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Measurements and predictions of nitric oxide and particulates emissions from heavy fuel oil spray flames
Twenty-Sixth Symposium (International) on Combustion/The Combustion Institute, 1996/pp. 2241–2250
MEASUREMENTS AND PREDICTIONS OF NITRIC OXIDE AND PARTICULATES EMISSIONS FROM HEAVY FUEL OIL SPRAY FLAMES M. A. BYRNES, E. A. FOUMENY, T. MAHMUD and A. S. A. K SHARIFAH Department of Chemical Engineering The University of Leeds Leeds LS2 9JT, UK T. ABBAS, P. G. COSTEN, S. HASSAN and F. C. LOCKWOOD Department of Mechanical Engineering Imperial College of Science, Technology, and Medicine London SW7 2BX, UK
The results of extensive experimental and predictive studies of nitric oxide (NO) and particulates (unburned coke) emissions from a large-scale laboratory furnace, fired by a heavy fuel oil (HFO) swirl burner with a rotary cup air blast atomizer are presented. A detailed in-flame data archive of gas temperature and O2, CO, CO2, and NO concentrations has been obtained for five flames for differing excess air levels (15% and 20%), swirl numbers (1.05 and 1.2), primary air-to-fuel ratios (2.5 and 3.0), and atomizer cup speeds (1.0 2 104 and 2.0 2 104 rpm). A wider range of operating parameters has been established to quantify their effects on NO and particulate concentrations at the exit of the furnace. In a parallel modeling study, a two-dimensional computational fluid dynamics code for the prediction of HFO spray combustion and NO and particulates emissions has been constructed. Validation of the code against the experimental data reveals reasonably good quality predictions in the near burner region. The code is capable of simulating the measured trends of flue-gas NO and particulates emissions with useful precision for a wide range of atomizer/burner operating conditions. A scrutiny of the in-flame and flue-gas data, with the aid of the predictions, has provided an enhanced understanding of combustion and combined NO/particulate emissions characteristics of the HFO flames generated by the rotary cup atomizer and establishes the foundation for future work in optimizing combustion and emissions performance.
Introduction To meet the increased demand for light distillate products, refineries have moved toward the production of “heavier” fuel oil. This oil has a high asphaltene content, typically about 15%, which has a strong influence on the amount of coke formed and hence the particulates emissions. Unfortunately, most methods of ameliorating the particulate burden favor an increase in NOX production and vice versa. The ideal oil burner design should be capable of producing a high turn-down, stable flame with minimal NOX and particulates emissions from these lowercut fuels. The combustion and emissions performance of a heavy fuel oil (HFO) flame is established by the initial spray characteristics of the atomization method used in the burner and the aerodynamics inherent in the near burner region (NBR) of the furnace in which it is located. Performance optimization, therefore, requires a thorough knowledge of the relevant physical and chemical processes. To gather such information in a full-scale plant is both difficult and
expensive, with the resulting data being specific to that furnace rather than having a universal application. An alternative is the use of a large-scale laboratory furnace in which the turbulence levels, radiation heat transfer, and residence timescales of the full-scale plant are replicated under well-controlled conditions. By allying such data to a validated mathematical model, the performance of any full-scale oil-fired furnace can be predicted with confidence. Comprehensive combustion data sets, including NOX and particulate concentrations, for HFO flames in such furnaces are, however, very rare; some notable data sets have been collected in earlier studies, for example by the authors’ group [1] and by other groups [2,3]. This same dearth of information is also true regarding the applications of generalized computational fluid dynamics (CFD) models to predict the combustion performance, particularly the NO and particulates emissions, of HFO-fired furnaces. Previous relevant modeling studies include, for example, predictions of combustion performances of HFO-fired furnaces [4,5] and NO emissions from medium fuel oil flames [6]. In a relatively recent
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Fig. 1. Schematic diagram of the furnace roof and burner arrangement.
study [7], a one-dimensional model, based on prescribed O2 concentration and residence time distributions, for the prediction of particulates emissions from residual fuel oil fired utility boilers has been presented. To the authors’ knowledge, no CFDbased modeling study pertaining to combined NO and particulates emissions from HFO-fired furnaces has been reported in the open literature. In the initial investigations in this work, isothermal measurements of droplet size distribution in the hollow-cone sprays generated by the rotary cup atomizer were obtained using a Malvern particle-size analyzer. The flames of several of the spray distributions were established in the large-scale laboratory furnace at Imperial College (ICSTM), where detailed in-flame measurements of NO and major species (O2, CO, CO2) concentrations and gas temperature, as well as NO and particulates emissions in the flue-gas, were recorded. The experimental data supplement that previously collected [1,8] for a twin-fluid atomizer giving a solid-cone distribution included detailed in-flame combustion but only fluegas NO and particulate measurements. A complementary CFD model of HFO spray combustion capable of predicting the emissions of NO and particulates was constructed at the University of Leeds. The Experiments The Atomizer/Burner Gun Assembly and Spray Test Rig The burner gun consisted of the rotary cup atomizer, along with oil and air (primary) supply conduits within a support pipe of external diameter 35 mm. The atomizer design was a necessary compromise between the accuracy of replicating an existing
full-scale unit and operational reliability and durability; the cup drive was provided by an electric motor normally found in a dentist’s drill. The HFO-spray characterization tests were carried out under nonreacting conditions in a vertical, cylindrical chamber [8], 0.6 m in diameter and 1.5 m in height. The droplet size distributions at different radial and axial locations were recorded using the Malvern 2600 Particle Size Analyser for various changes in the input parameters, such as the atomizer cup speed, primary air-to-fuel ratio (PAFR), and oil viscosity. PAFR refers to the ratio of the mass flow rate of the primary air jet surrounding the atomizer cup to the mass flow rate of fuel. The measurement techniques are described fully in Ref. 8. The Furnace and Instrumentation A detailed description of the ICSTM furnace and its instrumentation and oil supply system can be found in many previous publications, e.g., [1,8]. The vertical furnace is of modular construction comprised of 10, 0.6 m internal diameter, cylindrical water-cooled steel sections, each of a height 0.3 m. The burner is mounted in the roof section and fires downward (see Fig. 1). The five uppermost sections and the roof have an internal lining of castable silicon carbide refractory. The burner consists of a gun and a secondary combustion air nozzle in a conventional double-concentric configuration. Swirl is imparted to the secondary air using a movable-block swirl generator developed by the International Flame Research Foundation. In-flame gas temperatures were measured using 40 lm Pt/Pt:13%Rh thermocouples. The uncertainty in temperature measurements caused by radiation losses was about 10% in the regions of highest temperatures. The sampling for gas species concentration was achieved through the use of a water-cooled, water-quenched, stainless
