Energy Fuels 2010, 24, 1696–1703 Published on Web 01/25/2010
: DOI:10.1021/ef901236e
Experimental Study of NO Removal by Gas Reburning and Selective Noncatalytic Reduction using Ammonia in a Lab-Scale Reactor D. Quang Dao, L. Gasnot,* K. Marschallek, A. El Bakali, and J. F. Pauwels PhysicoChimie des Processus de Combustion et de l’Atmosph ere (PC2A), UMR CNRS 8522, Universit e de Lille 1-Sciences et Technologies, 59655, Villeneuve d’Ascq Cedex, France Received October 28, 2009. Revised Manuscript Received December 23, 2009
An experimental investigation has been performed to study the efficiency of NO reduction by three technological approaches: reburning by methane, selective noncatalytic reduction (SNCR) by ammonia, and a hybrid approach by coupling reburning and SNCR. Experiments were performed on a lab-scale reactor equipped with a McKenna flat flame burner able to generate the flue gas with well-known features. The desired levels of initial NO were achieved by seeding the flame with known amounts of nitric oxide. Experiments were performed throughout the temperature range of interest, i.e., from 973 to 1213 K, to investigate the effects of main working parameters on the performance of the NO reduction process. As a result, very high efficiencies of NO reduction have been obtained using, respectively, methane (up to 90%) and ammonia (up to 75%) as a reducing agent. When these two processes are coupled in a reburn/SNCR hybrid technique, higher efficiencies in NO reduction have been obtained by a comparison to the classical SNCR technique. The computational fluid dynamic (CFD) modeling has also been investigated to characterize the fluid dynamics in the reactor, notably the gas homogeneity. Furthermore, the modeling results obtained with the commercial CFD code FLUENT 6.3 showed a good agreement with measurements.
generated in this zone. The next stage consists of the injection of a secondary hydrocarbon fuel into a zone called the reburning zone to generate a fuel-rich mixture, where most NOx are reduced to form N2 by reactions with hydrocarbon radicals. The optimal temperature range in the reburning zone is situated from 1173 to 1573 K.3 Finally, in a third zone, additional air is injected into the flue gas to complete the combustion process by notably oxidizing the remaining hydrocarbon fragments. Natural gas, pulverized coal, and biomass can be used as reburning fuel.4-6 NOx reduction efficiencies up to 90% can be achieved with this technological approach. The principle of the SNCR process is similar to the reburning one. Instead of the injection of a secondary hydrocarbon fuel, this process requires the injection into flue gas of specific nitrogenous species containing -NH- or -CN- functions, such as ammonia (NH3), urea [(NH2)2CO], or cyanuric acid [(HNCO)3]. The NOx removal efficiency of the SNCR process is influenced by various working parameters, such as the flue gas temperature, the gas residence time, the amount of reducing agent [we will speak below of the normalized stoichiometric ratio (NSR) of the reducing agent], the initial NOx concentration, the residual oxygen concentration in flue gas, the mixing conditions, and the additives. The NSR
1. Introduction Nitric oxides (NOx), mainly including nitric oxide (NO) and nitrogen dioxide (NO2), are one of the most toxic pollutants formed during combustion processes. An important part of NOx emissions is believed to come from automobiles, industrial boilers, and incineration plants. NOx play important roles in the environment through acidification, forest damage, smog formation, damage to human health, depletion of the stratospheric ozone layer, and the greenhouse effect.1 Various technological approaches have been applied for NOx control. The two major categories of NOx control for fuel stationary combustion are (i) combustion control (low NOx burner, over-fire air, exhaust gas recirculation, and control of combustion parameters) and (ii) postcombustion treatments [selective catalytic reduction, selective noncatalytic reduction (SNCR), reburning, and advanced reburning]. Among the postcombustion technologies, the reburning and SNCR processes have been showed to be very effective techniques with low investment costs. Reburning is an effective post-treatment technology for NOx control, which has been studied in laboratory reactors and large-scale boilers. In principle, the reburning process can be divided into three zones.2 In the primary zone, most of the primary fuel is burnt with a lightly air excess. The typical combustion products including NOx are considered to be
(3) Hampartsoumian, E.; Forlayan, O. O.; Nimmo, W.; Gibbs, B. M. Optimisation of NOx reduction in advanced coal reburning systems and the effect of coal type. Fuel 2003, 82, 373–384. (4) Liesa, F.; Alzueta, M. U.; Millera, A.; Bilbao, R. Influence of reactant mixing in a laminar flow reactor: The case of gas reburning. 1. Experimental study. Ind. Eng. Chem. Res. 2007, 46, 3520–3527. (5) Lu, P.; Xu, S.-R.; Zhu, X.-M. Study on NO heterogeneous reduction with coal in an entrained flow reactor. Fuel 2009, 88, 110–115. (6) Casaca, C.; Costa, M. The effectiveness of reburning using rice husk as secondary fuel for NOx reduction in a furnace. Combust. Sci. Technol. 2005, 177, 539–557.
