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Application and Development of Biomass Fuels for Transportation in China
TSINGHUA SCIENCE AND TECHNOLOGY I S S N 1 0 0 7 - 0 2 1 4 1 3 / 1 4 pp 2 2 3 - 2 3 0 Volume 12, Number 2, April 2007
Application and Development of Biomass Fuels for Transportation in China WANG Jianxin (王建昕), SHUAI Shijin (帅石金)**, CHEN Hu (陈 虎) State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, China Abstract: Biomass fuels have become a big concern due to the large increase in green house gases and the rapid rise of petroleum prices around the world. This paper reviews recent developments in biomass fuels, such as ethanol and biodiesel, in China. Ethanol-gasoline mixture (E10) for vehicles is currently distributed in nine provinces while biodiesel is under development. One way to extend the application of ethanol is to burn it in diesel engines to lower soot emissions. The effects of the different methods blending ethanol with fossil diesel, and blending biodiesel with fossil diesel and ethanol-diesel on the combustion and emissions are investigated. The test results show that ethanol and biodiesel can be mixed with fossil diesel to greatly reduce particulate matter and soot emissions from diesel engines. But the application of ethanol blending with fossil diesel is more difficult than that of ethanol blending with gasoline, and biodiesel blending with fossil diesel. The dual-fuel injection of ethanol and diesel systems has the highest smoke reduction effect for a high ethanol fraction. Key words: biomass fuel; ethanol; biodiesel; diesel; blending; emissions
Introduction Currently, energy consumption in China accounts for 13.6% of the total worldwide energy consumption. China’s coal consumption is the largest in the world, and its petroleum and electric power consumption ranks second, just behind the USA. However, in terms of the averaged resources consumed per person, China has a very low level of consumption[1]. China has abundant biomass resources. There are 1024 billion m2 infield, which produces about 700 million tons of the straw every year[2]. Some of this straw is used for paper and feedstuff, but there are still about 200 million tons left. According to preliminary statistics, the biomass used to produce the fuel is more than 50% in total. With the issuance of renewable energy regulations in China, the percent of renewable energy Received: 2006-10-01
﹡﹡ To whom correspondence should be addressed. E-mail: [email protected]; Tel: 86-10-62772515
consumption may increase from 7% at present to 15% in 2020. Up to now, the most feasible bio-fuels for vehicles have been ethanol and biodiesel. Ethanol can be made from corn, sorgum, grains, potatos, sugar cane, etc. In China, ethanol is made from grain which has been stored for many years. Biodiesel can be produced from vegetable oils, such as rape oil and cottonseed oil, and animal fat. Since 2002, demonstrations of ethanol-gasoline vehicles have been carried out in Heilongjiang, Jilin, Liaoning, Henan, and Anhui Provinces. In 2004, the government issued several regulations about the ethanol-gasoline and decided to extend the demonstration projects to four other provinces, Hubei, Shandong, Hebei, and Jiangsu. There are now nine provinces using ethanol-gasoline with ethanol percentage in the fuel of 10% (E10). China currently has 1.02 million tons of ethanol production capability every year, with 0.3 million tons of
224
the ethanol produced by the TianGuan Company in Henan, 0.1 million tons by the Zhaodong Company in Heilongjiang, 0.3 million tons by the Tianhe Company in Jilin and 0.32 million tons by the Fengyuan Company in Anhui. In 2005, the E10 gasoline consumption was up to 700 million tons. The E10 gasoline will be widely used and encouraged by the government. In 2004, 46.26 million tons of main food oil plants (including soybean, rapeseed, peanut, and sesame) were produced in China, while the consumption was only 24 million tons. The consumption per person is close to 19 kg/(person ⋅ year−1), higher than the world average of 2 kg/(person ⋅ year−1). The edible oil market has become saturated so many companies are focusing on the production of biodiesel from vegetable oils. In addition, a large amount of wasted food oil can be collected from restaurants with many studies of production of the biodiesel from the used food oil to reduce environmental pollution. In recent years, the biodiesel industry has been mainly funded by private enterprises. Some production lines, with 10 000-20 000 tons yearly production capability, have been built by companies in Hainan and Sichuan Provinces. In addition, one new production line is being built with 30 000 tons capability in Fujian Province. Xinjiang Uygur Autonomous Region plans to develop a biodiesel plant to produce 100 000 tons annually from cottonseed oil. Recently, the Biolu Company from Austria announced that a biodiesel plant will be built in Weihai, Shandong Province in China with production capability of 250 000 tons per year from rapeseed oil. In 2006, the price of crude oil has been very high, and will continue to increase, resulting in vegetable oil and ethanol prices close to that of diesel, which will contribute to the commercialization of bio-fuels. In this study, the ethanol and biodiesel biomass fuels were used in diesel engines with different technologies. In addition, the effects of biomass fuels on the engine performance, economy, combustion characteristics, and emission characteristics were investigated. Furthermore, the particulate matter (PM) emissions, detailed components, and PM size distribution were analyzed for biomass fuels.
