Ind. Eng. Chem. Res. 1996,34, 4540-4548
4540
Simplified Kinetic Model of the Chemistry in the Reburning Zone Using Natural Gas Rafael Bilbao,* Maria U. Alzueta, Angela Millera, and Mario Duarte Department of Chemical and Environmental Engineering, University of Zaragoza, 50009 Zaragoza, Spain
A simplified kinetic model of the chemistry of NO reduction by the action of natural gas has been obtained. This model consists of 87 elementary reactions and 38 chemical species. It takes into account the natural gas decomposition and oxidation, the interaction between NO and hydrocarbons, and the conversion of the intermediate nitrogenous species. The concentrations of the species calculated with the model have been compared with those obtained experimentally using the operating conditions for which a good efficiency in the reburning zone is achieved. The comparison between the theoretical and experimental data of NO, HCN, CO, and COZhas been extended t o the results obtained over a wide range of values of the operating conditions in the reburning zone.
Introduceion Reburning with natural gas is an attractive technique that can be applied to reduce NO, emissions in coal combustion systems. The chemistry of the reburning process involves a large number of reactions of formation and consumption of many nitrogenous and hydrocarbon species. Many of these species are very reactive and are present only in very low concentrations and during short periods of time. The kinetic modeling of the reburning process involves the description of the chemical conversion processes using elementary reactions. This modeling can be very complex. To the high number of reactions and species that must be included in the modeling, and to the very low concentrations of many species, is added the discrepancy observed in the values of the kinetic parameters available in the literature. Different kinetic models have been reported t o describe the reburning process with natural gas, as well as the interactions between nitrogenous and hydrocarbon species. Depending on their complexity and description level, these models can be classified as detailed, simplified, and reduced models. Detailed models are composed of a large set of reactions and species, and they include all the possible reactions that can intervene in the process studied. The kinetic model of Miller and Bowman (19891, which consists of 307 elementary reactions and 48 chemical species, can be considered as a detailed model of nitrogen chemistry in combustion. This model studies the gas phase reactions of the nitrogenous species and the reactions involving hydrocarbons. It also includes the formation of thermal and prompt NO, the conversion of fuel nitrogen, staged combustion, and reburning processes, and the formation and elimination of NO2 and NzO. Starting from the model of Miller and Bowman (1989) and taking into account the kinetic data review of Glarborg and Hadvig (1991),Kilpinen et al. (1992) developed a kinetic model centered on NO reduction by the action of natural gas. This model consists of 225 reactions and 48 chemical species. Glarborg et al. (1992) also proposed a model for methane combustion and for the interactions existing between nitrogenous species
* To whom correspondence
should be addressed.
0888-5885/95/2634-454Q~Q9.QOJQ0
and hydrocarbon radicals. This model includes a global mechanism of 234 reactions and 48 species and is similar t o that proposed previously by Glarborg and Hadvig (19891,which took as the starting point the model of Miller and Bowman (1989),jointly with the model of Glarborg et al. (1986). In general, apart from the suitability of the kinetic parameters used, the detailed models have the problem of being very complex. They include a large number of reactions among a high number of species (stable and radicals), and therefore their resolution incorporated in complex flow models, to be applied to a boiler, requires a lengthy computationtime. Nevertheless, these models are very important because they usually represent the starting point for obtaining other kinds of models. The simplified models, also named “short” or “skeletal”, consist of a reactions subset from an initial detailed model. These models are constituted by the minimum reactions set which allows an acceptable description of the process considered. No mathematical simplification is carried out in the resolution of these models. They provide an intermediate solution between the obtaining of a prediction of the experimental results in a large range of operating conditions and the computation time necessary for the resolution of the kinetic model when interconnected with flow models. Different simplified reaction mechanisms have been proposed in order to facilitate the application of a determined model to a real installation and in a range of operating conditions of interest where good results can be obtained. One example of these simplified models is that obtained by Glarborg et al. (1992),which considers a reduced four-step mechanism taking into account their detailed model mentioned above. The corresponding Simplified model, described by them as skeletal, includes the interaction between nitrogenous species with hydrocarbon radicals but does not include the chemistry of the CZspecies. This model consists of 77 elementary reactions and 26 chemical species. The reduced models consist of a small number of reactions and species which are considered as the most relevant in the process studied. In general, these models include significant simplifications and suppositions. They sometimes assume partial equilibrium for some reactions andlor steady state for some species whose concentration is difficult to measure. A model that uses this approach is that proposed by Glarborg et al. (1992),which provides a good description of the
1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995 4541 methane oxidation process as well as of the formation and destruction of nitrogen oxides, although a t low temperatures or very fuel-rich conditions the reduced model predictions are not so good. Furthermore, some reduced models can often require the use of empirical equations for the determination of certain parameters or the inclusion of certain fitting parameters. As an example, the equations of Mereb and Wendt (1994) can be cited that refer t o the interactions of hydrocarbon radicals and NO in the reburning zone. In general, the application of these kinds of models is limited to some specific physical configurations of the experimental system as well as to a small range of operating conditions. Taking all these points into account, it has been considered useful to obtain an alternative kinetic model of the chemistry of NO reduction by the action of natural gas. Such a model must be as simple as possible in order that it can be applied with a reasonable computational time even with the joint use of complicated flow models; it should not include fitting parameters depending on the experimental system used; and it must predict the experimental results obtained with the operating conditions for which a good efficiency in the reburning zone is achieved.
