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Ind. Eng. Chem. Res. 2009, 48, 10994–11001
Development and Implementation of Numerical Simulation for a Selective Noncatalytic Reduction System Design Wei Zhou,* David Moyeda, Vitali Lissianski,† and J.-Y. Chen‡ General Electric Energy, 1831 Carnegie AVenue, Santa Ana, California 92705
Selective noncatalytic reduction (SNCR) technology is an effective and economical method of reducing NOx emissions from a wide range of industrial combustion systems. It is widely known that the SNCR process is primarily effective in a narrow temperature window, around 1200-1255 K, and that high CO concentrations can both shift the temperature window and limit the process’ effectiveness. To ensure proper design and application of SNCR technology, it is critical to understand the flow and temperature fields, SNCR kinetics, and species concentrations in the combustion system and to design an injection system that provides good mixing and distribution of the reagent with the furnace gases. The work summarized in this article developed and incorporated a reduced SNCR chemical mechanism into a commercial computational fluid dynamics (CFD) model. Three main results are reported: (1) the reduced mechanism is validated by comparisons to a detailed mechanism using a plug-flow reactor and a perfectly stirred reactor, (2) the SNCR modeling approach with the reduced mechanism is validated by comparing the three-dimensional modeling results with test data from a pilot-scale combustion furnace, and (3) the integrated CFD modeling approach is applied to designing an SNCR system for an industrial furnace. The SNCR system was installed and has been in operation for several years. The NOx reduction and ammonia slip performance for the full-scale system agreed well with the CFD predictions. 1. Introduction Selective noncatalytic reduction (SNCR) technology has been applied to utility boilers, waste incinerators, and other stationary combustion systems for NOx control. It is a flue-gas treatment process in which a nitrogen-containing agent, such as ammonia (NH3) or urea [CO(NH2)2], is injected into the combustion gases to react with and reduce NOx formed during the combustion process.1 In the proper temperature window, NH2, NH, and N generated from decomposition of the injected reagent react directly with NO to form N2. Although the optimum process temperature depends on the agent and furnace quench rate, the accepted window for SNCR application is generally at temperatures between 1200 and 1255 K. Figure 1 shows the impacts of gas temperature on the NOx reduction achieved with urea [CO(NH2)2] and ammonia (NH3) in tests performed in a 300 kW combustion facility. In these tests, the nitrogen agent was injected into the flue gas resulting from natural gas combustion at 3% excess oxygen. To form a controlled NOx level in the flue gas, ammonia was injected into the combustion zone. The nitrogen stoichiometric ratio (NSR) is defined as the ratio of nitrogen in the nitrogen agent to that in the NOx in the flue gas. As shown in this figure, at the proper flue-gas temperature (approximately 1200-1255 K), NOx reduction is optimal. As the gas temperatures increase, the NOx reduction performance is reduced, and a portion of the reagent can be oxidized to NO. As the temperature decreases, a portion of the reagent exits the process unconverted. The unconverted reagent exiting the process is referred to as ammonia slip. In full-scale applications, it is difficult to achieve the performance levels seen in Figure 1. One factor that limits the * To whom correspondence should be addressed. E-mail: wei.zhou@ ge.com. Tel. :949-794-2600. † Current address: General Electric Global Research Center, 18A Mason, Irvine, CA 92618. ‡ Current address: Department of Mechanical Engineering, University of California at Berkeley, Berkeley, CA 94720.
performance is the rapid temperature quench that is experienced in the convective section of a boiler. To account for the effects of mixing and the rapid temperature quench on the SNCR performance, numerical simulations are needed to facilitate the SNCR system design. Considerable effort has been spent to understand SNCR chemistry and key parameters that control the SNCR process.2-7 SNCR reduced chemical mechanisms have also been developed to describe the process.8,9 Cremer et al.10-12 applied a reduced SNCR chemical mechanism in conjunction with computational fluid dynamics (CFD) simulation software, BANFF and GLACIER, to predict SNCR performance. Their work demonstrated that incorporating the reduced SNCR mechanism into a CFD model can be an effective approach for improving the confidence in designing an SNCR system. Other groups have followed similar approaches in CFD modeling to predict SNCR performance.13,14
Figure 1. Pilot-scale test results.
