Mixing Effects in the Selective Noncatalytic Reduction of NO

Ind. Eng. Chem. Res. 2000, 39, 3221-3232

3221

Mixing Effects in the Selective Noncatalytic Reduction of NO Hanne Røjel, Anker Jensen, Peter Glarborg, and Kim Dam-Johansen Department of Chemical Engineering, Technical University of Denmark, Building 229, DK-2800, Lyngby, Denmark

An engineering model that combines a simple mixing model and a detailed reaction mechanism has been used to investigate selective noncatalytic reduction (SNCR) of NO. In this process a jet of NH3 is injected at high temperatures into a flue gas containing NO and O2. The mixing model used is based on the “maximum mixedness” model proposed by Zwietering. The chemical kinetic model of Miller and Glarborg was validated against experimental data obtained in a flow reactor over a range of NH3/NO/O2 compositions corresponding to conditions ranging from early jet entrainment to full mixing between reactants. The effect of mixing was investigated on three different experimental scales: A laboratory-scale diffusion-mixing reactor, a benchscale setup, and a full-scale grate-fired furnace. The results show that finite rate mixing affects the SNCR process efficiency at high temperatures where it may cause a narrowing or a widening of the temperature window, depending on the NO concentration. The effect of mixing on the selectivity for NO reduction could be modeled qualitatively in all scales with the proposed model, using mixing times estimated from simple jet correlations. Calculations for a full-scale woodfired grate fumace indicate that for this system with NO concentrations of 50-100 ppm the initial segregation of reactants may enhance the process efficiency. In systems with higher NO levels finite rate mixing may have an adverse effect on the SNCR process. Introduction Selective noncatalytic reduction (SNCR) of NOx is a widespread secondary measure for NOx control. In this process NO is reduced to N2 by injection of a reducing agent such as NH3 into the flue gas in a narrow temperature range around 1200 K. The SNCR reaction has been studied extensively (e.g., Lyon and Hardy, 1986; Miller and Bowman, 1989; Duo et al., 1992; Kasuya et al., 1995; Miller and Glarborg, 1996, 1999) and the detailed chemistry is fairly well established. The process is characterized by a selectivity in the reaction pathways as shown by the overall steps (Duo et al., 1992):

4NH3 + 4NO + O2 f 4N2 + 6H2O 4NH3 + 5O2 f 4NO + 6H2O The selectivity toward NO or N2 depends mainly on the temperature and gas composition. In the absence of combustibles, reduction of NO by NH3 is dominant around 1200 K, while oxidation of NH3 to NO becomes increasingly important with increasing temperature and may dominate. However, also the mixing of reactants is conceivably important because changes in the local conditions may favor different reaction pathways. The injection of NH3 to the flue gas is an example of injection of a high-velocity stream with a small mass flow rate into a low-velocity stream with a large mass flow rate. When the reactions are fast, both micromixing (on the molecular scale) and macromixing (on scales comparable to the physical dimension of the system) may be rate limiting. Experimental and theoretical results (Banna and Branch, 1981; Branch et al., 1982; Østberg et al., 1997; Rota et al., 1999) indicate that mixing affects the SNCR process at higher temperatures, but results appear to be contradictory. Branch et al. (1982) observed that

under non-premixed conditions the temperature window for NO reduction widens, indicating that incomplete mixing may extend the temperature range in which NO removal is effective to a higher temperature. In contrast, Østberg et al. (1997) found from bench-scale experiments that delayed mixing tended to narrow the temperature window. A reliable engineering model for the SNCR process would need to account for both chemical and fluid dynamic effects. Due to the coupling between fluid dynamics, thermodynamics, and chemical kinetics, a comprehensive detailed model may be computationally prohibitive. Turbulent fluctuations affect the chemical reaction rates as well as the flow, for example, by changing the local densities of the mixing fluids. The change in the species observed for a reactive jet along its axis is thus due to both mixing of the fluids, which introduces concentration gradients, and chemical conversion of the species. When the chemistry is described by a detailed reaction mechanism, the number of reacting species and thereby the number of partial differential equations (PDEs) in the model becomes large. This causes the solution procedure to be more demanding and results in an increased solution time. To reduce the computational time, two possibilities exist: a simplification of the reaction scheme by reducing the number of species that describe the chemical kinetics or a simplification of the flow description. The application of CFD modeling combined with a simplified kinetic scheme for SNCR has been investigated by Brouwer et al. (1996). Alternatively, the flow field can be approximated as a combination of idealized chemical reactors (Pedersen et al., 1998). The purpose of investigating simple modeling approaches is to develop a tool that can be used for the simulation of reactive flows using detailed chemistry, but at the same time avoid a large computation time. This will allow the implementation of the calculation procedures in large simulation programs for practical

10.1021/ie000049d CCC: $19.00 © 2000 American Chemical Society Published on Web 08/12/2000

3222

Ind. Eng. Chem. Res., Vol. 39, No. 9, 2000

tM )

∫0L u M

dx center(x)

)

c1 LM2 ujo dnozzle

x

Fjo Fb

(2)

Here, cl is a constant that depends on the densities of the jet and bulk (Gu¨nther, 1974):

()

c1 ) 0.07 - 0.0103 ln

Figure 1. Zwietering reactor (nonideal plug flow reactor). The entrained gas is added continuously to the jet.

design purposes. The objective of the present work is to investigate the combination of a detailed reaction mechanism with a model that describes mixing between an NH3 jet with high initial velocity and a low-velocity bulk gas flow. The chemical kinetic model is validated against experimental data obtained in a flow reactor. The effect of mixing is investigated in three different experimental scales: A laboratory-scale flow reactor, a bench-scale reactor, and a full-scale grate-fired furnace. Modeling Approach To choose an appropriate modeling approach, it is important to determine whether the conversion of the species is controlled by the rate of the chemical reaction or by the mixing process. This can be estimated from the Damko¨hler number, which is defined as the ratio between the mixing time and the characteristic time scale for the chemical conversion:

Da )

τf τc

(1)

