Prediction of Nitric Oxide Destruction by Advanced Reburning

Energy & Fuels 2001, 15, 541-551

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Prediction of Nitric Oxide Destruction by Advanced Reburning Hongjie Xu† and L. Douglas Smoot* Chemical Engineering Department, Brigham Young University, Provo, Utah 84602

Dale R. Tree Department of Mechanical Engineering, Brigham Young University, Provo, Utah 84602

Scott C. Hill Advanced Combustion Engineering Research Center, Brigham Young University, Provo, Utah 84602 Received June 5, 2000. Revised Manuscript Received December 2, 2000

Advanced reburning refers to a process wherein injection of a hydrocarbon fuel such as natural gas aft of the combustion zone is followed by injection of a nitrogen-containing species such as ammonia. In recent work, the authors used a systematic reduction method to develop a fourstep, eight-species reduced mechanism from a 312-step, 50-species full mechanism for advanced reburning processes. The four-step model has been integrated into a comprehensive computational fluid dynamics combustion code, PCGC-3. In this work, the integrated model for advanced reburning has been evaluated through comparisons with experimental data for species and temperature profiles as well as effluent NO concentrations. Comparisons are shown herein for a base case, with reburning only (natural gas addition) and with advanced natural gas reburning (natural gas addition followed by NH3 addition). Profile comparisons show that the predicted flow-average axial NO concentration with advanced reburning followed the trends of experimental data, though the predicted initial NO level was lower than experimental data by 20-30%, partly due to both inaccurate prediction of flame structure and experimental error. Comparisons of effluent molar flux of NO with variation in swirl number, (NH3/NO)in and location of NH3 injection show that predicted trends and magnitudes of change were consistent with measured trends, though the predicted NO reduction was typically much higher than measured. Measurements and predictions both showed that NO reduction increased with increasing swirl number, (NH3/ NO)in and reburning zone residence time. Most of the NO reduction is caused by chemical reaction, and only a small part of the NO reduction is due to dilution. Study results provide directions for further research.

Introduction Nitrogen oxides (NOx) are precursors to acid rain. Increasingly stringent standards imposed on NOx emissions from combustion sources have resulted in a greater effort to develop novel reduction approaches. Among the most recent developments for reducing NOx emissions from coal combustion systems are (1) the reburning technologies, wherein gaseous, liquid, or solid hydrocarbon fuels are injected downstream of the main combustion zone to react with NO and produce HCN and eventually N2,1-3 and (2) the advanced reburning technologies, wherein ammonia, urea, or similar sub* To whom correspondence should be addressed at: Chemical Engineering Department, 435T CTB, Brigham Young University. Phone: 801-378-8930. Fax: 801-378-1131. E-mail: [email protected]. † Current address: Fuel Tech., Inc., Batavia, Chicago, IL. (1) Wendt, J. O. L.; Mereb, J. B. DOE Final Report, DE-AC2287PC79850; University of Arizona, Tucson, AZ, September, 1991. (2) Boardman, R. D.; Smoot, L. D. In Fundamentals of Coal Combustion; Smoot, L. D., Ed.; Elsevier: Amsterdam, The Netherlands, 1993.

stance is injected after hydrocarbon injection to further reduce NOx species.4 Up to 85∼95% reduction of NOx can be achieved by combining reburning and advanced reburning technologies.5 Smoot6 et al., recently published a review of NOx control through reburning and advanced reburning technologies which identifies much of the early process work on these technologies. The main reactions of the advanced reburning5 are NH3 + OH, O, H f NH2 + H2O, OH, H2, and NH2 + NO f N2 + H2O. With increased CO present due to upstream reburning fuel injection, a larger temperature window (3) Folsom, B. A.; Sommer, T.; Ritz, H.; Pratapas, J.; Bautista, P.; Facchiano, T. EPRI/EPA 1995 Joint Symposium on Stationary Combustion NOx Control, Kansas City, MO, May 19, 1995. (4) Folsom, B. A.; Payne, R.; Moyeda, D.; Vladimir Zamansky; Golden, J. EPRI/EPA 1995 Joint Symposium on Stationary Combustion NOx Control, Kansas City, MO, May 19, 1995. (5) Zamansky, V. M.; Ho, L.; Maly, P. M.; Seeker, W. R. 26th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1996; pp 2075-2082. (6) Smoot, L. D.; Hill, S. C.; Xu, H. Prog. Energy Combust. Sci. 1998, 24, 385-408.