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TABLE 1 Fuel properties and the atomizer/burner operating conditions Property % Composition by mass Carbon Hydrogen Sulphur Nitrogen Oxygen Ash Density at 288 K (kg/m3): Gross calorific value (MJ/kg): Atomizer/burner operating conditions Flame Fuel flow rate (kg/h) Air flow rate (kg/h) Secondary air temperature (K) Swirl number Excess air (%) PAFR Cup speed (21000, rpm)
86.9 10.3 2.35 0.36 0.037 0.053 988 42.54
A
B
C
D
E
12 187.6 573 1.2 15 2.5
12 195.8 573 1.2 20 2.5
12 187.6 573 1.05 15 2.5
12 187.6 573 1.2 15 2.5
12 187.6 573 1.2 15 3.0
20
20
20
10
20
steel probe of 22 mm overall diameter. A Land Ltd. probe was used to sample back-end solids concentrations. Reproducibility of the data was, on average, within 10%. Test Conditions and Experimental Data The properties of the HFO and burner operating conditions are given in Table 1. For all experiments, an oil flow rate of 12 kg/h at 368 K was used. Detailed in-flame measurements of gas temperature, NO, and major species concentrations were carried out for five flames, referred to as Flames A–E, for different excess air level, swirl number, PAFR, and atomizer cup speed. Measurements of NO and particulate concentrations at the furnace exit were also carried out to quantify the effects of atomizer/burner operating parameters on the pollutants’ emissions. The Mathematical Modeling of HFO Combustion A parallel study for the prediction of the data was effected at the University of Leeds. An existing twodimensional CFD code [9,10] for pulverized-coal combustion was adapted by incorporating relevant physical models to handle HFO spray combustion. This included modeling of the droplet heating and evaporation, coke and soot particle formation, and
the coke and soot burnout. A NO postprocessor, simulating the thermal-, prompt- and fuel-NO mechanisms, was developed. The Aerodynamics and Combustion Models The gas-phase conservation equations are solved in a conventional finite-volume Eulerian treatment, while Lagrangian calculations resolve the particulate-phase equations. The turbulence is handled by the standard k-e model [11], without any swirl-related modification. The QUICK scheme [12] is used for the discretization of the convective terms in the gas-phase momentum equations, while the hybrid scheme [13] is employed for the scalar equations. The continuity and momentum equations are solved using the PISO algorithm [14]. The modeling of the turbulent combustion of droplet evaporation products presents a major difficulty in the prediction of HFO flames. This is also the case for the gas-phase volatiles combustion of pulverized-coal flames [9,10]. The evaporation products are represented by a single chemical species and their combustion processes are simulated by a twostep reaction mechanism. The time–mean burning rate of the fuel vapor is obtained using the widely employed eddy-dissipation combustion model [15]. The CO formed from the fuel vapor combustion is oxidized to CO2 and the time–mean oxidation rate
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is taken as the smaller of the turbulent mixing rate [15] and an Arrhenius chemical kinetic rate [16]. The radiation heat transfer is simulated by a nonequilibrium diffusion model [17]. The presence of soot has been accounted for. A global Arrhenius-type rate expression [18] has been used to model the soot formation, and a turbulent mixing rate expression [15] is employed to determine its burning rate. This approach has been found adequate for HFO and hydrocarbon flames [4,19]. The absorption coefficient of the medium is determined using the mixed gray gas model of Ref. 20. The Coke Formation and Burnout Models The emission of coke particles from HFO-fired systems involves the formation of coke, following the rapid evaporation of the low-boiling components of the oil, and the subsequent combustion of these particles. The processes involved in coke formation are extremely complex and the mechanism is far from established [21]. A simple model has been formulated to predict the coke formation and burnout and is fully described in Ref. 22. It is assumed that a single coke particle is formed from each oil droplet after evaporation is completed [23]. The portion of the initial mass of an oil droplet converted to coke has been correlated in terms of the Coke Formation Index (CFI), defined as the ratio of the initial mass of coke particle to the initial mass of oil droplet [24]. By assuming a spherical shape, this correlation allows the calculation of the initial mass and diameter of an individual coke particle to be made. The value of CFI used in the calculation was obtained from measurements [24] for residual fuel oils with similar asphaltene contents to that of the oil used in this study. The coke burnout has been handled in a manner similar to pulverized-coal char combustion. The heterogeneous combustion rate of the coke particles with O2 is controlled by the rate of external diffusion of O2 to the particle surface and by the rate of chemical reaction at the external surface. The apparent reaction order with respect to O2 partial pressure is taken as unity [25]. The kinetic parameters recommended by Northrop et al. [25] have been employed. The NO Postprocessor NO emissions from HFO flames arise from both atmospheric (thermal- and prompt-NO) and fuelbound nitrogen (fuel-NO). The relative contributions of each to the total NO emission depend on the nitrogen content of the fuel and on the operating conditions of the atomizer/burner. The NO postprocessor [22,26] employed in this study accommodates all three mechanisms. The thermal-NO model is that of Zeldovich [27]
