Comparison of the Spray Combustion Characteristics and Emissions


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Comparison of the Spray Combustion Characteristics and Emissions of a Wood-Derived Fast Pyrolysis Liquid-Ethanol Blend with Number 2 and Number 4 Fuel Oils in a Pilot-Stabilized Swirl Burner Tommy Tzanetakis, Sina Moloodi, Nicolas Farra, Brian Nguyen, Arran McGrath, and Murray J. Thomson* Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto, Ontario M5S 3G8, Canada ABSTRACT: Biomass fast pyrolysis liquid (bio-oil) is a cellulose-based alternative fuel with the potential to displace fossil fuels in stationary heat and power applications. To understand the combustion behavior and emissions of bio-oil, a 10 kW spray burner was designed and constructed. The effect of swirl, atomization quality, ignition (pilot) energy, and total primary combustion air on the stability and emissions of bio-oil spray flames was investigated. A blend of 80% pyrolysis liquid and 20% ethanol by volume was used during the tests, and the results were compared to the burner operation with number 2 and number 4 fuel oils. Bio-oil exhibits higher emissions than number 2 fuel oil at any given operating point. This is primarily due to better atomization quality with number 2 fuel oil, although not as a consequence of viscosity differences, which are minor at the measured fuel temperature in the nozzle (>80 °C). The disparity in atomization quality is caused by differences in the relative amount of atomizing air to liquid fuel flow rate provided to the nozzle at a fixed energy throughput. Another factor that contributes to higher bio-oil blend emissions is worse overall distillation characteristics compared to number 2 fuel oil. As a fully distillable fuel that evaporates high-energy compounds, number 2 fuel oil is far less sensitive to changes in flame stability or blow-off brought upon by varying the swirl number, atomizing air, pilot energy, or primary combustion air flow rate. Because of a combination of these atomization quality and fuel volatility effects, the bio-oil blend cannot operate over as wide of a range of primary air or atomizing air flow rates as number 2 fuel oil. The bio-oil blend has a lower boiling point than diesel and is much more susceptible to flashing-induced combustion instabilities, which lead to intermittent blowout and higher CO emissions. The NOx and particulate emissions of number 2 fuel oil are lower than bio-oil because of the negligible fuel nitrogen and ash contents in the fuel, respectively. Number 4 fuel oil is more comparable to bio-oil because of its nondistillable fraction, fuel nitrogen, and ash contents. CO and hydrocarbon emissions are lower than the bio-oil blend, but total particulates and carbonaceous residues are higher for number 4 fuel oil. This is despite better atomization and a lower nondistillable fraction, suggesting that the fuel-oil residues are more difficult to burn out completely. Fuel NOx conversion efficency of number 4 fuel oil is similar to the bio-oil blend. There are differences in fly ash mineral composition between the two fuels, as well as a much higher sulfur content for number 4 fuel oil. Carbon burnout analysis indicates that all fuels can achieve high carbon conversion efficiency (>99%) at the best operating conditions. The bio-oil blend has the highest amount of unburned carbon, of which the majority is in the form of gaseous CO.

1. INTRODUCTION Global climate change has driven the development of various renewable biofuels. One such alternative fuel is fast pyrolysis liquid or bio-oil. It is a cellulose-derived fuel created by thermally cracking biomass and rapidly condensing the stream of product vapors and aerosols into a liquid.1 Without additional refining, bio-oil is high in water and solid content and not fully distillable and has a heating value less than half that of conventional petroleum liquid fuels.2,3 Wood-derived bio-oil is typically a dark brown, single-phase liquid consisting of aqueous and tar fractions. The aqueous fraction contains water and low-molecularweight (LMW) oxygenated compounds. This polar fraction makes up 75 wt % bio-oil and renders it immiscible with hydrocarbon fuels. The remaining tar fraction contains high-molecularweight (HMW), water-insoluble lignin fragments (or pyrolytic lignin). The presence of nonvolatile compounds in bio-oil means that it cannot be used in combustion applications that require complete evaporation of the fuel. Boilers and furnaces that employ the spray combustion of various liquid fuel oils are some of the most robust burner r 2011 American Chemical Society

systems available. Their versatility in terms of handling viscous, nondistillable fuels make them well-suited for operation with biooil. There have been several prior studies that evaluate the feasibility of firing spray burner systems with pyrolysis liquid for energy throughputs ranging between 5 kW and 50 MW.418 These burners typically use air- or steam-assisted nozzles and swirling flows to achieve adequate combustion quality and stable flames. The most common practical problems encountered during bio-oil operation include poor ignition, fuel nozzle clogging, and corrosion of mild steel components. Ignition problems are mitigated by preheating combustion air and warming the refractory lining/combustion chamber to sufficiently high temperatures with a petroleum starter fuel. Nozzle plugging in larger systems is due to the polymerization of bio-oil and can be remedied by flushing the fuel system with an appropriate solvent, such as ethanol (EtOH). In smaller systems, high organic char content Received: June 21, 2011 Revised: September 19, 2011 Published: October 05, 2011 4305

