Article pubs.acs.org/EF
Combustion of Lignocellulosic Biomass Based Oxygenated Components in a Compression Ignition Engine Marc E. Baumgardner,† Timothy L. Vaughn,‡ Arunachalam Lakshminarayanan,‡ Daniel Olsen,‡ Matthew A. Ratcliff,§ Robert L. McCormick,§ and Anthony J. Marchese*,‡ †
Department of Mechanical Engineering, Gonzaga University, Spokane, Washington 99258, United States Department of Mechanical Engineering, Colorado State University, Fort Collins, Colorado 80523, United States § National Renewable Energy Laboratory, Golden, Colorado 80401, United States ‡
S Supporting Information *
ABSTRACT: Processes such as fast pyrolysis of whole biomass or base-catalyzed depolymerization of lignin produce complex mixtures of oxygenated compounds that must be upgraded to be suitable for blending with petroleum and processing in a refinery. Complete removal of these oxygenated compounds is exceedingly energy intensive, and it is likely that upgraded pyrolysis oils will contain up to 2% oxygen content to be economically viable. The purpose of this study was to evaluate the effect of the presence of oxygenated chemical components representative of those present in upgraded pyrolysis oil on diesel engine performance and emissions. Engine testing was performed by blending seven different oxygenated components and one multicomponent blend with certification ultralow sulfur diesel fuel and quantifying the performance and emissions from the combustion of these fuels in a four-cylinder, turbocharged, 4.5 L John Deere PowerTech Plus common rail, direct injection diesel engine that meets Tier 3 off-highway emissions specifications. The properties of the oxygenated fuel components were fully characterized in accordance with ASTM diesel fuel standards. Gaseous emissions measurements included CO, CO2, NO, NO2, and total hydrocarbons; particulate measurements were performed on a PM10 basis. The residual oxygenate blends exhibited very few statistically significant differences compared to diesel at lower blend levels (2 vol %) but negative effects were observed at higher blend levels (5−6 vol %). drop-in replacements for existing petroleum-based fuels.6,7 Accordingly, biofuels derived via these processes must go through upgrading processes to remove these oxygenated compounds and produce a fuel that contains only hydrocarbons.8 Upgrading processes are expensive, and their costs increase as the levels of oxygenated compounds in the final fuel product decrease.9 As a result, these upgrading processes will result in exceedingly high prices for renewable hydrocarbon fuel compared to petroleum-derived fuel. To reduce upgrading costs and produce drop-in biofuels at a market-competitive price, it is therefore economically desirable to leave a small fraction of oxygenated compounds in the final upgraded fuels. However, for this approach to be technically viable, it must be shown that the presence of these oxygenated compounds in the final fuel blend does not adversely affect the operation of existing engines. To this end, the U.S. Department of Energy (DOE) has sponsored a study at the National Renewable Energy Laboratory (NREL) in conjunction with Colorado State University to determine the effects of small percentages of oxygenated compounds present in upgraded advanced biofuels on fuel properties,8 performance, emissions, and durability of diesel and spark ignited engines. This report presents the results of the work on emissions from the
1. INTRODUCTION The renewable fuel standard (RFS) mandates that 36 billion gallons/year of renewable fuels be blended annually with traditional petroleum fuels in the United States by the year 2022. Of this annual total, 21 billion gallons/year must qualify as “advanced” or “cellulosic” biofuels, of which corn ethanol would not qualify.1 Because of the low productivity of many of today’s crop-based biofuels, as well as their potential competition with the global food supply, the motivation exists to develop advanced biofuels from the vast quantity of biomass that can be produced sustainably in the U.S. Indeed, recent studies suggest that by the year 2030 the U.S. will be capable of annually producing 1.3 billion tons of sustainable forest and agricultural waste as well as perennial crops that can be processed into biofuel.2 Such a quantity of biomass can potentially yield up to 45 billion gallons/year of liquid transportation fuels, which is far greater than that available from conversion of vegetable oil or animal fat or via fermentation of starch-derived sugars. Although vast quantities of sustainable biomass exist for conversion into advanced biofuels, a major imperative for the transportation sector is that these advanced biofuels can be classified as “drop-in” fuels; i.e., they are directly compatible with existing refinery and distribution infrastructure as well as existing engine technology. However, biofuels derived via basecatalyzed depolymerization of lignin3 or fast pyrolysis4,5 contain numerous oxygenated compounds that are difficult to remove but potentially impact the ability of these fuels to be used as © XXXX American Chemical Society
Received: July 14, 2015 Revised: September 23, 2015
A
DOI: 10.1021/acs.energyfuels.5b01595 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 1. Properties of the Tested Fuel Blendsa
a All fuels were blended with certification diesel, which had a derived cetane number (DCN) of 43 and a lower heating value (LHV) of 42.79 MJ/kg. The left most column shows the name of the fuel additive and abbreviation of the resulting fuel blend. bValues for DCN and lower heating value are estimated via a molar blending rule. cComp06 is a blend of certification diesel with 2% each of 2-methoxy-4-methylphenol, 4-methylacetophenone, and 4-propylphenol.
