Energy Fuels 2010, 24, 4812–4819 Published on Web 08/10/2010
: DOI:10.1021/ef100103z
A Comparison of Diesel and Biodiesel Emissions Using Dimethyl Carbonate as an Oxygenated Additive P. Rounce,† A. Tsolakis,*,† P. Leung,† and A. P. E. York‡ †
School of Mechanical Engineering, University of Birmingham, Birmingham B15 2TT, U.K., and ‡Johnson Matthey Technology Centre, Blount’s Court, Sonning Common, Reading RG4 9NH, U.K. Received January 28, 2010. Revised Manuscript Received June 28, 2010
Diesel vehicles account for the majority of new vehicles sales in Europe. This is due to inherent fuel efficiency and high reliability. Global warming concerns have seen demand for renewable alternatives to fossil diesel with low carbon dioxide (CO2) producing emissions. Oxygenated biodiesel fuels such as rapeseed methyl ester (RME) can be utilized in an unmodified conventional diesel engine. RME combustion produces low emissions of unburnt total hydrocarbons (THCs), carbon monoxide (CO), and particulate matter (PM). This is due in part to fuel-born oxygen content (10.8% wt). This study examines the effect of adding fuel-born oxygen in the form of dimethyl carbonate (DMC) (a nontoxic potentially bioderived 53.3% wt oxygenated additive) to conventional pump diesel. It was found that nitrogen oxides (NOx) increased and that THCs, CO, and PM were reduced by up to 50% with a 96% diesel, 4% DMC blend. Interestingly 2% DMC in diesel can generate comparable particulate, THCs and CO emissions to RME combustion, at just 1.1% wt oxygen. A DMC blend may also have potential in the reduction of as yet unregulated carcinogenic emissions such as benzene and 1,3-butadiene.
an increased possibility for complete diesel combustion. In the simplest terms, this is due to oxygen availability (reactants). When there is not enough oxygen, incomplete combustion can dominate. This is of course unfavorable, as less fuel burnt means less useful power, as well as potentially dangerous emissions of unburnt hydrocarbons (HCs) and CO. It has been observed that a reduction in both the formation and growth of soot nuclei occurs due to fuel-born oxygen.7,8 Indeed studies have shown that when fuel-born oxygen content is above 30%, combustion is smokeless.9-11 There are several compounds that can be used to add oxygen to a fuel. These include esters, ethers, alcohols glycols, acetates, and carbonates.12 Some researchers have found that there is a small difference in the effect upon emissions due to the structure and size of the molecule used to add oxygen to a fuel and that the controlling factor is the quantity of oxygen added.13-16
1. Introduction Diesel engines have high thermal efficiency, are reliable, and have improved torque characteristics in comparison with spark ignition (SI) engines. Global warming concerns and the need to move away from nonrenewable fossil fuels have seen the progressive introduction of conventional fossil road transport fuel replacement targets. An example is biofuels directive 2009/28/EC.1 Clearly biofuels have attracted keen interest in Europe. The European road transport market alone is approximately 270 m tonnes (2004). This is projected to rise to 325 m tonnes by 2020,2 when the biofuels directive 2009/28/EC should see 10% biofuel replacement. This could mean that biofuel production will have to be of the order of >30 m tonnes/y (as biofuels are often of lower heating value). With the use of biofuels comes fuel-born oxygen, 10.8% wt in the case of rapeseed methyl ester (RME) (currently the most popular in Europe). Combustion studies have shown that fuel-born oxygen aids fuel oxygen entrainment, so that even in fuel rich localities there is oxygen available from the fuel.3-6 This improved fuel oxygen entrainment is thought to enhance the combustion in several ways: The improved mixture allows
(7) Eastwood, P. Critical Topics in Exhaust Gas Aftertreatment; Research Studies Press LTD: Philadelphia, PA, 2000; ISBN 0863802427. (8) Kitamura K.; Ito, T.; Kitamura, Y.; Ueda, M.; Senda, J.; Fujimoto, H.; JSAE Paper No. 20030082, Society of Automotive Engineers of Japan: Tokyo, 2003. (9) Siebers, D.; Higgins, B. SAE Paper No 2001-01-0530; Society of Automotive Engineers: Warrendale, PA, 2001. (10) Miyamoto, N.; Ogawa, H.; Nurun, N. M.; Obata, I.; Arima T. SAE Paper No. 980506; Society of Automotive Engineers: Warrendale, PA, 1998. (11) Chen, H.; Wang, J.; Shuai, S.; Chen, W. Fuel 2008, 87, 3462– 3468. (12) Teng, H.; McCandless, J. C. SAE Paper No. 2006-01-0053; Society of Automotive Engineers: Warrendale, PA, 2006. (13) Stoner, M.; Litzinger, T. SAE Paper No. 1999-01-1475; Society of Automotive Engineers: Warrendale, PA, 1999. (14) Tsurutani, K.; Takei, Y.; Fujimoto, Y.; Matsudaira, J.; Kumamoto, M. SAE Paper No. 952349; Society of Automotive Engineers: Warrendale, PA, 1995. (15) Spreen, K. B.; Ullman, T. L.; Mason, R. L. SAE Paper No. 950250; Society of Automotive Engineers: Warrendale, PA, 1995. (16) Ren, Y.; Huang, Z.; Miao, H.; Di, Y.; Jiang, D.; Zeng, K.; Liu, B.; Wang, X. Fuel 2008, 87, 2691–2697.
