Spray Combustion Characteristics and Gaseous Emissions of a Wood


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Energy Fuels 2010, 24, 5331–5348 Published on Web 10/06/2010

: DOI:10.1021/ef100670z

Spray Combustion Characteristics and Gaseous Emissions of a Wood Derived Fast Pyrolysis Liquid-Ethanol Blend in a Pilot Stabilized Swirl Burner Tommy Tzanetakis, Nicolas Farra, Sina Moloodi, Warren Lamont, Arran McGrath, and Murray J. Thomson* Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto, Ontario, Canada, M5S 3G8 Received May 31, 2010. Revised Manuscript Received September 10, 2010

Biomass fast pyrolysis liquid (or bio-oil) is a cellulose based alternative fuel with the potential to displace fossil fuels in stationary heat and power applications. To better 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 source energy, air/fuel preheat, and equivalence ratio 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. Since the fuel is not fully distillable, it is important to have good atomization, thorough mixing, and increased recirculation to promote the burnout of nonvolatile material and decrease CO and hydrocarbon emissions. Air and fuel preheat are also important for reducing these emissions, although subsequent fuel boiling within the nozzle should be avoided in order to maintain flame stability. The amount of total primary air and atomizing air that can be used to improve turbulence, mixing, droplet burnout, and overall combustion quality is limited by the low volatility and tighter lean blow-out limit associated with bio-oil. The NOx produced in these flames is dominated by the conversion of fuel bound nitrogen. In order to reduce the NOx emissions without refining the fuel, the use of staged combustion is recommended.

The earliest detailed studies on bio-oil as an alternative fuel focused on single droplet combustion.6,7 Since then, there has been a great deal of work investigating the feasibility of operating furnaces/boilers,8-15 gas turbines,16-19 and diesel

1. Introduction The global issue of climate change and the need for energy independence have driven the development of various renewable biofuels and technologies. 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, acidic, not fully distillable, and has a heating value less than one-half that of conventional liquid petroleum fuels on a volumetric basis.1-3 It is typically a dark brown, single phase liquid consisting of both an aqueous and tar fraction.2,4,5 The aqueous fraction contains water and low molecular weight, oxygenated compounds. This polar fraction makes up about 75 wt % of bio-oil and renders it immiscible with conventional liquid petroleum fuels. The remaining tar fraction contains high molecular weight, water-insoluble lignin fragments (or pyrolytic lignin). Although bio-oil has been produced commercially for about 30 years within the chemical industry, it has only recently been considered as a viable biofuel.

(7) Shaddix, C. R.; Hardesty, D. R. Combustion Properties of Biomass Flash Pyrolysis Oils: Final Project Report, SAND99-8238; Sandia National Laboratories: Livermore, CA, 1999. (8) Shihadeh, A.; Lewis, P.; Manurung, R.; Beer, J. Combustion characterization of wood-derived flash pyrolysis oils in industrial-scale turbulent diffusion flames. In Proceedings of the Biomass Pyrolysis Oil Properties and Combustion Meeting, Estes Park, CO, NREL-CP-430-7215, September 26-28, 1994; pp 281-295. (9) Rossi, C. Bio-oil combustion tests at ENEL. In Proceedings of the Biomass Pyrolysis Oil Properties and Combustion Meeting, Estes Park, CO, NREL-CP-430-7215, September 26-28, 1994; pp 321-328. (10) Barbucci, P.; Costanzi, F.; Ligasacchi, S.; Mosti, A.; Rossi, C. Bio-fuel oil combustion in a 0.5 MW furnace. In Proceedings of the Second Biomass Conference of the Americas, Golden CO, NREL-CP-2008098, 1995; pp 1110-1120. (11) Huffman, D. R.; Vogiatzis, A. J.; Clarke, D. A. Combustion of bio-oil. In Bio-oil Production and Utilization; Bridgwater, A. V., Hogan, E. H., Eds.; CPL Press: Newbury, U.K., 1996; pp 227-235. (12) Gust, S. Combustion experiences of flash pyrolysis fuel in intermediate size boilers. In Developments in Thermochemical Biomass Conversion; Bridgwater, A. V., Boocock, D. G. B., Eds.; Blackie Academic & Professional: London, 1997; pp 481-488. (13) Huffman, D. R.; Freel, B. A. RTP Biocrude: a combustion/ emissions review. In Developments in Thermochemical Biomass Conversion; Bridgwater, A. V., Boocock, D. G. B., Eds.; Blackie Academic & Professional: London, 1997; pp 489-494. (14) Oasmaa, A.; Kyt€ o, M.; Sipil€a, K. Pyrolysis oil combustion tests in an industrial boiler. In Progress in Thermochemical Biomass Conversion; Bridgwater, A. V., Ed.; Blackwell Science: Oxford, 2001; pp 1468-1481. (15) Kyt€ o, M.; Martin, P.; Gust, S. Development of combustors for pyrolysis liquids. In Pyrolysis and Gasification of Biomass and Waste; Bridgwater, A. V., Ed.; CPL Press: Newbury, U.K., 2003; pp 187-190. (16) Andrews, R. G.; Fuleki, D.; Zukowski, S.; Patnaik, P. C. Results of industrial gas turbine tests using a biomass-derived fuel. In Making a Business from Biomass in Energy, Environment, Chemicals, Fibres and Materials; Overend, R. P., Chornet, E., Eds.; Elsevier Science Inc.: New York, 1997; pp 425-435.

*To whom correspondence should be addressed. Telephone: þ1 416 580 3391. Fax: þ1 416 978 7753. E-mail: [email protected] (1) Bridgwater, A. V. Renewable fuels and chemicals by thermal processing of biomass. Chem. Eng. J. 2003, 91, 87–102. (2) Oasmaa, A.; Czernik, S. Fuel oil quality of biomass pryolysis oils-state of the art for the end users. Energy Fuels 1999, 13, 914–921. (3) Czernik, S.; Bridgwater, A. V. Overview of applications of biomass fast pyrolysis oil. Energy Fuels 2004, 18, 590–598. (4) Oasmaa, A.; Kuoppala, E.; Gust, S.; Solantausta, Y. Fast pyrolysis of forestry residue. 1. Effect of extractives on phase separation of pyrolysis liquids. Energy Fuels 2003, 17, 1–12. (5) Oasmaa, A.; Kuoppala, E.; Solantausta, Y. Fast pyrolysis of forestry residue. 2. Physiochemical composition of product liquid. Energy Fuels 2003, 17, 433–443. (6) Wornat, M. J.; Porter, B. G.; Yang, N. Y. C. Single droplet combustion of biomass pyrolysis oils. Energy Fuels 1994, 8, 1131–1142. r 2010 American Chemical Society

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engines with bio-oil. However, there are fewer detailed studies23-28 that examine the stability and emissions of bio-oil spray combustion under well controlled conditions. In order to address this disparity in the literature, a 10 kW atmospheric pressure spray burner, capable of handling nondistillable, low calorific value fuels like bio-oil, was designed and constructed. The basic elements of the burner’s design that help to ensure good combustion are variable swirl, a continuous methaneoxygen pilot flame ignition source, an air-blast atomizer, and variable air/fuel preheat. The objective of this investigation is to study the effect of swirl number, atomization air flow, pilot energy, air/fuel preheat, and equivalence ratio on the stability and emissions of bio-oil spray flames. The observed trends in combustion behavior can then be used to determine the dominant physical processes in bio-oil flames, help designers optimize or retrofit burners to work with this fuel, and provide validation data for future model development. 2. Experimental Methodology

Figure 1. Main burner assembly.

2.1. Burner Assembly. Figure 1 shows an overall drawing of the main burner assembly. The air box and swirl blocks are constructed from aluminum and mild carbon steel since they are only exposed to air. All other burner sections downstream of the nozzle are constructed from 316 stainless steel to avoid any potential corrosion problems due to the acidic nature of the fuel. The combustion chamber walls are 3.2 mm thick and have no refractory lining. Important design aspects regarding the variable swirl generator, fuel nozzle, pilot flame, diffuser section, and extended exhaust section are discussed below. 2.1.1. Variable Swirl Generator. Figure 2 shows dimensions for the air intake, swirl generator, nozzle, and conical diffuser geometries of the burner. Heated primary combustion air is brought in through the insulated air box. From there, it is forced to go through a movable block type swirl generator.

Figure 2. Detail of swirl generator, nozzle, and burner inlet geometries (in millimeters).

(17) Andrews, R. G.; Zukowski, S.; Patnaik, P. C. Feasibility of firing an industrial gas turbine using a bio-mass derived fuel. In Developments in Thermochemical Biomass Conversion; Bridgwater, A. V., Boocock, D. G. B., Eds.; Blackie Academic & Professional: London, 1997; pp 495-506. (18) Strenziok, R.; Hansen, U.; K€ unstner, H. Combustion of bio-oil in a gas turbine. In Progress in Thermochemical Biomass Conversion; Bridgwater, A. V., Ed.; Blackwell Science: Oxford, U.K., 2001; pp 1452-1458. (19) L opez Juste, G.; Salva Monfort, J. J. Preliminary test on combustion of wood derived fast pyrolysis oils in a gas turbine combustor. Biomass Bioenergy 2000, 19, 119–128. (20) Solantausta, Y.; Nylund, N.-O.; Marten, W.; Kolijonen, T.; Oasmaa, A. Wood-pyrolysis oil as fuel in a diesel-power plant. Bioresour. Technol. 1993, 46, 177–188. (21) Solantausta, Y.; Nylund, N.-O.; Gust, S. Use of pyrolysis oil in a test diesel engine to study the feasibility of a diesel power plant concept. Biomass Bioenergy 1994, 7, 297–306. (22) Shihadeh, A.; Hochgreb, S. Diesel engine combustion of biomass pryolysis oils. Energy Fuels 2000, 14, 260–274. (23) Krumdieck, S. P.; Daily, J. W. Evaluating the feasibility of biomass pyrolysis oil for spray combustion applications. Combust. Sci. Technol. 1998, 134, 351–365. (24) Stamatov, V.; Honnery, D.; Soria, J. Combustion properties of slow pyrolysis bio-oil produced from indigenous Australian species. Renewable Energy 2006, 31, 2108–2121. (25) Nguyen, D.; Honnery, D. Combustion of bio-oil ethanol blends at elevated pressure. Fuel 2008, 87, 232–243. (26) Sequera, D.; Agrawal, A. K.; Spear, S. K.; Daly, D. T. Combustion performance of liquid biofuels in a swirl-stabilized burner. J. Eng. Gas Turbines Power 2008, 130. (27) Khodier, A.; Kilgallon, P.; Legrave, N.; Simms, N.; Oakey, J.; Bridgwater, T. Pilot-scale combustion of fast-pyrolysis bio-oil: ash deposition and gaseous emissions. Environ. Prog. Sustainable Energy 2009, 28, 397–403. (28) Zheng, J. L.; Kong, Y.-P. Spray combustion properties of fast pyrolysis bio-oil produced from rice husk. Energy Convers. Manage 2010, 51, 182–188.

