Environ. Sci. Technol. 2003, 37, 4512-4516
Formation and Destruction of CH2O in the Exhaust System of a Gas Engine M A R IÄ A U . A L Z U E T A * , † A N D PETER GLARBORG‡ Department of Chemical and Environmental Engineering, Centro Polite´cnico Superior (Torres Quevedo Building), Maria de Luna 3, 50018 Zaragoza, Spain, and Department of Chemical Engineering, Technical University of Denmark, 2800 Lyngby, Denmark
A computational study of chemical reactions occurring in the exhaust system of natural gas engines has been conducted, emphasizing the formation and destruction of formaldehyde. The modeling was based on a detailed reaction mechanism, developed for describing oxidation of C1-C2 hydrocarbons and formaldehyde. The mechanism was validated against data from laboratory flow reactors and from the exhaust system of a full-scale gas engine. A parametric study of the exhaust system chemistry was performed, investigating the effect of temperature, stoichiometry, pressure, and exhaust gas composition. The results indicate a complex interaction between unburned hydrocarbons (UHC), formaldehyde, and nitrogen oxides. Above 850 K, partial oxidation of unburned hydrocarbons may occur, resulting in net formation or net destruction of CH2O depending on the unburned hydrocarbons/CH2O ratio and the reaction conditions. At the typical unburned hydrocarbons/CH2O ratio of 1.0-1.5% for gas engines, net formaldehyde formation may occur in the exhaust system if temperatures above 850 K are reached.
Introduction Formaldehyde is emitted from a number of natural and anthropogenic sources. It is an intermediate oxidation product of hydrocarbon fuels, and emissions of formaldehyde from combustion processes are a significant environmental concern. Formaldehyde emissions have adverse effects on the near environment due to smell and may have carcinogenic effects. Furthermore, once emitted, the lifetime of formaldehyde in the atmosphere is considerable, on the order of magnitude of hours or even days. It is a very active compound in the tropospheric chemistry, participating in chain-propagating reactions through photolysis and by interaction with OH radicals, thereby contributing to photochemical smog. Nowadays, one of the major concerns related to formaldehyde refers to its emission from engines, in particular lean-burn natural gas engines. These engines are becoming increasingly important as the use of natural gas and other substitutes becomes more significant in power production (1, 2). In lean-burn gas engines a significant fraction of the * Corresponding author fax: +34 976 761879; e-mail: uxue@ unizar.es. † Centro Polite ´ cnico Superior. ‡ Technical University of Denmark. fax: +45 45 882258; e-mail:
[email protected]. 4512
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fuel may be emitted in the form of unburned hydrocarbons, mainly as methane since it is the main constituent of natural gas, about 90% in many blends. Even though the emission of methane from combustion is significantly lower than other natural and anthropogenic sources, it is of concern since methane is a strong greenhouse gas. However, also the emission of partly oxidized combustion products such as formaldehyde from these engines is of increasing concern because of its role on photochemical smog, even though the levels are typically much smaller than those of methane. A number of strategies to minimize the problems both with unburned hydrocarbons and emissions of aldehydes are being considered. These include modifications of the fuel, modifications of the engine itself, modifications of the engine operation, or application of catalytic and noncatalytic postcylinder control techniques. Measures taken to minimize unburned hydrocarbons will also affect formaldehyde in the exhaust, but the correlation between these emissions is presently not clear. Formaldehyde emissions are apparently caused by partial oxidation events in the cylinder or in the exhaust. While conditions inside the self-propagating flame in the cylinder ensure rapid oxidation of CH2O, substantial amounts of CH2O may be formed in the outflow from protected regions such as crevice volumes and quench zones during blow-down and later stages of the gas exchange process in the engine (3, 4). Recent in-cylinder sampling experiments (5) indicate that most engine-out formaldehyde either emerges from quench zones or is formed from unburned hydrocarbons emitted from quench zones. The importance of formaldehyde formation in the exhaust flow is presently not clear. It is possible for CH2O to be formed or destroyed during the flow out of the exhaust port and through the exhaust system (3). At low temperatures, chemistry is too slow for significant reaction to occur, and at high temperatures, oxidation will proceed to carbon oxides, reducing levels of both unburned hydrocarbons and CH2O. However, in an intermediate temperature window net formation of CH2O may be possible. The temperature window and the extent of CH2O formation result from a complex chemical interaction between the components in the gas. Not only the methane and oxygen level, but also the concentrations of higher hydrocarbons and nitrogen oxides are known to affect the extent of reaction and its selectivity (1, 6-8). The objective of the present work is to use a reliable chemical kinetic model to identify the temperature and composition limits for the CH2O formation window under conditions representative of the exhaust system in a gasfired engine. For this purpose we use a reaction mechanism describing the conversion of C1 and C2 hydrocarbons (7, 8). This mechanism is updated with the results from a recent experimental and kinetic modeling study of the oxidation of formaldehyde and its interaction with nitric oxide under flow reactor conditions (9). On the basis of the results, the importance of formaldehyde formation in the exhaust system of gas engines is discussed.