*To whom correspondence should be addressed. Telephone: (33)321-01-70-46. E-mail:
[email protected]. (1) Lee, G.-W.; Shon, B.-H.; Yoo, J.-G.; Jung, J.-H.; Oh, K.-J. The influence of mixing between NH3 and NO for a de-NOx reaction in the SNCR process. J. Ind. Eng. Chem. 2008, 14, 457–467. (2) Bilbao, R.; Millera, A.; Alzueta, M. U. Influence of the temperature and oxygen concentration on NOx reduction in the natural gas reburning process. Ind. Eng. Chem. Res. 1994, 33, 2846–2852. r 2010 American Chemical Society
1696
pubs.acs.org/EF
Energy Fuels 2010, 24, 1696–1703
: DOI:10.1021/ef901236e
Dao et al.
characterizes the amount of the reducing agent injected into the SNCR zone. Using ammonia as the reducing agent, the NSR is defined to be the ratio of the moles of NH3 to the moles of initial NO present in the flue gas (i.e., NSR = NH3/NO).7 The SNCR process is characterized by a very narrow temperature window located in the 1123-1373 K range.1 Below the limit temperature of 1123 K, the reaction is too slow to provide any NOx reduction. At a higher temperature than 1373 K, the oxidation phenomenon of reducing agents is dominant and leads to NOx formation instead of reducing this pollutant. Nevertheless, this narrow and relatively high temperature interval may be widened and also lowered by additive effects. Several additives have been evaluated in the literature, such as CO and H2,8 alkanes (CH4, C2H6, and C4H10),9 alcohols [CH3OH, C2H5OH, and C2H4(OH)2],10 volatile organic compounds (C6H5OH and C7H8),11 or sodium compounds (Na2CO3, HCOONa, CH3COONa, and NaNO3).12 NOx reduction efficiencies up to 80% have been obtained in these studies. Although a highly effective performance of NOx abatement has been achieved using reburning and SNCR classical technologies, further NOx reduction can be obtained by coupling these two processes in a hybrid approach. This hybrid strategy consists of either a staged injection of a reburning fuel and a SNCR fuel, which corresponds to the advanced gas reburning (AGR) process,13,14 or an injection of a hybrid reburning/ SNCR fuel mixture that is named the hybrid reburn/SNCR process.10,15,16 These coupling techniques can potentially provide a NOx reduction performance up to 90%, and they are very promising, especially for large-scale applications. The experimental results presented in this paper are the first ones obtained from a long-term research program focused on the study of the main NOx control technologies. The NOx control technologies concerned are the gas-reburning approach, the SNCR process, and the hybrid reburn/SNCR scheme. This paper first presents the influence of the principal operating parameters on the reburning and SNCR processes using, respectively, methane and ammonia as reducing agents
in a lab-scale reactor. The performance of a hybrid reburn/ SNCR technology that uses a mixture of CH4/NH3/N2 as reducing agents is also discussed in this paper. A complementary numerical study has been realized by simulating the experimental observations using the commercial computational fluid dynamic (CFD) code FLUENT 6.3. 2. Experimental Section The experiments have been performed in a lab-scale reactor, which consists of a 1.1 m long tube, with an inner diameter of 8 cm. As shown in Figure 1, the experimental setup first includes a McKenna flat flame burner, which allows for the stabilization of a methane-air flame to generate flue gas with known compositions and features. The flame is seeded with a known amount of NO (1370 ppm) to control the NO initial concentration [NO]0 in flue gas. A total of 13 accesses are located on the reactor at different heights and positions to allow the temperature measurements and gas sampling for analysis. Average residence times up to 7.3 s are obtained according to the access used. Five electric heaters are located along the reactor to control the working temperature in the 973-1213 K range. A total of 40 thermocouples (type K) are positioned on the surface outside of the reactor to measure and control the device temperature. The reactor wall is insulated by thermal glass wool to prevent heat losses. The reducing agent mixture (CH4/N2 or NH3/N2) is injected in the flue gas using a specific device constituted of four