Here, τf is the time scale for the turbulent mixing, defined from different length scales of mixing (Galmarini et al., 1995). τc, the chemical reaction time scale, can be calculated from the rate of reaction of a key species or an overall reaction rate at a well-defined temperature and conversion. Most combustion systems are characterized by high Damko¨hler numbers. Consequently, reactant segregation must be accounted for; models that use ideal plug flow in conjunction with the detailed chemistry are often not appropriate. One approach for modeling both mixing and reaction is the “Maximum Mixedness model” proposed by Zwietering (1959). This approach, which we employ in the present work, will be referred to as the Zwietering model. The reactor, which is used to describe the flow pattern of the jet mixing into the bulk gas, is the nonideal plug flow reactor shown in Figure 1. Instead of premixing the jet and bulk flow, the bulk flow is added to the jet over the mixing time, τmix. The mixing time accounts for the segregation in the reactor due to macro- or micromixing. Thereby, an increase in τmix corresponds to a slower rate of mixing of the reactants. The mixing rate of the jet and thus τmix may be estimated from experiments, from CFD calculations by integrating the flow field, or calculated from empirical entrainment correlations for turbulent jets (Gu¨nther, 1974; Simpson, 1975). Taking the characteristic length scale LM for macromixing to be the entrainment length for a free jet, the corresponding time scale can be estimated as (Simpson, 1975)

()

Fjo Fjo - 0.00184 ln2 Fb Fb

(3)

The time scale in (2) is defined as the time a fluid parcel takes to flow from the nozzle to the point of complete mixing based on the center velocity. The characteristic time for micromixing may be found as (Brodkey, 1975)

tm )

() ( ) 5 π

2/3

LM2 k

1/3

(4)

Here, k is the rate of kinetic energy dissipation per unit mass. The mathematical description of the Zwietering reactor for a constant feed rate is derived by Rota et al. (1997). An alternative approach is to use an exponential entrainment rate (Alzueta et al., 1998; Østberg et al., 1998). An exponential entrainment rate can be described by a pseudo-first-order reaction and thereby for isothermal conditions easily incorporated into a chemical kinetic mechanism, for instance, NO/ (nonreactive, in bulk flow) f NO (reactive, entrained in jet flow). It is important to reduce the effect of dilution resulting from the premixing of the jet and bulk flows when the bulk flow rate is significant compared to that of the jet flow. This can be achieved by converting each pseudocomponent into a number of reactive species, e.g., (NO)x (nonreactive) f xNO (reactive). For large values of x the dilution effect is minimized. The exponential entrainment rate and the linear rate of Ricou and Spalding (1961) are very similar until about 90% of the total gas flow is entrained in the jet volume. The time to obtain 90% mixing as calculated from the more correct linear entrainment models may be used to define the entrainment rate constant K in the exponential model to obtain the same degree of mixing in the same time:

˘ joz ) m ˘ jo + m ˘ e)m ˘ jo + mb(1 - exp(-Kt)), m ˘ ) CRSm t e τ90 (5) z ) x/d/ is the axial distance from the nozzle normalized by

x

d/ ) d0

Fjo Fb

and

mjo )

Fjoujoπd02 4

The residence time as a function of the axial position along the jet axis is obtained from

Ind. Eng. Chem. Res., Vol. 39, No. 9, 2000 3223

t)

∫0xdx u j

∫0r ru(r,x) dr u j (x) ) ∫0r r dr j

(6)

j

A combination of (5) and (6) leads to

(

ln 1 K)-

)

m ˘ jo(Ka′xτ90 - 1) m ˘b τ90

(7)

Ka′ is found from the expression

Ka′ )

x

CRS d/a

K′d0U0(1 - e-Ba ) B 2

where a, B, and K′ are constants describing the velocity field u(t,x) (Blevins, 1984). m ˘ e is the flow rate of the entrained gas. The constant CRS is 0.32 for a nonreacting free jet, while for a jet in cross-flow a value of CRS twice as high, i.e., 0.64, may be used (Cha and Kramlich, 1999). If reactions taking place in the jet cause significant heat release, CRS should be lowered compared to these values (Ricou and Spalding, 1961). For turbulent jets the mixing times calculated with an appropriate CRS value are expected be representative for the mixing process. Problems may arise, however, if several jets are positioned close together so the flow fields of the jets will interfere and jet-jet and jet-bulk mixing will occur simultaneously. The Zwietering approach together with full chemistry has been applied to model natural gas reburning (Rota et al., 1997; Alzueta et al., 1998; Cha and Kramlich, 1999) and very recently also SNCR (Rota et al., 1999). Generally, the results have been encouraging. The simple Zwietering model considers only macromixing; micromixing is assumed to be instant. A number of more complex reactor configurations have been proposed in the literature (Mehta, 1981; Mehta and Tarbell, 1983; Broadwell and Lutz, 1998; El-Hamouz and Mann, 1998). However, results obtained for reburning indicate that as long as the initial segregation between reactants is accounted for, the actual mixing model applied is less significant (Cha and Kramlich, 1999). In the present work the equations were solved using software that runs in conjunction with the CHEMKIN subroutine library (Kee et al., 1990). For plug flow simulations SENKIN (Lutz et al., 1990) was used. For Zwietering calculations under nonisothermal conditions, i.e., with a temperature difference between bulk and jet flows, a specially developed code was employed (Kristensen and Sarbaek, 1998). In this code the mixing rate estimated by eq 7 is written as a first-order differential equation that is solved separately. Experimental Section The present analysis of the influence of mixing on the SNCR process was based on experiments carried out at three scales: laboratory scale, bench scale, and full scale. In addition, experiments were conducted in a flow reactor under conditions with no mixing influence, to characterize the chemistry and validate the chemical kinetic model. The setups are sketched in Figure 2. The

Figure 2. Setups. (a) The homogeneous flow reactor (length of reactor tube, 190 mm). (b) The mixing diffusion reactor (length of reaction zone, 250 mm). (c) The bench-scale setup. (d) A model of the full-scale furnace (dimensions: height, 17 m; width, 8 m; depth, 6 m). For the bench scale setup: (1) natural gas burner; (2) combustion chamber; (3) water cooling probe; (4) reactor tube (length, 5 m.(meters)); (5) injection point; (6) probe for sampling gas to analysis; (T) temperature measurements.

laboratory- and full-scale experiments were carried out in the present work, while the bench-scale results were adopted from Østberg et al. (1997). The laboratory setup used for both the flow reactor and the diffusion experiments consisted of a gas supply section, a reactor section, and a gas analysis section. This setup and the experimental procedure have been described in detail elsewhere (Duo et al., 1991; Glarborg et al., 1994; Kristensen et al., 1996). The flow reactor (Figure 2a) was designed to obtain ideal plug flow. The temperature was measured by a thermocouple placed in a quartz tube with no access for the reactant gases. The gaseous components were fed to the reactor in four separate streams. The main flow usually contained nitrogen, oxygen, and water. The reactor tube had a radius of 0.45 cm and a length of 19 cm. The total gas flow was about 1200 NmL/min (at 1 atm and 273 K). It was kept constant during the experiments, leading to different residence times, depending on the temperature in the reaction zone. The residence time was approximately 150 ms at 1200 K. The laboratory diffusion experiments were carried out in a laminar diffusion reactor built of two concentric tubes. The injector tube had an inner diameter of 11 mm and the outer tube had an inner diameter of 28 mm (Figure 2b). The length of the reaction zone was 250 mm. The reactor was designed to obtain a laminar flow and similar volume flows in the tubes. The reactor was placed in the electrically heated oven described above. The length of the injector tube was designed from the