Reaction Mechanism and Validation The chemistry occurring in the exhaust system is potentially very complex. The reactivity of methane is quite low below 1000 K, but the presence of higher hydrocarbons such as ethane and of nitrogen oxides may significantly enhance the reaction rate (1, 6-8). Also, the presence of partially oxidized hydrocarbons such as formaldehyde is expected to lower the temperature window for reaction since they are more reactive than methane. To obtain reliable model predictions, the reaction mechanism must provide a satisfactory description 10.1021/es026144q CCC: $25.00
2003 American Chemical Society Published on Web 09/05/2003
FIGURE 1. Comparison between experimental results (8) and calculations for CH4 conversion in a flow reactor. Symbols denote experimental data and lines modeling predictions. Initial conditions: 2276 ppm CH4, 3.69% O2, 4% H2O, and N2 to balance. tr (s) ) 249.6/T (K), P ) 1 bar. of this complex chemistry. The mechanism used in the computational study was based on previous work by the authors on hydrocarbon oxidation (7,8). The formaldehyde oxidation subset of this mechanism was then modified according to the findings in our recent study on formaldehyde oxidation chemistry (9). Calculations were performed using Senkin (10), a plug-flow code that runs in conjunction with the Chemkin library (11). To evaluate the capability of the reaction mechanism to describe the exhaust oxidation chemistry, we compared model predictions against selected experimental results from literature. Suitable experimental data for model validation under conditions of an engine exhaust system are scarce. However, we use a combination of laboratory data obtained under well-controlled reaction conditions (8) and data obtained from a lean-burn gas engine equipped with a specially designed exhaust reactor (1), i.e., data that are both accurate and representative of the practical system. The laboratory set contains measurements of CH4, CH2O, CO, and CO2 in low-temperature oxidation of methane in a flow reactor under lean conditions (8), while the engine data describe conversion of unburned hydrocarbons and CO as a function of nitrogen oxide level in the exhaust system (1). Figure 1 shows the comparison between experiments (8) and calculations for CH4 oxidation in a flow reactor. The model captures the main features of the experiments. Even though slight discrepancies in the specific values are seen, the main trends are well predicted by the model, both in relation to methane conversion and for formation of CH2O and CO/CO2. The results of Figure 1, together with results reported elsewhere (8, 9), indicate that the reaction mechanism is appropriate for the simulation of natural gas and formaldehyde oxidation under flow reactor conditions. Consequently, we proceed to test the mechanism against data obtained in the exhaust system of a real lean-burn natural gas engine (1). The engine, a Ford diesel engine converted for natural gas operation, was equipped with an extended exhaust reactor to study the influence of the exhaust operating conditions on the conversion of unburned hydrocarbons. The data selected from this study concern the influence of NO level on the conversion of unburned hydrocarbons and on the concentration of CO in the exhaust reactor. Following the discussion in ref 1, the exhaust reactor was approximated as a plug flow reactor and the hydrocarbon partitioning at the inlet to the exhaust reactor was assumed to be the same as that in the fuel, i.e., 90% CH4 and 10% C2H6. The NO level
FIGURE 2. Comparison between experimental results obtained in the exhaust system of a real lean-burn natural gas engine (1) and calculations for the concentrations of unburned hydrocarbons and CO. Symbols denote experiments and lines modeling predictions. Initial conditions: 4410 ppm CH4, 490 ppm C2H6, 8.5% O2, 13% H2O, 6.5% CO2, 10-1813 ppm NO, 3-544 ppm NO2, and balance N2. tr ) 210 ms, T ) 953 K, P ) 1.7 bar. in the inlet varied between 10 and 1813 ppm, with NO2 between 3 and 544 ppm. Figure 2 compares the experimental data obtained at the outlet of the exhaust system (1) with calculations on the concentration of unburned hydrocarbons and CO as a function of NOx concentration. It can be seen that the level of unburned hydrocarbons is predicted with significant accuracy, while the CO level is slightly overpredicted as the inlet NO concentration increases. On the basis of these results, we conclude that the present model captures the main features of the complex chemistry occurring in low-temperature oxidation of natural gas and formaldehyde under engine-related conditions. For this reason, it can be used with some confidence to assess formation and destruction of formaldehyde in the exhaust system of a gas engine as a function of reaction conditions and exhaust gas composition.