water-cooled perforated tubes located 10 cm above the flat flame burner. Gas sampling is performed using a water-cooled probe17 connected to an experimental setup, allowing for continuous measurements by Fourier transform infrared spectroscopy (FTIR). Chemical species, such as NO, CH4, CO, CO2, and NH3, which play important roles in the studied processes, are quantitatively measured. Because of the uncertainties inherent in the FTIR measurements (mainly induced by spectral interferences and the calibration procedure), we estimate the accuracy of the FTIR measurements to be near (10%. For the flue gas temperature measurements, the accuracy is about 4%, which corresponds to an uncertainty of (20 K in our temperature range. Some previous experiments were performed to characterize the reactor. The objective of these experiments was notably to determine the axial flue gas temperature profile in the reactor. These measurements were investigated using a specific temperature probe, which was constituted of two Pt/Pt-10% Rh thermocouples with different diameters. The axial temperature profiles were measured for different set-point values and are displayed in Figure 2. The results showed that the shape of the temperature profiles were very similar for all of the temperature set-point values from 973 to 1173 K. The experimental temperature profiles allow us to determine the flue gas residence time in the reactor. To calculate this important parameter, the reactor is divided into several subvolumes according to the number of measurements (corresponding to the number of sampling holes). The residence time corresponding to a sampling hole is the sum of the times spent by flue gas in each subvolume of the reactor below this sampling hole. Therefore, the flue gas residence time (τ) corresponding to the nth sampling access from the burner surface is obtained from the following relation: n n n X X X Vi Hi τ ¼ ¼K ðR1Þ Δτi ¼ Ti V_ i ¼1 i ¼1 i ¼1
(7) Javed, M. T.; Irfran, N.; Gibbs, B. M. Control of combustiongenerated nitrogen oxides by selective non-catalytic reduction. J. Environ. Eng. 2007, 83, 251–290. (8) Javed, M. T.; Nimmo, W.; Gibbs, B. M. Experimental and modeling study of the effect of CO and H2 on the urea deNOx process in a 150 kW laboratory reactor. Chemosphere 2008, 70, 1059–1067. (9) Leckner, B.; Karlsson, M.; Dam-Johansen, K.; Wenell, C. E.; Kilpinen, P.; Hupa, M. Influence of additives on selective non-catalytic reduction of NO with NH3 in circulating fluidized bed boilers. Ind. Eng. Chem. Res. 1991, 30, 2396–2404. (10) Bae, S. W.; Roh, S. A.; Kim, S. D. NO removal by reducing agents and additives in the selective non-catalytic reduction (SNCR) process. Chemosphere 2006, 65, 170–175. (11) Noda, S.; Harano, A.; Hashimoto, M.; Sadakata, M. Development of selective non-catalytic reduction by ammonia in the presence of phenol. Combust. Flame 2000, 122, 439–450. (12) Zamansky, V. M.; Lissianski, V. V.; Maly, P. M.; Ho, L.; Rusli, D.; Gardiner, W. C. Reactions of sodium species in the promoted SNCR process. Combust. Flame 1999, 117, 821–831. (13) Han, X.; Wei, X.; Schnell, U.; Hein, K. R. G. Detailed modeling of hybrid reburn/SNCR processes for NOx reduction in coal-fired furnaces. Combust. Flame 2003, 132, 374–386. (14) Lee, C. Y.; Baek, S. W. Effects of hybrid reburning/SCNR strategy on NOx/CO reduction and thermal characteristics in oxygenenriched LPG flame. Combust. Sci. Technol. 2007, 179, 1649–1666. (15) Zhang, Y.; Cai, N.; Yang, J.; Xu, B. Experimental and modeling study of the effect of CH4 and pulverized coal on selective non-catalytic reduction process. Chemosphere 2008, 73, 650–656. (16) Cao, Q.; Wu, S.; Lui, H.; Liu, D.; Qui, Q. Experimental and modeling study of the effects of multicomponent gas additives on selective non-catalytic reduction process. Chemosphere 2009, 76, 1199– 1205.
In the relation R1, Δτi is the residence time of each subvolume (i) located below the nth sampling access, Vi and Hi refer to the volume and height of the ith subvolume, respectively, and V_ and Ti are the flow rate and average temperature of the flue gas in the (17) Sch€ obel, A.; Class, A. G.; Krebs, L.; Braun-Unkhoff, M.; Wahl, C.; Frank, P. Thermal destruction of benzene. Chemosphere 2001, 591– 599.