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Energy & Fuels can also plug fine nozzle orifices and passages and requires some form of prefiltering. Corrosion can be mitigated by constructing wetted components from stainless steel. Bio-oil spray combustion may be described by the following simplified stages: (i) evaporation and combustion of LMW volatiles, (ii) evaporation of water, cracking, gasification, and combustion of HMW compounds, and (iii) burnout of nonvolatile constituents.7 Visual observations of self-sustaining bio-oil flames indicate the presence of a stable combustion zone, followed by a region where char particles or individual droplet residues are undergoing burnout.4,13,16 Flame length depends upon a number of parameters, but most studies report bio-oil as having a similar or longer flame length compared to petroleum oils.7,13,14,17 The flame temperature is also reported as being lower than hydrocarbon flames.8,14,18 Generally, the emissions for bio-oil fall between those of light (number 2) fuel oil and heavy residual (number 6) fuel oil, except for particulate matter (PM).6,13,17,18 If atomization quality, residence time, and mixing are sufficient, nearly complete combustion can be achieved for bio-oil. This is demonstrated by the low CO (3050 ppm) and near zero total hydrocarbon (THC) emissions that have been observed under good combustion conditions. SOx emissions are lower for pyrolysis liquids (250 ppm) because they usually contain less sulfur than petroleum fuels. Several studies have confirmed that the NOx emissions from biooil combustion are dominated by the conversion of fuel-bound nitrogen.8,10,19 Bio-oil typically contains a nitrogen content between that of number 2 and number 6 fuel oils and, therefore, exhibits intermediate NOx emissions (60280 ppm). The PM in bio-oil exhaust is composed primarily of carbonaceous cenosphere residues, char and ash. With very good carbon burnout, the flue gas particulates are essentially all ash.7,10 The wide range in the PM concentration exhibited by bio-oil (30400 mg/m3) is thus a consequence of variability in fuel ash. The initial market for bio-oil is considered to be intermediatesize boilers, with a thermal output between 0.2 and 1 MW.20 These boilers typically operate with number 2 or number 4 fuel oil grades. These specifications are strongly related to the maximum allowable viscosity, ash content, and nonvolatile residue in the fuel that ensures good combustion quality and low emissions.21 The number 4 fuel oil grade is a blend between number 2 and number 6 fuel oils, which achieves an intermediate viscosity (525 cSt at 40 °C). To understand the effect of displacing these fuel oils with bio-oil in intermediate-size boilers, a 10 kW spray burner was constructed. The elements of the design of the burner that help to ensure good combustion quality with fuel oil and bio-oil are variable swirl, a continuous methane oxygen pilot flame ignition source, an air blast atomizer, and variable fuel/air preheat. The objective of this work is to study the effect of the swirl number, atomizing air flow rate, pilot energy, and primary air flow rate (equivalence ratio) on the combustion stability and emissions of an 80:20 (% by volume) bio-oil/EtOH blend and number 2 and number 4 fuel oils. Comparing the results provides insight on the relative differences between bio-oil and petroleum liquid combustion, an important step toward the displacement of fossil fuels in typical thermal/power generation applications.

2. EXPERIMENTAL METHODOLOGY 2.1. Burner Design. Primary combustion air goes through a set of adjustable swirl blocks, which can vary the theoretical swirl number

