Article pubs.acs.org/EF
Formulation and Combustion of Glycerol−Diesel Fuel Emulsions Scott J. Eaton,*,†,‡ George N. Harakas,†,§ Richard W. Kimball,†,§ Jennifer A. Smith,∥ Kira A. Pilot,§ Mitch T. Kuflik,§ and Jeremy M. Bullard§ †
SeaChange Group LLC, 8 Venture Avenue, Brunswick Landing, Brunswick, Maine 04011, United States Forest Bioproducts Research Institute, University of Maine, Orono, Maine 04469-5717, United States § Marine Engine Testing and Emissions Laboratory, Maine Maritime Academy, Castine, Maine 04420, United States ∥ Department of Chemical Engineering, Rochester Institute of Technology, 1 Lomb Memorial Drive, Rochester, New York 14623-5603, United States ‡
ABSTRACT: Diesel fuel emulsions have demonstrated reductions of unwanted combustion emissions. Glycerol, a renewable and abundant resource, is an attractive fuel component because it is a low-cost energy source. In this paper, the glycerol−diesel emulsion system is characterized and resultant fuel properties are presented. Surface analytical techniques and long-term stability evaluations are used to identify optimal surfactant composition. Emulsions are prepared using batch ultrasonic processing to produce a narrow droplet size distribution with a mean of approximately 3.3 μm. The glycerol water content tends to increase the droplet size distribution. Glycerol−diesel emulsions prepared at 10 and 20 vol % glycerol phase are combusted in a naturally aspirated single-cylinder diesel engine. Oxides of nitrogen and particulate matter emissions are reduced by 5−15 and 25−50%, respectively. Indicated fuel consumption is increased corresponding to an increased glycerol concentration because of a reduction in the emulsion energy density. Thermal efficiency improvements are observed at high loads.
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INTRODUCTION 1,2,3-Propanetriol (glycerol) is a plentiful biorenewable resource that has received significant attention for its potential as a cost-effective feedstock in chemical processing and fuel applications.1,2 It is primarily produced during tranesterification of plant lipids (triglycerides), notably as the byproduct of biodiesel production. In 2013, nearly 1.8 billion gallons of biodiesel were produced domestically, which equates to approximately 132 million gallons of glycerol.3 Transesterification results in a crude glycerol, containing many impurities, such as methanol, water, soap, ash, and other organic materials. The composition of the crude glycerol can vary markedly depending upon the feedstock used and the method of manufacture.4 In most large-scale biodiesel operations, the glycerol is purified and released to market as any number of purified products ranging from crude (50−85% purity) to United States Pharmacopeia (USP) (96−99.8% purity) grades for consumption as rosin additives or for food and pharmaceutical applications. Crude glycerol is mostly commonly traded at or near 80% purity level and, at the time of this publication, could be obtained within the price range of $0.08− 0.15/lb. Costs associated with downstream purification are considerable and depend upon starting crude quality.5,6 As a result, current USP-grade glycerol costs are approximately $0.69/lb. Therefore, purification of glycerol remains an active field of industrial research, and producers continue to seek high-volume markets to unload their products. Glycerol as a Burner Fuel. In comparison to traditional petroleum-derived fuels, glycerol offers many material property limitations, as seen Table 1. It is a three-carbon triol that is water-soluble with high fuel-bound oxygen (52 wt %). This oxygen lowers its overall high heating value (∼19 MJ/kg) and results in a high kinematic viscosity (∼741 cP at 25 °C). © XXXX American Chemical Society
Table 1. Comparison of Relevant Fuel Properties for Glycerol and #2 Distillates
a
property
glycerol
#2 distillates
flash point (°C) kinematic viscosity at 25 °C (cP) higher heating value (MJ/kg) specific gravity at 25 °C (g/mL) adiabatic flame temperature (K) surface tension at 25 °C (mN/m) estimated energy cost ($/GJ)
160 741 19 1.26 2210 62.5 9.28−17.4a
60 4 44.3 0.85 2380 28.2 23.5−26.9b
Crude glycerol at $0.08−0.15/lb. bDistillates at $0.88−1.00/L.