1
Ethanol Use in Diesel Engines
1.1
Methods
Three methods for using ethanol in diesel engines are
Tsinghua Science and Technology, April 2007, 12(2): 223-230
direct blending[3], online blending[4] (shown in Fig. 1a) and dual-fuel (shown in Fig. 1b)[5]. The directly blended fuel does not require any modifications to diesel engines, but stable, high-ethanol percentage blended fuels are difficult to produce since the ethanol and diesel are not miscible. Even with a cosolvent, the ethanol percentage in diesel is normally less than 15%. Online blending and dual-fuel systems can more easily adjust the ethanol percentage in the diesel, but they need modified fuel injection systems, especially for the dual-fuel injection method which required an additional fuel injection system on the engine. 1.2
Combustion and emission characteristics using online blended fuel
The effect of ethanol-diesel produced by online blending on diesel engine smoke emission is shown in Fig. 2. The engine smoke is reduced with increasing ethanol percentage in the diesel fuel, with more significant reductions with increasing load. For example, at 1540 r/min and brake mean effective pressure (BMEP) =0.7 MPa, the E10 reduces the smoke emissions by 42% while the E30 fuel reduces smoke by 74%.
Fig. 1 Methods for using ethanol in diesel engines
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Fig. 2 Effect of ethanol-diesel on smoke
Figure 3 shows the NOx emissions for different percentages of ethanol online blends. The addition of ethanol in the diesel reduces the NOx emissions by 10%-30% at low loads, but increases NOx emissions by 20%-30% at high loads. Ethanol is an oxygenated fuel, which increases the local air/fuel ratio due to more embedded oxygen and suppresses the soot formation, but increases the NOx emissions especially at high loads. However, the ethanol reduces the mixture temperature in the cylinder because of the high latent heat of ethanol; therefore, the combustion temperature is reduced, which reduces the NOx production, especially at low loads. To reduce NOx emissions at high loads, engines must use EGR technology or retard the injection timing.
Fig. 3 Effect of ethanol-diesel on NOx
Figure 4 compares the cylinder pressure and heat release ratio (HRR) for different online blending fuels at moderate loads. Pe indicates the effective power. As the ethanol percentage increases in the diesel, the combustion ignition is retarded, which increases the maximal HRR for premixed combustion, the in-cylinder pressure peak, and the pressure gradient. These trends further explain the smoke reduction and NOx increases with the high percentage of ethanol.
Fig. 4 Cylinder pressure and heat release ratio for various fuels
1.3
Combustion and emission characteristics of dual-fuel systems
In this paper, dual-fuel systems refer to systems where the ethanol is injected into the intake manifold port to form a premixed ethanol mixture and the diesel is directly injected into the cylinder to ignite the ethanol mixture, which is called the homogeneous charge induced ignition (HCII) combustion mode. In this combustion mode a homogeneous mixture is formed first with a high octane number fuel like ethanol and then the mixture is ignited by the high cetane number fuel like diesel. The formation of multiple ignition kernels shortens the flame propagation and diffuse combustion times. This combustion mode can be only realized with high ethanol percentage in the fuel. Figure 5 shows the energy consumption of various ethanol injection fractions into the intake port. BSEC indicates the brake specific energy consumption. The 100% indicates pure ethanol and 0% indicates pure diesel. When the ethanol injection fraction is in the range of 10%-90%, the HCII energy consumption is almost the same or less than with diesel. The biggest reduction is at about 10%.