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for the NOx reduction reactions and reduction of unreacted NH3 carryover (i.e., slip) can be achieved. Several full elementary mechanisms for hydrocarbon flames and NH3 oxidation are available for description of advanced reburning.7 However, unacceptable computer times result when large kinetic schemes are used with turbulent combustion models. Brouwer et al.8 developed a reduced mechanism for the closely related selective, noncatalytic reduction (SNCR) process in practical systems, including the influence of CO. This simplified SNCR mechanism was derived from the Miller and Bowman mechanism9 through sensitivity analysis and curve fitting. However, this model may be too specific to generally consider important factors such as the effects of O2, CO, and H2O concentrations. Han et al.10 recently applied a two-stage Lagrangian model to simulate the basic and advanced reburning processes in a 300 kw boiler simulation facility (7 m high × 56 cm i.d.). This model divides the combustion and NOx reduction processes into two stages: flame sheet and homogeneous core. Both stages are described by perfectly stirred reactor systems. Entrainment rates of the transverse jets rely on independent experimental data, while flame sheet residence time requires an experimental point to establish the proportionality constant. A detailed chemical mechanism for gas-phase reactions is employed without accounting for turbulence effects. Variation in N-agent injection temperature, percent reburn heat input, reburn zone residence time, and initial NO concentration capture the observed trends and magnitudes reasonably well. This model is not a comprehensive or a priori predictor and does not provide detailed multidimensional fluid structure within the furnace. Dahm and Tryggvason11 used the novel local integral moment (LIM) technique to simulate turbulent flow and complex chemistry for reburning processes. Unlike the traditional approach to turbulence modeling, LIM simulations involve no time-averaging and, instead, are based on a Lagrangian front-tracking method in which the time-evolution of flow, mixing, and detailed elementary mechanisms are computed directly. At the present time, coal combustion models are not included in their 2-D code. Comparato et al.12 used the PHOENICS CFD code to simulate full-scale furnaces to obtain timetemperature characteristics and then used a chemical kinetic model to determine the final NOx emission values and ammonia slip. However, the interaction between turbulence and chemical reaction was not considered. Xu et al.13 postulated a seven-step, 11-species reduced mechanism for the prediction of nitric oxide concentrations for advanced reburning, derived from a 62-step, (7) Xu. H., Dissertation, Department of Chemical Engineering, Brigham Young University, Provo, UT, August, 1999. (8) Brouwer, J.; Heap, M. P.; Pershing, D. W.; Smith, P. J. TwentySixth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1996; pp 2117-2124. (9) Miller, J. A.; Bowman, C. T. Prog. Energy Combust. Sci. 1989, 15, 287-338. (10) Han, D.; Mungal, M. G.; Zamansky, V. M.; Tyson, T. J. 1998 Fall Meeting, Western States Section of the Combustion Institute; Seattle, WA, Oct. 26-27, 1998. Combust. Flame 1999, 119, 483-493. (11) Dahm, W. J. A.; Tryggvason, G. Proceeding of the 1996 International Gas Research Conference, Cannes, France, 1996; Gas Research Institute: Chicago, IL, 1996. (12) Comparato, J. R.; Boyle, J. M.; Micheals, W. F. Presented at the ICAC Forum, 1998.

Xu et al.

20-species skeletal mechanism, which itself was based on a 312-step, 50-species full mechanism.7 The derivation of this reduced model by using a systematic reduction method14 was described in detail, including the selection of the full mechanism, the development of the skeletal mechanism, and the selection of steadystate species. The predictions of the seven-step reduced mechanism for laminar systems were in very good agreement with those of the full mechanism over a wide range of parameters applicable to coal-based, gas-based, and oil-based combustion cases. Further, Xu et al.15 postulated a simpler four-step, eight-species reduced mechanism with the revised NO rate. The eight nonsteady-state species are H2O, CO2, N2, O2, CO, NO, NH3, and OH, which represent the independent variables. The four overall reactions used in the mechanism are

O2 + CO + H2O f 2OH + CO2

(1)

O2 + 2CO f 2CO2

(2)

O2 + N2 f 2NO

(3)

OH + 3NO + NH3 f O2 + 2H2O + 2N2

(4)

The rates of these reactions were solved implicitly on the basis of the 62-step, 20-species skeletal mechanism.13 The four-step reduced submodel predictions were in reasonable agreement with the skeletal mechanism for most of the laminar cases where comparisons were made.15 It is also qualitatively in agreement with three independent sets of experimental data,16-18 including the influences of temperature, CO concentration, O2 concentration, and the ratio of NH3in to NOin (NSR). The four-step model was integrated into a comprehensive CFD combustion code, PCGC-3.19 PCGC-3 has been extensively evaluated by comparison of predictions of composition, temperature, and velocity with a substantial number of data sets as documented in the published literature (e.g., refs 2, 19, and 20). Figure 1 shows the global NO reactions included in the PCGC-3 NO submodel, including fuel-NO (i.e., NO formation from volatile-N through HCN and NH3),21 thermal-NO (i.e., NO formation from air-N2-O2 reaction at high temperature),21 reburning-NO (i.e., NO reduction from CiHj reaction with NO),22 advanced reburning-NO (i.e., NH3 + NO),13 and char-NO (i.e., formation and reduction of NO from char-N oxidation and char-NO (13) Xu, H.; Smoot, L. D.; Hill, S. C. Energy Fuels 1998, 12, 12781289. (14) Peters, N. In Reduced Kinetic Mechanisms and Asymptotic Approximations for Methane-Air Flames, Smooke, M. D., Ed.; SpringerVerlag: Heidelberg, Germany, 1991; Chapter 3. (15) Xu, H.; Smoot, L. D.; Hill, S. C. Energy Fuels 1999, 13, 411420. (16) Dill, J. W.; Sowa, W. A. 1992 Fall Meeting, Western States Section of the Combustion Institute; Berkeley, CA, Oct. 12-13, 1992. (17) Suhlmann, J.; Rotzoll, G. Fuel 1993, 72, 175-179. (18) Caton, J. A.; Siebers, D. L. Combust. Sci. Technol. 1989, 65, 277-293. (19) Hill, S. C.; Smoot, L. D. Energy Fuels 1993, 7, 874-883. (20) Eaton, A. M.; Smoot, L. D.; Hill, S. C.; Eatough, C. N. Prog. Energy Combust. Sci. 1999, 25, 387-436. (21) Smoot, L. D.; Boardman, R. D.; Brewster, B. S.; Hill, S. C.; Foli, A. K. Energy Fuels 1993, 7, 786-795. (22) Chen, W.; Smoot, L. D.; Hill, S. C.; Fletcher, T. H. Energy Fuels 1996, 10, 1046-1052.