3224


temperature profile of the oven to obtain an isothermal reaction zone. The residence time was about 1 s at 1200 K calculated from the end of the injector tube to the end of the isothermal zone. The Reynolds number of the main flow was in the range from 60 to 40 in the temperature range considered and the laminar flow theory may be used to investigate the jet. For the flow reactor the product gases were quenched at the outlet of the reactor by heat exchange with cooling air. The concentrations of NO, CO, CO2, and O2 were measured continuously by UV, IR, and paramagnetic analyzers with an accuracy of 3% but no less than 5 ppmv. The bench-scale experiments (Østberg et al., 1997) were carried out in the setup shown in Figure 2c. The flue gas to the reactor was produced by a 100-kWth burner (1) fired with natural gas and pressurized air. The burner was operated at about 45-kW thermal input and an O2 concentration of 3.4 ( 1 vol %, whereby sufficient amounts of NO for the experiments were formed. The typical flue gas composition during the experiments was (on a volume basis) 73.4% N2, 15.1% H2O, 8.0% CO2, 3.4% O2, and 550 ppmv NO. After the combustion chamber (2), the flue gas flowed through a tube with a heat exchanger (3) to control the temperature at the inlet to the reaction tube. The length of the heat exchanger was adjustable to allow variation in the inlet temperature of approximately 200 K. The reactor tube consisted of five sections with a length of each 1 m and an inner diameter of 0.05 m. During the experiments the temperature was measured at 0.25 and 4.25 m from the inlet. The residence time was approximately 100 ms at a temperature at the injection point of 1200 K. A temperature difference of about 75 K between the two measuring points was observed. The injection of NH3 (5) was placed 1.25 m downstream from the inlet; at this location, the bulk flow was fully developed after the 90° bend. The NH3 flow (1 NL/min) was injected in cross-flow into the flue gas through a nozzle 1.9 mm in diameter. Two carrier gases, N2 and air, were used. A water-cooled sampling probe (6) was positioned 4.5 m downstream from the inlet. The sampled gas was analyzed for O2, CO, CO2, and NO by continuous gas analyzers. The concentration of NH3 was measured by absorption in water and titration with HCl. The outlet concentrations with and without NH3 injection were measured. The full-scale furnace was a 78-MWth wood chip fired grate furnace. A sketch of the furnace is shown in Figure 2d. The NOx emission was controlled by SNCR with injection of NH3. The jets shown on the sketch only identify some of the injection points for the secondary and tertiary air and for the NH3. The temperature and concentrations were measured in the fuel-rich zone above the grate, just above the injection of secondary air and at positions just below and above the injection of NH3. The flue gas was sucked from the furnace through a water-cooled probe. It was transported through heated tubes that ensured a gas temperature above 180 °C, preventing condensation of water. The water was then condensed at 0 °C in a refrigerator. Finally, the gas was led to the individual gas analyzers. The flue gas components CO, CO2, O2, SO2, and NO were measured continuously with gas analyzers (UV, IR, and paramagnetic). No SO2 was detected.

Table 1. Inlet Reaction Conditions for the Validation of the Thermal DeNOx Mechanism of Miller and Glarborg (1996) NH3 NO O2 H 2O figure (ppmv) (ppmv) (vol %) (vol %) A B C D E F

2000 1950 1950 1900 2000 1950

520 168 85 16 46 168

3.02 1.00 0.52 0.1 0.25 0.25

3.0 3.0 3.0 3.0 3.0 3.0

N2 balance balance balance balance balance balance

NH3/NO O2/NO 4 12 23 119 43 12

58 59 61 62 54 15

Results Validation of the Kinetic Model. The chemistry of the SNCR process with ammonia has been extensively studied and both our understanding of the detailed chemistry and the reliability of the reaction mechanisms have improved considerably. In the present work we have adopted the chemical kinetic model of Miller and Glarborg (1996), referred to below as the MG mechanism. This scheme has been validated against flow reactor data over a wide range of oxygen concentrations (Kasuya et al., 1995), but with a fixed NH3/NO ratio of about 2. However, in a mixing configuration with slow entrainment of the bulk flow, extreme ratios between the reactants in the jet and bulk gas streams will occur. Consequently, reaction conditions may locally deviate significantly from those of a premixed reaction system. Since the MG mechanism similar to other available chemical kinetic models for SNCR has been validated against premixed data covering a limited range of NH3/NO ratios, model predictions may be inaccurate for the local conditions of the jet. For this reason experimental work was carried out to validate the MG mechanism for reactant ratios typical of the near nozzle conditions in an NH3 jet in a flue gas containing NO, O2, H2O, and N2. The reaction conditions for the validation experiments are shown in Table 1. Conditions were chosen to simulate ratios between the initial NH3 and NO level in the range 20-100 as found close to the NH3 injection nozzle in practical systems. These ratios significantly exceed final values obtained at full mixing. Figure 3 compares the model results and experimental data. It was chosen to vary the NO level at a constant NO/O2 ratio (Figure 3a-e). If no O2 is present in the carrier gas, the NO/O2 ratio will be equal to the ratio in the bulk flow; the absolute levels, however, will depend on the entrainment rate. The data show that when the NO and O2 levels are decreased (corresponding to a position closer to the nozzle), the initiation temperature for reaction increases. This indicates that comparatively little reaction will take place early in the NH3 jet where the temperatures are comparatively low; reaction will be delayed until sufficient amounts of heat and reactants (NO and O2) are supplied to the jet. In an additional experiment, the NO/O2 molar ratio was lowered (Figure 3f) while the NO level was kept the same as that in Figure 3b. With a lower ratio of NO/ O2 the initiation temperature as well as the temperature of maximum NO reduction increase due to the lack of O2. This is consistent with the findings of Kasuya et al. (1995). In general, the model predicts the flow reactor data fairly well. The best agreement is obtained at the higher oxygen concentrations, i.e., at conditions that correspond to full premixing (Figure 3a). For lower NO and O2 levels deviations between experimental data and model-


Figure 3. Comparison between experimental data obtained in the flow reactor and the mechanism of Miller and Glarborg (1996). In each experiment the inlet concentration of NH3 was 2000 ppmv, [H2O] ) 3 vol %. The O2/NO ) 60 for (a)-(e). In (f) O2/NO ) 15. The residence time was 150 ms at 1200 K. The reaction conditions are described in Table 1.

