ARTICLE pubs.acs.org/EF
Operational, Combustion, and Emission Characteristics of a Small-Scale Combustor A. S. Veríssimo, A. M. A. Rocha, and M. Costa* Mechanical Engineering Department, Instituto Superior Tecnico, Technical University of Lisbon, Avenida Rovisco Pais, 1049-001 Lisboa, Portugal ABSTRACT: This article examines the operational, combustion, and emission characteristics of a small-scale combustor. Flue-gas composition data and hydroxyl radical chemiluminescence (OH*) imaging are reported as a function of the excess air coefficient (λ), which in the present configuration implies also changes in the inlet air velocity. For two of these combustor operating conditions, spatial distributions of temperature and of O2, CO2, unburned hydrocarbons, CO, and NOx concentrations are also reported. The OH* images showed that as λ increases the main reaction zone moves progressively closer to the burner presumably due to the increase in the central jet momentum, which leads to a faster entrainment of fuel and burnt gases, and due to the increase in the oxygen concentration in the recirculated flue-gas. The OH* images also reveal that the structure of the main reaction zone and the combustion regime change with λ. For low values of λ the reaction zone is uniformly distributed over a relatively large volume of the combustor (flameless combustion, also known as MILD combustion, HiTAC, or colorless distributed combustion), whereas for high values of λ, the OH* images suggest and still photographs confirm the presence of a flame front located at the strong shear region between the central jet and the external recirculation zone (conventional lean combustion). The present combustor yields very low NOx (< 10 ppm @ 15% O2) and CO emissions (< 12 ppm @ 15% O2) for all conditions studied, which is attributed to the suppression of the thermal mechanism brought about by the flameless oxidation and conventional lean combustion modes. Finally, the detailed measurements made inside the combustor for the two operating conditions, a flameless oxidation condition and a conventional lean combustion condition, confirmed the observations based on the OH* images.
1. INTRODUCTION Flameless oxidation (FLOX),1 also called moderate or intense low oxygen dilution (MILD) combustion,2 high temperature air combustion (HiTAC)3,4 or colorless distributed combustion,5 is a combustion regime characterized by oxidation of the fuel in an atmosphere with relatively low oxygen concentration, due to previous mixing between the oxidizer and the combustion products, a distributed reaction zone instead of a thin flame front, relatively uniform temperatures, no visible flame, low noise, negligible soot formation, and very low NOx and CO emissions.1,6,7 This technology has been successfully applied in heating and heat-treating furnaces of the metal and steel industry8 and has potential for implementation into many other applications.9 This requires a better fundamental understanding of the flameless oxidation phenomena, which can be achieved through fundamental studies such as this one. In the past few years a number of articles presented experimental726 and computational6,2736 studies related to the flameless oxidation of various fuels. An early work of Plessing et al.10 carried out in a combustion chamber with highly preheated air and strong exhaust gas recirculation showed that flameless oxidation takes place in the well-stirred reactor regime, with the hydroxyl radical (OH*) concentration in the reaction zone being lower than in nonpreheated undiluted turbulent premixed flames. Subsequently, Weber et al.12 investigated the combustion of natural gas with high-temperature air and large quantities of flue-gas in a semi-industrial furnace. They reported comprehensive in-furnace combustion measurements and noticed that the furnace operated under conditions resembling a well-stirred r 2011 American Chemical Society
reactor, with almost all furnace volume filled with combustion products containing 23% oxygen, and inexistence of visible flame. In a separated study, Weber et al.16 used the same furnace to examine the fundamental and industrial application aspects of the combustion of natural gas, heavy and light fuel oils, and coal in highly preheated air. The authors concluded that the technology offers the potential of high furnace efficiencies, uniform heat flux distribution, and dramatic reductions in NOx, CO, and CO2 emissions and, thus, should be considered for future design of industrial furnaces. More recently, L€uckerath et al.21 investigated the flameless oxidation phenomena at high pressure (20 bar) in order to assess its applicability for gas turbine combustors. They demonstrated that low NOx emissions (< 10 ppm @ 15% O2) were achieved for excess air coefficients λ < 2.1, when the reaction zones were distributed relatively homogeneously over a large volume. For these conditions, the temperature distributions were also quite homogeneous. CO emissions were also low until the lean extinction limit of the flames was approached at λ ≈ 2.7. Dally et al.14 investigated the effects of the fuel mixture on the establishment of MILD combustion in a recuperative furnace that has the combustion air entering the burner preheated by the exhaust gases passing through a heat exchanger at the bottom of the furnace. They found that fuel dilution with CO2 or N2 reduces NOx emissions and makes the flame invisible. The Received: February 18, 2011 Revised: May 2, 2011 Published: May 03, 2011 2469
dx.doi.org/10.1021/ef200258t | Energy Fuels 2011, 25, 2469–2480
Energy & Fuels authors also found numerically that this dilution causes the stoichiometric mixture fraction to shift toward the rich side where the scalar dissipation is highest. This implies that premixing of the fuel stream with recirculated exhaust gases can have beneficial effects on the establishment of MILD combustion without the need for higher fuel jet momentum. Medwell et al.9 examined the structure of the reaction zone of a jet in a heated and diluted coflow, which emulates MILD conditions, using planar laser imaging techniques. They found that reducing the oxygen level leads to a suppression of OH* as a result of the reduced temperatures in the reaction zone. Associated with the drop in OH* levels the authors found a broadening of the OH* distribution. Li et al.18 also made use of measurements of OH chemiluminescence, along with measurements of exhaust gas species and velocity and temperature fields, to examine this combustion mode in a gas turbine combustor operating at atmospheric conditions. They found that the flameless combustion mode occurred only for a limited range of conditions at fuel lean conditions, high preheat temperature, and high air flow rates. The transition to this combustion mode from conventional combustion was gradual and a definite transition point cannot be well-defined. The authors observed that high air mass flow rates helped in promoting mixing and strong reaction resulting in a high temperature field, thus higher NO level, when compared with lower flow rate at the same excess air coefficient. However, at the same flame temperature, high air flow rate formed less NOx because of the more evenly distributed flame. Szeg€o et al.22 examined the performance and stability characteristics of a parallel jet MILD combustion burner system in a recuperative furnace. They found that a certain fuel jet momentum threshold was needed to achieve MILD conditions. This momentum ensured the penetration of the fuel jets to a region classified