and includes the effect of the reverse reactions. The rate constants were obtained from Ref. 28. The contribution of prompt-NO to the total formation is relatively small under fuel-lean or near stoichiometric conditions [29]. Hence, the global rate expression proposed by DeSoete [30] is employed. The formation and destruction of fuel-NO involve a complex series of radical reactions. A simple chemical scheme, again suggested by DeSoete [30], has been adopted here. In this approach, the assumed first product of the evolved fuel-bound nitrogen in the gas phase is HCN, which is subsequently either oxidized by an oxygen-containing species to NO or reduced by NO to N2. The kinetic parameters are those of DeSoete except for the preexponential factor for the HCN oxidation reaction, which, in common with other studies [31,32], was adjusted to achieve best overall agreement with the data. Following numerical experiments [22], the preexponential factor was assigned a value of 0.15 2 1011 s11, which is within the range of values (0.1 2 1011 to 1.0 2 1011 s11) reported in the literature [30–32]. Computational Details The computations were performed on a two-dimensional, nonuniform 61 2 37 grid. This specification is the result of extensive experience of predicting the ICSTM furnace [9,10]. The inlet boundary conditions for the predictions were specified, wherever possible, using the measured atomizer/burner operating parameters as given in Table 1. The inlet axial and swirl velocities were obtained from the measured flow rate and swirl number, and the radial velocity was taken as zero. In the absence of measurements, the shapes of the inlet velocity profiles and turbulence levels were specified based on previous sensitivity studies [9,10] of these parameters. The measured droplet size distributions were represented by four discrete sizes, and the droplets were injected at the measured spray cone angle from the periphery of the atomizer cup. In the absence of measurements, the droplet injection velocities were estimated from the rotational speed of the cup, assuming that the liquid discharged at a velocity corresponding to the cup speed [33]. The sensitivity of the predictions to the number of discrete droplet sizes and their injection velocities was examined, and these parameters were optimized with data to achieve best overall predictions. Discussion of Experimental and Predicted Results Although an extensive in-flame data archive was compiled from the experimental investigation, only selected results are presented because of space limitations. Additional isothermal atomization and com-
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Fig. 2. Predicted gas flow pattern and droplet/particle trajectories in the NBR.
bustion data and predictions are reported in Ref. 34 and 22, respectively. General Aerodynamic Characteristics Figure 2 shows typical predictions of the gas flow pattern and trajectories of the oil droplets/coke particles in the NBR for Flame A. The gas flow pattern reveals the formation of an internal recirculation zone (IRZ) caused by the high level of swirl imparted to the secondary air and a large external recirculation zone resulting from the flow separation due to the sudden expansion of the furnace configuration. As the spray droplets emerge from the periphery of the atomizer cup, they are entrained by the secondary air jet and travel around the boundary of the IRZ. Evaporation is completed within an axial displacement of 0.03–0.13 m from the burner throat, depending on the droplet size, which is in general agreement with the experimental observations. The coke particles penetrate further into the furnace with particles produced from the smaller droplets (,100 lm) being completely burned before the furnace exit. It is important to note that in the absence of gas velocity data, a deficiency common to many twophase reacting flow experiments, the quality of the predicted flow field cannot be evaluated quantitatively. However, nonreacting swirling flow measurements in sudden expansion configurations (see, for example, Ref. 35) suggest that the general features of the predicted flow pattern are correct. The predictions for Flames B–E (not shown here) reveal that the basic aerodynamic characteristics of this burner are rather insensitive to the inlet conditions. Flame Characteristics The measured and predicted radial profiles of O2 and CO concentrations and gas temperature for
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Flame A are presented in Figs. 3 and 4. As can be seen, the predictions are, generally, in good agreement with the measurements and match the general trends. However, at the first station, x/D 4 0.71 (where x is the axial distance from the burner throat and D is the diameter of the secondary nozzle), fairly uniform measured O2 concentration near the furnace axis suggests that the width of the IRZ is somewhat underpredicted. As a consequence, the radial location of the predicted peak is nearer to the axis compared with the measured one. The predicted droplets’ trajectories and the measured peaks in the CO concentration profiles suggest that the droplet evaporation and subsequent combustion of fuel vapor occur in the shear region around the boundary of the IRZ. The measured high CO concentrations at and about the furnace axis in the NBR also reveal that a proportion of droplets, generated in the hollow-cone distribution by the action of the rotary cup, diffuse into the IRZ, and the evaporation products combust there. This feature is not replicated in the prediction, possibly because of the droplets/turbulence interaction being neglected in the model. At x/D 4 6.6, the uniform O2 concentration and temperature profiles indicate that the fuel vapor combustion has already been completed. At this location and further downstream, the measured and predicted CO concentrations are both negligible. The gas temperature predictions also show a reasonable correlation with the experimental data. The lack of data at x/D 4 0.71 is due to unacceptable errors caused by droplets impinging on the fine-wire thermocouples. In-flame NO Formation The measured and predicted radial profiles of NO concentration for Flame A are presented in Fig. 4b. The overall agreement is reasonably good. The measured level of NO emission at the furnace exit is well reproduced by the prediction. In the NBR, the measured peak values of NO concentration at the furnace axis are also well predicted. However, the discrepancy observed between the measured and predicted O2 concentrations near the axis at x/D 4 0.71 is also reflected in the NO concentrations. At the next two downstream stations (x/D 4 1.25 and 1.79), differences between the measurements and predictions are evident in the forward flow region, which is dominated by the secondary air flow. This may be attributed to the overestimation of the secondary air jet spreading and the rate of mixing by the existing physical models. The steep gradients of the measured and predicted NO concentrations near the axis suggest that the NO formation occurs mainly within the IRZ and the surrounding forward flow zone. In this region, the gas temperatures are high and sufficient O2 is available, as shown in Figs. 3a
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Fig. 3. Measured and predicted (a) O2 and (b) CO concentration profiles (n experimental; —— prediction).
and 4a, to encourage the formation of NO by both the thermal and fuel mechanisms. The calculated rates of NO formation indicate that the main reaction zone extends up to an axial distance of about x/ D 4 3.5. In the downstream region of this zone, the NO concentration distribution is fairly uniform, as shown by the measured and predicted radial profiles at x/D 4 6.6.