1697
Energy Fuels 2010, 24, 1696–1703
: DOI:10.1021/ef901236e
Dao et al.
Figure 1. Pilot-scale experimental setup: (A) schematic view of the semi-industrial reactor and (B) global experimental system. Table 1. Experimental Configurations reburning process reducing agent mixture temperature range (K) total flow rate (L/h) inlet air (L/h) inlet CH4 (L/h) seeded NO (L/h) flame stoichiometry (φ)
Figure 2. Axial profiles of the gas temperature in the reactor for different set-point values.
total flow rate (L/h) injected CH4 (L/h) reburn zone stoichiometry (φ*) CH4/NO injected NH3 (L/h) NSR (NH3/NO) injected N2 (L/h) initial NO concentration (ppm)
ith subvolume, respectively. The K factor takes into account the geometric properties of the rector and the total flow rate in standard temperature and pressure (STP) conditions. Table 1 lists the typical experimental conditions for each of the three processes studied in this work. As shown in Table 1, the three processes are studied over a flue gas temperature range from 973 to 1213 K. Different total flow rates of the methane-air mixture, which vary from 738.1 to 741.7 L/h (L/h in STP conditions), feed the McKenna flat flame burner to generate flue gas. Moreover, the flow rate of inlet air is fixed at 670 L/h, and different flame equivalence ratios φ (i.e., φ = 0.9 and 1.0) are achieved by adjusting the methane flow rate. The flame is seeded with a known flow rate of NO (1.1 L/h) to adjust an initial NO molar fraction [NO]0 near 1370 ppm. To achieve the reduction process, a fixed flow rate of 60 L/h of reducing agent mixture (CH4/N2, NH3/N2, or CH4/NH3/N2) is injected in the burnt gas. In the reburning process, various amounts of methane are injected in the flue gas to generate a fuel-rich zone with a specific stoichiometry (φ*), which varies
SNCR process
reburn/ SNCR process
CH4/N2
NH3/N2
NH3/CH4/N2
973-1213
973-1193
973-1193
Burner Features 738.1 741.6 672 670 65 70.5 1.1 1.1 0.9 1.0 Injectors Features 60 60 0-60 0 0-10 0-54.5 balance 1378
1.1-5.5 1.0-5.0 balance 1370
741.6 670 70.5 1.1 1.0 60 1.1-22 1-20 5.5 5.0 balance 1370
from 0 to 10. This factor is defined as the molar ratio of CH4 in the reburning zone to O2 in the flue gas. The value of the O2 concentration in the wet flue gas is measured by a classical industrial analyzer and is typically near 1.5%. In the same way, each amount of injected CH4 corresponds to a specific [CH4]/[NO]0 molar ratio. In the SNCR process, the amount of injected reducing agent (here, NH3) is controlled via the NSR, which is defined to be the ratio of the reducing agent mole fraction to the initial NO 1698
Energy Fuels 2010, 24, 1696–1703
: DOI:10.1021/ef901236e
Dao et al.
concentration ([NH3]/[NO]0). In this work, NSR varies from 1.0 to 5.0 and corresponds to the NH3 flow rate from 1.1 to 5.5 L/h. For the hybrid reburn/SNCR process, NH3/CH4/N2 mixtures are used and injected in flue gas to reduce NO. In this hybrid configuration, the NSR of NH3 is fixed to the value of 5, which corresponds to the optimal condition of the SNCR process in our experimental conditions. The NSR of methane ([CH4]/[NO]0) varies from 1 to 20. The flow rate of nitrogen is adjusted to assume a constant total flow rate of 60 L/h for the injected mixture.
3. Computational Modeling A CFD modeling using the commercial CFD package FLUENT 6.3 has been investigated. This modeling phase is very helpful in the purpose of characterizing the fluid dynamics of gas flow in the reactor. The numerical results allow us to evaluate the effect of the mixing and injection conditions on the reducing agent homogeneity in the NO reduction zone and, thus, the NOx reduction efficiency. A 180 000 cells three-dimensional unstructured mesh was created using Gambit 2.3.16. This mesh generator divides the computational zone of the reactor into cells, upon which conservation equations of mass, momentum, and energy are solved. The velocity, temperature, and species concentrations are computed for each cell. To characterize the gas flow in the reactor, nonreactive calculations were first performed in the atmospheric pressure condition and with the flue gas temperature of 1173 K. The experimental flue gas was simulated by a carrier gas (i.e., N2) that was introduced at the inlet of the burner with a constant flow rate of 738 L/h, which is representative of our experimental conditions. A CH4 flow rate of 60 L/h is introduced into the injectors to consider the injection of the reducing agent mixtures. This modeling configuration is believed to be able to simulate the fluid dynamics characteristics of the gas flow in the reactor. The standard k-ε model is chosen as the turbulence model. In this work, a value of the time average fluctuation of the velocity in the fluid was proposed to be 7% for the injection jet and for the gas flow at the inlet of the burner. The mass flow inlet, temperature, and mass fraction of the mixture are defined as the boundary conditions at the inlet.