10.1021/ie9004089 CCC: $40.75 2009 American Chemical Society Published on Web 10/27/2009
Ind. Eng. Chem. Res., Vol. 48, No. 24, 2009 Table 1. Applicable Range of Reduced SNCR Chemistry
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Table 2. Flue-Gas Conditions
parameter
units
minimum
maximum
species
mass fraction
mole fraction
CO flue-gas temperature NSRa SRb
ppm K
0 600 0 0.7
20 000 2000 5 1.2
CO2 H2O N2 O2 NO
0.146 0.081 balance 0.024 1.5 × 10-4
0.093 0.126 balance 0.021 1.4 × 10-4
a Ratio of nitrogen in reagent to nitrogen in NO in flue gas. Actual oxygen-to-fuel ratio versus stoichiometric oxygen-to-fuel ratio in the combustion zone. b
Table 3. Test Conditions fixed temperature
This article expands the knowledge gained from previous studies by providing a comprehensive review of the SNCR modeling approach development, validation, and implementation. The reduced SNCR mechanism implemented in the CFD model was developed for a range of industrial applications, includes the effects of temperature and CO concentration on NOx reduction, and was validated by comparing its predictions with those from a detailed chemical mechanism in a plug-flow reactor and a perfectly stirred reactor and the data from pilot-scale testing during the development stage. The modeling approach was then successfully used in a number of industrial applications. This article reviews one of the industrial applications and compares CFD projections with actual full-scale data. 2. Reduced SNCR Mechanism Modeling 2.1. Development of SNCR Reduced Chemical Mechanism. The detailed kinetic mechanism of Glarborg et al.,15 consisting of 65 species and 477 reactions, was used to describe SNCR chemistry. The mechanism was previously used to describe promoted SNCR16 and advanced reburning17 processes. The detailed chemical mechanism can be used in one-dimensional simulations, such as in a plug-flow reactor (PFR) or a perfectly stirred reactor (PSR) to account for the species and temperature variations as a function of time. Overall, the chemistry consists of multiple reaction time scales and is numerically challenged when coupled with spatial variations. Therefore, a reduced SNCR chemical mechanism is desired for coupling chemical kinetics within a CFD framework to account for the SNCR chemistry in a three-dimensional application. The size of the reduced chemical mechanism has a significant impact on the computational time. Each species included in the reduced mechanism requires solving an additional transport equation, and each reaction step adds an additional reaction time scale to the application and, therefore, results in a stiffer numerical problem. Based on an evaluation of several reduced mechanisms, this study constrained the reduced chemical mechanism to contain 10 or fewer species and 6 or fewer global reactions for threedimensional CFD simulation. Depending on the application, a reduced chemical mechanism can be developed for a specific range of temperatures and species concentrations. The reduced chemistry in the current study is targeted for applications in utility boiler and process furnace retrofits. The applicable range of process conditions is summarized in Table 1. Additional requirements in the SNCR reduced chemical mechanism development include taking into account CO chemistry and its impact on SNCR performance and minimizing NOx prediction error within 20% when comparing with detailed chemistry over the range of process conditions. The computer-assisted reduction method (CARM) software developed by Chen and Tham18-20 was used to create a 29-
parameter
units
residence time temperature NSR CO
PSR
s K
PFR
1, 0.1, 0.01 0-2 600-1500 800, 900, 1000 0.8, 1.0, 1.2, 5 10 000, 20 000
ppm
CO dependence parameter
units
residence time temperature NSR CO
s K
PSR
PFR
1 600-1500 1.0
0-2 1200 0.8, 1.0
ppm
