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Development of selective noncatalytic reduction by ammonia in the presence of phenol
Development of Selective Noncatalytic Reduction by Ammonia in the Presence of Phenol SEIJI NODA, AZUCHI HARANO, MITSUO HASHIMOTO, and MASAYOSHI SADAKATA*
Department of Chemical System Engineering, Faculty of Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan; and Nippon Oil Company, Ltd., Central Technical Research Laboratory, Yokohama, Japan The thermal de-NOx process is one of the most effective methods of reducing the NOx in flue gas. A shortcoming in this process is the narrow temperature range of 870°C to 1100°C in which effective reduction of NO occurs. We approach this problem by adding an agent to the system in order to shift the temperature range to a lower one. The agent that was added was an aqueous solution containing trace amounts of phenolic compounds which is one of the industrial waste products of fluidized catalytic cracking (FCC) in petroleum refinement. The effect of the addition of an aqueous phenol solution on the thermal de-NOx process was investigated in a quartz flow reactor at a temperature, pressure, and residence time of 650 – 820°C, 1.0 atm, and 1.0 –1.2 s, respectively. In the case of 0 – 400 ppm phenol addition at NO/NH3 ⫽ 1.0, O2 ⫽ 4%, NO ⫽ 400 ppm, and H2O ⫽ 7.4%, profiles of NO, N2O, NO2, NH3, CO, CO2, C2H2, and phenol concentrations were experimentally obtained. The NO concentration significantly decreased over the wide temperature range of 675– 820°C. © 2000 by The Combustion Institute
INTRODUCTION Several types of NOx (nitrogen oxide) removal systems using catalysts are used in various combustion processes. However, these catalytic methods have problems that prevent wider application, such as blockage of the catalyst by particulates, deterioration under hostile conditions, and the high cost of catalysts. Both with and without a catalyst, ammonia is widely used as an effective agent to reduce the NOx in flue gas in the presence of oxygen. A disadvantage of the noncatalytic process, called the “thermal de-NOx process” [1–3], is the narrow temperature range of 870 to 1100°C in which the effective reduction of NO (nitrogen monoxide) occurs. If one can decrease the reaction temperature to around 600°C, this process may be applicable to NOx removal from the flue gas of stationary sources, such as a diesel power generator or a gas turbine system. The influence of additives, such as H2 [4], CO [5], CH4 [6], C2H6 [6], HNCO [7], and amines [8] on the reduction of NO in the thermal de-NOx process has been investigated. Hashimoto et al. [9] also examined how an aqueous *Corresponding author. E-mail: [email protected] COMBUSTION AND FLAME 122:439 – 450 (2000) © 2000 by The Combustion Institute Published by Elsevier Science Inc.
solution containing trace amounts of phenolic compounds affected NO concentration in a NOx noncatalytic reduction process by ammonia. This phenolic solution is one of the industrial waste products of fluidized catalytic cracking (FCC) in the petroleum refinery process. When the wastewater was directly sprayed into the heating furnace, the conversion ratio of NO increased to about 30% at around 700°C. In this study, the effect of adding a phenolic solution to the thermal de-NOx process is experimentally investigated. The outlet gas composition is examined in detail to determine whether or not the addition of the phenol solution produces pollutants. Finally, we discuss the mechanism by which the NO decrease occurs. EXPERIMENTAL PROCEDURE The experimental work was performed in a flow reactor made of quartz, which was operated at atmospheric pressure and temperatures from 650°C to 850°C, with residence times of 1.0 –1.2 s. Part of the apparatus is illustrated in Fig. 1. Our system consisted of three sections: a preheating section of 100 mm length and 22 mm i.d., a reaction section of 300 mm length and 31 0010-2180/00/$–see front matter PII S0010-2180(00)00119-X
440
Fig. 1. Experimental apparatus. A: preheat furnace, B: electric heater, C: reactor, D: water cooler, E: stainless steel capillary, F: microfeeder.
mm i.d., and a section for quenching with water cooling. An electric heater was used to control the temperature of the reactor section between 650 and 850°C. Gas temperature profiles were measured with a thermocouple along the central axis of the flow reactor (from x ⫽ 0 mm to 300 mm in Fig. 1). A microfeeder constantly supplied phenol aqueous solution through a glass-coated stainless tube of 0.3 mm i.d. The phenol solution was vaporized in the preheating section, the temperature of which was controlled at around 200°C. Reaction during preheating can be ignored because it was observed that no phenol decomposed during the preheating section. Gaseous phenol and other gases (NO, NH3, and O2) were supplied through a pinhole of 1.5 mm i.d. for good mixing, and introduced to the reaction section. Argon gas was used as a diluent. Flow rates of reactants were adjusted using four mass flow controllers. Gas concentrations were measured using a Fourier transform infrared analyzer (Shimadzu FTIR8000-PC) for N2O, and a chemiluminescence analyzer (Shimadzu NOA-7000) for NO and NO2. The characteristic absorption line (2240 cm⫺1) was adopted for determination of N2O concentration. Carbon compounds were analyzed by a gas chromatograph (Shimadzu GC-14A) equipped with a flame ionization detector (FID) using Porapack Q as a separation column for CO, CO2, and C1–C3 hydrocarbons, and using Unisole F-200 as a separation column for phenol. More than 99% of the ammonia was
S. NODA ET AL.
Fig. 2. The effect of phenol solution addition. Initial mole fractions: [NO]0 ⫽ 400 ppm, [NH3]0 ⫽ 400 ppm, [O2]0 ⫽ 4 vol%, [phenol]0 ⫽ 100 ppm, [H2O]0 ⫽ 7.4 vol%, residence time ⫽ 1072/(T ⫹ 273) s. (T: [°C]). F Without phenol solution. Œ With phenol solution.
absorbed in a water scrubber after passing through the reactor, and this ammonia solution was analyzed by an ammonium ion electrode. The following chemical species were detected: NO, CO, CO2, C2H2, phenol, N2O, NO2, and NH3. The concentrations of compounds except for NH3 and NO2 were measured with an error of ⫾5%. The uncertainty in the NO2 measurement was ⫾10%. The error in the ammonium ion electrode method was ⫾15%. RESULTS Plug flow experiments with or without phenol were performed under the following conditions: [NO]0 ⫽ 400 ppm, [NH3]0 ⫽ 400 ppm, [O2]0 ⫽ 4.0%, [phenol]0 ⫽ 0 – 400 ppm, [H2O]0 ⫽ 1.7– 7.4%, residence time ⫽ 1.0 –1.2 s, temperature ⫽ 650 – 820°C (phenol was injected into the reactor as phenol aqueous solution). Phenol (C6H5OH) is highly soluble in water because of its OH substituent. The Thermal De-NOx Process for a Residence Time of Approximately 1 s In the case of no phenol addition, NO conversion was 45% at 816°C at a residence time of ⫽ 0.98 s. Neither NO2 nor N2O was detected. As shown in Fig. 2, the NO profile is lower at the low temperature end of the temperature
ADDITION OF PHENOL TO NONCATALYTIC REDUCTION
Fig. 3. Concentrations of N2O and NO2. Initial mole fractions are the same as in Fig. 2. F N2O. ΠNO2. The solid line shows the total amount of N2O and NO2.
window than that of previous studies [1, 2, 10] because of the longer residence time of around 1 s in our experiments. The Effect of Phenol Addition The effect of phenol addition on the NO profile is shown in Fig. 2. Compared with the case of no phenol addition, the NO concentration significantly decreased over a wide temperature range, 675– 820°C, and an NO conversion ratio of about 69% was obtained at 728°C by addition of a small amount of phenol (phenol/NO ⫽ 0.25). This phenol effect was drastically suppressed for either (1) addition of phenol solution in the absence of ammonia, or (2) water addition in the presence of ammonia. Both phenol and ammonia are thought to play important roles in this marked effect of NO decrease.
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Fig. 4. Effect of phenol solution addition on NH3 concentration. Initial mole fractions are the same as in Fig. 2. F Without phenol solution. ΠWith phenol solution.
ppm in spite of the temperature rise, whereas some of the NH3 was converted to other nitrogen compounds without reducing NO above 730°C. This means that the addition of phenol solution also has the merit of reducing the amount of residual ammonia between 700 – 780°C, where about 90% of ammonia was converted at 776°C. Concentrations of carbon compounds, such as CO, C1–C3, CO2, and phenol, were experimentally determined in the case of phenol addition, as shown in Fig. 5. Only C2H2 was detected as C1–C3 hydrocarbons. When the
Outlet Gas Composition Figure 3 shows concentrations of NO2 and N2O under the same conditions as in Fig. 2. The fact that the total amount of NO2 and N2O reached about 80 ppm in terms of N atoms at 728°C, indicates that 27% of NO was not converted to nitrogen, but to other nitrogen oxides. The ammonia profile is shown in Fig. 4. The result indicates that the NH3 profile is similar to the NO profile in Fig. 2. However, in the temperature range between 730 –780°C, the NO concentration remained constant at around 140
Fig. 5. C composition in outlet gas. Initial mole fractions: NO ⫽ 400 ppm, NH3 ⫽ 400 ppm, O2 ⫽ 4 vol%, phenol ⫽ 100 ppm, H2O ⫽ 7.4 vol%, reaction time ⫽ 1072/(T ⫹ 273) s (T: [°C]).
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S. NODA ET AL. TABLE 1 The Thermal De-NOx Kinetic Mechanism [2] No.
Reaction
R1 R2 R3 R4 R5 R6 R7 R8 R9
NH3 ⫹ OH ⫽ NH2 ⫹ H2O NH2 ⫹ NO ⫽ NNH ⫹ OH NNH ⫽ N2 ⫹ H O ⫹ OH ⫽ H ⫹ O2 O ⫹ H2 ⫽ OH ⫹ H NH2 ⫹ NO ⫽ N2 ⫹ H2O H ⫹ O2 ⫹ Ma ⫽ HO2 ⫹ M OH ⫹ HO2 ⫽ H2O ⫹ O2 HNO ⫹ OH ⫽ NO ⫹ H2O
a
Fig. 6. Dependence of phenol concentration on NH3 concentration. Initial mole fractions: NO ⫽ 400 ppm, NH3 ⫽ 400 ppm, O2 ⫽ 4 vol%, H2O ⫽ 3.1–7.4 vol%, reaction time ⫽ 1.07 s, F T ⫽ 728°C, Œ T ⫽ 776°C.
injected phenol was completely oxidized, the calculated value of the total carbonaceous gases from the reactor would be 650 ppm in terms of carbon atoms. The results from 681– 815°C satisfy the mass balance of carbonaceous compounds within experimental uncertainties. With increasing temperature, more phenol was decomposed to produce CO, CO2, and C2H2. More than 99.9% of phenol was oxidized at temperatures higher than 728°C. Around 10 ppm of C2H2 was detected between 639 –728°C, while the CO concentration increased with increasing temperature in this regime, reaching 475 ppm at 728°C. At temperatures higher than 728°C, further oxidation of CO to CO2 was observed.
M represents the third body.
If one applies this result at 776°C, it is expected that no more than 60 ppm phenol addition is enough to obtain NO conversion of 69%, and results in lower emissions of pollutants such as CO, NO2, and N2O than 100 ppm phenol addition. DISCUSSION The Thermal De-NOx Mechanism The thermal de-NOx process [1–3] is a selective, noncatalytic technique for removal of nitrogen oxides. Important elementary reactions are shown in Table 1. The NH2 ⫹ NO reaction (R2 and R6), a key reaction in the thermal de-NOx process, has a radical branching channel as well as a molecular product channel:
Dependence of Concentrations of Phenol and Water on the Phenol Effect
NH2 ⫹ NO 3 N2H ⫹ OH
(R2)
3 N 2 ⫹ H 2O
(R6)
The influence of the concentrations of water and phenol on NO concentration in the thermal de-NOx process was also investigated. There was a negligible effect of water concentration in the range of 1.7% to 7.4% water. Figure 6 shows that the NO conversion ratio was enhanced with increasing phenol concentration between 0 ppm and 100 ppm at 728°C, and reached a plateau at concentrations higher than 100 ppm. In the case of 776°C, 60 ppm of phenol addition had the same effect on NO as 100 ppm phenol injection. It was observed that phenol was oxidized to form 47 ppm of CO and 316 ppm of CO2.
