Energy Fuels 2010, 24, 1487–1494 Published on Web 01/08/2010
: DOI:10.1021/ef901209q
Burning Velocity of Liquefied Petroleum Gas (LPG)-Air Mixtures in the Presence of Exhaust Gas Domnina Razus,*,† Venera Brinzea,† Maria Mitu,† and Dumitru Oancea‡ † ‡
“Ilie Murgulescu” Institute of Physical Chemistry, 202 Spl. Independentei, 060021 Bucharest, Romania and Department of Physical Chemistry, University of Bucharest, 4-12 Bd. Elisabeta, 030018 Bucharest, Romania Received October 23, 2009. Revised Manuscript Received December 22, 2009
The burning velocities of liquefied petroleum gas (LPG)-air and LPG-air-exhaust mixtures were calculated from pressure-time records obtained in a spherical vessel with central ignition, using a recent correlation based on the cubic law of pressure rise during the early stage of explosion. The burning velocities of LPG-air mixtures with a variable fuel/oxygen ratio at ambient initial conditions are compared to burning velocities of propane-air and n-butane-air mixtures, determined in the same conditions. All experimental data are examined against computed burning velocities, obtained for free laminar premixed flames using a reaction mechanism with 592 elementary reactions and 53 species. The measured and calculated burning velocities are used to examine the inerting effect of exhaust gas.
parameters associated with explosion ignition by high- and low-voltage electric sparks (quenching distances, minimum ignition energies, and minimum ignition currents) were measured for a LPG blend produced in Romania for domestic use (approximately 12 vol % propane, 87 vol % butane, and 1 vol % C5 fraction).4 A study on explosion propagation of LPG-air mixtures with various equivalence ratios, initial temperatures, and pressures in a closed cuboid vessel5,6 delivered values of burning velocity, corrected from the stretch effect, making use of optical records of the flame radius. The data refer to a LPG blend typical for engine fuels, containing approximately 28 vol % propane and 68 vol % butane (n-C4H10 þ i-C4H10). An experimental study on LPG-air combustion in a closed cylindrical vessel reported burning velocities in typical conditions of a heavy-duty engine cylinder (initial pressures between 1.5 and 4 bar and initial temperatures between 330 and 380 K).7 A comprehensive set of data (burning velocities and their dependencies on the initial temperature, pressure, and composition of LPG-air mixtures) was given by Huzayyin et al.2 from pressure-time records obtained in a closed vessel and discussed versus similar values for propaneair mixtures. Numerous studies are recently focused on the additive effect on LPG-air combustion, usually in conditions typical for spark ignition and/or for diesel engines. Exhaust gas, left usually from insufficient cylinder evacuation, is the typical additive of LPG-air mixtures. Dilution by exhaust gas leads to important power loss in spark-ignited engines5 but may result in a lower NOx concentration, because of the reduction of the flame temperature.8,9 Other interesting additives for LPG-air, with a beneficial effect on engine efficiency, are hydrogen,10
1. Introduction Liquefied petroleum gas (LPG) is extensively used as an alternative “clean” fuel in automotive engines, replacing gasoline and diesel, and as a fuel for domestic use. A large amount of information on its combustion characteristics is available, required mostly for analysis and prediction of the performances of various engines and/or combustors.1-8 The flammability limits in air of a LPG blend, formed from 70 vol % propane, 29 vol % butane, and 1% other hydrocarbons, used as motor fuel, were reported by Mishra and Rahman.1 Characteristic parameters of LPG-air explosions in closed vessels of various geometries and dimensions (explosion pressures, rates of pressure rise, explosion times, and severity factors) were determined for various initial pressures and temperatures2,3 and examined in comparison to similar values obtained for propane-air and butane-air. Other important *To whom correspondence should be addressed: “Ilie Murgulescu” Institute of Physical Chemistry, 202 Spl. Independentei, 060021 Sector 6, P.O. Box 12-197, Bucharest, Romania. Telephone: þ40-21-3167912. Fax: þ40-21-3121147. E-mail:
[email protected]. (1) Mishra, D.; Rahman, A. An experimental study of flammability limits of LPG/air mixtures. Fuel 2003, 82, 863–866. (2) Huzayyin, A.; Moneib, H.; Shehatta, M.; Attia, A. Laminar burning velocity and explosion index of LPG-air and propane-air mixtures. Fuel 2008, 87, 39–57. (3) Razus, D.; Brinzea, V.; Mitu, M.; Oancea, D. Explosion characteristics of LPG-air mixtures in closed vessels. J. Hazard. Mater. 2009, 165, 1248–1252. (4) Munteanu, V.; Musat, N.; Razus, D.; Oancea, D. Ignition of LPG/ air mixtures by high voltage and low voltage sparks. Rev. Chim. (Bucharest, Rom.) 2005, 56, 951–954. (5) Liao, S.; Jiang, D.; Gao, J.; Huang, Z.; Cheng, Q. Measurements of Markstein numbers and laminar burning velocities for liquefied petroleum gas-air mixtures. Fuel 2004, 83, 1281–1288. (6) Liao, S.; Jiang, D.; Cheng, Q.; Gao, J.; Hu, Y. Correlation for laminar burning velocities of liquefied petroleum gas-air mixtures. Energy Convers. Manage. 2005, 46, 3175–3184. (7) Lee, K.; Ryu, J. An experimental study of the flame propagation and combustion characteristics of LPG fuel. Fuel 2005, 84, 1116–1127. (8) Liao, S.; Jiang, D. M.; Cheng, Q.; Gao, J.; Hu, Y. Approximations of flammability characteristics of liquefied petroleum gas-air mixture with exhaust gas recirculation (EGR). Energy Fuels 2005, 19, 324–325. r 2010 American Chemical Society
(9) Razus, D.; Brinzea, V.; Mitu, M.; Movileanu, C.; Oancea, D. Inerting effect of the combustion products on the confined deflagration of liquefied petroleum gas-air mixtures. J. Loss Prev. Process Ind. 2009, 22, 463–468. (10) Wang, B.; Qiu, R.; Jiang, Y. Effects of hydrogen enhancement in LPG/air premixed flame. Acta Phys.-Chim. Sin. 2008, 24, 1137–1142.
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Energy Fuels 2010, 24, 1487–1494 11
: DOI:10.1021/ef901209q
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dimethyl ether, or diethyl ether, which act as ignition enhancers in diesel engines and allow important reduction of NOx emissions as well. Earlier studies on the exhaust gas influence on characteristic parameters of flammable gaseous mixtures used propane as test fuel. An experimental study on explosion propagation in closed cylindrical vessels of the propane-air mixture in the presence of dry exhaust gas was focused on the influence of the equivalence ratio and initial temperature on normal burning velocities,13 making use of pressure-time records. The normal burning velocity variation of propane-air resulting from dilution by exhaust gases was also reported from a computational study on premixed laminar flames, using an extended kinetic scheme.14 On the basis of the hypothesis of a limit value of burning velocity at the flammability limit, this approach allowed the authors the evaluation of the critical concentration of exhaust gas for the complete inerting of the system. The influence of exhaust gas on characteristic parameters of flame propagation in propylene-air mixtures with variable initial pressure and equivalence ratio was examined by means of experiments in a closed spherical vessel and kinetic modeling.15,16 Laminar burning velocities of LPG-air mixtures diluted or not by exhaust gas were determined by Liao et al.6 using an optical method of monitoring spherically expanding flames, in a closed cuboid vessel. In the present paper, burning velocities of a complex LPG blend (typical for domestic use) mixed with air, at various initial pressures (between 0.4 and 1.3 bar) and LPG/O2 ratios (equivalence ratio j between 0.80 and 1.97) are reported. The burning velocities are obtained from both experimental measurements of pressure variation during the early stage of closed vessel explosions and a detailed modeling of a free laminar premixed flame. Similar data referring to propaneair and n-butane-air at 1 bar total initial pressure within the same concentration range are reported. The results are examined in comparison to literature data, obtained by other experimental techniques or other computing packages. Furthermore, the burning velocities of several LPG-air mixtures in the presence of variable concentrations of their own exhaust gas, at various initial pressures (0.4-1.3 bar), are reported and discussed.