This design imparts different degrees of angular momentum to the flow by varying the opening size of radial and tangential air passages. A general schematic of the block geometry and the corresponding parameter values for the swirl generator used in this study are given in Figure 3 and Table 1, respectively. When the tangential passages are completely closed off, all the air is forced through the radial inlets and down into the burner without any angular motion (zero swirl). At the other extreme, the tangential inlets are fully open and incoming air is imparted with the maximum amount of swirl for the given geometry. With this design, the degree of swirl may be continuously varied during burner operation by adjusting the opening angle between the radial and tangential inlets, ξ. Swirling flows are important to combustion systems for several reasons. First, if the swirl is strong enough, a central recirculation zone (CRZ) is established that recycles hot exhaust products back toward the base of the flame.29 This helps with ignition and overall stability. Second, swirling flows generate a great deal of turbulence and promote mixing between fuel and air.29 This helps to shorten the length and extend the lean blowout limit of the flame. Finally, because of recirculation, the hot residence time of gases and small particles is increased, leading to more thorough burnout of the fuel and reduced total hydrocarbon (THC), CO, and particulate matter (PM) emissions in the exhaust.30 Swirling flows may be characterized by the nondimensional swirl number, S. This number represents the ratio of angular (29) Gupta, A. K.; Lilley, D. G.; Syred, N. Swirl Flows; Abacus Press: Tunbridge Wells, Kent, England, 1984. (30) Beer, J. M.; Chigier, N. A. Combustion Aerodynamics; Applied Science Publishers Ltd: London, 1972.

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Figure 3. Geometry of a movable block type swirl generator. Figure 4. Internal mix nozzle tip assembly33 (not to scale).

Table 1. Geometric Parameters for Movable Block Type Swirl Generator parameter

description

value

B Rh R n ξm R

block thickness hub (nozzle) radius outer radius number of fixed blocks maximum opening angle tangential angle

38.1 mm 9.53 mm 76.2 mm 8 12° 60°

2.1.2. Fuel Nozzle. An internal mix, air-blast nozzle provided by BEX Engineering Ltd. (model 1/4 in. JX6BPL11 with a 152 mm extension tube and 2  2JPL back-connect body) is used to atomize the fuel. The entire assembly is constructed from 316 stainless steel. It is placed along the centerline of the burner and immersed to a depth that brings the initial droplets of the fuel-air spray very close to the pilot flame. As indicated in Figure 2, there are only 15.9 mm of clearance between the atomizer tip and the centerline of the pilot port. Figure 4 shows the internal construction and assembly of the nozzle tip. Fuel enters the internal mixing chamber through a single liquid cap orifice that is 1.0 mm in diameter. After mixing with air, the spray is forced through six evenly spaced air cap discharge orifices that are 0.89 mm in diameter. The overall spray pattern may be described as six individual fuel-air jets with a total angle of about 65° and a hollow or quiescent core. One important aspect of this particular pattern is that the center of the spray does not introduce any substantial axial momentum along the centerline of the flow. In fact, the burner was initially designed to work with a nozzle having one central discharge orifice that produces a full cone spray pattern. However, it was found that a stable bio-oil flame could not be achieved, even with ethanol (EtOH) blend concentrations of up to 50% by volume. Preliminary tests showed that changing the nozzle to yield the current spray pattern resulted in improved flame stability and lower THC emissions. It is well-known that centerline spray jets can introduce enough axial momentum to penetrate right through the internal recirculation zone of a swirling flow field.34-36 Therefore, the observed improvements to combustion were attributed to the formation of a CRZ in the hollow core region of the spray which did not likely exist in the original single fuel jet design. 2.1.3. Pilot Flame. The pilot flame system is comprised of an oxy-fuel torch body and standard no. 7 tip with a 1.2 mm orifice diameter (Hoke model no. 110-406). Although originally designed for oxygen and natural gas, the pilot is run using pure methane and produces a fully premixed CH4/O2 flame. The tip also has a hexagonal slit or “rosebud” pattern that surrounds the central orifice. The multiple flames issuing from these small openings produce a wider and more distributed overall flame shape than that which would issue from just a single orifice. The tubular body leading to the tip is sealed against the burner using a 6.4 mm bore-through compression fitting at the pilot flame port (see Figure 1). The tip itself is immersed to a radial depth of only 5 mm in order to avoid any solid body flow obstruction or spray impingement. The maximum flow rate of methane through the pilot is 0.88 standard liters per minute (SLPM), corresponding to an energy input of 0.5 kW.

momentum flux (Gφ) to axial momentum flux (Gx) at a given cross section of the flow field. The swirl number at the exit plane of the swirl blocks (see Figure 2) can be estimated using the following theoretical expression developed for movable block type geometries.30,31 "  2 # Gφ R Rh 1A S ¼ ð1Þ 2B Gx R R A ¼

2π cos R½1 þ tan R tanðξ=2Þðξ=ξm Þ sin R nξm f1 - ½1 - cos R ð1þtan R tanðξ=2ÞÞξ=ξm g2 ð2Þ

For the geometric parameters specified in Table 1, the current design is theoretically capable of generating flows with 0 e S e 5.41. However, it should be noted that this expression has been developed assuming idealized conditions, such as a uniform axial velocity distribution at the plane of interest.30 Furthermore, since the derivation does not include the contribution to axial momentum flux from the radial pressure distribution in the fluid, S is not conserved along the length of the burner.32 In fact, others have shown that S can decay by up to 35% in the constant diameter ducting between the swirl block exit plane and the diffuser inlet.31 In confined burner geometries, such as the one used herein, there is loss of both angular and axial momentum due to friction effects with the walls.32 Finally, the increased temperatures during combustion substantially reduce the density of gases within the swirling flow. For a fixed mass flow rate, this results in axial acceleration of the fluid and a further reduction in the actual swirl number compared to an analogous isothermal (cold flow) case.32 In light of this discussion, it is likely that the swirl number in the vicinity of the flame (i.e., at the 130 mm i.d. burner throat) is much lower than the theoretical value calculated at the exit plane of the swirl blocks. As a result, the swirl numbers reported here should only be used as a relative measure of swirl intensity at different conditions as they are not likely applicable to other swirling flows categorized by an experimentally measured S value.

(34) Altgeld, H.; Jones, W. P.; Wilhelmi, J. Velocity measurements in a confined swirl driven recriculating flow. Exp. Fluids 1983, 1, 73–78. (35) Weber, R.; Visser, B. M.; Boysan, F. Assessment of turbulence modeling for engineering prediction of swirling vortices in the near burner zone. Int. J. Heat Fluid Flow 1990, 11, 225–235. (36) Vanoverberghe, K. P.; Van Den Bulck, E. V.; Tummers, M. J. Confined annular swirling jet combustion. Combust. Sci. Technol. 2003, 175, 545–578.

(31) Fudihara, T. J.; Goldstein, L., Jr.; Mori, M. The three-dimensional numerical aerodynamics of a movable block burner. Braz. J. Chem. Eng. 2003, 20, 391–401. (32) Weber, R.; Dugue, J. Combustion accelerated swirling flows in high confinements. Prog. Energy Combust. Sci. 1992, 18, 349–367. (33) Catalog No. JPL99C, JPL Series Air Atomizing Nozzles, BEX Engineering Ltd.: Mississauga, ON, Canada.

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Figure 5. (a) Good and (b) poor pilot flame alignment configurations (view is looking up into the diffuser).

Figure 6. Overall schematic of experimental setup.

section, however, these external zones are minute or completely nonexistent and a large CRZ is allowed to form.32,35 2.1.5. View-Port and Extended Exhaust Section. The main section of the burner, between the diffuser outlet and the support table, is 305 mm long. A 3.2 mm thick quartz window is used to directly view the flame and monitor the quality of combustion during testing. After the support table, the burner extends another 508 mm to the exhaust outlet. The extension was originally designed to accommodate the possibility of very long bio-oil flames. This particular situation can occur for burners operating with a central fuel jet and no CRZ. If the fuel-air spray has enough momentum to penetrate the CRZ, the flame may actually stabilize as a long nonrecirculating jet.23,36 As previously mentioned, this type of flame configuration was not achievable and the subsequent use of the current six-jet, wide angle nozzle tends to confine the flame within the diffuser and view-port sections of the burner. However, despite shorter overall flame lengths, it is still possible to observe the burning of a few individual droplets and fuel residues well beyond the support table and into the exhaust extension section (see Figure 1). This section is also outfitted with various sampling ports. The last one is used to insert a borescopic probe for taking unobstructed photographs of the flame that look up along the central axis of the burner. 2.2. Overall Experimental Setup. Figure 6 shows the various inputs, outputs, and sampling lines for the burner. Fuel is dispensed using peristaltic pumps since the flow rates for 10 kW operation are very low (about 30 mL/min). A pressure relief valve is installed upstream of the nozzle in order to avoid the dangerous buildup of fuel line back-pressure. Some bio-oils

Although the position of the pilot flame is fixed by the location of the pilot port, an alignment procedure is employed whereby the burner is operated on pure EtOH and the nozzle is rotated until a stable and even spray flame configuration is achieved. The quality of alignment is judged visually through the quartz view-port. Figure 5a,b shows the position of the pilot flame relative to the fuel jets under both “good” and “poor” alignment conditions. It is obvious that good alignment refers to the condition for which all the fuel jets are evenly ignited by the pilot flame. A poor alignment occurs when the pilot preferentially ignites or impinges on one of the individual fuel jets. Once alignment is completed, the nozzle is locked into place and its position remains unchanged throughout the duration of the testing period. 2.1.4. Diffuser Section. Beyond the nozzle and pilot flame, the burner opens up into a conical diffuser with a half angle of 35°. The internal throat (inlet) diameter of the diffuser is 130 mm and the final burner diameter it expands to is 221 mm. Therefore, at an expansion ratio of only 1.7, the swirling flow inside the burner may be classified as being highly confined.35 A port at the bottom of the diffuser section is used to insert a sheathed thermocouple for taking near flame, gas temperature measurements. As a general guideline, Gupta et al.29 recommend that a conical diffuser with a half angle between 20 and 35° be used in order to promote the onset of a central recirculation zone (CRZ) with respect to a sudden expansion geometry. Others have also confirmed, at least in confined geometries, that the presence of a diffuser considerably increases the mass of fluid undergoing reverse flow.30,36 In a sudden expansion geometry, the external recirculation zones occurring in the corner regions just beyond the burner throat interact strongly with the CRZ and tend to limit its expansion.35 With a conical diffuser 5334