The product branching ratio strongly influences NO reduction, because aminogen (NH2) is formed principally by OH through oxidation of ammonia. N2H decomposition rate followed by R2 also affects radical reactions, because N2H leads to N2 ⫹ H (R3 in Table 1). In the viewpoint of reactions between H and O2, H ⫹ O2 3 OH ⫹ O (-R4 in Table 1) can proceed at a slower rate than H ⫹ O2 ⫹ M 3 HO2 ⫹ M (R7 in Table 1) at temperatures lower than 1000 K. HO2 radicals decrease the number of OH radicals through R8. As a result OH is quickly decreased by ammonia and the
ADDITION OF PHENOL TO NONCATALYTIC REDUCTION TABLE 2 The Overall Benzene Oxidation Mechanism [14] Reactiona
No. R10 R11 R12 R13 R14 a
C6H6 ⫹ X 3 C6H5O (phenoxy radical), C6H5OH (phenol) C6H5O 3 C5H5 (cyclopentadienyl) ⫹ CO C5H5 ⫹ X 3 C4H5 (butadienyl radical) ⫹ CO C4H5 ⫹ X 3 C2H4, C2H2 C2H4, C2H2 ⫹ X 3 CO, CO2, H2O
X represents OH, H, O, O2.
radical pool is not sustained at lower temperatures. Consequently, the thermal de-NOx process requires a high temperature in order to obtain high branching ratios of R2 and/or R4. If effective NO reduction is required at lower temperatures, a new source of OH radicals needs to be found to overcome radical termination. Kinetic models [2, 10] of the thermal de-NOx describe well the experimental data conducted in laminar quartz reactors. The branching ratio defined as ␣ ⫽ R2/(R2 ⫹ R6), and the lifetime of N2H were estimated to be 0.0022 ⫻ T 0.70 (T: temperature [K]) and 10⫺6 [s], respectively [2]. The reaction channels of NH2 ⫹ NO have been experimentally investigated in recent studies [11–13]. Park and Lin [11] determined total rate constants and the branching ratio of NH2 ⫹ NO reaction, and also confirmed that no other product channel exists except R2 and R6 below 1060 K. Phenol Oxidation Mechanism Phenol is formed via one of the important routes of benzene oxidation. Brezinsky [14] discussed chemical mechanisms of the gasphase oxidation of benzene and toluene at atmospheric pressure and high temperature (600 –1200°C) in detail. The overall benzene oxidation mechanism is shown in Table 2. Since phenol is oxidized to form phenoxy radical (C6H5O 䡠 ) at the first stage, the mechanism described by R11–14 in Table 2 should govern phenol oxidation. Cyclopentadienyl (C5H5) formed via the following step (R11) is expected to react with radicals and molecules because of its stable characteristics [15]. The main products of reactions (R12) of C5H5 are C5H6, C4H5, and
443
CO. C4H5 is oxidized to form C2 hydrocarbons, resulting in CO, CO2, and H2O as final products. Emdee et al. [16] developed a kinetic model for the oxidation of benzene and toluene near 1200 K, which agrees well with experimental data including lean and rich equivalence ratios with temperatures in the range of 1100 –1190 K. Emdee’s model assumed several elementary steps for R12 and R13, and evaluated thermodynamic properties of each compound and reaction coefficients of each step, compared to those of similar reactions that had previously been well elucidated. Initial Steps of Phenol Oxidation Phenol is oxidized to form phenoxy radical (C6H5O 䡠 ) at the first stage of its oxidation process. Two oxidation steps are reported [17] as follows: C6H5OH ⫹ M 3 C6H5O ⫹ H ⫹ M
(R17)
C6H5OH ⫹ OH 3 C6H5O ⫹ H2O
(R18)
M: the third body The rate of the initiation step is not important at higher temperatures as discussed in Emdee’s model, because chain-branching steps generate more radicals than chain initiation. On the other hand, the initial step of phenol oxidation at lower temperatures, as in our study, is important in total oxidation rate. Especially, the pyrolysis reaction (R17) could be the most important step in the absence of OH radicals in the system. Bruinsma et al. [18] carried out an experimental study of gas-phase pyrolysis of coalrelated aromatic compounds, including phenol, in a flow tube reactor. Experimental conditions were P ⫽ 0.125 MPa, T ⫽ 1040–1140 K, and reaction time ⫽ 5 s. Their reactor was designed to suppress secondary reactions by using low concentrations, less than 500 ppm, and by depositing a graphite layer inside the reactor. Arrhenius parameters for phenol pyrolysis were evaluated to be 4.34 ⫻ 1011 as a frequency factor and 62.6 kcal kJ/mol as an activation energy. On the other hand the data of Lovell et al. [19] which the Emdee model adopted for phe-
444 nol pyrolysis, was estimated by experiments in a turbulent flow reactor for conditions of P ⫽ 1 atm, T ⫽ 1064–1162 K, initial phenol concentration ⫽ 500 –2016 ppm, and reaction time ⫽ 170 ms. In Ref. 19, the rate for R17 was calculated using the reverse rate for R17 and the equilibrium constant. Though the rate for R17 is very sensitive to variation of the equilibrium constant, the evaluated data described experimental results well. The time constant of phenol pyrolysis in Ref. 18 is shorter than that in Ref. 19, e.g. time constants at 1050 K are 27.9 s in Ref. 18 and 484 s in Ref. 19, respectively. Analysis of Reaction Pathways in Phenol Oxidation In order to analyze the main reaction paths in phenol oxidation, the reaction system was numerically treated using SENKIN [20]. Emdee’s model [16] was basically used for the kinetic modeling except two reactions of phenoxy radical pyrolysis and phenol pyrolysis (A1–A20, B1–B6, B10 –B50 in Table 3). As discussed above, the rate for phenol pyrolysis (B8 in Table 3) reported by Bruinsma et al. [18] is adopted for our experimental conditions of longer residence time (about 1 s). The unimoleculer decomposition rate of phenoxy radical (B7 in Table 3) was determined by ab initio calculations [21], which evaluated the activation energy to be 55.4 kcal/mol. Calculations were done for the conditions of [phenol]0 ⫽ 200 ppm, [O2]0 ⫽ 4%, i.e. ⫽ 0.4, Ar remainder, Pressure ⫽ 1 atm, T ⫽ 1000–1100 K, and reaction time ⫽ 1 s. Calculation results showed that 39% of phenol was decomposed at 1050 K, and the decreased phenol was mainly oxidized to CO, partially converted to C4H4, C2H2, and CO2 under 1050 K. At higher temperatures of 1050 – 1100 K, phenol was completely oxidized to CO and CO2. The ratio of CO2 to CO in very lean condition was reported to be less than 2:3 in the oxidation of cyclopentadiene (C5H6) at 1063 K [22]. The composition of byproducts in phenol oxidation must be similar to that in C5H6 oxidation, so that C5H6 plays a key role in hightemperature oxidation processes of aromatic compounds. Our calculation results agreed with
S. NODA ET AL. the characteristics of CO and CO2 formation in the oxidation of C5H6. A linear sensitivity analysis for phenol concentration at 1050 K indicated that three reactions have a high positive sensitivity of S ⫽ 0.1 to 0.25, and one reaction has a high negative sensitivity of S ⫽ ⫺0.05 to ⫺0.1. Reactions of a positive sensitivity: C6H5OH 3 C6H5O ⫹ H
B8 in Table 3
C5H5 ⫹ OH 3 C5H4OH ⫹ H C5H5O 3 C4H5 ⫹ CO
B20 in Table 3
B29 in Table 3
A reaction of a negative sensitivity: C6H5OH ⫹ OH 3 C6H5O ⫹ H2O B10 in Table 3 Since the temperature of 1050 K was relatively low, the sensitivity of chain-branching steps such as H ⫹ O2 3 O ⫹ OH was under 0.05. It is predicted that the initial step of phenol oxidation at lower temperatures is important in the total oxidation rate. The pyrolysis reaction (B8 in Table 3) could be the most important step in the case of the absence of OH radicals in the system. A negative sensitivity coefficient of B10 is understandable, because B10 is a radical terminating step, resulting in suppressing oxidation reactions of C2–C5 hydrocarbons by radicals. It is considered that B20 is followed by B30 and B31 in Table 3 to form C2H2 and CO as follows: C5H5 ⫹ OH 3 C5H4OH ⫹ H
B20 in Table 3
C5H4OH 3 C5H4O ⫹ H B30 in Table 3 C5H4O 3 C2H2 ⫹ C2H2 ⫹ CO
B31 in Table 3
The pyrolysis reaction of C5H5O should follow B19, because B19 is only one source of C5H5O formation. C5H5 is also oxidized through B19 while relatively unreactive radical HO2 is converted to a reactive one OH as follows: C5H5 ⫹ HO2 3 C5H5O ⫹ OH C5H5O 3 C4H5 ⫹ CO
B19 in Table 3
B29 in Table 3
ADDITION OF PHENOL TO NONCATALYTIC REDUCTION
445
TABLE 3 Reaction Rate Constants Used for Calculation of NO/NH3/O2/Phenol System Units: cm3, moles, seconds, calories, kelvin. No.
Reactiona
A1 A2 A3 A4 A5
O ⫹ OH ⫽ O2 ⫹ H O ⫹ H2 ⫽ OH ⫹ H OH ⫹ H2 ⫽ H2O ⫹ H 2OH ⫽ O ⫹ H2O 2H ⫹ M ⫽ H2 ⫹ M H2/0.0/H2O/0.0/CO2/0.0/ 2H ⫹ H2 ⫽ 2H2 2H ⫹ H2O ⫽ H2 ⫹ H2O H ⫹ O ⫹ M ⫽ OH ⫹ M H2O/5.0/ H ⫹ OH ⫹ M ⫽ H2O ⫹ M H2O/5.0/ 2O ⫹ M ⫽ O2 ⫹ M H2 ⫹ O2 ⫽ 2OH H ⫹ O2 ⫹ M ⫽ HO2 ⫹ M H2O/18.6/CO2/4.2/H2/2.9/CO/2.1/N2/1.3 H ⫹ HO2 ⫽ H2 ⫹ O2 H ⫹ HO2 ⫽ 2OH O ⫹ HO2 ⫽ O2 ⫹ OH OH ⫹ HO2 ⫽ H2O ⫹ O2 2HO2 ⫽ H2O2 ⫹ O2 H2O2 ⫹ M ⫽ 2OH ⫹ M H2O2 ⫹ H ⫽ HO2 ⫹ H2 H2O2 ⫹ OH ⫽ H2O ⫹ HO2 NH3 ⫹ M ⫽ NH2 ⫹ H ⫹ M NH3 ⫹ H ⫽ NH2 ⫹ H2 NH3 ⫹ O ⫽ NH2 ⫹ OH NH3 ⫹ OH ⫽ NH2 ⫹ H2O NH3 ⫹ HO2 ⫽ NH2 ⫹ H2O2 NH2 ⫹ H ⫽ NH ⫹ H2 NH2 ⫹ O ⫽ HNO ⫹ H NH2 ⫹ O ⫽ NH ⫹ OH NH2 ⫹ OH ⫽ NH ⫹ H2O NH2 ⫹ HO2 ⫽ NH3 ⫹ O2 NH2 ⫹ HO2 ⫽ H2NO ⫹ OH H2NO ⫹ O ⫽ NH2 ⫹ O2 NH2 ⫹ N ⫽ N2 ⫹ O2 NH2 ⫹ NH ⫽ N2H2 ⫹ H NH2 ⫹ NH2 ⫽ N2H2 ⫹ H2 NH2 ⫹ NH2 ⫽ NH3 ⫹ NH NH2 ⫹ NH2 ⫽ N2H4 NH2 ⫹ NO ⫽ NNH ⫹ OH NH2 ⫹ NO ⫽ N2 ⫹ H2O NH ⫹ H ⫽ N ⫹ H2 NH ⫹ O ⫽ NO ⫹ H NH ⫹ OH ⫽ HNO ⫹ H NH ⫹ OH ⫽ N ⫹ H2O NH ⫹ O2 ⫽ HNO ⫹ O NH ⫹ O2 ⫽ NO ⫹ OH NH ⫹ NH ⫽ N2 ⫹ 2H NH ⫹ N ⫽ N2 ⫹ H NH ⫹ NO ⫽ N2O ⫹ H NH ⫹ NO ⫽ N2O ⫹ H NH ⫹ NO ⫽ N2 ⫹ OH NH ⫹ NO2 ⫽ N2O ⫹ OH
A6 A7 A8 A9 A10 A11 A12 A13 A14 A15 A16 A17 A18 A19 A20 A21 A22 A23 A24 A25 A26 A27 A28 A29 A30 A31 A32 A33 A34 A35 A36 A37 A38 A39 A40 A41 A42 A43 A44 A45 A46 A47 A48 A49 A50 A51
a (terms of mole-cm-sec-k)
Ea cal/mol
Reference
14 04 09 08 18
⫺0.5 2.67 1.3 1.3 ⫺1
0 6,290 3,626 0 0
10 10 10 10 10
9.20E ⫹ 16 6.00E ⫹ 19 6.20E ⫹ 16
⫺0.6 ⫺1.25 ⫺0.6
0 0 0
10 10 10
1.60E ⫹ 22
⫺2
0
10
1.89E ⫹ 13 1.70E ⫹ 13 3.61E ⫹ 17
0 0 ⫺0.72
⫺1,788 47,780 0
10 10 10
0 0 0 ⫺0.61 0 0 0 0 0 2.39 1.94 2.04 0 0 ⫺0.5 0 2 0 0 0 0 0 0 0 0 ⫺3.02 ⫺0.98 0 0 0 0.5 2 1.5 0 0 ⫺0.4 ⫺0.23 ⫺0.23 0
0 1,073 1,073 340 0 45,500 3,800 1,800 93,470 10,171 6,460 566 22,000 3,650 0 0 1,000 0 0 0 0 0 0 10,000 0 9,589 ⫺2,605 0 0 0 2,000 6,500 100 0 0 0 0 0 0
10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 11 11 10 10 10 10 10 10 10 10 10 10 10 10
4.00E 5.06E 1.17E 6.00E 1.00E
1.25E 1.40E 1.40E 2.10E 2.00E 1.30E 1.60E 1.00E 2.20E 6.36E 9.40E 2.04E 3.00E 4.00E 6.63E 6.75E 4.00E 1.00E 2.50E 7.50E 7.20E 5.00E 8.50E 5.00E 1.50E 9.19E 3.40E 3.00E 9.20E 2.00E 5.00E 4.60E 1.30E 2.50E 3.00E 2.90E ⫺2.20E 2.20E 1.00E
⫹ ⫹ ⫹ ⫹ ⫹
n
⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹
13 14 13 15 12 17 12 13 16 05 06 06 11 13 14 12 06 13 13 13 13 13 11 13 13 22 14 13 13 13 11 05 06 13 13 14 13 13 13
446
S. NODA ET AL. TABLE 3 (Continued)
No.
