This study examines the composition and combustion performance of biodiesel produced from waste cooking oil. Six fuel batches produced from waste oil used in dining-hall fryers were examined to determine their physical and chemical properties, including their elemental and fatty acid methyl ester composition. Oleic and linoleic methyl esters accounted for more than 70% of the fuel composition, while the oxygen content averaged 10.2% by weight. Exhaust emissions were monitored for 5–100% biodiesel blends using two off-road engines: a 2007 Yanmar diesel generator and a 1993 John Deere front mower. Increasing biodiesel content resulted in reduced emissions of partial combustion products from the diesel generator but a rise in NO
Rising fuel costs and energy demands, combined with growing concern over greenhouse gas emissions, have led to increased interest in the use of renewable fuels to help meet increasing worldwide fuel demand and reduce atmospheric CO2 emissions from transportation sources [
Biodiesel can be produced from a range of vegetable oils and animal fats. The use of soybean oil and other high-quality food-grade vegetable oils presents economic difficulties because of competition with use for food products. One more economically feasible source for biodiesel is waste cooking or frying oils, also known as yellow grease. As a waste product, used cooking oil is a potentially cheaper feedstock than edible vegetable oils [
The composition of WCO biodiesel varies somewhat depending on the oil feedstock and the fuel production process. Most vegetable oil feedstocks consist primarily of C16 and C18 fatty acids, but can also include smaller contributions from other C8 to C22 compounds [
Several comprehensive reviews of the existing literature have concluded that biodiesel use will generally reduce exhaust emissions of particulate matter, total hydrocarbons, and carbon monoxide [
The effects of WCO biodiesel on emissions of nitrogen oxides (
In the mid 1990s, U. S. EPA began implementing emissions standards for off-road diesel engines that established maximum permitted levels for CO,
One particular area of concern for off-road equipment is emissions produced during idling. Since idling engine speed is relatively low, and only a small amount of fuel is added in order to maintain engine crankshaft revolution, combustion efficiency drops significantly. This results in higher hydrocarbon emissions and partial combustion products that are hazardous to the environment and the health of the user [
The current study examines biodiesel produced from waste cooking oil feedstocks at the University of Kansas using a small-scale batch reaction process. Multiple batches of WCO biodiesel were analyzed to determine batch-to-batch variation in physical and chemical properties. One batch of the WCO biodiesel was then blended with number 2 diesel to create fuels with 5–100% biodiesel by volume. These fuels were used to power two nonroad engines under idling conditions. Exhaust emissions were analyzed to determine concentrations of CO2, CO,
Biodiesel used in this study was produced by the University of Kansas Biodiesel Initiative, utilizing used cooking oil obtained from the University of Kansas dining halls and local restaurants. This used oil was processed to biodiesel in an on-campus pilot scale facility by conversion of the triglyceride fats to fatty acid methyl esters via base-catalyzed transesterification. Each batch of fuel produced was tested for compliance with ASTM standards for density, viscosity, and flash point. Samples from six biodiesel batches produced in 2009 and 2010 were analyzed to determine their hydrocarbon composition. For the emissions experiments, one batch of WCO biodiesel was blended with number 2 petroleum diesel fuel purchased commercially by the KU Maintenance and Operations Department. The fuels were mixed by hand to create five gallons each of 5%, 25%, 50%, and 75% by volume biodiesel blends (identified in this paper as B5, B20, B50 and B75 fuels, resp.). The blended fuels were used in the engine tests within ten days after mixing.
An Agilent 6890 gas chromatograph coupled with an Agilent 5973N mass spectrometer (GC-MS) was used for fatty acid methyl ester (FAME) analysis of the WCO biodiesel fuels. The chromatographic column was an HP-INNOWax Polyethylene Glycol column of 15 m length
Fuels were prepared for analysis by adding 0.1 mL biodiesel samples to 100 mL of n-hexane. One microliter of this mixture was injected into the GC-MS, which was programmed at 120°C for 1 min, then ramped at 6°C/min to 180°C, 1.5°C/min to 198°C, 5°C/min to 228°C, and then held at 228°C for five minutes. The total run time was 34 minutes. The injection port and transfer line were held at 250°C. Samples were injected in splitless mode with helium as the carrier gas with a flow through the column of 1.4 mL/min. The mass spectrometer used electron ionization at 70 eV, with an ion source temperature of 230°C and quadrupole temperature of 150°C. The electron multiplier was operated at 1482 V, and the solvent delay was 2.5 min.
