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Energy consumption and GHG emissions of GTL fuel by LCA: Results from eight demonstration transit buses in Beijing
Applied Energy 87 (2010) 3212–3217
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Energy consumption and GHG emissions of GTL fuel by LCA: Results from eight demonstration transit buses in Beijing Han Hao a,b, Hewu Wang a,b,*, Lingjun Song a, Xihao Li a, Minggao Ouyang a,b a b
State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, China China Automotive Energy Research Center (CAERC), Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 21 January 2010 Received in revised form 28 March 2010 Accepted 30 March 2010 Available online 5 May 2010 Keywords: GTL Alternative fuel Transit bus Life cycle assessment Energy consumption GHG emissions
a b s t r a c t Gas-to-liquids (GTL) as an alternative to diesel is considered to be one of the technical options to reduce petroleum consumption in the on-road transportation sector. Between May and August 2007, a joint demonstration program by Tsinghua University, Beijing Transit, Cummins Corporation and Shell Corporation was carried out in Beijing. The program focused on the supply systems and vehicle use of GTL fuel. The demonstration fleet was formed by four transit buses fueled with GTL and four with diesel. It was demonstrated that GTL has good compatibility with diesel in terms of fuel supply system and vehicle use. This paper compares the energy consumption and GHG emissions of diesel and GTL fuel supply chains by life cycle analysis based on demonstration results. The results indicate GTL’s large range (reported 54–70%) in synthesis efficiency, as the key factor in determining energy consumption and GHG emissions within the GTL fuel supply chain. For the probable case (GTL synthesis efficiency: 65%), the life cycle energy consumption and GHG emissions of GTL fuel are 42.5% and 12.6% higher than that of diesel. For two sensitivity analysis cases (GTL synthesis efficiency: 54% and70%), energy consumptions are 74.2% and 31.2% higher and GHG emissions are 27.3% and 7.4% higher than that of the diesel fuel supply chain. If the efficiency of the GTL synthesis process is improved to 75%, then the GHG emissions level of the GTL fuel supply chain can be reduced to the same level as the diesel fuel supply chain. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction With the rapid increase of vehicle stock in China, oil consumption and GHG emissions associated with on-road transportation are rising dramatically [1]. Increasing petroleum demand by the on-road transportation sector in China has raised the import dependence of petroleum from 19.7% in 1995 to 51.3% in 2008 [2]. Technology options for vehicles and fuels are in urgent need to establish a stainable and low-carbon transportation energy supply system. Gas-to-liquids (GTL) as an alternative to diesel is considered to be one of the technology options to reduce petroleum consumption of on-road transportation [3]. GTL technology is recovering and enjoying growth due to recent technology and catalyst advancements. Current forecasts estimate that the world’s GTL capacity could increase from 35,000 B/D to 1–2 million B/D by 2015 [4]. Between May and August of 2007, a joint demonstration program by Tsinghua University, Beijing Transit, Cummins Corporation and Shell Corporation was carried out in Beijing. The demon* Corresponding author at: State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, China. E-mail address: [email protected] (H. Wang). 0306-2619/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2010.03.029
stration fleet was made up of four transit buses fueled with GTL. Four buses fueled with traditional diesel were also included for comparison purposes. This paper analyzes the demonstration results regarding energy consumption and the associated GHG emissions of diesel and GTL fuel supply chains using life cycle assessment methods.
2. Methodology and data 2.1. Model and system boundary The LCA analysis section of this study is based on the well-towheels (WTW) analysis module of the Tsinghua-CA3EM (China Automotive Energy, Environment and Economy Model) model [5–7], which is an integrated computerized model that includes a specialized module for China’s automotive energy supply and demand balance calculation and analysis. The model is based on China’s national conditions and integrates the widely known and used transportation energy micro-level computing GREET model [8]. As described in Fig. 1, WTW analysis of GTL fuel chain is divided into well-to-pump (WTP) and pump-to-wheels (PTW) stages.
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Fig. 1. LCA diagram of GTL fuel chain, adapted from Ref. [5].
Table 1 LCA PEa use factors and direct and indirect GHG emission factors for PFs, adapted from Ref. [5]. PFa
Crude coalb Crude NGb Crude oilb Coal NG Diesel Gasoline Residual oil Electricity a b c
PE consumption factor
Indirect emission factor
Direct emission factor
Coal (MJ/MJ)
NG (MJ/MJ)
Oil (MJ/MJ)
CO2 (g/MJ)
CH4 (g/MJ)
N2O (mg/MJ)
CH4 (g/MJ)
N2O (g/MJ)
CCa (g C/MJ)
FORa
1.053 0.080 0.097 1.061 0.081 0.156 0.164 0.139 2.506
0.000 1.011 0.023 0.001 1.015 0.027 0.049 0.026 0.015
0.002 0.064 1.047 0.110 0.065 1.119 1.130 1.055 0.115
4.259 11.909 15.998 5.733 13.544 28.287 30.506 25.323 265.218
0.422 0.072 0.054 0.425 0.110 0.078 0.086 0.071 1.010
0.062 0.154 0.265 0.172 0.161 0.441 0.472 0.409 3.917
0.001 0.001 0.002 0.001 0.001 0.004 0.080 0.002 –
0.001 0.001 0.000 0.001 0.001 0.002/0.028c 0.002 0.000 –
26.35 15.30 20.00 24.74 24.70 20.20 18.90 21.10 –
0.90 0.99 0.98 0.90 0.99 0.98 0.98 0.98 –
PE: primary energy, PF: process fuel, FOR: oxidation rate, CC: carbon content factor. These fuels are mined and transported but not refined. For vehicles, the utilization value is 0.002, while for other applications this value is 0.028.