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Tsinghua Science and Technology, April 2007, 12(2): 223-230
Fig. 5 Energy consumption for various ethanol injection fractions
Figure 6 shows the ignition delay and combustion duration for various ethanol fractions. With increasing ethanol injection fraction, the ignition is retarded and the combustion duration is significantly shortened. When the ethanol percentage is 87% of the total fuel, the combustion lasts only 20° crank angle (CA), about half of the diesel’s duration (39° CA), which shows that the HCII combustion mode has nearly constant volume combustion. Figure 7 shows the smoke emission changes for the different ethanol fractions at middle and high loads. The smoke is reduced almost linearly with increasing ethanol fraction in the fuel. When the ethanol percentage is over 60%-70%, there is no smoke emission. Figure 8 shows the smoke emission changes with the oxygen content by mass. Smoke free combustion can be realized with oxygen content over 20%-25%. The investigation also showed that NOx emissions of the dual-fuel system were the same behavior as for the online blended fuel. The NOx emissions were reduced at low loads and increased moderately at high loads, with the HC emissions increased more or less because of the homogeneous combustion.
Fig. 6 Combustion characteristics for different ethanol proportions
Fig. 7 Smoke emissions for different ethanol fractions
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fraction, but also decreased at high load. There was no influence of the ethanol increase on the formaldehyde emissions. 1.5
Fig. 8 Smoke emissions for different oxygen content
1.4
Unregulated emissions of ethanol-diesel blend fuels
Unregulated emissions of acetaldehyde and unburned ethanol occur when ethanol is combusted with diesel. The ethanol and acetaldehyde in the emissions were analyzed using a gas chromatograph (GC). Figure 9 shows the emissions of formaldehyde, acetaldehyde, and unburned ethanol at different loads. The acetaldehyde emissions increase significantly with increasing ethanol percentage in the blended fuel, but the acetaldehyde emissions decreased with increasing load due to oxidation of the acetaldehyde at high combustion temperature. The unburned ethanol also increased obviously to 100×10−6-400×10−6 with increasing ethanol
When ethanol is directly added to the diesel fuel, the vapor pressure of the blended fuels will increase. Therefore, the safety of the blended fuels will be a concern in the distribution and transportation[6]. Considering the increase of HC, including the acetaldehyde and unburned ethanol in the emission, it is necessary to equip the diesel oxidation catalyst in the exhaust of diesel engines. In addition, previous studies have shown that ethanol will contaminate the lubricant and cause the abrasion, but there was no detailed investigation.
2
Biodiesel Use in Diesel Engines
2.1
Methods
Biodiesel is actually a saturated fatty acid resin with a slightly higher cetane value and viscosity than diesel and basically without sulphur. Biodiesel can be directly used in diesel engines, or mixed with any proportion of diesel in engines. Since the cost of biodiesel is higher than that of diesel, the biodiesel proportion is commonly 10% (B10) or 20% (B20). The biodiesel properties are very close to those of diesel; therefore, online blending or dual-fuel mixing is not generally used as with ethanol-diesel[7,8]. In order to reduce the difficulties of mixing ethanol with diesel, biodiesel was added as a solvent to increase the ethanol proportion in diesel, which is the mixture of biodiesel, ethanol, and diesel. The mixture will also improve the low cetane of ethanol and high viscosity of biodiesel, and raise the total oxygen content in the mixed fuel. 2.2
Fig. 9 Unregulated emissions for various ethanol proportion fuels
Difficulties of using ethanol in diesel engines
Emission characteristics of biodiesel-diesel blend
The performance of B5 (5% biodiesel) and B20 (20% biodiesel) blended fuels was evaluated in a turbocharged diesel engine which can meet Euro 3 emission regulations. The results show that the engine maximum torque with the blended fuels decreased while the fuel consumption increased with increasing biodiesel proportion due to the lower heat value of the biodiesel
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compared to diesel. Figure 10 shows the variation of the particulate matter (PM) emission with engine speed at full load and with load at 1200 r/min for different fuels. The PM emissions decrease with increasing biodiesel fraction in the diesel. The B20 reduced about 25% of the PM of the on average compared to the diesel.