Figure 3. Modeled and experimental radial CH4 mole fraction profiles obtained in the reactor with a total flow rate of 798 L/h and a gas temperature of 1173 K.
Figure 4. Evolution of NO reduction efficiency for different flue gas temperatures. The gas residence time is varied from 6 s (at 1213 K) to 7.2 s (at 973 K), and the initial NO concentration is 1370 ppm.
4.2. NO Control by the Reburning Process. The parametric study of NO reduction by reburning has been performed in the lab-scale reactor described above. The main working parameters, such as the flue gas temperature, the reburning zone stoichiometry (φ*), the flue gas residence time (τ), and the initial NO concentration [NO]0 were evaluated. The flue gas residence time is calculated via the relation R1 previously described. The effect of the flue gas temperature on NO reduction using methane as the reducing agent is presented in Figure 4. For all of the measurements, the gas sampling is realized at the same 11th access from the burner. As a typical result, the NO reduction efficiency increases with increasing the flue gas temperature (from about 16% at 973 K to 88% at 1213 K). In the low temperature condition of 973 K, an interesting NO reduction efficiency is obtained by coupling a high residence time (up to 7.2 s) to a very high stoichiometry (φ*) in the reburn zone. Always in Figure 4, it can be noticed that the effect of the reburn zone stoichiometry (φ*) on the NO reduction efficiency depends in a significant way upon the flue gas temperature. For the lower temperatures (from 973 or 1073 K), the NO reduction increases with increasing the reburn zone stoichiometry (φ*). However, for a higher
4. Results and Discussion 4.1. Characteristics of Fluid Dynamics in the Reactor. The modeled distribution of the CH4 mole fraction is shown in Figure 3. In this figure, we can clearly identify the position of the four injectors, which are located 0.10 m above the burner. Indeed, the analysis of the modeling profiles obtained at 0.105 and 0.11 m points out four peaks of mole fraction that were located at the positions of the four injection tubes. The radial distribution of the CH4 mole fraction is found to be strongly perturbed from the injectors to the first sampling access, which is located 0.16 m above the burner. Higher in the reactor, methane is found to be rapidly mixed with the flue gas to obtain a stable flow with a better homogeneity. Thus, it can be considered that, above the position of 0.16 m, the homogeneity is effective enough for not influencing the reduction process efficiency. To validate this observation, we have compared the calculations obtained at 0.26 m (i.e., the third sampling access) to the CH4 experimental measurements obtained with the same gas flow rate and temperature. As shown in Figure 3, the comparison between the experiment and modeling points out a very good agreement, validating the calculations. 1699
Energy Fuels 2010, 24, 1696–1703
: DOI:10.1021/ef901236e
Dao et al.
Figure 6. NO reduction efficiency as a function of the initial NO concentration [NO]0. The flue gas temperature is 1123 K, and φ* = 3.3.
Figure 5. NO reduction efficiency as a function of the residence time for different values of φ*. The flue gas temperature is 1173 K, and [NO]0 = 1370 ppm.
