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Properties and Performance of Levulinate Esters as Diesel Blend Components Earl Christensen,† Aaron Williams,† Stephen Paul,‡ Steve Burton,§ and Robert L. McCormick*,† †
National Renewable Energy Laboratory (NREL), Golden, Colorado 80401, United States Trenton Fuel Works, LLC, Princeton, New Jersey 08543, United States § MeadWestvaco Corporation, Raleigh, North Carolina 27606, United States ‡
bS Supporting Information ABSTRACT: The properties of ethyl (EL) and n-butyl levulinate (BL), two potential cellulose-derived diesel blend components, were assessed as both neat oxygenates and blends with diesel fuel. The samples tested were produced commercially from cellulose and alcohols but were not reagent-grade samples. They were relatively free of impurities, although EL contained some acidic compounds and both contained parts-per-million levels of calcium. Both esters exhibited a very low cetane number. The melting points of both esters were less than 60 °C. The water solubility of EL was 15.2 wt %, while that of BL was only 1.3 wt %. Blends of diesel fuel with EL were found to have an elevated cloud point, despite the extremely low melting point of this compound, because EL separates from diesel fuel as a separate liquid phase at low temperatures. This can be mitigated to some extent by including biodiesel in the blend. BL remained in solution and raised the diesel cloud point only when blended into 45 °C cloud point/15% aromatic no. 1 diesel fuel. Both esters were found to significantly increase diesel lubricity and conductivity. The esters were treated with the cetane-enhancing compound 2-ethyl hexyl nitrate and were tested as blends with diesel fuel in a 2008 model year Cummins ISB engine with the measurement of regulated pollutant emissions over the federal heavy duty diesel transient cycle. Fuel chemistry had no effect on tailpipe total hydrocarbons, carbon monoxide, or particulate matter for this diesel oxidation catalyst and particle filter equipped engine. The engine-out smoke number was reduced by 41.3% with a 10% blend of EL (EL10) and reduced by 55% with a blend of 20% BL (BL20). EL10 had no effect on emissions of nitrogen oxides (NOx), while BL20 increased NOx by 4.6%. Because of the poor solubility of EL in diesel fuel at low temperatures, its use as a diesel blend component will be technically challenging. The low cetane number of both esters can be addressed with cetane improver additives.
’ INTRODUCTION To reduce petroleum dependence and emissions of fossil carbon, there has been a long-term effort to understand the chemistry and process engineering of the conversion of lignocellulosic biomass into liquid transportation fuels. The various approaches to biomass conversion can be divided into two general types: thermochemical (gasification, pyrolysis, acid hydrolysis, combustion, and liquefaction) and biochemical (fermentation, enzymatic hydrolysis, and anaerobic and aerobic digestion).1 As shown in Figure 1, levulinic acid (4-oxopentanoic acid) is one product of the acid-catalyzed hydrolysis of cellulose, an abundant component of biomass. Levulinic acid is a γ keto acid, and the presence of multiple functional groups makes it a versatile intermediate chemical, especially because it can be made from many carbohydrates.2 It has been suggested as an alternative starting material for many compounds used in a number of largevolume chemical markets.3 The processing conditions of one proposed method of high-temperature acid hydrolysis, the Biofine process,4,5 favor high yields of levulinic acid and formic acid from hexose polymers, including cellulose (composed of D-glucose units) and six-carbon hemicellulose (composed of D-galactose, D-glucose, and D-mannose). Pentose polymers from five-carbon hemicellulose (composed of D-xylose and L-arabinose) will react to form furfural. The reaction conditions minimize the formation of undesirable material known as char. r 2011 American Chemical Society
Figure 1. Overall conversion of cellulose to levulinic acid and formic acid.
High yields of levulinic acid have also been shown by Mascal and Nikitin under less severe conditions but using dichloromethane as a solvent.6 A recent review describes many approaches for the conversion of lignocellulosic biomass to levulinic acid.7 The esters of levulinic acid have been proposed for use as gasoline and diesel additives.8 The blending octane number [(R + M)/2] for methyl levulinate is reported as 106.5, for ethyl levulinate (EL) as 107.5, and for iso- and sec-butyl levulinate as 102.5.8 Methyl levulinate is fully miscible with water and can separate from gasoline at cold temperatures.9 Because watersoluble fuels are more difficult to handle, interest in levulinate esters as fuels has focused on esters of ethanol and higher molecular-weight alcohols. With boiling points of over 200 °C,10 Received: August 12, 2011 Revised: September 26, 2011 Published: October 05, 2011 5422
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Energy & Fuels close to that of the heaviest components of gasoline, and a flash point of 91 °C11 for the ethyl ester, there has also been interest in the examination of these oxygenates as diesel-blending components. Grove and co-workers have reported data on the properties of ethyl, n-butyl, and other ester blends with diesel fuel.12 EL was found to separate from diesel fuel at temperatures below 0 °C; however, the n-butyl and higher esters remained in solution down to the diesel fuel cloud point. Grove and co-workers also note that incompatibility with elastomers, measured as percent volume swell and percent hardness change, is minimized for esters of n-butyl and higher molecular-weight alcohols. The very low cetane number of EL (less than 5) limits the amount that can be blended into diesel fuel.13 Lake and Burton report the use of biodiesel as a blend component with EL to both mitigate the low cetane number and enhance the solubility at low temperatures.14 The use of cetane improver additives to mitigate the low cetane number of the levulinate esters has also been described in the patent literature.15 Some information has recently been presented on levulinate ester effects on diesel engine emissions. Lake and Burton14 tested a blend of 80% ultralow sulfur diesel (ULSD) fuel, 13.33% biodiesel, and 6.47% EL in a 3.1 L turbocharged engine. Results for this fuel were compared to emissions for the ULSD and for 20 vol % biodiesel (B20). The EL blend showed greater reduction in particulate matter (PM) emissions than was observed for the B20 and also eliminated the small increase in nitrogen oxides (NOx) that is observed for B20 in many studies.16 Janssen and co-workers investigated high levels of n-butyl levulinate (BL) (60 80%) blended with n-tetradecane (with a cetane number of 95) in a fundamental single-cylinder engine combustion study.17 Virtually soot-free combustion was obtained with this high blend level; however, engine fuel system hoses and seals were not compatible with the high BL content fuel. The same research group has also examined EL as a 10% blend with diesel fuel and found that PM emission reductions as high as 50% could be obtained.18 Additional details on the properties of the ethyl and n-butyl esters of levulinic acid as both pure compounds and blends with several diesel fuels are presented below. Effects on regulated emissions were assessed for blends in a modern diesel engine.