Reactiona
A52 A53 A54 A55 A56 A57 A58 A59
N ⫹ NO ⫽ N2 ⫹ O N ⫹ O2 ⫽ NO ⫹ O N ⫹ OH ⫽ NO ⫹ H NO ⫹ HO2 ⫽ NO2 ⫹ OH NO2 ⫹ H ⫽ NO ⫹ OH NO2 ⫹ O ⫽ NO ⫹ O2 NO2 ⫹ M ⫽ NO ⫹ O ⫹ M HNO ⫹ M ⫽ H ⫹ NO ⫹ M H2O/10.0/O2/2.0/N2/2.0/H2/2.0/ HNO ⫹ H ⫽ NO ⫹ H2 HNO ⫹ O ⫽ NO ⫹ OH HNO ⫹ OH ⫽ NO ⫹ H2O HNO ⫹ O2 ⫽ NO ⫹ HO2 HNO ⫹ NH2 ⫽ NH3 ⫹ NO HNO ⫹ NO ⫽ N2O ⫹ OH HNO ⫹ HNO ⫽ N2O ⫹ H2O H2NO ⫹ M ⫽ HNO ⫹ H ⫹ M H2NO ⫹ H ⫽ HNO ⫹ H2 H2NO ⫹ H ⫽ NH2 ⫹ OH H2NO ⫹ O ⫽ HNO ⫹ OH H2NO ⫹ OH ⫽ HNO ⫹ H2O H2NO ⫹ NO ⫽ HNO ⫹ HNO H2NO ⫹ NH2 ⫽ HNO ⫹ NH3 N2H4 ⫹ H ⫽ N2H3 ⫹ H2 N2H4 ⫹ O ⫽ N2H2 ⫹ H2O N2H4 ⫹ OH ⫽ N2H3 ⫹ H2O N2H4 ⫹ NH2 ⫽ N2H3 ⫹ NH3 N2H3 ⫹ M ⫽ N2H2 ⫹ H ⫹ M N2H3 ⫹ H ⫽ NH2 ⫹ NH2 N2H3 ⫹ O ⫽ N2H2 ⫹ OH N2H3 ⫹ O ⫽ NH2 ⫹ HNO N2H3 ⫹ OH ⫽ N2H2 ⫹ H2O N2H3 ⫹ OH ⫽ NH3 ⫹ HNO N2H3 ⫹ NH ⫽ N2H2 ⫹ NH2 N2H2 ⫹ M ⫽ NNH ⫹ H ⫹ M H2O/15.0/O2/2.0/N2/2.0/H2/2.0/ N2H2 ⫹ H ⫽ NNH ⫹ H2 N2H2 ⫹ O ⫽ NH2 ⫹ NO N2H2 ⫹ O ⫽ NNH ⫹ OH N2H2 ⫹ OH ⫽ NNH ⫹ H2O N2H2 ⫹ NO ⫽ N2O ⫹ NH2 N2H2 ⫹ NH ⫽ NNH ⫹ NH3 N2H2 ⫹ NH2 ⫽ NNH ⫹ NH2 NNH ⫽ N2 ⫹ H NNH ⫹ NO ⫽ N2 ⫹ HNO NNH ⫹ H ⫽ N2 ⫹ H2 NNH ⫹ OH ⫽ N2 ⫹ H2O NNH ⫹ NH2 ⫽ N2 ⫹ NH3 NNH ⫹ NH ⫽ N2 ⫹ NH2 NNH ⫹ O ⫽ N2O ⫹ H N2O ⫹ OH ⫽ N2 ⫹ HO2 N2O ⫹ H ⫽ N2 ⫹ OH N2O ⫹ M ⫽ N2 ⫹ O ⫹ M H2O/10.0/O2/1.5/N2/1.5/ N2O ⫹ O ⫽ N2 ⫹ O2 N2O ⫹ O ⫽ 2NO
A60 A61 A62 A63 A64 A65 A66 A67 A68 A69 A70 A71 A72 A73 A74 A75 A76 A77 A78 A79 A80 A81 A82 A83 A84 A85 A86 A87 A88 A89 A90 A91 A92 A93 A95 A96 A97 A98 A99 A100 A101 A102 A103 A104 A105
a (terms of mole-cm-sec-k)
n
Ea cal/mol
Reference
3.27E 6.40E 3.80E 2.10E 3.50E 1.00E 1.10E 1.50E
⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹
12 09 13 12 14 13 16 16
0.3 1 0 0 0 0 0 0
0 6,280 0 ⫺480 1,500 600 66,000 48,680
10 10 10 10 10 10 10 10
4.40E 1.00E 3.60E 1.00E 2.00E 2.00E 4.00E 5.00E 3.00E 5.00E 3.00E 2.00E 2.00E 3.00E 1.30E 8.50E 5.00E 3.90E 3.50E 1.60E 5.00E 1.00E 1.00E 1.00E 2.00E 5.00E
⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹
11 13 13 13 13 12 12 16 07 13 07 07 07 12 13 13 12 12 16 12 12 13 13 12 13 16
0.72 0 0 0 0 0 0 0 2 0 2 2 2 0 0 0 0 0 0 0 0 0 0 0 0 0
650 0 0 25,000 1,000 26,000 5,000 50,000 2,000 0 2,000 1,000 13,000 1,000 2,500 1,200 1,000 1,500 46,000 0 5,000 0 1,000 15,000 0 0
10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10
5.00E 1.00E 2.00E 1.00E 3.00E 1.00E 1.00E 1.00E 5.00E 1.00E 5.00E 5.00E 5.00E 1.00E 2.00E 7.60E 4.00E
⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹
13 13 13 13 12 13 13 06 13 14 13 13 13 14 12 13 14
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 1,000 1,000 1,000 0 1,000 1,000 0 0 0 0 0 0 0 10,000 15,200 56,100
10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10
1.40E ⫹ 12 2.90E ⫹ 13
0 0
10,800 23,150
10 10
ADDITION OF PHENOL TO NONCATALYTIC REDUCTION
447
TABLE 3 (Continued) Reactiona
No. B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 B13 B14 B15 B16 B17 B18 B19 B20 B21 B22 B23 B24 B25 B26 B27 B28 B29 B30 B31 B32 B33 B34 B35 B36 B37 B38 B39 B40 B41 B42 B43 B44 B45 B46 B47 B48 B49 B50 a
a (terms of mole-cm-sec-k)
C6H5 ⫹ H ⫽ C6H6 C6H6 ⫹ O2 ⫽ C6H5 ⫹ HO2 C6H6 ⫹ OH ⫽ C6H5 ⫹ H2O C6H6 ⫹ O ⫽ C6H5O ⫹ H C6H6 ⫹ H ⫽ C6H5 ⫹ H2 C6H5 ⫹ O2 ⫽ C6H5O ⫹ O C6H5O ⫽ C5H5 ⫹ CO C6H5OH ⫽ C6H5O ⫹ H C6H5OH ⫹ O2 ⫽ C6H5O ⫹ HO2 C6H5OH ⫹ OH ⫽ C6H5O ⫹ H2O C6H5OH ⫹ H ⫽ C6H6 ⫹ OH C6H5OH ⫹ H ⫽ C6H5O ⫹ H2 C6H5OH ⫹ O ⫽ C6H5O ⫹ OH C6H5OH ⫹ C2H3 ⫽ C2H4 ⫹ C6H5O C6H5OH ⫹ C4H5 ⫽ C4H6 ⫹ C6H5O C6H5OH ⫹ C6H5 ⫽ C6H6 ⫹ C6H5O C5H5 ⫹ H ⫽ C5H6 C5H5 ⫹ O ⫽ C4H5 ⫹ CO C5H5 ⫹ HO2 ⫽ C5H5O ⫹ OH C5H5 ⫹ OH ⫽ C5H4OH ⫹ H C5H6 ⫹ O2 ⫽ C5H5 ⫹ HO2 C5H6 ⫹ HO2 ⫽ C5H5 ⫹ H2O2 C5H6 ⫹ OH ⫽ C5H5 ⫹ H2O C5H6 ⫹ H ⫽ C5H5 ⫹ H2 C5H6 ⫹ O ⫽ C5H5 ⫹ OH C5H6 ⫹ C2H3 ⫽ C5H5 ⫹ C2H4 C5H6 ⫹ C4H5 ⫽ C5H5 ⫹ C4H6 C6H5O ⫹ C5H6 ⫽ C6H5OH ⫹ C5H5 C5H5O ⫽ C4H5 ⫹ CO C5H4OH ⫽ C5H4O ⫹ H C5H4O ⫽ CO ⫹ C2H2 ⫹ C2H2 C4H5 ⫽ C2H2 ⫹ C2H3 C4H5 ⫹ M ⫽ C4H4 ⫹ H ⫹ M C4H5 ⫹ O2 ⫽ C4H4 ⫹ HO2 C2H3 ⫹ M ⫽ C2H2 ⫹ H ⫹ M C2H3 ⫹ O2 ⫽ C2H2 ⫹ HO2 C2H2 ⫹ O ⫽ HCCO ⫹ H C2H2 ⫹ O ⫽ CH2 ⫹ CO CH2 ⫹ O2 ⫽ H ⫹ OH ⫹ CO CH2 ⫹ O2 ⫽ CO ⫹ H2O HCCO ⫹ O2 ⫽ OH ⫹ CO ⫹ CO CO ⫹ O ⫹ M ⫽ CO2 ⫹ M CO ⫹ O2 ⫽ CO2 ⫹ O CO ⫹ OH ⫽ CO2 ⫹ H CO ⫹ HO2 ⫽ CO2 ⫹ OH HCO ⫹ M ⫽ H ⫹ CO ⫹ M HCO ⫹ O2 ⫽ CO ⫹ HO2 HCO ⫹ H ⫽ CO ⫹ H2 HCO ⫹ O ⫽ CO ⫹ OH HCO ⫹ OH ⫽ CO ⫹ H2O
Reaction coefficient is described as aT n exp(⫺Ea/RT).