0, 100
species skeletal mechanism from the detailed chemistry set, which was then further reduced to a 10-species reduced mechanism. CARM reduces mechanisms using the quasisteady-state assumption for a number of species including radicals. The species assumed to be in quasi-steady state are H, O, HO2, H2O2, H2, CH2O, N, CN, NCO, NH2, NNH, HCN, HOCN, HCO, HNO, H2NO, NH, HONO, and N2O. The species remaining in the mechanism are directly solved in CFD. The resulting mechanism consists of global reactions with reaction rates that are functions of individual rate coefficients of the detailed set. The final reduced SNCR mechanism consists of 10 species and 6 reactions as follows: NO + CO + OH ) O2 + HNCO
(R1)
NO + CO + CO2 + NH3 + OH ) O2 + H2O + 2HNCO (R2) CO2 + 2OH ) O2 + CO + H2O
(R3)
2O2 + 3HNCO ) 2NO + CO + 2CO2 + NH3
(R4)
NO + O2 + CO ) CO2 + NO2
(R5)
2O2 + 4HNCO ) NO + 2CO + 2CO2 + NH3 + N2 + OH (R6) The reaction rate of each step can be calculated as Ri )
∑w
i,j
(1)
j
where i ) 1-6; Ri is the reaction rate of the ith global step, gmol/(cm3 s); and wi,j is the net reaction rate of the jth reaction step in the detailed mechanism that is included in the ith global reaction. 2.2. Validation of Reduced SNCR Chemistry. Validation of the reduced mechanism was done by comparing its predictions in both a PFR and a PSR with the predictions from the detailed mechanism. The flue-gas composition and conditions used in the validation and the test conditions are presented in Tables 2 and 3, respectively. Example comparisons of the PSR testing cases are illustrated in Figure 2. The NOx emissions at the reactor exit are plotted
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against the reactor temperature. The agreement between the reduced chemistry and the detailed chemistry is quite satisfactory and is within 15%. The charts in Figure 2 demonstrate the effect of CO on NOx reduction. The temperature window for NOx reduction shifts to a lower range as the initial CO concentration increases. The reduced mechanism reproduces this trend with reasonable accuracy. Figure 3 shows the NH3 concentration at the reactor exit. As expected, NH3 slip increases significantly with decreased temperature. Sample comparisons of PFR testing cases are illustrated in Figures 4 and 5. The performance of the reduced mechanism is consistent with observations from the PSR test runs. Figure 4 compares the NO emissions for NSRs of 1 and 5, and Figure 5 shows the impact of CO on SNCR performance. High NSR improves NOx reduction substantially at flue-gas temperatures of 900 and 1000 K. In the cases with 0% CO, the performance of the reduced chemistry deteriorates somewhat but is acceptable. The reduced mechanism also slightly deviates from the detailed mechanism at low-
Figure 3. PSR exit NH3 versus reactor temperature. Conditions: residence time ) 1 s, CO ) 10 000 ppm, NSR ) 1.
Figure 4. Comparison of PFR time evolution of NO under flue-gas conditions in Table 2 for NSR ) 1 and NSR ) 5 with 10 000 ppm CO.
Figure 2. Comparison of PSR exit NO for flue-gas conditions in Table 2 with residence time ) 1 s, NSR )1.
NSR (NSR ) 1) and low-temperature (800 K) conditions (Figure 4a). Overall comparisons between the detailed mechanism and the reduced mechanism demonstrate that the 10-species reduced mechanism is accurate for most cases of practical interest. 2.3. SNCR CFD Model Development and Validation in a Pilot-Scale Furnace. The reduced mechanism can be implemented in a CFD code that solves continuity, momentum, energy, and species transport equations. This study incorporated the reduced mechanism into FLUENT,21 a widely used commercial program. The modeling approach consisted of two steps: The first step solves the main combustion problem and provides a converged solution for temperature and main species profiles.