Prediction of NO Destruction by Advanced Reburning

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Figure 1. Advanced NOx submodels in PCGC-3.

reduction).2 This global NO submodel is executed as a postprocessor to PCGC-3 after the flame structure has been predicted. To integrate the four-step, advanced reburning reduced mechanism into the PCGC-3, the local equilibrium assumption is applied for CO2, H2O, and N2, and five species continuity equations are solved for O2, CO, NO, NH3, and OH. To consider effects of turbulence, mean reaction rates are obtained by using a probability density function (PDF)23 with the original rates of the four-step model. The advanced reburning submodel element of Figure 1 is activated immediately upon NH3 injection. Model calculations for turbulent coal combustion cases showed that the submodel integrated with PCGC-3 gave realistic predictions though no comparisons with measurement were made.15 The predicted impacts of temperature, CO, O2, and NSR on advanced reburning were consistent with observations based on laminar flame mechanism studies.13 Experimental Data In recent measurements, Tree and Clark24 have shown that an 85% reduction in effluent NO concentration can be achieved within a pilot-scale furnace, with combined reburning and advanced reburning technologies. They also reported species, velocity and temperature profile data, and effluent measurements. These data, thought to be the only such profile data available for advanced reburning, are used herein for evaluation of the NO submodel of Figure 1. Reactor. The advanced reburning experimental operating conditions and results were reported in detail by Tree and Clark.24 The experimental data for base line, reburning, and advanced reburning cases were obtained in an axisymmetric, 0.2 MWt, pulverized coal, down-fired reactor known as the controlled (wall temperature) profile reactor (CPR), shown in Figure 2. This reactor is so termed because independent electrical wall heating elements in each of the sections permit the wall temperature profile to be set and controlled during a (23) Smith, P. J.; Hill, S. C.; Smoot, L. D. 19th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1982; pp 1263-1270. (24) Tree, D. R.; Clark, A. W. Fuel 2000, 79, 1687-1695; see also Clark, A. W. Master Thesis, Department of Mechanical Engineering, Brigham Young University, Provo, UT, December, 1998.

test. The internal diameter and total length of the reactor are 0.75 and 2.9 m, respectively. Axial wall temperatures were controlled with segmented wall heaters, providing known wall boundary conditions for test and modeling purposes. The primary stream pulverized-coal feed rate was established with a constant speed motor auger. A preheated secondary air stream passed through a moveable-block swirl generator and entered the combustion chamber around the primary air/coal outlet. Four vertical windows, one in each quadrant of each reactor section, allowed access to the flame for measurement and injection of reburning fuel, the advanced reburning agent, and tertiary air. The momentum of these flows injected at the centerline of the reactor is dependent on the injector opening pintle positions and flow rate. Both inlet and outlet flow rates were measured for use as code input and mass balance evaluation. Wyodak, low-sulfur, sub-bituminous coal was used, with proximate and ultimate analyses given in Table 1. Natural gas was taken to be CH4 in PCGC-3 calculations. Natural gas used in these tests contained approximately 93 vol. % CH4 with 5% ethane and propane and 1% N2/CO2. No tests were made to evaluate the impact of this assumption, though it is thought to be small. Reactor Profile Tests. The CPR was operated at an overall stoichiometric ratio of 1.1 for all tests. More detailed information about operating conditions for in-situ measurements is shown in Table 2 for three species profile Cases 1, 2, and 3. For the base line case 1, neither reburning fuel nor ammonia were injected. The measured data for the reburning case 2 were obtained by Nazeer.25 For the advanced reburning case 3, three different tests were performed for common test conditions, including two for species measurements (test A and test B) and one for temperature measurements (test C). The gas temperatures and concentrations of gaseous species (i.e., NO, NH3, O2, CO2, CO, and HCN) were sampled over a grid of points within the combustion chamber. The in-situ probe location consisted of six radial points at each of nine axial locations. The flow-rate averaged (i.e., mixing-cup) concentrations of these species were obtained from the radial species concentrations and velocity profiles. The reburning zone in the reburning case was fuel-rich, while this zone in the advanced reburning case was fuel-lean, so the flow rates of tertiary air required to burn out the rest of the carbon were different in the two cases. (25) Nazeer, W. A., Master Thesis, Department of Mechanical Engineering, Brigham Young University, Provo, UT, August, 1997.

