Ind. Eng. Chem. Res. 1996,34, 1882-1888
1882
Mutually Promoted Thermal Oxidation of Nitric Oxide and Organic Compounds Klaus HjulerJ Peter Glarborg," and Kim Dam-Johansen Department of Chemical Engineering, Building 229, Technical University of Denmark, DK-2800Lyngby, Denmark
The mutually promoted oxidation of various organic compounds and nitric oxide in the presence of oxygen was studied in a laboratory scale flow reactor. Experiments were performed at atmospheric pressure in the temperature range 650-1300 K. The results show that in the proper temperature range a simultaneous oxidation of the organic compound and nitric oxide occurs. Nitric oxide is oxidized to nitrogen dioxide, while the combustible is converted mainly to CO. Methanol and methylamine were found to be effective oxidizers; the highest conversion was obtained with methanol, while methylamine has the widest temperature window for oxidation. Acetaldehyde, acetone, and ethane were less efficient in oxidizing NO. The efficiency of a given compound in oxidizing NO can be attributed to its oxidation mechanism as it depends on the production of peroxyl radicals, HOz. Temperature, molar ratio of the additive to NO, and reaction time were shown to be important parameters for the process performance. For a methanoV nitric oxide ratio close to 1, a n oxidation efficiency of 80-90% could be obtained in a narrow temperature regime around 950 K. At lower molar ratios, the conversion of NO decreased. Very short reaction times (10-15 ms) caused the temperature window to shift to higher values while the NO oxidation potential decreased. The presence of water vapor appears to have a small beneficial impact on the oxidation efficiency. The implications of the present results for the practical application of the process are discussed.
Introduction The oxidation of nitric oxide to nitrogen dioxide by a combustible additive may constitute a promising NO, control strategy if nitrogen dioxide can be removed economically, for instance, in existing wet scrubbers for flue gas desulfurization. Moreover, the presence of nitrogen dioxide may enhance the performance of nitric oxide reduction processes where surface plays an essential role, such as selective catalytic reduction by ammonia and sodium oxide sorption processes. The oxidation of NO t o NO2 by agent injection has been investigated previously for methanol (Murakami et al., 1982a; Lyon et al., 1990; Hjuler and DamJohansen, 1993) and vari0u.s other compounds (Murakami et al. 1982b). The results show that a number of combustible species are able to promote the oxidation of NO. Simultaneously with the conversion of NO to NOz, oxidation of the added compound is enhanced. The degree of nitric oxide conversion varies, however, as it depends on the production rate of peroxyl (HO2') radicals during combustion. The results of Murakami et al. (1982a,b) show that, for methanol, formaldehyde, and hydrogen peroxide, a molar ratio to NO of about 1was enough for reaction to take place, but ratios several tens of times higher were needed for methane and hydrogen. Except for hydrogen peroxide, the presence of oxygen was needed to promote the oxidation process. In the present work, a number of combustibles (methanol, ethane, acetone, acetaldehyde, and methylamine) are tested for their ability t o oxidize nitric oxide in the 650-1300 K temperature range in laboratory flow reactors. These organic compounds represent various types of reactants, i.e. alcohol, alkane, ketone, aldehyde, and amine, and the data obtained significantly expand the range of components tested as oxidizing agents. Furthermore, the present study establishes an experimental data base that describes the oxidation behavior
* To whom correspondence +
should be addressed.
Present address: dk-TEKNIK, DK-2860 S~borg,Denmark. 0888-5885/95/2634-1882$09.00/0
of the tested components, in both the absence and presence of nitric oxide. The experiments are performed under well-controlled conditions (isothermal, plugflow), and the data are accurate enough to be used for the development and testing of detailed oxidation schemes for the components investigated. For several of the fuels tested, few reliable oxidation data are available in the literature. The present results may also be helpful in understanding the proportion of NO2 to NO, in the exhaust from combustion systems. This ratio is important not only in designing exhaust gas treatment systems but also in the analysis of atmospheric and indoor NO, behavior (Hori et al., 1992). NO2 formation in the exhaust gas has been shown t o be affected by the presence of unburnt fuel fragments (Hori et al., 1992; Bromly et al., 1992 and references therein), and the chemistry involved is expected to be similar to the chemistry of the present study.