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Figure 6. Test-furnace temperature distributions.
Figure 5. FPR exit NO. Conditions: T ) 1200 K, NSR )1.
The second step assumes that the SNCR chemistry has a minimum impact on combustion and applies user-defined subroutines to calculate the source terms from eq 1 for the SNCR species. The main combustion processes are simulated in this work by the eddy-dissipation combustion model, which assumes that the reaction rate is controlled by the turbulent mixing rate.21 This approach has been applied successfully in a number of industrial applications that involve the nonpremixed fossil fuel combustion process.22,23 Initially, a three-dimensional CFD/SNCR model was developed for understanding the overfire-air (OFA)/SNCR process24 in a 300 kW combustion test facility.15,16 The test furnace was fired on natural gas. The heat input was set at a nominal value of 200 kW. Ammonia was doped into the combustion air to provide a controlled initial NOx level of approximately 200 ppm, corrected to 0% O2. A specially constructed overfire-air (OFA) injector was placed at a specific flue-gas temperature. Ammonia was added to the overfire-air streams to reduce NOx emissions. The ammonia was injected in gaseous form. The ratio of the nitrogen in the reagent to the NO in the flue gas (NSR) was approximately 1 for all tests. Two variables evaluated during the tests were (1) flue-gas temperature at injection, which was 1565 or 1618 K, and (2) burner stoichiometric ratio (SR1), which was 0.90, 0.95, or 1.05. A three-dimensional CFD model was built for the test furnace. The flue-gas temperature is specified at the model inlet as shown in Figure 6. The higher flue-gas temperature (1618 K) is specified for SR1 values of 0.95 and 1.05, and the lower flue-gas temperature (1565 K) is specified for an SR1 value of 0.9. Initial NOx is introduced uniformly at the furnace inlet. The cooler OFA mixes with the hot flue gas
Figure 7. Comparison of NOx reduction between test measurements and CFD predictions.
and results in a mixing temperature that favors the SNCR process.24 The predicted NOx reductions are compared with the measurements shown in Figure 7. The comparison shows that these two data sets are in good agreement. The results indicate that, when SR1 is 0.9 (i.e., combustion is under fuelrich conditions), negative NOx reduction is obtained from both the experiments and the CFD simulations. The poor SNCR performance is due to the high CO concentration generated in the incomplete combustion region, which shifts the SNCR temperature window toward lower temperature, as discussed earlier. However, when SR1 is greater than 1, 20-40% NOx reduction can be achieved, depending on the furnace temperature. Lower flue-gas temperature results in better NOx reduction performance. Figure 8 illustrates the predicted species profiles in the furnace. This exercise has demonstrated that the SNCR CFD model is capable of predicting NOx performances and trends due to injection of nitrogen reagent. 3. SNCR System Design and Performance Projections The CFD/SNCR model approach developed in the preceding section was applied to a number of industrial applications to facilitate SNCR design and optimization. A recent successful
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Figure 10. CFD model of the CO heater.
Figure 8. Contours of nitrogen-containing species at SR1 ) 1.05.
Figure 9. Schematic of CO heater.