Individual fatty acid methyl esters present in each biodiesel sample were identified based on GC retention time compared to the known FAME standard compounds and mass spectrum analysis. The response factors of each compound relative to the internal standard were then used to quantify the mass present. The mass detection limit was
Two engines were used in these experiments: a Yanmar Model 3TN75RJ installed in a 1993 F1145 John Deere Front Mower, and a Yanmar L100V6 installed in a 2007 YDG 5500EV diesel generator. Specifications for both engines are listed in Table
Test engine properties.
Front Mower | Generator | |
---|---|---|
Manufacturer | Yanmar | Yanmar |
Model | 3TN75RJ | L100V6-GY |
Year | 1993 | 2007 |
Number of cylinders. | 3 | 1 |
Displacement, L | 0.994 | 0.435 |
Bore, mm | 75 | 86 |
Stroke, mm | 75 | 75 |
Rated power, hp | 24 | 9.1 |
Compression ratio | 17.8 : 1 | 21.2 : 1 |
The Yanmar generator was located in a test cell under controlled atmospheric conditions, with an ambient temperature between 23.0 and 24.5°C during all tests. Five different biodiesel blends (Petroleum Diesel, B5, B20, B50, and B100) were tested in a rotating order each day for six days, ensuring that each fuel was in each position once. During the final two experiments, a sixth blend, B75, was also tested. Between each test, the gas tank was completely drained and rinsed several times with the fuel for the next test. During each test, the engine was fully warmed up for 30 minutes prior to the beginning of data collection. Emissions were then monitored for 30 minutes. All experiments were performed with the generator at zero added loading.
Biodiesel combustion experiments with the mower were performed outdoors at the University of Kansas. Ambient temperatures during the test period varied from 13.5°C to 28.0°C. The variation within a given day was between 3 and 4°C, while the largest mean temperature difference between the six test days was 10°C. Four different biodiesel blends, were used in these tests: B5, B20, B50, and B100. Since the mower had been operating on a B5 mixture for more than a year before the beginning of the experiment, no tests were performed with 100% petroleum diesel. Daily testing followed the same procedure as described above for the generator tests, except that not all fuels were tested on each day. The engine was set to idle throughout each experiment.
The exhaust pipe from each engine was attached to a high-speed exhaust flow meter (EFM-HS; Sensors, Inc.) using one foot (0.30 m) of metal tubing attached to seven feet (2.1 m) of silicon hose connectors. Heated exhaust was pulled from the flow meter at a rate of eight L/min and transferred by heated line to the emission analyzer. The heated line maintained the exhaust at 192°C to prevent water and total hydrocarbon condensation. Ambient temperature, absolute humidity, and exhaust gas temperature were continuously monitored and recorded via an external weather probe throughout each experiment.
Analysis of gas-phase exhaust constituents was carried out using a SEMTECH-DS portable emissions analyzer (Sensors, Inc.). This instrument measures CO2 and CO concentrations by using nondispersive infrared spectroscopy (NDIR), NO and NO2 by nondispersive ultraviolet spectroscopy (NDUV), and THC using a heated flame ionization detector (FID). An auxiliary electrochemical sensor provided simultaneous O2 measurements. The analyzers were calibrated at the beginning and end of each day using ambient air as the reference zero condition.
Raw concentrations were converted to fuel-specific mass emissions using the SEMTECH analytical software. This is accomplished by composing an overall carbon balance in the exhaust to estimate the total amount of fuel consumed. The calculation for the fuel-specific emissions is shown below for NO [
An analysis of variance (ANOVA) test was performed to estimate the effects of biodiesel content and other factors on the fuel-specific emissions. The investigators used a general linear model, with biodiesel percentage and the test order of the different blends as the fixed effects. Ambient temperature, exhaust temperature (measured at the exhaust flow meter and used as a surrogate for engine temperature during combustion), and absolute humidity were chosen as covariates. A Pearson correlation analysis was performed on the influence factors that were reported to be statistically significant for two or more pollutants for each engine to test for the linearity and direction of the correlation. All statistical analyses were performed using MiniTab and all variables were tested at the 95% confidence interval.
The waste cooking oil biodiesel consisted primarily of C16 to C20 esters, with oleic and linoleic acids accounting for at least 75% of the total mass of each sample (Table
Composition (as % weight) for six waste cooking oil biodiesel samples.