as the oxidation zone. Very recently, Mi et al.23 reported an investigation on the importance of the initial airfuel injection momentum rate and the airfuel premixing on the MILD combustion in a recuperative furnace. They observed that various patterns of partially and fully premixed reactants work extremely well in the configuration studied. Furthermore, the authors concluded, numerically, that there is a critical momentum rate of the inlet fuelair mixture below which the MILD combustion cannot occur. Also, they found, both experimentally and numerically, that, above this critical rate, both the momentum rate and the inlet fuelair mixedness affect only marginally the stability of and emissions from the MILD combustion. The effect of the initial airfuel jet momentum on the establishment of MILD combustion has been studied both experimentally and numerically in various combustor configurations, evidencing the threshold below which MILD combustion cannot occur not only for gaseous hydrocarbon fuels but also for highly reactive fuels, as hydrogen-containing fuel mixtures.19,20 This article examines the operational, combustion, and emission characteristics of a small-scale combustor, which is able to work under flameless and conventional combustion modes. Measurements of flue-gas composition and hydroxyl radical chemiluminescence (OH*) were performed as a function of the excess air coefficient, which in the present configuration implies also changes in the inlet air velocity. For two of these combustor operating conditions, spatial distributions of temperature and of O2, CO2, CO, unburned hydrocarbons (HC), and NOx concentrations were also carried out. This work intends not only to extend the current understanding of the chemical and physical processes that occur under flameless oxidation conditions,
ARTICLE
Figure 1. Schematic of the combustor.
but it intends also to provide data of special value to modelers for the critical evaluation and development of advanced turbulence and combustion models.
2. TEST FACILITY AND EXPERIMENTAL METHODS Figure 1 shows a schematic of the combustor used in this study. The combustion chamber is a quartz-glass cylinder with an inner diameter of 100 mm and a length of 340 mm. During the tests, the quartz cylinder was well-insulated with a 30-mm-thick ceramic fiber blanket. The burner is placed at the top end of the combustion chamber and the exhaustion of the burned gases is made by the bottom end through a convergent nozzle with a length of 150 mm and an angle of 15°. As seen in Figure 1, the burner consists of a central orifice of 10-mm inner diameter, through which the combustion air is supplied, surrounded by 16 small orifices of 2-mm inner diameter each, positioned on a circle with a radius of 15 mm, for the fuel (methane) supply. The combustion air is preheated by an electrical heating system that allows inlet air temperatures up to 700 °C, which are monitored by a type K thermocouple installed at the entrance of the burner. Local mean temperature measurements were obtained using 76-μm-diameter fine wire platinum/platinum: 13% rhodium (type R) thermocouples. The hot junction was installed and 2470
dx.doi.org/10.1021/ef200258t |Energy Fuels 2011, 25, 2469–2480
Energy & Fuels
ARTICLE
Table 1. Test Conditionsa inlet air velocity (m/s)
inlet air momentum (N)c
residence time (s)d
flue-gas temperature (°C)
1.1
96.2
0.38
0.153
1094
1.3
113.2
0.52
0.131
1060
3
1.5
126.5
0.65
0.115
1035
4e
1.7
143.0
0.83
0.102
992
5
1.9
162.8
1.08
0.092
965
6
2.1
178.6
1.30
0.084
936
7
2.2
184.8
1.39
0.080
931
run
excess air coefficientb
1 2e
For all conditions: fuel thermal input = 10 kW, inlet fuel velocity = 6.2 m/s, inlet air temperature = 400 °C. b Excess air coefficient, λ = actual air-fuel ratio/stoichiometric air-fuel ratio. c Inlet air momentum = air mass flow rate inlet air velocity. d Residence time = combustion chamber volume/ reactants volume flow rate. e Conditions for which measurements of local mean gas temperatures and local mean major gas species concentrations throughout the combustor have been carried out. a
Figure 2. Mean OH* images at the combustor symmetry plane for various excess air coefficients: (a) run 1 (λ = 1.1), (b) run 2 (λ = 1.3), (c) run 3 (λ = 1.5), (d) run 4 (λ = 1.7), (e) run 5 (λ = 1.9), (f) run 6 (λ = 2.1).
supported on 350-μm wires of the same material located in a twin-bore alumina sheath with an external diameter of 5 mm. The uncertainty due to radiation heat transfer was estimated to be less than 5% by considering the heat transfer by convection and radiation between the thermocouple bead and the surroundings. Sampling of the gases for the measurement of local mean O2, CO2, HC, CO, and NOx concentrations was achieved using a stainless steel water-cooled probe, which has been designed to minimize the major sources of uncertainty in the concentration measurements inside the combustor, namely the quenching of chemical reactions and the aerodynamic disturbances of the flow.37 The probe was composed of a central 1.3-mm inner diameter tube through which quenched samples were evacuated. This central tube was surrounded by two concentric tubes for probe cooling. The gas sample was drawn through the probe and part of the system by an oil-free diaphragm pump. A condenser removed the main particulate burden and condensate. A filter and a drier removed any residual particles and moisture so that a constant supply of clean dry combustion gases was delivered to the analyzers through a manifold to give species concentration on a dry basis. The analytical instrumentation included a magnetic pressure analyzer for O2 measurements, a non-dispersive infrared gas analyzer for CO2 and CO measurements, a flame ionization detector for HC measurements, and a chemiluminescent analyzer
for NOx measurements. Quenching of the chemical reactions was rapidly achieved upon the samples being drawn into the central tube of the probe due to the high water cooling rate in its surrounding annulus—our best estimate indicated quenching rates of about 107 to 108 K/sec. No attempt was made to quantify the probe flow disturbances. On average, the repeatability of the gas species concentration data was within 10%. Flue-gas composition data, obtained at the beginning of the exhaust duct, were obtained using the procedures described above for the concentration measurements inside the combustor. At the combustor exit, probe effects were negligible and errors arose mainly from quenching of chemical reactions, which was found to be adequate. Repeatability of the flue-gas data was, on average, within 5%. Both the temperature and the gas species probes were inserted into the quartz-glass combustion chamber through holes made on the bottom end of the combustor. The analog outputs of the thermocouple and of the analyzers were transmitted via A/D boards to a computer where the signals were processed and the mean values were computed. The OH* images were collected on an ICCD camera (FLAMESTAR II, LaVision, 286 384 pixels), equipped with an UV lens (UV Nikkor, 105 mm, f/4.5) and a bandpass interference filter (Melles Griot) centered at 310 nm with a bandwidth of 10 nm. To eliminate the dark signal, background 2471
dx.doi.org/10.1021/ef200258t |Energy Fuels 2011, 25, 2469–2480
Energy & Fuels
ARTICLE
Figure 3. Typical appearance of the combustion modes as viewed along all length of the combustion chamber for various excess air coefficients: (a) run 1 (λ = 1.1), (b) run 2 (λ = 1.3), (c) run 3 (λ = 1.5), (d) run 4 (λ = 1.7), (e) run 5 (λ = 1.9), (f) run 6 (λ = 2.1).