Experimental Method and Optimum Operating Conditions An extensive experimental study was performed in order to obtain those operating conditions for which a good efficiency (low NO and HCN concentrations at the exit) was obtained in the reburning zone. The experimental system used is described elsewhere (Bilbao et al., 1994, 19951, and it allows us to simulate the gas composition exiting from the primary zone of a pulverized coal boiler and to inject different amounts of natural gas. In the reburning experiments a gas consisting of 0 2 , C02, and N2 was prepared and bubbled into a water container, in order t o reach a desired moisture. The gas was later mixed with NO and natural gas, and the mixture was fed into the reburning reactor. The natural gas used had an average volume composition of 90.5% CH4,8.5% CzHs, 0.5% C3H8,0.4% N2, and 0.1% C, (n = 4-7). In all the experiments, a fixed concentration of C02 (20%)in dry basis was used, and the amount of steam added represented an extra 6% of the total flow rate in dry basis. Experiments were carried out to analyze the influence of the different operating conditions in the reburning zone over a wide range of variable values. The variables analyzed were as follows: the temperature (1200-1500 "C), the natural gas concentration (14.5%) or reburning fuel fraction (0.08-0.50), the oxygen concentration at the inlet of the reburning zone (0-5%), the stoichiometric ratio in the reburning zone (0.751.05), the primary NO concentration (100-1200 ppm), and the gas residence time (94-280 ms). At the outlet of the reburning zone, the concentrations of 0 2 , H2, N2, HCN, CO, C02, and different hydrocarNO, NO2, "3, bons were determined using different methods of analysis. From the results obtained (Bilbao et al., 19951, it can be concluded that the values of the operating conditions that allow us t o reach the highest efficiencies in the reburning zone are temperatures between 1300 and 1400 "C, stoichiometric ratios in the reburning zone between 0.93 and 0.95, natural gas concentration between 1.5 and 2%, oxygen concentration between 1.5
and 2.5%, and residence times higher than 120 ms. In the present work, the kinetic model proposed has been applied mainly to these ranges of the variable values, although the comparison between the theoretical and experimental data has been also extended to the results obtained in all the experiments.