’ EXPERIMENTAL SECTION Fuels and Blend Components. EL was obtained commercially, while BL was synthesized from levulinic acid, and n-butanol was obtained from the Penta International Corporation. The reactants were esterified using hydrochloric acid as a catalyst, after which the resulting ester was washed, neutralized, and dried. The resulting BL product was distilled, removing most of the remaining water, butanol, and other residues. The base diesel fuel used in blend tests of properties and engine performance was a certification ULSD from Halterman Products. Commercially obtained no. 1 and no. 2 diesel fuels were also used in some blending experiments. A soy-based biodiesel was also used for blending in some experiments. For engine tests and some performance tests, the levulinate esters were additized with the cetane enhancer 2-ethylhexyl nitrate (2-EHN). The additive was blended with the esters at a ratio of 30 mL/gallon (0.79%, v/v) prior to blending with diesel fuel. Chemical Analysis and Performance Tests. Ester purity was determined by gas chromatography mass spectrometry (GC MS) using a method provided in the Supporting Information. Dissolved water content was determined by Karl Fischer titration using a Metrohm 831 KF coulometer and following American Society for Testing and
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Materials (ASTM) method D6304. The water solubility of the esters was assessed by saturating the samples with water at room temperature and measuring the water content of the ester by Karl Fischer titration. The amount of ester absorbed by the aqueous layer remaining after water saturation was determined by GC flame ionization detector (FID) using the same column as used for the GC MS analysis (see the Supporting Information). Potential for the esters to be extracted out of diesel fuel into water was assessed by exposing blends of 10% EL and 20% BL in certification ULSD to 10% volume of water and measuring the ester in the water phase by GC. The amount of water absorbed by the diesel phase was measured by Karl Fischer titration. Low-temperature solubility and phase separation experiments were conducted by blending the neat esters with certification ULSD or commercial no. 1 and no. 2 diesel fuels at varying concentrations and temperatures. Observations of miscibility were made by visual inspection of blends. Initial miscibility observations were made at room temperature. Blends that were soluble at room temperature were placed in graduated, conical centrifuge tubes and exposed to temperatures ranging from 10 to 10 °C in 5 °C increments by placing the blends in a TestEquity 1007C temperature chamber for 30 min at each temperature. The impact on diesel stability was assessed by analyzing blends of EL and BL with certification ULSD by ASTM method D2274. Additionally, neat esters and blends of EL and BL with certification diesel were tested by Rancimat, test method EN 15751. Other performance properties were measured using ASTM test methods. Emission Testing. Regulated emission measurements were performed using procedures consistent with the Code of Federal Regulations Title 40, Section 86, Subpart N. The test setup consisted of a 2008 model year 6.7 L 330 hp Cummins ISB. The properties of the test engine are shown in the Supporting Information. The engine employs cooled high-pressure exhaust gas recirculation, a variable geometry turbocharger, electronic control, and high-pressure common rail direct fuel injection. The engine, designed and calibrated to meet the 2007 U.S. heavy-duty emission standards, also uses an actively regenerated diesel particulate filter (DPF). Testing was conducted over the heavy duty diesel transient (HDDT) cycle in accordance with the federal test procedure. In addition, an eightmode steady-state test was conducted to measure engine-out smoke number emissions using an AVL smoke meter. Following the HDDT cycle, two hot-start repeats of this test were conducted for the EL10 and BL20 test fuels. Four hot-start repeats were conducted with the certification ULSD. Because this engine is equipped with a DPF, which results in nearly undetectable tailpipe PM emission levels, all measurements are made at the inlet to the DPF (engine-out). The smoke number is not an approximation of grams per brake horsepower per hour (g bhp 1 h 1) PM emissions but provides a value that is proportional to PM emissions and allows us to quantify reduction in engine-out smoke for oxygenated fuels. These reductions may have implications on the overall performance of the DPF. Details of emission testing procedures are provided in the Supporting Information.
’ RESULTS AND DISCUSSION Properties and Analysis of Neat Esters. Properties of EL and BL are shown in Table 1. EL and BL have a very low melting point, below 60 °C. The exact melting point could not be determined in a cloud point/melting point apparatus. Experiments were performed holding the neat esters in an environmental chamber for several hours at various temperatures. Esters were solid at 70 °C but liquid at 60 °C. Both esters boil in the diesel range and have flashpoints that exceed the minimum no. 2 diesel fuel requirement. The derived cetane number (DCN) of both neat esters was very low; however, the DCN of BL was measurably higher at 14. Note that the D6890 method precision 5423
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Table 1. Properties of the Levulinate Ester Samples Tested method
description
units
EL
BL C9H16O3
chemical formula
C7H12O3
CAS number
539-88-8
2052-15-5
D6890
derived cetane number