images were taken with capped camera, under the same integration time and gain of the measured images, for subsequent subtraction from the original measured images. In all chemiluminescence experiments, 500 single instantaneous images were recorded and averaged. The signal was detected with an exposure time of 10 μs. The signal-to-noise ratio of the instantaneous images was better than 8:1 for excess air coefficients e1.6 and 12:1 for excess air coefficients >1.6. For the calibration of the spatial resolution and depth of field, an object with known dimensions was placed inside the combustion chamber, in front of the ICCD, and moved along the camera lens axis in a range of 100 mm. This was made to verify if the size of the image from the object viewed by the ICCD was the same regardless its position. The size of the image of the object captured by the ICCD showed maximum differences of about 1 pixel for different positions of the object in the 100-mm interval. Since each pixel has about 10 μm, the deviation error was negligible. Each image obtained, which maintains the spatial resolution, showed an area of 90 105 mm2. Repeatability of the photometric data was, on average, within 5%. The ICCD camera collected the signal from the entire combustion chamber so that the signals were spatially integrated in depth. To obtain local information instead of line-of-sight information, each averaged OH* image was subsequently tomographically reconstructed using the inverse transform of Abel.38 The results of this procedure are presented in the following section.
3. RESULTS AND DISCUSSION Table 1 summarizes the test conditions used in this study. Methane (purity 99.5%) was used as fuel. Flue-gas measurements were obtained for all runs listed in Table 1, OH* imaging was carried out for runs 1 to 6 and detailed measurements of local mean gas temperatures and local mean major gas species concentrations were performed for runs 2 and 4. Before examining the data, a note concerning the nature of the flow in the present combustor will be of use in understanding the discussion of the results presented below. The note is based on the fundamental knowledge gained from the literature and from the present results. In brief, the momentum of the central air jet is large enough to generate a strong reverse flow zone that recirculates hot flue-gas back toward the near burner region so that combustion will take place with a relatively low oxygen
Figure 4. NOx and CO emissions as a function of the excess air coefficient.
concentration in the oxidizer. This means that the establishment of conditions for flameless oxidation to occur in the present configuration is largely determined by the momentum of the central air jet (the inlet fuel velocity is much lower than the inlet air velocity—see Table 1). Note that the 16 fuel jets are directly injected into the recirculation zone, which will also contribute to achieve flameless oxidation conditions. In this configuration, however, one would expect that operation with high excess air levels may lead to departure from flameless combustion due to the mixing between the fuel jets and the hot flue-gas, in this case, with a relatively high oxygen concentration. Figure 2 displays mean OH* images at the combustor symmetry plane for various excess air coefficients (runs 1 to 6 in Table 1). It should be noted that, due to the short lifetime of OH*, the chemiluminescence originates only from the reaction zone so that this technique yields information about the position and size of the reaction zone.39 At λ = 1.1 (run 1), Figure 2a, the main reaction zone, as typified by the OH* distribution, is located between z ≈ 150 and 250 mm, and is uniformly distributed around the combustor axis. As the excess air coefficient (and, consequently, the inlet air velocity and the initial central air jet momentum) increases, the main reaction zone moves progressively closer to the burner: specifically, at λ = 1.3 (run 2), Figure 2b, the main reaction zone is located between z ≈ 110 and 210 mm; at λ = 1.5 (run 3), Figure 2c, between z ≈ 95 and 200 mm; at λ = 1.7 (run 4), Figure 2d, between z ≈ 60 and 160 mm; at λ = 1.9 (run 5), Figure 2e, between z ≈ 50 and 130 mm; and at λ = 2.1 (run 6), Figure 2f, between z ≈ 40 and 2472
dx.doi.org/10.1021/ef200258t |Energy Fuels 2011, 25, 2469–2480
Energy & Fuels
ARTICLE
Figure 5. Contours of temperature and O2, CO2, HC, CO, and NOx concentrations at the combustor symmetry plane for runs 2 (λ = 1.3) and 4 (λ = 1.7).
