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Energy & Fuels 1999, 13, 650-654
Comparison of CO and NO Emissions from Propane, n-Butane, and Dimethyl Ether Premixed Flames Christopher A. Frye and Andre´ L. Boehman*1, Fuel Science Program, Department of Energy and Geo-Environmental Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802
Peter J. A. Tijm Air Products and Chemicals, Incorporated, 7201 Hamilton Boulevard, Allentown, Pennsylvania 18195-1501 Received September 24, 1998. Revised Manuscript Received January 18, 1999
We report a comparison of CO and NO emissions from dimethyl ether (DME), propane, and n-butane laminar premixed flames. Measurements were made with a water-cooled stainless steel sampling probe situated above the visible reaction zone of a co-flow burner. Species were measured by a Nicolet Magna 550 FTIR spectrometer. The fuels were compared on the basis of constant mass flow, constant C-atom flow, and constant firing rate. Results were corrected for dilution by entrained air. Our results indicate that on all bases considered, DME demonstrated lower CO emission than propane and n-butane over a broad range of stoichiometries. NO production from DME was generally less than or similar to propane and n-butane over the same stoichiometric range. We conclude that in terms of its relative CO and NO production, DME is a viable alternative utility fuel.
Introduction Because of their effects on human health and property, emissions of carbon monoxide (CO) and nitric oxide (NO) from combustion sources are regulated by the Environmental Protection Agency.1 Alternative fuels are one means of reducing emissions from mobile combustion sources.2 Chen and Niu have shown that when used indoors as a utility fuel, DME demonstrated concentration levels of methanol, formaldehyde, and CO below acceptable limits established for residential areas and houses.3 The present study seeks to build on this work while answering a fundamental but important question: How different is DME (CH3OCH3) from conventional utility fuels such as propane and n-butane in its combustion properties? DME, used predominantly as an aerosol propellant, has been shown to have excellent properties as an alternative diesel fuel, yielding emission levels at or below those proscribed by the California ULEV standard.4-10 If commercialization of a new, lower cost production method is successful, DME may also find use (1) De Nevers, N. Air Pollution Control Engineering; McGraw-Hill: New York, 1995; pp 6-28. (2) The ABC’s of AFVs-A Guide to Alternative Fuel Vehicles, 3rd ed., California Energy Commission: Sacramento, CA, 1996. (3) Chen, Z. H.; Niu, Y. Q. Coal Conv. 1996, 19, 14. (4) Fleisch, T. H. Diesel Prog. Engines Drives 1995, 42-45. (5) Kapus, P. E.; Cartellieri, W. P. SAE paper no. 952754; Society of Automotive Engineers: Warrendale, PA, 1995. (6) Kapus, P.; Ofner, H. SAE paper no. 9500062; Society of Automotive Engineers: Warrendale, PA, 1995. (7) Karpuk, M. E.; Cowley, S. W. SAE paper no. 881678; Society of Automotive Engineers: Warrendale, PA, 1988. (8) Sorenson, S. C.; Mikkelsen, S.-V. SAE paper no. 950064; Society of Automotive Engineers; Warrendale, PA, 1995. (9) Wilson, R. Diesel Prog. Engines Drives 1995, 108-109.
as an alternative utility fuel. The primary commercial process for production of DME today involves the dehydration of methanol. Dehydration is satisfactory for DME production geared toward current demand but it is not cost-effective for the mass production of DME required for widespread fuel use.10 The recent and intense interest in DME as a transportation fuel has arisen from development of new methods to produce DME on a larger scale from natural gas10 and from syngas in a one-step slurry phase process.11 The latter approach is a promising new method of producing DME directly from syngas in a single, slurry-phase reactor using a physical mixture of a commercial methanol synthesis catalyst and a proprietary dehydration catalyst, slurried in mineral oil.11 This liquid-phase dimethyl ether (LPDME) process can be incorporated into an integrated gasification combined cycle (IGCC) plant to fully utilize the capacity of the gasifier(s). It can also be a stand-alone process to produce DME for use as a replacement diesel fuel, a domestic fuel, or a chemical building block.11 Laminar premixed flames are common in residential and commercial equipment and have been widely used for combustion and flame studies. Chen and Niu have explored the suitability of DME as a utility fuel.3 Building on this work, this study compares the CO and NO emissions from directly above propane, n-butane, and DME laminar premixed flames on a co-flow burner. (10) Hansen, J. B.; Voss, B.; Joensen, F.; Siguroardottir, I. D. SAE paper no. 950063; Society of Automotive Engineers: Warrendale, PA, 1995. (11) Tijm, P. J. A.; Waller, F. J.; Toseland, B. A.; Peng, X. D. Presented at the Energy Frontiers International Conference, Alaska, July, 1997.