Experimental Section The flow reactor setup, which has previously been used to study various aspects of nitrogen chemistry ( e g . Duo et al., 1990; Johnsson et al., 1992; Hulgaard and Dam-Johansen, 1993; Glarborg et al., 1994a), is described in detail elsewhere (Kristensenet al., 19941, and only a brief description is given here. Figure 1 shows the reactor design. The reactor was designed to minimize dispersion, and a very good plug-flow approximation is obtained (Glarborg et al., 1994b; Kristensen et al., 1994). The reactor is placed in an electrically heated oven with three separately controlled heating zones, securing a uniform temperature profile within f 7 K throughout the reaction zone. The reactor has one inlet for the main stream (containing nitrogen, oxygen, and water vapor, if desired) and three side stream inlets for the injection of reactants. The main stream is preheated internally before being mixed with the reactant gases at the inlet point of the flow reactor tube. This secures a well-
0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 5, 1995 1883 Table 1. Experimental Conditione set additive (ppm) NO (ppm) HzO ( ~ 0 1 % )residence time (s) ____
1 2 3 4 5 6 7 8
-20 E m
l+U -
-10 E m
-
0 om
-
10 cm
-
20 em
I
0 0
477 580 580 580 0
504 0
508 0
485 0
47 1
trace trace trace 0.7 0.7 0.7 trace trace trace trace trace trace trace trace
187 WT 182 WT 199 WT 218 WT 219 WT 13.3 WT 202 WT 202 WT 205 WT 205 WT 200 WT 201 WT 203 WT 203 WT
a The experiments are conducted at constant mass flow. Thereby, the residence time is dependent on the temperature, as listed (T denotes the temperature; K is the unit, Kelvin).
The concentrations of CO, C02, and NO were measured by means of W and IR range spectrophotometers. The uncertainty on these measurements was f3% but not less than 5 ppm. 0 2 was measured thermomagnetically with an accuracy of f 3 % (relative). NO2 was converted to NO a t 613 K using a molybden oxide converter and measured by the NO analyzer. The uncertainty in the NO2 measurement is largely due to the uncertainty in the conversion efficiency; we estimate it to be &20% and not less than 20 ppm. In selected experiments, HCN (FTIR),NH3 (pH sensitive electrode or FTIR), and N2O (spectrophotometric or FTIR) were measured. Except for the spectrophotometric measurement of NzO ( f 3 %but not less than 5 ppm), we estimate an uncertainty for these species of f20% but not less than 10 ppm.
-
9 10 11 12 13 14
CH30H (486) CH30H (558) CH30H (419) CH30H (525) CH30H (278) CH30H (283) CH3CHO (239) CH3CHO (238) (CHdzCO (243) (CHdzCO (243) CzHs (237) (C2Hs) (270) CH3NH2 (261) CH3NH2 (260)
30 em
6
Figure 1. Quartz flow reactor design: (1)injectors, (2) main flow inlet, (3) cooling air inlet, (4) reactor outlet, (5) premixing of injected flows, and (6) reactor tube.
defined reaction volume. The reaction mixture is quenched at the reactor tube outlet using cooling air. The flow system was designed to mix a simulated flue gas (0.2-2.0 nUmin) containing up to eight different components and water. The individual gases were supplied from gas cylinders using mass flow controllers. Water vapor was added by saturating a nitrogen stream. Methanol and acetone were fed into the system by equilibrating a small stream of nitrogen in a thermostated bubble flask at 263 K. The flow rate of nitrogen necessary was determined by the vapor pressures at the thermostat temperature and the desired inlet concentrations. The inlet concentrations of the organic compounds were checked as the sum of the outlet CO and C02 concentrations at about 1300 K where complete oxidation takes place. The flow system and reactor temperature were controlled automatically by data acquisition and control software. The reactor temperature was measured with a NiCr-Ni thermocouple protected by a quartz tube.