As a result, use of glycerol as a stand-alone fuel requires equipment modifications to achieve adequate combustion. Bohon et al. demonstrated that USP-grade glycerol can be efficiently combusted in a refractory-lined burner with aggressive air recirculation.7 Air-assisted atomization and fuel preheating were used to obtain proper fuel spray characteristics. Oxides of nitrogen (NOx) emissions were reduced by over 90% compared to #2 distillates when operated at congruent energy rates of 7.3 kW. It was speculated that lower flame temperatures in glycerol combustion quenched prompt NOx formation rates. This result was not repeated in subsequent investigations by Steinmetz et al. using an 82 kW burner of similar design.8 Their work examined the impacts of fuel ash on resulting combustion and emission behaviors. NO x was found to increase approximately 50% (v/v) compared to #2 distillates. Methanol contamination in the crude further increased NOx. Particulate Received: March 26, 2014 Revised: May 11, 2014
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dx.doi.org/10.1021/ef500670d | Energy Fuels XXXX, XXX, XXX−XXX
Energy & Fuels
Article
70/220 test procedures.10 The emission reductions are attributed to water−fuel droplet interactions in the cylinder. A detailed examination by Mura et al. showed that emulsified water induces microexplosions of the fuel droplets, which reduce mean fuel droplet diameters.11 The droplet size reduction offers many benefits. First, droplet size reduction increases overall droplet surface area, improving combustion efficiency. Second, the reduced size droplets limit pyrolysis of fuel prior to combustion, which is a dominant pathway to soot formation.17 Lastly, water offers a high heat of evaporation (∼2250 kJ/kg), which reduces overall flame temperatures and overall cylinder temperatures, reducing both prompt and thermal NOx emissions. These interactions were confirmed by Ghojel et al. in comparing combustion dynamics of water-indiesel fuel to #2 distillates on a four-cylinder, direct-injection diesel with crank-angle-resolved heat-release monitoring.12 Emulsification of glycerol has been investigated for it applicability to burner systems. Mize et al. conducted emulsification experiments at over 50 vol % level in #2 distillates using a variety of crude glycerols and found that both water and salt contents play an important role in the stabilization of the glycerol−diesel system.13 Optimal surfactant hydrophilic−lypophilic balance (HLB) rating for glycerol was found to be higher (5−10 HLB rating) than required for water−diesel emulsions. Impurities, such as salt and soap, in the glycerol were found to reduce overall emulsion stability. Bombos et al. concluded that glycerol emulsified at 60−65 vol % in #2 distillate yielded a flash point of approximately 149 °C, well above the #2 distillate ASTM specification, and increased fuel dynamic viscosity. Combustion of glycerol−diesel fuel emulsions has had limited treatment in the literature. It is anticipated that glycerol−diesel emulsions will not require significant fuel heating prior to atomization because glycerol−diesel emulsions offer a 10-fold decrease in kinematic viscosity. Further, glycerol combustion can be piloted by the ignition of the diesel components, thus limiting the need for intake manifold heating. These interactions were demonstrated by Striu̅gas et al. when examining the combustion properties of glycerol−heavy fuel oil emulsions in the range of 27−34 vol % in a burner system. It was found that glycerol emulsions reduced the NOx and SO2 emissions compared to the heavy fuel oil.16 These results are consistent with speculated glycerol decomposition and combustion dynamics during droplet burning. Investigations on the pyrolysis of glycerol have shown that dehydration and unimolecular decomposition reactions can occur at temperatures between 525 and 725 °C, well below the sooting temperatures of hydrocarbon fuels, which can help droplet breakup by producing volatile species and promoting side reactions.18−20 In the present study, the glycerol−diesel emulsion system is further examined for optimal surfactant systems and their impacts in long-term stability. Emulsions using a sonic processor are created at the lab scale and monitored for shelf stability. Glycerol droplet size distributions are characterized, and the effect of the glycerol water content is examined. Finally, glycerol−diesel emulsions containing 10 and 20 vol % glycerol are combusted in a single-cylinder diesel engine to determine combustion and emission behavior. This work is thought to be the first report on the combustion characteristics on glycerol− diesel emulsion fuels and is considered an advance in alternative fuels because they offer the potential to reduce fuel costs while offering environmental benefits.