10.1021/ef980196c CCC: $18.00 © 1999 American Chemical Society Published on Web 02/26/1999
CO and NO Emissions
Figure 1. Schematic of the co-flow burner.
CO and NO emissions are reported over a broad range of stoichiometries for each fuel. Experimental Section A co-flow burner was designed to produce laminar premixed flames that could be studied over a wide range of stoichiometries. Figure 1 shows a schematic diagram of this burner. The burner consists of a central tube through which premixed fuel and air are passed in a manner similar to a Bunsen burner.12 Surrounding the central tube is an annular region of co-flow air. Uniform exit velocity of the co-flow air is ensured by packing with approximately 3 cm of 2-mm diameter borosilicate spheres beneath a monolith insert. A similar burner was recently employed by Nguyen et al.13 at Berkeley for RamanLIF measurements of temperature and major species in a methane-air flame. Fuel of 99.5% purity is supplied to the burner from compressed cylinders. Air is delivered to the burner assembly from a “purge gas generator” that filters moisture, carbon dioxide, and particulate matter from compressed laboratory air. Flow is metered through a combination of rotameters and electronic mass flow controllers. The burner stand consists of a stainless steel plate mounted on a metal table by four metal legs to ensure burner stability while allowing access to the inlet tubing. Fuel flows into the burner stand through a flame arrestor consisting of a stainless steel fitting with a single passage hole that is below the quenching diameter for all fuels under consideration. Air mixes with the fuel in a tee-fitting, and the mixture flows into the burner element. The degree of mixedness was not measured analytically and was assumed to be complete for all experiments. The appearance and length of the flames varied with fuel type and stoichiometry. For stoichiometric or fuel-lean flames, a single cone was observed for all fuels under consideration. For fuel-rich flames, a dual-cone structure was observed and consisted of a premixed, fuel-rich inner flame and a stoichiometric outer diffusion flame. As a result of the greater mass flow rate, fuel-lean flames were longer than fuel-rich flames. The appearance of the DME flame is noteworthy in that even at fuel-rich conditions, only a small, fuel-rich yellow inner cone was observed. Fuel-lean DME flames were pale blue and only moderately luminous. Product gas composition was analyzed by a Nicolet Magna 550 FTIR spectrometer equipped with a 2 m path length gas (12) Warnatz, J.; Mass, U.; Dibble, R. W. Combustion. Physical and Chemical Fundamentals Modeling and Simulation, Experiments, Pollutant Formation; Springer: New York, 1996; pp 4-5. (13) Nguyen, Q. V.; Edgar, B. L.; Dibble, R. W.; Gulati, A. Combust. Flame 1995, 100, 395-406.