ing predictions are observed for both the location and width of the temperature window. This is most pronounced for Figure 3c,d,f. However, the experimental trends are predicted correctly. Despite the shortcomings, we believe the mechanism to be sufficiently accurate to be used with the Zwietering approach to investigate the SNCR process at partial mixing conditions. The uncertainties in the mechanism mainly relate to conditions with low oxygen and nitric oxide concentrations, i.e., early in the jet. In bench-scale and full-scale experiments, the initial low temperature of the NH3 will prevent reaction in the early phases of entrainment and the model deficiencies will be of minor importance. However, the uncertainties in the mechanism may influence the modeling of the isothermal laminar diffusion experiments performed in the laboratory. Investigation of the Influence of Mixing on SNCR. The mixing model with exponential entrainment described in the previous paragraph was used in conjunction with the detailed chemical kinetic model to simulate the three experimental scales. The results are discussed below.

Laboratory-Scale Conditions. Figure 4 shows experimental data for SNCR obtained in the laminar diffusion reactor. The data are compared with experimental results obtained under plug flow conditions for the same inlet composition. The plug flow experiments were carried out by Duo et al. (1991) with a reactor similar to the one shown in Figure 2a. At lower temperatures, the two sets of data are not directly comparable due to significant differences in reactor residence time. The higher temperature of initiation for the plug flow results is due to a shorter residence time in the mixing reactor compared to the data obtained in the diffusion reactor. However, when the temperature increases, the chemical reactions become faster. Thereby, the characteristic chemical time scale will decrease and eventually become much shorter than the reactor residence time. At high temperatures, i.e., above 1250 K, deviations between the two sets of experimental data can be attributed to mixing effects. According to the experimental results in Figure 4, mixing has a distinct (but not drastic) effect on the SNCR chemistry at higher temperatures. The data show

3226


Figure 4. Variation of the mole fraction of NO with the temperature and the mixing rate for the laboratory-scale reactor. The inlet mole fractions for the mixing experiment were NO, 500 ppmv; O2, 4 vol %; NH3, 1000 ppmv. The residence time was 1274/T(K) s. The inlet mole fractions for the plug flow experiment were (Duo et al., 1991) NO 496 ppmv, O2 3 vol %, and NH3 1021 ppmv. The residence time was 93/T(K) s. Lines are modeling results and symbols are experimental data.

that the temperature window becomes narrower when the mixing rate is decreased. The experimental data were compared with modeling predictions (shown as lines in Figure 4). To estimate the entrainment rate of the “bulk flow”, correlations for laminar jets were employed. For a laminar submerged jet Klaminar may be expressed as (Blevins, 1984)

(

ln Klaminar )

m ˘b m ˘ b+m ˘ jo - 8πµjx90 τ90

)

(8)

µj is the viscosity of the jet and τ90 was obtained as

τ90 ) t(x90) )

∫0

x90dx

u j

)

5.924µx902 (m ˘ b+m ˘ jo)F

(9)

For the diffusion reactor the use of laminar flow theory results in a value of K ) 33, independent of temperature. This value corresponds to a mixing time of about 70 ms. The plug flow simulation is in reasonable agreement with the experimental plug flow data, despite a notable shift in the initiation temperature. Note that the residence time of the plug flow simulations corresponds to the flow reactor experiments, while the mixing calculations were performed with the longer residence time of the laminar diffusion experiments. Zwietering modeling predictions are performed for mixing times of 5 and 50 ms, respectively. The latter value is comparable to the estimate of 70 ms discussed above. At low temperatures, no effect of mixing is predicted from the modeling results. When the characteristic chemical time scale becomes small at higher temperatures, the mixing time eventually becomes limiting and affects reaction selectivity. Consistent with the experimental data, the results obtained from the Zwietering model deviate from the plug flow results. The model predicts correctly that the temperature window for NO reduction becomes narrower for longer mixing times. However, the Zwietering predictions tend to slightly overestimate the effect of mixing compared to the experimental data. The

Figure 5. Variation of the normalized outlet mole fraction of NO and NH3 with the injection temperature and the mixing rate for the bench-scale reactor. Inlet mole fractions of NO 550 ppmv, O2 3 vol %, and NH3 1000 ppmv. Residence time: 95 ms at 1200 K. Carrier gas: Nitrogen. Lines are modeling results and symbols are experimental data.

difference may be attributed to the simplified mixing approach as well as uncertainties in the reaction mechanism and in the data. Also, these experiments are characterized by a low Peclet number and axial diffusion not accounted for in the model may affect the experimental results. Bench-Scale Conditions. The influence of initial momentum and composition (N2 or air) of the carrier gas for the NH3 jet in SNCR was investigated in a benchscale experiment by Østberg et al. (1997). Ammonia and the carrier gas were injected into the flue gas from the gas burner to obtain a molar ratio of NH3/NO ) 2 at complete mixing conditions. The inlet conditions varied with the temperature of the flue gas from the natural gas burner. The experimental temperature gradient and the variation in inlet concentrations were accounted for in the model. The flow of the carrier gas and thereby the jet momentum was varied from 3 to 11 NL/min while keeping the flow of NH3 at 1 NL/min. This way the influence of mixing on the process could be assessed. The results of the bench-scale experiments are shown in Figure 5 for N2 as the carrier gas. Neither the benchscale results (Østberg et al., 1997) nor the present calculations indicate any effect of the carrier gas composition. The concentration of NO is shown as a function of the temperature at the injection point. When the flow of the carrier gas was increased and the mixing time thereby decreased, the NO reduction window broadened. The mixing only affected the high-temperature branch of the NO reduction window. Data for the ammonia concentration (Østberg, 1996) showed little effect of jet momentum. At lower temperatures ammonia consumption is governed by the slow chemical reactions and mixing has little effect. At high temperatures where mixing may become rate limiting, the ammonia is largely oxidized. In all experiments the Reynolds numbers of the jet and bulk gas were above 7000 and 30 000, respectively, and K could be obtained from the empirical correlations for turbulent jets for different temperatures and carrier flows. For the experimental conditions investigated the specific momentum (Fu2)jet/(Fu2)cross is small compared to the jets present in full-scale experiements ((Fu2)jet/ (Fu2)bulk > 10 000) and so the effect of the cross-flow is expected to be large. The specific momentum of the ammonia jet increased from 2.4 to 22 when the carrier