Effects of Operating Conditions on NO and Particulates Emissions The measured and predicted furnace-exit NO and particulate concentrations for a range of the operating conditions are shown in Fig. 5. As can be seen, the measured trends of NO emissions are well simulated, whereas the particulates emissions are qualitatively reproduced by the model.
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Fig. 4. Measured and predicted (a) gas temperature and (b) NO concentration profiles (n experimental; —— prediction).
The improvement in spray quality with primary air is reflected in an initial decrease in the particulate burden and a corresponding increase in NO. Beyond a PAFR of 2.5, no further improvement in spray quality was observed in the isothermal spray characterization experiments. The consequence in the flame is a leveling off of the particulate emissions at
approximately 55 mg/m3. The further increase in NO emissions with PAFR is due to the increased availability of O2 in the evaporation region. The effect of increased O2 availability, on this occasion due to the increase in excess air, is shown in Figs. 5aii and 5bii. In this instance, the reduction in particulates burden is due to the augmented heter-
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Fig. 5. Measured and predicted (a) NO and (b) particulates emissions at the furnace exit (M experimental; —j —— prediction).
ogeneous oxidation of the coke particles. Beyond 15% excess air, there is no further reduction in the particulate concentration because of the complete burnout of carbon in the particles. It should be noted that a particulate concentration of about 55 mg/m3 corresponds to complete carbon burnout. Additional O2 leads, however, to a further increase in the NO emissions. The effect of secondary air swirl on NO and particulates emissions is shown in Figs. 5aiii and 5biii . As can be seen, both the NO and particulate concentrations decrease slightly between swirl numbers of 1.05 and 1.2. The effect of this parameter is not readily apparent, as a larger range of swirl set-
tings were not possible due to flame stability problems. The final effect is that of rotating cup speed, which is shown in Figs. 5aiv and 5biv. As expected, increasing cup speed enhances the spray fineness, which leads to a more intense flame resulting in an increased level of NO emissions. However, this has an opposite effect on particulates emissions, which decrease because of the enhanced coke burnout rate of smaller-size particles. Concluding Remarks The experimental data collected for HFO swirling flames, generated by a rotary cup atomizer, comple-
POLLUTANTS EMISSIONS FROM SPRAY FLAMES
ment and extend the data previously gathered for a twin-fluid atomizer at ICSTM. The additional computational input from the University of Leeds further enhances the value of the work, with the code being validated against the axisymmetric experimental data. The resulting code is capable of simulating with useful precision the measured trends of NO and particulates emissions for a wide range of operating conditions of the atomizer and the burner. It is readily apparent from this work, and the work of others using different types of atomizers (e.g., pressure-jet, twin-fluid), that the “holy grail” of low-NO and low-particulates emissions appears to be mutually exclusive, and only a compromised operation is possible. A minimum particle concentration of about 55 mg/m3, achieved by increasing the quality of the atomization and using high levels of excess air, corresponds to the desired complete burnout of carbon in the fuel. Unfortunately, this is only achieved at the expense of high NO emissions. From the predicted oil droplets’ trajectories and our knowledge of low NOx technologies used for solid fuels, the atomization and evaporation must be established in an O2-lean atmosphere if NO emissions are to be reduced. Although the rotary cup, and to a greater extent the pressure-jet atomizer, can separate atomization quality from primary air-to-fuel ratio, low NO emissions demand that the droplets penetrate into the rich IRZ. Unfortunately, the fine droplets are more likely to follow the gas stream readily and diffuse into the O2-rich secondary air stream. A fundamental rethinking of the design of conventional atomizers is implied to achieve the target of combined low NO and particulates emissions. To this end, further modification of the present atomizer is being undertaken, while the furnace and the CFD code are being adapted to accommodate nonambient air concentrations prevalent in flue-gas recirculation or oxygen injection as a means of satisfying the dual demands of high efficiency and low emissions per unit energy input. Acknowledgment The authors are indebted to the Engineering and Physical Sciences Research Council of the United Kingdom for financial support (GR/H 15585) of this research project.
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COMMENTS Dr. Josette Bellan, Jet Propulsion Laboratory, USA. During combustion of asphaltene—containing oils, cenospheres are formed. These cenospheres have a central “blow hole” and burn very differently from coal particles, with transport processes being very important. We have published about 10 years ago a model (Lowenberg, Bellan and Gavales in Chemical Eng. Communications) describing precisely cenosphere combustion. Why are you using a coal-burning model? You certainly cannot hope to obtain a predictive model with a model that is so far from reality. Author’s Reply. Comprehensive simulation of heavy fuel oil spray combustion in a practical system requires modelling of a large number of physical and chemical processes. In order to construct an economical model for engineering predictions we have chosen to incorporate simple yet realistic models for these processes where possible. We agree
in general with you that it would be more realistic to use a detailed coke combustion model which accounts for the evolution of the coke structure and intricate transport processes within the porous particle during combustion. However, to date, studies on coke oxidation have been rather limited, and it is extremely difficult to quantify these effects. As a consequence, we have adopted a simplified model, as described in the text, where the kinetic rate constants are those evaluated for coke [25] and not for coal chars. A similar approach has also been used in Ref. 7 to model coke emissions from residual fuel oil fired boilers. It is of interest to note that the present coke combustion model is able to predict the measured trends of particulates emissions for a wide range of operating conditions of the atomizer and the burner. We shall bear your comments in mind and try your coke combustion model when time permits.