Energy & Fuels, Vol. 13, No. 3, 1999 651 cell and a liquid nitrogen-cooled MCT detector. The spectrometer and the ancillary control devices are connected to a 100 MHz Pentium PC that provides integrated control and monitoring of most aspects of the experiment. Flow is induced by a vacuum pump downstream of the FTIR. A MKS type 248 pressure control valve, connected to a computer interface, adjusts the pressure in the gas cell of the FTIR spectrometer. Pressure in the gas cell is sensed with a MKS “Baratron” type 121pressure transducer. The combination of the pump, pressure control, and flow control allows gas to be sampled into the spectrometer under flowing conditions while the pressure and temperature of the gas in the cell is strictly controlled. The pressure and temperature of the cell were 680 Torr and 135 °C during all experiments. A water-cooled stainless steel sampling probe extracted combustion gases directly above the flame. Drake et al.14 have shown that water-cooled stainless steel sampling probes do not alter the sample composition in the post-flame gases of nonpremixed turbulent flames. This is presumably also the case for the laminar premixed flames of the present study. However, recent work by Nguyen et al.13 using in situ laser measurements suggests that extractive sampling probes may underestimate the CO concentration by a factor of 10 in fuellean flames. For all experiments, the probe is located just above the visible section of the flame, whether the flame is luminous or nonluminous. Because of the broad range of fuel stoichiometries employed, the probe height was not held constant with respect to the burner but was maintained approximately constant with respect to the end of the visible reaction zone. Water at 90 °C was pumped through a jacket surrounding the sampling tube within the probe, preventing condensation within the sample tube. This probe is also connected to the FTIR spectrometer through an electronic mass flow controller. All connecting tubing was maintained at a temperature in excess of 100 °C to prevent condensation. For all runs the inlet reactant temperature was maintained at 60 °C. Individual experiments were begun at the lean combustion limit for the fuel on the co-flow burner. The air flow in the premixed fuel/air flow was decreased in order to obtain samples from the lean combustion limit for the particular fuel on this burner to a fuel-rich flame, allowing CO and NO emissions to be measured over a broad range of stoichiometries. Samples were drawn approximately every 5 min. The cell was purged with nitrogen for 1 min between runs to reduce the possibility of residual contributions from the previous sample. Prior to sampling each flame, the FTIR was purged for 10 min with nitrogen and a new background sample was taken. The sampling rate was approximately 0.5 L per min. Three data points were taken for each value of fuel stoichiometry to assess reproducibility of the data. For all data reported, repeatability of the CO and NO concentrations is approximately (5%. Emission sampling from above the flame zone raises the issue of dilution of the combustion products. The measured amounts of CO and NO at the probe height were related to the amount produced in the flame in the following manner. Assuming complete combustion, one can determine the expected molar fraction of carbon dioxide produced in the flame. By comparing this figure with the amount of carbon dioxide detected by the probe, a dilution factor is determined. Emissions measurements in parts per million (ppm) volume are multiplied by the dilution factor in order to estimate CO and NO production in the absence of dilution. These adjusted values are reported as at the flame. (14) Drake, M. C.; Correa, S. M.; Pitz, R. W.; Shyy, W.; Schefer, R. W. Combust. Sci. Technol. 1990, 69, 347-365.
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Figure 2. Carbon monoxide concentration above the flame on the co-flow burner for DME, propane, and n-butane, corrected for dilution by ambient air. Compared on the basis of a constant mass flow of 0.30 g/min.
Results and Discussion The fuels were compared on three different bases: constant fuel mass flow, constant energy release (firing) rate, and constant C-atom flow. For each basis, trends in CO and NO emissions over a range of fuel stoichiometries were obtained. The results are reported as lines to allow for comparison of emission trends. It should be remembered that each line is composed of no more than six individual points and that each of these points is an average of three samples. For all experiments, the relative flame height of the propane and n-butane flames increased with increasing fuel equivalence ratio (φ), as expected.15 The DME flame height, however, was less sensitive to stoichiometric changes, likely owing to the enhanced chemical reaction rates and more compact reaction and post-flame zones for the ether molecule. Figure 2 presents the comparison of CO emissions for the three fuels under a constant mass flow of 0.3 g/min. It should be noted that under this basis of comparison, the resulting energy release rate of the flames decreases as propane > n-butane > DME. It is clear from this figure that DME produces considerably less CO over the entire range of fuel stoichiometry studied, reducing CO emissions by 50 ppm in fuel-lean regions to 250 ppm in fuel-rich regions. Of particular interest is that for DME, CO production remains fairly flat over the entire stoichiometric range. This effect may be the result of the relatively low flow rates employed under this basis of comparison. It is reasonable to expect that at lower flow rates, the flame chemistry is sufficiently rapid to allow combustion to proceed to approximately the same degree of completion for all points considered. That is, the quenching mechanism of CO production is avoided. Indeed, Turns and Brooks16 have suggested that CO emissions can be considered as an indicator of rapid mixing and quenching. Figure 3 presents the results of NO production above the flames under constant mass flow. Two interesting (15) Blevins, L. G.; Gore, J. P. Combust. Sci. Technol. 1995, 109, 255-271. (16) Turns, S. R.; Brooks, B. K. Combust. Sci. Technol. 1994, 103, 175-189.