2.20E 6.30E 2.11E 2.78E 2.50E 2.09E 7.94E 4.34E 1.81E 6.00E 2.21E 1.15E 2.81E 6.00E 6.00E 4.91E 1.00E 1.00E 3.00E 3.00E 2.00E 1.99E 3.43E 2.19E 1.81E 6.00E 6.00E 3.16E 7.94E 2.10E 1.00E 3.98E 2.98E 1.20E 2.98E 1.20E 5.80E 1.40E 6.02E 2.41E 1.46E 2.51E 2.51E 1.50E 6.03E 1.86E 4.17E 7.24E 3.02E 3.02E
⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹
14 13 13 13 14 12 14 11 14 12 13 14 13 12 12 12 14 14 13 13 13 12 09 08 13 12 12 11 14 13 15 11 33 11 33 11 06 06 11 11 12 13 12 07 13 17 12 13 13 13
n
Ea cal/mol
Reference
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1.18 1.77 0 0 0 0 0 0 0 0.7 ⫺5 0 ⫺5 0 2.09 2.09 0 0 0 0 0 1.3 0 ⫺1 0 0 0 0
0 60,000 4,570 4,910 16,000 7,470 55,400 62,892 40,658 0 7,910 12,400 7,352 0 0 4,400 0 0 0 0 25,000 11,660 ⫺447 3,000 3,080 0 0 8,000 55,400 48,000 78,000 42,260 44,320 0 44,320 0 1,562 1,562 0 0 2,500 ⫺4,540 47,690 ⫺765 22,950 17,000 0 0 0 0
16 16 16 16 16 16 21 18 see text 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 21 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16
448
S. NODA ET AL.
The Effect of Phenol Addition on the Thermal De-NOx Process As shown in the results section, the important role of NH3 suggests that the mechanism of the phenol addition should be similar to the effect of other additives on the thermal de-NOx process. Kinetic Model for Analyzing the Effect of Phenol Addition The CHEMKIN simulations [23] were carried out, based on the following kinetic models: the benzene oxidation model (B1–B6, B10 –B50 in Table 3) proposed by Emdee et al. [16] and the NO/NH3/O2 system model (A1–A37, A40 – A105 in Table 3) proposed by Glarborg et al. [10]. The data of Park and Lin [11] and Bruinsma et al. [18] were used for NH2 ⫹ NO reactions (A38, A39) and phenol pyrolysis (B8), as discussed in previous sections. The unimoleculer decomposition rate of phenoxy radical (B7) was determined by ab initio calculations [21], which was used instead of that in the Emdee model. Comparison of the Model Calculation and Experimental Data Figure 7 shows the calculated results of NO profile in the case of phenol addition and no phenol addition. Phenol addition causes only 50 K shift of the NO profile to lower temperatures, and has almost no effect under 1023 K (750°C). In phenol oxidation chemistry, the following reactions are important as discussed in “Phenol Oxidation Mechanism”: C6H5OH 3 C6H5O ⫹ H
B8 in Table 3
C5H5 ⫹ OH 3 C5H4OH ⫹ H C5H5O 3 C4H5 ⫹ CO
B20 in Table 3
B29 in Table 3
Though the most sensitive rates (B8 and B20 in Table 3) for phenol oxidation were changed by a factor of 2, NO profile varied within 10%. The Effect of Phenol Addition on the Thermal De-NOx Process above 1050 K (777°C) Schuhlmann and Rotzoll [5] investigated the influence of CO on the thermal de-NOx process.
Fig. 7. Calculation result for the effect of phenol solution addition. (Reaction B9 in Table 3 not included.) Reaction rate constants are listed in Table 3 (B9 is not included). Initial mole fractions: [NO]0 ⫽ 400 ppm, [NH3]0 ⫽ 400 ppm, [O2]0 ⫽ 4 vol%, [H2O]0 ⫽ 7.4%, residence time ⫽ 1072/(T ⫹ 273) (T: [°C]). The dotted, dashed, and solid line show NO profiles (1) without phenol solution, (2) with CO 600 ppm addition, and (3) with phenol 100 ppm addition, respectively.
According to their report, the addition of CO promotes radical reactions and shifts the temperature range to 777– 827°C. The following mechanism of CO addition was proposed. CO ⫹ OH 3 CO2 ⫹ H
(R15)
H ⫹ O2 3 OH ⫹ O
(R16)
CO is oxidized in R15. Since reaction R15 is followed by R16, CO provides additional radicals that overcome the radical termination reactions. Because CO is also generated in the process of phenol oxidation in our system as shown in Fig. 5, the effect of the produced CO on the thermal de-NOx process was treated using the kinetic model (A1–A105, B42–B45 in Table 3). In order to estimate the upper limit of the CO effect on the thermal de-NOx process, it was assumed that 100 ppm of phenol was momentarily oxidized to form 600 ppm of CO. The initial condition of the calculation was [NO]0 400 ppm, [NH3]0 400 ppm, [O2]0 4%, [CO]0 600 ppm, [H2O]0 7.4%, and reaction time 1.0 s. The calculated result is also shown in Fig. 7. The kinetic model indicated that the calculated NO profile in the case of phenol addition approached that in CO addition, when the rate
ADDITION OF PHENOL TO NONCATALYTIC REDUCTION
449
constants of phenol oxidation were increased by a factor of 2. It is considered that CO produced through phenol oxidation, caused the NO decrease at higher than 1050 K (777°C). The Effect of Phenol Addition on the Thermal De-NOx Process under 1050 K (777°C) Hjuler et al. [24] investigated the mutually promoted oxidation of various organic compounds and NO in the presence of oxygen, using laboratory flow reactors. Various kinds of hydrocarbons (methanol, ethane, acetone, acetylene, acetaldehyde, and methylamine) were tested in the temperature range of 650 –1300 K. Nitrogen oxide is oxidized to NO2, whereas the hydrocarbons are converted mainly to CO at relatively low temperature, e.g. from 850 to 1050 K in the case of methanol. For a methanol/NO ratio close to 1, an oxidation efficiency was reported to be 80 –90% at around 950 K. Hjuler et al. explained experimental results for mutual oxidation of CH3OH and NO, using the following elementary reactions: NO ⫹ HO2 3 NO2 ⫹ OH
(R19)
CH3OH ⫹ OH 3 CH2OH ⫹ H2O
(R20)
CH2OH ⫹ O2 3 CH2O ⫹ HO2
(R21)
NO is oxidized almost solely through R19 whereas the relatively unreactive HO2 radical is converted to a reactive one OH. The main source of HO2 is hydrogen abstraction by O2 from CH2OH, not from CH3OH nor CH3O at 850 –1050 K. Consequently, NO oxidation by hydrocarbons strongly depends on the rate of the corresponding abstraction reactions. Figure 3 shows NO2 formation while NO decrease occurs by phenol addition. It could be considered that NO2 formation results from reactions of NO and phenol and/or byproducts during phenol oxidation. NO is oxidized to NO2, whereas OH would react not only with hydrocarbons but also with NH3 to form the selective reduction agent NH2. In terms of reactions of hydrocarbons with molecular oxygen, reactions of benzene (C6H6), phenyl radical (C6H5), cyclopentadiene (C5H6), butadienyl radical (C4H5), C2H3, and HCO were included in Emdee’s model [16], while a
Fig. 8. Calculated results for the effect of phenol solution addition. (Reaction B9 in Table 3 included). Reaction rate constants are listed in Table 3 (B9 is included). Initial mole fractions: [NO]0 ⫽ 400 ppm, [NH3]0 ⫽ 400 ppm, [O2]0 ⫽ 4 vol%, [H2O]0 ⫽ 7.4%, residence time ⫽ 1072/(T ⫹ 273) (T: [°C]). The dotted and the solid line show the calculated profiles in the kinetic model (1) where B9 is not included, and (2) B9 is included.
reaction with phenol was ignored because the bond energy of O™H in phenol has a high value of 87 kcal/mol. H abstraction reactions from hydrocarbons become dominant at relatively low temperature range (650 –1000 K) where chain-branching steps cannot sustain the number of radicals. In order to estimate the abstraction reaction from phenol, CHEMKIN simulation was carried out, based on the kinetic model (A1–A105, B1–B8, B10 –B50 in Table 3) discussed above, and the phenol ⫹ O2 reaction (B9). By analogy to the reaction of CH2O ⫹ O2 [25], the rate constant for phenol ⫹ O2 reaction was assumed to be 6.03 ⫻ 10 13 ⫻ exp(⫺20,460/T), because the reaction enthalpy for CH2 ⫹ O2 (33.8 kcal/mol at 1050 K) is close to that for phenol ⫹ O2 (152 kJ/mol at 1050 K). Calculated profiles of NO and NO2 are shown in Fig. 8. The rate for the phenol ⫹ O2 reaction was changed within the reported uncertainty of 3. Simulation results indicated that NO was converted mainly to N2, partially to NO2, whereas NH3 was constantly oxidized, resulting in reducing NO through the thermal de-NOx mechanism. In terms of the mass balance of nitrogen atoms, 52 ppm of NO and 33 ppm of NH3 were converted to 19 ppm of NO2 and 33 ppm of N2 (66 ppm in terms of N atoms) at 723°C, in the event that the rate for phenol ⫹
450 O2 was increased by a factor of 3. It should be noted that mutual oxidation of NO and phenol could generate additional OH radical along the similar steps to R19 –R21, and oxidize NH3 even under 1050 K (777°C), while some amount of NO is oxidized to NO2. The simulation suggested the potential of phenol for NO reduction at lower than 1050 K, but indicated less effect of phenol addition than that in the experimental results (Fig. 2). This is because of the complicated channel of either the H abstraction reactions from phenol and/or byproducts during phenol oxidation by O2, or the interaction of phenol fragments with the NO/NH3/O2 system. Further experimental and theoretical research is needed to clarify the effect of phenol addition on the thermal deNOx process.
S. NODA ET AL. 4. 5. 6. 7. 8.
9. 10. 11. 12. 13. 14.
CONCLUSIONS We investigated a new approach for reducing the NO concentration in combustion products using phenol aqueous solution, which is one of the industrial waste products in the petroleum refinement process. The effect of adding phenol solution on the thermal de-NOx process was experimentally investigated in detail. The experimental conditions were [NO]0 ⫽ 400 ppm, [NH3]0 ⫽ 400 ppm, [O2]0 ⫽ 4.0%, [phenol]0 ⫽ 0 – 400 ppm, [H2O]0 ⫽ 1.7–7.4%, residence time ⫽ 1.0 –1.2 s, and temperature ⫽ 650 – 820°C. A small amount of phenol (phenol/ NO ⫽ 0.25) decreased the NO concentration significantly in the temperature range of 680 – 820°C, e.g. about 69% of NO was converted at 728°C. The injected phenol was completely oxidized to form CO or CO2 at temperatures higher than 728°C. REFERENCES Kinball-Linne, M. A., and Hanson, R., Combust. Flame 64:337–351 (1986). 2. Miller, J. A., and Bowman, C. T., Prog. Energy Combust. Sci. 15:287–338 (1989). 3. Lyon, R. K., US Patent 3,900554, Aug. 19, 1975.
1.
15. 16. 17.
18. 19. 20.
21. 22.
23.
24. 25.