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Figure 2. Schematic diagram of controlled profile reactor. Parametric Tests. Several effluent tests were conducted at 1.5 and 0.5 swirl number to better evaluate advanced reburning characteristics. Parameters varied included: swirl number (SN), reburning zone SR, NSR (i.e., NH3in/NOin), and ammonia injection location (LNH3). The input conditions for these parametric tests were the same as case 3 in Table 2, except for the varied parameters. At 1.5 SN, the test program is shown in Table 3. Those combinations noted with an “x” were the conditions tested. Two different LNH3 values, 1.30 and 1.47 m, were chosen to study the effects of injection location at the matrix of conditions shown in Table 3.

Table 1. Properties of WYODAK Black Thunder Coal (Sub-bituminous)24 moisture ash

Proximate Analysis (%) 23.7 fixed carbon 4.7 volatile matter

H2O C H N

Ultimate Analysis (%) 23.7 S 55.8 O 3.7 ash 1.0

36.7 34.9 0.4 10.7 4.7



Table 2. Input Conditions for Base Line, Reburning, and Advanced Reburning Cases with Profile Measurements24,25 case 1 case 2 case 3 (base line) (reburning) (adv reburning)

variables primary zone SRa reburning zone SRa final zone SRa swirl number

1.1 0.90 1.1 0.5

1.1 1.05 1.1 0.5

temperature (K) air flow rate (kg/s × 103) mean particle size (µm) mass coal/mass air

Primary Stream 310 310 4.44 4.44 55.3 55.3 1.5 1.5

310 4.44 55.3 1.5

temperature (K) air flow rate (kg/s × 102)

Secondary Stream 585 585 4.89 4.89

585 4.89

Reburning Stream temperature (K) 300 CH4 flow rate (kg/s × 104) 7.64 b axial injection location (m) 0.88

300 1.44 0.88

Advanced Reburning Stream temperature (K) NH3 flow rate (kg/s × 105) mole ratio of NH3 to N2 axial injection locationb (m)

300 3.04 1:3 1.49

Tertiary Stream temperature (K) 300 3 air flow rate (kg/s × 10 ) 13.33 axial injection locationb (m) 1.89 wall temperature (K) 1275 1275

300 5.25 1.89 1275

a

1.1 1.1 0.5

Table 3. Parametric Test Program for 1.5 SN at LNH3 ) 1.3 and 1.49 m24 reburning SR NSR

1.5 2.0 2.5

1.0

1.05

x

x x

x

x

Table 4. Parametric Test Program for 0.5 SN22 reburning SR LNH3 (m)

1.28 1.49 1.69

all five streams is to identify a single, flow-averaged temperature for three of the streams and treat the remaining two in the normal fashion. In these simulations, a flow-averaged temperature was used for the primary, secondary, and tertiary air streams, while using the measured inlet temperature for the reburning and advanced reburning streams. The flow-averaged temperature is obtained from the mixture law for ideal gases with constant heat capacity:

Tav )

SR, stoichiometric ratio. b From the burner outlet.

0.95

Figure 3. Strategy of PCGC-3 application for the advanced reburning cases.

0.95

1.0

1.05

1.10

2.0

1.0, 1.5, 2.0 2.0 2.3, 2.5

1.0, 1.5, 2.0, 2.5 1.5, 2.0, 2.5 1.6, 2.0

2.0 1.2 1.75

The test matrix for 0.5 SN is shown in Table 4. The numbers inside of the table indicate the range of NSR values at which the tests were conducted. A reburning SR value of 1.10 was included to test the effect of ammonia injection with both reburning and tertiary air turned off. The natural gas and tertiary air injection locations were kept constant at 0.88 and 1.89 m, respectively, during all parametric tests. Detailed results are provided by Tree and Clark,24 while comparative tables that follow in this discussion contain much of the data.

Results and Discussion Strategy for PCGC-3 Application to Laboratory Test Data. In PCGC-3, effects of turbulence on reaction rates can only be accounted for in three inlet streams; however, five inlet streams are required to describe advanced reburning cases, namely, primary (i.e., air + coal), secondary (i.e., preheated air), reburning (i.e., CH4 + N2), advanced reburning (i.e., NH3 + N2), and tertiary (i.e., preheated air) streams. An approximate way to allow for consideration of turbulence-reaction effects in