MEASUREMENTS AND PREDICTIONS OF NITRIC OXIDE AND PARTICULATES EMISSIONS FROM HEAVY FUEL OIL SPRAY FLAMES M. A. BYRNES, E. A. FOUMENY, T. MAHMUD and A. S. A. K SHARIFAH Department of Chemical Engineering The University of Leeds Leeds LS2 9JT, UK T. ABBAS, P. G. COSTEN, S. HASSAN and F. C. LOCKWOOD Department of Mechanical Engineering Imperial College of Science, Technology, and Medicine London SW7 2BX, UK
The results of extensive experimental and predictive studies of nitric oxide (NO) and particulates (unburned coke) emissions from a large-scale laboratory furnace, fired by a heavy fuel oil (HFO) swirl burner with a rotary cup air blast atomizer are presented. A detailed in-flame data archive of gas temperature and O2, CO, CO2, and NO concentrations has been obtained for five flames for differing excess air levels (15% and 20%), swirl numbers (1.05 and 1.2), primary air-to-fuel ratios (2.5 and 3.0), and atomizer cup speeds (1.0 2 104 and 2.0 2 104 rpm). A wider range of operating parameters has been established to quantify their effects on NO and particulate concentrations at the exit of the furnace. In a parallel modeling study, a two-dimensional computational fluid dynamics code for the prediction of HFO spray combustion and NO and particulates emissions has been constructed. Validation of the code against the experimental data reveals reasonably good quality predictions in the near burner region. The code is capable of simulating the measured trends of flue-gas NO and particulates emissions with useful precision for a wide range of atomizer/burner operating conditions. A scrutiny of the in-flame and flue-gas data, with the aid of the predictions, has provided an enhanced understanding of combustion and combined NO/particulate emissions characteristics of the HFO flames generated by the rotary cup atomizer and establishes the foundation for future work in optimizing combustion and emissions performance.
Introduction To meet the increased demand for light distillate products, refineries have moved toward the production of “heavier” fuel oil. This oil has a high asphaltene content, typically about 15%, which has a strong influence on the amount of coke formed and hence the particulates emissions. Unfortunately, most methods of ameliorating the particulate burden favor an increase in NOX production and vice versa. The ideal oil burner design should be capable of producing a high turn-down, stable flame with minimal NOX and particulates emissions from these lowercut fuels. The combustion and emissions performance of a heavy fuel oil (HFO) flame is established by the initial spray characteristics of the atomization method used in the burner and the aerodynamics inherent in the near burner region (NBR) of the furnace in which it is located. Performance optimization, therefore, requires a thorough knowledge of the relevant physical and chemical processes. To gather such information in a full-scale plant is both difficult and
expensive, with the resulting data being specific to that furnace rather than having a universal application. An alternative is the use of a large-scale laboratory furnace in which the turbulence levels, radiation heat transfer, and residence timescales of the full-scale plant are replicated under well-controlled conditions. By allying such data to a validated mathematical model, the performance of any full-scale oil-fired furnace can be predicted with confidence. Comprehensive combustion data sets, including NOX and particulate concentrations, for HFO flames in such furnaces are, however, very rare; some notable data sets have been collected in earlier studies, for example by the authors’ group [1] and by other groups [2,3]. This same dearth of information is also true regarding the applications of generalized computational fluid dynamics (CFD) models to predict the combustion performance, particularly the NO and particulates emissions, of HFO-fired furnaces. Previous relevant modeling studies include, for example, predictions of combustion performances of HFO-fired furnaces [4,5] and NO emissions from medium fuel oil flames [6]. In a relatively recent
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Fig. 1. Schematic diagram of the furnace roof and burner arrangement.
study [7], a one-dimensional model, based on prescribed O2 concentration and residence time distributions, for the prediction of particulates emissions from residual fuel oil fired utility boilers has been presented. To the authors’ knowledge, no CFDbased modeling study pertaining to combined NO and particulates emissions from HFO-fired furnaces has been reported in the open literature. In the initial investigations in this work, isothermal measurements of droplet size distribution in the hollow-cone sprays generated by the rotary cup atomizer were obtained using a Malvern particle-size analyzer. The flames of several of the spray distributions were established in the large-scale laboratory furnace at Imperial College (ICSTM), where detailed in-flame measurements of NO and major species (O2, CO, CO2) concentrations and gas temperature, as well as NO and particulates emissions in the flue-gas, were recorded. The experimental data supplement that previously collected [1,8] for a twin-fluid atomizer giving a solid-cone distribution included detailed in-flame combustion but only fluegas NO and particulate measurements. A complementary CFD model of HFO spray combustion capable of predicting the emissions of NO and particulates was constructed at the University of Leeds. The Experiments The Atomizer/Burner Gun Assembly and Spray Test Rig The burner gun consisted of the rotary cup atomizer, along with oil and air (primary) supply conduits within a support pipe of external diameter 35 mm. The atomizer design was a necessary compromise between the accuracy of replicating an existing
full-scale unit and operational reliability and durability; the cup drive was provided by an electric motor normally found in a dentist’s drill. The HFO-spray characterization tests were carried out under nonreacting conditions in a vertical, cylindrical chamber [8], 0.6 m in diameter and 1.5 m in height. The droplet size distributions at different radial and axial locations were recorded using the Malvern 2600 Particle Size Analyser for various changes in the input parameters, such as the atomizer cup speed, primary air-to-fuel ratio (PAFR), and oil viscosity. PAFR refers to the ratio of the mass flow rate of the primary air jet surrounding the atomizer cup to the mass flow rate of fuel. The measurement techniques are described fully in Ref. 8. The Furnace and Instrumentation A detailed description of the ICSTM furnace and its instrumentation and oil supply system can be found in many previous publications, e.g., [1,8]. The vertical furnace is of modular construction comprised of 10, 0.6 m internal diameter, cylindrical water-cooled steel sections, each of a height 0.3 m. The burner is mounted in the roof section and fires downward (see Fig. 1). The five uppermost sections and the roof have an internal lining of castable silicon carbide refractory. The burner consists of a gun and a secondary combustion air nozzle in a conventional double-concentric configuration. Swirl is imparted to the secondary air using a movable-block swirl generator developed by the International Flame Research Foundation. In-flame gas temperatures were measured using 40 lm Pt/Pt:13%Rh thermocouples. The uncertainty in temperature measurements caused by radiation losses was about 10% in the regions of highest temperatures. The sampling for gas species concentration was achieved through the use of a water-cooled, water-quenched, stainless
