ARTICLE pubs.acs.org/EF
Emission Analysis of Alternative Diesel Fuels Using a Compression Ignition Benchtop Engine Generator Gregory S. Bugosh, Rachel L. Muncrief, and Michael P. Harold* Department of Chemical and Biomolecular Engineering, University of Houston, Houston, Texas 77204-4004, United States ABSTRACT: A compact diesel-powered generator (5 kW) experimental setup was used to systematically evaluate the emissions generated by the combustion of several commercially produced alternative diesel fuels. The measured diesel engine exhaust components include oxides of nitrogen (NOx), carbon monoxide (CO), particulate matter (PM), volatile hydrocarbons (HCs), and carbon dioxide (CO2). Alternative diesel fuels, including biodiesel made from various feedstocks (soy, canola, palm, and tallow) and gas-to-liquid (GTL) diesel fuel, were compared to Texas low-emission diesel (TxLED), a petroleum-based ultra-low sulfur diesel (ULSD), in terms of emissions and fuel consumption. The biodiesel feedstock oil identity is an important variable affecting the level of NOx emissions, because increases were found for soy biodiesel and canola biodiesel, which are the most common feedstocks in the United States and Europe, respectively. At low engine load, NOx emissions increased for all biodiesels when compared to the ULSD, with the soy-based fuel giving the largest increase, followed by canola-based fuel and then tallow-based fuel, with palm-based biodiesel producing the smallest increase in NOx. At high engine load, NOx emissions for biodiesels followed the same relative ranking, but the increases were smaller for biodiesel made from the soy and canola feedstocks and decreases were observed for palmand tallow-based biodiesel. NOx emissions for the GTL decreased at both low and high load. Emissions of CO and HCs decreased for all alternative diesel fuels at both load points. Quantification of several exhaust HC species was accomplished, with the highest concentrations belonging to ethene, pentane, formaldehyde, and acetaldehyde. Biodiesels produced slightly more CO2 compared to ULSD, while GTL produced slightly less. The small differences in CO2 emissions were attributed to the carbon/hydrogen (C/H) ratio of the fuels and to the conversion of CO, HCs, and PM to CO2. This study provides further evidence that alternative diesel fuels can be used in compression ignition engines designed for conventional diesel without modification and that emission reductions can be achieved in some cases.
1. INTRODUCTION Diesel-fueled, compression ignition engines are generally more efficient, have increased torque, and are more durable than their gasoline-powered, spark-ignited counterparts. This has led to their use in heavy-duty vehicles for transportation and construction, with an increased presence in passenger vehicles worldwide. However, emissions from diesel engines contribute significantly to pollution, especially particulate matter (PM) and oxides of nitrogen (NOx).1,2 Carbon monoxide (CO) and volatile hydrocarbons (HCs) are also present in the exhaust. In addition, the generation of carbon dioxide (CO2) by fossil fuel combustion is of a growing concern because of its potential role in climate change. To take full advantage of the fuel efficiency afforded by diesel, a host of engine and emission aftertreatment technologies are being developed and commercialized that reduce the emissions of NOx, particulates, and HCs from diesel exhaust. A potentially less costly approach to reduce harmful emissions is to change the fuel type, without engine or emission control modifications. Combustion engines have been continuously developed and refined to perform optimally on traditional petroleum-based fuels that meet specific standards.3 Diesel fuel must currently conform to American Society for Testing and Materials (ASTM) D975-11 in North America,4 while in Europe, it must conform to EN 590.5 The most popular alternative diesel fuels now have their own standards; for example, biodiesel follows ASTM D6751-11a.6 Compositional differences between alternative r 2011 American Chemical Society
non-petroleum fuels and petroleum-based fuels can result in a cleaner combustion. For example, biomass-derived fuels typically contain oxygen (oxygenates); oxygenates are known to reduce emissions of PM, CO, and HCs.711 Direct substitution of petroleum-derived fuels with alternative fuels in vehicles can provide immediate emission reduction benefits and can be more economical than other options. Understanding the physical and chemical properties of alternative fuels, their combustion properties, and resulting emissions is needed in the development of new engines and emission aftertreatment systems. A leading petroleum diesel alternative is biodiesel, typically made from vegetable oils or animal fat (tallow) by way of the transesterification reaction with methanol.12,13 The feedstock oil is a mixture of triglycerides of varying carbon number, typically in the 1618 range.14 The transesterification produces monoalkyl esters, which have lower viscosity than the feedstock oil, giving properties very similar to that of petroleum-based diesel fuel. The byproduct of the reaction is glycerol, for which uses are being developed.15 In 2002, the United States Environmental Protection Agency (U.S. EPA) compiled and analyzed published data on how using biodiesel in existing engines impacted fuel economy and exhaust emissions.16 The majority of the data was for heavy-duty highway Received: June 28, 2011 Revised: August 29, 2011 Published: August 29, 2011 4704