as the reducing agent was also carried out in the present study. The experimental configurations that we used to study the SNCR process are presented in Table 1. Four main working parameters have been analyzed to evaluate their respective impact on the SNCR efficiency: the NSR ratio ([NH3]/[NO]0), the gas residence time (τ), the initial NO concentration [NO]0, and the flue gas temperature. The effect of the NSR ratio and the flue gas residence time on the NO reduction efficiency is presented in Figure 7. It can be shown that the NO concentration in the flue gas first increases to reach a maximum for a NSR value close to 3. For higher values of NSR, the NO concentration decreases to become lower than the initial [NO]0 for NSR values up to 4. Globally, in our experimental conditions, the NRS optimal interval for NO removal is located from 4 to 5 depending upon the average residence time. For a flue gas temperature of 1173 K, the best conditions for SNCR reduction are obtained for a NSR ratio of 5 and with a residence time of 6.2 s. The evolution of the NO concentration presented in Figure 7 can notably be explained by the presence of significant residual oxygen in the flue gas. Actually, because of favorable conditions of temperature, the ammonia injected into flue gas is first oxidized in the presence of O2 to form NO. When the NSR ratio reaches about a value close to 3, the kinetic behavior of the NH3/flue gas mixture seems to enhance and favor the NO reduction process. Furthermore, some complementary measurements have clearly pointed out the effect of slight variation of the O2 concentration (about 5% for an initial O2 concentration of 1.5%) in the flue gas on the NO reduction efficiency. Similar results that agree with our experimental observations have been reported in previous works by Han et al.13 The effect of the gas residence time on NO reduction by ammonia for various values of NSR is presented in Figure 8. The SNCR process has been evaluated in a large range of residence times from 0.8 to 6.2 s. As shown in Figure 8, the NO concentration is not a linear function of the residence time. For optimal conditions of the NSR ratio (i.e., NSR = 5), the NO reduction is effective with a short residence time (less than 2.5 s) or a residence time higher than 4.5 s. The effect of the initial NO concentration [NO]0 is also evaluated and displayed in Figure 9. When the flue gas temperature was fixed at 1123 K and the residence time was fixed at 2.4 s, the amount of injected ammonia was systematically adjusted for each value of [NO]0 to assume a constant [NH3]/[NO]0 molar ratio equal to 5. As seen in Figure 9, the effect of [NO]0 on the NO reduction efficiency is
temperature (1173 K), a peak of NO reduction is clearly observed at the stoichiometry of 1.7. Thus, in the stoichiometry interval from 1.7 to 3.3, a new kinetic phenomena may contribute to the decrease of the NO reduction efficiency, while φ* increases. The phenomenon is more interesting with the increasing again of NO reduction efficiency in the higher reburn zone stoichiometry. This peak of NO reduction was previously observed by Dagaut et al., who used a CH4/C2H6 (10:1) mixture as a reducing agent.18 In our experimental conditions, the optimal flue gas temperature is found to be 1213 K. In these conditions, the increase of the reburn zone stoichiometry induces first an increase of the NO removal performance, which tends to reach a maximum value of 90% for a value of the φ* coefficient near 3.0. The use of higher amounts of CH4 does not facilitate the NO reduction process. We have studied the effect of the residence time on the NO reduction efficiency for different values of the reburn zone stoichiometry. The experimental results that we obtained are displayed in Figure 5. It can be noticed that, when the reburn zone stoichiometry φ* is maintained constant, the increase of the residence time induces an increase of the NO reduction efficiency. This typical residence time behavior is found to be reproductive with each value of the reburn zone stoichiometry. Figure 6 presents the effect of the initial NO concentration in flue gas [NO]0 on the NO reduction efficiency. At the flue gas temperature of 1123 K, the NO reduction efficiency first strongly increases with increasing the initial NO concentration from 200 to 800 ppm. Nevertheless, [NO]0 values higher than 800 ppm do not promote the NO reduction and induce a stabilization of the process efficiency. Our observations agree with previous results reported by Casaca and Costa, who used natural gas and ethylene as reburn fuels.6 Table 2 summarizes the experimental evolution of the NO reduction efficiencies using the reburning process as a function of the flue gas temperature, the [CH4]/[NO]0 ratio, the reburn zone stoichiometry φ*, and the average residence time. For each flue gas temperature (973, 1073, and 1173 K), the NO reduction efficiencies are displayed for four average residence times, which correspond to the 5th, 7th, 9th, and 11th sampling access of the reactor (Figure 1). 4.3. NO Control by the SNCR Process Using Ammonia. The experimental study of NO removal by SNCR with NH3 (18) Dagaut, P.; Lecomte, F.; Chevailler, S.; Cathonnet, M. Experimental and detailed kinetic modeling of nitric oxide reduction by a natural gas blend in simulated reburning conditions. Combust. Sci. Technol. 1998, 139, 329–363.
1700
Energy Fuels 2010, 24, 1696–1703
: DOI:10.1021/ef901236e
Dao et al.