Frye et al.
Figure 3. Nitrogen oxide concentration above the flame on the co-flow burner for DME, propane, and n-butane, corrected for dilution by ambient air. Compared on the basis of a constant mass flow of 0.30 g/min.
features are that DME produces lower NO emissions over the entire range of stoichiometry and that the NO peak is visible for DME but not for n-butane and propane. It is clear from the figure that for n-butane and propane, the measured NO concentrations occur on the fuel-rich side of the peak NO production, while for DME, the peak production was observed. This effect is caused by the higher flame speed of DME compared to propane and n-butane. Under very fuel-lean conditions, the tendency is for the flow of reactants to cause the flame to detach from the burner orifice; indeed, this highest possible reactant flow rate prior to detachment of the flame was used as the starting point for the sample runs. As a result, the higher flame speed of DME in the fuel-lean region17 allowed its flame to remain attached to the burner under higher reactant flow rates (and hence, under a more fuel-lean condition), so that it was possible to gather data points on the lean side of peak NO production. This was not possible for propane and n-butane on our co-flow burner. Figure 4 compares the CO emissions on the basis of constant C-atom flow. The fuels were normalized to n-butane (C4) so that, for example, approximately 1.6 times the mass of DME (compared to n-butane) is flowing through the burner when compared on this basis. Nevertheless, Figure 4 shows that CO production for DME is lower than that for the other fuels over much of the range considered. The modest decline in emissions from propane and n-butane near stoichiometric combustion is somewhat puzzling. The total decline is just within the expected experimental error, so the effect may not be real. If it is a real effect, it may be an artifact of the burner configuration such that the fuel and air mixing under these particular conditions of reactant flow and burner design is optimized for n-butane and propane. Fuel and air mixing would be expected to be less critical with DME owing to the oxygen atom present in the ether molecule. Other than the single point where propane dips slightly below DME (and which is not significant in (17) Glassman, I. Combustion; Academic Press: New York, 1987; pp 461-463.
CO and NO Emissions
Figure 4. Carbon monoxide concentration above the flame on the co-flow burner for DME, propane, and n-butane, corrected for dilution by ambient air. Compared on the basis of constant C-atom flow, normalized to n-butane (C4).
Figure 5. Nitrogen oxide concentration above the flame on the co-flow burner for DME, propane, and n-butane, corrected for dilution by ambient air. Compared on the basis of constant C-atom flow, normalized to n-butane (C4).
terms of experimental error), CO production for DME is lower than that for propane and n-butane despite the increased mass flow. Again, this is likely the result of the relatively rapid combustion chemistry of DME compared to propane and n-butane. The results of the constant C-atom NO emissions are presented in Figure 5. For values of φ less than 1.7, all three fuels produced approximately equal amounts of NO. For larger values of φ, the fuels demonstrate the familiar pattern of DME < propane < n-butane. It is interesting that under constant C-atom flow, it is the NO production that is similar for the three fuels and not the CO production. As seen in the previous figure, CO emissions follow the typical pattern, while this figure shows that NO emissions are similar for all three fuels. There are several possible explanations for the comparatively high NO production. First, because the reactant flow rate is considerably higher for DME, a longer flame is produced. This might seem to suggest increased residence time in the high-temperature region, which would in turn increase NO production. However, this explanation is not consistent with the
Energy & Fuels, Vol. 13, No. 3, 1999 653
Figure 6. Carbon monoxide concentration above the flame on the co-flow burner for DME, propane, and n-butane, corrected for dilution by ambient air. Compared on the basis of constant firing rate of 100 J/s.