2.2. Basic data In this study, we use previously conducted research as a source for process fuels’ life cycle energy consumption and GHG emissions in China [5–7]. Detailed data are presented in Table 1.
3. GTL supply and its characteristic 3.1. GTL supply All the GTL fuel consumed by the demonstration buses was supplied by Shell Corporation’s Malaysia plant. The GTL fuel was first shipped from Malaysia to Tianjin harbor by ocean tanker, and then distributed to Beijing’s Looan fuel storage site by road tankers. A dedicated refueling truck was used to fuel the four GTL transit buses. Given that the tanks were previously used for storage of regular diesel fuel, the oil tank for GTL storage was treated using a mild cleaning procedure beforehand to eliminate contamination of the GTL fuel. Firstly, both manhole covers and the tank outlet valve were removed. The tank force was vented to remove hydrocarbon vapors. The remaining fluid and sludge were removed by vacuumpumping. Secondly, a person was lowered into the tank to rinse it with kerosene. All associated pipe work and pumps were flushed with kerosene. The small road tanker and hoses used for local
distribution of the GTL were cleaned in the same manner as above. Finally, all associated pipe work, pumps and flexible hoses were back-flushed with GTL fuel before filling the storage tank. About 500 L of GTL fuel was used for flushing of pipe work and associated equipments. All filters in the lines and manifold system were removed and replaced after flushing. To determine the effectiveness of the cleaning process, the storage tank was pre-filled with around one sixth of an ISO tank of GTL fuel. Some of this fuel was run off via the manifold and a visual inspection of color was made until the fuel ran clear and colorless. After cleaning, all manholes were closed to prevent any contamination from dust, rain, etc. before loading the fuel (see Fig. 2).
3.2. GTL characteristic and its influence on vehicle performance GTL synthetic diesel fuel is derived from the hydrocarbon compounds from natural gas using a Fischer–Tropsch chemical reaction process [9]. It has been demonstrated that GTL has good compatibility with petroleum derived diesel fuel and no vehicle modifications are needed when using GTL on a traditional diesel vehicle [10]. However, due to the different characteristics of the two fuels, vehicle performance can be different. Table 2 presents the characteristics of GTL produced by Shell Corporation in the Malaysia plant and the No. 0 diesel vehicle with sulfur content of 50 ppm produced by Beijing’s Yanshan Petrochemical Corporation.
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Fig. 2. Picture of GTL storage station and transport vessel.
Table 2 Comparison of fuel characteristic between GTL and diesel. Item
Unit
Density (15 °C) Kinematic viscosity (40 °C) Lower heating value (LHV) Total aromatics Total sulfur Cetane index
g/cm3 ASTM D4052-96(2002)e1 0.7773 0.8241 mm2/s ASTM D445-04e2 2.568 3.728
Measurement method
GTL
Diesel
kJ/kg
ASTM D4868-00(2005)
43,709 42,500
% mg/kg
EN 12916-2000 ASTM D5453-05 ASTM D976-04a
<0.1 <1 74.7
17.4 50 53.4
26 June 2006. The odometer readings ranged from 35,800 to 66,000 km. The demonstration offered the prospect of checking the injectors for fouling both for conventional fuel and GTL fuel. A procedure is required to change injectors. At the start of the trial, all the injectors for four of the eight GTL buses were replaced with brand new injectors (six injectors per vehicle). At the end of the test, the ‘‘trial” injectors were removed for examination. It is determined that all injectors worked properly on the buses and no unusual signs of wear were discovered in the follow-up examination. 4.2. Demonstration route
The different characteristic of GTL can influence vehicle performance in the following ways. Compared with diesel, the lower heating value (LHV) of GTL is 2.8% higher by weight but 3.0% lower by volume, resulting in less power for a fixed volume injection into the engine. The kinematic viscosity of GTL is 31% lower than diesel, implying an advantage to fuel spraying optimization. The aromatics and sulfur content of GTL are far lower than diesel, which can decrease the emission of total hydrocarbons (THC) and particulate matter (PM). The cetane index of GTL is higher than diesel by 20 units, which is beneficial to improve fuel combustion performance [11–15].
All eight buses were running on the same transit route, named Beijing No. 731 transit route, as shown in Fig. 3. The total length of
4. Demonstration buses and route 4.1. Demonstration buses The vehicles evaluated were manufactured by the Chinese company King Long with Euro III common rail diesel engines by Cummins. All eight demonstration buses were the same model as described in Table 3. The 11.5-m long transit buses were chosen for the demonstration because this type of bus (by dimensions and engine type) represents most of the Beijing transit bus fleet. All buses were nine months old and have been in operation since
Fig. 3. Transit route of the demonstration program.
Table 4 Distance covered and fueling volume of demonstration buses.
Table 3 Model and specification of demonstration buses.
Fuel
Bus No.
Distance covered (km)
Fuel filled (L)
Fuel consumption (L/100 km)
Average (L/100 km)
GTL
GLT-1 GLT-2 GLT-3 GLT-4
2660 3442 2523 2391
1085 1422 1000 968
40.8 41.3 39.6 40.5
40.55
Diesel
Diesel-1 Diesel-2 Diesel-3 Diesel-4
2508 2772 2530 2618
1078 1100 1012 990
43.0 39.7 40.0 37.8
40.13
Diesel/GTL bus Model Dimensions (mm) Max total mass (kg) Max passenger number Engine model Max power/speed (kW/rpm) Max torque/speed (N m/rpm) Displacement (mL) Compression ratio
XMQ6118G 11,480 2490 3220 16,100 78 Cummins ISBe220 162/2500 820/1500 5883 17.6
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350
GTL-1
600
GTL-2
GLT-2
Fueling volume /L
Distance /km
500
GTL-3 GTL-4
400 300 200
250
GTL-3 GTL-4
200 150 100 50
100 0
GTL-1
300
0 0
10
20
30
40
50
60
70
80
90
100 110
0
10
20
30
40
50
60
70
80
90
100 110
Days
Days Fig. 4. Distance covered and fueling volume of the four GTL fueled buses.