Experimental Results The experiments were first carried out with an organic compound and oxygen only and then with nitric oxide present. The inlet conditions for the various experiments are listed in Table 1. Concentrations are given as volume fractions (ppm, dry gas basis) for convenience. The effect of methanol was studied at two inlet additive/NO molar ratios (about 1 and 0.51, with and without water present, and a t two residence times (about 200 and 13 ms). Two quartz flow reactors with different reaction volumes (0.4015.6 and 0.89118.5 cm, respectively, i.d./length) were used in the present study, both designed t o give a minimum of axial dispersion at essentially isothermal conditions. The 200 ms residence time experiments were all carried out in the large reactor, while the short residence time sets were conducted in the small reactor. The experiments with organic compounds other than methanol were carried out at an inlet additive/NO molar ratio of about 0.5 and at residence times of about 200 ms. All experiments were conducted with about 4 vol % of oxygen present, and the temperature range covered was typically 673 1273 K. The total flow rate was about 1 nUmin (Le. at 298 K and 1 atm). The results for methanol oxidation in the absence of NO are shown in Figure 2. The figure shows the conversion of methanol to CO and CO2 as a function of temperature for two sets of experiments with similar conditions. The results show a characteristic narrow temperature range where significant CO emission occurs. CO formation is initiated above 850 K and peaks around 1000 K. Above this temperature, CO is rapidly converted to CO2. The agreement between the two data sets is fairly good, indicating a good reproducibility of the experiments.
1884 Ind. Eng. Chem. Res., Vol. 34, No. 5, 1995 1.2
1.0
0.8
,
I
**-
1
C O ‘(set ~ 2) ,0”
f.”
v
-
1.4 -AAAAACO
(set 3)
COB (set 3)
1.2 -
CO (set 4) C O ~(set
4)
Figure 4. Impact of the CH30I+NO molar ratio on the oxidation of NO and CH30H in the large reactor. The solid symbols denote an inlet C H 3 0 ~ O molar ratio of 0.9, and the open symbols denote an inlet ratio of 0.5. The inlet conditions are given in Table
1.0 E
35 0.8
0, u
3:0.6
1.
b0.4
low-temperature boundary for the oxidation of NO to NO2 coincides with the onset of CO formation, while the upper bound for oxidation apparently is linked to the onset of CO to CO2 conversion. This suggests that the NO oxidation process is closely coupled to the fuel oxidation chemistry. The presence of water vapor (about 0.7 vol %) has little impact on the location and size of the temperature for NO oxidation (Figure 3). The addition of water has no appreciable effect on the CO formation, while it increases the CO2 formation rate significantly above 1050 K. Furthermore, the presence of HzO appears to enhance slightly the oxidation of NO to NOz. Figure 4 shows the effect of the molar ratio of CH3OH to NO on the system (in the presence of water). The results are shown for additive/NO ratios of approximately 0.9 (open symbols, also shown on Figure 3) and 0.5 (solid symbols). The lower stoichiometric ratio results in a decrease in NO to NO2 conversion potential, from about 90 to below 60%. However, the temperature regime for oxidation is not affected, and the change in molar ratio has little impact on the CO and COZprofiles. Figure 5 shows the effect of reaction time on the system. Shifking from the large to the small reactor corresponds to a change in reactor residence time of about a factor of 16 at constant mass flow. In these experiments, the inlet CH30WN0 molar ratio was about 0.5 and water was added to the system. Lowering the residence time from about 200 to about 13 ms causes the optimal temperature for NO oxidation to shift upward by about 100 K, while the fractional conversion of NO is lowered from about 0.55 to about 0.35. The CO maximum is also shifted about 100 K upward. Figure 6 shows the results obtained with acetaldehyde with and without NO. Notice that the fractional conversions of the organic compound to CO and C02 have been normalized with a factor of 2 as acetaldehyde
0
0.2
1
A
- d L .
0.0 1.4
1.2
00.8 h0.6 0.4
0.2 0.0 600
700
800
900
1000
Temperature
1100
/
1200
1:
D
K
Figure 3. Oxidation of NO and CH30H in the large reactor with (solid symbols) and without (open symbols) water present. The inlet CH30H/NO molar ratio was about 1. The inlet conditions are given in Table 1.