120 mm. This shift is presumably due to the increase in the central jet momentum with the increase in the excess air, while the fuel jet momentum remains constant, which leads to a faster entrainment of fuel and burnt gases, and also due to the increase in the oxygen concentration in the recirculated flue-gas. Note that in the present investigation the diameter of the air injection port is kept constant so the entrainment ratio is expected to be similar regardless of the air velocity. The faster entrainment combined with the higher oxygen concentration in the recirculed flue-gas may result in earlier inception of the reaction in the combustion chamber. Apart from the differences in the location of the main reaction zone, Figure 2 also reveals that the structure of the main reaction zone changes as the excess air increases. The chemiluminescence images show that for λ > 1.5 the reaction zone becomes less evenly distributed, with much less intense or even no reaction
around the combustor axis. In fact, at λ = 1.7, 1.9, and 2.1 (near the lean extinction limit), Figure 2df, the OH* distributions suggest the presence of a flame front located at the strong shear region between the central jet and the external recirculation zone. Figure 3 shows the appearance of the combustion mode for runs 1 to 6 as viewed along all length of the present combustion chamber. It is seen that the flame is hardly visible in runs 1 and 2 (flameless combustion), the flame is visible in a small region in run 3 (transition from flameless to conventional combustion), and the flame is visible in runs 4 to 6 (conventional lean combustion). It is concluded that in the present configuration flameless conditions occur only for low excess air levels. As mentioned above, for high excess air levels hot recirculated gases with relatively high oxygen concentrations mix with the fuel jets creating conditions for the establishment of conventional (lean) combustion. Moreover, Figures 2 and 3 2473
dx.doi.org/10.1021/ef200258t |Energy Fuels 2011, 25, 2469–2480
Energy & Fuels
Figure 6. Axial profiles of mean gas temperature and gas species concentration for run 2 (λ = 1.3).
Figure 7. Axial profiles of mean gas temperature and gas species concentration for run 4 (λ = 1.7).
suggest that the transition from flameless to conventional combustion in the present combustor is gradual, as also observed by Li et al.18 Figure 4 shows the NOx and CO emissions as a function of the excess air coefficient (λ). HC emissions were not detected for any of the test conditions. As can be observed in Figure 4 very low emissions were measured: specifically, NOx emissions below 10 ppm @ 15% O2 and CO emissions below 12 ppm @ 15% O2, regardless of the value of λ. In addition, Figure 4 reveals that the NOx emissions increase as the excess air coefficient increases up to λ = 1.7, beyond which they level off. This seems to contradict what one could expect since the adiabatic temperature decreases
ARTICLE
Figure 8. Contours of temperature variance (T*) at the combustor symmetry plane for runs 2 (λ = 1.3) and 4 (λ = 1.7).
as λ increases, which should originate lower NO formation via the thermal mechanism. The lower NOx emissions observed for runs 1 and 2 can be attributed to the suppression of the thermal mechanism brought about by the flameless oxidation regime, while the values and evolution of the NOx emissions beyond λ > 1.7 are consistent with those expected for conventional lean combustion—in this case the high excess air levels should also minimize the NO formation via the thermal mechanism and promote the formation of NO via the N2O intermediate mechanism. It is seen that the CO emissions increased for λ > 1.9—this is because the lean extinction limit was approached at λ ≈ 2.2, which is consistent with the data of L€uckerath et al.21 obtained at high pressure (20 bar). Figure 5 shows the contours of temperature and O2, CO2, NOx, HC, and CO concentrations at the combustor symmetry plane for runs 2 and 4 (see also Figures 2b,d and 3b,d). In Figure 5, the left-hand side of each illustration corresponds to run 2 and the right-hand side corresponds to run 4. In the experiments, radial profiles of temperature and gas species concentration were measured at 10 positions (radial distance r = 0, 5, 10, 15, 20, 25, 30, 35, 40, and 45 mm) at 10 axial distances (axial distance z = 11, 45, 79, 113, 147, 181, 215, 250, 280, and 310 mm from the burner), as schematically represented on the top left of Figure 5. These data can be found in Table A1 for run 2 and Table A2 for run 4 in the Appendix. To help the discussion that follows, Figures 6 and 7 show the profiles of mean gas temperature and concentrations of O2, CO2, NOx, HC, and CO along the axis of the combustor also for runs 2 and 4, respectively. Note that in Figures 57 the HC, CO, and NOx concentrations are reported as measured. Recall that combustion has been recognized in run 2 as flameless combustion and in run 4 as conventional combustion. Consistently with the OH* images (see Figure 2b and d), Figures 6 and 7 reveal immediately that a major difference between runs 2 and 4 is the location of the main reaction, which has shifted upstream for run 4, as typified by the temperature profiles. It is interesting to observe that the on-axis measured 2474