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TABLE 1 Fuel properties and the atomizer/burner operating conditions Property % Composition by mass Carbon Hydrogen Sulphur Nitrogen Oxygen Ash Density at 288 K (kg/m3): Gross calorific value (MJ/kg): Atomizer/burner operating conditions Flame Fuel flow rate (kg/h) Air flow rate (kg/h) Secondary air temperature (K) Swirl number Excess air (%) PAFR Cup speed (21000, rpm)
86.9 10.3 2.35 0.36 0.037 0.053 988 42.54
A
B
C
D
E
12 187.6 573 1.2 15 2.5
12 195.8 573 1.2 20 2.5
12 187.6 573 1.05 15 2.5
12 187.6 573 1.2 15 2.5
12 187.6 573 1.2 15 3.0
20
20
20
10
20
steel probe of 22 mm overall diameter. A Land Ltd. probe was used to sample back-end solids concentrations. Reproducibility of the data was, on average, within 10%. Test Conditions and Experimental Data The properties of the HFO and burner operating conditions are given in Table 1. For all experiments, an oil flow rate of 12 kg/h at 368 K was used. Detailed in-flame measurements of gas temperature, NO, and major species concentrations were carried out for five flames, referred to as Flames A–E, for different excess air level, swirl number, PAFR, and atomizer cup speed. Measurements of NO and particulate concentrations at the furnace exit were also carried out to quantify the effects of atomizer/burner operating parameters on the pollutants’ emissions. The Mathematical Modeling of HFO Combustion A parallel study for the prediction of the data was effected at the University of Leeds. An existing twodimensional CFD code [9,10] for pulverized-coal combustion was adapted by incorporating relevant physical models to handle HFO spray combustion. This included modeling of the droplet heating and evaporation, coke and soot particle formation, and
the coke and soot burnout. A NO postprocessor, simulating the thermal-, prompt- and fuel-NO mechanisms, was developed. The Aerodynamics and Combustion Models The gas-phase conservation equations are solved in a conventional finite-volume Eulerian treatment, while Lagrangian calculations resolve the particulate-phase equations. The turbulence is handled by the standard k-e model [11], without any swirl-related modification. The QUICK scheme [12] is used for the discretization of the convective terms in the gas-phase momentum equations, while the hybrid scheme [13] is employed for the scalar equations. The continuity and momentum equations are solved using the PISO algorithm [14]. The modeling of the turbulent combustion of droplet evaporation products presents a major difficulty in the prediction of HFO flames. This is also the case for the gas-phase volatiles combustion of pulverized-coal flames [9,10]. The evaporation products are represented by a single chemical species and their combustion processes are simulated by a twostep reaction mechanism. The time–mean burning rate of the fuel vapor is obtained using the widely employed eddy-dissipation combustion model [15]. The CO formed from the fuel vapor combustion is oxidized to CO2 and the time–mean oxidation rate
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is taken as the smaller of the turbulent mixing rate [15] and an Arrhenius chemical kinetic rate [16]. The radiation heat transfer is simulated by a nonequilibrium diffusion model [17]. The presence of soot has been accounted for. A global Arrhenius-type rate expression [18] has been used to model the soot formation, and a turbulent mixing rate expression [15] is employed to determine its burning rate. This approach has been found adequate for HFO and hydrocarbon flames [4,19]. The absorption coefficient of the medium is determined using the mixed gray gas model of Ref. 20. The Coke Formation and Burnout Models The emission of coke particles from HFO-fired systems involves the formation of coke, following the rapid evaporation of the low-boiling components of the oil, and the subsequent combustion of these particles. The processes involved in coke formation are extremely complex and the mechanism is far from established [21]. A simple model has been formulated to predict the coke formation and burnout and is fully described in Ref. 22. It is assumed that a single coke particle is formed from each oil droplet after evaporation is completed [23]. The portion of the initial mass of an oil droplet converted to coke has been correlated in terms of the Coke Formation Index (CFI), defined as the ratio of the initial mass of coke particle to the initial mass of oil droplet [24]. By assuming a spherical shape, this correlation allows the calculation of the initial mass and diameter of an individual coke particle to be made. The value of CFI used in the calculation was obtained from measurements [24] for residual fuel oils with similar asphaltene contents to that of the oil used in this study. The coke burnout has been handled in a manner similar to pulverized-coal char combustion. The heterogeneous combustion rate of the coke particles with O2 is controlled by the rate of external diffusion of O2 to the particle surface and by the rate of chemical reaction at the external surface. The apparent reaction order with respect to O2 partial pressure is taken as unity [25]. The kinetic parameters recommended by Northrop et al. [25] have been employed. The NO Postprocessor NO emissions from HFO flames arise from both atmospheric (thermal- and prompt-NO) and fuelbound nitrogen (fuel-NO). The relative contributions of each to the total NO emission depend on the nitrogen content of the fuel and on the operating conditions of the atomizer/burner. The NO postprocessor [22,26] employed in this study accommodates all three mechanisms. The thermal-NO model is that of Zeldovich [27]