2. METHODS
combustion of diesel fuels doped with small percentages of various oxygenates in a modern diesel engine. Engine testing was performed by blending seven different oxygenated components and a multicomponent blend with certification ULSD diesel fuel and quantifying the performance and emissions from the combustion of these fuels in a fourcylinder, turbocharged, 4.5 L John Deere PowerTech Plus common rail, direct injection diesel engine that meets Tier 3 emissions specifications. The properties of the oxygenated fuel components were fully characterized in accordance with ASTM diesel fuel standards. Gaseous emissions measurements included CO, CO2, NO, NO2, and total hydrocarbons. Particulate measurements consisted of total PM mass emissions done on a PM10 basis. In-cylinder pressure measurements were conducted and used to evaluate combustion parameters such as apparent rate of heat release and bulk mean cylinder temperature. These measurements are used along with fuel property test results to assess the effects of the oxygenated components on the measured emissions.
In a recent study by Christensen and co-workers,7 three hydrotreated bio-oils with different oxygen contents (8.2%, 4.9%, and 0.4%) were distilled to produce light, naphtha, jet, diesel, and gas oil fractions. These hydrotreated bio-oils were characterized for oxygen-containing species using a variety of techniques including 13C nuclear magnetic resonance, acid number, GC/MS, and liquid chromatography. Christensen et al.7 found numerous oxygenated compounds present in the upgraded fractions including carboxylic acids, carbonyls, aryl ethers, phenols, and alcohols. Based on these results and ongoing work at NREL,8 seven oxygenated compounds and one multicomponent blend were identified as indicative of those present in upgraded advanced biofuels. These compounds were blended with 2007 certification ultralow sulfur diesel (Johann Haltermann, Ltd., Product Code HF0582) at blends of 2−20 vol %. The tested fuel blends are described in Table 1. Each of the blending components listed in the left-hand column was blended with certification diesel in the volume percent shown in the table. An abbreviation is also provided for each fuel blend. For example, PG05 represents a 5 vol % blend of 4-propylguaiacol with 95% certification diesel. The specific gravity, viscosity, derived cetane number (DCN), and lower heating value (LHV) of the blended fuel, as well as the molecular structure of each blending component, are also shown. Note that 2,4-dimethylphenol was mixed in two percentages (2 and 5 vol %) B
DOI: 10.1021/acs.energyfuels.5b01595 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels to investigate the effect on emissions of even a slight increase in percentage. Composite blend 06 (Comp06) is a blend of certification diesel with 2 vol % each of 2-methoxy-4-methylphenol, 4-methylacetophenone, and 4-propylphenol, which simulates the composition of oxygenated residual components from upgraded pyrolysis oil. The fuel blends listed in Table 1 were tested in a four-cylinder, turbocharged, 4.5 L John Deere PowerTech Plus common rail, direct injection diesel engine that meets Tier 3/Stage IIIA emissions specifications. The engine was not equipped with a diesel particulate filter, diesel oxidation catalyst, or selective catalytic reduction system, although it did include a variable geometry turbo and exhaust gas recirculation. An eddy current dynamometer (Midwest Inductor Dynamometer, 1014A) and dynamometer controller (Dyn-LocIV) were used to load the engine and maintain a constant engine speed for a given fueling request. Each of the nine fuel blends was tested at 50%, 75%, and 100% loadall load points correspond to an engine speed of 2400 rpm. The torque output and resulting power produced by the engine at each load are listed in Table 2. More details on this engine can be found elsewhere.10,11