(mT)primary + (mT)secondary + (mT)tertiary (5) mprimary + msecondary + mtertiary

where m is the air flow rate. This approximation may result in earlier ignition because Tav is higher than Tprimary, which may lead to lower NO formation from fuel nitrogen due to a somewhat lower extent of mixing with secondary air. However, comparisons of predicted NO formation and coal burnout with measured results have shown that inlet temperatures have a weak effect on these parameters, which indicates that the use of an average inlet temperature will have minimal effect on the predicted results. Figure 3 shows the strategy for implementation of the NO submodel components in PCGC-3. Near the point of injection of the advanced reburning agent, the advanced reburning submodel was activated. In this study, the location of activation of this submodel was one axial cell aft of the NH3 injection. The fuel-NO and thermal-NO submodels were turned off in this aft zone, as illustrated in Figure 3. The reasons for this strategy of sequential NO submodel component application have been described elsewhere.15 One problem is the possible existence of recirculation of products and injected agents upstream of the injection point. If such species are transported upstream, the starting location can be set upstream of the location of the advanced reburning injection. However, for the test data included herein, the observed internal recirculation zone (IRZ) was just upstream of natural gas injection and well upstream of the ammonia injection. The IRZ was located in the top 70 cm. Natural gas was injected at 88 cm. The NH3 injection location was varied from 128, 149, and 169 cm. Profile Comparisons. In-situ measurements for base line, reburning, and advanced reburning cases from Tree and Clark24 and Nazeer25 data were compared with integrated model predictions, as shown in Figures 4-7. In all model predictions herein, no fitting param-

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Figure 4. Base line case: Profile comparisons of predictions with experimental data.

eters were used and no adjustments were made to the physical and chemical coefficients to improve the agreement with data. These profile comparisons are more comprehensive and illustrative than effluent measurement comparisons and provide valuable information for computer code validation and for identification of research needs. Base Line Case. For the base line case shown in Figure 4, the flow-average profiles of predicted temperature, CO2 concentration, and NO concentration vs axial location are compared with experimental data. The predicted NO concentration was lower than the experimental data by about 30%, with 520 ppm of NOout predicted and 720 ppm of NOout measured. This difference could have resulted for numerous reasons, but three more likely causes are (1) inaccuracies in the kinetic parameters for the NO model, (2) measurement errors, and (3) incorrect upstream flow field prediction. There are limitations in the kinetic parameters for the thermal-NO and fuel-NO global submodels in PCGC-3 because they were derived through a data fitting method. Second, about 15% relative error in NO measurement exists in these data because of an unstable coal feed rate, a long period for data collection (over 12 h), and equipment measurement error ((5 ppm).24 These experimental errors may account for up to half of the observed difference. Further, accurate prediction of NO emissions, which are predicted in a postprocessing

fashion once a fully converged flow-field solution is obtained, are dependent on the quality of the flow-field solution.20 Comparisons in Figure 5 show that predictions were not correct for radial temperature distribution near the burner in this strongly swirling flow. An earlier ignition was predicted than was observed, which can also be seen in Figure 4b where higher upstream CO2 concentrations were predicted. Measured temperatures near the burner were flatter than those in the calculation, which may have resulted from the approximate treatment of incoming air streams mentioned above. Also the narrower calculated flame width indicates that predictions do not adequately simulate these strongly swirling flows. This near-burner flow field discrepancy between predictions and measurements was also observed by Eaton et al.20 Incorrect flow field predictions may also have caused the noted temperature differences between data and predictions, as shown in Figure 5c,d. However, the predicted axial temperature profile in the downstream zone was quite flat and agreement with experimental data was much improved, as shown in Figure 5e-f. The acceptable agreement of flow-field properties in the postcombustion zone where reburning and advanced reburning agents are injected was considered to be adequate for the purposes of this study. Also, the general trend of predicted NO concentration was similar to that measured. Reburning Case. For the reburning case of Figure 6,



Figure 5. Base line case: Predicted and measured radial temperature profiles.

Figure 6. Reburning case: Profile comparisons of predictions with experimental data.

about 10% of the total heating value of the coal, in the form of natural gas, was injected at 0.88 m from the burner outlet to create a fuel-rich reburning zone (SR ) 0.9). Tertiary air was introduced into the postcombustion zone at 1.89 m to burn out the remaining CH4

and carbon in the coal char. The prediction of average axial temperature values matched well with experimental data, as shown in Figure 6a, but the NO concentrations did not compare as well except for the effluent concentration, as shown in Figure 6b. Lower NO concentrations were predicted upstream. It was also noticed that earlier NO reduction occurred experimentally, but in the predicted case, significant NO reduction started well beyond 0.88 m where the reburning fuel was injected. Both experimental and predicted values showed large NO reduction in the postcombustion zone where the tertiary air was introduced. Part of this NO reduction was due to the combustion gas dilution by tertiary air. Incorrect upstream flame structure predicted by the CFD-based combustion model may have caused the difference between experimental data and predictions. Measured velocity profiles with 0.5 swirl number in the CPR26 indicated that there was a strong central recirculation zone upstream of CH4 injection by about 10 cm. However, there was no central recirculation zone predicted by the flow model. Further, without a predicted central recirculation zone, less O2 than observed was predicted near the CH4 injection location, which leads to the decomposition of CH4 to C and H2, rather than reaction with O2 to generate CO2 and H2O. This result then leads to a lower local temperature due to the endothermic dissociation reactions, which reduces the reburning reaction rate. Advanced Reburning Case. Figure 7 shows the comparisons for the advanced reburning case. Natural gas was injected at 0.88 m from the burner outlet to create a fuel-lean reburning zone (SR ) 1.05), followed by NH3 injection at 1.49 m and tertiary air injection at 1.89 m. (26) Pickett, L. M.; Jackson, R. E.; Tree, D. R. Combust. Sci. Technol. 1999, 143, 79.