stainless-steel electrodes; the spark gap was located in the geometrical center of the bomb. The sparks have energies between 1 and 5 mJ. The pressure-time records were obtained with a piezoelectric pressure transducer (Kistler 601A), connected to a charge amplifier (Kistler 5001SN). The signals of the ionization probe amplifier and charge amplifier were recorded with an acquisition data system TestLab Tektronix 2505, by means of an acquisition card type AA1, usually at 104 signals per second. A vacuum and gas-feed line, tight at pressures between 0.5 mbar and 1.5 bar, connected the vacuum pump, the gas cylinders with fuel and air, the metallic cylinder for mixture storage, and the explosion vessel. Details on the experimental setup were given recently.3,9 The fuel-air gaseous mixtures were obtained by the partial pressure method and used 48 h after mixing the components, at a total pressure of 4 bar. The initial pressures of fuel-air mixtures were measured by a strain gauge manometer (Edwards type EPS-10HM). 2.2. Examined Systems. LPG (99.5%) (Arpechim, Romania) contained 12 vol % propane, 62.6 vol % n-butane, 24.4 vol % i-butane, and 1 vol % pentane. Propane and n-butane (both 99.99% purity, SIAD, Italy) were used without further purification. LPG-air mixtures with fuel concentrations between 2.60 and 6.15 vol % (j = 0.80-1.97) were investigated, at total initial pressures between 0.4 and 1.3 bar. Experimental measurements on LPG-air and LPG-air-exhaust mixtures at higher initial pressures (1.5 and 2.0 bar) were made only for fuel-lean mixtures. Propane-air and n-butane-air mixtures, at fuel concentrations within the same concentration range, were studied at ambient initial conditions. LPG-air-exhaust mixtures (concentration of exhaust gas of 2-10 vol %) were prepared directly in the explosion vessel, from fresh LPG-air mixtures (0.883H2O>CO2. The role played by radicals OH, O, H, and HO2 in flame propagation is outlined by sensitivity analyses for burning velocity, reported in the literature for several alkane-air mixtures.20,23 The analyses showed similar features of mixtures of C1-C3 alkanes with air. Regardless of the fuel-air ratio, a small number of sensitive reactions were found ðR1Þ H þ O2 f OH þ O CO þ OH f CO2 þ H
ðR2Þ
OH þ O f H þ O2
ðR10 Þ
H þ O2 þ M f HO2 þ M
ðR3Þ
The burning velocities of LPG-air mixtures obtained from the detailed modeling of combustion have overestimated values in comparison to both the present measured velocities and other literature data, especially for fuel-rich mixtures. A better match of computed and experimental results should be obtained by revision of input data. A fairly good agreement of experimental and predicted burning velocities was found for lean LPG-air and LPG-air-exhaust mixtures. The normal burning velocities decrease by progressive dilution of LPG-air mixtures with their own exhaust gases, at all initial pressures. The effectiveness of exhaust gas for burning velocity deceleration and explosion inerting as well is dependent to a large extent upon the [CO2]/[H2O] ratio, determined by the initial equivalence ratio. Larger variations of propagation parameters are observed by dilution of lean mixtures as compared to the stoichiometric one. Acknowledgment. The authors gratefully thank Prof. U. Maas (Heidelberg University, Germany) and Dr. D. Markus (Physikalisch-Technische Bundesanstalt, Braunschweig, Germany) for the permission to run INSFLA and the provided assistance. The results reported in the present study were partly financed by the Romanian Ministry of Education and Research, through Grants 42/2005-2007 and ID_458/2009.
Nomenclature A = fit parameter, in eq 5 (Su versus j) (cm s-1) a and b = fit parameters, in eq 5 (Su versus j) k = coefficient of the cubic law (bar s-3) K = dimensionless constant p = pressure (bar) R = radius (m) S = velocity (cm s-1) t = time (s) T = temperature (K) V = volume (cm3) x = mole fraction
Both reactions R1 and R2 are rate-limiting elementary reactions and have a positive sensitivity; reaction R1 is important, as a chain-branching step, while reaction R2 controls a large part of the heat release. Reactions R10 and R3 have negative sensitivities: reaction R10 is the reverse of reaction R1, and reaction R3 is a chain-terminating step.20
Greek Letters
5. Conclusions
R = pressure correction (bar) β = time correction (s) Δ = variation j = equivalence ratio of a fuel-oxidant mixture ν = baric exponent of normal burning velocity
The burning velocities of LPG in air diluted or not by a mixture of carbon dioxide, water vapor, and nitrogen in the form of exhaust gas were obtained from experimental records of pressure during explosions in a spherical vessel with central ignition, using the coefficients of the cubic law in the early stage of the process. The burning velocities of LPG in air agree well with literature data, obtained by other measuring techniques. Because the burning velocity calculation does not require values of thermo-physical properties of the fuel, the early stage method appears to be suitable for complex systems, formed by a composite fuel with air or single fuel-air mixed with complex additives.
Subscripts max = maximum value ref = reference value rel = relative u = unburned gas 0 = initial value
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