and includes the effect of the reverse reactions. The rate constants were obtained from Ref. 28. The contribution of prompt-NO to the total formation is relatively small under fuel-lean or near stoichiometric conditions [29]. Hence, the global rate expression proposed by DeSoete [30] is employed. The formation and destruction of fuel-NO involve a complex series of radical reactions. A simple chemical scheme, again suggested by DeSoete [30], has been adopted here. In this approach, the assumed first product of the evolved fuel-bound nitrogen in the gas phase is HCN, which is subsequently either oxidized by an oxygen-containing species to NO or reduced by NO to N2. The kinetic parameters are those of DeSoete except for the preexponential factor for the HCN oxidation reaction, which, in common with other studies [31,32], was adjusted to achieve best overall agreement with the data. Following numerical experiments [22], the preexponential factor was assigned a value of 0.15 2 1011 s11, which is within the range of values (0.1 2 1011 to 1.0 2 1011 s11) reported in the literature [30–32]. Computational Details The computations were performed on a two-dimensional, nonuniform 61 2 37 grid. This specification is the result of extensive experience of predicting the ICSTM furnace [9,10]. The inlet boundary conditions for the predictions were specified, wherever possible, using the measured atomizer/burner operating parameters as given in Table 1. The inlet axial and swirl velocities were obtained from the measured flow rate and swirl number, and the radial velocity was taken as zero. In the absence of measurements, the shapes of the inlet velocity profiles and turbulence levels were specified based on previous sensitivity studies [9,10] of these parameters. The measured droplet size distributions were represented by four discrete sizes, and the droplets were injected at the measured spray cone angle from the periphery of the atomizer cup. In the absence of measurements, the droplet injection velocities were estimated from the rotational speed of the cup, assuming that the liquid discharged at a velocity corresponding to the cup speed [33]. The sensitivity of the predictions to the number of discrete droplet sizes and their injection velocities was examined, and these parameters were optimized with data to achieve best overall predictions. Discussion of Experimental and Predicted Results Although an extensive in-flame data archive was compiled from the experimental investigation, only selected results are presented because of space limitations. Additional isothermal atomization and com-
POLLUTANTS EMISSIONS FROM SPRAY FLAMES
Fig. 2. Predicted gas flow pattern and droplet/particle trajectories in the NBR.
bustion data and predictions are reported in Ref. 34 and 22, respectively. General Aerodynamic Characteristics Figure 2 shows typical predictions of the gas flow pattern and trajectories of the oil droplets/coke particles in the NBR for Flame A. The gas flow pattern reveals the formation of an internal recirculation zone (IRZ) caused by the high level of swirl imparted to the secondary air and a large external recirculation zone resulting from the flow separation due to the sudden expansion of the furnace configuration. As the spray droplets emerge from the periphery of the atomizer cup, they are entrained by the secondary air jet and travel around the boundary of the IRZ. Evaporation is completed within an axial displacement of 0.03–0.13 m from the burner throat, depending on the droplet size, which is in general agreement with the experimental observations. The coke particles penetrate further into the furnace with particles produced from the smaller droplets (,100 lm) being completely burned before the furnace exit. It is important to note that in the absence of gas velocity data, a deficiency common to many twophase reacting flow experiments, the quality of the predicted flow field cannot be evaluated quantitatively. However, nonreacting swirling flow measurements in sudden expansion configurations (see, for example, Ref. 35) suggest that the general features of the predicted flow pattern are correct. The predictions for Flames B–E (not shown here) reveal that the basic aerodynamic characteristics of this burner are rather insensitive to the inlet conditions. Flame Characteristics The measured and predicted radial profiles of O2 and CO concentrations and gas temperature for
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Flame A are presented in Figs. 3 and 4. As can be seen, the predictions are, generally, in good agreement with the measurements and match the general trends. However, at the first station, x/D 4 0.71 (where x is the axial distance from the burner throat and D is the diameter of the secondary nozzle), fairly uniform measured O2 concentration near the furnace axis suggests that the width of the IRZ is somewhat underpredicted. As a consequence, the radial location of the predicted peak is nearer to the axis compared with the measured one. The predicted droplets’ trajectories and the measured peaks in the CO concentration profiles suggest that the droplet evaporation and subsequent combustion of fuel vapor occur in the shear region around the boundary of the IRZ. The measured high CO concentrations at and about the furnace axis in the NBR also reveal that a proportion of droplets, generated in the hollow-cone distribution by the action of the rotary cup, diffuse into the IRZ, and the evaporation products combust there. This feature is not replicated in the prediction, possibly because of the droplets/turbulence interaction being neglected in the model. At x/D 4 6.6, the uniform O2 concentration and temperature profiles indicate that the fuel vapor combustion has already been completed. At this location and further downstream, the measured and predicted CO concentrations are both negligible. The gas temperature predictions also show a reasonable correlation with the experimental data. The lack of data at x/D 4 0.71 is due to unacceptable errors caused by droplets impinging on the fine-wire thermocouples. In-flame NO Formation The measured and predicted radial profiles of NO concentration for Flame A are presented in Fig. 4b. The overall agreement is reasonably good. The measured level of NO emission at the furnace exit is well reproduced by the prediction. In the NBR, the measured peak values of NO concentration at the furnace axis are also well predicted. However, the discrepancy observed between the measured and predicted O2 concentrations near the axis at x/D 4 0.71 is also reflected in the NO concentrations. At the next two downstream stations (x/D 4 1.25 and 1.79), differences between the measurements and predictions are evident in the forward flow region, which is dominated by the secondary air flow. This may be attributed to the overestimation of the secondary air jet spreading and the rate of mixing by the existing physical models. The steep gradients of the measured and predicted NO concentrations near the axis suggest that the NO formation occurs mainly within the IRZ and the surrounding forward flow zone. In this region, the gas temperatures are high and sufficient O2 is available, as shown in Figs. 3a
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Fig. 3. Measured and predicted (a) O2 and (b) CO concentration profiles (n experimental; —— prediction).
and 4a, to encourage the formation of NO by both the thermal and fuel mechanisms. The calculated rates of NO formation indicate that the main reaction zone extends up to an axial distance of about x/ D 4 3.5. In the downstream region of this zone, the NO concentration distribution is fairly uniform, as shown by the measured and predicted radial profiles at x/D 4 6.6.