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Figure 7. Advanced Reburning Case: Profile Comparisons of Predictions with Experimental Data.

The amount of reburning fuel in case 3 was less than that in the reburning case 2 discussed above (see Table 2). Two sets of reproduced NO concentration data were provided by Tree and Clark24 for case 3 (i.e., test A and test B), while only one set of temperature profiles (test C) was measured. The predicted axial temperature profile was quite consistent with measured values, as shown in Figure 7a. In Figure 7b, observed differences in NO concentration between test A and test B give some indication of experimental variation. The percentage of total NO reduction from advanced reburning was computed by

∆NO )

(NOmax - NOout) × 100% NOmax

(6)

From the Figure 7b, ∆NOtest A ) 90% and ∆NOtest B ) 77%, while ∆NOpredicted ) 89%. The predicted percentage reduction value was within the two experimental values. The pattern of the predicted NO reduction basically followed the experimental data along the combustor length. As noted for the base line case, lower initial NO concentration was also predicted here. Less NO reduction before 0.88 m where the reburning fuel was injected was measured, compared with that in the reburning case 2. This is because less reburning fuel was used in the advanced reburning case. Unlike the two sets of experimental data, no NO reduction was predicted before the reburning zone. Because no central recirculation zone was predicted, the reburning fuel could not be carried upstream and the reburning reaction was predicted later than observed experimentally. After NH3 was introduced, substantial NO reduction was both observed and predicted. De-

creasing of the NO concentration due to dilution of tertiary air was also observed and predicted after 1.89 m. Parametric Comparisons. While comparisons of measured and predicted temperature and species profile concentrations provide detailed information about the advanced reburning submodel predictions inside a furnace, comparisons with effluent properties provide insights regarding parametric effects of advanced reburning. As noted above, Tree and Clark24 conducted a series of tests wherein only effluent temperatures and species concentrations were measured. These data provided the basis for parametric comparisons of measured and predicted values. Two techniques were chosen for calculating the percent NO reduction for advanced reburning tests. The first compares the base line effluent molar NO flux to the advanced reburning effluent NO molar flux. This technique is used to eliminate the effect of NO reduction due to dilution caused by advanced reburning gas injection (i.e., reburning fuel, advanced reburning agent, and tertiary air). These values are normalized by the molar flux of NO in the base line case as shown in eq 7, where the subscript B is for base line and AR for advanced reburning.

[NO]% ) (molar flux)B [NO]B - (molar flux)AR[NO]AR (molar flux)B[NO]B

×

100% (7) With the Tree and Clark assumption of constant product molecular weight of 30 kg/kmol, [NO]% values computed by eq 7 are the same, whether molar or mass flux values



Table 5. Effect of Swirl Number on NO Reduction SN ) 0.5 conditions: SR ) 1.05; NSR ) 1.5; LNH3 ) 1.49 m

SN ) 1.5

[NO]B (ppm)

[NO]AR (ppm)

[NO]% (%)

[NO]# (%)

[NO]B (ppm)

[NO]AR (ppm)

[NO]% (%)

[NO]# (%)

660 520

302 46

50 90

54 91

520 432

143 23

70 94

73 95

measured predicted

Table 6. Effect of Reburning Zone Stoichiometric Ratio on NO Reduction SR ) 0.98 conditions: SN ) 0.5; NSR ) 1.5; LNH3 ) 1.49 m measured predicted conditions: SN ) 0.5; NSR ) 1.5; LNH3 ) 1.49 m

SR ) 1.00

[NO]B (ppm)

[NO]AR (ppm)

[NO]% (%)

[NO]# (%)

[NO]B (ppm)

[NO]AR (ppm)

[NO]% (%)

[NO]# (%)

520 SR ) 1.025

39

92

92

660 520 SR ) 1.05

211 43

65 91

68 92

[NO]B (ppm)

[NO]AR (ppm)

[NO]% (%)

[NO]# (%)

[NO]B (ppm)

[NO]AR (ppm)

[NO]% (%)

[NO]# (%)

520

47

90

91

520 520

143 53

70 88

73 90

measured predicted

are used. Thus mass flux values were used herein. (mass flux)B and (mass flux)AR were 6 × 10-2 and 6.55 × 10-2 kg/s, respectively. In the second method, the percentage of NO reduction (i.e., [NO]#) includes both dilution and reaction, and was calculated by

[NO]# )

[NO]B - [NO]AR [NO]B

× 100%

(8)