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Energy & Fuels engines without exhaust gas recirculation (EGR), a method that reduces the production of NOx but increases the generation of soot or diesel particulate filtration in the diesel particulate filter (DPF). Volumetric fuel consumption increased by 4.610.6% when using biodiesel. The emissions of PM and CO were reduced by about 48%, while HCs were reduced by approximately 67%, when using biodiesel. NOx emissions generally increased by roughly 10% but varied depending upon the source of feedstock oil used to produce the biodiesel. Neat soy-based biodiesel produced the largest NOx increase of ca. 15%; rapeseed-based biodiesel produced a NOx increase of ca. 12%; and animal-based biodiesel produced an approximate 4% increase in NOx. A more recent review by Lapuerta et al.17 reported similar effects of biodiesel in terms of fuel consumption and PM, CO, HCs, and NOx emissions. This review goes into more detailed analysis of the results. The reduction in fuel economy is attributed to the lower heating value (mass basis) of biodiesel compared to petroleum diesel. Lower CO, PM, and HCs are generally attributed to the higher cetane number and oxygen content of the biodiesel, which results in more complete combustion. There is not a similar consensus on the differences in NOx emissions, however. Nitric oxide (NO) and nitrogen dioxide (NO2) that comprise NOx in vehicle exhaust form during combustion inside the engine cylinders.18 Quality biodiesel does not contain nitrogen species; therefore, fuel-bound NOx formation is generally not a concern. The combustion air contains nitrogen (N2) and excess oxygen (O2), which combines at high temperatures by the Zeldovich mechanism and is referred to as thermal NOx. The amount of fuel-borne nitrogen to NOx produced is a function of the temperature, excess O2 concentration, and the residence time. NO formation through the prompt mechanism, where radicals formed during combustion react with N2, is a small contributor to total NOx emission but might be significant for biodiesel.17 Thus, reasons for biodiesel producing more NOx include both physical and chemical properties of the fuel and are still being explored. McCormick et al. investigated the effect of the fatty acid chain length and degree of unsaturation (number of double bonds) on the emissions of NOx.19 They reported that the biodiesel density, cetane number, and degree of saturation are interrelated. Pure fatty acid esters that were highly saturated, with no double bonds in the fatty acid chain, resulted in lower density, higher cetane number, and lower NOx emissions than unsaturated esters. A longer fatty acid chain length also reduced density, increased cetane number, and lowered NOx emissions compared to shorter chain lengths. Biodiesel contains a natural mixture of the esters depending upon the feedstock source, leading to average density and cetane number for a particular fuel and resulting in an intermediate range of emissions. This finding was further supported by Benjumea et al.20 in their study of the degree of unsaturation of biodiesel, comparing palm and linseed biodiesels. The palm biodiesel methyl esters are more saturated, which leads to higher cetane number, lower density, and lower HC and NOx emissions compared to the linseed biodiesel emissions. Other studies have focused on the altered fuel injection characteristics as a result of using biodiesel. A review by Graboski et al.21 reported that retarding the injection timing by 14° when using B20 blends decreased the NOx emission to levels for ultralow sulfur diesel (ULSD) with unaltered timing; however, the reduction in PM was also eliminated. The density, bulk modulus, and speed of sound of biodiesel were reported by Tat et al.22 to
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cause advancement in the injection timing of approximately 1°, and NOx emissions are known to increase with advancement in injection timing. Subsequent studies2330 have shown that injection timing, injection pressure, injection duration, ignition delay, and combustion duration can be altered when using biodiesel and contribute to observed increases in NOx emission. Another type of alternative diesel fuel is gas-to-liquid (GTL) fuel. This is a specific type of FischerTropsch (FT) fuel made from natural gas, rather than coal or biomass. In the FT process, synthesis gas reacted over Co-based catalysts produces straightchain alkanes, which can be used as liquid fuel or feedstock to produce other chemicals.31 Sulfur must be removed prior to this reaction because it will poison the catalyst. The result is a high cetane number, sulfur-free fuel with virtually no aromatic compounds. Emissions of PM, CO, and HCs are generally reduced.3234 A study by Szybist et al.35 compared NOx emissions produced by FT diesel and soy-based biodiesel to petroleum diesel fuel. They concluded that the higher bulk modulus of the biodiesel compared to traditional diesel led to an advancement in the start of injection (SOI) and resulted in higher NOx emissions. The lower bulk modulus of FT diesel led to a delay in SOI and resulted in lower NOx. Emissions of NOx by diesel engines receive a lot of attention because of the difficulty of the reduction to N2 in the oxidizing environment of diesel exhaust, compared to the capability of three-way catalysts (TWCs) available for gasoline-powered engines operating under stoichiometric conditions. The emissions of HCs by vehicles are also