Lyon, R., and Hardy, E., Ind. Eng. Chem. Fundam. 25:19 –24 (1986). Schulmann, J., and Rotzoll, G., Fuel 72:175–179 (1993). Hemberger, R., Muris, S., Pleban, K.-U., and Wolfrum, J., Combust. Flame 99:660 – 668 (1994). Miller, J. A., and Bowman, C. T., Int. J. Chem. Kinetics 23:289 –313 (1991). Wenli, D., Dam-Johansen, K., and Ostergaard, K., Twenty-Third Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1990, pp. 297–303. Hashimoto, M., Watanabe, Y., Mama, F., and Odan, A., UK patent GB 208901B (1985). Glarborg, P., Dam-Johansen, K., et al., Int. J. Chem. Kinetics 26:421– 436 (1994). Park, J., and Lin, M. C., J. Phys. Chem. A 101:5–13 (1997). Stephans, J. W., et al., J. Phys. Chem. 97:8944– 8951 (1993). Diau, E. W., and Lin, M. C., J. Phys. Chem. 98:4034 – 4042 (1994). Brezinsky, K., Prog. Energy Combust. Sci. 12:1–24 (1986). Bozzeli, J. W., et al., the ACS Fuels division, National Meeting, New York, 1494 –1498 (1991). Emdee, J. L., Brezinsky, K., and Glassman, I., J. Phys. Chem. 96:2151–2161 (1992). Venkat, C., Brezinsky, K., and Glassman, I., Nineteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1982, pp. 143–152. Bruinsma, O. S. L., et al., Fuel 67:327–333 (1988). Lovell, A. B., Brezinsky, K., and Glassman, I., Int. J. Chem. Kinetics 21:547–560 (1989). Lutz, A. F., et al. (1995). SENKIN Sandia Rept. SAND87-8248. Sandia National Laboratories, Livermore, CA. Olivella, S., Sole, A., and Garcia-Raso, A., J. Phys. Chem. 99:10549 –10556 (1995). Brezinsky, K., Butler, R. G., and Glassman, I., Symp. Mechanisms and Chemistry of Pollutant Formation and Control from Internal Combustion Engines. Division of Petroleum Chem., ACS, Washington, D.C., 1467–1472 (1992). Kee, R. J., Ruply, F. M., and Miller, J. A. (1989). CHEMKIN, Sandia Report SAND89-8009. Sandia National Laboratories, Livermore, CA. Hjuler, K., Glarborg, P., and Dam-Johansen, K., Ind. Eng. Chem. Res. 34:1882–1888 (1995). Baulch, D. L., Cobos, C. J., Cox, R. A., et al., J. Phys. Chem. Ref. Data 21:411– 429 (1992).
Received 27 May 1997; revised 7 January 2000; accepted 16 February 2000
Department of Chemical System Engineering, Faculty of Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan; and Nippon Oil Company, Ltd., Central Technical Research Laboratory, Yokohama, Japan The thermal de-NOx process is one of the most effective methods of reducing the NOx in flue gas. A shortcoming in this process is the narrow temperature range of 870°C to 1100°C in which effective reduction of NO occurs. We approach this problem by adding an agent to the system in order to shift the temperature range to a lower one. The agent that was added was an aqueous solution containing trace amounts of phenolic compounds which is one of the industrial waste products of fluidized catalytic cracking (FCC) in petroleum refinement. The effect of the addition of an aqueous phenol solution on the thermal de-NOx process was investigated in a quartz flow reactor at a temperature, pressure, and residence time of 650 – 820°C, 1.0 atm, and 1.0 –1.2 s, respectively. In the case of 0 – 400 ppm phenol addition at NO/NH3 ⫽ 1.0, O2 ⫽ 4%, NO ⫽ 400 ppm, and H2O ⫽ 7.4%, profiles of NO, N2O, NO2, NH3, CO, CO2, C2H2, and phenol concentrations were experimentally obtained. The NO concentration significantly decreased over the wide temperature range of 675– 820°C. © 2000 by The Combustion Institute
INTRODUCTION Several types of NOx (nitrogen oxide) removal systems using catalysts are used in various combustion processes. However, these catalytic methods have problems that prevent wider application, such as blockage of the catalyst by particulates, deterioration under hostile conditions, and the high cost of catalysts. Both with and without a catalyst, ammonia is widely used as an effective agent to reduce the NOx in flue gas in the presence of oxygen. A disadvantage of the noncatalytic process, called the “thermal de-NOx process” [1–3], is the narrow temperature range of 870 to 1100°C in which the effective reduction of NO (nitrogen monoxide) occurs. If one can decrease the reaction temperature to around 600°C, this process may be applicable to NOx removal from the flue gas of stationary sources, such as a diesel power generator or a gas turbine system. The influence of additives, such as H2 [4], CO [5], CH4 [6], C2H6 [6], HNCO [7], and amines [8] on the reduction of NO in the thermal de-NOx process has been investigated. Hashimoto et al. [9] also examined how an aqueous *Corresponding author. E-mail: [email protected] COMBUSTION AND FLAME 122:439 – 450 (2000) © 2000 by The Combustion Institute Published by Elsevier Science Inc.
solution containing trace amounts of phenolic compounds affected NO concentration in a NOx noncatalytic reduction process by ammonia. This phenolic solution is one of the industrial waste products of fluidized catalytic cracking (FCC) in the petroleum refinery process. When the wastewater was directly sprayed into the heating furnace, the conversion ratio of NO increased to about 30% at around 700°C. In this study, the effect of adding a phenolic solution to the thermal de-NOx process is experimentally investigated. The outlet gas composition is examined in detail to determine whether or not the addition of the phenol solution produces pollutants. Finally, we discuss the mechanism by which the NO decrease occurs. EXPERIMENTAL PROCEDURE The experimental work was performed in a flow reactor made of quartz, which was operated at atmospheric pressure and temperatures from 650°C to 850°C, with residence times of 1.0 –1.2 s. Part of the apparatus is illustrated in Fig. 1. Our system consisted of three sections: a preheating section of 100 mm length and 22 mm i.d., a reaction section of 300 mm length and 31 0010-2180/00/$–see front matter PII S0010-2180(00)00119-X
440
Fig. 1. Experimental apparatus. A: preheat furnace, B: electric heater, C: reactor, D: water cooler, E: stainless steel capillary, F: microfeeder.
mm i.d., and a section for quenching with water cooling. An electric heater was used to control the temperature of the reactor section between 650 and 850°C. Gas temperature profiles were measured with a thermocouple along the central axis of the flow reactor (from x ⫽ 0 mm to 300 mm in Fig. 1). A microfeeder constantly supplied phenol aqueous solution through a glass-coated stainless tube of 0.3 mm i.d. The phenol solution was vaporized in the preheating section, the temperature of which was controlled at around 200°C. Reaction during preheating can be ignored because it was observed that no phenol decomposed during the preheating section. Gaseous phenol and other gases (NO, NH3, and O2) were supplied through a pinhole of 1.5 mm i.d. for good mixing, and introduced to the reaction section. Argon gas was used as a diluent. Flow rates of reactants were adjusted using four mass flow controllers. Gas concentrations were measured using a Fourier transform infrared analyzer (Shimadzu FTIR8000-PC) for N2O, and a chemiluminescence analyzer (Shimadzu NOA-7000) for NO and NO2. The characteristic absorption line (2240 cm⫺1) was adopted for determination of N2O concentration. Carbon compounds were analyzed by a gas chromatograph (Shimadzu GC-14A) equipped with a flame ionization detector (FID) using Porapack Q as a separation column for CO, CO2, and C1–C3 hydrocarbons, and using Unisole F-200 as a separation column for phenol. More than 99% of the ammonia was
S. NODA ET AL.
Fig. 2. The effect of phenol solution addition. Initial mole fractions: [NO]0 ⫽ 400 ppm, [NH3]0 ⫽ 400 ppm, [O2]0 ⫽ 4 vol%, [phenol]0 ⫽ 100 ppm, [H2O]0 ⫽ 7.4 vol%, residence time ⫽ 1072/(T ⫹ 273) s. (T: [°C]). F Without phenol solution. Œ With phenol solution.
absorbed in a water scrubber after passing through the reactor, and this ammonia solution was analyzed by an ammonium ion electrode. The following chemical species were detected: NO, CO, CO2, C2H2, phenol, N2O, NO2, and NH3. The concentrations of compounds except for NH3 and NO2 were measured with an error of ⫾5%. The uncertainty in the NO2 measurement was ⫾10%. The error in the ammonium ion electrode method was ⫾15%. RESULTS Plug flow experiments with or without phenol were performed under the following conditions: [NO]0 ⫽ 400 ppm, [NH3]0 ⫽ 400 ppm, [O2]0 ⫽ 4.0%, [phenol]0 ⫽ 0 – 400 ppm, [H2O]0 ⫽ 1.7– 7.4%, residence time ⫽ 1.0 –1.2 s, temperature ⫽ 650 – 820°C (phenol was injected into the reactor as phenol aqueous solution). Phenol (C6H5OH) is highly soluble in water because of its OH substituent. The Thermal De-NOx Process for a Residence Time of Approximately 1 s In the case of no phenol addition, NO conversion was 45% at 816°C at a residence time of ⫽ 0.98 s. Neither NO2 nor N2O was detected. As shown in Fig. 2, the NO profile is lower at the low temperature end of the temperature
ADDITION OF PHENOL TO NONCATALYTIC REDUCTION
Fig. 3. Concentrations of N2O and NO2. Initial mole fractions are the same as in Fig. 2. F N2O. ΠNO2. The solid line shows the total amount of N2O and NO2.
window than that of previous studies [1, 2, 10] because of the longer residence time of around 1 s in our experiments. The Effect of Phenol Addition The effect of phenol addition on the NO profile is shown in Fig. 2. Compared with the case of no phenol addition, the NO concentration significantly decreased over a wide temperature range, 675– 820°C, and an NO conversion ratio of about 69% was obtained at 728°C by addition of a small amount of phenol (phenol/NO ⫽ 0.25). This phenol effect was drastically suppressed for either (1) addition of phenol solution in the absence of ammonia, or (2) water addition in the presence of ammonia. Both phenol and ammonia are thought to play important roles in this marked effect of NO decrease.
441
Fig. 4. Effect of phenol solution addition on NH3 concentration. Initial mole fractions are the same as in Fig. 2. F Without phenol solution. ΠWith phenol solution.
ppm in spite of the temperature rise, whereas some of the NH3 was converted to other nitrogen compounds without reducing NO above 730°C. This means that the addition of phenol solution also has the merit of reducing the amount of residual ammonia between 700 – 780°C, where about 90% of ammonia was converted at 776°C. Concentrations of carbon compounds, such as CO, C1–C3, CO2, and phenol, were experimentally determined in the case of phenol addition, as shown in Fig. 5. Only C2H2 was detected as C1–C3 hydrocarbons. When the
Outlet Gas Composition Figure 3 shows concentrations of NO2 and N2O under the same conditions as in Fig. 2. The fact that the total amount of NO2 and N2O reached about 80 ppm in terms of N atoms at 728°C, indicates that 27% of NO was not converted to nitrogen, but to other nitrogen oxides. The ammonia profile is shown in Fig. 4. The result indicates that the NH3 profile is similar to the NO profile in Fig. 2. However, in the temperature range between 730 –780°C, the NO concentration remained constant at around 140
Fig. 5. C composition in outlet gas. Initial mole fractions: NO ⫽ 400 ppm, NH3 ⫽ 400 ppm, O2 ⫽ 4 vol%, phenol ⫽ 100 ppm, H2O ⫽ 7.4 vol%, reaction time ⫽ 1072/(T ⫹ 273) s (T: [°C]).
442
S. NODA ET AL. TABLE 1 The Thermal De-NOx Kinetic Mechanism [2] No.
Reaction
R1 R2 R3 R4 R5 R6 R7 R8 R9
NH3 ⫹ OH ⫽ NH2 ⫹ H2O NH2 ⫹ NO ⫽ NNH ⫹ OH NNH ⫽ N2 ⫹ H O ⫹ OH ⫽ H ⫹ O2 O ⫹ H2 ⫽ OH ⫹ H NH2 ⫹ NO ⫽ N2 ⫹ H2O H ⫹ O2 ⫹ Ma ⫽ HO2 ⫹ M OH ⫹ HO2 ⫽ H2O ⫹ O2 HNO ⫹ OH ⫽ NO ⫹ H2O
a
Fig. 6. Dependence of phenol concentration on NH3 concentration. Initial mole fractions: NO ⫽ 400 ppm, NH3 ⫽ 400 ppm, O2 ⫽ 4 vol%, H2O ⫽ 3.1–7.4 vol%, reaction time ⫽ 1.07 s, F T ⫽ 728°C, Œ T ⫽ 776°C.
injected phenol was completely oxidized, the calculated value of the total carbonaceous gases from the reactor would be 650 ppm in terms of carbon atoms. The results from 681– 815°C satisfy the mass balance of carbonaceous compounds within experimental uncertainties. With increasing temperature, more phenol was decomposed to produce CO, CO2, and C2H2. More than 99.9% of phenol was oxidized at temperatures higher than 728°C. Around 10 ppm of C2H2 was detected between 639 –728°C, while the CO concentration increased with increasing temperature in this regime, reaching 475 ppm at 728°C. At temperatures higher than 728°C, further oxidation of CO to CO2 was observed.
M represents the third body.