Both [NO]% and [NO]# are presented and discussed below. Effect of Swirl Number. Swirling secondary flow strongly impacts the character of the flame in the CPR. Low swirl settings produce a dominant axial flow with little recirculation. As the swirl level is increased, the flame is moved closer to the burner with increasingly larger recirculation zones. Therefore, the mixture fraction, ignition location and temperature profile are all affected with a change of the swirl number. Swirl effects on NO concentration are complex, as discussed by Smith et al.27 Measured and predicted effects of swirl number on NO concentration reduction are listed in Table 5. The ammonia was initially stored in compressed tanks as 25% NH3 and 75% N2. During injection, the NH3/N2 mixture (to be referred to as NH3 injection) was mixed with additional N2 as a carrier gas to increase the momentum of the injected flow to improve mixing. The flow rate of N2 as a carrier gas was 3.2 kg/h, and the flow rate of NH3 was listed in Table 2 (i.e., case 3). Experimentally, [NO]% increased from 50% to 70% with an increase of swirl number from 0.5 to 1.5, compared with the predicted [NO]% increase from 90% to 94%. Predictions showed an increase of [NO]% with increasing SN as observed, but predicted values were higher than observed, and the incremental change was less than observed. Experimental error in these values was up to (15%, explaining part of the differences between predicted and observed [NO]%. Also, inaccuracies in the kinetic parameters for the NO model, and incorrect flow field prediction likely contribute to these differences. The measured differences in [NO]% and [NO]# in Table 5 (i.e., 50% to 54% at 0.5 SN and 70% to 73% at 1.5 SN) are quite small, suggesting that only a small part of NO reduction is due to dilution (i.e., reburning fuel, ad-

vanced reburning fuel, and tertiary air injection). Most of the NO reduction is caused by chemical reaction. This trend is also predicted by the model (i.e., at SN ) 0.5, NO% - NO# ) 90-91%; SN ) 1.5, NO% - NO# ) 9495%). Thus, the predictive method reliably identifies the smaller role of dilution. Effect of SR. There are conflicting evidences about the effects of the reburning stoichiometric ratio on advanced reburning. A stoichiometric ratio of 0.99 (i.e., fuel-rich reburning zone) was the optimum value cited by Chen et al.28 for advanced reburning, while Folsom et al.29 indicated that SR ) 1.05 (i.e., fuel-lean reburning zone) was the optimum value for the advanced reburning process. Tree and Clark’s24 tests indicated that a reburning SR of 1.05 may be closer to the value for maximum NO reduction, which supported Folsom’s observation, as shown in Table 6 without any carrier gas. These data show that NO reduction, [NO]%, increased from 65% to 70% as SR increased from 1.0 to 1.05. However, the predicted [NO]% showed a reverse trend from 92% to 88% as SR increased from 0.98 to 1.05 (Note: no experimental data were available for 0.98 SR and 1.025 SR). Also, the predicted values were higher than observed. Predictions indicated that the fuel-rich reburning zone is better than the fuel-lean reburning zone for the advanced reburning process. If Tree and Clark’s24 and Folsom’s29 experimental results are correct, the reverse predicted trend in [NO]% with SR change is possibly caused by the sequential use of separate reburning and advanced reburning submodels in PCGC-3 (see Figure 3). Sequential use of reburning and advanced reburning submodels may have masked some information about interaction between the hydrocarbon (CH4) and ammonia. No difference is noted between predicted [NO]% and [NO]# at 0.98 SR in Table 6, suggesting that this difference becomes smaller as [NO]% or [NO]# increase. Only when the percentage of NO reduction is low will dilution play a significant role. (27) Smith, P. J.; Smoot; L. D.; Hill, S. C. AIChE J. 1986, 32, 421423. (28) Chen, S. L.; Lyon, R. K.; Seeker, W. R. Environ. Prog. 1991, 10, 182-185. (29) Folsom, B. A.; Sommer, T. M.; Latham. C. E.; Moyeda, D. K.; Gaufillet. G. D.; Janik. G. S.; Whelan, M. P. 1997 Joint Power Generation Conference, Nov. 3-6, 1997.

550


Xu et al.

Table 7. Effect of NSR on NO Reduction NSR ) 1.5 conditions: SR ) 1.05; SN ) 0.5; LNH3 ) 1.49 m measured predicted

NSR ) 2.5

[NO]B (ppm)

[NO]AR (ppm)

[NO]% (%)

[NO]# (%)

[NO]B (ppm)

[NO]AR (ppm)

[NO]% (%)

[NO]# (%)

660 520

211-151 53

65-75 88

68-77 90

520 520

119 46

75 90

77 91

Table 8. Effect of Location of NH3 Injection on NO Reduction LNH3 ) 1.49 m conditions: SR ) 1.05; NSR ) 1.5; SN ) 0.5 measured predicted

LNH3 ) 1.69 m

[NO]B (ppm)

[NO]AR (ppm)

[NO]% (%)

[NO]# (%)

[NO]B (ppm)

[NO]AR (ppm)

[NO]% (%)

[NO]# (%)