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between 0 and 5.41. The nozzle is an air-blast, internal mix atomizer with six individual discharge orifices, 0.89 mm in diameter. It creates a spray pattern with a quiescent core and a total angle of about 65°. The burner opens up into a diffuser section with a half angle of 35° to promote recirculation (along with the quiescent core pattern of the spray). The throat diameter of the diffuser is 130 mm, and the final burner diameter that it expands to is 221 mm. A premixed CH4/O2 pilot flame is positioned at the diffuser throat and run continuously to stabilize combustion. A quartz window is used to view the flame and monitor combustion quality during testing. Further downstream, a single-access port is used to insert a borescopic probe for taking photographs of flames at various operating conditions. Specific details regarding the burner assembly and experimental methodology discussed below are provided in prior studies.22,23 2.2. Experimental Setup. Liquid flow rates are metered between 17 and 35 mL/min to provide 10 kW operation based on the lower heating values (LHVs) of the fuels. Pure EtOH is a good solvent for biooil and is used to warm up the burner prior to taking data and to flush the fuel line during shutdown. The pyrolysis liquid is blended with 20% EtOH by volume to promote flame stability and study a wide range of operating conditions. Primary air preheat temperatures between 230 and 250 °C are achieved using a 1.5 kW electric element. The primary air is drawn through a downstream stack fan to keep the system under negative pressure. The burner is always warmed up to a steady state prior to changing parameters (i.e., swirl number and atomizing air) and making any measurements. 2.3. Exhaust Gas Speciation. Exhaust gas is tapped using a heated sample line between 190 and 195 °C and filtered to remove any PM. THC emissions are measured using a flame ionization detector (FID) and reported in parts per million (ppm) volume of methane. CO, NOx, CH4, formaldehyde (CH2O), and acetaldehyde (C2H4O) emissions are measured using a Fourier transform infrared (FTIR) spectrometer. The percentage of O2 in the exhaust is continuously monitored using a zirconia oxygen sensor. The equivalence ratio is back-calculated using this measured value and assuming complete combustion. 2.4. PM Measurement and Analysis. Particulates are collected with an isokinetic sampling probe placed directly in the unfiltered exhaust stream. A vacuum pump is used to equalize the static pressures in the stack pipe and probe, so that a representative distribution of PM is collected. The PM is deposited on a 47 mm diameter borosilicate microfiber filter, which has a 99.9% retention efficiency for 0.3 μm particles. The filters are analyzed gravimetrically to determine a PM emissions index in milligrams of PM per kilogram of fuel input. The morphology of particulates is examined with a scanning electron microscope (SEM), and the chemical composition of ash is investigated using energy-dispersive X-ray (EDX) analysis. The amount of carbonaceous residue versus ash in a PM sample is determined by burning off the noncombustible material in an oven between 725 and 800 °C. 2.5. Flame Visualization. Photographs are taken looking up along the central axis of the burner and used to help understand the qualitative trends in mixing quality and atomization as conditions are varied. The borescope consists of a rigid fiberoptic member coupled to a digital camera. The unit is on a rail-guide system, so that it may be quickly immersed and removed from the burner. A sheath of compressed air is used to protect the probe from PM impingement and to keep it cool. The probe only remains immersed for 25 s to take a single photograph. Light sensitivity, aperture, and shutter speed settings are adjusted on the camera to achieve the best quality picture. 2.6. Fuel Analysis. The bio-oil used in this study was produced from a mixed hardwood feedstock. The liquid properties are summarized in Table 1 along with number 2 and number 4 fuel oils. Two distinct batches of bio-oil were used for different phases of the combustion tests. Batch 1 was used to make gaseous pollutant emission measurements, 4306

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Table 1. Liquid Properties of Bio-oil Batches and Fuel Oils property

a

batch 1 bio-oil

batch 2 bio-oil

number 2 fuel oil

number 4 fuel oil

CHON (wt % dry)a

556380.5

537400.1

861400

861130.3

solids (wt %)

0.15

0.07

0.000

NMb

ash (wt %)

0.17

0.06

0.000

0.024

water (wt %)

28.5

27.6

0.017

NM

LHV (MJ/L)

17.3

16.6

35.4

36.8

density (kg/m3)

1195

1163

828

872

kinematic viscosity at 40 °C (cst)

25.5

12.1

1.93

7.02 (80 °C)

Oxygen calculated by difference. b NM = not measured.

Figure 1. Burner test procedure flow diagram.

Table 2. Base Point Burner Operating Conditions parameter

a

value(s)

fuel flow (bio-oil/EtOH and number 2 and number 4 fuel oils)

33.3, 16.9, and 16.3 mL/min (10 kW)

swirl number (S)

5.41

pilot methane flow pilot oxygen flow

0.88 SLPMa 2.3 SLPM

primary air preheat temperature

230250 °C

O2 concentration in exhaust

6.88.1% by volume

equivalence ratio (from O2 above)

0.60.64

atomizing air flow rate

23 SLPM

primary air flow rate

250260 SLPM

Run continuously, represents 5% of the total energy (0.5 kW).

and batch 2 was used to measure PM emissions. Both batches were blended with 20% EtOH by volume prior to running a test. The solid contents in the bio-oil batches are those of the “as-received” fuel without any further filtration. The addition of EtOH to the pyrolysis liquid changes the overall volatility and viscosity of the fuel. The volatility and evaporation characteristics of the bio-oil/EtOH blends and fuel oils are evaluated using thermogravimetric (TG) analysis in a nitrogen atmosphere at a heating rate of 10 °C/min. The kinematic viscositytemperature relationships of all fuels are determined according to standard American Society for Testing and Materials (ASTM) D445. 2.7. Test Procedure. Figure 1 shows the operating procedures adopted for each fuel. For the bio-oil blend, the burner is first warmed

with EtOH, switched over to the main fuel, and then run at the base point conditions in Table 2 for 1530 min to achieve steady-state operation before taking measurements. A similar procedure is employed for the number 4 fuel oil but using number 2 fuel oil as the warm-up fuel. It is important to note that all of the number 2 fuel oil tests were conducted without the use of a separate warm-up fuel or primary air preheat. Furthermore, the number 4 fuel oil was run only at base point operating conditions, while a complete parametric study (varying the swirl number, atomizing air, etc.) was performed with bio-oil/EtOH and number 2 fuel oil. Burner parameters are varied one at a time to determine their effect on combustion, while all other parameters listed in Table 2 are kept at their respective base point values. 4307

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Figure 2. Kinematic viscosity versus temperature.

Figure 4. DTG curves in nitrogen for a heating rate of 10 °C/min.