dx.doi.org/10.1021/ef200258t |Energy Fuels 2011, 25, 2469–2480
Energy & Fuels temperature maximum is about the same in both runs (≈ 1500 °C), with the levels of HC and CO in run 2 being slightly higher than those in run 4 because of the lower excess air level. Figure 5 reveals that the temperature fields are consistent with the main reaction zones identified through the OH* images (see Figure 2b and d), where the temperature reaches its maxima values around 1530 °C for run 2 and 1510 °C for run 4. The main reaction zone of run 2, located between z ≈ 110 and 210 mm, presents lower temperature gradients across the combustor than that for run 4, located between z ≈ 60 and 160 mm. To assess the uniformity of the temperature field, Figure 8 shows the contours of the temperature variance (T*) at the combustor symmetry plane for runs 2 and 4. The temperature variance T* is here defined as: _ jTmeasured T j _ ð1Þ T ¼ T _ where Tmeasured is the measured local temperature and T is the _ average of all measured _local temperatures in each run (T = 1240 °C for run 2 and T = 1218 °C for run 4). Figure 8 reveals that the reaction zone, as typified by the values of T*, is more homogeneous for run 2 than for run 4. It is interesting to note that in run 4 the deviation from homogeneity occurs only in a very small region, near the combustor axis, due to the presence of a flame front (see Figure 3d). In contrast, in run 2 the deviation from homogeneity is less marked and distributed over a larger region, as expected when operating under flameless oxidation conditions. It should be pointed out that a number of authors evidenced experimentally this behavior with different gaseous1,7,12,13,19,33 and solid16 fuels and, very recently, also with liquid and more reactive fuels.26 Figure 5 also reveals that the chemistry fields are consistent with the main reaction zones identified through the OH* images (see Figure 2b and d). In both runs (runs 2 and 4), as expected, the highest concentrations of HC appear in front of the fuel entrance orifices (Figure 5), but its presence along the centerline of the combustor up to axial distances beyond z = 200 mm (Figure 6) for run 2 reveals slower combustion for this condition than for run 4 where the presence of HC is noticeable up to z ≈ 140 mm (Figure 7). Consistently with the location of the main reaction zone, as identified by the OH* imaging, the highest CO concentrations appear in the region 110 < z < 210 mm (Figure 5) for run 2 and 60 < z < 160 mm (Figure 5) for run 4, with a maximum of about 1% being reached at z ≈ 150 mm and r = 0 (Figure 6) for run 2 and of about 0.7% being reached at z ≈ 110 mm and r = 0 (Figure 7) for run 4. Nonetheless, at the combustor exit CO emissions were ≈4 and 0 ppm @ 15% O2 for runs 2 and 4, respectively, with no HC emissions detected for neither case, as referred to earlier. The NOx concentrations in both runs are low throughout the entire combustion chamber (Figure 5), which is consistent with the comparable NOx emissions; specifically, 6.7 and 8.7 ppm @ 15%O2 for runs 2 and 4, respectively. The NOx measured profiles displayed in Figures 6 and 7 show a progressive increase of the concentrations along the combustor axis reaching a steady value very close to the combustor exit (z = 340 mm). This suggests that some of the slowest reactions involved in the NO formation, typically those controlled by chemical equilibrium, reach their final state close to z = 340 mm. This could imply that an increase of the reactants' residence times would yield larger NOx emissions than those reported in Figure 4. For the conditions studied
ARTICLE