Simplified Kinetic Model Proposed Prior to the obtention of the kinetic model, a study of the equilibrium of the reactions involved was performed. A Gibbs free energy minimization was carried out in order to determine the equilibrium concentrations of the most important species considered in the reburning process. These equilibrium studies were performed using the software of chemical processes simulation PRO-11, marketed by Simulation Sciences Inc. The SRK (Soave-Redlich-Kwong) method, which applies a cubic state equation to nonideal mixtures containing light hydrocarbons, was chosen. For these studies, a total gas flow rate in dry basis of 900 N U , initial concentrations of 20% COZ and of 900 ppm NO, and different amounts of natural gas (0-4.5%) and oxygen (04%) were considered. Moreover, an extra 6% steam was added to the total flow rate in dry basis. The trends observed in the equilibrium and experimental concentration values of the different species were quite similar, but the values of calculated equilibrium concentrations were quite different from the experimental values. Therefore, it was concluded that in the reburning experiments the equilibrium for the species considered was not reached, and therefore a model considering the kinetics of the different reactions had to be chosen. The model should be as simple as possible and should not require the use of fitting parameters. The model must include the kinetic equations corresponding t o those reactions which are usually considered as the most important in the natural gas reburning process. In order to obtain the simplified kinetic model, the starting point was a detailed kinetic model composed by the reactions included in the model of Kilpinen et al. (1992) and other reactions proposed by Glarborg and Hadvig (1991) that are not in the Kilpinen model. This initial model consists of 298 elementary reactions and 48 chemical species. From the analysis of the kinetic parameters existing in the bibliography (Glarborg et al., 1986; Glarborg and Hadvig, 1991; Hanson and Salimian, 1985; Kilpinen et al., 1992; Miller and Bowman, 1989; Tsang and Hampson, 1986), a significant discrepancy among the values of these parameters has been observed for the different reactions. Therefore, a prior selection was made of the values of the kinetic parameters used. First, the kinetic parameters that had been obtained in the same temperature intervals as those used in this work were chosen. Where different kinetic parameters were proposed for the same temperature interval, the value obtained by several different authors andlor methods was selected. Where no suitable parameters were found for the temperature range considered, the parameters recommended by Glarborg and Hadvig (1991) were selected. This detailed kinetic model was solved using the chemical kinetic software CHEMKIN (Kee et al., 1989a), jointly with LSODE (Livermore Solver for Ordinary Differential Equations), and utilizing the thermodynamic data from the Sandia CHEMKIN data base (Kee et al., 198913). In the model, the expression of the molar production rate of a chemical species, hk,is:
4642 Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995 K
K
i=l k = l
k=l
I
K k=l
The units of Li)k are mol cm3 s-l. The values of the rate constants kfi are given in the form:
k,
= F@ exp(-EIRT)
(2)
T is given in K and E in cal mol-l. In the model resolution, the following assumptions were taken into account: (a) There is no pressure drop through the reactor. (b) A length of 800 mm is considered as the reaction zone length (Bilbao et al., 1995). (c) The nominal temperature used is taken to be the initial temperature a t the inlet of the reaction zone. Temperatures along this zone are calculated through heat balances. Data of the concentrations of several products (NO, HCN, “3, CO, C02, ...) were calculated at the outlet of the reburning zone for different operating conditions. As is shown below, the results of the concentrations of NO and HCN obtained with this model show an acceptable concordance with the experimental results in some cases, although a higher discrepancy is obtained in others. From this initial model, the simplification was performed taking into account the following points: (a) Having determined the relevance of the different reactions with respect to the global reaction rate by means of a reaction rate analysis, the slowest reactions, which should not have any great importance for the residence times considered, were excluded. (b) The species whose concentrations were always very low ( 1ppm) were also eliminated. For each kinetic model configuration, the concentrations of different products at the outlet of the reburning zone were calculated using the same method described above for the initial detailed model. For each elimination carried out, and in order to check if some significant reaction or species had been excluded, the results obtained were compared with those corresponding to the detailed model. This comparison has been done for all the experimental conditions used in the reburning experiments, although with an emphasis on the interval of the operating conditions that allows us to reach the highest effkiencies in the reburning zone (as has been mentioned above). The simplified kinetic model thus obtained includes 87 elementary reactions and 38 chemical species, and its description is shown in Table 1. The values of F , /3, and E corresponding to the different reactions are also shown in Table 1. The species M which appears in some reactions corresponds to a so-called third-body species, with its corresponding third-body efficiency. These reactions can be lumped together in three important stages considered to take place sequentially. These reaction stages correspond to (i)the natural gas decomposition and oxidation to give mainly hydrocarbon radicals, CO and C02, (ii) the reactions corresponding to the interaction between NO and the hydrocarbon radicals originated, and (iii) the reactions of the intermediate nitrogenous species to produce the final products.
Figure 1. Preferential reaction pathways for the natural gas decomposition and oxidation.