Table 5 Efficiency and composition of process fuels for life cycle steps of diesel and GTL fuel chains. Efficiency
Diesel fuel chain Crude extraction [8,16] Transportation [8] Production [6,17] Distribution [6,7,17]
95 98.9 95 99
GTL fuel chain NG extraction [8,16] NG processing [16] GTL production [9,16] Transportation [6,17] Distribution [7,17]
99 98 54/65/70 98.5 97.8
Composition of process fuels CO
RO
Di
El
Ga
31 0 0 0
1 0 20 0
15 2 0 70
18 6 15 10
4 0 0 0
0 0 0 0 0
1 0 0 0 0
10 1 0 2 70
1 3 0 6 10
1 0 0 0 0
NG
HO
CH4
31 0 55 0
0 92 10 20
0 0 0 0
77 90 100 0 0
0 0 0 92 20
10 6 0 0 0
CO: crude oil, RO: residual oil, Di: diesel, El: electricity, Ga: gasoline, NG: natural gas, HO: heavy oil.
the route is 44 km. The route was designed with 20% considered a fast section (green1) and 80% a slow section (yellow), which is very typical for Beijing transit conditions. The buses covered approximately 160–240 km/day. 4.3. Demonstration result Drivers of the demonstration buses were required to record the fueling volume and driving mileage everyday. The ratio of total fuel filled versus mileage covered was evaluated to determine the fuel consumption rate. The results are presented in Table 4 (see Fig. 4). Table 4 indicates that all eight buses had similar fuel consumption rates, ranging from 37.8 L/100 km (Diesel-4) to 43 L/100 km (Diesel-1). The difference of fuel consumption rates can be attributed to different driving habits such as the use of air conditioning. The average fuel economy of the GTL transit buses was slightly lower (1%) than the diesel buses.
LPG are produced in refineries. Diesel is then distributed by pipeline or trucks. The efficiencies of each stage are presented in Table 5 and all of them are based on literature review. The GTL supply chain hasn’t been evaluated for a long time and there is large potential for efficiency improvement. In this study, the GTL fuel supply chain starts with natural gas extraction and processing in Malaysia. GTL fuel is produced in plants located near the gas field. After production, GTL fuel is transported by sea tanker to Tianjin harbor located on the northeast coast of China. Distribution forms the last stage of the GTL supply chain. Efficiencies of each stage are presented in Table 5. In particular, to reflect the possible efficiency improvement of the GTL synthesis process, three efficiencies of low (54%), probable (65%) and high (70%) are assumed and a sensitivity analysis is conducted. For each process fuel, the related life cycle energy consumption (sorted by coal, natural gas and petroleum), GHG emission (CH4 and N2O equated as CO2 by global warming potential value) are all calculated base on values given by OU from his CA3EM model.
5. Life cycle assessment 5.2. Energy consumption 5.1. Diesel and GTL supply The traditional diesel supply chain is very efficient since this fuel supply chain has dominated on-road transportation fuels for a long time. In this study, the diesel fuel chain starts with crude oil extraction in the mid-east. After extraction, the crude oil is transported by sea tanker to China where gasoline, diesel and 1 For interpretation of color in Fig. 3, the reader is referred to the web version of this article.
Table 6 presents the life cycle energy consumption and GHG emissions of diesel and GTL fuel supply chains. The results are also presented in Fig. 5. As shown in Table 6 and Fig. 5, for the probable case, the life cycle fossil consumption of the GTL fuel supply chain is 42.5% higher than that of a traditional diesel fuel supply chain. The higher consumption of the GTL fuel supply chain can be mostly attributed to the low efficiency of the GTL synthesis process. And, with the same end use technology (diesel bus), the GTL production process is far
H. Hao et al. / Applied Energy 87 (2010) 3212–3217
Table 6 Life cycle energy consumption and GHG emissions of diesel and GTL fuel chains.
Fossil Petroleum NG Coal GHG
Diesel
MJ/100 km
1667.5 1510.8 69.3 87.4 124.0
kg/100 km
GTL Low
Probable
High
2904.3 118.0 2666.2 120.0 157.9
2376.6 87.8 2206.3 82.4 139.6
2191.6 77.3 2045.1 69.3 133.2
GTL synthesis efficiency improvement
180 160
GHG emissions/(Kg/100km) .
Unit
54%
54%
65% 65%
140
70%
70%
120
GTL synthesis efficiency improvement
100 80 Fossil consumption-diesel Fossil consumption-GTL Petroleum consumption-diesel Petroleum consumption-GTL
60 40 20 0
0
500
1000
1500
2000
2500
3000
Table 6 and Fig. 5 indicate that, for the probable case, the GHG emissions of the GTL fuel supply chain is 12.6% higher than that of a traditional diesel fuel supply chain. Compared with the diesel fuel supply chain, GTL needs more process fuels due to lower synthesis efficiency. However, among all the process fuels used in the GTL fuel supply chain, natural gas, which is lower in carbon content than petroleum, is consumed in large percentage. Therefore, the GHG emissions level of the GTL fuel supply chain is decreased. If the synthesis efficiency is improved up to 70%, then the GHG emissions of the GTL fuel supply chain can be decreased by 4.6%, still 7.4% higher than that of the diesel fuel supply chain. To decrease the GHG emissions of the GTL fuel supply chain to the same level as diesel, the synthesis efficiency has to be improved up to 75%. GTL as an alternative to diesel is beneficial to reduce the reliance on petroleum. However, the GTL fuel supply chain tends to increase the resulting GHG emissions. The trade off between petroleum conservation and GHG reduction must be addressed when promoting the mass use of GTL. Of all the factors affecting the result, the synthesis efficiency is the key factor. GTL production as a newly introduced technology has large potential for efficiency improvement. With higher efficiency, the GTL fuel supply chain results in better performance in terms of life cycle fossil consumption and GHG emissions. Therefore, GTL becomes competitive as an alternative to diesel.