here, however, the flue-gas data, measured at the beginning of the exhaust duct, revealed that the NOx variations beyond z = 310 mm are insignificant. Note that the data in Figures 57 are reported as measured while that reported in Figure 4 has been corrected to 15% of O2 in the combustion products. In addition, it is observed that NOx concentrations within the main reaction zone, where the temperatures are the highest of all, but always inferior to ≈1530 °C for run 2 and ≈1510 °C for run 4, and the O2 concentrations are relatively high, are low for both cases, which reveals that NO formation via the thermal mechanism is strongly inhibited there, in agreement with the very low NOx emissions measured. In addition, for the present flameless oxidation condition (run 2, λ = 1.3) and present conventional lean combustion condition (run 4, λ = 1.7) NO formation via the prompt mechanism is expected also to be insignificant.33,36 Under these circumstances and given the absence of nitrogen in the fuel, the formation of NO via the N2O intermediate mechanism is expected to have an important role in the present reacting flows (runs 2 and 4).
4. CONCLUSIONS Measurements of flue-gas composition and OH chemiluminescence have been obtained in a small-scale combustor as a function of the excess air coefficient, which in the present configuration implied also changes in the air inlet velocity. For two of these combustor operating conditions, spatial distributions of temperature and of O2, CO2, HC, CO, and NOx concentrations have also been obtained. The main conclusions from this work can be summarized as follows: 1. The OH* images showed that as the excess air coefficient increases the main reaction zone moves progressively closer to the burner. This shift is presumably due to the increase in the central jet momentum, which leads to a faster entrainment of fuel and burnt gases, and due to the increase in the oxygen concentration in the recirculated flue-gas. 2. The structure of the main reaction zone and the combustion regime also change with the excess air. Accordingly with the OH* images, for low excess air coefficients the reaction zone is uniformly distributed over a relatively large volume of the combustor (flameless combustion), while for high excess air levels, they suggest and still photographs reveal the presence of a flame front located at the strong shear region between the central jet and the external recirculation zone (conventional lean combustion). This is because at high excess air levels hot recirculated gases with relatively high oxygen concentrations mix with the fuel jets creating conditions for the establishment of conventional combustion. 3. The present combustor yields very low NOx (< 10 ppm @ 15% O2) and CO emissions (< 12 ppm @ 15% O2) for all conditions studied, which is attributed to the suppression of the thermal mechanism brought about by the flameless oxidation and conventional lean combustion modes. 4. The detailed measurements made inside the combustor for the two operating conditions, a flameless oxidation condition and a conventional lean combustion condition, confirmed the observations based on the OH* images. In particular, the former condition presents a main reaction zone with lower temperature gradients than the latter condition. 2475
dx.doi.org/10.1021/ef200258t |Energy Fuels 2011, 25, 2469–2480
Energy & Fuels
ARTICLE
’ APPENDIX Table A1. Experimental Results for Run 2 O2
CO2
NOx
HC
CO
z (mm)
r (mm)
T (°C)
(dry volume %)
(dry volume %)
(dry volume ppm)
(dry volume ppm)
(dry volume ppm)
11
0 5
484.0 950.0
20.9 18.1
0 1.1
4.3 5.8
1 2287
45
79
113
147
1 550
10
997.1
6.6
4.8
9.0
22667
2560
15
1018.4
5.2
7.2
6.4
11343
3180
20
1036.9
5.3
8.2
7.7
4000
1890
25
1035.5
5.9
8.1
9.0
733
950
30
1016.4
6.1
8.0
11.3
200
665
35
1009.2
5.8
8.2
12.3
107
680
40 45
1004.8 990.6
6.3 7.2
7.9 7.4
12.8 13.5
67 25
480 230 172
0
600.5
20.3
0.2
4.3
667
5
967.3
18.4
1.0
4.4
2657
535
10
1134.7
9.8
4.7
5.7
9335
4040
15
1119.2
4.0
8.2
6.6
5329
6751
20
1106.1
4.2
8.5
8.2
1738
3700
25
1092.6
4.5
8.6
10.5
733
2002
30 35
1071.6 1048.9
4.8 4.9
8.6 8.5
12.2 13.2
280 160
1412 1053
40
1051.7
4.9
8.6
13.9
70
752
45
1054.8
5.0
8.6
14.2
33
491
0
1157.0
17.4
1.4
5.6
2666
1540 1231
5
1124.7
17.3
1.5
5.7
2300
10
1132.0
12.6
3.6
6.1
5005
4373
15
1145.8
6.3
7.0
6.6
2620
7280
20 25
1149.9 1143.8
4.4 4.6
8.6 8.7
7.8 11.2
611 153
2967 1370
30
1128.0
4.9
8.5
13.7
67
861
35
1107.7
5.0
8.5
14.5
47
645
40
1100.0
5.2
8.5
14.9
17
420
45
1101.8
5.4
8.4
14.9
11
251
0
1356.9
13.8
2.9
5.9
2533
8007 4026
5
1334.6
15.0
2.5
5.8
2401
10
1287.9
11.6
4.1
6.1
2313
9171
15
1220.9
6.0
7.1
6.9
1212
11802
20
1188.3
4.6
8.4
8.7
207
4917
25
1176.3
4.9
8.5
11.8
43
1315
30
1164.4
5.2
8.4
14.3
10
542
35
1150.8
5.5
8.2
15.8
5
263
40
1129.2
5.6
8.2
16.3
2
116
45
1128.4
5.7
8.1
16.5
2
80
0
1466.0
8.7
5.7
7.7
511
11171
5
1470.6
10.7
4.6
7.5
1067
10612
10
1418.0
8.8
5.7
7.8
723
10124
15
1365.3
5.8
7.6
9.4
178
6932
20
1303.3
4.8
8.3
11.6
58
2815
25
1241.3
5.4
8.2
13.4
16
1016
30
1210.5
5.6
8.2
15.0
4
395
35
1179.7
5.7
8.2
15.9
1
156
40
1147.7
5.8
8.1
16.0
1
78
45
1146.5
5.9
8.0
16.3
0
48
2476