660 520

194 54

68 88

71 90

660 520

169 58

72 88

74 89

Effect of NSR. The most commonly cited NSR values (i.e., the ratio of NH3 injected to local NO concentration where NH3 is injected) in the literature for both SNCR and advanced reburning are 1.0-1.7.24 This range of values is high enough to supply an ample amount of ammonia to react with NO in the combustion gases and still minimize the possibility of ammonia slip. Also, Caton and Siebers’18 experimental data indicated that increasing NSR above 3 resulted in no additional NO reduction. Data24 shown in Table 7 without carrier gas also indicated that increasing NSR from 1.5 to 2.5 resulted in a small increase in [NO]% reduction from 65-75% to 75%. Predicted values also show only a small increase in percentage reduction with NSR increase, though the predicted values are higher. The ammonia slip was not investigated in the predictions because the predicted NH3 concentration in the advanced reburning submodel has not been validated, and the Tree and Clark experiments only focused on NO reduction. Model calculations show that the efficiency of NO reduction will decrease until no additional reduction results from the increase of NSR. This prediction is not verified by the Tree and Clark24 data because only two different NSR values were investigated. For the same reason, Suhlmann and Rotzoll’s 17 experimental observation from laminar flames that there is no upper limit of NSR on the NO reduction was not confirmed experimentally by Tree and Clark.24 Effect of the Location of NH3 Injection. The experimental data show that NO reduction is sensitive to the location of NH3 injection and that the optimal location changes with swirl number. For the data of Tree and Clark,24 but not shown herein, at 1.5 SN, the optimum location was 1.28 m, while at 0.5 SN, it shifted to 1.69 m. Furthermore, the experimental data show that the sensitivity to location is nonlinear. That is, within a region near the optimum, NO reduction is fairly insensitive to change in location. Model predictions were compared to measurements at two positions that were near the optimal injection location. These were the only points where NSR was constant. Ammonia injection at 1.69 m produced only slightly higher reduction in [NO]% than at 1.49 m from 72% to 68%, as shown in Table 8. No additional carrier gas flow for ammonia injection was used in these tests. Also the predicted values showed little difference at 1.49 m and at 1.69 m, confirming the measurements. Finally, the role of dilution was small compared to the role of reaction. The location for optimum NH3 injection is thought to be strongly related to reburning zone residence time and local temperature. When reburning fuel has been con-

sumed entirely before the end of the reburning zone, the change of LNH3 would not affect NO reduction. Investigation of local temperature within the zone may be more important for the advanced reburning process than the location of NH3 injection. Conclusions The four-step integrated submodel for advanced reburning with PCGC-3 has been compared with profile and effluent data of nitric oxide reduction. The predicted flow-average axial profiles of temperature and CO2 concentration were reasonably good, and the predicted average axial NO concentration with advanced reburning (NH3 injection) followed experimental trends but were lower than data by ∼30%, due to in part inaccurate prediction of flame structure and to experimental error. Experiments and predictions both showed that NO reduction increased with increasing swirl number, NSR, and reburning zone residence time. However, for reburning zone SR, this was not the case. Further, the magnitudes of the predicted NO reduction were much higher than measured, while the incremental changes in predicted NO reduction with changes in SN, SR, NSR, and LNH3 were somewhat less than observed. Experimental data24 showed that a slightly fuel-lean reburning zone is better for advanced reburning processes, while predictions show that a slightly fuel-rich reburning zone is better. Independently measured data also differ on the value of this variable for optimum NO reduction. The roles of chemical reaction and dilution on NO reduction were distinguished by comparison of [NO]% and [NO]#. The former eliminates the effect of NO reduction due to dilution, while the latter includes both reaction and dilution. The difference, [NO]% - [NO]#, represents the NO reduction from dilution. For the test data and predictions herein, most of NO reduction is caused by chemical reaction, and only a small part of the NO reduction is due to dilution. Results of this work, which is thought to be the first comparison of advanced reburning profile data with comprehensive CFD-based combustion model predictions, provide insights into this complicated process while identifying directions for further research. The key issue to improved agreement is thought to be improved prediction of the flame structure in the nearfield. Acknowledgment. This research was conducted in the Advanced Combustion Engineering Research Center at Brigham Young University, and financially supported


in part by the National Science Foundation (NSF), Engineering Education and Centers Division, through an Engineering Research Center award, by the participating industrial companies and by Brigham Young University. Nomenclature AR B CFD CPR in LNH3 LIM max

advanced reburning base line case computational fluid dynamics controlled profile reactor at advanced combustion engineering research center inlet location of NH3 injection from the burner outlet (m) local integral moment maximum value

Energy & Fuels, Vol. 15, No. 3, 2001 551 air flow rate (kg.s-1) percentage of NO reduction without dilution effect percentage of NO reduction with dilution effect ratio of NH3in/NOin outlet pulverized coal gasification and combustionsthree dimension PDF probability density function primary primary air secondary secondary air SN secondary swirl number SNCR selective noncatalytic reduction SR reburning zone stoichiometry tertiary tertiary air T temperature (K)

m [NO]% [NO]# NSR out PCGC-3

EF000120S

Prediction of Nitric Oxide Destruction by Advanced Reburning

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