3500
Energy consumption/(MJ/100km)
6. Discussion
Fig. 5. Comparison of energy consumption and GHG emissions between diesel and GTL fuel chains.
less efficient than diesel. For the probable case, the efficiency of the GTL synthesis process is only 65%, consuming large amounts of process fuels. If the synthesis efficiency is improved up to 70%, the life cycle fossil consumption can be decreased by 7.8%. However, even with high synthesis efficiency, the life cycle fossil consumption of the GTL fuel supply chain is 31.4% higher than that of the diesel fuel supply chain. Although results indicate higher fossil consumption, GTL as a natural gas derived fuel can be utilized to reduce petroleum demand. Even with low synthesis efficiency, the petroleum consumption of the GTL fuel supply chain is less than 1/10 of that of the diesel fuel supply chain. Some petroleum is still necessary for the GTL fuel supply chain because it is assumed that gasoline, diesel and residual oil are consumed by some life cycle steps such as transportation and distribution.
Energy consumption, MJ/km
5.3. GHG emissions
Several studies have been conducted to address the energy efficiency and GHG emissions of diesel and GTL fuel chains, as presented in Fig. 6 (only for probable cases assumed for each study). The two cited studies are based on car platforms with significantly higher fuel economy, making life cycle energy consumption and GHG emissions of both diesel and GTL fuel chains lower. For these three studies, energy consumption of GTL fuel chains are 42.9%, 28% and 42.5% higher than diesel fuel chains. GHG emissions of GTL fuel chains are 18.3%, 10.5% and 12.6% higher than diesel fuel chains. For the second study, it is assumed that diesel is refined domestically with energy efficiency of 91.46% (95% for the other two studies), resulting in relatively higher energy consumption and GHG emissions of the diesel fuel chain. This explains why exceeding levels of energy consumption and emissions in the GTL fuel chain in the second study are lower than the other two studies. Life cycle studies on other alternative fuels have assessed several possible fuel chains [17–26]. Among related fuel chains, other studies indicate that on a bus platform with fuel consumption of
Diesel fuel chain
30 42.5%
25
1600 12.6%
GTL fuel chain
1400 1200
20
1000
15
800 600
10 5 0
42.9%
18.3%
28%
400
10.5%
GHG emissions, g/km
3216
200 Ref.[16]
Ref.[17]
This study
Energy consumption
Ref.[16]
Ref.[17]
This study
0
GHG emissions
Fig. 6. Comparison of energy consumption and GHG emissions between diesel and GTL fuel chains by different studies (all for probable cases).
H. Hao et al. / Applied Energy 87 (2010) 3212–3217
45 L diesel (or 45 kg CNG or 80 L DME) per 100 km, the life cycle energy consumption and GHG emissions of the CNG fuel chain is slightly lower than the diesel fuel chain while the DME fuel chain is about two times higher than the diesel fuel chain. It can be concluded that for the two natural gas derived fuels (CNG and GTL), the CNG fuel chain is better than the GTL fuel chain in terms of energy consumption and GHG emissions while the GTL fuel chain is better in terms of compatibility with traditional oil supply system. For the two compression ignition (CI) platform based alternative fuels (GTL and DME), GTL exhibits better performance than DME in terms of energy consumption and GHG emissions. 7. Conclusion (1) GTL’s supply system is compatible with the traditional diesel supply system. It is demonstrated that, with proper procedure, GTL fuel can be stored and supplied by the same infrastructure as diesel fuel. (2) The use of GTL fuel is compatible for use in diesel vehicles. No modifications are needed when fueling a diesel bus with GTL fuel. The fuel economy of GTL transit buses is slightly lower (1%) than diesel buses. (3) GTL synthesis efficiency is the key factor to determine the energy consumption and GHG emissions of the GTL fuel supply chain. For GTL synthesis efficiency of 54%, 65% and 70%, the energy consumption of the GTL fuel supply chain is 74.2%, 42.5% and 31.2% higher than that of the diesel fuel supply chain. However, GTL as a natural gas derived fuel, consumes little petroleum within its life cycle, that is, less than 6% of the diesel fuel supply chain. (4) For GTL synthesis efficiencies of 54%, 65% and 70%, the GHG emissions of the GTL fuel supply chain are 27.3%, 12.6% and 7.4% higher than that of the diesel fuel supply chain. If the efficiency of the GTL synthesis process is improved to 75%, then the resulting GHG emission levels of the GTL fuel supply chain can be reduced to the same level as that of the diesel fuel supply chain.
Acknowledgements The project is co-supported by Beijing Transit, Cummins Corporation and Shell Corporation. The authors would like to thank Dr. Xunmin Ou of Tsinghua University, Dr. Kristin. B. Zimmerman of GM and Dr. Michael Wang of Argonne National Laboratory for their generous help. References [1] Huo H, Wang M, Johnson L, He DQ. Projection of Chinese motor vehicle growth, oil demand, and CO2 emissions through 2050. Transport Res Rec 2007:69–77. [2] National Bureau of Statistics. China statistical yearbook 2009. Beijing: China Statistic Press; 2009 [in Chinese].