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Table 2. Liquid Fuel Properties of the Bio-Oil Used in This Study and Other Typical Wood Derived Bio-Oils property

test method (this study)

bio-oil (this study)

typical bio-oil (from literature)a

C-H-O-N (wt % dry) solids (wt %) ash (wt %) water (wt %) acidity (pH) LHV (MJ/L) density (kg/m3) kinematic viscosity at 40 °C (cSt)

ASTM D5291 MeOH-DCM insolubles ASTM D482 ASTM E203 ASTM E70-07 ASTM D240 N/A ASTM D445

55-6-38-0.5 0.15 0.17 28.5 2.6 17.3 1195 25.5

55-6-38-0.15b 0.2-1 0-0.3 15-30 2-3 15.6-21.6 1200 10-100

a

Taken from various literature sources.2,3,7,37,38 bAverage values.

content.38,39 The value reported in Table 2 is that of the “as received” fuel sample for which no subsequent liquid filtration process is employed prior to combustion testing. The ash content falls within the typical range but is slightly higher than that of the total solid content. Although much of the ash is likely bound up within solid char fines, some inorganic material may leach into the liquid and exist as dissolved mineral content.40,41 Water content is at the higher end for wood derived bio-oils and is primarily responsible for the fuel’s reduced heating value and degraded ignition quality. The low pH value leads to corrosion problems and requires wetted parts to be made from chemically inert materials such as high-density plastics and stainless steel. The reported viscosity is relatively low and is likely a consequence of the low solid and high water contents in the fuel. 2.3.2. Pyrolysis Liquid-Ethanol Blends. In this study, bio-oil is blended with 20% EtOH by volume (14 wt %). The primary reason for this is to increase the overall volatility of the fuel and promote flame stability. Although the EtOH content is relatively high, other researchers have also used this particular concentration in combustion studies.19,24 It should be noted that the burner is capable of operating at a blend of 10% EtOH by volume (6.8 wt %) and even 100% pure bio-oil. However, preliminary tests at these concentrations indicate that THC emissions become quite high (>150 ppm) and flame stability is poor, making it more susceptible to blow-out. Therefore, in order to accommodate a wider range of operating conditions, a higher concentration of EtOH was chosen to help stabilize combustion. Alcohol addition also has a beneficial effect on the storage stability of bio-oil. Some of the chemical compounds in pyrolysis liquid can interact with one another via polymerization and polycondensation reactions which “age” the fuel by increasing its viscosity over time, even at ambient storage conditions.42 Eventually, complete phase separation of the tar and aqueous fractions may occur.2 This process is highly sensitive to temperature and is considerably accelerated in the presence of heat. However, when alcohol is added between 5 and 10 wt %, the aging rate can be drastically reduced, delaying an equivalent change in viscosity of the pure bio-oil by up to a year at the higher concentration.43 Alcohols prevent aging reactions by several mechanisms:42,43 (i) dilution of the compounds responsible for aging, (ii) stabilization of the pyrolytic lignin fraction within the liquid (i.e., changing the ratio of polar/nonpolar compounds), and (iii) chemical reactions that lead to the prevention of polymerization (or the growth of high molecular weight compounds). MeOH has been identified as the

contain a significant amount of solid char particles that could potentially plug the liquid nozzle orifice while fuel is flowing. During testing, EtOH is used to warm up the burner and to flush the fuelling system/nozzle upon shut-down. Bio-oil is a thermally sensitive fuel, and if left stagnant in a hot environment, it may polymerize and clog the fuel-line.7 All fuel-line components are constructed from 316 stainless steel or Teflon tubing in order to ensure maximum chemical resistance. The primary combustion air is provided by a downstream stack fan so that the entire burner operates at a negative pressure of 170-350 Pa (depending on equivalence ratio) and exhaust leaks are avoided. Different primary air preheat temperatures are achieved by varying the voltage to a resistive heating unit that has a maximum power input of 1.5 kW. Atomizing air is provided from a compressed air source and metered to the nozzle using a pressure regulator and rotameter. Since the extended nozzle body sheath (which conveys fuel and air to the tip) has an exposed length of 113 mm beyond the exit plane of the swirl blocks (see Figure 2), both the atomizing air and liquid fuel are subsequently heated by the incoming primary combustion air. Figure 6 indicates that exhaust is tapped from the burner via a 6.4 mm diameter stainless steel heated sample line that maintains the gas at 190-195 °C in order to avoid water and hydrocarbon condensation. Before any gas phase speciation diagnostics are entered, the exhaust sample is passed through a heated glass microfiber/Teflon bounded filter element that removes most of the PM (95% retention efficiency at 0.03 μm particle size). The exhaust gases pass through a water cooled heat exchanger that condenses out most of the moisture prior to reaching the stack fan. This is done to reduce the temperature of gases reaching the fan and to monitor the amount of heat that can be extracted downstream of the main burner assembly. 2.3. Fuel Analysis. 2.3.1. Pure Pyrolysis Liquid. The bio-oil used in this study was pyrolyzed from a mixed hardwood feedstock. The most pertinent liquid fuel properties are summarized in Table 2 and compared to typical ranges of other wood derived bio-oils found in the literature. Elemental analysis indicates that the pyrolysis liquid contains very consistent amounts of carbon, hydrogen, and oxygen but a relatively large amount of fuel bound nitrogen. The solid content is determined using a specialized filter washing technique with methanoldichloromethane (MeOH-DCM) as the solvent. The pyrolytic lignin fraction strongly interacts and binds to the organic char fines within the liquid, and this particular solvent is capable of readily dissolving these high molecular weight tar compounds to provide a more accurate measurement of total solid

(40) Elliot, D. C. Water, alkali and char in flash pyrolysis oils. Biomass Bioenergy 1994, 7, 179–185. (41) Agblevor, F. A.; Besler, S. Inorganic compounds in biomass feedstocks. 1. Effect on the quality of fast pyrolysis oils. Energy Fuels 1996, 10, 293–298. (42) Diebold, J. P.; Czernik, S. Additives to lower and stabilize the viscosity of pyroylsis oils during storage. Energy Fuels 1997, 11, 1081– 1091. (43) Oasmaa, A.; Kuoppala, E.; Selin, J.-F.; Gust, S.; Solantausta, Y. Fast pyrolysis of forestry residue and pine. 4. Improvement of the product quality by solvent addition. Energy Fuels 2004, 18, 1578–1583.

(37) Boucher, M. E.; Chaala, A.; Roy, C. Bio-oils obtained by vacuum pyrolysis of softwood bark as a liquid fuel for gas turbines. Part I: properties of bio-oil and its blends with and pyrolytic aqueous phase. Biomass Bioenergy 2000, 19, 337–350. (38) Bridgwater, A. V., Ed. Fast Pyrolysis of Biomass: A Handbook, Vol. 3; CPL Press: Tall Gables, The Sydings, Speen, Newbury, Burks, UK., 2005. (39) Oasmaa, A.; Meier, D. Norms and standards for fast pyrolysis liquids 1. Round robin test. J. Anal. Appl. Pyrol. 2005, 73, 323–334.

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most effective compound for improving bio-oil liquid stability, although other alcohols, including EtOH, have a very similar effect. Besides increasing the overall volatility and promoting flame stability, the addition of EtOH reduces the viscosity and increases the heating value of the fuel. A shift in volatility and change in viscosity have very specific consequences for combustion. These details have been analyzed in the results section using the experimental techniques described below. 2.3.3. Thermogravimetric (TG) Analysis. Performing TG analysis on a liquid fuel sample provides detailed information about its volatility distribution and its solid residue formation/ coking potential.44 A TA Instruments model Q50 analyzer is used to measure the weight loss of a 15-20 mg sample of fuel at a constant heating rate of 10 °C/min. The sample is exposed to a continuous flow of nitrogen gas at 100 mL/min under atmospheric pressure. The tests begin at ambient conditions and finish at a maximum temperature of 600 °C. Since the amount of material used during these tests is very small, the fuel batches are thoroughly mixed to ensure that a homogeneous, representative sample is extracted and analyzed. 2.3.4. Viscosity-Temperature Measurements. Viscosity is one of the most important parameters controlling the atomization quality of liquid fuel sprays. The kinematic viscosity of the 80/20 bio-oil/EtOH blend is measured according to standard ASTM D445 at temperatures of 20, 40, 60, and 80 °C. This particular method uses a calibrated glass capillary tube and is suitable for opaque, Newtonian fluids. Other researchers have shown that the method produces accurate and repeatable viscosity measurements for a wide range of bio-oils.39 Special attention should be paid to any measurements made above 40 °C as bubble formation from the vaporization of lighter volatiles may affect the results. However, no bubbles were observed during the viscosity measurements at higher temperatures for any of the samples that were measured. 2.4. Combustion Diagnostics and Instrumentation. 2.4.1. Total Hydrocarbons. THC emissions are measured using a California Analytical Instruments model 600 flame ionization detector (FID). The analyzer works by passing exhaust through a small hydrogen-air flame where it burns and subsequently produces an ionized current proportional to the number of carbon atoms in the sample. The unit features a built-in sample pump that draws gas at a rate of 1.5 SLPM. The temperature of exhaust entering the FID is kept at 190-195 °C in order to avoid the condensation of heavier hydrocarbons.45 The instrument is calibrated using a pressurized cylinder certified to 104 parts per million (ppm) methane in nitrogen. The linear range of the FID is extrapolated to a maximum reading of 300 ppm. Single value results are reported as parts per million methane with an accuracy of (3 ppm. 2.4.2. Detailed Exhaust Gas Speciation. A Nicolet 380 Fourier transform infrared spectrometer (FTIR) is used to measure CO, NOx, CH4, formaldehyde (CH2O), acetaldehyde (C2H4O), CO2, and H2O emissions. The analyzer works by comparing the absorbance spectrum of a gas sample in the mid infrared region (500-4000 cm-1) against known standards. The FTIR gas cell has a 2 m long path length and a sample volume of 0.19 L. Each spectrum is acquired with 24 successive scans over 1 min at a wavenumber resolution of 1 cm-1. A partial least-squares (PLS) model employing gas mixtures of every compound considered is used to calibrate the instrument. The lower detection limit of each species is determined by evaluating the minimum concentration that produces a signalto-noise ratio of 4. The root mean squared error (RMSE) represents the deviation between the calibration model predic-