recent results of Feese and Turns18 on laminar premixed flames. They conclude that based on the work of Santoro et al.,19 the buoyancy force dominates the flame motions, increasing the flame velocity as more hot combustion products are produced. Thus, in their study, longer flames equate to less time in the high-temperature region and hence reduced NO production. Flame velocity measurements were not taken in our experiments. Another possible explanation is an error in measured flow rates. There was some question of the calibration of the rotameter at higher flow rates. Thus, the curve for DME may be shifted to the right in Figure 5. If it were shifted more to the left, the figure would show lower NO production over much of the range considered, though the differences would be minor. The final basis of comparison is perhaps the most interesting and relevant one for the purposes of a comparison of utility fuels. Figure 6 presents a comparison of CO emissions under a constant firing rate (energy release rate) of 100 J/s. Producing the same firing rate requires a larger mass of DME compared to propane and n-butane. Despite this, DME produces less CO than either propane or n-butane over the entire range of φ considered. The departure of the DME curve from the relative flatness of the previous CO figures is due solely to a single point at a φ ) 3 and, therefore, may be overstated in this figure. Figure 7 presents the NO comparison for the three fuels under a constant firing rate of 100 J/s. Under very lean conditions (φ ) 0.5), the three fuels produced an approximately equal amount of NO whereas DME produces lower NO emissions over the remainder of the range of stoichiometries considered, consistent with Figure 5. The differences between DME and the other fuels are not as significant in this comparison. The amount of NO produced by DME is no greater than that produced by propane and n-butane. Indeed, over much of the stoichiometric range, DME is seen to produce less NO within the expected experimental error. The results presented in Figures 6 and 7 are interesting when compared to Figures 8 and 9. Figure 8 (18) Feese, J. J.; Turns, S. R. Combust. Flame 1998, 113, 66-78. (19) Santoro, J. R.; Yeh, T. T.; Horvath, J. J.; Semerjian, H. G. Combust. Sci. Technol. 1987, 53, 89-99.
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Figure 7. Nitrogen oxide concentration above the flame on the co-flow burner for DME, propane, and n-butane, corrected for dilution by ambient air. Compared on the basis of constant firing rate of 100 J/s.
Figure 8. Comparison of equilibrium CO production for propane, n-butane, and DME.
Frye et al.
Figure 10. Comparison of the adiabatic flame temperatures of propane, n-butane, and DME.
contribute significantly to overall CO production.20 This mechanism may be at work for our system since the observed trends in CO concentration proceed in precisely the fashion predicted by the equilibrium production in the rich, premixed inner flame. Figure 9 yields the interesting result that DME’s equilibrium NO production is actually greater than that for propane and n-butane, differing from the results obtained on our burner. The cause for this effect is seen in Figure 10, which shows that the adiabatic flame temperature of DME is higher than that of the other fuels over the entire stoichiometric range. Our results generally demonstrate that DME’s NO production is similar to or less than that of propane and n-butane. Since DME requires less air flow than the other fuels at any given stoichiometry, DME’s residence time in the flame zone is actually somewhat longer at a given stoichiometry than for the other fuels. This adds credibility to our assertion that the reaction chemistry of the DME flame is more rapid than that of propane and n-butane. Conclusions
Figure 9. Comparison of equilibrium NO production for propane, n-butane, and DME.
presents a comparison of the equilibrium production of CO for the three fuels, while Figure 9 presents the same comparison for NO production. Figure 8 shows that for the premixed inner flame, CO production falls off as n-butane > propane > DME. This same general trend is observed in the previous figures comparing CO emissions, suggesting a potential CO formation mechanism. Recent work has suggested that fuel leakage from the quench zone at the burner lip may
DME generally demonstrated reduced CO production on all bases considered. Differences in NO production were less striking. At worst, DME produced a similar amount of NO as propane and n-butane. This is of particular interest in the case of constant firing rate, the most reasonable basis of comparison for utility fuels. While considerably more DME was flowing compared to propane and n-butane, measured CO emission was lower than that for propane and n-butane. NO production was similar to propane and n-butane when compared on this basis, despite the higher flow rate of DME. We finally conclude that in terms of its comparative CO and NO emissions, DME is a viable alternative utility fuel. Acknowledgment. We thank Air Products and Chemicals, Inc., and especially Dr. Bernie Toseland for supporting this research project. EF980196C (20) van der Meij, C. E.; Mokhov, A.; Jacobs, R. A. A. M.; Levinsky, H. B. Symposium on Combustion Proceedings of the 25th Symposium on Combustion, Irvine, CA, July 31-August 5, 1994, 243.