Chen et al. (1986) also consider these three stages for the reburning process description. Burch et al. (1991) stated that there are two energy barriers in the NO reduction by natural gas, which would correspond to stages i and ii. Glarborg and Hadvig (1989)take into account a fourth stage corresponding to the reactions among the air nitrogen and the hydrocarbon radicals. These reactions can be represented by
CH,
+ N, - HiCN + (N, NH)
(3)
which would correspond to the first stage in the prompt NO formation. In this work, these kinds of reactions were considered in the initial detailed model, but their elimination was found not to affect significatively the results obtained for the operating conditions analyzed. Nevertheless, experiments to analyze the possible formation of prompt NO were also performed. These experiments were carried out at different temperatures (1200-1500 “C), introducing into the reactor natural gas, nitrogen, and water (in concentrations similar to those used in the reburning experiments). In addition, air was introduced at a final stage at temperatures between 900 and 1100 “C in order to oxidize possible intermediate nitrogenous compounds. No appreciable amounts of NO or HCN were detected either at the outlet of the reducing zone or after the addition of air. From this simplified model, the preferential reaction pathways through the main species evolved were determined using a reaction rate analysis. Figure l shows the main reaction pathways for the natural gas decomposition and oxidation, which correspond to the first stage of the reaction mechanism mentioned above. Taking into account the composition of the natural gas used, CH4 and C2H6 can be considered as the most representative hydrocarbons of natural gas. At high temperatures, these hydrocarbons can decompose (by oxidation or pyrolysis), and hydrocarbon radicals, CO and C02, are produced with a product distribution which depends on the operating conditions. If the oxygen concentration is very low and the temperature is very high, the thermal decomposition of hydrocarbons is the main effect (Bilbao et al., 1994). In the presence of a certain amount of oxygen, a high concentration of radicals such as 0, H and OH is produced. These radicals participate in the hydrocarbon decomposition by oxidation, also giving hydrocarbon
Ind. Eng. Chem. Res., Vol. 34,No. 12, 1995 4543 Table 1. Reaction Set of the Kinetic Model Proposed elements C, H, N, 0 suecies
kinetic parameter reaction
+ +
CH4 + M = CH3 H M third-body efficiency CH4 + H = CH3 + Hz CH4 + 0 =CH3 + OH CH4 + OH = CH3 + HzO CH3 + M = CHz H + M CH3 0 = CHzO + H CH3 OH = CH2 + H2O CH3 + 02 = CH30 + 0 CH3 + HO2 = CH3O + OH CH3 CH2O = CH4 + HCO CH3 HCO = CH4 + CO CH3 + CH2 = CzH4 + H CH3 + CH = CzH3 + H CH3 + C = CzHz H CHzO + H = HCO + H2 CHzO + 0 =COz + H + H HCO M = CO H M third-body effciency CH2 + OH = CH + HzO CH2 + 0 2 = CHzO + 0 CH2 + 0 2 = C02 + H + H CH2 + 0 2 = COz + Hz CH2 + C02 = CO + CH2O CH +OH = HCO H C2H6 = CH3 + CH3 CzH6 + H = CZH5 + Hz CzH6 + CH3 = CzH5 CH4 C2H5 + M = C2H4 + H + M C2H4 + H = C2H3 + HZ CzH4 + CH3 = CzH3 + CH4 CzH3 + 0 2 = C2Hz HO2 CzHz + 0 = CHz + CO CHzCO M = CH2 CO + M CHzCO H = CH3 CO CHzCO + H = HCCO + H2 HCCO + H = CHz + CO CO + 0 + M = C02 + M CO OH = C02 + H CH3 + 0 2 = CHz0 + OH CH + OH = C + H2O CH2 + NO = HCN + OH CN Hz = HCN H HCN 0 = NCO + H HCN + OH = CN + H2O HCN + OH = HNCO+ H CN OH = NCO H NCO + OH = N O + CO + H NCO + Hz = HNCO + H NCO + NO = NzO + CO HNCO + 0 = NCO + OH HNCO + OH = NCO + HzO COz + N = N O + CO CH N = CN + H CN + N = C + N2 NO + N = 0 + Nz NO2 M = NO 0 M Nz+M=N + N + M H+ 0 2 M = HOz + M third-body effciency