Figure 3 shows the impact of NO on the system, with (solid symbols) and without (open symbols) addition of water. The upper part of the figure shows the conversion of CH30H to CO and COz. It is seen that the presence of NO has a significant effect on the CO profile. As in Figure 2, CO formation is initiated at around 850 K, but it now reaches a plateau peak level, which ranges from 900 to above 1000 K. Above 1050 K, oxidation of CO becomes competitive, and C02 is the major product. The lower part of Figure 3 shows the conversion of NO to NOz. For a molar ratio of CH3OH to NO of about 0.9, 80-90% oxidation of NO to NO2 can be achieved under the present conditions. The NO oxidation window is quite narrow; approximately from 850 to 1050 K. This temperature range corresponds to the CO plateau. The
Ind. Eng. Chem. Res., Vol. 34, No. 5, 1995 1885
-
m
1.4
c
-Coz
1.2
-Cot
o (set 5)
(set 5 )
CO (set 6) (set 6 )
1.o
-1.0
E
P
$-0.8 zm0.6
SO.8
N n
0, 3:0.6 u
V x0.4
20.4 0 0.2
m 0.0 1.4
0.0 1.4
I
NOZ '(set 10)
1.2
00.8 20.6
20.6
0.4
0.4
0.2
0.2
0.0
T
600
700
800
900
1000
Temperature
1100
/
1200
I
1:
K
Figure 5. Impact of the residence time on the oxidation of NO and CH30H. The solid symbols denote results from the large reactor (a residence time of 219 ms at 1000 K),and the open symbols denote results from the small reactor (a residence time of 13 ms at 1000 K). The inlet CH30H/NO molar ratio was about 0.5. The inlet conditions are listed in Table 1.
h
.I1.o
0
u,0.8
5
1
0.6
X K! 0.4
W
20.2
u 0.0 1.4
I
1.2
0.2 0.0
600
700
800
900
1000
Temperature
1100
/
1200
1300
K
Figure 6. Oxidation of NO and acetaldehyde in the large reactor. The inlet CH&HO/NO molar ratio was about 0.5. The solid lines indicate data obtained with NO present, whereas the dashed lines indicate data obtained without NO present. Some of the data were reproduced by decreasing the oven temperature stepwise after the normal heating procedure. The inlet conditions are listed in Table 1.
contains two carbon atoms. In the absence of NO (dashed lines, solid symbols), the oxidation behavior is similar to that obtained with methanol. CO peaks at a
600
700
800
900
1000
Temperature
1100
/
1200
1: 30
K
Figure 7. Oxidation of NO and acetone in the large reactor. The inlet (CH&COiNO molar ratio was about 0.5. The solid lines indicate data obtained with NO present, whereas the dashed lines indicate data obtained without NO present. The inlet conditions are given in Table 1.
slightly higher temperature (1075 K us 1000 K for methanol), but the CO and COSprofiles are qualitatively very similar. When NO is added, however, the results are quite different from those obtained with methanol. The presence of NO shifts the CO peak about 200 K, down to about 875 K, where it coincides with the maximum NO conversion. However, the CO peak does not exhibit a plateau; instead, CO declines steadily with increasing temperature. Contrary to the results for methanol (and several of the other compounds tested), there is no shift from a slow or negligible CO oxidation regime to a fast CO oxidation regime. It is interesting to note that the CO oxidation rate above 1100 K without NO present is higher than with NO, i.e. the presence of NO inhibits the CO oxidation under these conditions. Despite the significant impact of NO on the location and shape of the CO peak, the efficiency of acetaldehyde in oxidizing NO is apparently low, of the order of 20% under these conditions. Figures 7 and 8 show the results obtained with acetone and ethane, respectively. The CO and C 0 2 profiles of acetone have some similarities with methanol, while the profiles of ethane have similarities with acetaldehyde. The NO to NO2 conversion efficiencies of acetone and ethane are relatively low. The results obtained with methylamine differ significantly from those obtained with the other hydrocarbons (Figure 9). The NO oxidation potential is fairly high, approximately 50% with the additive/NO molar ratio of 0.5, and the temperature window is wider than for any of the other tested compounds, ranging from about 700 to 1075 K. The fractional conversion of CH3NH2 to CO is lower, however, and the measurements show that significant quantities of reactive nitrogen species other than NO, are released a t 785 K, z.e. about 80 ppm of HCN and 30 ppm of NH3. While only a few ppm of NzO
1886 Ind. Eng. Chem. Res., Vol. 34, No. 5, 1995 0.6
0"
Z 0.5
1 .o
A-
nlet _ _ _ _Molar _ _ - _Ratio _______
0
h . I
60.8
c 0.4
3
0 .+
0.6
\ \
v)
X cv -0.4
L,
u
\
* 0.3
>0.2 0
Methanol Acetaldehyde &&&A Acetone M Ethane weee~Methylamine _ _ Equilibrium OeeeD
i)
G
-
______
u
u
3
0.0 1.4
(c 0.2
j
1.2
..1
3
% 0.1
r,
E4
0.0 I
10.6
700
I
800
900
1000
Temperature
z 0.4
1100
/
1200
1300
K
Figure 10. Comparison of the nitric oxide oxidation capabilities of the compounds tested. Also shown (dashed line) is the calculated equilibrium fraction of NOz.