Figure 3. TG curves in nitrogen for a heating rate of 10 °C/min.

Figure 5. Base point gaseous emissions.

3. RESULTS AND DISCUSSION

batch 1 is shown for clarity). Their nondistillable nature potentially makes these fuels more susceptible to localized flame shear and lean blow-out effects.26,27 In addition, the 1720 wt % leftover residue at 600 °C is correlated to fuel coking tendency and indicates that bio-oil can form solid carbonaceous cenospheres during combustion.28 The addition of EtOH lowers the TG curve compared to pure bio-oil, which helps to mitigate the aforementioned blow-out effects and high PM emissions. In contrast, number 2 fuel oil is fully evaporated by a temperature of 200 °C. It exhibits a slightly lower volatility than the bio-oil blends between 120 and 140 °C. Although the relatively high volatility of the blends at low temperature promote evaporation and contribute to flame stabilization, there can also be a detrimental effect if the fuel reaches its boiling point within the nozzle. This leads to intermittent vapor discharge and flashing-induced combustion instabilities.22 Number 4 fuel oil shows a similar TG curve to bio-oil up to 400 °C because it contains a portion of nonvolatile (residual) compounds. After 400 °C, there is a marked drop in sample weight percent that is likely due to cracking of the residual material.29 This does not seem to be the case during bio-oil evaporation, which may be dominated by fuel polymerization. The 10 wt % leftover residue indicates that number 4 fuel oil also tends to form carbonaceous cenospheres during combustion. The differential thermogravimetric (DTG) curves in Figure 4 show that the main peak mass loss rates for bio-oil and bio-oil/ EtOH occur between 70 and 100 °C. This range covers the boiling points of various LMW oxygenated compounds, including

3.1. Fuel Properties. Figure 2 shows the kinematic viscosity of the fuels. The bio-oil blends are very sensitive to temperature and show a large reduction in viscosity between 20 and 80 °C. In contrast, number 2 fuel oil does not exhibit such a strong dependence. Although the plot for number 2 fuel oil is taken from the literature,24 it is considered indicative because of its consistency with the current measured value. To comment about the effect of viscosity on atomization quality, it is important to know the fuel temperature in the nozzle. This temperature exceeds 80 °C for the bio-oil blends at most of the operating conditions considered. Furthermore, number 2 and number 4 fuel oils operate at temperatures in excess of 90 and 110 °C, respectively. For the current burner system, this means that all fuels reach a similar viscosity in the nozzle and that viscosity does not play a significant role in determining the relative atomization quality between the fuels. However, one parameter that strongly affects spray quality in airblast nozzles is the air-to-liquid mass flow ratio (ALR).25 This parameter differs greatly between bio-oil and fuel oil because of the disparity in their heating values, which in turn controls the total mass flow rate of liquid fuel required to maintain 10 kW operation (see Tables 1 and 2). These aspects are treated more thoroughly in section 3.4. Figure 3 compares the distillation behavior of the fuels in a nitrogen environment. Bio-oil and bio-oil/EtOH do not achieve a zero sample weight percent, which means that they cannot be fully evaporated (both batches have similar TG curves, but only

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Table 3. Detailed NOx and PM Base Point Emissions fuel NOx conversion

theoretical ash emissions

measured ash emissions

unburned carbonaceous residues

efficiency (%)

(mg/kgfuel)

(mg/kgfuel)

(mg/kgfuel)

bio-oil/EtOH

38

518

532

243

number 4 fuel oil

43

254

295

494

fuel

Figure 6. Base point total PM emissions.

EtOH and water.30 Other sharp peaks beyond this temperature range are due to internally trapped vapor escaping the polymerized/HMW compound enriched surface layer of the TG sample. Number 2 fuel oil has a smooth evaporation history, with a peak rate twice that of the bio-oil samples at 155 °C. Number 4 fuel oil displays two distinct DTG peaks. The first corresponds directly to that of number 2 fuel oil because it makes up about 50% of the fuel blend. The second at 450 °C corresponds to the cracking point of HMW residual compounds. From a distillation point of view, number 4 fuel oil is more similar to bio-oil because it contains a nonvolatile fraction, which will likely undergo heterogeneous solid-phase combustion.31 3.2. Base Point Operation. Figure 5 compares gaseous emissions under the base point operating conditions in Table 2. Bio-oil/EtOH has the highest THC and CO emissions primarily because of worse atomization quality and overall volatility compared to the fuel oils. Details regarding these specific effects are discussed in subsequent sections. Number 2 fuel oil has the lowest THC and CO, despite the lack of primary air preheat. The NOx emissions for bio-oil/EtOH and number 4 fuel oil are higher than number 2 fuel oil because of their inherent nitrogen content. Table 3 shows that the conversion efficiencies of fuel nitrogen into NOx are similar for bio-oil/EtOH and number 4 fuel oil. The value for bio-oil/EtOH is calculated assuming 50 ppm combined flame and pilot thermal NOx, which is based on pure EtOH operation. The conversion efficiency for number 4 fuel oil is calculated assuming 80 ppm thermal NOx, corresponding to that which is measured during number 2 fuel oil baseline operation. Figure 6 compares total PM emissions under the same baseline conditions. Bio-oil/EtOH and number 4 fuel oil have higher emissions because of their ash content and nondistillable residual fractions. In contrast, the PM emissions for number 2 fuel oil are below the detection limit of the current system. The results in Table 3 show that the measured ash content in the collected biooil blend and number 4 fuel oil PM closely matches that which is expected on the basis of the inorganic content in the fuel. This means that, under base operating conditions, a representative