If one applies this result at 776°C, it is expected that no more than 60 ppm phenol addition is enough to obtain NO conversion of 69%, and results in lower emissions of pollutants such as CO, NO2, and N2O than 100 ppm phenol addition. DISCUSSION The Thermal De-NOx Mechanism The thermal de-NOx process [1–3] is a selective, noncatalytic technique for removal of nitrogen oxides. Important elementary reactions are shown in Table 1. The NH2 ⫹ NO reaction (R2 and R6), a key reaction in the thermal de-NOx process, has a radical branching channel as well as a molecular product channel:
Dependence of Concentrations of Phenol and Water on the Phenol Effect
NH2 ⫹ NO 3 N2H ⫹ OH
(R2)
3 N 2 ⫹ H 2O
(R6)
The influence of the concentrations of water and phenol on NO concentration in the thermal de-NOx process was also investigated. There was a negligible effect of water concentration in the range of 1.7% to 7.4% water. Figure 6 shows that the NO conversion ratio was enhanced with increasing phenol concentration between 0 ppm and 100 ppm at 728°C, and reached a plateau at concentrations higher than 100 ppm. In the case of 776°C, 60 ppm of phenol addition had the same effect on NO as 100 ppm phenol injection. It was observed that phenol was oxidized to form 47 ppm of CO and 316 ppm of CO2.
The product branching ratio strongly influences NO reduction, because aminogen (NH2) is formed principally by OH through oxidation of ammonia. N2H decomposition rate followed by R2 also affects radical reactions, because N2H leads to N2 ⫹ H (R3 in Table 1). In the viewpoint of reactions between H and O2, H ⫹ O2 3 OH ⫹ O (-R4 in Table 1) can proceed at a slower rate than H ⫹ O2 ⫹ M 3 HO2 ⫹ M (R7 in Table 1) at temperatures lower than 1000 K. HO2 radicals decrease the number of OH radicals through R8. As a result OH is quickly decreased by ammonia and the
ADDITION OF PHENOL TO NONCATALYTIC REDUCTION TABLE 2 The Overall Benzene Oxidation Mechanism [14] Reactiona
No. R10 R11 R12 R13 R14 a
C6H6 ⫹ X 3 C6H5O (phenoxy radical), C6H5OH (phenol) C6H5O 3 C5H5 (cyclopentadienyl) ⫹ CO C5H5 ⫹ X 3 C4H5 (butadienyl radical) ⫹ CO C4H5 ⫹ X 3 C2H4, C2H2 C2H4, C2H2 ⫹ X 3 CO, CO2, H2O
X represents OH, H, O, O2.
radical pool is not sustained at lower temperatures. Consequently, the thermal de-NOx process requires a high temperature in order to obtain high branching ratios of R2 and/or R4. If effective NO reduction is required at lower temperatures, a new source of OH radicals needs to be found to overcome radical termination. Kinetic models [2, 10] of the thermal de-NOx describe well the experimental data conducted in laminar quartz reactors. The branching ratio defined as ␣ ⫽ R2/(R2 ⫹ R6), and the lifetime of N2H were estimated to be 0.0022 ⫻ T 0.70 (T: temperature [K]) and 10⫺6 [s], respectively [2]. The reaction channels of NH2 ⫹ NO have been experimentally investigated in recent studies [11–13]. Park and Lin [11] determined total rate constants and the branching ratio of NH2 ⫹ NO reaction, and also confirmed that no other product channel exists except R2 and R6 below 1060 K. Phenol Oxidation Mechanism Phenol is formed via one of the important routes of benzene oxidation. Brezinsky [14] discussed chemical mechanisms of the gasphase oxidation of benzene and toluene at atmospheric pressure and high temperature (600 –1200°C) in detail. The overall benzene oxidation mechanism is shown in Table 2. Since phenol is oxidized to form phenoxy radical (C6H5O 䡠 ) at the first stage, the mechanism described by R11–14 in Table 2 should govern phenol oxidation. Cyclopentadienyl (C5H5) formed via the following step (R11) is expected to react with radicals and molecules because of its stable characteristics [15]. The main products of reactions (R12) of C5H5 are C5H6, C4H5, and
443
CO. C4H5 is oxidized to form C2 hydrocarbons, resulting in CO, CO2, and H2O as final products. Emdee et al. [16] developed a kinetic model for the oxidation of benzene and toluene near 1200 K, which agrees well with experimental data including lean and rich equivalence ratios with temperatures in the range of 1100 –1190 K. Emdee’s model assumed several elementary steps for R12 and R13, and evaluated thermodynamic properties of each compound and reaction coefficients of each step, compared to those of similar reactions that had previously been well elucidated. Initial Steps of Phenol Oxidation Phenol is oxidized to form phenoxy radical (C6H5O 䡠 ) at the first stage of its oxidation process. Two oxidation steps are reported [17] as follows: C6H5OH ⫹ M 3 C6H5O ⫹ H ⫹ M
(R17)
C6H5OH ⫹ OH 3 C6H5O ⫹ H2O
(R18)
M: the third body The rate of the initiation step is not important at higher temperatures as discussed in Emdee’s model, because chain-branching steps generate more radicals than chain initiation. On the other hand, the initial step of phenol oxidation at lower temperatures, as in our study, is important in total oxidation rate. Especially, the pyrolysis reaction (R17) could be the most important step in the absence of OH radicals in the system. Bruinsma et al. [18] carried out an experimental study of gas-phase pyrolysis of coalrelated aromatic compounds, including phenol, in a flow tube reactor. Experimental conditions were P ⫽ 0.125 MPa, T ⫽ 1040–1140 K, and reaction time ⫽ 5 s. Their reactor was designed to suppress secondary reactions by using low concentrations, less than 500 ppm, and by depositing a graphite layer inside the reactor. Arrhenius parameters for phenol pyrolysis were evaluated to be 4.34 ⫻ 1011 as a frequency factor and 62.6 kcal kJ/mol as an activation energy. On the other hand the data of Lovell et al. [19] which the Emdee model adopted for phe-
444 nol pyrolysis, was estimated by experiments in a turbulent flow reactor for conditions of P ⫽ 1 atm, T ⫽ 1064–1162 K, initial phenol concentration ⫽ 500 –2016 ppm, and reaction time ⫽ 170 ms. In Ref. 19, the rate for R17 was calculated using the reverse rate for R17 and the equilibrium constant. Though the rate for R17 is very sensitive to variation of the equilibrium constant, the evaluated data described experimental results well. The time constant of phenol pyrolysis in Ref. 18 is shorter than that in Ref. 19, e.g. time constants at 1050 K are 27.9 s in Ref. 18 and 484 s in Ref. 19, respectively. Analysis of Reaction Pathways in Phenol Oxidation In order to analyze the main reaction paths in phenol oxidation, the reaction system was numerically treated using SENKIN [20]. Emdee’s model [16] was basically used for the kinetic modeling except two reactions of phenoxy radical pyrolysis and phenol pyrolysis (A1–A20, B1–B6, B10 –B50 in Table 3). As discussed above, the rate for phenol pyrolysis (B8 in Table 3) reported by Bruinsma et al. [18] is adopted for our experimental conditions of longer residence time (about 1 s). The unimoleculer decomposition rate of phenoxy radical (B7 in Table 3) was determined by ab initio calculations [21], which evaluated the activation energy to be 55.4 kcal/mol. Calculations were done for the conditions of [phenol]0 ⫽ 200 ppm, [O2]0 ⫽ 4%, i.e. ⫽ 0.4, Ar remainder, Pressure ⫽ 1 atm, T ⫽ 1000–1100 K, and reaction time ⫽ 1 s. Calculation results showed that 39% of phenol was decomposed at 1050 K, and the decreased phenol was mainly oxidized to CO, partially converted to C4H4, C2H2, and CO2 under 1050 K. At higher temperatures of 1050 – 1100 K, phenol was completely oxidized to CO and CO2. The ratio of CO2 to CO in very lean condition was reported to be less than 2:3 in the oxidation of cyclopentadiene (C5H6) at 1063 K [22]. The composition of byproducts in phenol oxidation must be similar to that in C5H6 oxidation, so that C5H6 plays a key role in hightemperature oxidation processes of aromatic compounds. Our calculation results agreed with
S. NODA ET AL. the characteristics of CO and CO2 formation in the oxidation of C5H6. A linear sensitivity analysis for phenol concentration at 1050 K indicated that three reactions have a high positive sensitivity of S ⫽ 0.1 to 0.25, and one reaction has a high negative sensitivity of S ⫽ ⫺0.05 to ⫺0.1. Reactions of a positive sensitivity: C6H5OH 3 C6H5O ⫹ H
B8 in Table 3
C5H5 ⫹ OH 3 C5H4OH ⫹ H C5H5O 3 C4H5 ⫹ CO
B20 in Table 3
B29 in Table 3
A reaction of a negative sensitivity: C6H5OH ⫹ OH 3 C6H5O ⫹ H2O B10 in Table 3 Since the temperature of 1050 K was relatively low, the sensitivity of chain-branching steps such as H ⫹ O2 3 O ⫹ OH was under 0.05. It is predicted that the initial step of phenol oxidation at lower temperatures is important in the total oxidation rate. The pyrolysis reaction (B8 in Table 3) could be the most important step in the case of the absence of OH radicals in the system. A negative sensitivity coefficient of B10 is understandable, because B10 is a radical terminating step, resulting in suppressing oxidation reactions of C2–C5 hydrocarbons by radicals. It is considered that B20 is followed by B30 and B31 in Table 3 to form C2H2 and CO as follows: C5H5 ⫹ OH 3 C5H4OH ⫹ H
B20 in Table 3
C5H4OH 3 C5H4O ⫹ H B30 in Table 3 C5H4O 3 C2H2 ⫹ C2H2 ⫹ CO
B31 in Table 3
The pyrolysis reaction of C5H5O should follow B19, because B19 is only one source of C5H5O formation. C5H5 is also oxidized through B19 while relatively unreactive radical HO2 is converted to a reactive one OH as follows: C5H5 ⫹ HO2 3 C5H5O ⫹ OH C5H5O 3 C4H5 ⫹ CO
B19 in Table 3
B29 in Table 3
ADDITION OF PHENOL TO NONCATALYTIC REDUCTION
445
TABLE 3 Reaction Rate Constants Used for Calculation of NO/NH3/O2/Phenol System Units: cm3, moles, seconds, calories, kelvin. No.