0.2
,
0.0
600
3oQoIscQ 700
800
900
1000
Temperature
1100
/
1200
1
I
K
Figure 8. Oxidation of NO and ethane in the large reactor. The inlet C z H O O molar ratio was about 0.5. The solid lines indicate data obtained with NO present, whereas the dashed lines indicate data obtained without NO present. Some of the data were reproduced by decreasing the oven temperature stepwise after the normal heating procedure. The inlet conditions are given in Table 1. 1.4 1.2
A
1.o e
kO.8 30.6
E
The NO oxidation capabilities and the position of the temperature windows of the various organic compounds tested are compared in Figure 10. The methylamine window is significantly wider than the others, while methanol is the most efficient oxidizing agent in the proper temperature range. The windows of acetaldehyde, acetone, and ethane are positioned within that of methanol. Also shown is the calculated thermodynamical equilibrium fraction of NO2 at 4 vol % of 0 2 , using data from Barin (1989). Notice that the conversions obtained are in excess of the corresponding equilibrium values. This occurs because the NO oxidation is controlled by superequilibrium concentrations of radicals (HOz' as discussed below). Both in the present experiments and in practical systems, the characteristic reaction times are too short for the NOM02 ratio to reach equilibrium.
?/
TOS4 0.2 0
Discussion The mechanism for the mutual oxidation of CH30H and NO has been discussed by Lyon et al. (1990), and the overall chemistry for this process is fairly wellestablished. The oxidation of NO to NO2 proceeds almost solely through the reaction.
u 0.0 1.4 1.2
1.o
NO
G
00.8 2
+ HO,'
* NO,
(1)
Methanol is suited to being an oxidizing agent because HOz' is readily formed during its oxidation. This happens through the sequence (Lyon et al., 1990)
h0.6 0
0.4
+ OH' * CH,OH' + H,O CH,OH' + 0, CH,O + HO,'
CH30H
0.2 0.0
700
800
900
1000
Temperature
1100
/
1200
1300
K
Figure 9. Oxidation of NO and methylamine in the large reactor. The inlet CHsNHflO molar ratio was about 0.5. The solid lines indicate data obtained with NO present, whereas the dashed lines indicate data obtained without NO present. The inlet conditions are given in Table 1.
is observed a t this temperature, our results show that NzO constitutes an important product above 1075 K (Figure 9).