Table 3. Detection Limits and RMSE for FTIR Analysis Species Using a PLS Calibration Model species

units

low detection limit

high detection limit

RMSE

CO NOx CH4 CH2O C2H4O CO2 H2O

ppm ppm ppm ppm ppm % %

10 10 10 10 30 2 0.5

1500 300 250 150 150 15 15

25.5 6.3 3.1 1.7 5.2 0.19 0.12

tion and the actual concentration in a standard gas sample mixture. Details of the detection limits and calibration model accuracy are summarized in Table 3. The RMSE values may be used to put error bars on all the measured data. However, none of the errors reported in Table 3 are significant enough to cause ambiguity in any of the trends observed and are therefore not included in the results section. During an actual test, exhaust gas is continuously drawn through the gas cell by a vacuum pump at 10.3 SLPM and a pressure of 86.3 kPa. The gas cell temperature is maintained at 115-120 °C in order to avoid water condensation. The cell volume is refilled about 50 times during the collection of a single spectrum and is thus an averaged representation of the exhaust gas. Two consecutive spectra are acquired at each operating point of interest. The data presented in the results is thus an arithmetic average of the emissions values determined from each pair of spectra. In some instances, the standard deviation between two consecutive spectral measurements can exceed the instrumental error reported in Table 3. The ranges for the standard deviation of CO and NOx are 1-150 ppm and 1-12 ppm, respectively. The highest deviations occur for specific cases, typically corresponding to poor combustion conditions. When accounted for, these errors still do not affect the trends observed in the data and are not included in the emissions plots of the results section. 2.4.3. Oxygen Concentration. A Zirconia (ZrO2) model OXY6200 oxygen sensor from Engine Control and Monitoring (ECM) is used to continuously determine the % O2 in the exhaust. The sensor is calibrated to 21% oxygen (by volume) using ambient room air and is provided with 1.8 SLPM of sample. Since the sensor uses a high temperature element that can oxidize exhaust gas species such as hydrocarbons and CO, it is not placed in-line with the FID or FTIR. The real-time equivalence ratio during a test is calculated by using the measured % O2 value and the atomic composition of the fuel mixture and assuming complete combustion. It is difficult to include the measured CO2 and CO emissions in this methodology since they are not continuously monitored. However, the combustion efficiency is generally quite high (even at the worst case, CO only makes up 0.22% of the total exhaust volume) and back-calculating the equivalence ratio in this way is considered accurate. 2.4.4. Primary Air, Nozzle Sheath, and Fuel Temperatures. An exposed bead, J-type thermocouple with fiberglass insulated leads is used to measure the primary combustion air temperature. Figure 1 shows that the thermocouple itself is fed through the air box and positioned at the exit of the swirl block assembly. Since the electric preheat unit is located upstream of the air box, this particular thermocouple placement accounts for heat losses through the box and accurately conveys the temperature of air just prior to entering the main combustion zone. A bolt-on, J-type thermocouple, designed for measuring surface temperatures, is clamped to the outer sheath of the nozzle extension body. The sheath temperature is lower than the primary air temperature because relatively cool atomizing air flows through the inside of this tube on its way to the nozzle tip. Preliminary tests have shown that this surface temperature is within 10-20 °C of the actual temperature of air that exits the

(44) Garcia-Perez, M.; Lappas, P.; Hughes, P.; Dell, L.; Chaala, A.; Kretschmer, D.; Roy, C. Evaporation and combustion characteristics of biomass vacuum pyrolysis oils. IFRF Comb. J. 2006, 200601. (45) Adachi, M. Emission measurement techniques for advanced powertrains. Meas. Sci. Technol. 2000, 11, R113–R129.

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nozzle. The sheath measurement is therefore used as an estimate of the atomizing air temperature. To measure fuel temperature, a 305 mm long, J-type thermocouple is fed through the liquid passages of the nozzle. The probe is grounded to a bendable, 1.6 mm thick sheath made of 316 stainless steel. As shown in Figure 4, the tip of the thermocouple is positioned just prior to the 1.0 mm diameter liquid cap orifice and provides an accurate measurement of the fuel temperature just prior to entering the atomizer mixing chamber. 2.4.5. Near Flame Gas Temperature. A 457 mm long, K-type thermocouple is inserted at the bottom of the diffuser section in order to take gas temperature measurements (see Figure 1). The probe is grounded to a 3.2 mm thick sheath made of a special alloy (CHROMEGA-ALOMEGA-XL) that allows for continuous exposure at 1200 °C. The thermocouple is immersed at 5 radial distances between 1 and 21 cm so that a temperature profile across the entire inner diameter of the burner is measured (see the thermocouple immersion path in Figure 5a). A radial distance of 11 cm corresponds to the centerline of the burner assembly. The probe takes about 30 s to reach equilibrium at each immersion distance after which the temperature does not change. There are many errors associated with this type of temperature measurement. Some of these include radiation losses from the probe and conduction losses along the extended sheath.46 In addition, the 3.2 mm sheath is immersed through a 6.4 mm tube fitting and does not fully seal with the port. The tube fitting is only opened during the temperature measurement, but this method still introduces a certain amount of cold, room air dilution near the flame region. Because of these issues, the temperature profiles shown herein should only be considered as relative measurements. 2.4.6. Flame Visualization. The stability of bio-oil combustion is evaluated by monitoring pressure fluctuations in the burner but also by taking borescopic photographs of the flame under different conditions. The photographs help in understanding the qualitative trends in mixing and atomization quality as operating conditions are changed. Figure 7 shows a schematic of the flame visualization system used in this study. The probe itself is a 4 mm diameter Lenox Instrument Co. direct view borescope. It is essentially a rigid fiber-optic member that is coupled to a 10 megapixel digital camera. A 90° mirror tube is installed over the direct view member in order to produce an unobstructed, upward-looking view of the flame along the central axis of the burner. The optical assembly is installed along a rail-guide system so that the borescope may be rapidly and steadily inserted/removed from the burner. A 9.5 mm diameter, insulated tube is used to provide continuous cooling air to the probe whenever it is inserted. Cooling air is metered at about 250-300 SLPM and forms a high velocity jet around the optical components exposed to exhaust gas. This also helps to protect the components from any damage due to PM impingement. The borescope air is introduced far downstream of the combustion zone and therefore has no effect on the flame dynamics being observed. Prior to a test performed, the camera is set to maximum optical zoom and focused with the borescope installed. During a test, the borescope is inserted through a tube fitting on the end of the cooling sheath and sealed against a rubber gasket. The probe remains inserted for only 2-5 s, just long enough to take a single picture. Once the borescope is removed, the tube fitting is immediately capped since the cooling air is run continuously. The luminosity from the flames themselves provides sufficient light for clear pictures. Several photographs are taken at a single operating condition while the aperture opening, shutter speed, and ISO value (light sensitivity) of the camera are manually adjusted to achieve the best picture quality possible. No post

Figure 7. Borescopic probe assembly.

process image treatment (besides cropping and resizing) has been applied to any of the presented photographs. 2.5. Overall Test Plan. The objective of this particular study is to measure the gaseous emission trends of bio-oil/EtOH blend combustion with respect to the following variable burner parameters: (1) swirl number, (2) atomization air flow rate (spray quality), (3) pilot fuel flow rate (or energy throughput), (4) primary air and fuel preheat temperature, (5) oxygen content in the exhaust (or equivalence ratio). The observed trends are related to fundamental phenomenon such as atomization quality, evaporation, residence time, ignition, and fuel-air mixing in order to determine the dominant physical mechanisms that govern stability, blow-out, and the overall flame dynamics in bio-oil spray combustion systems. 2.6. Testing Procedure. Prior to a run with fuel, heated primary air is drawn through the burner until the desired steady state preheat temperature is reached. The EtOH is blended with bio-oil and thoroughly mechanically mixed at room temperature conditions just prior to testing. The burner is then warmed up with pure EtOH combustion for 15 min. Between the 15-20 min mark, the fuel is switched over to the 80/20 bio-oil/EtOH blend. Ethanol is an excellent solvent for bio-oil and is therefore an ideal start-up fuel, providing a smooth and continuous transition to bio-oil combustion. The bio-oil blend is allowed to run for an additional 30 min at the base point operating conditions shown in Table 4. This allows temperatures and emissions to reach a steady state before any operating parameters are varied. One parameter (i.e., swirl number, atomization air flow) is varied over the course of a single test while all other conditions are kept as constant as possible. After a parameter is changed, 5 min are allowed for the new operating point to stabilize prior to taking data. Once all the data is collected, the fuel is switched back to pure EtOH in order to flush the fuel-line system and nozzle. The nozzle tip is also mechanically cleaned and flushed with acetone between each test in order to eliminate the effect of accumulated solid deposits on atomization and burner performance. Borescope pictures are taken during a completely separate test because the cooling sheath air dilutes the burner exhaust and affects gas phase measurements. It should be noted that the steady state primary air temperature achievable using the maximum heat input of 1.5 kW and without combustion is 170 °C. However, during combustion, radiant energy from the spray flame increases the local temperature of the surrounding burner walls. This energy is eventually conducted to the air box and increases the air preheat temperature even further. After EtOH warm up and base point operation with the bio-oil blend for 30 min, the incoming primary air reaches the range reported in Table 4. Therefore, even for the case with no added preheat power, the minimum achievable primary air temperature for the system is 86 °C. Air preheat is also the only parameter that requires a completely separate test for every temperature considered. Because of the burner’s thermal inertia, it is difficult to set several

(46) Figliola, R. S.; Beasley, D. E. Theory and Design for Mechanical Measurements, 4th ed.; John Wiley & Sons: Hoboken, NJ, 2006.

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Table 4. Base Point Burner Operating Conditions parameter

value

fuel blend fuel flow swirl number (S) pilot methane flow pilot oxygen flow primary air preheat temperature O2 concentration in exhaust atomizing air flow primary combustion air flow

20% EtOH by vol 33.2 mL/min (10 kW) 5.41 0.88 SLPM (0.5 kW) 2.3 SLPM 236-249 °C 7.3% by vol 22.8 SLPM 255 SLPM

Table 5. Comparison of Bio-Oil Fuel Properties Blended with 20% EtOH by Volume fuel blend (% by volume)

LHV (MJ/L)

viscosity at 40 °C (cSt)

residual fraction (wt %)

100% bio-oil 80/20 bio-oil/EtOH

17.3 18.1

25.5 11.4

20.3 17.4

Figure 8. Kinematic viscosity of 80/20 bio-oil/EtOH fuel blend vs temperature.

different primary air temperatures within the duration of a typical test run. The data for each preheat temperature is achieved by simply warming the incoming primary air with different heat inputs (from 0 to 1.5 kW) after which an identical test procedure of EtOH combustion, base point blend combustion at the reduced preheat level, and steady state measurements is carried out.