Fig. 11 NOx emissions for different biodiesel blends
Fig. 10 PM emissions of different biodiesel blends
Figure 11 gives NOx emission variations with engine speed at full load and with loads at 1200 r/min for different fuels. The addition of biodiesel to the diesel had no significant effect on the NOx emissions with the NOx emissions of B5 and B20 almost the same as for diesel alone, with the biggest difference less than 5%. 2.3
Emission characteristics of biodiesel-ethanol diesel blends
Figure 12 shows the smoke and PM variations with engine load for the different biodiesel-ethanol-diesel blends. The blended fuels consisted of 10% biodiesel, 10%, 20%, or 30% ethanol, and 60%-100% diesel as shown in Table 1[9]. Figure 12 shows that the exhaust smoke and PM are reduced with increasing biodiesel and ethanol fractions, and that the reductions are more obvious at high loads than at low loads. At high loads, the smoke from the E10B, E20B, and E30B are lower than that of diesel about 30%, 55% and 85%, respectively. The PM
Fig. 12 Smoke and PM emissions of diesel and blended fuels Table 1
Compositions of the blended fuel
Blended fuel
Diesel (%)
Ethanol (%)
Biodiesel (%)
E0
100
0
0
E10B
80
10
10
E20B
70
20
10
E30B
60
30
10
WANG Jianxin (王建昕) et al：Application and Development of Biomass Fuels …
emissions of the blended fuels are also significantly reduced by increasing fractions of biodiesel and ethanol. At low loads, the PM emissions are reduced more than the smoke with the largest PM reduction at moderate loads[10]. For example, for BMEP=0.46 MPa, the PM emissions of E20B and E30B are reduced about 50% compared to E0, but at high loads the reductions are not obvious. The dry soot (DS), soluble organic fraction (SOF), and the sulfate in the PM are also analyzed. Figure 13 shows that the proportions of three components in PM for different fuel due to the different oxygen contents in the fuels. Increased oxygen content in the fuel reduced the DS but increased the SOF generation.
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the development and production of biomass fuels has expanded rapidly, with ethanol production reaching
Fig. 14 PM size distribution of diesel and E30 blends for different loads
Fig. 13 fuels
Component proportion of PM for different
Figure 14 compares the PM size distributions for E30B and E0. Only a small percentage of the particles had the sizes larger than 0.5 µm, while a high percentage had sizes smaller than 0.2 µm. The percentage of particles less than 0.2 µm was greater for the E30 fuel which means that the increased oxygen content in the blended fuel increased the percentage of very small particles.
3
Conclusions
(1) With continual increases in crude oil prices in recent years, the prices of ethanol and biodiesel are approaching the price of gasoline and diesel, which means that biomass fuels will soon enjoy widespread use. With the support from the Chinese government,
1 000 000 tons annually and biodiesel production reaching 100 000 tons annually. (2) The technology for using ethanol in gasoline engines is very mature so ethanol can provide a good substitute for oil in gasoline engines. In diesel engines, ethanol cannot only reduce the oil usage but also significantly reduce PM and dry soot emissions. Among the three blending methods, the dual-fuel injection system uses the highest ethanol fraction, has the highest efficiency and highest smoke reduction effect. However, ethanol utilization in diesel engines still has some obstacles to overcome. (3) Biodiesel can also be used to reduce oil usage and emissions, and is more easily used in diesel engines than ethanol. The mixing of biodiesel and ethanol with diesel forms a high oxygen content blended fuel which significantly reduces smoke and PM emissions.
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References [1] BP statistical review of world energy. June 2005, http://www.bp.com, 2005-06-15.
Tsinghua Science and Technology, April 2007, 12(2): 223-230 [6] McCormick R L, Parish R. Technical barriers to the use of ethanol in diesel fuel. Report from National Renewable Energy Laboratory, NREL/MP-540-32674, 2001. [7] Sharp C A, Howell S A, Jobe J. The effects of biodiesel fu-
[2] Shi Yuanchun. Developing the biomass industrials. Review
els on transient emissions from modern diesel engines, Part
of China Agricultural Science and Technology, 2006, 8(1):
I: Regulated emissions and performance. SAE, 2000-01-
1-5. (in Chinese)
1967.
[3] He Bangquan, Wang Jianxin, Yan Xiaoguang. Study on
[8] Mei Deqing, Sun Ping, Yuan Yinnan. Investigation on ex-
combustion and emission characteristics of diesel engines
haust emission characteristics of compression ignition en-
using ethanol blended diesel fuels. SAE, 2003-01-0762.
gine fueled with biodiesel. Transactions of CSICE, 2006,
[4] He Bangquan, Wang Jianxin, Yan Xiaoguang. Combustion and emission characteristics of engines using ethanol-diesel fuels blended on line. J. Tsinghua Univ. (Sci. & Tech.), 2003, 43: 1523-1525. (in Chinese) [5] He Bangquan, Wang Jianxin, Shuai Shijin. Homogeneous charge combustion and emissions of ethanol ignited by pi-
24: 331-335. [9] Wang Jianxin, He Bangquan, Shuai Shijin. One blended diesel fuel. Patent: ZL031211445, 2003-03-28. (in Chinese) [10] Chen Hu, Wang Jianxin, Shuai Shijin. Effects of ethanol in ester-ethanol-diesel blended fuels on spray behavior and PM emissions. SAE, 2006-01-0236.
lot diesel on diesel engines. SAE, 2004-01-0094.