(2)
(3) Reactions 1-3 form a chain-propagating sequence. Apart from oxidizing NO to NOz, eq 1 converts a relatively unreactive radical, HOz', to a reactive one, O H . Thereby, the reaction enhances the oxidation rate of the methanol, shifting the temperature regime for oxidation toward lower values. The sequence of reactions 1-3 is useful in illustrating the characteristics of the mutual oxidation of methanol and nitric oxide. However, a quantitative description of the process requires detailed knowledge of the =L
600
+ OH'
Ind. Eng. Chem. Res., Vol. 34,No. 5, 1995 1887 complex chain-branching mechanism by which methanol oxidizes. One complexity that was not considered in previous work (Lyon et al., 1990)involves competition between the formation of CH20H' and CH30' (Norton and Dryer, 1989; Grotheer et al., 1992)
-
+ OH' CH30H + OH'
CH30H
CH,OH'
--L
CH3U
+ H20
+ H,O
(2) (4)
It is important to distinguish between CH20H' and CH30' because the reaction characteristics of these isomers are different. CH20H' reacts preferably with 0 2 , leading to HOf (eq 3), while CH30' easily dissociates, forming H' atoms, CH30'
+ M-
+
CH20 H'+ M
(5) Thereby, CHzOH', but not CH30, acts to promote the oxidation of NO, and the significant potential of methanol for oxidizing NO is due to a fairly high selectivity for producing CH20H' a t the current temperatures. Work is currently underway in our laboratory to develop a detailed chemical kinetic model for the process. The potential of a particular compound for oxidizing NO depends on the ability of this species to produce significant amounts of HOz' radicals during its oxidation. By analogy with reaction 3, the most likely candidate for HO2' production is hydrogen abstraction by 0 2 from a fuel-derived radical, R. Direct reaction between the fuel compound and 0 2 is generally negligible a t the present temperatures due to a high activation energy. An alternative formation pathway for HOz' is the recombination of H' atoms with 0 2 ,
H' + 0,
- + -+
+M
HO,' M (6) This reaction is most important at comparatively low temperatures and high pressures, where it competes favorably with the chain-branching reaction
H'+ 0,
0' OH' (7) However, eq 6 alone cannot provide sufficient HO2' radicals for a significant oxidation of NO. The potential for NO oxidation of a given compound can ideally be assessed from analysis of the corresponding R 0 2 reaction. Complications often arise, however, either because the fuel such as methanol fragments into several isomers with different reaction characteristics or because the R + 0 2 reaction is a complicated multiproduct channel reaction, for which little information is available a t higher temperatures. For many organic compounds, it is thus difficult a priori to evaluate their ability for oxidizing NO and experimental work is generally required. The reactions of the initial derivatives (R)of acetaldehyde (CH3CO), acetone (CH2COCH3'), and ethane (C2H5') with 0 2 all have several competing product channels, with the addition channel dominating at low temperatures (Mallard et al., 1994). Neither of these reactions is well-characterized at higher temperatures. However, it is obvious from our experiments that the HO$-producing channels of these reactions are of minor importance under the present conditions. Reactions of methylamine (CH3NH2)with 0' and OH' can potentially lead to both CH2NH2' and CH3NH'. The selectivity for each of these isomers and the characteristics of the subsequent reactions of the isomers with 0 2 are unknown at present. However, from the rela-
+
tively high efficiency of methylamine in oxidizing NO, it would be expected that significant amounts of peroxyl radicals are formed. It is evident from Figure 9 that the interaction of methylamine with NO is much more complicated than that of the other organic compounds tested. Two separate temperature regimes for NO removal can be identified. The lower temperature window can be associated with NO to NO, conversion, as discussed above. Significant oxidation of the NH3 and HCN formed in this regime appears to be inhibited, probably due to insufficient temperature and reaction time. In the upper window, NH3 and HCN would be expected to react readily, and the observed removal of NO is probably due to reaction with these species. This suggestion is supported by the detection of N20, which is known to be a byproduct of the HCN/NO and NHd NO reactions (Hulgaard and Dam-Johansen, 1993). Lyon et al. (1990) report that the CH30W02/N0 system interacts with the sulfur chemistry if S02/S03 is present. Besides oxidizing NO, methanol acts to reduce sulfur trioxide t o sulfur dioxide. To assess the impact of SO2 on the present systems, a few experiments with methanol and methylamine were carried out with SO2 addition. Approximately 500 ppm of SO2 was added a t the optimum temperature for NO oxidation: 950 K for methanol (conditions correspondingto set 5,Figures 4 and 5) and 785 K for methylamine (conditions corresponding t o set 14, Figure 9). The results showed that, under the conditions investigated, addition of SO2 had no observable effect on the results. This observation is in agreement with chemical kinetic considerations. To have a significant impact on the system, SO2 must either affect the radical pool (particularly HO2') or interact directly with NO or NO2. However, the reactions SO2 HOn', SO2 NO, or SO2 NO2 are all slow at the present temperatures (Mallard et al., 1994). Contrary to S02, so3 reacts fairly rapidly with HO2' (Mallard et al., 19941, leading eventually to SO,, as observed by Lyon and co-workers (1990).