Table 2. Global Summary of NO Reduction Efficiencies by Reburning φ* [CH4]/[NO]0 973 K
1073 K
1173 K
2.8 s 4.3 s 5.8 s 7.2 s 2.6 s 3.9 s 5.3 s 6.6 s 2.4 s 3.7 s 5.0 s 6.2 s
0.8
1.7
2.5
3.3
4.2
5.0
5.8
6.7
7.5
8.4
10.0
4.5 1.9 1.0 0.2 1.0 3.0 2.7 3.5 2.9 3.2 4.8 7.8 9.3
9.1 0.4 2.0 1.2 8.2 2.9 1.6 1.4 13.6 27.1 49.8 56.5 60.1
13.6 0.7 1.5 3.1 11.1 0.4 8.4 2.3 15.1 8.4 31.6 27.4 36.5
18.2 0.8 0.6 3.9 9.2 3.2 0.7 6.5 14.7 9.5 20.3 20.6 24.9
22.7 0.6 1.0 4.6 6.5 7.8 8.1 4.7 18.6 12.9 19.1 26.8 30.6
27.3 2.2 1.6 5.8 11.7 10.2 11.9 10.4 20.8 18.6 19.7 38.5 36.8
31.8 2.4 2.5 4.0 11.5 10.7 14.4 10.2 27.7 23.7 31.0 55.8 50.2
36.4 5.5 3.0 8.3 13.4 13.9 21.0 10.9 27.2 29.5 33.8 66.0 66.9
40.9 3.0 4.5 7.4 11.2 15.4 20.2 13.7 30.7 37.7 45.1 72.3 75.8
45.5 3.1 6.1 7.4 12.7 16.5 18.9 17.6 30.4 41.2 56.6 74.1 77.9
54.5 5.3 7.5 8.4 16.6 18.8 22.6 26.8 36.1 49.5 70.6 75.5 77.3
Figure 9. Effect of [NO]0 on the NO reduction efficiency. Gas temperature = 1123 K; residence time = 2.4 s; NSR = 5.0. Figure 7. Evolution of the NO concentration as a function of the NSR ratio for different gas residence times (τ). The flue gas temperature is fixed at 1173 K.
Figure 10. Evolution of the NO concentration as a function of the gas temperature for different residence times (τ). NSR is fixed to 5.0.
concentration increases with the flue gas temperature. It seems that, in low-temperature conditions, the NH3 kinetic favors the NO formation process rather than the NO reduction process. For the gas temperatures higher than 1073 K, the NO reduction becomes dominant and reaches a maximum NO reduction up to 75% at 1193 K. The kinetic modeling of the NO reduction processes is in progress using the SENKIN-CHEMKIN II code and the detailed kinetic mechanism developed recently by Coda Zabetta and Hupa.19 This modeling phase should provide very interesting information about the reaction pathways involved in the reduction processes. Some primary results of sensibility analysis allow us to identify the main reaction paths for NO formation and NO reduction with ammonia
Figure 8. Evolution of the NO concentration as a function of the residence time at different NSR ratios. The flue gas temperature is fixed at 1173 K.
significant and differs from the one observed using methane as the reducing agent (Figure 6). In the case of the SNCR process, it seems that no saturation effect occurs when increasing [NO]0. Thus, for [NO]0 values higher than 1000 ppm, the NO reduction efficiency increases linearly with a significant slope. The effect of the gas temperature (from 873 to 1193 K) on the NO abatement is presented in Figure 10. The experiments have been realized with different average residence times from 2.4 to 6.2 s. As a result, we can notice that the behavior of the NO concentration is very similar for each residence time. For temperatures lower than 1073 K, the NO
(19) Coda Zabetta, E.; Hupa, M. A detailed kinetic mechanism including methanol and nitrogen pollutants relevant to the gas phase combustion and pyrolysis of biomass-derived fuels. Combust. Flame 2008, 152, 14–27.
1701
Energy Fuels 2010, 24, 1696–1703
: DOI:10.1021/ef901236e
Dao et al.
Figure 12. Effect of the CH4 addition on the NO concentration for various residence times. NSR ([NH3/NO]0) = 5; temperature = 1173 K; [NO]0 = 1370 ppm.
Figure 11. Kinetic behavior of ammonia during the SNCR process in the present experimental conditions. [NO]0 = 1370 ppm; [NH3]/ [NO]0 = 5; τ = 2.4 s.