dx.doi.org/10.1021/ef200258t |Energy Fuels 2011, 25, 2469–2480
Energy & Fuels
ARTICLE
Table A1. Continued z (mm) 181
215
250
280
310
r (mm)
T (°C)
O2
CO2
NOx
HC
CO
(dry volume %)
(dry volume %)
(dry volume ppm)
(dry volume ppm)
(dry volume ppm)
0
1515.8
6.8
7.2
9.82
37
3572
5
1524.0
7.5
6.8
9.0
65
4191
10
1526.4
6.7
7.3
9.6
47
3368
15
1479.6
5.8
7.9
11.7
13
2156
20
1414.5
5.2
8.3
13.9
4
1259
25
1335.0
5.4
8.3
15.1
2
614
30
1299.4
5.5
8.2
16.1
1
281
35
1267.2
5.7
8.1
16.9
0
139
40
1244.2
5.7
8.1
17.2
0
81
45
1232.6
5.8
8.0
17.3
0
55
0
1459.5
6.1
7.8
15.7
1
1181
5
1467.0
6.4
7.6
14.5
2
1223
10
1471.3
6.2
7.8
14.6
2
1117
15
1443.9
5.9
8.0
15.7
3
811
20
1402.6
5.7
8.1
16.4
1
542
25
1364.3
5.7
8.1
16.8
1
344
30
1329.6
5.8
8.0
17.1
1
194
35
1294.2
5.8
8.1
17.3
0
122
40
1267.7
5.8
8.0
17.5
0
79
45
1245.0
5.8
8.0
17.5
0
62
0
1408.6
6.0
7.9
17.9
0
515
5
1431.2
6.0
7.9
17.4
0
553
10
1433.7
6.0
7.9
17.3
0
484
15
1435.7
5.9
8.0
17.6
0
406
20
1410.6
5.8
8.0
18.2
0
318
25
1381.8
5.8
8.0
18.5
0
225
30
1347.1
5.8
8.0
18.7
0
157
35
1313.4
5.8
8.0
19.1
0
102
40
1279.9
5.9
7.9
19.3
0
73
45
1252.7
6.0
7.9
19.6
0
58
0
1431.6
5.9
7.9
19.7
0
291
5
1427.3
5.9
7.9
19.2
0
299
10
1411.0
5.9
7.9
18.9
0
273
15
1391.0
5.9
8.0
18.9
0
254
20
1363.0
5.8
8.0
18.9
0
206
25
1335.4
5.9
7.9
19.0
0
155
30
1299.1
5.9
7.9
19.2
0
119
35
1269.8
5.9
7.9
19.2
0
88
40
1247.6
5.9
7.9
19.5
0
69
45
1226.5
5.9
7.9
19.8
0
57
0
1355.3
5.9
7.9
20.4
0
182
5
1359.9
5.8
8.0
20.0
0
197
10
1360.5
5.7
8.0
19.8
0
264
15
1354.1
5.8
8.0
19.9
0
181
20
1340.6
5.9
8.0
20.0
0
151
25
1328.1
5.9
8.0
20.3
0
125
30
1305.0
5.9
7.9
20.4
0
107
35
1272.3
5.9
7.9
20.6
0
84
40
1250.5
6.0
7.9
21.0
0
65
45
1224.4
6.0
7.9
21.4
0
52
2477
dx.doi.org/10.1021/ef200258t |Energy Fuels 2011, 25, 2469–2480
Energy & Fuels
ARTICLE
Table A2. Experimental Results for Run 4 z (mm) 11
45
79
113
147
r (mm)
T (°C)
O2
CO2
NOx
HC
CO
(dry volume %)
(dry volume %)
(dry volume ppm)
(dry volume ppm)
(dry volume ppm)
0
450.0
20.9
0.1
2.4
73
17
5
1171.0
16.5
2.0
6.5
5514
1817
10
1167.5
7.5
5.7
10.8
15018
6575
15
1042.9
7.4
6.7
9.2
6142
5971
20
1113.1
8.3
6.8
12.0
12667
2821
25
1115.2
8.3
7.1
14.4
3667
1387
30
1096.2
9.4
6.7
13.5
1062
973
35
1060.2
8.6
7.2
14.6
867
868
40
1053.5
9.2
6.8
15.1
34
532
45
1066.0
9.7
6.6
13.4
11
235
0
650.0
20.6
0
2.2
465
108
5
1286.9
18.8
0.9
2.3
2984
1171
10
1215.2
8.9
5.4
4.0
48435
11265
15
1195.0
5.9
7.8
9.3
12673
7496
20
1169.5
7.2
7.3
15.5
5015
4910
25
1141.7
7.9
7.1
17.1
193
2707
30
110.3
7.9
7.2
18.4
95
1615
35
1099.1
7.4
7.6
18.4
52
1063
40
1093.7
8.2
7.1
19.2
13
672
45
1093.7
8.0
7.1
17.9
6
244
0
1390.1
18.6
0.8
6.4
1923
1763
5
1486.7
17.6
1.3
7.5
2133
2671
10
1399.4
7.7
6.6
11.3
536
7614
15
1254.2
5.8
7.9
14.6
84
4758
20
1195.2
7.8
7.2
16.6
37
1363
25
1160.5
8.2
7.0
18.0
14
645
30
1141.1
8.7
6.6
18.8
6
395
35
1124.5
8.2
6.9
19.7
5
321
40
1104.8
8.4
6.9
18.6
2
269
45
1106.8
7.8
7.4
17.9
0
85
0
1480.8
15.1
2.4
6.3
1 267
7662
5
1510.9
15.6
2.3
6.2
1617
6178
10
1407.5
9.6
5.8
7.9
265
5452
15
1326.5
7.6
7.1
11.2
87
2244
20
1251.4
7.9
7.1
14.9
20
801
25
1195.2
8.7
6.6
16.2
9
315
30
1161.8
8.7
6.7
18.2
2
157
35
1135.1
9.0
6.5
18.7
1
92
40
1126.7
8.6
6.8
19.1
0
61
45
1122.4
9.0
6.5
19.0
0
42
0
1437.1
11.3
4.8
8.6
141
5961 7572
5
1432.9
12.0
4.3
8.4
294
10
1339.4
10.2
5.5
9.5
143
4492
15
1334.8
7.9
7.0
13.2
27
1614
20 25
1247.2 1229.0
8.7 8.6
6.6 6.7
13.8 15.7
26 8
857 384
30
1196.2
8.9
6.5
16.4
4
192
35
1171.3
8.9
6.5
18.0
1
92
40
1148.5
8.3
6.9
18.2
0
63
45
1150.7
9.1
6.4
18.1
0
45
2478
dx.doi.org/10.1021/ef200258t |Energy Fuels 2011, 25, 2469–2480
Energy & Fuels
ARTICLE
Table A2. Continued z (mm) 181
215
250
280
310
r (mm)
T (°C)
O2
CO2
NOx
HC
CO
(dry volume %)
(dry volume %)
(dry volume ppm)
(dry volume ppm)
(dry volume ppm)
0
1381.4
9.9
5.7
10.2
14
5
1377.8
10.2
5.5
9.3
37
2199 2701
10
1347.9
9.7
5.9
9.9
36
2163
15
1309.6
8.2
6.4
11.5
17
1365
20
1260.4
8.6
6.7
14.7
4
537
25
1229.7
9.1
6.4
15.5
3
335
30
1203.1
9.1
6.4
16.3
2
197
35
1176.8
9.2
6.4
16.7
1
129
40
1163.7
9.2
6.3
17.2
1
81
45
1155.7
9.0
6.5
18.3
0
48
0
1341.4
9.4
6.2
13.6
2
869
5
1335.0
9.6
6.1
12.8
3
1021
10
1322.3
9.4
6.2
13.3
3
854
15
1300.5
9.3
6.3
14.5
2
575
20
1273.9
9.1
6.3
15.9
1
335
25
1242.9
9.1
6.4
17.1
0
185
30
1220.3
8.7
6.6
17.8
0
118
35
1200.1
9.5
6.2
17.0
0
109
40
1177.9
9.6
6.1
17.9
0
66
45
1164.2
9.6
6.1
18.2
0
48
0
1313.0
9.3
6.3
15.4
0
292
5
1313.8
9.2
6.3
15.2
0
318
10
1304.2
9.2
6.3
15.3
0
291
15
1293.2
9.2
6.3
15.5
0
246
20
1271.6
9.2
6.3
15.9
0
189
25
1246.5
9.1
6.4
16.4
0
135
30
1224.2
9.1
6.4
16.8
0
97
35
1204.9
9.1
6.3
17.3
0
71
40
1186.6
9.1
6.3
17.8
0
55
45
1169.9
9.2
6.3
18.0
0
44
0
1299.2
9.1
6.3
17.0
0
143
5
1293.9
9.1
6.3
16.7
0
146
10
1287.2
9.1
6.3
16.8
0
107
15
1276.6
9.1
6.3
16.6
0
126
20
1251.1
9.1
6.3