emissions associated with caustic fuel ash were elevated and noted as a concern for continuous operations, leading toward poor burner durability and health concerns. Volatile organic emissions were consistent with #2 distillate fuels, including acrolein, a known toxic decomposition product of glycerol, which was found at 20 ppb, well below health concern limits. This work identified a need to purify crude glycerol to meet optimal fuel ash and methanol limits. Glycerol as a Diesel Fuel. McNeil et al. extended the use of glycerol to internal combustion engines by examining its combustion and emission behaviors in both naturally aspirated and turbo-charged engines with intake manifold heating.9 USPgrade glycerol was used and found to offer efficient combustion in each engine configuration. Natural aspiration required intake air heating to 144 °C to sustain combustion compared to turbocharged operation at 100 °C because glycerol has poor ignitability (cetane = 0) and volatility rates. In all tests, acrolein emissions are comparable to that of #2 distillates with an associated 35% NOx reduction. Use of glycerol as a stand-alone fuel, however, has many material-handling considerations. Lower energy density requires nearly double fueling rates to achieve equivalent power output compared to diesel fuel. Viscosity and surface tension requires increased fuel rail pressure and/or heating to achieve comparable fuel atomization to #2 distillates. Lastly, storage stability concerns because of microbial activity and adsorption of moisture may require specialized conditioning. Glycerol Emulsification. Emulsification of glycerol is one method of reducing problems associated with stand-alone glycerol fuel use. Emulsification is a widely used technique for incorporating aqueous-phase materials into a petroleum oil.10−16 This process uses intense liquid shearing forces to create a fine dispersion of droplets, typically on the micro- or nanoscale, dispersed within the bulk petroleum phase. Surfactant molecule(s) stabilize the oil−water interface and limit droplet coalescence. This process is not thermodynamically stable. However, emulsions have been prepared that are shelf-stable for many years for well-suited surfactant systems. Figure 1 provides a pictorial representation of possible water-inoil emulsion system dynamics over time. Water−diesel emulsions have shown NOx and particulate matter (PM) emission reductions compared to straight diesel. Armas et al. demonstrated that 10 vol % water in diesel fuel provided engine thermal efficiency improvement in a turbocharged indirect injection diesel engine operating on European
Figure 1. Pictorial representation of pathways observed in diesel emulsion aging. B
dx.doi.org/10.1021/ef500670d | Energy Fuels XXXX, XXX, XXX−XXX
Energy & Fuels
Article
Table 2. Gibbs Adsorption Isotherm Results for the Non-ionic Surfactant Screening System: Glycerol−Diesel surfactant
HLB
MW (g/mol)
CMC (mM/dm3)
γCMC (mN/m)
Γmax (×10−10, M/m2)
Amin (nm2/molecule)
ΔG (kJ/mol)
Span 85 Span 80 Tween 20 Tween 80 technical glycerides Merpol SE Merpol A Span 20
1.5 4.5 16.7 15 3.4 11 6 8
1005 429 1228 1310 357 660 421 346
8.66 2.03 0.71 3.32 12.2 6.5 2.1 12.6
4.88 2.35 1.01 0.32 4.01 8.06 1.04 1.51
1.08 1.26 0.71 0.09 1.15 1.10 0.15 0.20
154 132 232 1908 144 154 1124 821
−11.7 −15.3 −17.8 −14.0 −10.8 −11.3 −15.2 −10.7
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correspondingly high surface coverage, Γmax. Surfactant Tween 80 offered the greatest interfacial surface tension reduction. These data suggest that a surfactant system consisting of a mixture of Tween 20, Tween 80, and Span 80 may yield optimal stability for the glycerol− diesel system. Glycerol−Diesel Emulsification. Glycerol−diesel emulsions were created in 200 mL batches in a 300 mL wide mouth beaker using a Misonix S-4000 ultrasonic processor with a 1 in. horn. The processor operates at 20 kHz and is capable of continuously outputting up to 100 W of energy. A total of 70 mL of glycerol was first added to the beaker. The surfactants Tween 80 and Span 80 where premixed to obtain a HLB rating of 8. A total of 4.5 mL of the surfactant mixture was then dissolved in 125 mL of ULSD and layered on top of the glycerol phase. The entire mixture was processed under agitated conditions using a stirring plate during emulsification with the sonic processor operating with a total energy input of nominally 1600 J at a rate of 45 W. The resulting glycerol droplet size distributions were determined using a Horiba LA-300 particle size analyzer. The analyzer uses a dynamic laser light scattering flow cell and continuous stirring to minimize droplet flocculation. Using this technique, a mean droplet size of 3.35 μm was achieved with a 10−90% droplet size range between 1.98 and 5.12 μm, respectively. Emulsification experiments were repeated, as described above, at various concentrations of water in the glycerol phase to determine its effect on the resulting droplet size. In these experiments, the glycerol phase was maintained at 35 vol % of the overall mixture, with water comprising 0−40 vol % of the glycerol phase. The water concentration was found to broaden the droplet size distribution, as seen in Figure 3. The mean droplet size diameter increased from 3.35 to 8.95 μm for the 0 and 40 vol % water samples, respectively. The 10−90% droplet size range for 40 vol % expanded to 3.91−15.17 μm. This is attributed to surface tension and surfactant HLB mismatching, which has further implications when considering emulsions from crude glycerol sources, which may have contaminants (water, methanol, etc.) and may contain
EXPERIMENTAL SECTION
Surfactant Screening. A total of eight surfactants were characterized for their utility in the glycerol/diesel system, as listed in Table 2. Experiments were conducted according to the Gibbs interfacial adsorption isotherm technique.21 A Sigma 70 surface tensiometer with a Du Noüy ring microbalance was used to obtain interfacial surface tension values over a range of surfactant concentrations. In each test, 40 mL of reagent glycerol obtained from Sigma-Aldrich Co., containing