CERNET2 Accredited China’s Top Scientific Development in 2006 The backbone network of the China next-generation Internet, CNGI-CERNET2/6IX, was accredited China’s top scientific development in 2006, as the annual list of “Top Ten Scientific Events” was unveiled in Beijing January 21，2007, voted out by 565 scientists from the Chinese Academy of Sciences and the Chinese Academy of Engineering. CNGI-CERNET2/6IX, undertaken by Tsinghua and 24 other universities, runs IPv6 protocol and connects 25 PoPs distributed in 20 cities around China with the speed of 2.5 Gbps/10 Gbps.
（From http://news.tsinghua.edu.cn，2007-01-24）
Application and Development of Biomass Fuels for Transportation in China WANG Jianxin (王建昕), SHUAI Shijin (帅石金)**, CHEN Hu (陈 虎) State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, China Abstract: Biomass fuels have become a big concern due to the large increase in green house gases and the rapid rise of petroleum prices around the world. This paper reviews recent developments in biomass fuels, such as ethanol and biodiesel, in China. Ethanol-gasoline mixture (E10) for vehicles is currently distributed in nine provinces while biodiesel is under development. One way to extend the application of ethanol is to burn it in diesel engines to lower soot emissions. The effects of the different methods blending ethanol with fossil diesel, and blending biodiesel with fossil diesel and ethanol-diesel on the combustion and emissions are investigated. The test results show that ethanol and biodiesel can be mixed with fossil diesel to greatly reduce particulate matter and soot emissions from diesel engines. But the application of ethanol blending with fossil diesel is more difficult than that of ethanol blending with gasoline, and biodiesel blending with fossil diesel. The dual-fuel injection of ethanol and diesel systems has the highest smoke reduction effect for a high ethanol fraction. Key words: biomass fuel; ethanol; biodiesel; diesel; blending; emissions
Introduction Currently, energy consumption in China accounts for 13.6% of the total worldwide energy consumption. China’s coal consumption is the largest in the world, and its petroleum and electric power consumption ranks second, just behind the USA. However, in terms of the averaged resources consumed per person, China has a very low level of consumption[1]. China has abundant biomass resources. There are 1024 billion m2 infield, which produces about 700 million tons of the straw every year[2]. Some of this straw is used for paper and feedstuff, but there are still about 200 million tons left. According to preliminary statistics, the biomass used to produce the fuel is more than 50% in total. With the issuance of renewable energy regulations in China, the percent of renewable energy Received: 2006-10-01
﹡﹡ To whom correspondence should be addressed. E-mail: [email protected]; Tel: 86-10-62772515
consumption may increase from 7% at present to 15% in 2020. Up to now, the most feasible bio-fuels for vehicles have been ethanol and biodiesel. Ethanol can be made from corn, sorgum, grains, potatos, sugar cane, etc. In China, ethanol is made from grain which has been stored for many years. Biodiesel can be produced from vegetable oils, such as rape oil and cottonseed oil, and animal fat. Since 2002, demonstrations of ethanol-gasoline vehicles have been carried out in Heilongjiang, Jilin, Liaoning, Henan, and Anhui Provinces. In 2004, the government issued several regulations about the ethanol-gasoline and decided to extend the demonstration projects to four other provinces, Hubei, Shandong, Hebei, and Jiangsu. There are now nine provinces using ethanol-gasoline with ethanol percentage in the fuel of 10% (E10). China currently has 1.02 million tons of ethanol production capability every year, with 0.3 million tons of
224
the ethanol produced by the TianGuan Company in Henan, 0.1 million tons by the Zhaodong Company in Heilongjiang, 0.3 million tons by the Tianhe Company in Jilin and 0.32 million tons by the Fengyuan Company in Anhui. In 2005, the E10 gasoline consumption was up to 700 million tons. The E10 gasoline will be widely used and encouraged by the government. In 2004, 46.26 million tons of main food oil plants (including soybean, rapeseed, peanut, and sesame) were produced in China, while the consumption was only 24 million tons. The consumption per person is close to 19 kg/(person ⋅ year−1), higher than the world average of 2 kg/(person ⋅ year−1). The edible oil market has become saturated so many companies are focusing on the production of biodiesel from vegetable oils. In addition, a large amount of wasted food oil can be collected from restaurants with many studies of production of the biodiesel from the used food oil to reduce environmental pollution. In recent years, the biodiesel industry has been mainly funded by private enterprises. Some production lines, with 10 000-20 000 tons yearly production capability, have been built by companies in Hainan and Sichuan Provinces. In addition, one new production line is being built with 30 000 tons capability in Fujian Province. Xinjiang Uygur Autonomous Region plans to develop a biodiesel plant to produce 100 000 tons annually from cottonseed oil. Recently, the Biolu Company from Austria announced that a biodiesel plant will be built in Weihai, Shandong Province in China with production capability of 250 000 tons per year from rapeseed oil. In 2006, the price of crude oil has been very high, and will continue to increase, resulting in vegetable oil and ethanol prices close to that of diesel, which will contribute to the commercialization of bio-fuels. In this study, the ethanol and biodiesel biomass fuels were used in diesel engines with different technologies. In addition, the effects of biomass fuels on the engine performance, economy, combustion characteristics, and emission characteristics were investigated. Furthermore, the particulate matter (PM) emissions, detailed components, and PM size distribution were analyzed for biomass fuels.