sample of exhaust particulates was acquired with minimal loss of material to the burner walls. However, number 4 fuel oil exhibits the highest PM emissions, with 2 times the unburned carbonaceous residue of bio-oil/EtOH. This is despite the fact that biooil/EtOH has worse atomization quality and a larger TG residue than number 4 fuel oil and suggests that fuel oil residues are more difficult to burn off completely. Figure 7 shows images of the flames. The fuel oil flames are generally more luminous than bio-oil/EtOH, which is probably caused by soot radiation. The number 2 fuel oil demonstrates a faint blue color at the base of the spray jets, suggesting a certain degree of lean or partially premixed combustion in the flame. The bio-oil/EtOH flame indicates the presence of distinctly bright streaks, which are attributed to the combustion of individual droplet residues.32 These streaks are not as prominent in the number 4 fuel oil photos because the droplets are smaller and the flame is much brighter. Such streaks do not typically appear outside of the flame envelope of number 2 fuel oil because the fuel can fully evaporate. 3.3. Swirl Number. Figure 8 shows that bio-oil/EtOH and number 2 fuel oil follow the same trend in CO with respect to the swirl number. This is due to the effect that swirl has on the flow patterns and flame dynamics within the burner. Swirling flows induce a central recirculation zone (CRZ) that recycles hot exhaust gases to the base of the flame, which in turn help to stabilize ignition and combustion.33 As S is decreased, the size and streangth of the CRZ is diminished and ignition quality is compromised. This leads to an increase in CO emissions as S is reduced from 5.41 to 1.46. The degradation of combustion quality is clearly demonstrated in Figure 9, which shows that all flames are less stable at lower swirl numbers. However, the bio-oil blend seems to be much more sensitive to these changes because there are barely any jet flames visible at S = 1.46 in Figure 9f. The combustion instability at this particular operating point has been previously attributed to the lack of a CRZ.22 It should be noted that the swirl numbers used herein are calculated using an idealized expression for moveable block-type swirl generators and are likely much higher than the physical swirl numbers in the burner. This is why recirculation is reported to occur at a relatively high swirl number of S = 1.46. The CO emissions in Figure 8 decrease at S = 0 for both the bio-oil blend and number 2 fuel oil, despite the poor mixing and stability associated with this operating condition. This may be due to larger amounts of fuel being bound up in polymerized or pyrolyzed residues that escape complete combustion. Number 4 fuel oil exhibits CO emissions that are similar to number 2 fuel oil at maximum swirl. Although not shown here, THC emissions for bio-oil/EtOH and number 2 fuel oil follow the same trend as CO with respect to the swirl number. Bio-oil/EtOH shows higher emissions than number 2 fuel oil across the entire swirl number range. This is due to a combination of differences in atomization quality (discussed in the next section) and distillation characteristics. Even though Figure 3 shows that bio-oil/EtOH has superior low-temperature volatility 4309

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Figure 7. Flame images at base case operating conditions.

Figure 8. CO versus swirl number (S).

compared to number 2 and number 4 fuel oils, the compounds that evaporate consist primarily of water and low-energy content oxygenates.30,34 These compounds do not make as significant of a contribution to early flame heat release and stabilization as the pure hydrocarbons in fuel oil. The bio-oil blends are therefore more susceptibile to localized flame shear and lean blow-out effects, which lead to increased emissions (flame shear mechanism). Evidence of these mechanisms is provided in Figure 9, which shows that number 2 fuel oil flames remain relatively coherent and anchored to the nozzle as S is decreased, while bio-oil/EtOH flames do not. Furthermore, the presence of non-evaporative compounds in both the bio-oil blend and number 4 fuel oil do not seem to have a significant effect on localized fuelair spray overleaning and CO emissions. This is demonstrated in Figure 8 by the similar CO for both number 2 and number 4 fuel oils at maximum swirl number, even though one fuel is fully distillable and the other is not. Another possible contribution to the higher CO may be related to the heterogeneous surface combustion that nondistillable residues undergo to achieve complete burnout. Without adequate residence time, temperature, and oxygen availability, these surface reactions can be readily quenched (residue quenching mechanism).31 The presence of individual droplet residues that persist outside of the main flame envelope are observable as bright dots and streaks in Figure 9d. As previously mentioned, however,