Reactiona
A1 A2 A3 A4 A5
O ⫹ OH ⫽ O2 ⫹ H O ⫹ H2 ⫽ OH ⫹ H OH ⫹ H2 ⫽ H2O ⫹ H 2OH ⫽ O ⫹ H2O 2H ⫹ M ⫽ H2 ⫹ M H2/0.0/H2O/0.0/CO2/0.0/ 2H ⫹ H2 ⫽ 2H2 2H ⫹ H2O ⫽ H2 ⫹ H2O H ⫹ O ⫹ M ⫽ OH ⫹ M H2O/5.0/ H ⫹ OH ⫹ M ⫽ H2O ⫹ M H2O/5.0/ 2O ⫹ M ⫽ O2 ⫹ M H2 ⫹ O2 ⫽ 2OH H ⫹ O2 ⫹ M ⫽ HO2 ⫹ M H2O/18.6/CO2/4.2/H2/2.9/CO/2.1/N2/1.3 H ⫹ HO2 ⫽ H2 ⫹ O2 H ⫹ HO2 ⫽ 2OH O ⫹ HO2 ⫽ O2 ⫹ OH OH ⫹ HO2 ⫽ H2O ⫹ O2 2HO2 ⫽ H2O2 ⫹ O2 H2O2 ⫹ M ⫽ 2OH ⫹ M H2O2 ⫹ H ⫽ HO2 ⫹ H2 H2O2 ⫹ OH ⫽ H2O ⫹ HO2 NH3 ⫹ M ⫽ NH2 ⫹ H ⫹ M NH3 ⫹ H ⫽ NH2 ⫹ H2 NH3 ⫹ O ⫽ NH2 ⫹ OH NH3 ⫹ OH ⫽ NH2 ⫹ H2O NH3 ⫹ HO2 ⫽ NH2 ⫹ H2O2 NH2 ⫹ H ⫽ NH ⫹ H2 NH2 ⫹ O ⫽ HNO ⫹ H NH2 ⫹ O ⫽ NH ⫹ OH NH2 ⫹ OH ⫽ NH ⫹ H2O NH2 ⫹ HO2 ⫽ NH3 ⫹ O2 NH2 ⫹ HO2 ⫽ H2NO ⫹ OH H2NO ⫹ O ⫽ NH2 ⫹ O2 NH2 ⫹ N ⫽ N2 ⫹ O2 NH2 ⫹ NH ⫽ N2H2 ⫹ H NH2 ⫹ NH2 ⫽ N2H2 ⫹ H2 NH2 ⫹ NH2 ⫽ NH3 ⫹ NH NH2 ⫹ NH2 ⫽ N2H4 NH2 ⫹ NO ⫽ NNH ⫹ OH NH2 ⫹ NO ⫽ N2 ⫹ H2O NH ⫹ H ⫽ N ⫹ H2 NH ⫹ O ⫽ NO ⫹ H NH ⫹ OH ⫽ HNO ⫹ H NH ⫹ OH ⫽ N ⫹ H2O NH ⫹ O2 ⫽ HNO ⫹ O NH ⫹ O2 ⫽ NO ⫹ OH NH ⫹ NH ⫽ N2 ⫹ 2H NH ⫹ N ⫽ N2 ⫹ H NH ⫹ NO ⫽ N2O ⫹ H NH ⫹ NO ⫽ N2O ⫹ H NH ⫹ NO ⫽ N2 ⫹ OH NH ⫹ NO2 ⫽ N2O ⫹ OH
A6 A7 A8 A9 A10 A11 A12 A13 A14 A15 A16 A17 A18 A19 A20 A21 A22 A23 A24 A25 A26 A27 A28 A29 A30 A31 A32 A33 A34 A35 A36 A37 A38 A39 A40 A41 A42 A43 A44 A45 A46 A47 A48 A49 A50 A51
a (terms of mole-cm-sec-k)
Ea cal/mol
Reference
14 04 09 08 18
⫺0.5 2.67 1.3 1.3 ⫺1
0 6,290 3,626 0 0
10 10 10 10 10
9.20E ⫹ 16 6.00E ⫹ 19 6.20E ⫹ 16
⫺0.6 ⫺1.25 ⫺0.6
0 0 0
10 10 10
1.60E ⫹ 22
⫺2
0
10
1.89E ⫹ 13 1.70E ⫹ 13 3.61E ⫹ 17
0 0 ⫺0.72
⫺1,788 47,780 0
10 10 10
0 0 0 ⫺0.61 0 0 0 0 0 2.39 1.94 2.04 0 0 ⫺0.5 0 2 0 0 0 0 0 0 0 0 ⫺3.02 ⫺0.98 0 0 0 0.5 2 1.5 0 0 ⫺0.4 ⫺0.23 ⫺0.23 0
0 1,073 1,073 340 0 45,500 3,800 1,800 93,470 10,171 6,460 566 22,000 3,650 0 0 1,000 0 0 0 0 0 0 10,000 0 9,589 ⫺2,605 0 0 0 2,000 6,500 100 0 0 0 0 0 0
10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 11 11 10 10 10 10 10 10 10 10 10 10 10 10
4.00E 5.06E 1.17E 6.00E 1.00E
1.25E 1.40E 1.40E 2.10E 2.00E 1.30E 1.60E 1.00E 2.20E 6.36E 9.40E 2.04E 3.00E 4.00E 6.63E 6.75E 4.00E 1.00E 2.50E 7.50E 7.20E 5.00E 8.50E 5.00E 1.50E 9.19E 3.40E 3.00E 9.20E 2.00E 5.00E 4.60E 1.30E 2.50E 3.00E 2.90E ⫺2.20E 2.20E 1.00E
⫹ ⫹ ⫹ ⫹ ⫹
n
⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹
13 14 13 15 12 17 12 13 16 05 06 06 11 13 14 12 06 13 13 13 13 13 11 13 13 22 14 13 13 13 11 05 06 13 13 14 13 13 13
446
S. NODA ET AL. TABLE 3 (Continued)
No.
Reactiona
A52 A53 A54 A55 A56 A57 A58 A59
N ⫹ NO ⫽ N2 ⫹ O N ⫹ O2 ⫽ NO ⫹ O N ⫹ OH ⫽ NO ⫹ H NO ⫹ HO2 ⫽ NO2 ⫹ OH NO2 ⫹ H ⫽ NO ⫹ OH NO2 ⫹ O ⫽ NO ⫹ O2 NO2 ⫹ M ⫽ NO ⫹ O ⫹ M HNO ⫹ M ⫽ H ⫹ NO ⫹ M H2O/10.0/O2/2.0/N2/2.0/H2/2.0/ HNO ⫹ H ⫽ NO ⫹ H2 HNO ⫹ O ⫽ NO ⫹ OH HNO ⫹ OH ⫽ NO ⫹ H2O HNO ⫹ O2 ⫽ NO ⫹ HO2 HNO ⫹ NH2 ⫽ NH3 ⫹ NO HNO ⫹ NO ⫽ N2O ⫹ OH HNO ⫹ HNO ⫽ N2O ⫹ H2O H2NO ⫹ M ⫽ HNO ⫹ H ⫹ M H2NO ⫹ H ⫽ HNO ⫹ H2 H2NO ⫹ H ⫽ NH2 ⫹ OH H2NO ⫹ O ⫽ HNO ⫹ OH H2NO ⫹ OH ⫽ HNO ⫹ H2O H2NO ⫹ NO ⫽ HNO ⫹ HNO H2NO ⫹ NH2 ⫽ HNO ⫹ NH3 N2H4 ⫹ H ⫽ N2H3 ⫹ H2 N2H4 ⫹ O ⫽ N2H2 ⫹ H2O N2H4 ⫹ OH ⫽ N2H3 ⫹ H2O N2H4 ⫹ NH2 ⫽ N2H3 ⫹ NH3 N2H3 ⫹ M ⫽ N2H2 ⫹ H ⫹ M N2H3 ⫹ H ⫽ NH2 ⫹ NH2 N2H3 ⫹ O ⫽ N2H2 ⫹ OH N2H3 ⫹ O ⫽ NH2 ⫹ HNO N2H3 ⫹ OH ⫽ N2H2 ⫹ H2O N2H3 ⫹ OH ⫽ NH3 ⫹ HNO N2H3 ⫹ NH ⫽ N2H2 ⫹ NH2 N2H2 ⫹ M ⫽ NNH ⫹ H ⫹ M H2O/15.0/O2/2.0/N2/2.0/H2/2.0/ N2H2 ⫹ H ⫽ NNH ⫹ H2 N2H2 ⫹ O ⫽ NH2 ⫹ NO N2H2 ⫹ O ⫽ NNH ⫹ OH N2H2 ⫹ OH ⫽ NNH ⫹ H2O N2H2 ⫹ NO ⫽ N2O ⫹ NH2 N2H2 ⫹ NH ⫽ NNH ⫹ NH3 N2H2 ⫹ NH2 ⫽ NNH ⫹ NH2 NNH ⫽ N2 ⫹ H NNH ⫹ NO ⫽ N2 ⫹ HNO NNH ⫹ H ⫽ N2 ⫹ H2 NNH ⫹ OH ⫽ N2 ⫹ H2O NNH ⫹ NH2 ⫽ N2 ⫹ NH3 NNH ⫹ NH ⫽ N2 ⫹ NH2 NNH ⫹ O ⫽ N2O ⫹ H N2O ⫹ OH ⫽ N2 ⫹ HO2 N2O ⫹ H ⫽ N2 ⫹ OH N2O ⫹ M ⫽ N2 ⫹ O ⫹ M H2O/10.0/O2/1.5/N2/1.5/ N2O ⫹ O ⫽ N2 ⫹ O2 N2O ⫹ O ⫽ 2NO
A60 A61 A62 A63 A64 A65 A66 A67 A68 A69 A70 A71 A72 A73 A74 A75 A76 A77 A78 A79 A80 A81 A82 A83 A84 A85 A86 A87 A88 A89 A90 A91 A92 A93 A95 A96 A97 A98 A99 A100 A101 A102 A103 A104 A105
a (terms of mole-cm-sec-k)
n
Ea cal/mol
Reference
3.27E 6.40E 3.80E 2.10E 3.50E 1.00E 1.10E 1.50E
⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹
12 09 13 12 14 13 16 16
0.3 1 0 0 0 0 0 0
0 6,280 0 ⫺480 1,500 600 66,000 48,680
10 10 10 10 10 10 10 10
4.40E 1.00E 3.60E 1.00E 2.00E 2.00E 4.00E 5.00E 3.00E 5.00E 3.00E 2.00E 2.00E 3.00E 1.30E 8.50E 5.00E 3.90E 3.50E 1.60E 5.00E 1.00E 1.00E 1.00E 2.00E 5.00E
⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹
11 13 13 13 13 12 12 16 07 13 07 07 07 12 13 13 12 12 16 12 12 13 13 12 13 16
0.72 0 0 0 0 0 0 0 2 0 2 2 2 0 0 0 0 0 0 0 0 0 0 0 0 0
650 0 0 25,000 1,000 26,000 5,000 50,000 2,000 0 2,000 1,000 13,000 1,000 2,500 1,200 1,000 1,500 46,000 0 5,000 0 1,000 15,000 0 0
10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10
5.00E 1.00E 2.00E 1.00E 3.00E 1.00E 1.00E 1.00E 5.00E 1.00E 5.00E 5.00E 5.00E 1.00E 2.00E 7.60E 4.00E
⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹
13 13 13 13 12 13 13 06 13 14 13 13 13 14 12 13 14
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 1,000 1,000 1,000 0 1,000 1,000 0 0 0 0 0 0 0 10,000 15,200 56,100
10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10
1.40E ⫹ 12 2.90E ⫹ 13
0 0
10,800 23,150
10 10
ADDITION OF PHENOL TO NONCATALYTIC REDUCTION
447
TABLE 3 (Continued) Reactiona
No. B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 B13 B14 B15 B16 B17 B18 B19 B20 B21 B22 B23 B24 B25 B26 B27 B28 B29 B30 B31 B32 B33 B34 B35 B36 B37 B38 B39 B40 B41 B42 B43 B44 B45 B46 B47 B48 B49 B50 a
a (terms of mole-cm-sec-k)
C6H5 ⫹ H ⫽ C6H6 C6H6 ⫹ O2 ⫽ C6H5 ⫹ HO2 C6H6 ⫹ OH ⫽ C6H5 ⫹ H2O C6H6 ⫹ O ⫽ C6H5O ⫹ H C6H6 ⫹ H ⫽ C6H5 ⫹ H2 C6H5 ⫹ O2 ⫽ C6H5O ⫹ O C6H5O ⫽ C5H5 ⫹ CO C6H5OH ⫽ C6H5O ⫹ H C6H5OH ⫹ O2 ⫽ C6H5O ⫹ HO2 C6H5OH ⫹ OH ⫽ C6H5O ⫹ H2O C6H5OH ⫹ H ⫽ C6H6 ⫹ OH C6H5OH ⫹ H ⫽ C6H5O ⫹ H2 C6H5OH ⫹ O ⫽ C6H5O ⫹ OH C6H5OH ⫹ C2H3 ⫽ C2H4 ⫹ C6H5O C6H5OH ⫹ C4H5 ⫽ C4H6 ⫹ C6H5O C6H5OH ⫹ C6H5 ⫽ C6H6 ⫹ C6H5O C5H5 ⫹ H ⫽ C5H6 C5H5 ⫹ O ⫽ C4H5 ⫹ CO C5H5 ⫹ HO2 ⫽ C5H5O ⫹ OH C5H5 ⫹ OH ⫽ C5H4OH ⫹ H C5H6 ⫹ O2 ⫽ C5H5 ⫹ HO2 C5H6 ⫹ HO2 ⫽ C5H5 ⫹ H2O2 C5H6 ⫹ OH ⫽ C5H5 ⫹ H2O C5H6 ⫹ H ⫽ C5H5 ⫹ H2 C5H6 ⫹ O ⫽ C5H5 ⫹ OH C5H6 ⫹ C2H3 ⫽ C5H5 ⫹ C2H4 C5H6 ⫹ C4H5 ⫽ C5H5 ⫹ C4H6 C6H5O ⫹ C5H6 ⫽ C6H5OH ⫹ C5H5 C5H5O ⫽ C4H5 ⫹ CO C5H4OH ⫽ C5H4O ⫹ H C5H4O ⫽ CO ⫹ C2H2 ⫹ C2H2 C4H5 ⫽ C2H2 ⫹ C2H3 C4H5 ⫹ M ⫽ C4H4 ⫹ H ⫹ M C4H5 ⫹ O2 ⫽ C4H4 ⫹ HO2 C2H3 ⫹ M ⫽ C2H2 ⫹ H ⫹ M C2H3 ⫹ O2 ⫽ C2H2 ⫹ HO2 C2H2 ⫹ O ⫽ HCCO ⫹ H C2H2 ⫹ O ⫽ CH2 ⫹ CO CH2 ⫹ O2 ⫽ H ⫹ OH ⫹ CO CH2 ⫹ O2 ⫽ CO ⫹ H2O HCCO ⫹ O2 ⫽ OH ⫹ CO ⫹ CO CO ⫹ O ⫹ M ⫽ CO2 ⫹ M CO ⫹ O2 ⫽ CO2 ⫹ O CO ⫹ OH ⫽ CO2 ⫹ H CO ⫹ HO2 ⫽ CO2 ⫹ OH HCO ⫹ M ⫽ H ⫹ CO ⫹ M HCO ⫹ O2 ⫽ CO ⫹ HO2 HCO ⫹ H ⫽ CO ⫹ H2 HCO ⫹ O ⫽ CO ⫹ OH HCO ⫹ OH ⫽ CO ⫹ H2O
Reaction coefficient is described as aT n exp(⫺Ea/RT).