1
Ethanol Use in Diesel Engines
1.1
Methods
Three methods for using ethanol in diesel engines are
Tsinghua Science and Technology, April 2007, 12(2): 223-230
direct blending[3], online blending[4] (shown in Fig. 1a) and dual-fuel (shown in Fig. 1b)[5]. The directly blended fuel does not require any modifications to diesel engines, but stable, high-ethanol percentage blended fuels are difficult to produce since the ethanol and diesel are not miscible. Even with a cosolvent, the ethanol percentage in diesel is normally less than 15%. Online blending and dual-fuel systems can more easily adjust the ethanol percentage in the diesel, but they need modified fuel injection systems, especially for the dual-fuel injection method which required an additional fuel injection system on the engine. 1.2
Combustion and emission characteristics using online blended fuel
The effect of ethanol-diesel produced by online blending on diesel engine smoke emission is shown in Fig. 2. The engine smoke is reduced with increasing ethanol percentage in the diesel fuel, with more significant reductions with increasing load. For example, at 1540 r/min and brake mean effective pressure (BMEP) =0.7 MPa, the E10 reduces the smoke emissions by 42% while the E30 fuel reduces smoke by 74%.
Fig. 1 Methods for using ethanol in diesel engines
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225
Fig. 2 Effect of ethanol-diesel on smoke
Figure 3 shows the NOx emissions for different percentages of ethanol online blends. The addition of ethanol in the diesel reduces the NOx emissions by 10%-30% at low loads, but increases NOx emissions by 20%-30% at high loads. Ethanol is an oxygenated fuel, which increases the local air/fuel ratio due to more embedded oxygen and suppresses the soot formation, but increases the NOx emissions especially at high loads. However, the ethanol reduces the mixture temperature in the cylinder because of the high latent heat of ethanol; therefore, the combustion temperature is reduced, which reduces the NOx production, especially at low loads. To reduce NOx emissions at high loads, engines must use EGR technology or retard the injection timing.
Fig. 3 Effect of ethanol-diesel on NOx
Figure 4 compares the cylinder pressure and heat release ratio (HRR) for different online blending fuels at moderate loads. Pe indicates the effective power. As the ethanol percentage increases in the diesel, the combustion ignition is retarded, which increases the maximal HRR for premixed combustion, the in-cylinder pressure peak, and the pressure gradient. These trends further explain the smoke reduction and NOx increases with the high percentage of ethanol.
Fig. 4 Cylinder pressure and heat release ratio for various fuels
1.3
Combustion and emission characteristics of dual-fuel systems
In this paper, dual-fuel systems refer to systems where the ethanol is injected into the intake manifold port to form a premixed ethanol mixture and the diesel is directly injected into the cylinder to ignite the ethanol mixture, which is called the homogeneous charge induced ignition (HCII) combustion mode. In this combustion mode a homogeneous mixture is formed first with a high octane number fuel like ethanol and then the mixture is ignited by the high cetane number fuel like diesel. The formation of multiple ignition kernels shortens the flame propagation and diffuse combustion times. This combustion mode can be only realized with high ethanol percentage in the fuel. Figure 5 shows the energy consumption of various ethanol injection fractions into the intake port. BSEC indicates the brake specific energy consumption. The 100% indicates pure ethanol and 0% indicates pure diesel. When the ethanol injection fraction is in the range of 10%-90%, the HCII energy consumption is almost the same or less than with diesel. The biggest reduction is at about 10%.