Sample | Palmitic |
Palmitoleic |
Stearic |
Oleic (C18:1) | Linoleic |
Linolenic |
Arachidic |
Behenic |
Others |
---|---|---|---|---|---|---|---|---|---|
WCO1 | 12.9 | n | 2.4 | 54.3 | 21.4 | 5.3 | 0.7 | n | 3.1 |
WCO2 | 14.4 | n | 4.8 | 51.6 | 21.6 | 3.5 | n | n | 4.1 |
WCO3 | 7.1 | 0.4 | 3.0 | 45.2 | 25.9 | 11.5 | 0.8 | 0.4 | 5.7 |
WCO4 | 7.0 | 0.4 | 3.3 | 45.9 | 26.5 | 12.4 | 0.9 | 0.6 | 2.6 |
WCO5 | 7.2 | 0.4 | 3.3 | 46.4 | 26.2 | 12.2 | 0.9 | 0.6 | 2.8 |
WCO6 | 7.4 | 0.3 | 3.0 | 46.3 | 23.4 | 12.7 | 0.8 | 0.5 | 5.7 |
n: not present at detectable levels.
For the 2010 fuel samples, the bulk fuel composition results obtained from GC-MS analysis were verified by estimating the calculated H : C ratio and fuel oxygen content based on the FAME composition of the fuel and comparing these predicted values to actual values determined by direct elemental analysis (Table
Measured and estimated H : C ratios and oxygen contents.
Sample | H : C Ratio | Oxygen content (% weight) | ||
---|---|---|---|---|
Measureda | Predicted | Measureda | Predicted | |
WCO 3 | 1.82 | 1.85 | 10.5 | 10.3 |
WCO 4 | 1.86 | 1.85 | 10.5 | 10.6 |
WCO 5 | 1.86 | 1.85 | 10.5 | 10.6 |
WCO 6 | 1.86 | 1.85 | 10.5 | 10.3 |
aUncertainties in the measured H : C ratio and oxygen content are
A single batch of the waste cooking oil biodiesel (WCO1) was used in all of the emissions experiments, in order to minimize any effects due to fuel quality variations. Table
Fuel properties of biodiesel blends used in the emissions study.
Fuel type | H : C | Oxygen (% wt) | Density (g/L) | Flash point (°C) | Viscosity |
---|---|---|---|---|---|
B100 | 1.87 | 10.5 | 881 | 158 | 4.99 cST/s |
B75 | 1.85 | 7.9 | 870 | 98 | 4.17 cST/s |
B50 | 1.84 | 5.3 | 859 | 75 | 3.47 cST/s |
B20 | 1.81 | 2.1 | 847 | 65 | 2.81 cST/s |
B5 | 1.80 | 0.53 | 840 | 62 | 2.61 cST/s |
# 2 diesel | 1.80 | 0 | 825 | 23 | 2.17 cST/s |
Figure
Fuel-specific CO2 emissions from (a) generator and (b) front mower for biodiesel blends.
Figure
Gas-phase pollutant emissions from the generator engine (error bars indicate one standard deviation).
The fuel-specific emissions reported here do not account for any changes in fuel consumption rates related to the lower energy content of the biodiesel blends. This is due to the difficulty in accurately measuring output power at “no load” conditions. However, the effect of these changes can be estimated. For a 10% decrease in energy content between the petroleum diesel and the B100 fuel, we estimate a 10% increase in fuel consumption to achieve the same power output. This would also increase total emissions of all constituents by 10% on a brake-specific basis (g fuel/power × time; e.g., g/kWh). As the changes in fuel-specific emissions for CO, THC, and NO are substantially larger than 10%, these constituents will be reduced (or increased, for NO) even after accounting for increased fuel consumption.
The higher bulk modulus of compressibility of biodiesel fuels can result in advanced fuel injection timing by as much as one to two crank-angle degrees [
Results from the ANOVA analysis of the generator emissions data showed that the fuel biodiesel content was significantly correlated to changes in emissions of all compounds except for NO2. Nitrogen dioxide formation and destruction reactions will both be influenced by biodiesel-induced changes in combustion temperature and timing. Overall, we observed no consistent net effect due to the presence of biodiesel in the fuel. Variations in ambient temperature and humidity in the test cell were small during these tests (temperatures from 22–24°C and humidity from 55–60 grains/lb dry air) and did not impact the exhaust composition, while the exhaust temperature was significantly correlated with CO2 concentrations only. The measured exhaust temperature did decrease with higher biodiesel content, from a high of 106°C with number 2 diesel to a low of 98°C with the B100 fuel, a change directly related to the advanced injection timing. While biodiesel may burn hotter during the combustion process, this combustion occurs earlier in the expansion stroke of the engine. Since combustion is completed closer to top dead center, more work can be done by the engine (longer effective expansion stroke) before the exhaust valve opens and the cylinder walls see a larger temperature gradient for a longer time, promoting convective heat transfer. As a result, the exhaust temperature will decrease even though the peak combustion temperature is higher.