Ind. Eng. Chem. Res., Vol. 39, No. 9, 2000 3227 Table 2. Time Scales for Complete Mixing of the NH3 Jet Injected into the Bench-Scale Reactor CRS ) 0.32 temp. (K)

carrier gas (NL/min)

tm (ms)

tM (ms)

1200 1400 1200 1400

7 7 11 11

41 35 12 10

74 64 21 18

CRS ) 0.64 KM

tm (ms)

tM (ms)

KM

30 35 105 122

10 8 3 3

18 16 5 5

123 143 421 491

flow increased from 3 to 11 NL/min. The estimated mixing times and entrainment rate constants are shown in Table 2. For all combinations of temperature and jet momentum considered the time scale of macromixing time exceeds the micromixing time and consequently the macromixing is expected to dominate the mixing process. However, the estimated time scales are of the same order of magnitude and therefore none of them may be neglected. This is consistent with the findings of Østberg et al. (1997); i.e., a good prediction of the mixing effect on the NO reduction is found when mixing times comparable to the micromixing times listed in Table 2 are used. The macro- as well as micromixing time decrease when the jet momentum and the temperature increase. This is consistent with the experimental data shown in Figure 5. The modeling results for mixing times of τ90 ) 0 (plug flow), 50, and 100 ms are shown in Figure 5 as lines. Following Østberg et al. (1997) and Rota et al. (1999), we have assumed isothermal conditions in the modeling. The model captures the essential trends of the benchscale experiments, i.e., that mixing mainly affects the high-temperature branch of the NO reduction window and that the improved mixing at increased momentum causes a widening of the temperature window. The model predicts some effect of mixing, even at lower temperatures, because the estimated mixing times are comparable to the reactor residence time. Thereby, unmixed reactants present in the outlet may affect the process efficiency. This effect is not discernible in the experimental data. Full-Scale Conditions. In the full-scale application, the ammonia was injected into an aqueous solution, using preheated air as the carrier gas. The injection system involved a number of nozzles, located on both sides of the furnace. The effect of NH3 injection was investigated for a full load and a low load case, i.e., 78 and 27 MWth. The concentrations of NO, O2, CO, and CO2 were measured above the injection system and at the injection level while the ammonia injection was switched off. The average values without injection are shown in Table 3. The temperatures at the injection point at full load and low load were approximately 1200 and 1023 K, respectively. The high load experiments were performed to investigate the effect of nozzle configuration. In these tests both injection sides or either the left or right side were used for ammonia injection. Typical results for the exit NO mole fraction are shown in Figure 6. Independent of injection strategy, only small changes in the average NO concentration were observed. Due to the presence

Figure 6. Variation of the mole fraction of NO with time and injection configuration for the full-scale case. Carrier gas: air. The nozzle was placed in three rows. Upper level is the upper row. The total amount of ammonia injected was unchanged.

of CO (750 ppm), the injection temperature of 1200 K was too high to be favorable for the SNCR process. As shown in previous work (Duo et al., 1991), the presence of significant amounts of combustibles causes a shift in the process window toward lower temperatures. At low load, the effect of NH3 injection was investigated by switching it off while the air injection was maintained. These conditions proved slightly more favorable for the SNCR process; a reduction of the NO emission of about 20% was observed. To explain the full-scale test results and to assess the effect of mixing on the SNCR process in the plant, the Zwietering approach was used. In the modeling the heating of the cold ammonia jet was attributed to the physical mixing with the hot bulk gas. For the conditions considered the heat of reaction would not significantly affect the temperature in the jet. The calculations were carried out for single jets, neglecting any interference between nozzles. It was assumed that each of n nozzles covered a fraction of 1/n of the bulk gas. This is an acceptable assumption for the furnace geometry because the jets will act like one jet at a certain distance downstream of the nozzle (Holdeman, 1993). The length and time scales for the jets are shown in Tables 4 and 5. Only the flue gas entrainment from the half of the cross section near the nozzle was considered due to symmetry of the furnace and the flow at the injection location. Figures 7 (full load) and 8 (low load) compare the predicted NO concentration in the measuring point with the corresponding plug flow calculations and with the mean measured values (shown as lines). The jet ammonia concentration was assumed to be 2800 ppmv, corresponding to an overall NH3/NO ratio of 1.5. Both a low and a high value of the entrainment constant CRS were used, i.e., CRS ) 0.32 (free jet) and CRS ) 0.64 (jet in cross-flow). From the velocity ratio (u0/ub ) 50) CRS ) 0.32 is expected to provide the best prediction (Blevins, 1984).

Table 3. Inlet Concentrations for Calculations on the Ammonia Injection at Low and High Load

low load high load

NO (ppmv)

CO (ppmv)

O2 (vol %)

CO2 (vol %)

NH3 in jet (vol %)

Tbulk

70.1 ( 5 81.3 ( 4

1040 ( 400 854 ( 300

3.75 ( 0.5 2.52 ( 0.4

16.6 ( 0.2 17.7 ( 0.2

2.0/0.2 2.0/0.2

1023 1200

3228


Figure 7. Modeling results of a variation in the NO mole fraction at high load. Carrier gas: air. Center line: τmix was calculated from (4). Integrated: τmix was calculated from (6). CRS is the constant in the expression of Ricou and Spalding (1961). NH 3/NO ) 1.5 at premixed conditions was used for the calculations.

Figure 8. Modeling results of a variation in the NO mole fraction at low load. Carrier gas: air. Center line: τmix was calculated from (4). Integrated: τmix was calculated from (6). CRS is the constant in the expression of Ricou and Spalding (1961). NH3/NO ) 1.5 at premixed conditions was used for the calculations. Table 4. Time and Length Scales for Complete Mixing of the NH3 Jet Located at the Grate Fired Boiler at High Load (Jet/Bulk Flow Interaction: Jet Entrains Half the Cross Section) CRS ) 0.32

CRS ) 0.32

CRS) 0.64

tM (ms)

KM

L e ) LM

tM (ms)

KM

899a 756

3 3

8 8

225a 189

10 12

a

Table 5. Time and Length Scales for Complete Mixing of the NH3 Jet Located at the Grate Fired Boiler at Low Load (Jet/Bulk Flow Interaction: Jet Entrains Half the Cross Section)

Le ) LM

Based on the average velocity.