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Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Energy consumption and GHG emissions of GTL fuel by LCA: Results from eight demonstration transit buses in Beijing Han Hao a,b, Hewu Wang a,b,*, Lingjun Song a, Xihao Li a, Minggao Ouyang a,b a b
State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, China China Automotive Energy Research Center (CAERC), Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 21 January 2010 Received in revised form 28 March 2010 Accepted 30 March 2010 Available online 5 May 2010 Keywords: GTL Alternative fuel Transit bus Life cycle assessment Energy consumption GHG emissions
a b s t r a c t Gas-to-liquids (GTL) as an alternative to diesel is considered to be one of the technical options to reduce petroleum consumption in the on-road transportation sector. Between May and August 2007, a joint demonstration program by Tsinghua University, Beijing Transit, Cummins Corporation and Shell Corporation was carried out in Beijing. The program focused on the supply systems and vehicle use of GTL fuel. The demonstration fleet was formed by four transit buses fueled with GTL and four with diesel. It was demonstrated that GTL has good compatibility with diesel in terms of fuel supply system and vehicle use. This paper compares the energy consumption and GHG emissions of diesel and GTL fuel supply chains by life cycle analysis based on demonstration results. The results indicate GTL’s large range (reported 54–70%) in synthesis efficiency, as the key factor in determining energy consumption and GHG emissions within the GTL fuel supply chain. For the probable case (GTL synthesis efficiency: 65%), the life cycle energy consumption and GHG emissions of GTL fuel are 42.5% and 12.6% higher than that of diesel. For two sensitivity analysis cases (GTL synthesis efficiency: 54% and70%), energy consumptions are 74.2% and 31.2% higher and GHG emissions are 27.3% and 7.4% higher than that of the diesel fuel supply chain. If the efficiency of the GTL synthesis process is improved to 75%, then the GHG emissions level of the GTL fuel supply chain can be reduced to the same level as the diesel fuel supply chain. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction With the rapid increase of vehicle stock in China, oil consumption and GHG emissions associated with on-road transportation are rising dramatically [1]. Increasing petroleum demand by the on-road transportation sector in China has raised the import dependence of petroleum from 19.7% in 1995 to 51.3% in 2008 [2]. Technology options for vehicles and fuels are in urgent need to establish a stainable and low-carbon transportation energy supply system. Gas-to-liquids (GTL) as an alternative to diesel is considered to be one of the technology options to reduce petroleum consumption of on-road transportation [3]. GTL technology is recovering and enjoying growth due to recent technology and catalyst advancements. Current forecasts estimate that the world’s GTL capacity could increase from 35,000 B/D to 1–2 million B/D by 2015 [4]. Between May and August of 2007, a joint demonstration program by Tsinghua University, Beijing Transit, Cummins Corporation and Shell Corporation was carried out in Beijing. The demon* Corresponding author at: State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, China. E-mail address: [email protected] (H. Wang). 0306-2619/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2010.03.029
stration fleet was made up of four transit buses fueled with GTL. Four buses fueled with traditional diesel were also included for comparison purposes. This paper analyzes the demonstration results regarding energy consumption and the associated GHG emissions of diesel and GTL fuel supply chains using life cycle assessment methods.
2. Methodology and data 2.1. Model and system boundary The LCA analysis section of this study is based on the well-towheels (WTW) analysis module of the Tsinghua-CA3EM (China Automotive Energy, Environment and Economy Model) model [5–7], which is an integrated computerized model that includes a specialized module for China’s automotive energy supply and demand balance calculation and analysis. The model is based on China’s national conditions and integrates the widely known and used transportation energy micro-level computing GREET model [8]. As described in Fig. 1, WTW analysis of GTL fuel chain is divided into well-to-pump (WTP) and pump-to-wheels (PTW) stages.
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Fig. 1. LCA diagram of GTL fuel chain, adapted from Ref. [5].
Table 1 LCA PEa use factors and direct and indirect GHG emission factors for PFs, adapted from Ref. [5]. PFa
Crude coalb Crude NGb Crude oilb Coal NG Diesel Gasoline Residual oil Electricity a b c
PE consumption factor
Indirect emission factor
Direct emission factor
Coal (MJ/MJ)
NG (MJ/MJ)
Oil (MJ/MJ)
CO2 (g/MJ)
CH4 (g/MJ)
N2O (mg/MJ)
CH4 (g/MJ)
N2O (g/MJ)
CCa (g C/MJ)
FORa
1.053 0.080 0.097 1.061 0.081 0.156 0.164 0.139 2.506
0.000 1.011 0.023 0.001 1.015 0.027 0.049 0.026 0.015
0.002 0.064 1.047 0.110 0.065 1.119 1.130 1.055 0.115
4.259 11.909 15.998 5.733 13.544 28.287 30.506 25.323 265.218
0.422 0.072 0.054 0.425 0.110 0.078 0.086 0.071 1.010
0.062 0.154 0.265 0.172 0.161 0.441 0.472 0.409 3.917
0.001 0.001 0.002 0.001 0.001 0.004 0.080 0.002 –
0.001 0.001 0.000 0.001 0.001 0.002/0.028c 0.002 0.000 –
26.35 15.30 20.00 24.74 24.70 20.20 18.90 21.10 –
0.90 0.99 0.98 0.90 0.99 0.98 0.98 0.98 –
PE: primary energy, PF: process fuel, FOR: oxidation rate, CC: carbon content factor. These fuels are mined and transported but not refined. For vehicles, the utilization value is 0.002, while for other applications this value is 0.028.