Effects of Operating Conditions on NO and Particulates Emissions The measured and predicted furnace-exit NO and particulate concentrations for a range of the operating conditions are shown in Fig. 5. As can be seen, the measured trends of NO emissions are well simulated, whereas the particulates emissions are qualitatively reproduced by the model.
POLLUTANTS EMISSIONS FROM SPRAY FLAMES
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Fig. 4. Measured and predicted (a) gas temperature and (b) NO concentration profiles (n experimental; —— prediction).
The improvement in spray quality with primary air is reflected in an initial decrease in the particulate burden and a corresponding increase in NO. Beyond a PAFR of 2.5, no further improvement in spray quality was observed in the isothermal spray characterization experiments. The consequence in the flame is a leveling off of the particulate emissions at
approximately 55 mg/m3. The further increase in NO emissions with PAFR is due to the increased availability of O2 in the evaporation region. The effect of increased O2 availability, on this occasion due to the increase in excess air, is shown in Figs. 5aii and 5bii. In this instance, the reduction in particulates burden is due to the augmented heter-
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Fig. 5. Measured and predicted (a) NO and (b) particulates emissions at the furnace exit (M experimental; —j —— prediction).
ogeneous oxidation of the coke particles. Beyond 15% excess air, there is no further reduction in the particulate concentration because of the complete burnout of carbon in the particles. It should be noted that a particulate concentration of about 55 mg/m3 corresponds to complete carbon burnout. Additional O2 leads, however, to a further increase in the NO emissions. The effect of secondary air swirl on NO and particulates emissions is shown in Figs. 5aiii and 5biii . As can be seen, both the NO and particulate concentrations decrease slightly between swirl numbers of 1.05 and 1.2. The effect of this parameter is not readily apparent, as a larger range of swirl set-
tings were not possible due to flame stability problems. The final effect is that of rotating cup speed, which is shown in Figs. 5aiv and 5biv. As expected, increasing cup speed enhances the spray fineness, which leads to a more intense flame resulting in an increased level of NO emissions. However, this has an opposite effect on particulates emissions, which decrease because of the enhanced coke burnout rate of smaller-size particles. Concluding Remarks The experimental data collected for HFO swirling flames, generated by a rotary cup atomizer, comple-
POLLUTANTS EMISSIONS FROM SPRAY FLAMES
ment and extend the data previously gathered for a twin-fluid atomizer at ICSTM. The additional computational input from the University of Leeds further enhances the value of the work, with the code being validated against the axisymmetric experimental data. The resulting code is capable of simulating with useful precision the measured trends of NO and particulates emissions for a wide range of operating conditions of the atomizer and the burner. It is readily apparent from this work, and the work of others using different types of atomizers (e.g., pressure-jet, twin-fluid), that the “holy grail” of low-NO and low-particulates emissions appears to be mutually exclusive, and only a compromised operation is possible. A minimum particle concentration of about 55 mg/m3, achieved by increasing the quality of the atomization and using high levels of excess air, corresponds to the desired complete burnout of carbon in the fuel. Unfortunately, this is only achieved at the expense of high NO emissions. From the predicted oil droplets’ trajectories and our knowledge of low NOx technologies used for solid fuels, the atomization and evaporation must be established in an O2-lean atmosphere if NO emissions are to be reduced. Although the rotary cup, and to a greater extent the pressure-jet atomizer, can separate atomization quality from primary air-to-fuel ratio, low NO emissions demand that the droplets penetrate into the rich IRZ. Unfortunately, the fine droplets are more likely to follow the gas stream readily and diffuse into the O2-rich secondary air stream. A fundamental rethinking of the design of conventional atomizers is implied to achieve the target of combined low NO and particulates emissions. To this end, further modification of the present atomizer is being undertaken, while the furnace and the CFD code are being adapted to accommodate nonambient air concentrations prevalent in flue-gas recirculation or oxygen injection as a means of satisfying the dual demands of high efficiency and low emissions per unit energy input. Acknowledgment The authors are indebted to the Engineering and Physical Sciences Research Council of the United Kingdom for financial support (GR/H 15585) of this research project.
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COMMENTS Dr. Josette Bellan, Jet Propulsion Laboratory, USA. During combustion of asphaltene—containing oils, cenospheres are formed. These cenospheres have a central “blow hole” and burn very differently from coal particles, with transport processes being very important. We have published about 10 years ago a model (Lowenberg, Bellan and Gavales in Chemical Eng. Communications) describing precisely cenosphere combustion. Why are you using a coal-burning model? You certainly cannot hope to obtain a predictive model with a model that is so far from reality. Author’s Reply. Comprehensive simulation of heavy fuel oil spray combustion in a practical system requires modelling of a large number of physical and chemical processes. In order to construct an economical model for engineering predictions we have chosen to incorporate simple yet realistic models for these processes where possible. We agree
in general with you that it would be more realistic to use a detailed coke combustion model which accounts for the evolution of the coke structure and intricate transport processes within the porous particle during combustion. However, to date, studies on coke oxidation have been rather limited, and it is extremely difficult to quantify these effects. As a consequence, we have adopted a simplified model, as described in the text, where the kinetic rate constants are those evaluated for coke [25] and not for coal chars. A similar approach has also been used in Ref. 7 to model coke emissions from residual fuel oil fired boilers. It is of interest to note that the present coke combustion model is able to predict the measured trends of particulates emissions for a wide range of operating conditions of the atomizer and the burner. We shall bear your comments in mind and try your coke combustion model when time permits.