*To whom correspondence should be addressed. E-mail: a.tsolakis@ bham.ac.uk. (1) Directive 2009/28/EC of the European Parliament and of the Council of the 23rd April 2009. Off. J. Eur. Commun. 2009, 140 (16), 2 (8). (2) Ho, M.-W.; Biodiesel Boom in Europe? ISIS Report 06/03/06, 2006. (3) Hribernik, A.; Kegl, B. Energy Fuels 2007, 21, 1760–1767. (4) Yoshida, K.; Taniguchi, S.; Kitano, K.; Tsukasaki, Y.; Hasegawa R.; Sakata, I. SAE Paper No 2008-01-2499; Society of Automotive Engineers: Warrendale, PA, 2008. (5) Nanjundaswamy, H.; Tatur, M.; Tomazic, D.; Koerfer, T.; Lamping, M.; Kolbec, A. SAE Paper No. 2009-01-0488; Society of Automotive Engineers: Warrendale, PA, 2009. (6) Lapuerta, M.; Armas, O.; Rodrıguez-Fernandez, J. Prog. Energy Combust. Sci. 2008, 34, 198–223. r 2010 American Chemical Society
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pubs.acs.org/EF
Energy Fuels 2010, 24, 4812–4819
: DOI:10.1021/ef100103z
Rounce et al. Table 1. Engine Specification Lister Petter TR1 injector bore stroke displacement volume maximum torque maximum power compression ratio con-rod length
3 hole Ø 0.25 mm 98.4 mm 101.6 mm 773 cm3 39.2 N m at 1800 rpm 8.6 kW at 2500 rpm 15.5:1 165.0 mm
Figure 1. Dimethyl carbonate (C3H6O3).
Dimethyl carbonate (DMC) is a nontoxic compound, C3H6O3 (Figure 1). The principal interest in DMC is generated by its high oxygen weighting at around 53%. There have been many previous studies that have attempted to study the effect of fuel-born oxygen.8-18 Perhaps the most difficult problem that these studies have encountered is the decoupling of the effect that the additive has in changing various fuel properties simultaneously. DMC, though still not ideal, has a low impact upon the other various important characteristics of the fuel. Previous studies have indicated the potential of DMC in emissions reduction of PM, HCs, CO, and in some cases NOx and thermal efficiency.19,20 In recent years it has become possible to produce DMC by catalytic oxidative carbonylation of methanol with oxygen.21,22 This step has seen the elimination of dangerous phosgene which was a component of the DMC synthesis. Using a feedstock of bioderived methanol and potentially waste supercritical CO2 from CO2 sequestration in the latest generation power stations, it could be possible to generate DMC as a green biofuel in an economical way. Previous studies have focused on comparisons between diesel and biodiesel combustion.23-25 We have seen that biodiesel fueling can give improvements in smoke and CO and total unburnt hydrocarbons (THCs). However, biodiesel fueling often gives increased NOx. The primary objective of this study is to further our understanding of the role that fuel-born oxygen can play in diesel combustion by comparing diesel combustion with dieselcontaining DMC and biodiesel. The secondary objective is to examine the potential of DMC as an oxygenated additive for conventional fossil diesel.