2.20E 6.30E 2.11E 2.78E 2.50E 2.09E 7.94E 4.34E 1.81E 6.00E 2.21E 1.15E 2.81E 6.00E 6.00E 4.91E 1.00E 1.00E 3.00E 3.00E 2.00E 1.99E 3.43E 2.19E 1.81E 6.00E 6.00E 3.16E 7.94E 2.10E 1.00E 3.98E 2.98E 1.20E 2.98E 1.20E 5.80E 1.40E 6.02E 2.41E 1.46E 2.51E 2.51E 1.50E 6.03E 1.86E 4.17E 7.24E 3.02E 3.02E
⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹ ⫹
14 13 13 13 14 12 14 11 14 12 13 14 13 12 12 12 14 14 13 13 13 12 09 08 13 12 12 11 14 13 15 11 33 11 33 11 06 06 11 11 12 13 12 07 13 17 12 13 13 13
n
Ea cal/mol
Reference
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1.18 1.77 0 0 0 0 0 0 0 0.7 ⫺5 0 ⫺5 0 2.09 2.09 0 0 0 0 0 1.3 0 ⫺1 0 0 0 0
0 60,000 4,570 4,910 16,000 7,470 55,400 62,892 40,658 0 7,910 12,400 7,352 0 0 4,400 0 0 0 0 25,000 11,660 ⫺447 3,000 3,080 0 0 8,000 55,400 48,000 78,000 42,260 44,320 0 44,320 0 1,562 1,562 0 0 2,500 ⫺4,540 47,690 ⫺765 22,950 17,000 0 0 0 0
16 16 16 16 16 16 21 18 see text 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 21 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16
448
S. NODA ET AL.
The Effect of Phenol Addition on the Thermal De-NOx Process As shown in the results section, the important role of NH3 suggests that the mechanism of the phenol addition should be similar to the effect of other additives on the thermal de-NOx process. Kinetic Model for Analyzing the Effect of Phenol Addition The CHEMKIN simulations [23] were carried out, based on the following kinetic models: the benzene oxidation model (B1–B6, B10 –B50 in Table 3) proposed by Emdee et al. [16] and the NO/NH3/O2 system model (A1–A37, A40 – A105 in Table 3) proposed by Glarborg et al. [10]. The data of Park and Lin [11] and Bruinsma et al. [18] were used for NH2 ⫹ NO reactions (A38, A39) and phenol pyrolysis (B8), as discussed in previous sections. The unimoleculer decomposition rate of phenoxy radical (B7) was determined by ab initio calculations [21], which was used instead of that in the Emdee model. Comparison of the Model Calculation and Experimental Data Figure 7 shows the calculated results of NO profile in the case of phenol addition and no phenol addition. Phenol addition causes only 50 K shift of the NO profile to lower temperatures, and has almost no effect under 1023 K (750°C). In phenol oxidation chemistry, the following reactions are important as discussed in “Phenol Oxidation Mechanism”: C6H5OH 3 C6H5O ⫹ H
B8 in Table 3
C5H5 ⫹ OH 3 C5H4OH ⫹ H C5H5O 3 C4H5 ⫹ CO
B20 in Table 3
B29 in Table 3
Though the most sensitive rates (B8 and B20 in Table 3) for phenol oxidation were changed by a factor of 2, NO profile varied within 10%. The Effect of Phenol Addition on the Thermal De-NOx Process above 1050 K (777°C) Schuhlmann and Rotzoll [5] investigated the influence of CO on the thermal de-NOx process.
Fig. 7. Calculation result for the effect of phenol solution addition. (Reaction B9 in Table 3 not included.) Reaction rate constants are listed in Table 3 (B9 is not included). Initial mole fractions: [NO]0 ⫽ 400 ppm, [NH3]0 ⫽ 400 ppm, [O2]0 ⫽ 4 vol%, [H2O]0 ⫽ 7.4%, residence time ⫽ 1072/(T ⫹ 273) (T: [°C]). The dotted, dashed, and solid line show NO profiles (1) without phenol solution, (2) with CO 600 ppm addition, and (3) with phenol 100 ppm addition, respectively.
According to their report, the addition of CO promotes radical reactions and shifts the temperature range to 777– 827°C. The following mechanism of CO addition was proposed. CO ⫹ OH 3 CO2 ⫹ H
(R15)
H ⫹ O2 3 OH ⫹ O
(R16)
CO is oxidized in R15. Since reaction R15 is followed by R16, CO provides additional radicals that overcome the radical termination reactions. Because CO is also generated in the process of phenol oxidation in our system as shown in Fig. 5, the effect of the produced CO on the thermal de-NOx process was treated using the kinetic model (A1–A105, B42–B45 in Table 3). In order to estimate the upper limit of the CO effect on the thermal de-NOx process, it was assumed that 100 ppm of phenol was momentarily oxidized to form 600 ppm of CO. The initial condition of the calculation was [NO]0 400 ppm, [NH3]0 400 ppm, [O2]0 4%, [CO]0 600 ppm, [H2O]0 7.4%, and reaction time 1.0 s. The calculated result is also shown in Fig. 7. The kinetic model indicated that the calculated NO profile in the case of phenol addition approached that in CO addition, when the rate
ADDITION OF PHENOL TO NONCATALYTIC REDUCTION
449
constants of phenol oxidation were increased by a factor of 2. It is considered that CO produced through phenol oxidation, caused the NO decrease at higher than 1050 K (777°C). The Effect of Phenol Addition on the Thermal De-NOx Process under 1050 K (777°C) Hjuler et al. [24] investigated the mutually promoted oxidation of various organic compounds and NO in the presence of oxygen, using laboratory flow reactors. Various kinds of hydrocarbons (methanol, ethane, acetone, acetylene, acetaldehyde, and methylamine) were tested in the temperature range of 650 –1300 K. Nitrogen oxide is oxidized to NO2, whereas the hydrocarbons are converted mainly to CO at relatively low temperature, e.g. from 850 to 1050 K in the case of methanol. For a methanol/NO ratio close to 1, an oxidation efficiency was reported to be 80 –90% at around 950 K. Hjuler et al. explained experimental results for mutual oxidation of CH3OH and NO, using the following elementary reactions: NO ⫹ HO2 3 NO2 ⫹ OH
(R19)
CH3OH ⫹ OH 3 CH2OH ⫹ H2O
(R20)
CH2OH ⫹ O2 3 CH2O ⫹ HO2
(R21)
NO is oxidized almost solely through R19 whereas the relatively unreactive HO2 radical is converted to a reactive one OH. The main source of HO2 is hydrogen abstraction by O2 from CH2OH, not from CH3OH nor CH3O at 850 –1050 K. Consequently, NO oxidation by hydrocarbons strongly depends on the rate of the corresponding abstraction reactions. Figure 3 shows NO2 formation while NO decrease occurs by phenol addition. It could be considered that NO2 formation results from reactions of NO and phenol and/or byproducts during phenol oxidation. NO is oxidized to NO2, whereas OH would react not only with hydrocarbons but also with NH3 to form the selective reduction agent NH2. In terms of reactions of hydrocarbons with molecular oxygen, reactions of benzene (C6H6), phenyl radical (C6H5), cyclopentadiene (C5H6), butadienyl radical (C4H5), C2H3, and HCO were included in Emdee’s model [16], while a
Fig. 8. Calculated results for the effect of phenol solution addition. (Reaction B9 in Table 3 included). Reaction rate constants are listed in Table 3 (B9 is included). Initial mole fractions: [NO]0 ⫽ 400 ppm, [NH3]0 ⫽ 400 ppm, [O2]0 ⫽ 4 vol%, [H2O]0 ⫽ 7.4%, residence time ⫽ 1072/(T ⫹ 273) (T: [°C]). The dotted and the solid line show the calculated profiles in the kinetic model (1) where B9 is not included, and (2) B9 is included.
reaction with phenol was ignored because the bond energy of O™H in phenol has a high value of 87 kcal/mol. H abstraction reactions from hydrocarbons become dominant at relatively low temperature range (650 –1000 K) where chain-branching steps cannot sustain the number of radicals. In order to estimate the abstraction reaction from phenol, CHEMKIN simulation was carried out, based on the kinetic model (A1–A105, B1–B8, B10 –B50 in Table 3) discussed above, and the phenol ⫹ O2 reaction (B9). By analogy to the reaction of CH2O ⫹ O2 [25], the rate constant for phenol ⫹ O2 reaction was assumed to be 6.03 ⫻ 10 13 ⫻ exp(⫺20,460/T), because the reaction enthalpy for CH2 ⫹ O2 (33.8 kcal/mol at 1050 K) is close to that for phenol ⫹ O2 (152 kJ/mol at 1050 K). Calculated profiles of NO and NO2 are shown in Fig. 8. The rate for the phenol ⫹ O2 reaction was changed within the reported uncertainty of 3. Simulation results indicated that NO was converted mainly to N2, partially to NO2, whereas NH3 was constantly oxidized, resulting in reducing NO through the thermal de-NOx mechanism. In terms of the mass balance of nitrogen atoms, 52 ppm of NO and 33 ppm of NH3 were converted to 19 ppm of NO2 and 33 ppm of N2 (66 ppm in terms of N atoms) at 723°C, in the event that the rate for phenol ⫹
450 O2 was increased by a factor of 3. It should be noted that mutual oxidation of NO and phenol could generate additional OH radical along the similar steps to R19 –R21, and oxidize NH3 even under 1050 K (777°C), while some amount of NO is oxidized to NO2. The simulation suggested the potential of phenol for NO reduction at lower than 1050 K, but indicated less effect of phenol addition than that in the experimental results (Fig. 2). This is because of the complicated channel of either the H abstraction reactions from phenol and/or byproducts during phenol oxidation by O2, or the interaction of phenol fragments with the NO/NH3/O2 system. Further experimental and theoretical research is needed to clarify the effect of phenol addition on the thermal deNOx process.
S. NODA ET AL. 4. 5. 6. 7. 8.
9. 10. 11. 12. 13. 14.
CONCLUSIONS We investigated a new approach for reducing the NO concentration in combustion products using phenol aqueous solution, which is one of the industrial waste products in the petroleum refinement process. The effect of adding phenol solution on the thermal de-NOx process was experimentally investigated in detail. The experimental conditions were [NO]0 ⫽ 400 ppm, [NH3]0 ⫽ 400 ppm, [O2]0 ⫽ 4.0%, [phenol]0 ⫽ 0 – 400 ppm, [H2O]0 ⫽ 1.7–7.4%, residence time ⫽ 1.0 –1.2 s, and temperature ⫽ 650 – 820°C. A small amount of phenol (phenol/ NO ⫽ 0.25) decreased the NO concentration significantly in the temperature range of 680 – 820°C, e.g. about 69% of NO was converted at 728°C. The injected phenol was completely oxidized to form CO or CO2 at temperatures higher than 728°C. REFERENCES Kinball-Linne, M. A., and Hanson, R., Combust. Flame 64:337–351 (1986). 2. Miller, J. A., and Bowman, C. T., Prog. Energy Combust. Sci. 15:287–338 (1989). 3. Lyon, R. K., US Patent 3,900554, Aug. 19, 1975.
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Received 27 May 1997; revised 7 January 2000; accepted 16 February 2000