3. Results and Discussion 3.1. Fuel Properties. 3.1.1. Heating Value and Viscosity. As previously discussed, the addition of EtOH to bio-oil changes the liquid properties of the fuel. Most notably, the lower heating value (LHV) is slightly increased and the viscosity is significantly reduced. Table 5 indicates that the addition of 20% EtOH by volume more than halves the viscosity of the fuel blend compared to pure pyrolsyis liquid at 40 °C. A reduction in viscosity effectively translates into improved atomization quality for the liquid spray. Smaller fuel droplets can undergo more thorough burnout for a given amount of residence time. This helps to increase combustion efficiency while simultaneously reducing THC, CO, and PM emissions.19,47 Typically, the kinematic viscosity of bio-oil has a very strong dependence on temperature. Other researchers have demonstrated that pure pyrolysis liquid can undergo a 73% decrease in viscosity when its temperature is increased from 20 to 40 °C.39 Figure 8 shows that the measured viscosity of the bio-oil/EtOH blend decreases by 64% over the same temperature range, indicating that this strong dependence is preserved even with the addition of alcohol. 3.1.2. Volatility and Distillation. EtOH addition also improves the overall volatility of the fuel. This effect can be quantified by the TG plots in Figure 9. The 80/20 bio-oil/EtOH blend shows a lower TG curve than pure bio-oil over the entire temperature range, indicating a higher evaporation rate and larger fully distillable fraction. These properties benefit the stability of heterogeneous combustion systems (liquid sprays) where the lean blow-out limit is not only governed by the heat released into the incoming fuel-air mixture but also by the time required for the fuel to evaporate.48 In the flame zone (assuming

Figure 9. TG curves for pure and blended bio-oil at a heating rate of 10 °C/min.

similar atomization quality), a fuel of low volatility tends to form localized regions with a lower equivalence ratio than the nominal value (overlean). The net result is a reduced rate of heat release that promotes the onset of flame extinction and blow-out at higher overall equivalence ratios than more volatile, fully distillable fuels. With the addition of EtOH to bio-oil, the lean blow-out limit of the fuel can be extended and stable combustion may be achieved for a wider range of primary air and atomizing air flow rates. Evidence that these stability mechanisms play an important role in bio-oil combustion is provided in the results from preliminary burner tests that used 10% and 0% EtOH volume fractions in the fuel. Recall that decreasing the EtOH concentration results in degraded combustion stability as the flame becomes more susceptible to blow-out. The differential thermogravimetric (DTG) plots shown in Figure 10 provide even further insight regarding the evaporation process of the fuel. For the 80/20 blend, the highest observable mass loss rate occurs between temperatures of 90-100 °C and corresponds to an obvious inflection point in the TG curve of Figure 9. This event is likely dominated by the evaporation of water and EtOH, which together make up 38.5% of the blend sample mass. Up to a temperature of 100 °C, other low molecular weight volatiles also continuously evaporate from the surface of the TG sample.49 After this particular event, however, there is a sharp decrease in the mass loss rate followed by a steady decline and pronounced peak at

(47) Moses, C. Fuel-specification considerations for biomass liquids. In Proceedings of the Biomass Pyrolysis Oil Properties and Combustion Meeting, Estes Park, CO, NREL-CP-430-7215, September 26-28, 1994; pp 362-382. (48) Ateshkadi, A.; McDonnel, V. G.; Samuelsen, G. S. Lean blowout model for a spray fired swirl-stabilized combustor. Proc. Combust. Inst. 2000, 28, 1281–1288.

(49) Branca, C.; Di Colomba, B.; Elefante, R. Devolatilization and heterogeneous combustion of wood fast pyrolysis oils. Ind. Eng. Chem. Res. 2005, 44, 799–810.

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average (ppm)

standard deviation (ppm)

THC CO NOx

7 374 288

2 47 8

mentioned, the stability of the fuel could have been improved even further by adding EtOH at the very beginning of the storage period. This was not practiced here because the final blend concentration was not chosen until the preliminary testing period was over (after 4 months). The properties reported in Table 2 were measured immediately after the fuel was produced and represent those of the fresh bio-oil. The pure and blended fuel properties discussed in section 3.1 were measured once all of the combustion tests were completed. In order to verify that the bio-oil samples (still single phase) did not undergo any significant physiochemical changes during storage, the viscosity at the end of the 6 month period was measured via ASTM D445 and found to be 28.6 cSt at 40 °C. This is a 12% increase compared to the freshly pyrolyzed fuel, denoting only a minor degree of aging. 3.2. Base Point Operation and Repeatability. At the beginning of every parametric test (except for those of varying preheat levels), the bio-oil blend is run up for 30 min at the base case operating conditions shown in Table 4. Parameters are changed only after the burner has reached steady state at these conditions. This procedure provides a way to validate the repeatability of burner operation. The statistical analysis of all separate base operating points is summarized in Table 6. The repeatability of base point emissions is always confirmed before proceeding with parameter variation and data collection. It should also be noted that all the separate base point CO2 emissions lie within 5-10% of what is expected from theoretically complete combustion at the measured % O2 level. As previously mentioned, the combustion efficiency is considered to be generally high, so much of the 5-10% deviation is likely caused by experimental error. Given this, the measured CO2 and CO values confirm that good combustion quality is achieved during base point operation of the burner. 3.3. Swirl Number (S). 3.3.1. Combustion Chamber Flow Patterns. Before discussion of the effect of swirl number on the characteristics and emissions of bio-oil spray flames, it is important to develop a basic understanding of the flow patterns occurring inside the combustion chamber. The overall physical situation is rather complicated since it involves the interaction between a multiphase, chemically reacting fuel spray and a turbulent swirling flow. Although the flow field inside the burner has not been measured, there are several studies that can provide considerable insight on the expected flow patterns.32,34-36,51 One in particular, the investigation by Weber and Dugue,32 is aimed at understanding the effect of nonpremixed combustion on swirling flows in highly confined geometries. The furnace used in their study also employs a conical diffuser inlet followed by a narrow cylindrical chamber. Their results indicate that, under isothermal conditions, the size (based on the zero streamline boundary) and strength (defined as the ratio of mass undergoing reverse flow to the

Figure 10. DTG curves for pure and blended bio-oil at a heating rate of 10 °C/min.

about 170 °C. The sharp drop in mass loss is due to polymerization of the sample surface. In fact, both polymerization and pyrolysis (cracking) processes take place as the low molecular weight compounds evaporate and the surface becomes enriched with nonvolatile pyrolytic lignin.7,44 This results in the formation of a “skin” residue that prevents the evaporation of volatiles found within the interior of the sample. As the temperature increases, heat diffuses inward where the accumulation and pressure buildup of internally trapped vapor occurs. The peak at 170 °C is caused by the eventual rupturing of this skin layer and the subsequent loss of these vapors from the sample. The DTG curve for 100% pure bio-oil exhibits many similar features with that of the 80/20 blend. The broad peak associated with the loss of water and low molecular weight volatiles occurs within a similar temperature range but reaches a lower value since there is no EtOH. The sharp peaks associated with skin rupturing events begin at lower temperatures, indicating that the onset of polymerization and cracking take place earlier than in the 80/20 blend. It is interesting to note that the skin formation, internal vapor accumulation, and rupturing phenomena just described are analogous to those that a single pyrolysis liquid droplet would undergo during the combustion process, save oxidation.6,7,44,50 In the case of incomplete burnout, bio-oil droplets tend to form solid cenospheric residues. Indeed, inspection of the TG samples after each test revealed that they exhibit a similar structure to these particles, albeit at a larger scale. The leftover residue is bloated to a larger volume than the original sample because of the internal buildup of trapped vapors. The material is mechanically brittle and the surface is covered in macropores that indicate where skin rupturing occurred.49 The mass of residual material that remains at the end of TG analysis is also summarized in Table 5. Since similar evaporation processes take place in spray flames, the measured residual mass is a good indicator of the fuel’s tendency to form cenospheric PM during combustion. EtOH obviously reduces this tendency, likely by simple dilution of the residual forming compounds. 3.1.3. Storage Stability and Aging. The total storage period for the bio-oil used in this study was 6 months. The first 4 months were used to perform preliminary burner tests while all the finalized data was acquired in the last 2 months. During this period, the pure pyrolysis liquid was kept in sealed, 2 L glass bottles and maintained at a temperature of 2-8 °C so as to retard the aging process. As previously

(51) Syred, N. A review of oscillation mechanisms and the role of the precessing vortex core (PVC) in swirl combustion systems. Prog. Energy Combust. Sci. 2006, 32, 93–161.

(50) Hristov, J.; Stamatov, V. Physical and mathematical models of bio-oil combustion. Atomization Sprays 2007, 17, 731–755.

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Figure 12. CO emissions for varying swirl number (S).

of the combustion chamber were not carried out, it is not likely that a strong PVC exists in the vicinity of the flame. This is due to several reasons: (i) PVCs are damped during nonpremixed combustion, (ii) the presence of a conical diffuser tends to reduce the amplitude of combustion oscillations, and (iii) the presence of a cross-flowing pilot flame right at the throat of the burner helps to stabilize the original vortex breakdown point and location of the CRZ.51 As previously discussed, the actual swirl number at the diffuser throat is likely much lower than 5.41 at base case conditions. In turn, the phenomena associated with strong swirl intensities should play a less dominant role in the burner chamber flow patterns. 3.3.2. THC and CO Emissions. Figure 12 shows that CO emissions increase from 400 to 560 ppm as swirl number is decreased from 5.41 to 2.98. This difference is attributed primarily to changes in the size and strength of the recirculation zone. As S is decreased, both of these diminish and a lower mass of hot combustion products is recycled back toward the base of the jets.29,30,32,36 This can reduce the localized temperature of the incoming fuel-air mixture and destabilize ignition of the spray flames. Figure 13a shows a photograph of the base case operating condition and demonstrates that all the fuel-air jets are ignited very close to the center of the spray. Figure 13b shows the combustion situation at S = 2.98 and indicates that the anchor points for some of the jets have moved further downstream. This behavior occurs despite the presence of a continuous pilot flame and denotes an overall degradation in ignition quality. With reduced recirculation zone strength, the amount of CO that gets recycled back into the main combustion zone and oxidized to CO2 is also decreased. At a swirl number of 1.46, there is a large jump in CO emissions to 1400 ppm. This increase is likely caused by a dramatic change of the flow patterns inside the burner. Figure 13c shows that a stabilized six-jet flame does not exist at this swirl number and that combustion is solely maintained by the pilot. A similar combustion mode persists for lower swirl numbers all the way down to zero. This step change in flame stabilization behavior suggests the loss of a CRZ. Without hot, chemically active gases recycled back toward the incoming fuel-air mixture, ignition of the jets is no longer promoted and combustion quality is severely degraded. However, compared to the reported values of 0.4 < S < 0.6, it seems unreasonable to associate the onset of a central recirculation region with a swirl number of 1.46. Considering the decay of S through the inlet duct and an even further reduction from axial flow acceleration during