2.2. Basic data In this study, we use previously conducted research as a source for process fuels’ life cycle energy consumption and GHG emissions in China [5–7]. Detailed data are presented in Table 1.
3. GTL supply and its characteristic 3.1. GTL supply All the GTL fuel consumed by the demonstration buses was supplied by Shell Corporation’s Malaysia plant. The GTL fuel was first shipped from Malaysia to Tianjin harbor by ocean tanker, and then distributed to Beijing’s Looan fuel storage site by road tankers. A dedicated refueling truck was used to fuel the four GTL transit buses. Given that the tanks were previously used for storage of regular diesel fuel, the oil tank for GTL storage was treated using a mild cleaning procedure beforehand to eliminate contamination of the GTL fuel. Firstly, both manhole covers and the tank outlet valve were removed. The tank force was vented to remove hydrocarbon vapors. The remaining fluid and sludge were removed by vacuumpumping. Secondly, a person was lowered into the tank to rinse it with kerosene. All associated pipe work and pumps were flushed with kerosene. The small road tanker and hoses used for local
distribution of the GTL were cleaned in the same manner as above. Finally, all associated pipe work, pumps and flexible hoses were back-flushed with GTL fuel before filling the storage tank. About 500 L of GTL fuel was used for flushing of pipe work and associated equipments. All filters in the lines and manifold system were removed and replaced after flushing. To determine the effectiveness of the cleaning process, the storage tank was pre-filled with around one sixth of an ISO tank of GTL fuel. Some of this fuel was run off via the manifold and a visual inspection of color was made until the fuel ran clear and colorless. After cleaning, all manholes were closed to prevent any contamination from dust, rain, etc. before loading the fuel (see Fig. 2).
3.2. GTL characteristic and its influence on vehicle performance GTL synthetic diesel fuel is derived from the hydrocarbon compounds from natural gas using a Fischer–Tropsch chemical reaction process [9]. It has been demonstrated that GTL has good compatibility with petroleum derived diesel fuel and no vehicle modifications are needed when using GTL on a traditional diesel vehicle [10]. However, due to the different characteristics of the two fuels, vehicle performance can be different. Table 2 presents the characteristics of GTL produced by Shell Corporation in the Malaysia plant and the No. 0 diesel vehicle with sulfur content of 50 ppm produced by Beijing’s Yanshan Petrochemical Corporation.
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Fig. 2. Picture of GTL storage station and transport vessel.
Table 2 Comparison of fuel characteristic between GTL and diesel. Item
Unit
Density (15 °C) Kinematic viscosity (40 °C) Lower heating value (LHV) Total aromatics Total sulfur Cetane index
g/cm3 ASTM D4052-96(2002)e1 0.7773 0.8241 mm2/s ASTM D445-04e2 2.568 3.728
Measurement method
GTL
Diesel
kJ/kg
ASTM D4868-00(2005)
43,709 42,500
% mg/kg
EN 12916-2000 ASTM D5453-05 ASTM D976-04a
<0.1 <1 74.7
17.4 50 53.4
26 June 2006. The odometer readings ranged from 35,800 to 66,000 km. The demonstration offered the prospect of checking the injectors for fouling both for conventional fuel and GTL fuel. A procedure is required to change injectors. At the start of the trial, all the injectors for four of the eight GTL buses were replaced with brand new injectors (six injectors per vehicle). At the end of the test, the ‘‘trial” injectors were removed for examination. It is determined that all injectors worked properly on the buses and no unusual signs of wear were discovered in the follow-up examination. 4.2. Demonstration route
The different characteristic of GTL can influence vehicle performance in the following ways. Compared with diesel, the lower heating value (LHV) of GTL is 2.8% higher by weight but 3.0% lower by volume, resulting in less power for a fixed volume injection into the engine. The kinematic viscosity of GTL is 31% lower than diesel, implying an advantage to fuel spraying optimization. The aromatics and sulfur content of GTL are far lower than diesel, which can decrease the emission of total hydrocarbons (THC) and particulate matter (PM). The cetane index of GTL is higher than diesel by 20 units, which is beneficial to improve fuel combustion performance [11–15].
All eight buses were running on the same transit route, named Beijing No. 731 transit route, as shown in Fig. 3. The total length of
4. Demonstration buses and route 4.1. Demonstration buses The vehicles evaluated were manufactured by the Chinese company King Long with Euro III common rail diesel engines by Cummins. All eight demonstration buses were the same model as described in Table 3. The 11.5-m long transit buses were chosen for the demonstration because this type of bus (by dimensions and engine type) represents most of the Beijing transit bus fleet. All buses were nine months old and have been in operation since
Fig. 3. Transit route of the demonstration program.
Table 4 Distance covered and fueling volume of demonstration buses.
Table 3 Model and specification of demonstration buses.
Fuel
Bus No.
Distance covered (km)
Fuel filled (L)
Fuel consumption (L/100 km)
Average (L/100 km)
GTL
GLT-1 GLT-2 GLT-3 GLT-4
2660 3442 2523 2391
1085 1422 1000 968
40.8 41.3 39.6 40.5
40.55
Diesel
Diesel-1 Diesel-2 Diesel-3 Diesel-4
2508 2772 2530 2618
1078 1100 1012 990
43.0 39.7 40.0 37.8
40.13
Diesel/GTL bus Model Dimensions (mm) Max total mass (kg) Max passenger number Engine model Max power/speed (kW/rpm) Max torque/speed (N m/rpm) Displacement (mL) Compression ratio
XMQ6118G 11,480 2490 3220 16,100 78 Cummins ISBe220 162/2500 820/1500 5883 17.6
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350
GTL-1
600
GTL-2
GLT-2
Fueling volume /L
Distance /km
500
GTL-3 GTL-4
400 300 200
250
GTL-3 GTL-4
200 150 100 50
100 0
GTL-1
300
0 0
10
20
30
40
50
60
70
80
90
100 110
0
10
20
30
40
50
60
70
80
90
100 110
Days
Days Fig. 4. Distance covered and fueling volume of the four GTL fueled buses.