Computational Parametric Study The sources of aldehyde emissions from gas engines are not yet clarified. However, it has been proposed that partial oxidation of unburned hydrocarbons within the exhaust system could contribute to formaldehyde emission (1, 3, 8). To investigate this issue, we performed a parametric study of the oxidation of the main unburned hydrocarbons, i.e., CH4 and C2H6, as a function of temperature and flue gas composition under conditions resembling those in the exhaust system of a full-scale gas engine (1, 12). It is known that the composition of unburned hydrocarbons in the exhaust of gas engines largely resembles the parent fuel (1). The baseline case was an exhaust gas mixture with 1000 ppm natural gas (approximated as 90% methane and 10% ethane), 500 ppm CO (from incomplete combustion), 5% oxygen, and carbon dioxide and water according to stoichiometry. In the parametric study various amounts of formaldehyde (0, 50, and 100 ppm) and nitric oxide (0, 300, and 1000 ppm) were added to the exhaust gas. Also, the unburned hydrocarbon level in the exhaust gas was varied (1000-5000 ppm). The temperature was varied in the range 800-1000 K, while the pressure was held at 1.8 bar. We assumed a residence time in the exhaust port/system of 40 ms, a value representative of gas engines at the chosen temperature. The effects of changes in the oxygen concentration (5-9%) or pressure (minor variations) were investigated and found to be small. An increase in oxygen to 9% results in a slight shift (less than VOL. 37, NO. 19, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Simulation results of CH4, C2H6, and CH2O at the outlet of the exhaust system for different initial levels of CH2O. Initial conditions: 1000 ppm natural gas, 5% O2, 500 ppm CO, 7.25% CO2, 14.5% H2O, balance N2. tr ) 40 ms, P ) 1.8 bar. 25 K) of profiles to lower temperatures, while a decrease in pressure to 1.6 bar results in a similar shift toward higher temperatures. Figure 3 shows predicted concentrations of CH4, C2H6, and CH2O as a function of temperature for different initial concentrations of formaldehyde in the absence of NO. The unburned hydrocarbon conversion is negligible, independent of the CH2O level in the initial composition. Oxidation of formaldehyde, when present in the inlet, is observed only at the highest temperatures investigated. Calculations show that under the conditions of Figure 3 oxidation of CH2O proceeds more rapidly above 950 K than that of CH4 and C2H6. It involves the reactions
CH2O + OH h HCO + H2O
(1)
HCO + O2 h CO + HO2
(2)
HCO + M h H + CO + M
(3)
The presence of C2H6 in the gas would be expected to facilitate oxidation. However, under these conditions, the opposite effect is predicted. Modeling predictions indicate a higher reactivity of methane than that of natural gas in the presence of formaldehyde. The inhibition of reaction observed under the conditions of Figure 3 for the natural gas mixture is caused by recombination of radicals with the intermediates derived from C2H6 and/or recycle reactions back to CH4, C2H6, and CH2O. These reactions serve to remove most radicals that are formed. A similar behavior was predicted for an inlet concentration of natural gas of 5000 ppm. It can be concluded that in the absence of NO, little conversion of unburned hydrocarbons or CH2O can be expected in the exhaust flow and the presence of CH2O in the exhaust gas in realistic concentrations will not lead to any significant conversion of unburned hydrocarbons. 4514