Figure 11. Schematic representation of the flow patterns inside the burner during base point operation.

total mass flow) of the CRZ both increase with increasing swirl number. In fact, cold flows with high swirl numbers can exhibit recirculation zones that extend far beyond the diffuser section. Although this trend is preserved under combustion conditions, the recirculation zone shrinks substantially, forming an anchor point near the burner throat or bluff body (nozzle) and closing not very far downstream from the diffuser outlet. The study also concludes that the presence of a nozzle located at the throat of the burner increases the strength of the CRZ. However, others have reported an opposite trend,29,36 and it is difficult to determine whether or not the fuel nozzle is improving recirculation in the current burner. Despite the wide range of combustion situations considered in their work, Weber and Dugue showed that a CRZ consistently forms when the measured swirl number (not including the contribution from radial pressure distribution) is between 0.4 and 0.45 at the throat of the burner. This is in the same vicinity as other literature, which indicates a transition to reverse flow for S > 0.6.29,30 When a recirculation zone does exist and the flame front is located within or just beyond the diffuser section, combustion actually occurs along the boundary of the reverse flow region. A similar situation is likely in the current geometry since the spray angle is large and fuel is injected along the outer periphery of the combustion chamber. In light of these details, a representative diagram showing the location of the fuel spray, flame front and proposed recirculation zone boundary is given in Figure 11. The situation depicted is most indicative of the flow patterns expected at base case operating conditions (i.e., maximum swirl). These patterns would change dramatically with a decrease in S. In some situations, a weak, centralized region of forward axial flow may establish itself within the core of the CRZ.32,51 This phenomenon is most prevalent for strongly swirling flows and highly confined geometries. However, without detailed measurements or numerical modeling, it is difficult to ascertain the size and location of this flow regime in the current system. Strongly swirling flows in high confinements also allow angular momentum to persist far downstream of the combustion zone.34 This can cause a secondary vortex breakdown event near the exhaust outlet, resulting in the formation of a precessing vortex core (PVC).51 Although continuous pressure amplitude and frequency measurements 5340

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Figure 13. Borescopic images of bio-oil/EtOH flames at varying swirl numbers (S).

combustion, it is possible that the actual swirl number at the burner throat can reach a value as low as 0.4, making it consistent with the transition point to a reverse flow regime. Figure 12 also indicates that CO emissions drop off as S is decreased below a value of 1.46. Since stable combustion is not possible for these swirl numbers, it is likely that a large proportion of unburned carbon is bound up within fuel droplets that escape the flame zone and deposit along the inner surfaces of the burner. Even though the total amount of unburned fuel is likely increasing in this swirl number regime, the amount that is detectable via gas phase measurements decreases, causing the observed reduction in CO emissions. The THC emissions for varying swirl numbers are shown in Figure 14. These emissions are not measured at the same time as CO and yet they show the same trend, substantiating the phenomena discussed above. However, the values are much lower, only ranging between 8 and 28 ppm. Generally, THC emissions should be negligible for steady combustion systems. Exceptions occur when there is poor mixing which allows pockets of unburned fuel to directly escape the combustion zone or when reaction quenching takes place relatively quickly.52 With the absence of a stable flame below S = 1.46, it is likely that both of these mechanisms are playing a role. Both the THC and CO emissions show an identical 350% increase between their minimum and maximum reported values. This indicates that they exhibit a similar sensitivity to changes in swirl number. 3.3.3. NOx Emissions. NOx remains very close to 280 ppm between S = 1.46 and S = 5.41. Below a swirl number of 1.46, the emissions decrease only slightly to a value of 250 ppm. Varying S should produce a discernible decrease in NOx emissions since the entrainment of hot combustion products into the flame acts as a form of exhaust gas recirculation.36 The relatively constant NOx emissions observed over the entire range of swirl intensities suggests that the dominant formation mechanism is not thermal. The majority of NOx is likely due to conversion of the fuel bound nitrogen found in the bio-oil. However, total NOx is still a combination of that produced by thermal and fuel bound nitrogen mechanisms. Fuel NOx is insensitive to temperature,53,54 so the 30 ppm decrease observed

Figure 14. THC emissions for varying swirl number (S).

at lower swirl numbers is most likely due to lower temperatures and reduced thermal NOx production. 3.4. Atomizing Air Flow Rate. 3.4.1. Estimation of Mean Droplet Size. For air-blast or air-assist type atomizers, the mass flow of air through the nozzle has a direct effect on spray quality. In order to get a relative idea about how the droplet size distribution in the fuel spray varies with atomizing air flow rate, an appropriate Sauter mean diameter (SMD) correlation has been selected from the literature.55 " #  ðσm_ L Þ0:33 m_ L 1:70 1 þ SMD ¼ 0:95 FL 0:37 FA 0:30 UR m_ A !0:5   μL 2 do m_ L 1:70 þ 0:13 1þ ð3Þ σFL m_ A Several correlations (from the source55) developed for different nozzle types and geometries were investigated and compared. All of them yield a similar trend in the prediction of SMD over the entire range of atomizing air flow rates considered in this study. Equation 3 was ultimately chosen because it has been validated for a very wide range in both air flow conditions and fuel properties. The expression also takes on a general form that clearly shows the dependence of atomization quality on liquid density (FL), surface tension (σ), and dynamic viscosity (μL). Other important parameters for twin-fluid nozzles include the relative air-liquid velocity (UR) and the ratio of liquid-air mass flow rates (m_ L/m_ A).

(52) Flagan, R. C.; Seinfeld, J. H. Fundamentals of Air Pollution Engineering; Prentice Hall: Englewood Cliffs, NJ, 1990. (53) Williams, A. Combustion of Liquid Fuel Sprays; Butterworths: London, 1990. (54) Cooper, C. D.; Alley, F. C. Air Pollution and Control, A Design Approach; Waveland Press, Inc.: Prospect Heights, IL, 2002.

(55) Lefebvre, A. H. Atomization and Sprays; Taylor & Francis: New York, 1989.

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Table 7. Liquid Property and Atomizing Air Flow Parameters Used in the Calculation of SMD SMD correlation parameter

value

liquid density (FL) surface tension (σ) dynamic viscosity (μL) liquid mass flow (m_ L) air-liquid relative velocity (UR) liquid-air ratio (m_ L/m_ A) discharge orifice diameter (do)

1117 kg/m3 30 mN/m 3.39  10-3 Pa s 6.64  10-4 kg/s 79.1-191 m/s 0.98-2.5 1.0 mm

Table 7 summarizes the different liquid property and air flow ranges used in the calculation of SMD for the 80/20 biooil/EtOH blend. Viscosity is evaluated at 80 °C because the fuel temperature always exceeded this value during the parametric atomizing air flow test. The density of air (FA) is calculated using the ideal gas law and the nozzle sheath measurement which, as previously discussed, provides a good estimate of the actual atomizing air temperature. Although this density does not affect the total mass flow rate of air, it does strongly influence the relative air-liquid velocity inside the mixing chamber of the nozzle. The only liquid property not directly measured is surface tension. The constant value of 30 mN/m is taken from the literature56 and corresponds to the surface tension of a hardwood derived bio-oil at 80 °C. The effect of EtOH addition is not accounted for in this parameter. Figure 15 shows a plot of the calculated SMD at each individual atomizing air flow condition. 3.4.2. THC and CO Emissions. Figure 16 shows that THC emissions exhibit a strong dependence on atomizing air flow rate. As flow rates are reduced from the base condition of 22.8 SLPM, the SMD increases, primarily because of an increase in the liquid-air ratio in the nozzle. The larger droplets undergo less thorough burnout because they require a longer residence time to completely combust.47 Although the SMD trend in Figure 15 shows a smooth and steady trend in droplet size over the entire range of atomizing air flow rates, there is a sharp increase in emissions when the flow is reduced below 17.8 SLPM. In this regime, it is likely that another mechanism also contributes to increasing the emissions besides just changes in the mean droplet size. For such low atomizing air flow rates, the localized turbulence levels and mixing quality between fuel and air are probably reduced. This can lead to pockets of fuel that undergo partial burning or escape the main combustion zone completely, both of which would substantially increase THC emissions. Physical evidence that mixing quality reduces and droplet size increases at lower atomizing air flow rates is provided in Figure 17a. The image shows that jet flames become longer and wider than that of the base case condition in Figure 17b. Many more bright “streaks” may also be discerned from the low air flow case in Figure 17a. These streaks have been observed in other bio-oil combustion studies and are attributed to the individual burnout of polymerized char residues (nonvolatile content) that formed from the largest droplets within the fuel spray.23-25 Eventually, if the atomizing air is reduced below a value of about 10 SLPM, the atomization quality, droplet burnout, and fuel-air mixing become so poor that a stable flame cannot be sustained. Figure 16 also indicates that increasing the air flow from the base case

Figure 15. Calculated SMD of 80/20 bio-oil/EtOH fuel spray at varying atomizing air flow rates.

Figure 16. THC emissions for varying atomizing air flow rates.

condition gradually reduces THC emissions in step with the decrease in SMD. CO is plotted in Figure 18 and shows the same general trend for reductions in atomizing air flow rate. The phenomena of increased droplet size and poor mixing quality are also responsible for the increase in CO emissions at lower air flow rates.52 However, there is a marked difference in behavior as atomizing air is increased beyond the base case condition. Unlike THC emissions which gradually decrease in this regime, there is a discernible increase of 223 ppm in CO at the highest flow rate (33.9 SLPM). Clearly, another mechanism besides atomization quality is responsible. At higher atomizing air flows, the spray jet velocity increases (see range for relative air-liquid velocity in Table 7 as an indicator of this) and the flames themselves get anchored further downstream. Figure 17c substantiates this phenomenon by showing how the anchor points of some individual jet flames move away from the centerline of the burner and the pilot. The higher jet velocities increase flame shear and reduce the temperatures to which the incoming fuel-air mixture can be heated to. With further increases in the atomizing air flow, the flames become more and more detached as ignition quality continually degrades. Eventually, the heat release rate is not adequate enough to sustain combustion and a full blow-out condition is reached at an atomizing air flow rate of about 36 SLPM. THC emissions do not seem to be as sensitive to this process as CO. Perhaps smaller droplet sizes allow volatile content in the fuel to evaporate and convert into CO rather quickly, reducing the tendency for detectable THC emissions. Regardless, the upper bound imposed on the total

(56) Tzanetakis, T.; Ashgriz, N.; James, D. F.; Thomson, M. J. Liquid fuel properties of a hardwood-derived bio-oil fraction. Energy Fuels 2008, 22, 2725–2733.