Table 5 Efficiency and composition of process fuels for life cycle steps of diesel and GTL fuel chains. Efficiency
Diesel fuel chain Crude extraction [8,16] Transportation [8] Production [6,17] Distribution [6,7,17]
95 98.9 95 99
GTL fuel chain NG extraction [8,16] NG processing [16] GTL production [9,16] Transportation [6,17] Distribution [7,17]
99 98 54/65/70 98.5 97.8
Composition of process fuels CO
RO
Di
El
Ga
31 0 0 0
1 0 20 0
15 2 0 70
18 6 15 10
4 0 0 0
0 0 0 0 0
1 0 0 0 0
10 1 0 2 70
1 3 0 6 10
1 0 0 0 0
NG
HO
CH4
31 0 55 0
0 92 10 20
0 0 0 0
77 90 100 0 0
0 0 0 92 20
10 6 0 0 0
CO: crude oil, RO: residual oil, Di: diesel, El: electricity, Ga: gasoline, NG: natural gas, HO: heavy oil.
the route is 44 km. The route was designed with 20% considered a fast section (green1) and 80% a slow section (yellow), which is very typical for Beijing transit conditions. The buses covered approximately 160–240 km/day. 4.3. Demonstration result Drivers of the demonstration buses were required to record the fueling volume and driving mileage everyday. The ratio of total fuel filled versus mileage covered was evaluated to determine the fuel consumption rate. The results are presented in Table 4 (see Fig. 4). Table 4 indicates that all eight buses had similar fuel consumption rates, ranging from 37.8 L/100 km (Diesel-4) to 43 L/100 km (Diesel-1). The difference of fuel consumption rates can be attributed to different driving habits such as the use of air conditioning. The average fuel economy of the GTL transit buses was slightly lower (1%) than the diesel buses.
LPG are produced in refineries. Diesel is then distributed by pipeline or trucks. The efficiencies of each stage are presented in Table 5 and all of them are based on literature review. The GTL supply chain hasn’t been evaluated for a long time and there is large potential for efficiency improvement. In this study, the GTL fuel supply chain starts with natural gas extraction and processing in Malaysia. GTL fuel is produced in plants located near the gas field. After production, GTL fuel is transported by sea tanker to Tianjin harbor located on the northeast coast of China. Distribution forms the last stage of the GTL supply chain. Efficiencies of each stage are presented in Table 5. In particular, to reflect the possible efficiency improvement of the GTL synthesis process, three efficiencies of low (54%), probable (65%) and high (70%) are assumed and a sensitivity analysis is conducted. For each process fuel, the related life cycle energy consumption (sorted by coal, natural gas and petroleum), GHG emission (CH4 and N2O equated as CO2 by global warming potential value) are all calculated base on values given by OU from his CA3EM model.
5. Life cycle assessment 5.2. Energy consumption 5.1. Diesel and GTL supply The traditional diesel supply chain is very efficient since this fuel supply chain has dominated on-road transportation fuels for a long time. In this study, the diesel fuel chain starts with crude oil extraction in the mid-east. After extraction, the crude oil is transported by sea tanker to China where gasoline, diesel and 1 For interpretation of color in Fig. 3, the reader is referred to the web version of this article.
Table 6 presents the life cycle energy consumption and GHG emissions of diesel and GTL fuel supply chains. The results are also presented in Fig. 5. As shown in Table 6 and Fig. 5, for the probable case, the life cycle fossil consumption of the GTL fuel supply chain is 42.5% higher than that of a traditional diesel fuel supply chain. The higher consumption of the GTL fuel supply chain can be mostly attributed to the low efficiency of the GTL synthesis process. And, with the same end use technology (diesel bus), the GTL production process is far
H. Hao et al. / Applied Energy 87 (2010) 3212–3217
Table 6 Life cycle energy consumption and GHG emissions of diesel and GTL fuel chains.
Fossil Petroleum NG Coal GHG
Diesel
MJ/100 km
1667.5 1510.8 69.3 87.4 124.0
kg/100 km
GTL Low
Probable
High
2904.3 118.0 2666.2 120.0 157.9
2376.6 87.8 2206.3 82.4 139.6
2191.6 77.3 2045.1 69.3 133.2
GTL synthesis efficiency improvement
180 160
GHG emissions/(Kg/100km) .
Unit
54%
54%
65% 65%
140
70%
70%
120
GTL synthesis efficiency improvement
100 80 Fossil consumption-diesel Fossil consumption-GTL Petroleum consumption-diesel Petroleum consumption-GTL
60 40 20 0
0
500
1000
1500
2000
2500
3000
Table 6 and Fig. 5 indicate that, for the probable case, the GHG emissions of the GTL fuel supply chain is 12.6% higher than that of a traditional diesel fuel supply chain. Compared with the diesel fuel supply chain, GTL needs more process fuels due to lower synthesis efficiency. However, among all the process fuels used in the GTL fuel supply chain, natural gas, which is lower in carbon content than petroleum, is consumed in large percentage. Therefore, the GHG emissions level of the GTL fuel supply chain is decreased. If the synthesis efficiency is improved up to 70%, then the GHG emissions of the GTL fuel supply chain can be decreased by 4.6%, still 7.4% higher than that of the diesel fuel supply chain. To decrease the GHG emissions of the GTL fuel supply chain to the same level as diesel, the synthesis efficiency has to be improved up to 75%. GTL as an alternative to diesel is beneficial to reduce the reliance on petroleum. However, the GTL fuel supply chain tends to increase the resulting GHG emissions. The trade off between petroleum conservation and GHG reduction must be addressed when promoting the mass use of GTL. Of all the factors affecting the result, the synthesis efficiency is the key factor. GTL production as a newly introduced technology has large potential for efficiency improvement. With higher efficiency, the GTL fuel supply chain results in better performance in terms of life cycle fossil consumption and GHG emissions. Therefore, GTL becomes competitive as an alternative to diesel.