application is the design of an SNCR system for a CO heater with a steam generating capacity of 200 tonnes per hour (t/ h).25 3.1. CO Heater CFD Model. The CO heater processes COrich off-gas from a fuel catalytic cracking unit (FCCU) regenerator. The equipment arrangement, shown in Figure 9, processes the off-gas from the catalyst regenerator, which is operated in a partial combustion mode, resulting in the production of an off-gas containing CO concentrations on the order of 6% and relatively high levels of total fixed nitrogen (TFN)
species (TFN ) NH3 + HCN). The CO heater oxidizes CO in the off-gas with a support fuel and air, resulting in the conversion of a significant fraction of TFN to NOx. At the full steam load of 200 t/h, the CO heater is normally operated with 6% CO in the CO off-gas and 1% furnace excess O2 (wet, measured at the heater exit). A three-dimensional CFD model was developed for the furnace to estimate the flow, temperature, and species distribution under baseline operating conditions. The eddy-dissipation and k-ε turbulence models were applied to account for the turbulent combustion in the CO boiler. The combustion model was calibrated against the baseline field data for appropriate prediction of the main combustion process. The SNCR model was then used to predict the mixing and emissions performance of various ammonia injection concepts. The CFD model geometry, as shown in Figure 10, consists of four main components: the support fuel burners, the heater furnace, the CO gas plenum, and the convective pass section. Regenerator flue gas enters through the 12 CO ports located at the annulus of the circular burner face. Refinery fuel is injected through four staged-fuel, low-NOx burners arranged on a twoby-two array on the burner face. The CO gas and refinery fuel gas enter and combust in a 5.12-m-inner-diameter horizontal, refractory-lined furnace. The products of combustion then travel upward into a rectangular vertical, convective pass section, which consists of several horizontal heat exchanger tube banks. The flue gases finally exit the furnace through a horizontal circular duct. 3.2. Baseline Results. The baseline CFD studies were conducted to provide information about the flow distribution; temperature profile; and distribution of species, such as CO and NH3, along the gas path. These results provided a basis for placing the SNCR injectors on the furnace and optimizing the injector design. Figure 11 shows the flow features in the CO heater. The regenerator off-gas inlet location and velocity have a dominant effect on the major flow field patterns in the furnace. The high velocity of the regenerator off-gas tends to mix relatively quickly with the burner flow and the secondary air. The model predicts that peak gas velocities occur at the inside elbow where the flow turns from horizontal to vertical. Flow separation also occurs along the vertical wall due to the change of the flow direction. Substantially lower velocities and a large recirculation zone occur toward the back wall. Figure 12 shows the temperature distribution in the boiler. The flue-gas temperature is relatively uniform at the end of the combustion chamber and in the transition
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Figure 11. Side view of velocity contours.
Figure 14. CO contours at the injection plane. Table 4. Comparison with Field Data
Figure 12. Side view of temperature contours.
Figure 13. Temperature contours at the injection plane.
duct and varies from 1172 K near the wall to 1339 K at the core. The temperature range favors the SNCR process. The CFD analysis suggested that the injectors should be placed on an angle cross section originate on the inside elbow such that the following requirements are met: (1) high CO and temperature pockets are avoided, (2) reasonable residence times are achieved, and (3) easy access to the furnace side wall is obtained with minimum installation difficulty. Figure 13 shows the CFD-predicted temperature distribution in the proposed injection plane located on the elliptical cross section of the furnace. The maximum CFD-predicted flue-gas temperature in the proposed injection plane is about 1330 K and
parameter
units
test data
CFD
temperature CO O2 CO2
K ppm % %
1235 53 2.22 10.5
1236 36 0.65 11.2
is located in a core zone at the center of the plane slightly toward the top. The average temperature predicted at the injection plane is 1288 K, which is close to the optimal window for ammonia SNCR. The CFD calculations predict average CO levels in the injection plane of about 2600 ppmv, with a maximum of 6500 ppmv located near the top (Figure 14). The CO concentration varies with the CO content in the regenerator off-gas and the air-to-fuel ratio in the burners. The CFD prediction of 38 ppmv CO near the back wall is comparable to the measured value of 53 ppmv. The baseline predictions were compared against field test data measured at a traverse distance of 5 ft into the furnace from the back wall. The comparison is shown in Table 4. The predictions are in good agreement with measurements. The predicted oxygen is significantly lower than the measured data; however, this is believed to be due to the interference of the aspirator air over the 5 ft of probe insertion. 3.3. Injection System Modeling Results and Performance Predictions. The CFD results from the baseline studies suggested that the NH3 injectors should be placed at the oblique plane in the lower furnace, where the temperature is around 1288 ( 340 K, the CO concentration is relatively low (