+
+ +
+
+
+
+
+ +
+
+
+ +
+ + +
+
+
+
+
+
+
+
+ +
+
+
F 1017
B
kinetic parameter
E 88370
ref
2.0 x 0.0 a HzO (5) 2.2 x 104 3.0 8750 b 6.9 x lo8 1.56 8484 C 1.9 105 2.4 2110 d 1.9 x 10'6 0.0 91600 b 8.4 1013 0.0 0 e 7.5 107 2.0 5000 C 2.1 x 10'8 -1.57 29229 d 2.0 1013 0.0 0 d 5.5 x 103 2.81 5860 d 1.2 x 1014 0.0 0 d 4.2 x 1013 0.0 0 d 3.0 x 1013 0.0 0 b 5.0 x 1013 0.0 0 b 2.2 x 108 1.77 3000 b 3.5 x 105 2.42 1360 f 2.5 1014 0.0 16802 a CH4 (2.8), HzO (5.0),C02 (3.0),Hz (1.91, CO (1.9) 1.1 x 107 2.0 3000 R 5.0 x 1013 0.0 9000 b 1.6 x 10l2 0.0 1000 b 6.9 x 10" 0.0 500 b b 1.1 x 10'1 1000 0.0 3.0 1013 0.0 0 b 5.8 1013 0.0 75000 h 3.5 b 5200 3 1 4.0 8280 b 1.0 1017 31000 0.0 h 5.4 x 1014 0.0 15000 C 4.2 x 10l2 0.0 11100 C 1.6 1013 0.0 10000 i 7.0 x 103 2.8 500 C 3.6 1015 0.0 59300 a 1.1 1013 0.0 3430 b 8000 5.0 x 1013 0.0 g 1.4 1014 0.0 0 b 6.2 1014 0.0 3000 d -760 1.5 107 1.3 b 9000 3.2 x 10" 0.0 C 3000 4.0 x 107 2.0 C 1.0 1014 0.0 0 j 3.0 105 2.45 2237 b 1.4 x 104 2.64 4980 b 1.5 1013 10929 0.0 b 4.8 x 10" 11000 0.0 C 6.0 1013 0.0 0 b 1.0 x 1013 0.0 0 b 8.6 x 10l2 0.0 9000 b b 1.0x 1013 0.0 -390 3.2 x 10l2 0.0 10300 k 2.6 x 10l2 0.0 k 5540 1.4 x 10'2 0.0 5000 C 1.3 1013 0.0 b 0 1.9 x 1015 -0.6 0 e 3.3 x 10'2 0.3 0 b 1.1 x 10'6 0.0 66000 b 3.7 x 1021 -1.5 220000 C 7.0 x 1017 -0.8 0 a HzO (18.6), Hz (2.86), Nz (1.261, C02 (4.2), CO (2.11), CH4 (18.6)
::::
reaction
+
+
0 Hz = H OH OH+OH=H2O+O OH + Hz = HzO + H HOz+H=OH+OH H+H+HzO=Hz+H2O 0 + OH = H + 0 2 HO2 + OH = HzO + 0 2 NH3 + M = NHz + H M NH3+H=NH2 +H2 NH3+O=NHz+OH NH3+0H=NHz+HzO NH + 0 = N O H N + OH = NO + H NO + HO2 = NO2 + OH NO + NHz = Nz + HzO NO + NH2 = N" + OH NO + NH = NzO + H NO + N N H = N ~ + H N O HNO M = H NO M third-bods effciency NH2 M = NH M -iH HNO + H = H z + N O HNO OH = NO HzO NO2 + H =NO OH NH + N = Nz + H NH + N2O = Nz + HNO HNO + 0 = N O + OH CN + NzO = NCO + N2 NzO OH = Nz HOz NO2 + 0 = NO + 0 2 N + 0 2 = NO + 0
+
+
+ +
+
+ + + +
+
+
+
F
4
E
5.0 x lo4 2.1 x lo8 1.0 x lo8 1.7 x 1014 6.0 x 1019 4.5 x 1014 1.4 x 10l6 1.4 x 10l6 6.4 x lo5 9.4 x lo6 4.7 x lo6 2.0 x 1013 3.8 x 1013 2.1 x 10l2 6.2 x 1015 6.4 x 1015 2.4 x 1015 5.0 x 1013 1.5 x 10l6 H2O (10). 0 3.2 x 5.0 x 10l2 3.6 x 1013 3.5 x 1014 3.0 1013 2.0 x 10'2 1.0 1013 1.0 x 1013 2.0 x 1012 1.0 x 1013 6.4 x 109
2.67 1.4 1.6 0.0 -1.25
6290 -400 3300 874 0 60
-0.5 -1.0 0.0
ref
O
90600 2.39 10171 1.94 6461 1.9 500
0.0 0.0 0.0
-1.25 -1.25 -0.8 0.0 0.0
O O
-480
e
d c c
b d d b b 1 m b b b
o
g
o o O
g g b b
48680 (21, H2 (2). Nz (2) --2.0 -91400 n 0.0 O b 0.0 O b 0.0 1500 b 0.0 0 0 0.0 6000 n 0.0 O P 0.0 O P 0.0 10000 g 0.0 600 b 1.0 6280 b 2
*
a Warnatz (1985). Glarborg et al. (1986). Glarborg and Hadvig (1991). Tsang and Hampson (1986). e Westley et al. (1991). f Hsu et al. (1983). g Miller and Bowman (1989). Glarborg et al. (1987). Cooke and Williams (1971). Langford e t al. (1983). Tully et al. (1988). Sutherland et al. (1990). m Bltz et al. (1988). " Hanson and Salimian (1985). Miller e t al. (1983).p Miller and Bowman (1991). 4 Thorne et al. (1986). J
radicals according to the equation (4)
Other oxygenated intermediate products (CH30, CH20,
HCO, CHzCO, HCCO) are also produced, forming mainly CO and CO2. Concerning. the interactions among. NO and hvdrocarbon radicals, the reaction corresponding to CHziCHz