number 4 fuel oil exhibits low CO compared to bio-oil/EtOH, even though it contains a significant residual fraction and higher carbonaceous PM at the base case. This suggests that residue quenching is less significant than the flame shear mechanism. Another parameter that could be influencing CO is temperature within the spray flame. Centerline diffuser outlet gas temperatures range from 713 to 766 °C and from 743 to 762 °C for biooil/EtOH and number 2 fuel oil, respectively. These results indicate that gas temperature differences are minor. Figure 10 shows that NOx emissions are relatively insensitive to the swirl number. The 30 ppm decrease for bio-oil/EtOH below S = 1.46 may be due to poor combustion stability and reduced fuel N conversion efficiency. For number 2 fuel oil, the NOx increases by 20 ppm between S = 0.54 and 1.46. This may be due to reduced exhaust gas recirculation in the flame, which tends to increase thermal NOx.33 The 20 ppm decrease at S = 0 is also attributed to poor mixing/combustion stability, which can reduce thermal NOx formation.35 The NOx for number 4 fuel oil lies between that of bio-oil/EtOH and number 2 fuel oil because it has an intermediate fuel N content. 3.4. Atomizing Air Flow Rate. The Sauter mean diameter (SMD) of droplets in an airblast spray can be estimated using a correlation from the literature.22,25 Previous rheological analysis has shown that even pyrolysis liquids with a high char content and an inhomogeneous microstructure exhibit Newtonian behavior at high strain rates,36 indicating that it is appropriate to use a standard SMD correlation to estimate bio-oil droplet size. Table 4 summarizes the parameters used in the calculation, the range of applicability for which the correlation was developed, and the SMD results for baseline operation. The SMD for bio-oil/EtOH is 2.53 times higher than either the number 2 or number 4 fuel oil. This is not primarily a consequence of viscosity or surface tension, which are all similar, but of differences in the ALR. At base case conditions, for instance, the ALR is 0.70.75 and 2.0 for bio-oil/EtOH and the fuel oils, respectively. Calculations also reveal that the fuel oils have a lower SMD than bio-oil/EtOH at any given atomizing air flow rate. Smaller fuel droplets require less residence time to achieve full burnout, leading to lower THC, CO, and PM emissions.27 Figure 11 summarizes the effect of the atomization air flow rate and spray quality on CO emissions. For a SMD greater than 100 μm, droplet sizes are relatively large, leading to slower 4310

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Figure 9. Images of number 2 fuel oil and bio-oil/EtOH flames versus swirl number.

Figure 10. NOx versus swirl number (S).

evaporation and less thorough burnout. These conditions are achieved with low atomizing air flow rates [1018 standard liters per minute (SLPM)], which also reduce turbulent mixing and increase THC and CO emissions. Slow evaporation and poor mixing are qualitatively demonstrated by the number 2 fuel oil “fireball” flame in Figure 12a. Here, the jets become long enough to wrap around the CRZ, forming a closed ball. The THC emissions at this point exceed 300 ppm and also indicate that poor mixing is leading to incomplete combustion. Increased droplet sizes at lower atomizing air flow rates are demonstrated by the

increased presence of individual residue burning streaks in Figure 12d. Many of the streaks appear outside of the main flame envelope. For bio-oil/EtOH, this means that quenching of nonvolatile residues undergoing heterogeneous surface combustion could potentially be compounding the CO emissions at large SMD. In contrast, there are no bright streaks visible for the number 2 fuel oil low atomizing air case. This is likely due to the complete evaporation of fuel within the flame envelope. The CO for number 4 fuel oil is not discernible in Figure 11 because it overlaps emissions for number 2 fuel oil at a SMD of 30 μm, despite having a significant amount of carbonaceous PM. At a SMD of 100 μm, the CO of number 2 fuel oil and bio-oil/ EtOH are the same, even though the fuelair mixing quality is worse for number 2 fuel oil because of the lower atomizing air flow being provided.22 The overlapping emissions can be explained by number 2 fuel oil having both a faster evaporation rate and burning time compared to bio-oil,18 thereby compensating for the lower mixing rate. Below a SMD of 100 μm, bio-oil/EtOH shows an increasing trend in CO emissions. This is due to higher atomizing air flow rates, which increase jet velocities and flame shear effects, counteracting the benefits of a smaller droplet size. Evidence of this mechanism is given in Figure 12f, which shows that the bio-oil/EtOH flame becomes lifted off from the nozzle. However, the CO emissions for number 2 fuel oil continue to decrease within this regime. This is because of its higher energy content volatiles, which evaporate quickly and contribute to early 4311

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Table 4. Spray Atomization Parameters Used for the Calculation of SMD correlation range correlation parameter

units

batch 2 bio-oil/EtOH

number 2 fuel oil

number 4 fuel oil

of applicabilitya

kg/m3

1117

1088

828

872

surface tension (σ)

mN/m

30

30

21

20

2676

dynamic viscosity (μL)

Pa s

3.39  103

2.39  103

1.60  103

2.86  103

1.076  103

liquid mass flow (m_ L)

kg/s

6.64  104

6.04  104

2.33  104

2.37  104

N/A

airliquid relative velocity (UR)

m/s

79191

90161

52146

123

60180

0.401.0

0.510.91

0.852.9

1.9

116

1.0 91

1.0 77

1.0 31

1.0 30

N/A 20130

liquid density (FL) b

ALR (m_ A/m_ L) discharge orifice (do) SMD (base case) a

batch 1 bio-oil/EtoH

mm μm

7942180

Taken from ref 25. b Taken from ref 36.