16.8
0
104
25
1229.1
9.1
6.3
16.9
0
84
30
1209.5
9.1
6.3
17.2
0
68
35
1188.3
9.1
6.3
17.3
0
054
40
1171.8
9.2
6.3
17.6
0
45
45
1162.0
9.2
6.3
18.2
0
39
0
1233.9
9.1
6.3
17.6
0
84
5
1248.9
9.1
6.3
17.6
0
84
10
1264.9
9.1
6.3
17.5
0
88
15
1271.3
9.1
6.3
17.5
0
88
20
1274.1
9.1
6.3
17.5
0
74
25
1271.2
9.1
6.3
17.7
0
68
30
1266.4
9.1
6.3
18.2
0
57
35
1251.7
9.1
6.3
18.7
0
42
40
1243.1
9.2
6.3
19.1
0
38
45
1228.8
9.2
6.3
19.3
0
35
2479
dx.doi.org/10.1021/ef200258t |Energy Fuels 2011, 25, 2469–2480
Energy & Fuels
’ AUTHOR INFORMATION Corresponding Author
*Fax: þ 351 21 847 5545. E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was developed within the framework of project PTDC/EME-MFE/102997/2008, which is financially supported by Fundac-~ao para a Ci^encia e a Tecnologia (FCT). A.S.V. is pleased to acknowledge the Coordenac-~ao de Aperfeic-oamento de Pessoal de Nível Superior (CAPES) for the provision of the scholarship BEX:3909/05-0 and A.M.A.R. is pleased to acknowledge the FCT for the provision of the scholarship SFRH/BPD/40709/2007. ’ REFERENCES (1) W€unning, J. A.; W€unning, J. G. Prog. Energy Combust. Sci. 1997, 23, 81–94. (2) Cavaliere, A.; de Joannon, M. Prog. Energy Combust. Sci. 2004, 30, 329–366. (3) Katsuki, M.; Hasegawa, T. Proc. Combust. Inst. 1998, 27, 3135– 3146. (4) Tsuji, H.; Gupta, A. K.; Hasegawa, T.; Katsuki, M.; Kishimoto, K.; Morita, M. High Temperature Air Combustion; CRC Press: Boca Raton, FL, 2003. (5) Arghode, V. K.; Gupta, A. K. Appl. Energy 2010, 87, 1631–1640. (6) Coelho, P. J.; Peters, N. Combust. Flame 2001, 124, 503–518. (7) Effuggi, A; Gelosa, D.; Derudi, M.; Rota, R. Combust. Sci. Technol. 2008, 180, 481–493. (8) W€unning, J. G. FLOXÒ Flameless Combustion; Thermprocess Symposium, D€usseldorf, Germany, 2003. (9) Medwell, P. R.; Kalt, P. A. M.; Dally, B. B. Combust. Flame 2007, 148, 48–61. (10) Plessing, T.; Peters, N.; W€unning, J. G. Proc. Combust. Inst. 1998, 27, 3197–3204. (11) Yuan, J.; Naruse, I. Energy Fuels 1999, 13, 99–104. (12) Weber, R.; Orsino, S.; Lallemant, N.; Verlaan, A. Proc. Combust. Inst. 2000, 28, 1315–1321. (13) Cavigiolo, A.; Galbiati, M. A.; Effuggi, A; Gelosa, D.; Rota, R. Combust. Sci. Technol. 2003, 175, 1347–1367. (14) Dally, B. B.; Riesmeier, E.; Peters, N. Combust. Flame 2004, 137, 418–431. (15) Kumar, S.; Paul, P. J.; Mukunda, H. S. Proc Combust. Inst. 2005, 30, 2613–2621. (16) Weber, R.; Smart, J. P.; Kamp, W. Proc. Combust. Inst. 2005, 30, 2623–2629. (17) de Joannon, M.; Cavaliere, A.; Faravelli, T.; Ranzi, E.; Sabia, P.; Tregrossi, A. Proc. Combust. Inst. 2005, 30, 2605–2612. (18) Li, G.; Gutmark, E. J.; Stankovic, D.; Overman, N.; Cornwell, M.; Fuchs, L.; Vladimir, M. Experimental Study of Flameless Combustion in Gas Turbines Combustors; 44th AIAA Aerospace Sciences Meeting and Exhibit, Paper no. AIAA 2006-546, Reno, Nevada, USA, 2006. (19) Derudi, M.; Villani, A.; Rota, R. Ind. Eng. Chem. Res. 2007, 46, 6806–6811. (20) Mancini, M.; Schw€oppe, P.; Weber, R.; Orsino, S. Combust. Flame 2007, 150, 54–59. (21) L€uckerath, R.; Meier, W.; Aigner, M. J. Eng. Gas Turbines Power 2008, 130, 011505–17. (22) Szeg€o, G. G.; Dally, B. B.; Nathan, G. J. Combust. Flame 2009, 156, 429–438. (23) Mi, J.; Li, P.; Dally, B. B.; Craig, R. A. Energy Fuels 2009, 23, 5349–5356. (24) Krishnamurthy, N.; Paul, P. J.; Blasiak, W. Proc. Combust. Inst. 2009, 32, 3139–3146. (25) Colorado, A. F.; Herrera, B. A.; Ameli, A. A. Bioresour. Technol. 2010, 101, 2443–2449.
ARTICLE
(26) Derudi, M.; Rota, R. Proc. Combust. Inst. 2011, 33, 3325–3332. (27) Orsino, S.; Weber, R.; Bollettini, U. Combust. Sci. Technol. 2001, 170, 1–34. (28) Mancini, M.; Weber, R.; Bollettini, U. Proc. Combust. Inst. 2002, 29, 1155–1163. (29) Christo, F. C.; Dally, B. B. Combust. Flame 2005, 142, 117–129. (30) Kim, S. H.; Huh, K. Y.; Dally, B. Proc. Combust. Inst. 2005, 30, 751–757. (31) Galletti, C.; Parente, A.; Tognotti, L. Combust. Flame 2007, 151, 649–664. (32) Mancini, M.; Schwoppe, P.; Weber, R.; Stefano, O. Combust. Flame 2007, 150, 54–59. (33) Galletti, C.; Parente, A.; Derudi, M.; Rota, R.; Tognotti, L. Int. J. Hydrogen Energy 2009, 34, 8339–8351. (34) Schaffel, N.; Mancini, M.; Szlek, A.; Weber, R. Combust. Flame 2009, 156, 1771–1784. (35) Ihme, M.; See, Y. C. Proc. Combust. Inst. 2011, 33, 1309–1317. (36) Parente, A.; Galletti, C.; Tognotti, L. Proc. Combust. Inst. 2011, 33, 3343–3350. (37) Rocha, A. M. A.; Carvalho, J. A., Jr.; Lacava, P. T. Fuel 2008, 87, 3433–3444. (38) Bracewell, R. The Fourier Transform and Its Applications; McGraw-Hill: New York, 2000; pp 351356. (39) Lee, J. G.; Santavicca, D. A. J. Propul. Power 2003, 19, 735–750.
2480
dx.doi.org/10.1021/ef200258t |Energy Fuels 2011, 25, 2469–2480