4644 Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995
+
-
+
NO HCN NO) seems to be the most important. This conforms with the proposition put forward by several authors (Bowman, 1991; Houser et al., 1988; Smart and Weber, 1987; Toqan et al., 1987). The HCN produced can give other nitrogenous species. This stage is characterized by a low concentration of hydrocarbon radicals, because these species are consumed in previous stages (Kilpinen et al., 1992).The representative reactions of this HCN conversion include:
-
P
g
400
0
'.
In presence of 0,:
HCN
+ 0-NCO + H
(5)
+ OH * CN + H,O HCN + OH +. HNCO + H HCN
1.5
1.4
In absence of 0,:
1.7
1.6
1,8
2,O
1,9
2,1
Natural gas ( % )
(6)
Figure 2. Calculated and experimental NO values for different natural gas concentrations.
(7)
- Calc. (1300 "C)
Depending on the operating conditions and on the concentration of the different compounds in the reactant mixture, these intermediate nitrogenous species (NCO, CN, HNCO) can afterward evolve again into NO or Nz. Although the NH3 concentration obtained in the experiments was low, the NHi species existing during the process are important and have been taken into account in many reactions.
0
Exp. (1400 "C) )I'
/
(NO) p= 900 ppm 1.7 % natural gas
0
Experimental Verification of the Model The concentrations of the different species a t the exit of the reburning zone have been calculated using the simplified model proposed. The experimental verification of the model has been performed through the concentrations of NO, HCN, "3, CO, and C02. First, the calculated and experimental concentrations of the nitrogenous species obtained in the range of operating conditions allowing us to reach the highest efficiencies in the reburning zone were compared. The values of the NH3 concentration have not been included in this work because, in all cases, both the calculated and experimental values were low and similar results were obtained. Therefore, the comparison between the concentrations of NO and HCN, which can be considered as the most relevant species in the reburning zone, has been analyzed. A good efficiency in the reburning zone has been obtained when the natural gas concentration ranges between 1.5 and 2%, values which would correspond to the use of a reburn fuel fraction, ~ R B , between approximately 0.14 and 0.22 when a coal with a heating value of 3850 kcalkg is used in the primary combustion zone. Figure 2 shows the comparison between experimental and calculated NO concentrations, for different natural gas concentrations, temperatures of 1300 and 1400 "C,an inlet NO concentration of 900 ppm, and an oxygen concentration of 2%. A good agreement between experimental and calculated NO concentrations is observed for a temperature of 1300 "C. At a temperature of 1400 "C,discrepancies are observed. For low natural gas concentrations the experimental NO concentrations are lower than those calculated with the model. At these high temperatures (21400 "C) and low oxygen concentrations, the formation of a pulverized carbonaceous solid material was detected (Bilbao et al., 1994). The influence of this kind of material on the NO reduction is not clear. Kremer and Schulz (1986) stated that heterogeneous reactions between NO and char may
01 1,o
1,5
28 Oxygen (%)
2.5
38
Figure 3. Calculated and experimental NO values for different oxygen concentrations.