Figure 11. CO versus calculated SMD.

flame heat release, thereby reducing localized blow-out effects. The fact that number 2 fuel oil is less susceptible to flame shear than bio-oil/EtOH is demonstrated in Figure 12c, which shows that flames remain anchored to the nozzle even at high atomizing air flow rates. The predominantly blue jet color indicates good mixing and thorough evaporation of fuel, leading to lean/ premixed combustion, which helps to stabilize the flame. If the atomizing air is high enough, flame shear effects begin to increase CO for number 2 fuel oil as well, eventually leading to complete blow-out of the flame. In general, Figure 11 indicates that the upper limit of atomizing air that can be used to improve spray quality and promote fuelair mixing for bio-oil/EtOH is limited compared to fuel oil. Figure 13 shows that NOx emissions remain relatively level throughout most of the atomizing air flow rate range. Both biooil/EtOH and number 2 fuel oil exhibit a 50% decrease at 13 and 10 SLPM, respectively. For bio-oil, the drop is likely due to poor fuel burnout and reduced oxygen availability, which both compromise the conversion of fuel-bound nitrogen.35 Similarly, low localized oxygen availability in the number 2 fuel oil “fireball” flame is likely responsible for the observed thermal NOx reduction because centerline diffuser gas temperatures vary by only 21 °C for all atomizing air cases considered. 3.5. Pilot Flame Energy. Figure 14 shows that CO emissions for number 2 fuel oil fluctuate between 10 and 30 ppm with respect to pilot energy (i.e., changes in the CH4 flow rate). The lack of a discernible trend suggests that number 2 fuel oil is not particularly sensitive to this parameter. In contrast, bio-oil/EtOH shows a 245 ppm reduction in CO for a 0.3 kW increase in pilot

energy. Further evidence that the bio-oil blend is much more sensitive to pilot flame energy is provided in Figure 15. Even for the number 2 fuel oil pilot-off case, it is clear that the jets remain anchored to the nozzle by blue flames. However, bio-oil/EtOH flames become anchored further downstream when the pilot is turned off, indicating blow-off of the fuelair mixture. Part of the observed sensitivity to blow-off for bio-oil may be due to differences in atomization quality (larger droplets take a longer time to evaporate). However, there are several other factors that could explain why bio-oil/EtOH exhibits more significant blowoff compared to fuel oil. The first is its high water content, which absorbs latent heat to vaporize in the flame. This can reduce local temperatures and inhibit the activation of chemical reactions. Second, with lower heating value compounds evaporating from the bio-oil, there may be lower heat release rates within the initial regions of the spray jets, leading to blow-off. Finally, there could also be a detrimental chemical effect caused by compounds, such as LMW oxygenates, that may have inherently slower chemical kinetics compared to number 2 fuel oil. This raises the question of whether or not the addition of EtOH could be contributing to the observed sensitivity of bio-oil to pilot energy. EtOH has a high autoignition temperature and low cetane number, both of which are related to poor ignition characteristics and, thus, slower chemical kinetics.37,38 However, preliminary tests showed that the addition of EtOH reduces flame blow-off because higher ethanol concentrations in the bio-oil lead to flame anchor points closer to the nozzle and lower THC emissions. This suggests that blow-off in the current burner is governed by physical processes, such as atomization, mixing, fuel vaporization, and heat release, most of which are improved by EtOH addition. 3.6. Primary Air Flow Rate. Varying the total primary air flow rate changes the turbulence, mixing quality, and equivalence ratio in the burner. Table 5 shows the relatioinship between the measured percentage of O2 in the exhaust and the calculated equivalence ratio. All emissions are plotted against the oxygen content and normalized to 4% O2 to remove the effect of dilution. Figure 16 shows that bio-oil/EtOH CO emissions increase at the lower end of the oxygen content. Although gas and flame temperatures are hotter for these conditions (because of the fixed electric heater input), the CO emissions increase because of inadequate mixing. Evidence of this is given in Figure 17d, which shows that, at 3.6% O2, the bio-oil blend has a much “hazier” flame structure than the base case. The longer, more diffuse flames do not exhibit rapid fluctuations because there is less turbulent 4312

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Figure 12. Images of number 2 fuel oil and bio-oil/EtOH versus atomizing air flow rate.

Figure 13. NOx versus atomizing air flow rate.