important contributors to ozone formation. The U.S. EPA regulates total HCs, but some species have higher ozone-forming potential than others.36 The European Commission Ozone Directive requires its members to report air concentrations of 29 prevalent ozone precursors.37 The level of those HCs in vehicle exhaust was studied by Montero et al.38 The findings for diesel exhaust were that ethene, propene, acetylene, and isobutylene each contributed more than 10% of the total HC emission, with ethene consistently the largest fraction between 19.0 and 48.4%. HC emissions also directly affect human health because of their toxicity. Formaldehyde was recently declared carcinogenic by the U.S. Department of Health and Human Services.39 Formaldehyde and acetaldehyde, both carbonyls, are classified by the U.S. EPA as probable human carcinogens. The emission of carbonyl compounds by heavy-duty diesel trucks and diesel backup generators were studied by Sawant et al.40 It was found that formaldehyde and acetaldehyde were the largest contributors, averaging 53.7 and 18.4%, respectively. Specifically, for backup generators, carbonyls made up 6.59% of the total HC emission, with formaldehyde at 3.69% of the total HC emission. For the heavy-duty trucks, carbonyls made up 37.0% of the total HC emission and formaldehyde was 18.3% of the total HC emission. While many studies on the topic of biodiesel or GTL diesel combustion exist, many do not provide sufficient details on the engine used or the fuel properties, leading to difficulties when trying to further analyze results or cross-reference and compare to other experiments. Many research studies produce their own biodiesel fuels, which are not subject to the same quality testing as commercial fuels.41 Some studies use vehicles fitted with emission aftertreatment, such as diesel oxidation catalysts, and do not report raw engine-out emissions. Few studies compare both biodiesel and GTL to a petroleum baseline fuel. Moreover, 4705
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Figure 1. Schematic of the benchtop engine system.
Table 1. Specifications for the Diesel-Powered Electric Generator model
Yanmar YDG5500EV-6EI
type
revolving field-type AC generator (with brush)
regulation
automatic voltage regulator (AVR)
frequency
60 Hz
speed
3600 rpm
rated output rated voltage
5.0 AC kVA 120/240 VAC
rated current
41.7/20.8 A
phase
single phase
dry weight
238 lbs
engine
Yanmar L100 V engine
type
four-stroke, vertical cylinder, air-cooled
combustion system
direct injection, naturally aspirated
number of cylinders bore stroke
1 3.39 2.95 in. (86 75 mm)
displacement
26.5 in.3 (0.435 L)
compression ratio
21.1
rated output
8.3 hp (6.2 kW)
fuel injection timing
15.5°
governor
all speed type, mechanical
studies that carry out speciation of the HC emissions are uncommon. The objective of this study is to compare commercially produced alternative diesel fuels, including biodiesels made from various feedstocks (soy, canola, palm, and tallow), and GTL fuel to petroleum-based ULSD fuel. Using the benchtop engine system (BES) and operating at two load points (low and high), direct comparisons of the fuel efficiency and exhaust emissions were performed, including speciation of several key HC emissions. Where possible, trends are interpreted in terms of known differences in the fuels or features of the operating conditions.
2. EXPERIMENTAL SECTION 2.1. Engine and Load Control. The BES consists of a dieselpowered backup electricity generator with computer control (Labview 8.5) and a data acquisition system (National Instruments PXI-1033). Figure 1 is a schematic of the BES. Experimental details were previously described by An et al.42 The benchtop system gives results faster and at
less expense than full-scale dynamometer testing. The BES results were previously validated by comparing the effect of several fuel additives on emissions from the generator at several steady-state loads to the results from a heavy-duty diesel truck using the urban dynamometer drive schedule. The setup is best suited for directly comparing the effect of fuel type or fuel additives on fuel consumption and exhaust emissions during constant (steady-state) engine load. The BES includes the ability to switch between fuels without interrupting engine operation, enabling direct comparative results at constant load. The load is placed on the engine through a power controller, while an electric resistance heater consumes electricity produced by the generator. Exhaust gases are sampled though heated lines, with a heated filter removing particulates prior to analysis by Fourier transform infrared (FTIR) spectroscopy. Table 1 gives specifications for the generator and information on the diesel-powered compression ignition engine. The engine nameplate states that it conforms to 2005 MY California and U.S. EPA regulations for off-road compression ignition engines. 2.2. Test Fuels. A total of six fuels were used in this study, including four types of biodiesel and a GTL diesel fuel. These were directly compared to a baseline petroleum diesel fuel, called Texas low-emission diesel (TxLED). TxLED is an ULSD (47
n/a
76.0
cetane number (ASTM D613 or EN ISO 5165)
GTL
kinematic viscosity (ASTM D445 or EN ISO 3104)
cSt
2.585
4.067
4.483
5.494
n/a
3.425
density
kg/m3
822.4
868.8
861.6
858.8
868.4
769.2
volumetric energy density
MJ/m3
37650
34980
34620
34430
34470
36230
1
0.9290
0.9194
0.9143
0.9153
0.9621
relative energy density carbon
wt %
85.2
77.2
77.3
76.8
76.7
84.6
hydrogen nitrogen
wt % wt %
13.4