At the start of each test day, the engine was started and warmed up using the off-road diesel. Once the engine had stabilized at the desired load point, the solenoid valve was switched to deliver test fuel to the engine. The volume of the fuel delivery system in the engine and the time required to flush this system at each load was measured, and verified by recording the time when the color of the fuel in the return line changed. Once the line had been purged of all off-road diesel fuel and the engine was clearly operating on test fuel, the engine was allowed to stabilize and the 5 min test point was started. After each test, the engine was switched back to off-road diesel to purge the previous test fuel from the system. This process was repeated until the test matrix was completed. The order of the fuels tested was randomized to reduce any systematic error. All emissions were measured undiluted and on a dry basis. Carbon monoxide (CO) emissions were measured using nondispersive infrared spectroscopy (NDIR; Ultramat 6, Siemens, Munich, Germany). Total hydrocarbon (THC) emissions were measured using a flame ionization detector (FID; NGA 2000 FID hydrocarbon analyzer module, Rosemount, Solon, OH, USA). Oxides of nitrogen (NO, NO2, and NOx) were measured using a chemiluminescence detector (NOXMAT 600, Siemens). Emissions of formaldehyde were measured using Fourier transform infrared spectroscopy (FTIR; Nicolet 6700, Thermo Fisher Scientific Inc., Waltham, MA, USA). Fuel mass flow rate was measured with a Coriolis flow meter (Micro Motion) located downstream of the fuel return line and gravimetrically by measuring the instantaneous mass of the auxiliary fuel tank. A Kistler high speed pressure transducer (model 6056A) was installed in the glow plug port of cylinder 1 (per John Deere standard cylinder numbering scheme, cylinder 1 is the cylinder closest to the front of the engine). The in-cylinder pressure data were used to calculate the apparent rate of heat release (J/°crank angle (°CA)) and the bulk, mean in-cylinder temperature (K). All of the PM measurements were taken after the exhaust sample was diluted with clean air in a dilution tunnel (Figure 1see Bennett12,13 for more detailed information regarding the design and operation of the dilution tunnel). The PM sampling probe, placed at the center of the engine exhaust flow, was turned inward to the flow and sized such that exhaust was sampled isokinetically. A 150 °C heated sample line then delivered the exhaust sample to the dilution tunnel to prevent the gases from condensing. Large particulates were first separated from the sampling stream via a 10 μm cutpoint cyclone.
Table 2. Engine Operating Conditions at Each Load Step load (%)
power output (kW)
torque output (N·m)
speed (RPM)
50 75 100
56 85 115
226 339 452
2400 2400 2400
Data were acquired over a 5 min period for each individual fuel/ engine load condition. For each test point, data collection began once the operating parameters indicated that the engine was at steady state. The series of tests spanned 6 days. Certification diesel tests were performed near the start and end of each day or test period resulting in a total of 10 certification diesel tests for each of the three load points (i.e., 30 baseline diesel tests done in total). From the certification diesel tests, a mean and standard deviation were obtained for all of the examined emissions. The fuel system for the engine consisted of a standard fuel tank (20 gallons) filled with off-road red-dyed (ULSD) diesel and an auxiliary fuel tank (5 gal) to deliver the test fuel via a three-way solenoid valve. The smaller auxiliary fuel tank was utilized to minimize the volume of fuel required for the fuel blends given the high cost of the oxygenated fuel components.
