%. Because these parts of GHGs emission are static sources, they are easier to be captured and stored than those from movable sources. The application of CCS is helpful for coal-based vehicle fuel pathways to improve their global warming effect dramatically. Compared with CBD pathway, CBDME pathway consumes less PEC and emits less GHGs emission as well. Even though the CCS and CH4-fired generation are used, the advantages of CBDME are still kept. One of main reasons is that the energy efficiency of CBDME production plant is higher than that of CBD production plant. The other reason is that CBDME is a low-carbon fuel relative to CBD. It is important for vehicle—a movable emission source, to reduce GHGs emission. For saving petroleum energy and reducing global warming effect, CBDME has greater potential than CBD to substitute CPBD under current synthesis and vehicle technologies. However, CBDME is facing some problems. One of them is the maturity of engine and vehicle technologies in case of large scale production. Being short of corresponding regulations and standards, another big problem is infrastructures such as fuel transportation and fuel station. If these hurdles are reliably solved, CBDME will have better prospect in China.

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