3500
Energy consumption/(MJ/100km)
6. Discussion
Fig. 5. Comparison of energy consumption and GHG emissions between diesel and GTL fuel chains.
less efficient than diesel. For the probable case, the efficiency of the GTL synthesis process is only 65%, consuming large amounts of process fuels. If the synthesis efficiency is improved up to 70%, the life cycle fossil consumption can be decreased by 7.8%. However, even with high synthesis efficiency, the life cycle fossil consumption of the GTL fuel supply chain is 31.4% higher than that of the diesel fuel supply chain. Although results indicate higher fossil consumption, GTL as a natural gas derived fuel can be utilized to reduce petroleum demand. Even with low synthesis efficiency, the petroleum consumption of the GTL fuel supply chain is less than 1/10 of that of the diesel fuel supply chain. Some petroleum is still necessary for the GTL fuel supply chain because it is assumed that gasoline, diesel and residual oil are consumed by some life cycle steps such as transportation and distribution.
Energy consumption, MJ/km
5.3. GHG emissions
Several studies have been conducted to address the energy efficiency and GHG emissions of diesel and GTL fuel chains, as presented in Fig. 6 (only for probable cases assumed for each study). The two cited studies are based on car platforms with significantly higher fuel economy, making life cycle energy consumption and GHG emissions of both diesel and GTL fuel chains lower. For these three studies, energy consumption of GTL fuel chains are 42.9%, 28% and 42.5% higher than diesel fuel chains. GHG emissions of GTL fuel chains are 18.3%, 10.5% and 12.6% higher than diesel fuel chains. For the second study, it is assumed that diesel is refined domestically with energy efficiency of 91.46% (95% for the other two studies), resulting in relatively higher energy consumption and GHG emissions of the diesel fuel chain. This explains why exceeding levels of energy consumption and emissions in the GTL fuel chain in the second study are lower than the other two studies. Life cycle studies on other alternative fuels have assessed several possible fuel chains [17–26]. Among related fuel chains, other studies indicate that on a bus platform with fuel consumption of
Diesel fuel chain
30 42.5%
25
1600 12.6%
GTL fuel chain
1400 1200
20
1000
15
800 600
10 5 0
42.9%
18.3%
28%
400
10.5%
GHG emissions, g/km
3216
200 Ref.[16]
Ref.[17]
This study
Energy consumption
Ref.[16]
Ref.[17]
This study
0
GHG emissions
Fig. 6. Comparison of energy consumption and GHG emissions between diesel and GTL fuel chains by different studies (all for probable cases).
H. Hao et al. / Applied Energy 87 (2010) 3212–3217
45 L diesel (or 45 kg CNG or 80 L DME) per 100 km, the life cycle energy consumption and GHG emissions of the CNG fuel chain is slightly lower than the diesel fuel chain while the DME fuel chain is about two times higher than the diesel fuel chain. It can be concluded that for the two natural gas derived fuels (CNG and GTL), the CNG fuel chain is better than the GTL fuel chain in terms of energy consumption and GHG emissions while the GTL fuel chain is better in terms of compatibility with traditional oil supply system. For the two compression ignition (CI) platform based alternative fuels (GTL and DME), GTL exhibits better performance than DME in terms of energy consumption and GHG emissions. 7. Conclusion (1) GTL’s supply system is compatible with the traditional diesel supply system. It is demonstrated that, with proper procedure, GTL fuel can be stored and supplied by the same infrastructure as diesel fuel. (2) The use of GTL fuel is compatible for use in diesel vehicles. No modifications are needed when fueling a diesel bus with GTL fuel. The fuel economy of GTL transit buses is slightly lower (1%) than diesel buses. (3) GTL synthesis efficiency is the key factor to determine the energy consumption and GHG emissions of the GTL fuel supply chain. For GTL synthesis efficiency of 54%, 65% and 70%, the energy consumption of the GTL fuel supply chain is 74.2%, 42.5% and 31.2% higher than that of the diesel fuel supply chain. However, GTL as a natural gas derived fuel, consumes little petroleum within its life cycle, that is, less than 6% of the diesel fuel supply chain. (4) For GTL synthesis efficiencies of 54%, 65% and 70%, the GHG emissions of the GTL fuel supply chain are 27.3%, 12.6% and 7.4% higher than that of the diesel fuel supply chain. If the efficiency of the GTL synthesis process is improved to 75%, then the resulting GHG emission levels of the GTL fuel supply chain can be reduced to the same level as that of the diesel fuel supply chain.
Acknowledgements The project is co-supported by Beijing Transit, Cummins Corporation and Shell Corporation. The authors would like to thank Dr. Xunmin Ou of Tsinghua University, Dr. Kristin. B. Zimmerman of GM and Dr. Michael Wang of Argonne National Laboratory for their generous help. References [1] Huo H, Wang M, Johnson L, He DQ. Projection of Chinese motor vehicle growth, oil demand, and CO2 emissions through 2050. Transport Res Rec 2007:69–77. [2] National Bureau of Statistics. China statistical yearbook 2009. Beijing: China Statistic Press; 2009 [in Chinese].
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