Figure 1. Schematic diagram of dilution tunnel. C
DOI: 10.1021/acs.energyfuels.5b01595 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels The remainder of the sample stream then passed through a 47 mm PTFE filter made by Whatman.14 Each filter along with a control filter was preweighed (a minimum of three times each to ensure consistency) on a Mettler-Toledo MX-5 microbalance with a resolution of 1 μg. A Venturi flow meter (also heated to 150 °C) was used to measure the flow rate of the exhaust air sample entering the dilution tunnel. The dilution air was cleaned by a HEPA filter, followed by an activated charcoal filter. A turbine flow meter was used to measure the flow rate of clean dilution air. A valve located downstream of the turbine flow meter will be used to control the dilution ratio, which was maintained at 20:1. After each test point the PM filters were removed and weighed (along with their corresponding control filters) a minimum of three times on a Mettler-Toledo MX-5 microbalance (resolution of 1 μg). Muktibodh15 includes additional detail on the collection and weighing of PM filter samples from this dilution tunnel. 2.1. Emissions Calculations. The methods used to calculate brake specific fuel consumption, thermal efficiency, and brake specific emissions of CO, THC, NOx, CH2O, and PM10 are described later. Brake specific fuel consumption was calculated as shown in
BSFC =
ṁ f Ẇ b
fpm =
Ẇ b ṁ f LHV
ṁ ex = ṁ f (1 + A /F )
ṁ f αf Yi MWi Ẇ b MWf ∑ex Yjαj
BSE PM =
(7)
ṁ ex fpm (1 + DR) Ẇ b
(8)
where BSEPM is brake specific PM10 emissions in g/kWh and DR is the ratio of dilution air to exhaust air (which was ∼20:1). 2.2. Combustion Data Calculations. In-cylinder pressure measurements and calculated cylinder volume were used to calculate apparent rate of heat release as well as in-cylinder bulk mean temperature. Equation 9 shows the general formula16 for finding the apparent rate of heat release (dQ/dt) in J/°CA based on the measured pressure and volume readings and a calculated ratio of specific heats (γ).
(1)
dQ γ dV 1 dP = P + V dt γ − 1 dt γ − 1 dt
(9)
To determine the approximate bulk mean in-cylinder temperature, an approach similar to that of Mueller et al.17 was used in which the temperature is derived from the ideal gas law. The gas constant was assumed to be that of air, and the initial number of moles was determined based on the intake temperature and pressure at the time of intake-valve closure (IVC). Lastly, to capture the effect of mass addition during fuel injection a linear fuel addition rate was assumed based on the measured start of injection timing (SOI) and the measured fuel consumption rate. The end of injection was assumed to coincide with the point of maximum heat release during the nonpremixed combustion phase.
(2)
(3)
where BSEi is the brake specific emissions of species i in g/kWh, αf the average number of carbon molecules in the fuel, Yi the mass fraction of species i in the exhaust, MWi the molecular weight of species i in g/ mol, MWf the molecular weight of the fuel in g/mol, ∑exYjαj the sum of mass fraction times the number of carbon atoms in each carboncontaining species measured in the exhaust. Brake specific PM emissions were calculated by determining the fraction of PM in the PM sampling line, calculating the mass flow rate of the engine exhaust from the known air to fuel ratio, and then relating the fraction of PM in the sampling line to the mass of PM in the exhaust flow based on the known dilution ratio of the mixture of exhaust air and dilution air from which PM samples were taken. The mass flow rate of PM in the filter sample line was calculated as shown in
ṁ pm =
(6)
where ṁ ex is the mass flow rate of exhaust out of the engine in g/s and A/F the air to fuel ratio on a mass basis. Next, brake specific PM10 emissions were calculated using
where η is the thermal efficiency and LHV the lower heating value of the fuel blend in J/g. Brake specific emissions of CO, THC, NOx, and CH2O were calculated as shown in
BSEi =
ṁ air + ṁ pm
To calculate the total mass of PM in the exhaust from the mass fraction of PM calculated using eq 6, the mass flow rate of the exhaust leaving the engine was calculated using
where BSFC is the brake specific fuel consumption in g/kWh, ṁ f the rate at which the fuel was consumed in g/h, and Ẇ b the brake power output of the engine in kW. Thermal efficiency was calculated as shown in
η=
ṁ pm
3. RESULTS AND DISCUSSION In-cylinder pressure data were acquired for all fuels at a crank angle resolution of 0.5 °CA (the limit of resolution for the engine encoder). From this pressure data, the apparent net heat release rate was calculated using eq 9. Figure 2 and Figure 3 show the apparent heat release rate data for the 75% load point for all of the fuels tested. The apparent heat release curves were filtered using a low pass filter. The data were filtered in both the forward and backward directions (with respect to time) to
mpm t
(4)
where ṁ pm is the mass flow PM matter collected on the filter for that test in μg and t the duration of the test in seconds. The mass flow rate of exhaust mixed with dilution air through the filter sample line was calculated as shown in ̇ ṁ air = ρdil air Vair
(5)
where ṁ air is the mass flow rate of air through the filter sample line in kg/s, ρdil air the density of the dilution air in kg/m3, and V̇ air the volumetric flow rate of air through the filter sample line in m3/s. The density of the dilution air was calculated using the ideal gas law. It was assumed that the mass fraction of PM in the mixture of exhaust air and dilution air was equal to the mass fraction of PM in the PM sampling line ( f pm). The mass fraction of PM in the PM sampling line was calculated as shown in
Figure 2. Apparent heat release rate (J/deg) for certification diesel, PG05, Comp06, and DMP05 for the 75% load condition. D
DOI: 10.1021/acs.energyfuels.5b01595 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Figure 5. Brake specific NOx emissions from certification diesel baseline for each fuel blend at 75% load vs calculated maximum incylinder temperature.