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Figure 17. Borescopic images of bio-oil/EtOH flames at varying pilot energy throughput.

quality at this low flow rate is quite poor, leading to increased THC and CO emissions, as well as localized regions within the flame that are relatively depleted in oxygen. These fuel rich zones preferentially convert fuel bound nitrogen into N2, causing a substantial reduction in the total formation of NOx.52 This type of behavior also suggests that the NOx emissions in bio-oil spray flames may be controlled using a conventional staged combustion strategy typically employed for other fuels that contain nitrogen. 3.5. Pilot Flame Energy Throughput. 3.5.1. THC and CO Emissions. THC emissions do not exceed 6 ppm for pilot energies greater than 0.32 kW. Below this energy throughput, the emissions range between 10 and 30 ppm, including the case for which the pilot is completely turned off. Oxygen flow to the pilot is also changed along with the volumetric fuel flow rate so that they remain in the same proportions at different conditions. Below an energy throughput of 0.32 kW, the flame length of the pilot is substantially reduced and does not reach the centerline of the burner. The even ignition quality of all the spray jets becomes slightly degraded, showing up as a small shift in the THC emission range. CO emissions change steadily by a total of 318 ppm between the base case energy throughput (0.5 kW) and the condition for which the pilot is turned off. This means that although self-sustaining bio-oil/EtOH flames are achievable with the current burner design, there is a penalty to both THC and CO emissions without a continuous ignition source. The pilot is a premixed CH4/O2 flame and acts as a localized source of very high temperature gases that not only stabilizes ignition but also helps to oxidize CO. Figure 20a,b demonstrates that the presence of the pilot anchors the jet flames much closer to the nozzle tip (i.e., further upstream in the fuel-air spray). This increases the residence time of gases within the hot combustion zone and may be another mechanism by which CO emissions are reduced when using the pilot ignition source. 3.5.2. NOx Emissions. NOx is reduced by a total of only 15 ppm when the pilot is turned off. This difference may be attributed to the thermal NOx production from the pilot flame itself. Although there is likely a small decrease in the residence time of the bio-oil/EtOH spray for the no-pilot case, this is not expected to affect the production of fuel NOx within the flame. 3.6. Primary Combustion Air and Fuel Preheat Temperature. 3.6.1. THC and CO Emissions. Figure 21 indicates that the use of air preheat has a very strong effect on CO emissions, decreasing them by a factor of 3 for an increase

Figure 18. CO emissions for varying atomizing air flow rates.

Figure 19. NOx emissions for varying atomizing air flow rates.

amount of atomizing air that can be supplied to reduce SMD and improve combustion efficiency is governed by the lean blow-out limit of the fuel. A more volatile fuel would be expected to remain stable for a wider range of liquid-air mass flow rate ratios (m_ L/m_ A). 3.4.3. NOx Emissions. Figure 19 shows that NOx emissions range between 270 and 310 ppm for atomizing air flow rates higher than 17.8 SLPM. These emissions are scattered around the base point value of 280 ppm and show no discernible trend. The relatively stable NOx level observed throughout much of the air flow range further substantiates that the formation mechanism is dominated by the conversion of fuel bound nitrogen. However, there is a sharp reduction to 148 ppm once the atomizing air flow is brought down to 13.3 SLPM. As previously discussed, the mixing 5343

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Figure 20. Borescopic images of bio-oil flames at varying pilot energy throughput.

Figure 21. CO emissions for varying primary air preheat temperature.

Figure 22. Near flame burner temperature profiles at the diffuser exit plane for varying primary air preheat temperatures.

of 148 °C in temperature. A very similar trend is observed for THC emissions which range between 7 and 26 ppm at the highest and lowest preheat temperatures, respectively. There are a number of different mechanisms that are likely contributing to the observed changes in emissions with preheat temperature. Recall that the incoming primary combustion air heats the atomizing air and liquid fuel that both flow through the extended nozzle body. The atomizing air temperature (as measured by the nozzle sheath thermocouple) changes from 86 to 194 °C while the fuel temperature changes from 60 to 84 °C over the entire range of primary air preheat considered. The density of atomizing air is reduced at higher temperatures, leading to higher air-liquid relative velocities in the nozzle mixing chamber (note that the ratio of m_ L/m_ A is kept constant because the nominal flow rate is fixed at 22.8 SLPM for all preheat conditions). Higher fuel temperatures also decrease the viscosity of the bio-oil/EtOH blend. Both of these effects improve atomization quality in the spray as the preheat temperature is increased. When these mechanisms are incorporated into eq 3, the predicted range in SMD between the lowest and highest air preheat temperatures is 116-91 μm, respectively. This relatively small difference in atomization quality is likely not the only factor contributing to the observed changes in THC and CO. Raising the primary air temperature increases both the flame and exhaust gas temperatures, which promote overall oxidation and reduce the tendency for reaction quenching. Evidence of this effect is given in Figure 22 which shows that the near flame gas temperature measurements increase along with the degree of preheat. Recall that the sheathed thermo-

couple used to make these measurements is inserted at the outlet plane of the diffuser section. At this position, the spray jets are widely separated and the pathway of the immersed thermocouple passes between two individual flames at both ends of the burner (see Figure 5a). This results in relatively low temperatures compared to what is expected from direct thermocouple contact with the flame. Increasing both the primary and atomizing air temperatures also promotes the evaporation of the spray droplets, leading to more thorough burnout of the fuel and reduced THC and CO emissions. 3.6.2. NOx Emissions. As expected with a NOx formation mechanism dominated by fuel bound nitrogen, the emissions shown in Figure 23 change by only 30 ppm over the entire range of air preheat temperatures considered. Fuel NOx is rather insensitive to temperature, and the 30 ppm difference is likely due to changes in the thermal NOx contribution as air preheat; thus, flame temperature is varied. 3.6.3. Fuel Boiling and Combustion Stability. Figures 21 and 23 both indicate that a fuel boiling regime exists beyond the maximum primary air temperature of 235 °C. As previously discussed, high air preheat temperatures can lead to high liquid fuel temperatures. Typically, when the fuel temperature exceeds about 85 °C, random, spontaneous jumps to 95 or even 105 °C are observed in the reading. Preliminary tests with pure bio-oil and the 80/20 bio-oil/ EtOH blend indicate that the onset of boiling for these liquids (i.e., first signs of bubbles observed in a heated 15-20 mL sample) occurs at about 96 and 86 °C, respectively. The measured boiling point of the 80/20 blend closely 5344

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Figure 24. CO emissions for varying oxygen concentration in the exhaust (% by volume).

Figure 23. NOx emissions for varying primary air preheat temperature. Table 8. Relationship between Volumetric Oxygen Content in the Exhaust and Equivalence Ratio % O2 (by volume) measured in exhaust

corresponding equivalence ratio for 80/20 bio-oil/EtOH blend

9.4 7.3 5.5 3.6

0.5 0.6 0.7 0.8

matches the threshold at which fuel temperature fluctuations are observed and suggests that boiling is indeed the mechanism responsible. When such a condition exists, vapor can accumulate within the internal mixing chamber or fuel tube of the nozzle. Eventually, the sudden discharge of this fuel vapor causes a large, instantaneous increase in the flow rate and jet velocity through the nozzle that temporarily blows out the flame. In fact, if the pilot is not kept on as a continuous ignition source under such conditions, the flame is susceptible to complete blow-out. This phenomenon is generally characterized as “flashing”57 and will be discussed as it pertains to observed emission trends in section 3.8. Operating points with primary air preheat temperatures below that of the base (i.e., maximum) value do not exhibit any flashing induced flame blow-outs because the fuel does not reach its boiling point within the nozzle. The difference in boiling point temperature between pure pyrolysis liquid and the 80/20 blend suggests that the presence of EtOH promotes the onset of flashing induced combustion instability. This behavior is consistent with the fact that ethanol has a relatively low boiling point (78 °C) and that its addition increases the overall volatility of the fuel (see DTG curves in Figure 10). Since pure bio-oil has a higher boiling point than the 80/20 blend, decreasing the concentration of EtOH is expected to reduce the likelihood of fuel boiling and flashing at a given preheat temperature. 3.7. Exhaust Oxygen Content (Equivalence Ratio). 3.7.1. Note on Test Conditions. The overall equivalence ratio is adjusted by varying the total amount of primary combustion air provided to the burner. With this particular approach, mixing conditions and gas temperatures are also affected along with the concentration of oxygen measured in the exhaust. Table 8 shows all of the volumetric oxygen contents considered and their corresponding overall equivalence ratios. It is important to note that the burner test for this

Figure 25. THC emissions for varying oxygen concentration in the exhaust (% by volume).

parameter was conducted at a reduced level of preheat (1.11 kW) so as to avoid flashing instabilities, especially at lower total primary air flow rates. Even with taking such a precaution, the primary air temperature still varied between 198 and 222 °C and some instances of flashing were observed. 3.7.2. THC and CO Emissions. All the emissions data presented in this and the following sections are normalized to 4% O2 so as to remove the effect of dilution on the observed trends. Figure 24 shows that CO reaches a minimum of 461 ppm at the base point with 7.3% O2. Reducing the oxygen concentration below this value results in a steady increase in emissions to 642 ppm at 3.6% O2. A similar trend is shown for the THC emissions in Figure 25. The cases with lower oxygen concentrations have a lower total mass flow rate of air and reach higher overall combustion zone temperatures at a given level of preheat energy input, as indicated in Figure 26. These conditions are expected to inhibit quenching and promote oxidation, resulting in lower THC and CO emissions. The higher emissions are thus likely dominated by changes in mixing quality within the burner. Turbulence levels are reduced, and there is less thorough mixing between fuel and air at lower total air flow rates. Increasing the exhaust gas oxygen content to 9.4% also causes both THC and CO emissions to rise with respect to the minimum base point value. At this higher flow rate condition, improved turbulence and mixing quality are expected to reduce emissions. Although Figure 26 indicates that burner gas temperatures are the lowest at this point, there is only a 4 °C difference in primary air temperature compared to the case at 7.3% oxygen content in the exhaust. Once again, this

(57) Polanco, G.; Holdo, A. E.; Munday, G. General review of flashing jet studies. J. Hazard. Mater. 2010, 173, 2–18.

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Figure 26. Near flame burner temperature profiles at the diffuser exit plane for varying oxygen concentration in the exhaust (% by volume).

Figure 28. Primary air, nozzle sheath, and fuel temperatures during transient burner operation at base point conditions (Table 4).

Figure 27. NOx emissions for varying oxygen concentration (% by volume).