Figure 14. CO versus pilot energy.

and rapid mixing interaction between the spray jets and primary swirling air. Figure 17d also does not show any droplet residue burning streaks. With reduced mixing quality and oxygen availability in the flame zone, one explanation could be that larger droplet residues fail to ignite under these conditions. This is consistent with the observed increase in carbonaceous PM at low percentages of O2.23 At the higher end of oxygen, the rise in CO is likely due to a combination of reduced gas/flame temperatures

and flame shear effects from increased primary air velocities. The same trends are seen for THC emission as well. Figure 17b shows that number 2 fuel oil flames also suffer from less rapid mixing at lower oxygen contents. However, the CO emissions in Figure 16 remain relatively unaffected, indicating that the fuel oil is less sensitive to similar changes in the primary air flow rate compared to bio-oil/EtOH. As previously discussed, these results are likely a consequence of better overall distillation 4313

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Figure 15. Images of number 2 fuel oil and bio-oil/EtOH versus pilot energy.

Table 5. Equivalence Ratio and Percentage of O2 (by Volume) percentage of O2

percentage of O2

percentage of O2

equivalence ratio

for batch 1 bio-oil/ EtOH (%)

for number 2 fuel oil (%)

for number 4 fuel oil (%)

0.5

9.4

10.1

0.6

7.3

8.1

0.7

5.5

6.0

0.8

3.6

4.0

8.1

characteristics and atomization quality for number 2 fuel oil compared to bio-oil. Figure 18 shows that NOx plateaus for bio-oil/EtOH, a behavior that is indicative of nitrogen-containing fuels and that is caused by the saturation of fuel NOx conversion efficiency.19 In contrast, NOx for number 2 fuel oil increases steadily from 80 to 125 ppm over the same range. Higher oxygen contents lead to lower primary air and flame temperatures. However, excess air can lead to an increase in thermal NOx emissions for non-premixed combustion systems because of increased oxygen availability.35 The results in Figure 18 support this trend and suggest that the thermal NOx is oxygen-limited as opposed to temperature-limited in the current system. 3.7. Transient Base Point Operation. Figure 19 shows the CO emissions during transient warming of the burner. When the

Figure 16. CO versus oxygen content in exhaust.

fuel is switched to bio-oil/EtOH after running on EtOH for 20 min, CO rises and then tends toward the baseline value of 400 ppm within 40 min. The large fluctuations in CO are caused by flashing-induced combustion instabilities.22 When temperatures in the nozzle exceed the boiling point of the blend (86 °C), vapor can accumulate and then suddenly discharge, causing a temporary blow-out of the flame and rise in CO. This is due to the low boiling point of EtOH (78 °C) in the blend and is distinct from the mechanism that induces blow-out at high atomizing air flow rates. The pilot flame is turned off after 60 min and indicates 4314

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Figure 17. Images of number 2 fuel oil and bio-oil/EtOH versus oxygen content in exhaust.

Figure 18. NOx versus oxygen content in exhaust.

Figure 19. CO during transient bio-oil/EtOH and number 2 fuel oil operation.

that the current burner design is capable of producing selfsustaining bio-oil/EtOH flames. However, CO increases to an average of 680 ppm, which is consistent with the previously discussed sensitivity of bio-oil combustion to pilot flame energy input. Number 2 fuel oil drops to levels that do not exceed 30 ppm CO within the first 10 min of operation, remaining at that level for the duration of the test. There are also no large CO fluctuations as a result of flashing because the boiling point of diesel is higher (155 °C) than the fuel temperature reached in the

nozzle. However, the use of fuel preheat could cause the diesel to eventually flash given the very low pressure drops across the liquid nozzle orifice. There is only a modest shift in number 2 fuel oil CO to between 20 and 50 ppm when the pilot flame is turned off. Figure 20 shows a jump in NOx from 50 to 280 ppm when fuel is switched to bio-oil/EtOH. This demonstrates the large effect of fuel-bound nitrogen on total NOx emissions. Number 2 fuel oil is dominated by thermal NOx formation and remains at 80 ppm until the 60 min mark. The reduction in NOx for the bio-oil blend 4315

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Energy & Fuels after the pilot is turned off is only 20 ppm. The drop in NOx for number 2 fuel oil is 50 ppm, making the self-sustaining flame emissions for this fuel only 30 ppm. The lean/partially premixed combustion in the jets, recirculation of exhaust gas, lack of air preheat, and absence of refractory lining all tend to lower flame temperatures and suppress thermal NOx and could explain the relatively low value.33,35,39 Furthermore, small-scale combustors tend to strongly depend upon the flame residence time, with shorter times producing lower thermal NOx emissions.40 As a result, the therml NOx emissions reported for both fuels may not be indicative of what is typically observed in industrial practice. 3.8. Methane, Formaldehyde, and Acetaldehyde Emissions. CH4, CH2O, and C2H4O emissions remained below detection limits for all number 2 fuel oil cases, except the minimum

Figure 20. NOx during transient bio-oil/EtOH and number 2 fuel oil operation.

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atomizing air flow rate of 10 SLPM. This condition is characterized by poor atomization and mixing quality, giving rise to 17 ppm formaldehyde. CH4 and CH2O were detected in small amounts (