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FIGURE 4. Simulation results of CH4, C2H6, CH2O, CO, and NO at the outlet of the exhaust system for different initial levels of CH2O. Initial conditions: 300 ppm NO. The rest is equal to Figure 3. It is likely that the exhaust gas, in addition to unburned hydrocarbons, contains significant amounts of NO, formed at the high combustion temperatures in the cylinder. Nitric oxide is known to promote oxidation of hydrocarbons under lean conditions (6, 8, 13), and it has been shown to enhance the conversion of unburned hydrocarbons in the exhaust system (1). Figure 4 shows the effect of adding 300 ppm NO on the conversion of CH4, C2H6, and CH2O at varying amounts of CH2O. Consistent with the results of Kristensen et al. (1), the presence of a significant amount of NO results in a considerable promotion of methane and ethane oxidation, even in the absence of formaldehyde. The presence of CH2O further promotes reaction. It is noteworthy that above 925 K the predicted formaldehyde emission is more or less independent of the inlet level in the range investigated. In the absence of CH2O in the inlet gas, net formation of formaldehyde occurs above 875 K, peaking at about 950 K at 40 ppm. If formaldehyde is present initially in levels above this value, a net consumption of CH2O occurs, initiated above 850 K. The exit concentration of formaldehyde is determined by the competition between formation and destruction. Under the conditions of Figure 4, formation of formaldehyde proceeds mainly through the sequence
CH3 + NO2 h CH3O + NO
(4)
CH3O (+M) h CH2O + H (+M)
(5)
while it is consumed largely by the reaction with OH radicals, reaction 1. Reaction 4 is the most important consumption step for the methyl radical (CH3). This shows the importance of nitrogen oxides for the methane oxidation mechanism. Methyl and peroxide radicals, formed by recombination of CH3 and H, respectively, with O2, i.e.
H + O2 (+M) h HO2 (+ M)
(6)
CH3 + O2(+M) h CH3O2(+M)
(7)
are comparatively unreactive and serve to slow the fuel oxidation rate. The presence of nitrogen oxides promotes the oxidation rate because they participate in reactions that rapidly convert these radicals to reactive radicals such as H and OH. In addition to reactions 4 and 5, other important reactions include
NO + HO2 h NO2 + OH
(8)
NO + CH3O2 h NO2 + CH3O
(9)
Nitrogen dioxide is recycled to NO mainly by reaction with methyl radicals (reaction 4) but also by reaction with hydrogen atoms
NO2 + H h NO + OH
(10)
At higher levels of unburned hydrocarbons in the exhaust gas, the steady-state CH2O concentration increases. If the unburned hydrocarbon/CH2O ratio increases, formation of formaldehyde in the exhaust gas will be favored compared to its destruction. However, measurements from gas engines indicate that this ratio is roughly constant, with values ranging between 1.0% and 2.5% for a range of engines and operating conditions (3). If the amount of NO in the gas is increased up to 1000 ppm, the conversion of methane, ethane, and formaldehyde is reduced compared to the conditions of Figure 4. This is in agreement with the findings of Glarborg et al. (14), who observed an inhibition of CO oxidation in the presence of high NO concentrations. If present in sufficient amounts, NO may act as a radical scavenger. This effect is also indicated in the unburned hydrocarbon profile shown in Figure 2. Figures 5 and 6 show simulation results of CH4, C2H6, CH2O, and CO as a function of temperature, inlet formaldehyde concentration (0 and 100 ppm), and inlet NO concentrations (0, 300, and 1000 ppm). When no initial CH2O is present, a significant formation of this component is predicted. The amount of CH2O formed increases as the initial natural gas level is increased (from 1000 to 5000 ppm), and it is notable that the extent of conversion (in percentage) to CH2O is roughly independent of the initial natural gas concentration. The onset for CH2O formation coincides with the decrease in both methane and ethane. Again, as the NO concentration increases, here from 300 to 1000 ppm, the onset of reaction is shifted to higher temperatures. At lower temperatures, reaction is very sensitive to chain-terminating steps, such as
NO + O (+M) h NO2 (+M)
(11)
NO + OH(+M) h HONO (+M)
(12)
NO2 + O h NO + O2
(13)
HONO + OH h NO2 + H2O
(14)
FIGURE 5. Simulation results of CH4, C2H6, CH2O, and CO at the outlet of the exhaust system for different initial NO concentrations. Initial conditions: NO, 0 ppm CH2O. The rest is equal to Figure 3.