Figure 29. CO emissions during transient burner operation at base point conditions (Table 4).

indicates that another physical mechanism is likely responsible for the observed increase in emissions. Analogous to the behavior observed at high atomizing air flow rates, the additional primary air required to increase the total oxygen content subjects the spray flame to excessive shear, bringing it closer to lean blow-out. The tendency for localized flame quenching and the escape of unburned fuel from the main combustion zone both increase, leading to the observed increase in THC and CO emissions. Generally, it seems that the nonvolatile fraction within bio-oil tightly constrains the lean blow-out limit of the fuel and imposes a severe restriction on the amount of primary and atomizing air that can be supplied to improve combustion quality. 3.7.3. NOx Emissions. Figure 27 shows the NOx emissions for varying exhaust oxygen content. At 3.6% O2, the formation of NOx is inhibited by poor mixing quality and lower overall oxygen availability. As the primary air flow is increased, both the mixing rate and oxygen availability are sufficient enough to saturate the conversion of fuel bound nitrogen into NOx . This causes a leveling off in emissions, a type of behavior has been previously reported for other nitrogen containing fuels, including bio-oil.58 3.8. Transient Base Point Operation. 3.8.1. Burner Temperatures. Figure 28 shows the transient primary air, nozzle sheath and fuel temperatures during pure EtOH and bio-oil/

EtOH blend combustion under baseline operating conditions (Table 4). All these temperatures do not change substantially after the 50 min mark, substantiating that the burner reaches steady state operation around this time. Before combustion, the primary air and nozzle sheath temperatures are 170 and 145 °C, respectively. As previously discussed, these temperatures rise with combustion because of flame radiation effects. At the no combustion condition, the fuel temperature exceeds 100 °C because the nozzle passages are filled with stagnant air. Whenever there is a switch to liquid fuel, the temperature dips and then quickly stabilizes because of high heat transfer rates. 3.8.2. CO Emissions. Figure 29 shows the transient CO emissions under the same baseline operating conditions described above. After about 10 min of operation, pure EtOH combustion begins to exhibit large fluctuations in CO emissions. This occurs once the fuel exceeds its boiling point temperature of 78 °C. The subsequent flashing of vapor through the nozzle blows out the flame for a brief instant and registers as a sharp spike in CO. Because of the pilot however, the fuel spray is easily reignited and relatively continuous combustion is maintained. Fluctuations do not appear in the actual fuel temperature measurement shown in Figure 28 because the time interval between the data points is 5 min as opposed to 1 min for CO emissions. Once the fuel is switched over to the 80/20 blend, CO emissions initially increase and then generally tend toward the baseline value of about 400 ppm within the 50-60 min mark. The decrease in CO over the entire blend operating period is attributed to burner warm-up, which reduces reaction quenching, particularly in regions near the combustion chamber walls.

(58) Baxter, L.; Jenkins, B. Baseline NOx emissions during combustion of wood-derived pyrolysis oils. In Proceedings of Biomass Pyrolysis Oil Properties and Combustion Meeting, Estes Park, CO, NREL-CP-4307215, September 26-28, 1994; pp 270-280.

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emission of 50 ppm (including the pilot flame) and is primarily due to a thermal formation mechanism since the fuel contains no nitrogen. The contribution to total NOx caused by fuel bound nitrogen in bio-oil is therefore roughly 230 ppm. If all the nitrogen in the 80/20 blend were converted to NOx, the level measured in the exhaust at base point conditions would be about 635 ppm. The conversion efficiency is affected by the total amount of nitrogen within the fuel, overall equivalence ratio, and the degree of fuel-air mixing (i.e., local oxygen availability).52-54 Petroleum oil fired burners of various scales and with a comparable amount of fuel bound nitrogen (0.31 wt %, including dilution from EtOH) typically show a conversion efficiency of approximately 40-70%.52 At 230 ppm, bio-oil blend combustion at the base case seems fairly consistent with these observations, indicating that about 36% of the nitrogen content is converted to NOx. The conversion efficiency is likely limited to the lower threshold because of lower local oxygen availability compared to the experiments for which the reported range was determined. 3.9. CO Emissions for Small-Scale Burners. Generally, the CO emissions reported in this paper are high for a spray based burner. Other researchers have also reported high CO emissions for bio-oil combustion,7,11-13 but the values herein cannot be attributed to the quality of fuel alone. The 10 kW throughput of the burner is small, and it is well-known that smaller systems have shorter hot zone residence times that increase CO emissions.53 Furthermore, the burner is not lined with refractory. The absence of insulation increases heat losses and reduces temperatures in the burner, resulting in less effective CO oxidation. Although the absolute CO emissions are not totally indicative of a full-scale burner (the target for use with bio-oil being 0.2-1 MW38), the relative trends observed may still be extrapolated to industrial scale systems. 3.10. Methane and THC Emissions. Most of the parametric cases considered in this study show that THC emissions range between 5 and 30 ppm. The only case where the value of THC exceeds these bounds is for the minimum atomizing air flow rate of 13.3 SLPM (see Figure 16). As previously discussed, the mixing and atomization quality at this operating point are particularly poor, giving rise to THC emissions of 227 ppm. This is also the only operating condition that shows a detectable CH4 emission of 39 ppm. However, even for this poor combustion quality case, the total THC emissions are generally low compared to CO. The fuel that escapes complete combustion ultimately ends up as THC or CO emissions and may also form solid particulate matter. The results suggest that most of the unburned matter is partially converted to CO or forms solid residues. A possible explanation for this observation is that low to medium molecular weight volatiles in the aqueous fraction of the fuel evaporate first and burn off almost completely. The remaining higher molecular weight volatiles evaporate later and are partially converted to CO because of inadequate residence time at high temperatures. These compounds also likely interact with nonvolatile material in the fuel via polymerization and cracking processes that inevitably lead to the formation of PM. 3.11. Formaldehyde and Acetaldehyde Emissions. The use of oxygenated biofuels in combustion systems has led to an increased concern regarding aldehyde exhaust emissions. Both CH2O and C2H4O do not appear above their respective detection limits (see Table 3) throughout most of the steady state, parametric bio-oil tests. The only exception is the low

Figure 30. NOx emissions during transient burner operation at base point conditions (Table 4).

Throughout this transient however, there are still specific instances of CO fluctuations caused by flashing of the 80/20 blend. Near the end of the test, the pilot flame is gradually ramped down. The average CO measured in the no-pilot regime is 685 ppm, validating the previous observation that emissions increase in the absence of a continuous ignition source. It is important to note that flashing induced flame blow-out constitutes a separate mechanism of combustion instability than that encountered during the use of high primary or atomizing air flow rates. Although the physical process leading to excessive flame shear and the eventual blow-out event are similar, the actual root causes are different. In the case of flashing, it is fuel boiling and thus the presence of volatile compounds in the 80/20 blend (including EtOH) that leads to the high velocity discharge of vapor from the nozzle. This phenomenon can occur at base case operating conditions where the primary and atomizing air flow rates do not induce overall lean blow-out of the flame. The spray is easily reignited, and boiling within the nozzle can potentially be avoided by reducing the concentration of EtOH in the blend, cooling the liquid fuel, or increasing the liquid pressure inside the nozzle (i.e., the flashing problem is partly due to the near atmospheric pressures and high temperatures encountered in the atomizer mixing chamber as a result of low fuel flow rates). In the case of overall lean blow-out caused by high air flow rates, the fuel spray cannot be easily reignited, even in the presence of the pilot. This operational limit is governed by the portion of nonvolatile material within the fuel that cannot readily evaporate and mix with air in order to help sustain stable combustion. The lean blow-out limit may be extended to a certain degree by improving overall volatility via the addition of alcohol to the fuel blend, the opposite of what is required to avoid flashing. 3.8.3. NOx Emissions. Figure 30 indicates that EtOH produces a total of about 50 ppm NOx. After fuels are switched, the emissions change to a value of about 280 ppm within the span of 3 min and stay at that level throughout the remainder of the transient run-up. Although flame length and the hot residence time of gases likely increase during bio-oil blend combustion, it is difficult to attribute such a large jump in emissions to thermal NOx alone. Along with the observed insensitivity to temperature throughout the entire burner warm-up period, the transient results in Figure 30 further substantiate the dominant presence of fuel NOx. However, there is still a small contribution from thermal NOx that may be estimated from the pure EtOH warm-up results. Ethanol combustion shows a total NOx 5347

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atomizing air case discussed above, where CH2O emissions of 22 ppm are detected. About 10-20 ppm of formaldehyde and 40-50 ppm of acetaldehyde are also observed just prior to and during the transient test fuel switch from pure EtOH to the bio-oil blend. Prior to the fuel switch, these emissions are attributed to ethanol flashing within the nozzle and the subsequent combustion instabilities that ensue. Although the actual transition between fuels is very smooth, there are rapid and significant changes in the flame dynamics and combustion quality taking place that could be causing the increased emissions observed during the switch.

DCM = dichloromethane DTG = differential thermogravimetric EtOH = ethanol FID = flame ionization detector FTIR = Fourier transform infrared (spectrometer) i.d. = inner diameter ISO = international organization for standardization LHV = lower heating value MeOH = methanol NOx = NO þ NO2 o.d. = outer diameter PLS = partial least-squares PM = particulate matter ppm = parts per million volume PVC = precessing vortex core RMSE = root mean square error SLPM = standard liters per minute SMD = Sauter mean diameter SOC = start of combustion TG = thermogravimetric B = swirl block thickness do = discharge orifice diameter Gφ = axial flux of angular momentum Gx = axial flux of axial momentum m_ A = atomizing air mass flow rate m_ L = liquid fuel mass flow rate n = number of movable (or fixed) swirl blocks R = outer radius of movable block swirl block generator Rh = inner (hub or nozzle) radius S = swirl number UR = relative air to liquid velocity in nozzle R = tangential angle μL = liquid fuel viscosity ξ = adjustable opening angle ξm = maximum opening angle FA = atomizing air density FL = liquid fuel density σ = liquid-air surface tension

4. Conclusions The results discussed herein indicate that it is important to have good atomization, thorough mixing, and internal recirculation to promote the burnout of nonvolatile material and decrease CO and THC emissions. Air and fuel preheat are important for reducing CO emissions but only up to the point where subsequent fuel boiling is avoided in order to maintain spray flame stability. The amount of total primary air and atomizing air that can be used to improve turbulence, mixing, droplet burnout, and overall combustion quality is limited by the low volatility and narrow lean blow-out limit associated with bio-oil. The NOx produced in these flames is dominated by the conversion of fuel bound nitrogen. In order to reduce NOx formation without refining the fuel, the use of a staged combustion strategy would be needed. Acknowledgment. The authors thank Chris J. Green for designing and constructing the variable swirl generator, Prof. James S. Wallace from the University of Toronto for his helpful suggestions throughout the project, and Reza Rizvi for his assistance with the TG measurements. The authors also thank NSERC and Agriculture and Agri-Food Canada for funding this research.

Nomenclature ASTM = American society for testing and materials CRZ = central recirculation zone

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