(Figure 11). As can be seen, in our experimental conditions, the injection of NH3 may lead to the competition between two kinetic processes: the NO reduction by the SNCR mechanism (NH3 f NH2 f NNH f N2) and the NO formation by the NH3 oxidation mechanism (NH3 f HONO f NO and NH3 f NH2 f H2NO f HNO f NO). In our experimental conditions, the NO formation is favored for low values of NSR ([NH3]/[NO]0 < 3), residence time (τ < 4 s), and flue gas temperature (T < 1073 K). In these conditions, the remaining O2 concentration in flue gas may favor the NH3 oxidation in forming NO, as can be seen experimentally in Figures 7-9. The NO reduction by the SNCR mechanism is favored when the residual O2 is strongly consumed by the first steps of NH3 oxidation; these favorable conditions are obtained with high values of NSR, τ, and T parameters. This kinetic study has to be continued, and the detailed kinetic mechanism has to be optimized to be validated in all of our experimental conditions. 4.4. NO Removal by the Hybrid Reburn/SNCR Process. As for the two previous studies, the hybrid approach by coupling reburning and SNCR has been tested to evaluate the effect of the main working parameters on the NO reduction efficiency. The experimental configurations used are displayed in Table 1. Figure 12 presents the influence of CH4 addition into the reducing agent mixture (i.e., NH3/CH4/N2) on the hybrid process for different residence times. The NO concentration for the [CH4]/[NO]0 = 0 condition corresponds to the one obtained by the classical SNCR process with the [NH3]/[NO]0 ratio fixed to 5. As can be seen, the NO concentration decreases with increasing the CH4 addition. The experimental results show that just a small CH4 addition in the reducing agent mixture induces a great effect on NO reduction efficiency. Actually, when the [CH4]/[NO]0 molar ratio is increased from 0 to 3, the NO removal efficiency increases from 20 to 45% and from 67 to 80%, respectively, for a residence time of 2.4 and 6.2 s. Furthermore, when the [CH4]/[NO]0 ratio is increased from 3 to 20, the NO reduction performance becomes stable and only a slight increase of NO reduction is observed for each value of residence time. Therefore, it seems to be judicious to use a small amount of CH4 to enhance the hybrid reburn/SNCR process. In this
Figure 13. Effect of the temperature on the hybrid reburn/SNCR efficiency. Residence time = 2.4 s; [NO]0 = 1370 ppm; NSR (NH3/ NO) = 5.
kind of configuration, CH4 reburn fuel may be considered just as an additive. The experimental effect of the flue gas temperature on the hybrid reburn/SNCR process is presented in Figure 13. Experiments were carried out in a wide range of temperatures from 873 to 1198 K. A great influence of the methane addition is observed for all flue gas temperature set-point values. For temperatures lower than 1073 K, the addition of hydrocarbon has an inhibitive effect on the NH3 oxidation, inducing a significant decrease of the NO formation via NH3 oxidation. For higher temperatures, the hydrocarbon addition leads to a further NO reduction and lowers the optimal temperatures. 5. Conclusions Experimental and computational investigations have been performed to analyze the effectiveness of three NO reduction processes: methane gas reburning, SNCR by ammonia, and hybrid reburn/SNCR approach. The parametric study has been realized on a specific lab-scale reactor, and measurements were performed using FTIR analysis. The major findings are as follows: (1) A CFD modeling study has been investigated to characterize the fluid dynamics of gas flow in the reactor. Calculations showed that a good homogeneity of reducing agent could be achieved in flue gas from the third sampling access of the reactor. (2) Methane gas reburning, which was performed in relatively low-temperature conditions and in a large range of reburn zone stoichiometries was observed to be very effective. A maximum NO removal efficiency up to 90% has been obtained at optimal conditions. 1702
Energy Fuels 2010, 24, 1696–1703
: DOI:10.1021/ef901236e
Dao et al.
further NO reduction and lower the optimal temperature range. Only a small addition of hydrocarbon (CH4) to an initial NH3/N2 mixture was found to be effective in the NO reduction process.
The NO reduction performance increased with increasing the main working parameters, such as the reburn zone stoichiometry, the flue gas temperature, the average residence time, and the initial NO concentration. (3) The SNCR classical system could achieve a significant NO reduction. Although a kinetic competition between NO reduction and NH3 oxidation occurred, a very high performance of NO reduction up to 75% was obtained. The molar ratio of [NH3]/[NO0], the flue gas temperature, the initial NO concentration, and the residence time were found to be the main working parameters that have to be optimized to obtain better NO reduction efficiencies. (4) The hybrid reburn/SNCR strategy, which used a NH3/CH4/N2 mixture as the reducing agent, could provide
Acknowledgment. This work was supported by the Centre National de la Recherche Scientifique (CNRS) and the Air Quality Program of Institut de Recherche en Environnement Industriel (IRENI). We are grateful to the region Nord-Pas-deCalais and the European Funds for Regional Economic Development for their financial support. The authors also thank the Departement Energetique Industrielle of Ecole des Mines de Douai in France for their collaboration during CFD modeling.
1703