As the temperature increases, the radical pool is replenished and the inhibiting effect of reactions 11-14 is diminished. The reaction sequence 4-10 now becomes dominating, and the high NO level acts to further enhance the fuel oxidation rate. The increase in NO level from 300 to 1000 ppm (Figure 5) shifts the temperature for onset of reaction to slightly higher temperatures, but above 950 K it enhances the fuel oxidation rate. Therefore, the temperature window for CH2O formation becomes narrower. Two different oxidation regimes can be identified. The first one, which is the slowest, is responsible for a sharp increase in CH2O, which basically accumulates with little CO formation. The second regime is faster and coincides with a sharper decrease in CH4 and C2H6, accompanied by the oxidation of CH2O to CO. If formaldehyde is present in a quantity of 100 ppm (Figure 6), a slightly different behavior is predicted. Compared to Figure 5, the onset of CH4 and C2H6 conversion is shifted toward lower temperatures and their oxidation rate is increased, confirming the promoting effect of CH2O on unburned hydrocarbon oxidation. Contrary to Figure 5, no net formation of CH2O is predicted at any temperatures. If the initial natural gas concentration is increased to 5000 ppm, a net increase in CH2O is predicted at temperatures of 850VOL. 37, NO. 19, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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reported to be below 690 K near the exhaust port in the cylinder (3). Even accepting an error margin of 50 K in our calculations, this temperature is too low for significant reaction to occur in the exhaust system. For these engines the formaldehyde emitted can be expected to derive largely from in-cylinder processes, in agreement with conclusions in the literature (3-5). For lean-burn and rich-burn four-stroke engines, the temperatures are generally considerably higher. The exhaust temperatures for these engines may be 200 K higher than for the two-stroke engines (3). For this reason, we expect that temperatures approaching 900 K may be encountered in the exhaust system for these engines. Assuming a typical unburned hydrocarbons/CH2O ratio of 1.0-1.5% (4, 12), the formaldehyde concentration at the exhaust port would be 10-15 ppm at an unburned hydrocarbons level of 1000 ppm. With these values, our calculations indicate that net formation of CH2O may occur in the temperature range of about 850-950 K, even at a small fractional conversion of the unburned hydrocarbons. On the other hand, it is unlikely that the temperature in the exhaust system reaches values above 950 K, where a net reduction of CH2O could be expected.
Acknowledgments The work is part of the research programs of the DIQTMA (Department of Chemical and Environmental Engineering) of the University of Zaragoza (financial support from DGA, project P061/99-T is acknowledged) and of CHEC (Combustion and Harmful Emissions Control), which is co-funded by the Danish Technical Research Council, Elsam (the JutlandFunen Electricity Consortium), Elkraft (the Zealand Electricity Consortium), and the Danish Ministry of Energy.
Literature Cited
FIGURE 6. Simulation results of CH4, C2H6, CH2O, CO, and NO at the outlet of the exhaust system for different initial NO concentrations. Initial conditions: 100 ppm CH2O. The rest is equal to Figure 3. 950 K. The increase in CH2O coincides with the onset of natural gas conversion. This shows the significance of the unburned hydrocarbons/CH2O ratio for the competition between formation and destruction of formaldehyde in the exhaust channel.
Practical Implications The parametric study indicated that a significant conversion of unburned hydrocarbons may occur in the 800-1000 K range, promoted by the presence of CH2O and, more pronounced, NO. Depending primarily on the unburned hydrocarbons/CH2O ratio in the exhaust gas and the temperature, the oxidation of unburned hydrocarbons may increase or decrease the level of formaldehyde. However, other parameters such as pressure, oxygen concentration, nitrogen oxides concentration, and temperature gradient will also affect the reaction. The interaction between unburned hydrocarbons, formaldehyde, and nitric oxide in the oxidation process is quite complex. Due to inaccuracies in the reaction mechanism, our modeling predictions are probably only accurate within 25-50 K. Despite this uncertainty, some important points can be made on the importance of formaldehyde formation in the exhaust system of natural gas engines. Our modeling study indicates that very little reaction takes place below 850 K. Temperatures for two-stroke engines are 4516
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Received for review September 10, 2002. Revised manuscript received April 9, 2003. Accepted June 16, 2003. ES026144Q