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Fig. 5 Energy consumption for various ethanol injection fractions
Figure 6 shows the ignition delay and combustion duration for various ethanol fractions. With increasing ethanol injection fraction, the ignition is retarded and the combustion duration is significantly shortened. When the ethanol percentage is 87% of the total fuel, the combustion lasts only 20° crank angle (CA), about half of the diesel’s duration (39° CA), which shows that the HCII combustion mode has nearly constant volume combustion. Figure 7 shows the smoke emission changes for the different ethanol fractions at middle and high loads. The smoke is reduced almost linearly with increasing ethanol fraction in the fuel. When the ethanol percentage is over 60%-70%, there is no smoke emission. Figure 8 shows the smoke emission changes with the oxygen content by mass. Smoke free combustion can be realized with oxygen content over 20%-25%. The investigation also showed that NOx emissions of the dual-fuel system were the same behavior as for the online blended fuel. The NOx emissions were reduced at low loads and increased moderately at high loads, with the HC emissions increased more or less because of the homogeneous combustion.
Fig. 6 Combustion characteristics for different ethanol proportions
Fig. 7 Smoke emissions for different ethanol fractions
WANG Jianxin (王建昕) et al：Application and Development of Biomass Fuels …
227
fraction, but also decreased at high load. There was no influence of the ethanol increase on the formaldehyde emissions. 1.5
Fig. 8 Smoke emissions for different oxygen content
1.4
Unregulated emissions of ethanol-diesel blend fuels
Unregulated emissions of acetaldehyde and unburned ethanol occur when ethanol is combusted with diesel. The ethanol and acetaldehyde in the emissions were analyzed using a gas chromatograph (GC). Figure 9 shows the emissions of formaldehyde, acetaldehyde, and unburned ethanol at different loads. The acetaldehyde emissions increase significantly with increasing ethanol percentage in the blended fuel, but the acetaldehyde emissions decreased with increasing load due to oxidation of the acetaldehyde at high combustion temperature. The unburned ethanol also increased obviously to 100×10−6-400×10−6 with increasing ethanol
When ethanol is directly added to the diesel fuel, the vapor pressure of the blended fuels will increase. Therefore, the safety of the blended fuels will be a concern in the distribution and transportation[6]. Considering the increase of HC, including the acetaldehyde and unburned ethanol in the emission, it is necessary to equip the diesel oxidation catalyst in the exhaust of diesel engines. In addition, previous studies have shown that ethanol will contaminate the lubricant and cause the abrasion, but there was no detailed investigation.
2
Biodiesel Use in Diesel Engines
2.1
Methods
Biodiesel is actually a saturated fatty acid resin with a slightly higher cetane value and viscosity than diesel and basically without sulphur. Biodiesel can be directly used in diesel engines, or mixed with any proportion of diesel in engines. Since the cost of biodiesel is higher than that of diesel, the biodiesel proportion is commonly 10% (B10) or 20% (B20). The biodiesel properties are very close to those of diesel; therefore, online blending or dual-fuel mixing is not generally used as with ethanol-diesel[7,8]. In order to reduce the difficulties of mixing ethanol with diesel, biodiesel was added as a solvent to increase the ethanol proportion in diesel, which is the mixture of biodiesel, ethanol, and diesel. The mixture will also improve the low cetane of ethanol and high viscosity of biodiesel, and raise the total oxygen content in the mixed fuel. 2.2
Fig. 9 Unregulated emissions for various ethanol proportion fuels
Difficulties of using ethanol in diesel engines
Emission characteristics of biodiesel-diesel blend
The performance of B5 (5% biodiesel) and B20 (20% biodiesel) blended fuels was evaluated in a turbocharged diesel engine which can meet Euro 3 emission regulations. The results show that the engine maximum torque with the blended fuels decreased while the fuel consumption increased with increasing biodiesel proportion due to the lower heat value of the biodiesel
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Tsinghua Science and Technology, April 2007, 12(2): 223-230
compared to diesel. Figure 10 shows the variation of the particulate matter (PM) emission with engine speed at full load and with load at 1200 r/min for different fuels. The PM emissions decrease with increasing biodiesel fraction in the diesel. The B20 reduced about 25% of the PM of the on average compared to the diesel.