For a full load case (Figure 7), the initial segregation of reactants is important for the predicted process efficiency, but the rate of mixing (the value of CRS) has only a small effect on the modeling results. The Zwietering model predicts a modest reduction in NO, in agreement with the measured results, while a net formation of NO is predicted by the plug flow model. For these conditions, the initial segregation of reactants thus appears to promote the process performance. This effect, which is contrary to the observation from the laboratory- and bench-scale experiments, can mainly be attributed to the low initial NO concentration. This issue is discussed in more detail below. Due to the initial low temperature of the ammonia jet, a high mixing

CRS ) 0.64

tM (ms)

KM

L e ) LM

tM (ms)

KM

L e ) LM

723* 561

3 4

7 7

1818 140

12 16

4 4

a

Based on the average velocity.

fraction, about 80%, is necessary to initiate reaction. For this reason most of the chemical reaction actually occurs at fully mixed conditions. Still, the early conversion of NH3 determines whether a net formation or reduction of NO is predicted. At low load (Figure 8) the NO reduction is limited by a low bulk temperature. For these conditions, i.e., the low-temperature leg of the NO curve, mixing effects would be assumed to be minor. This is confirmed by the modeling predictions; as expected, the differences between Zwietering and plug flow modeling predictions are comparatively small. Still, the Zwietering calculations are in slightly better agreement with observations than plug flow calculations.


NH3/NO ) 1000/500. All other conditions are the same in the two cases. The two cases correspond to initiation at concentration levels representative of the almost premixed case and at levels when axial changes due to mixing are significant. The mixing times in the calculations were varied from 0 to 50 ms. Calculations are shown for both the full MG mechanism and the simple two-step kinetic model of Brouwer et al. (1996):

NH3 + NO f N2 + H2O + H NH3 + O2 f NO + H2O + H

Figure 9. Variation of the mole fraction of NO with the temperature and the mixing rate. The inlet mole fractions are: NO 50 ppmv, O2 4 vol %, and NH3 1000 ppmv for the full lines and NO 500 ppmv, O2 4 vol %, and NH3 1000 ppmv for the dotted lines. Thick lines are the curves obtained at plug flow. Residence time: 150 ms at 1200 K. H2O: 3 vol %. The detailed mechanism of Miller and Glarborg (1996) and the reduced model of Brouwer et al. (1996) were used.

For both full load and low load the model tends to overpredict the NO reduction obtained. The deviations from the measured concentrations may partly be attributed to uncertainties in reaction conditions caused by incomplete mixing between flue gas and jet and fluctuations in temperature and CO concentrations. Peaks of CO of up to 2000 ppm caused by the grate vibration or local changes in the flow will shift the optimum temperature for NO reduction considerably and render comparisons of the measurements and modeling predictions more difficult. Effect of Mixing on the SNCR Process The experimental and modeling results of the laboratory- and bench-scale experiments (Figures 4 and 5) indicate that the characteristic temperature window for SNCR that results from the selectivity in the chemistry will become narrower when the mixing rate decreases. This is also in agreement with the recent analysis of Rota et al. (1999). However, the opposite trend was found by Branch et al. (1982) based on laboratory-scale data as well as in our modeling of the full-scale data (Figure 7). On the basis of the experimental results of Banna and Branch (1981) together with modeling, Branch et al. proposed that finite rate mixing causes a broadening of the window. Figure 9 shows the Zwietering modeling results for conditions similar to the experiments of Banna and Branch, i.e., NH3/NO ) 1000/50, and for a case with

Due to its simplicity, this simple scheme is suitable for process simulations with the CFD codes where reduced mechanisms are preferred to reduce the computational time. The scheme has been validated against the detailed kinetic model using a plug flow assumption with good results. Figure 9 together with additional calculations indicate that when the ratio of NH3/NO is large or the initial NO concentration is low, mixing causes the window to widen. These trends are observed for calculations with both the full MG mechanism and the simple scheme of Brouwer et al. At a high ratio of NH3/NO or a low concentration of NO, an increase in the mixing time results in a suppression of the NH3 oxidation to NO. For full-scale SNCR process applications the molar ratio of NH3/NO at the temperature of initiation is usually limited to the range 1-3. For these conditions a NO concentration in the range 50-100 ppmv or below may result in a widening of the temperature window due to mixing effects. Such concentration levels may be found in biomass fired systems, such as that described in the present work. In a system with higher NO levels, such as pulverized coal combustion, the delayed mixing will result in a narrowing of the process temperature window, compared to that of the premixed conditions. Branch et al. (1982) explained the principal reason for the broader temperature window for NO removal with incomplete mixing as follows. In the plug flow calculations, chain carriers remain at their place of origin and promote the chain branching process, which results in OH growth and which, ultimately, leads to NO production at high temperatures. However, when gradients are introduced into the system, the chain carriers diffuse away from where they were formed and thus damp the branching process. Consequently, the formation of hydroxyl is impeded at high temperature by the diffusive removal of the active chain carriers from the reaction zone, and the temperature regime in which nitric oxide reduction is effective is extended. In the temperature range for the NO reduction, only the rate of the overall process is affected, although the same mechanism hampers the reduction process slightly near the lowtemperature limit. If the explanation of Banna and Branch (1982) was correct, a widening of the temperature window should also be observed at NH3/NO ratios of 2; this is not the case (Figure 4). Furthermore, the fact that the Zwietering model, which neglects diffusion, is capable of describing the experimental results indicates that diffusion of radicals is not the main explanation. A detailed analysis of concentration profiles over time calculated with the model suggests a different explanation. Figure 10 shows the transient behavior of the normalized mole flows of NO and NH3 through the

3230


Figure 10. Variation of the mole fraction of NO and NH3 with the residence time and the mixing rate. The modeling results were obtained with the Zwietering approach. The inlet mole fractions of NO at complete mixing were 24 ppmv, O2 2 vol %, and NH3 500 ppmv for the bold lines and NO 250 ppmv, O2 2 vol %, and NH3 500 ppmv for the thin lines. The temperature was 1500 K throughout the system. The detailed mechanism of Miller and Glarborg (1996) was used. β ) [NH3]i/[NO]i.