Figure 3. Apparent heat release rate for certification diesel and the low volume percent blends for the 75% load condition.
eliminate any phase shift associated with filtering. To facilitate interpretation of the results, Figure 2 includes the baseline diesel average, and those blends which had a 5 vol % or higher oxygenate fuel blend. Figure 3 shows the baseline diesel average, and the 2 vol % fuel blendsi.e., those fuels that consistently ignited after the certification diesel average. For all fuels shown in Figure 2 and Figure 3 the apparent heat release rate during the non-premixed burn phase does not vary substantially, which one might expect given that all of the fuels were run at a constant, high load point (75% load and 2400 rpm). The most obvious difference between these fuels is the crank angle corresponding to the maximum heat release rate (and magnitude of maximum heat release rate) during the premixed burn phase. Since all of the tested blends had very similar fuel energies (i.e., lower heating values), the timing of premixed energy release appears to have had the most significant impact on exhaust emissions. The ignition delay as controlled by both the individual blend chemistry as well as physical properties affecting fuel vaporization/mixing, and the adiabatic flame temperature all affect the location of the maximum premixed heat release rate, which in turn affects emissions. The pressure data were combined with calculated cylinder volume and the ideal gas law to calculate bulk mean cylinder temperature since previous studies have shown a correlation between bulk mean cylinder temperature and exhaust-out emissions.17−21 In Figure 4, Figure 5, and Figure 6 the maximum bulk mean temperature is plotted against the brake
Figure 6. Brake specific CH2O emissions from certification diesel baseline for each fuel blend at 75% load vs calculated maximum incylinder temperature.
specific CO, NOx, and CH2O emissions. For all three emissions species, the brake specific emissions exhibited a roughly linear correlation with bulk mean cylinder temperature (r2 = 0.80, 0.76, and 0.75 for CO, NOx, and CH2O, respectively). p-values for all three trends were much greater than 0.05 for a 95% confidence interval, so there is little statistical support of significance. However, theory does support increased bulk mean cylinder temperatures correlating with decreased CO emissions, increased NOx emissions, and decreased CH2O emissions. There are a variety of fuel attributes that affect the bulk mean temperature including the adiabatic flame temperature and the ignition timing (i.e., the location of maximum apparent rate of heat release during the premixed burn phase). For all fuel blends, the adiabatic flame temperature was calculated using Gaseq22 assuming an equivalence ratio of 1.0 and an extended list of complete combustion products. [Gaseq combustion products considered for equilibrium adiabatic temperature calculation: N2, H2O, CO2, CO, O2, OH, H, O, H2, NO, HCO, CH2O, CH4, CH3, HO2, NO2, NH3, NH2, N, HCN, CN, NO2, C2, and CH.] Using this approach, the adiabatic flame temperature for certification diesel at 1 bar was calculated to be 2288 K. As shown in Figure 7, the majority of the fuel blends had calculated adiabatic flame temperatures that were relatively close to the calculated certification diesel value (