Fig. 11 NOx emissions for different biodiesel blends
Fig. 10 PM emissions of different biodiesel blends
Figure 11 gives NOx emission variations with engine speed at full load and with loads at 1200 r/min for different fuels. The addition of biodiesel to the diesel had no significant effect on the NOx emissions with the NOx emissions of B5 and B20 almost the same as for diesel alone, with the biggest difference less than 5%. 2.3
Emission characteristics of biodiesel-ethanol diesel blends
Figure 12 shows the smoke and PM variations with engine load for the different biodiesel-ethanol-diesel blends. The blended fuels consisted of 10% biodiesel, 10%, 20%, or 30% ethanol, and 60%-100% diesel as shown in Table 1[9]. Figure 12 shows that the exhaust smoke and PM are reduced with increasing biodiesel and ethanol fractions, and that the reductions are more obvious at high loads than at low loads. At high loads, the smoke from the E10B, E20B, and E30B are lower than that of diesel about 30%, 55% and 85%, respectively. The PM
Fig. 12 Smoke and PM emissions of diesel and blended fuels Table 1
Compositions of the blended fuel
Blended fuel
Diesel (%)
Ethanol (%)
Biodiesel (%)
E0
100
0
0
E10B
80
10
10
E20B
70
20
10
E30B
60
30
10
WANG Jianxin (王建昕) et al：Application and Development of Biomass Fuels …
emissions of the blended fuels are also significantly reduced by increasing fractions of biodiesel and ethanol. At low loads, the PM emissions are reduced more than the smoke with the largest PM reduction at moderate loads[10]. For example, for BMEP=0.46 MPa, the PM emissions of E20B and E30B are reduced about 50% compared to E0, but at high loads the reductions are not obvious. The dry soot (DS), soluble organic fraction (SOF), and the sulfate in the PM are also analyzed. Figure 13 shows that the proportions of three components in PM for different fuel due to the different oxygen contents in the fuels. Increased oxygen content in the fuel reduced the DS but increased the SOF generation.
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the development and production of biomass fuels has expanded rapidly, with ethanol production reaching
Fig. 14 PM size distribution of diesel and E30 blends for different loads
Fig. 13 fuels
Component proportion of PM for different
Figure 14 compares the PM size distributions for E30B and E0. Only a small percentage of the particles had the sizes larger than 0.5 µm, while a high percentage had sizes smaller than 0.2 µm. The percentage of particles less than 0.2 µm was greater for the E30 fuel which means that the increased oxygen content in the blended fuel increased the percentage of very small particles.
3
Conclusions
(1) With continual increases in crude oil prices in recent years, the prices of ethanol and biodiesel are approaching the price of gasoline and diesel, which means that biomass fuels will soon enjoy widespread use. With the support from the Chinese government,
1 000 000 tons annually and biodiesel production reaching 100 000 tons annually. (2) The technology for using ethanol in gasoline engines is very mature so ethanol can provide a good substitute for oil in gasoline engines. In diesel engines, ethanol cannot only reduce the oil usage but also significantly reduce PM and dry soot emissions. Among the three blending methods, the dual-fuel injection system uses the highest ethanol fraction, has the highest efficiency and highest smoke reduction effect. However, ethanol utilization in diesel engines still has some obstacles to overcome. (3) Biodiesel can also be used to reduce oil usage and emissions, and is more easily used in diesel engines than ethanol. The mixing of biodiesel and ethanol with diesel forms a high oxygen content blended fuel which significantly reduces smoke and PM emissions.
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24: 331-335. [9] Wang Jianxin, He Bangquan, Shuai Shijin. One blended diesel fuel. Patent: ZL031211445, 2003-03-28. (in Chinese) [10] Chen Hu, Wang Jianxin, Shuai Shijin. Effects of ethanol in ester-ethanol-diesel blended fuels on spray behavior and PM emissions. SAE, 2006-01-0236.
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CERNET2 Accredited China’s Top Scientific Development in 2006 The backbone network of the China next-generation Internet, CNGI-CERNET2/6IX, was accredited China’s top scientific development in 2006, as the annual list of “Top Ten Scientific Events” was unveiled in Beijing January 21，2007, voted out by 565 scientists from the Chinese Academy of Sciences and the Chinese Academy of Engineering. CNGI-CERNET2/6IX, undertaken by Tsinghua and 24 other universities, runs IPv6 protocol and connects 25 PoPs distributed in 20 cities around China with the speed of 2.5 Gbps/10 Gbps.
（From http://news.tsinghua.edu.cn，2007-01-24）