reactor obtained at 1500 K for the reaction conditions corresponding to Figure 9. The plug flow case is compared to a nonpremixed case with a mixing time of 12 ms. The variable on the ordinate is independent of dilution of the jet flow due to entrainment. Thereby, a change is due to reaction only. At the high NH3/NO ratio under plug flow conditions the NO concentration is so low that oxidation of NH3 to NO is the dominating reaction pathway. Since the gases are premixed, the NO generated by NH3 oxidation will be present in a relatively low concentration and so the reduction of the formed NO by the remaining NH3 is slow compared to oxidation. In a jet mixing situation O2 and NO will be entrained into the NH3 jet and at the time where the reaction mixture ignites and NH3 oxidation is initiated only a small part of the bulk gas is entrained. For this reason the NO generated by NH3 oxidation reaches rather high concentration levels, which is beneficial for reduction of NO by the remaining NH3. Since O2 is present in excess, all NH3 is consumed long before all the bulk gas is entrained and the last part of the entrainment is a pure dilution of the jet. For the case of NH3/NO ) 2 in plug flow, the NO concentration is high enough for NO reduction to be dominating. During jet mixing insufficient amounts of NO are entrained into the jet at the point where the O2 concentration becomes high enough to cause ignition. As a result, NH3 oxidation is initially the dominating reaction. Although the NO concentration quickly increases, too much NH3 will be consumed by the oxidation reaction for the jet mixing case to be overall beneficial compared to that of the plug flow case. Conclusions The influence of finite rate mixing on the SNCR process was studied experimentally and theoretically. The experiments were carried out in three widely differing experimental scales: A laminar co-flow diffusion reactor, a bench-scale setup (Østberg et al.), and a 78 MW full-scale wood fired boiler. The detailed chemical kinetic model of Miller and Glarborg was validated in separate experiments under plug flow conditions using ratios of NH3 to NO and O2 specific to the jet. In general, the kinetic model compared fairly well with the experimental data, with the best agreement obtained at higher NO and O2 concentrations. The detailed reaction mechanism was combined with a simple model

for the mixing process based on the approach of Zwietering. The mixing model contained only one parameter that could be predicted from simple jet entrainment correlations. The experimental data showed that finite rate mixing influenced the observed selectivity for reduction of NO at high temperatures where the chemistry is fast compared to that of mixing. Experiments at the laboratory and bench scale showed that, at NO levels of about 500 ppmv, the effect of finite rate mixing was to narrow the temperature window where NO reduction took place. At low initial NO levels or at high NH3/NO ratios, simulations indicate that finite rate mixing may be beneficial for the SNCR process. Modeling of full-scale experiments, characterized by low NO concentrations, indicate that the initial segregation of reactants may enhance process efficiency, but the rate of mixing has only a small effect on the modeling results. The model presents a relatively simple tool for analysis of mixing-chemistry interactions using detailed chemical kinetic models in systems where a reactant is introduced into a system through a turbulent jet. Acknowledgment The authors would like to thank Martin Skov Rasmussen and Thomas Wolfe, CHEC, for assistance in carrying out laboratory-scale experiments. Thomas Wolfe and Jørn Hansen, CHEC, are gratefully acknowledged for their work during the measuring campaign at the wood chip fired boiler. This work was carried out as a part of the CHEC (Combustion and Harmful Emission Control) Research Program, which is financially supported by the Danish Ministry of Energy, Elsam (the Jutland-Funen Electricity Consortium), Elkraft (the Zealand Electricity Consortium), the Danish and Nordic Energy Research Programs, the European Union, and the Danish Technical Research Council. Nomenclature C ) entrainment constant d0 ) nozzle diameter d/ ) source diameter k ) turbulent kinetic energy K ) constant describing the exponential entrainment L ) the upper bound of the largest eddies l ) length scale

Ind. Eng. Chem. Res., Vol. 39, No. 9, 2000 3231 Lw ) length of impact Le ) length of entrainment LM ) length of macro mixing m0 ) mass flow rate at the nozzle m ) axial mass rate P ) pressure tM ) characteristic time for macromixing tm ) characteristic time for micromixing t ) residence time, characteristic time T ) temperature u ) average axial velocity u0 ) velocity at the nozzle x ) distance downstream of the nozzle z ) x/d ) nondimensional axial location Greek Letters π ) density of the fluid µ ) viscosity of the fluid τ ) time scale τc ) chemical time scale τf ) time scale for the turbulent mixing τmix ) time over which the bulk flow is added continuously to the jet Subscripts 90 ) condition where 90% of the bulk flow has been entrained b ) bulk fluid c ) chemical f ) flow i ) initial jo ) conditions at the nozzle outlet j ) jet m ) micromixing M ) macromixing 0 ) initial conditions x ) x-direction Superscript /

) nonreactive pseudo species

Abbreviations CFD ) computational fluid dynamics Da ) Damkohler number i.d. ) inner diameter IR ) infrared NL ) Normal liter (at 273 K and 1 atm) o.d. ) outer diameter PDE ) partial differential equation RS ) Ricou and Spalding SNCR ) selective noncatalytic reduction UV ) ultraviolet

Literature Cited Alzueta, M. U.; Bilbao, R.; Millera, A.; Glarborg, P.; Østberg, M.; Dam-Johansen, K. Modeling Low-Temperature Gas Reburning. NOx, Reduction Potential and Effects of Mixing. Energy Fuels 1998, 12, 329. Banna, S. M.; Branch, M. C. Mixing and Reaction of NH3 with NO in Combustion Products. Combust. Flame 1981, 42, 173. Blevins, R. D. Applied Fluid Dynamics Handbook; Van Nostrand Reinhold Co.: New York, 1984. Branch, M. C.; Kee, R. J.; Miller, J. A. A Theoretical Investigation of Mixing Effects in the Selective Reduction of Nitric Oxide by Ammonia. Combust. Sci. Technol. 1982, 29, 147. Broadwell, J. E.; Lutz, A. E. Turbulent Jet Chemical Reaction: NOx Production in Jet Flames. Combust. Flame 1998, 114, 319. Brodkey, R. S. Mixing in Turbulent Fields; Academic Press: New York, 1975. Brouwer, J.; Heap, M. P.; Pershing, D. W.; Smith, P. J. A. Model for Prediction of Selective Non-Catalytic Reduction of Nitrogen