be important, while for Bose and Wendt (1988) the NO reduction chemistry in reburning is governed mainly by the gas phase reactions. In spite of this controversy it could be assumed that, for high temperatures, heterogeneous reactions can contribute to the NO reduction. In the experimental study performed, it was also found that the optimum oxygen concentration at the inlet of the reburning zone ranges between 1.5 and 2%, values which correspond t o the use of an air excess in the combustion primary zone between approximately 10 and 20%. Figure 3 shows the comparison between the experimental and calculated NO concentrations for different oxygen concentrations, temperatures of 1300 and 1400 "C, a natural gas concentration of 1.7%, and an inlet NO concentration of 900 ppm. Again a good agreement is observed for 1300 "C, while the values calculated for 1400 "C are higher than those obtained experimentally. The results obtained for different natural gas and oxygen concentrations can be lumped together using the stoichiometric ratio in the reburning zone, SR:!. From the experimental results it was concluded that at high temperatures a good efficiency in the reburning zone was achieved until SR2 = 0.93 and that when the temperature decreases, the optimum value of SR:! depends on the oxygen concentration ranging between 0.93 and 0.95. Figure 4 shows the comparison between experimental and calculated NO concentrations for different SRz values and a temperature of 1300 "C. It can be observed that a good agreement is obtained in the range 0.92-0.95. The NO concentrations calculated
Ind. Eng. Chem. Res., Vol. 34,No. 12, 1995 4546
- Proposed model
-
90
Ta B
E
6
v
,Iy: , 1
60
fT=1300"C
30
,
,
,
,
,
1
J
I = 170ms 1.7 % natural gas
t
0
0,91
0,92
0,93
0.94
0,95
400
0,96
600
800
SR z Figure 4. Experimental and theoretical NO values calculated with the detailed and simplified models for T = 1300 "C.
5001 e
0
Experimental
--
Detailed model
t ,= 160ms
2O0
1200
1000
1400
(NO) p
0
Figure 6. Calculated and experimental NO values for different inlet NO concentrations.
0
t A
0,91
0,92
0,93
0.94
0,95
0,96
1.4
1.5
SR 2 Figure 5. Experimental and theoretical NO values calculated with the detailed and simplified models for T = 1400 "C.
with the initial detailed model are also shown in Figure 4. These values present a slightly better agreement with the experimental results, although the differences between the results calculated with both models are not significant and the use of the simplified model can be considered accurate. Figure 5 shows the results obtained for a temperature of 1400 "C. It is observed that for the lower SR2 values the results agree in a n acceptable way, but for higher SR2 values the NO concentrations calculated are higher than those experimentally determined. It can also be observed that the values calculated with the simplified model are similar to those obtained with the initial detailed one, which signifies that the disagreement is not due to the model simplification. The disagreement obtained could be due to the values of some kinetic and thermodynamic parameters used and to the possible contribution of heterogeneous reactions between the carbonaceoussolid and NO which would improve the NO reduction. Experiments have also been performed using other primary NO concentrations. Figure 6 shows the comparison between the NO concentrations obtained experimentally and those calculated with the simplified model. For a temperature of 1300 "C a good agreement is observed for all the (NO), values studied. The HCN concentrations calculated with the model have also been compared with those experimentally obtained for different operating conditions. Figures 7-11 show the results obtained for different temperatures, natural gas and oxygen concentrations, and stoichiometric ratios. Similar conclusions can be obtained for the results corresponding to 1300 and 1400
1.6
1.7 1.8 Natural gas ('3%)
1.9
2.0
2.1
Figure 7. Calculated and experimental HCN values for different natural gas concentrations. 500
I
I
(NO) p = 900 ppm 400 -
$
300 -
-
a
yz
A
200
Calc. ( 1 300 "C) Exp. (1400 "C)
-
1,4
1,6
1.8
2.0 2.2 Oxygen ( % )
2,4
2,6
Figure 8. Calculated and experimental HCN values for different oxygen concentrations.
"C. Thus, the calculated values agree with the experimental ones for low natural gas concentrations and high oxygen concentrations, and consequently for high stoichiometric ratios. For low SRZvalues the calculated HCN concentrations are higher than the experimental ones. The HCN concentration values calculated with the initial detailed model are also shown in Figures 9 and 10. It can be observed that the simplification does not introduce a significant worsening with respect t o the detailed model. Figure 11 shows the HCN concentrations obtained using different primary NO concentrations. An accept-
4646 Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995
I
1000 A
--
d
/
Q = 900 Nlh t
170ms
300 -
z
i
Experimental Proposed model Detailed model
200
-
100 -
2001
/
0
200
a
400 600 Calculated NO (ppm)
800
1000
Figure 12. Comparison between the experimental and calculated NO values.
T = 1400 "C (NO) p= 900 ppm Q = 900 Nlih
100 -
A A
A
n 0.91
0,92
0,93
0,94
0,95
0.96
"
0
L l Exp.
a
100 -
d
1
?