The effect of biodiesel blending on exhaust emissions of CO, THC, and
Gas-phase pollutant emissions from the mower engine (error bars indicate one standard deviation).
The ANOVA results for the mower study indicate that changes in fuel biodiesel content are significantly correlated with changes in emissions of all measured compounds, including NO2 (although the absolute changes in NO2 emissions were very small). Exhaust temperature was correlated with CO2, NO, and THC, but not with CO and NO2. The overall range of exhaust temperatures was smaller in the mower studies, with a maximum of 84°C during the B5 tests and a minimum of 81°C with B100. Ambient conditions also varied more during the mower tests, which were conducted outdoors. Temperatures during these tests ranged from 13–27°C and humidity from 23–73 grains/lb dry air. Even so, ANOVA regression indicated no significant effect for ambient temperature on emission results. Humidity, however, was correlated with changes in CO2, CO, and NO2 emissions. Changes in humidity can affect the fuel viscosity, which can in turn affect fuel injection and combustion patterns, resulting in fluctuations in CO and CO2 emissions. Increased humidity in the intake air has also been shown to decrease total
Fuel-specific emissions for all pollutant compounds except for CO were much higher from the mower in comparison to the generator at all biodiesel contents (Figures
Pearson correlation coefficients for significant influence factors. Bold values show significant linear correlation at 95% confidence.
CO2 | CO | NO | NO2 | THC | |
---|---|---|---|---|---|
Generator emissions | |||||
| |||||
Biodiesel content |
|
|
|
−0.369 |
|
| |||||
Mower emissions | |||||
| |||||
Biodiesel content |
|
0.126 |
|
|
|
Exhaust temperature | 0.006 |
|
−0.373 |
|
|
Absolute humidity | −0.152 |
|
−0.496 |
|
|
The generator and front mower used in this experiment represent two different engine designs with respect to their fuel injection control. The US EPA adopted off-road diesel engine emissions regulations for engines under 50 hp in 1996, after the front mower was put into service. The mower engine would therefore have had a fuel injection system optimized for performance at the expense of emission control. The generator, by contrast, was built in 2007 and therefore subject to Tier 2 emissions standards for engines under 11 hp [
This difference in injection timing is the most likely source for the different responses of the two engines to increased biodiesel content. Retarding the combustion process will produce lower overall
Increasing the fuel biodiesel content should result in an accelerated fuel injection sequence in both the generator and mower engines, as discussed previously. The change in injection timing, however, may have a much less significant effect on the combustion profile for the older mower engine. When fuel injection timing is optimized for performance, heat release is maximized closer to top dead center, when the piston is moving relatively slowly. Hence, changing by a few crank-angle degrees will have a smaller effect on conditions within the cylinder. Under these conditions, the lower energy content of biodiesel may play a more important role in nitrogen oxide formation by reducing the in-cylinder temperature, resulting in the decreased B100
While the physical properties of the six WCO biodiesel batches produced by the KU Biodiesel Initiative showed little variation, the FAME content of the four samples collected in 2010 included less saturated compounds while consisting of longer chain compounds than the 2009 samples. Elemental analysis of the fuel H : C ratio and oxygen content were generally in line with the FAME analysis, indicating that the GC-MS technique provided a good characterization of the overall fuel, despite the presence of some unquantifiable fragments. The small differences in fuel batch-to-batch variability suggest that changes in the WCO feedstock makeup will have relatively little effect on the resulting fuel.
Increased WCO biodiesel content in the fuel lowered emissions of total hydrocarbons in both engines under idle conditions, with greater reductions at lower biodiesel content in the generator engine. These decreases are consistent with similar results from other studies of waste cooking oil biodiesel and most likely result from compositional differences between biodiesel and petroleum diesel, particularly the higher oxygen level and lower aromatic content. The relationship between biodiesel content and CO and
These results indicate that the effects of biodiesel use in nonroad engines on emission profiles may depend greatly on the fuel injection strategy used, which in turn will be related to the age of the engine. In the United States, off-road vehicles and equipment less than 10–15 years old (depending on the specific engine class) are subject to the U.S. EPA’s tiered emissions standards for off-road engines. As a result, these engines will generally employ delayed fuel injection timing to control
Our results suggest that the use of biodiesel blends in these older off-road and stationary engines may actually result in the reduction of both
The authors thank Pavan Ilipilla of the University of Kansas Environmental Engineering program for assistance in data collection. This research was funded by the University of Kansas, Transportation Research Institute from Grant no DT0S59-06-G-00047, provided by the U.S. Department of Transportation Research and Innovative Technology Administration.