by Ammonia, Urea, and Cyanuric Acid with Mixing Limitations in the Presence of CO. Proc. Combust. Inst. 1996, 26, 2117. Cha, C. M.; Kramlich, J. C. An Investigation of the Effect of FiniteRate Mixing in Natural Gas Reburning Using a Simple Mixing Model. Presented at the Joint Meeting of the United States Sections of Combustion Institute, March 15-17, 1999, Session D-2, Practical Combustion 1, 1999. Duo W.; Dam-Johansen, K.; Østergaard K. Widening the Temperature Range of the Thermal DeNOx Process: An Experimental Investigation. Proc. Combust. Inst. 1991, 23, 297. Duo W.; Dam-Johansen, K.; Østergaard K. Kinetics of the GasPhase Reaction between Nitric Oxide, Ammonia and Oxygen. Can. J. Chem. Eng. 1992, 70, 1014. El-Hamouz, A. M.; Mann, R. Effect of Micro-Mixing on the Yield Intermediates in Triplet Consecutive/Competitive Reactions. Can. J. Chem. Eng. 1998, 76, 650. Galmarini, S.; Vila-Guerau de Arellano, J.; Duykerke, P. G. The Effect of Micro-Scale Turbulence on the Reaction Rate in a Chemically Reactive Plume. Atmos. Environ. 1995, 29, 87. Glarborg, P.; Johnsson, J. E.; Dam-Johansen, K. Kinetics of Homogeneous Nitrous Oxide Decomposition. Combust. Flame 1994, 99, 523. Gunther, R. Verbrennung und Feuerungen; Springer-Verlag: New York, 1974. Holdeman, J. D. Mixing of Multiple Jets with a Confined Subsonic Crossflow. Prog. Energy Combust. Sci. 1993, 19, 32. Hulgaard, T.; Dam-Johansen, K. Homogeneous Nitrous Oxide Formation and Destruction under Combustion Conditions. AlChE J. 1993, 39, 1342. Janssen, F.; van den Kerkhof, F.; Lefers, J.; Lodder, P.; Luierweert, L. The Determination of Ammonia in Flue Gas from the Selective Catalytic Reduction of Nitric Oxide with Ammonia. Anal. Chim. Acta 1986, 190, 245. Jødal, M.; Lauridsen, T. L.; Dam-Johansen, K. NOx Removal on a Coal-Fired Utility Boiler by Selective Non-Catalytic Reduction. Environ. Prog. 1992, 11, 296. Jødal, M.; Nielsen, C.; Hulgaard, T.; Dam-Johansen, K. Pilot-Scale Experiments with Ammonia and Urea as Reductants in Selective Non-Catalytic Reduction of Nitric Oxide. Proc. Combust. Inst. 1990, 23, 237. Johnsson, J. E.; Glarborg, P. Dam-Johansen, K. Thermal Dissociation of Nitrous Oxide at Medium Temperatures. Proc. Combust. Inst. 1992, 24, 917. Kasuya, F.; Glarborg, P.; Johnsson, J. E.; Dam-Johansen, K. The Thermal DeNOx Process: Influence of Partial Pressures and Temperature. Chem. Eng. Sci. 1995, 50, 1455. Kee, R. J.; Pupley, F. M.; Miller, J. A. The CHEMKIN Thermodynamic Database; Report SAND86-8209; Sandia National Laboratories, 1990. Kolb, T.; Jansohn, P.; Leukel, W. Reduction of NOx Emission in Turbulent Combustion by Fuel-Staging/Effects of Mixing and Stoichiometry in the Reduction Zone. Proc. Combust. Inst. 1988, 23, 1193. Kristensen, P. G.; Glarborg, P.; Dam-Johansen, K. Nitrogen Chemistry during Burnout in Fuel-Staged Combustion. Combust. Flame 1996, 10, 211. Kristensen, T.; Sarbaek, L. Reburning Med Faste Brandsler. Master’s Thesis, Department of Chemical Engineering, Technical University of Denmark, 1998 (in Danish). Lutz, A. E.; Kee, R. J.; Miller, J. A. SENKIN: a FORTRAN Program for Predicting Homogeneous Gas-Phase Chemical Kinetics with Sensitivity Analysis; Report SAND87-8248; Sandia National Laboratories, 1990. Lyon, R. K.; Hardy, J. E. Discovery and Development of the Thermal DeNOx Process. Ind. Eng. Chem. Fundam. 1986, 25, 19. Magel, H. C.; Schnell, U.; Hein, K. R. G. Simulation of Detailed Chemistry in a Turbulent Combustor Flow. Proc. Combust. Inst. 1996, 26, 67. Mehta, R. V. A Mathematical Model of Turbulent Mixing and Chemical Reaction; Ms. Thesis, Pennsylvania State University, 1981. Mehta, R. V.; Tarbell, J. M. Four Environment Model of Mixing and Chemical Reactions1. Model Development. AIChE J. 1983, 29, 320. Miller, J. A.; Bowman, C. T. Mechanism and Modelling of Nitrogen Chemistry in Combustion. Prog. Energy Combust. Sci. 1989, 15, 287.

3232


Miller, J. A.; Glarborg, P. In Gas-Phase Chemical Reaction Systems: Experiments and Models 100 years after Max Bodenstein; Wolfrum, J., Volpp, H.-R., Rannacher, R., Warnatz, J., Eds.; Springer Series in Chemical Physics; Springer: Berlin, 1996; Vol. 61, p 318. Miller, J. A.; Glarborg, P. Modelling the Thermal De-NOx Process: Closing in on a Final Solution. Int. J. Chem. Kinet. 1999, 31, 757. Østberg, M. Influence of Mixing in High-Temperature Gas-Phase ReactionssThe SNCR Process. Ph.D. Thesis, Department of Chemical Engineering, Technical University of Denmark, 1996. Østberg, M.; Dam-Johansen, K.; Johnsson, J. E. Influence of Mixing on the SNCR Process. Chem. Eng. Sci. 1997, 52, 2511. Østberg, M.; Glarborg, P.; Jensen, A.; Johnson, J. E.; Pedersen, L. S.; Dam-Johansen, K. A. Model of the Coal Reburning Process. Proc. Combust. Inst. 1998, 27, 3027. Pedersen, L. S.; Glarborg, P.; Dam-Johansen, K.; Hepburn P. W.; Hesselmann, G. A. Chemical Engineering Model for Predicting NO Emissions and Burnout from Pulverised Coal Flames. Combust. Sci. Technol. 1998, 127, 251. Ricou, F. P.; Spalding D. B. Measurements of Entrainment by Axisymetrical Jets. J. Fluid Mech. 1961, 11, 21.

Rota, R.; Bonini, F.; Servida, A.; Morbidelli, M.; Carra, S. Modelling of the Reburning Process. Combust. Sci. Technol. 1997, 123, 83. Rota, R.; Zanoelo, E. F.; Morbidella, M.; Carr, Y. S. Effect of Mixing on Selective Non-Catalytic Nitric Oxide Reduction. Presented at the Fifth International Conference on Technologies and Combustion for a Clean Environment, Portugal, July 12-15, 1999. Schetz, J. A. Injection and Mixing in Turbulent Flow. Progress in Astronautics and Aeronautics; American Institute of Aeronautics and Astronautics: New York, 1980; Vol. 68. Simpson, L. L. Mixing in Turbulent Fields; Academic Press: New York, 1975. Zwietering, Th. N. The Degree of Mixing in Continuous Flow Systems. Chem. Eng. Sci. 1959, 11, 1.

Received for review January 11, 2000 Revised manuscript received May 24, 2000 Accepted May 25, 2000 IE000049D

Mixing Effects in the Selective Noncatalytic Reduction of NO

Recommend Documents