+
+
+
Practical Implications Injection of a combustible compound in order to oxidize NO to NO2 in practical combustion systems has a significant potential as an integral part of various NO, reduction schemes (Lyon et al., 1990; Hjuler and DamJohansen, 1993; Pont et al., 1993). In the present work, a number of potential additives have been tested, and it is possible to assess their suitability for this purpose. Our results confirm the observations of other studies ( e g . Lyon et al., 1990) that methanol has a high potential for NO oxidation. Methanol was clearly the most efficient compound of those tested. Furthermore, methanol has a number of practical advantages compared t o other potential additives. The availability and cost of methanol is very competitive, and its presence as a liquid is advantageous in transport and handling. The major drawback for methanol is the narrow temperature regime for NO oxidation. Methylamine exhibits a wider temperature window for reaction but is less suitable due to an increased potential for pollutant byproduct emission. Methylamine exhibited a low conversion efficiency t o CO/C02, below 30%, a t the temperatures of interest. Furthermore, the use of methylamine involves the risk of emitting HCN and NH3 and possibly NzO. The parametric study of methanol addition has identified some of the important process parameters. P.part from temperature, both additive/NO molar ratio and
1888 Ind. Eng. Chem. Res., Vol. 34,No. 5, 1995
reaction time are important for the process performance. In the laboratory experiments of the present work, a methanoynitric oxide ratio of about 1was sufficient to obtain an 80-90% oxidation of NO (under isothermal conditions). In practical systems, problems with obtaining complete mixing between the oxidizing agent and the flue gas would be expected to lower the oxidation efficiency. Another limitation expected in practical systems is the time available for reaction, particularly when retrofitting the technique. The present results have shown that, at very short reaction times, the oxidation potential decreases significantly and the temperature regime for oxidation is shified to higher temperatures. Even though results indicate that 50-60 ms are sufficient for the process under bench scale conditions (Lyon et al., 19901, reaction time may still be of concern due to the mixing difficulties and the considerable temperature gradients encountered in practical units. The lower reduction potential expected in practice may to some extent be compensated for by increasing the additive/NO molar ratio. However, due to the characteristics of the process chemistry, a trade-off between NO reduction and CO emission is necessary. It is obvious from the present experiments as well as others that the oxidation of CO to CO2 is very slow under the conditions where NO2 formation is favored. Thereby, CO emissions become a practical concern when applying this technology. Pilot scale results confirm that CO emissions can be significant (Pont et al., 1993). Since the additive that promotes NO oxidation will have to be injected at fairly low temperatures, it will be very difficult to enhance CO oxidation downstream of the NO NO2 zone in practical systems.
-
Conclusions An experimental study of NO oxidation by addition of organic compounds has been conducted. Parameters investigated were the type of additive, additive/NO molar ratio, temperature, reaction time, and effect of water vapor. Our results confirm the findings of previous studies in that methanol has a high potential for oxidizing NO, approaching 90 and 60%, respectively, for m e t h a n o m 0 molar ratios of 0.9 and 0.5. Oxidation takes place in a narrow temperature regime, under our conditions typically 850-1050 K, where methanol is also converted t o CO. At short reaction times, the temperature regime for oxidation is shifted toward higher values and oxidation efficiency decreases. Addition of water vapor slightly enhances the oxidation efficiency but has no impact on the location of the temperature window. Of the other compounds tested, only methylamine was found to have a significant potential for oxidation of NO. Methylamine is active as an oxidizer in a fairly wide temperature window, but in this regime, significant amounts of hydrogen cyanide and ammonia were detected. Acetaldehyde, acetone, and ethane were less efficient in promoting NO oxidation. This is attributed t o their inability to produce significant amounts of the HO2 radical during their oxidation process.
by the Danish Technical Research Council, Elsam (the Jutland-Funen Electricity Consortium), Elkraft (the Zealand Electricity Consortium), and the Danish Ministry of Energy.
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Received for review August 22, 1994 Accepted February 10,1995@
Acknowledgment The authors would like to acknowledge the assistance of J ~ r Hansen n in carrying out the experiments. The work is part of the research program CHEC (Combustion and Harmful Emission Control), which is cofunded
IE940505W Abstract published in Advance ACS Abstracts, April 1, 1995. @