Figure 2. Diesel engine schematic with EGR. Table 2. Fuels abbreviation
% volumetric makeup
ULSD RME 1DMC 2DMC 4DMC 20DMC
100% 100% 1% 2% 4% 20.3%
ultra low sulfur diesel rapeseed methyl ester dimethyl carbonate 99% dimethyl carbonate 98% dimethyl carbonate 96% dimethyl carbonate 79.7%
ULSD ULSD ULSD ULSD
manufacturer’s settings. The fuel injector is located near the combustion chamber center and has an opening pressure of 180 bar. The combustion chamber is a bowl-in-piston design. Table 1 shows a detailed engine specification. The single cylinder diesel engine test rig (Figure 2) consists of a thyristor-controlled DC motor-generator machine dynamometer coupled to a load cell and is used to load and motor the engine. In-cylinder pressure traces were acquired by a Kistler 6125B quartz type pressure transducer with a Kistler 5011 charge amplifier at crank shaft positions determined by a 360-ppr incremental shaft encoder with data recorded by data acquisition board National Instruments PCI-MIO-16E-4 installed in a PC. In-house developed LabVIEW based software was used to obtain pressure data and analyze combustion parameters (e.g., coefficient of variation (COV) of indicated mean effective pressure (IMEP), peak pressure, indicated power, and heat release). Readings of atmospheric conditions, temperature, pressure, and humidity, were recorded and used for the combustion and emissions analysis. The experimental system also involved other standard engine test rig instrumentation, i.e. a fuel flow meter and several local thermocouples. The exhaust gas recirculation (EGR) flow was controlled manually by a valve. The EGR level was determined volumetrically as the percentage reduction in volume flow rate of air at a fixed engine operating point. Intake airflow is measured using a Romet G65 rotary airflow meter. The accuracy of this EGR method has been tested using the EGR function of the Horiba tower at each of our tested conditions, the maximum difference recorded between the analyzer and our volume reduction technique is 1DMC > 2DMC > 4DMC > RME though, the differences are small. Indeed fuel flow
(40) Report on Carcinogens, Eleventh ed.; U.S. Department of Health and Human Services, Public Health Service, National Toxicology Program, 2005.
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Energy Fuels 2010, 24, 4812–4819
: DOI:10.1021/ef100103z
Rounce et al.
rates (not shown) for the DMC blends compared to ULSD were in proportion to their heating values across all conditions. We have seen benefits in smoke, PM, and HC for DMC as an additive in ULSD. This could be used to help expand EGR limits (i.e., higher NOx reduction) and could also benefit the aftertreatment system’s performance, longevity, and cost.
in a controlled environment) and should be investigated. A more thorough miscibility study would also be advantageous. Finally, an optical study and the use of a modern multicylinder (higher injection pressure) engine are envisaged. Acknowledgment. Thanks to The Engineering and Physical Science Research Council for providing support for this work, Shell Global Solutions UK for providing fuels, and Johnson Matthey Plc in acknowledgement of the Industrial Case Studentship for P.R.
4. Conclusions To assess the role that fuel-born oxygen plays in combustion and to test the feasibility of dimethyl carbonate (DMC) as a diesel additive, a comparative combustion study has been conducted between diesel (ULSD), biodiesel (RME), and 4 blends of the oxygenated DMC additive in diesel. DMC at 53% wt oxygen can be mixed neat with diesel at low levels and utilized as a combustion fuel in a standard diesel engine. However, larger proportion DMC blends such as 20DMC (20.3% DMC in diesel) were not practicable. Fuel-born oxygen can help to improve engine out emissions of THCs, CO, and PM. Even low level additions of DMC (2 and 4DMC) can reduce carbon PM (soot) by as much as 50% in this engine. Emissions of NOx were slightly increased due to an enhanced, more premixed combustion for all the oxygenated fuels. An interesting observation is that 2% DMC in diesel can generate comparable particulate, THCs, and CO emissions to RME combustion, even though it has only 1.1% wt oxygen. DMC addition in diesel may also have potential to reduce the smaller potentially carcinogenic VOCs such as benzene and 1,3-butadiene, during EGR conditions this quality is especially apparent. Fuel-born oxygen reduces the level of VOCs emitted and also reduces the variety of VOCs within the tested (C1-C7) range. We have seen that there are several potential emissions benefits with dimethyl carbonate as a diesel additive. There may be potential water contamination issues not observed in this laboratory based study (using clean fuel tanks
Note Added after ASAP Publication. Reference 1 was updated in the version of this paper published ASAP August 10, 2010. The correct version published on August 25, 2010.
Nomenclature BSN = Bosch smoke number CAD = crank angle degree CO = carbon monoxide CO2 = carbon dioxide Cp = specific heat capacity DMC = dimethyl carbonate EGR = exhaust gas recirculation HC = hydrocarbon IMEP = indicated mean effective pressure LCV = lower calorific value NOx = nitrogen oxides O2 = oxygen PM = particulate matter RME = rapeseed methyl ester ROHR = rate of heat release SMPS = scanning mobility particle sizer SOC = start of combustion THCs = total hydrocarbons ULSD = ultra low sulfur diesel VOCs = volatile organic compounds VOC PM = volatile organic compound particulate matter water PM = water particulate matter
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