1000
Calculated HCN (ppm)
Figure 10. Experimental and theoretical HCN values calculated with the detailed and simplified models for T = 1400 "C.
150
800
600
400
200
SR 2
Figure 13. Comparison between the experimental and calculated HCN values.
a
I
/
T = 1300 "C Q = 900 Nl/h t r = 170111s 1.7 % natural gas 2%02 400
600
800
1000
1200
1400
(NO) (ppm)
"0
I
2
3
4
Calculated CO (76)
Figure 11. Calculated and experimental HCN values for different inlet NO concentrations.
Figure 14. Comparison between the experimental and calculated CO values.
able agreement is observed between the experimental results and those calculated with the simplified model. The comparison between the concentrations of the main species obtained experimentally and those calculated with the simplified model has been extended to the results corresponding to all the reburning experiments performed in the range of operating conditions mentioned in the experimental method. The comparisons among the NO, HCN, CO, and C02 concentrations are shown in Figures 12-15. In general, a good agreement is reached for the CO and C02 values. With respect to the NO and HCN concentrations, an acceptable agreement is achieved for values a t the exit of the
reburning zone lower than 300 ppm. It can also be observed that, in some cases with high predicted NO removal, measured data indicate significant NO emissions. Generally, these results correspond t o high natural gas concentrations (low stoichiometric ratios). It is possible that for these conditions the mixing of the experimental gas upstream of the reburning reaction zone is not quite complete. For high NO and HCN concentrations, the experimental values obtained are lower than those theoretically predicted, although it is worth bearing in mind that in these cases the reburning efficiency is low and therefore this technique should not be applied.
Ind. Eng. Chem. Res., Vol. 34,No. 12, 1995 4547
5
20-
8-
19-
N
01
5
18
-
.-E g
17
-
16
-
c)
4
(NO), = inlet NO concentration to the reburning zone Q = gas flow rate R = universal gas constant SR2 = stoichiometricratio in the reburning zone (available oxygedoxygen needed for the complete combustion) T = temperature xk = molar concentration of a species ai = enhanced third-body efficiency of the k species in the i reaction @ = temperature exponent v’ki = stoichiometriccoefficientof a reactant k in a reaction
i V”ki
= stoichiometriccoefficientof a product k in a reaction
i 15
16
17
18
19
20
21
22
Calculated CO 2 (5%)
Figure 15. Comparison between the experimental and calculated COz values.
Conclusions A detailed kinetic model of the chemistry in the reburning zone using natural gas consisting of 298 elementary reactions and 48 chemical species has been simplified to a model with 87 reactions and 38 species without loss of accuracy. This simplified model can be applied to a boiler with a reasonable computational time, even using complicated flow models. The NO values calculated with the model agree with the experimental ones for T = 1300 “C. At T = 1400 “C and high SR2 values, the NO experimental concentrations are lower than the calculated ones, which could be due to the effect of the carbonaceous solid formed a t high temperatures and to the inaccuracy of some of the kinetic parameters used. For both temperatures the HCN concentrations calculated agree with the experimental ones for high stoichiometric ratios. For low SR2 values, the calculated values are somewhat higher than the experimental ones. When the calculated and experimental results are compared for a wide range of values of operating conditions, a good agreement is reached for the CO and C02 values. The NO and HCN results agree for exit concentrations lower than 300 ppm, excepting the data corresponding to very low stoichiometric ratios, for which the experimental results are higher than those predicted; there is also some discrepancy for higher values of NO and HCN, but in such cases the reburning efficiency is low.
Acknowledgment The authors express their gratitude to the ENAGAS, ENDESA, and SEVILLANA companies, to OCIGAS, OCIDE, and CICYT (Project AMB92-0888)for providing financial support for this work, and also to Ministerio de Educaci6n y Ciencia (Spain) for a research grant awarded to M. U. Alzueta.
Nomenclature E = activation energy F = preexponential factor Z = total number of elementary reaction steps i = index of the elementary reaction step K = total number of species k = species index kfi = forward rate constant for an elementary reaction k,i = reverse rate constant for an elementary reaction
&i)k
= molar production rate of a species k
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Received for review J a n u a r y 27, 1995 Accepted July 20, 1995" IE950091M ~~~~
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Abstract published i n Advance ACS Abstracts, October 15, 1995. @
