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Prediction of the Autoignition Delay Time of Producer Gas fromBiomass Gasification

Juan J. Hernandez,* Clara Serrano, and Javier PerezUni V ersidad de Castilla-La Mancha, Escuela Te cnica Superior de Ingenieros Industriales,

Edificio Politecnico, A V enida Camilo Jose Cela S/N, 13071 Ciudad Real, Spain Recei V ed June 30, 2005. Re V ised Manuscript Recei V ed No V ember 14, 2005

The greenhouse effect and the increasing demand for energy have encouraged the use of alternative fuels,with agricultural and forestry biomass waste being two of the main renewable sources that European andSpanish policies have been promoting during the last several years. In this article, theoretical results for thechemical autoignition delay time of producer gas obtained from the gasification of lignocellulosic biomass arepresented. These results, together with those obtained in a previous work related to the laminar flame speedof the gas, are of great interest both for understanding the chemical kinetic mechanisms that control the fueloxidation process and for the development of combustion models that provide significant information to beused as a tool for the optimization and design of internal combustion engines. The CHEMKIN software, inconjunction with the GRI-Mech chemical reaction mechanism, has been used to compute the autoignitiondelay time for different producer gas compositions, different values of pressure and temperature, and differentproducer gas/air equivalence ratios. Correlations of the delay time as a function of those variables are proposed,and a sensitivity analysis of the main reactions affecting the autoignition process has been carried out. The

results have been compared with those obtained for conventional fuels (isooctane and methane), showing thepotential of producer gas to reduce the knock tendency in a spark ignition engine and to allow homogeneouscharge compression ignition (HCCI) combustion conditions at intake temperatures lower than those typicallyused in a natural gas HCCI engine. The reliability of the Livengood - Wu integral method, as a way to estimatethe ignition timing under engine conditions (variable pressure and temperature), has also been checked.Differences lower than 6% between the Livengood - Wu integral and the CHEMKIN method, the latter usinga complete chemical kinetic scheme, have been obtained for different engine operating conditions.

Introduction

Recent European and Spanish energy policies 1,2 stronglyencourage the use of biomass waste for energy purposes. Thesepolicies have three targets: the reduction of CO 2 emissions,the removal of wastes, and the use of indigenous fuels. Due tothe low heating value and the high geographic dispersion of forestry and agricultural lignocellulosic waste, gasificationconstitutes an efficient technology that permits use of thosewastes at the same location where they are produced, thuseliminating the cost derived from storage and transportation topower plants. The low energy content gas (producer gas), whichconsists mainly of CO, H 2, N2, CO2, H2O, and CH4, can bedirectly used as a fuel in internal combustion engines, such asspark ignition (SI) or homogeneous charge compression ignition(HCCI) engines, 3- 5 or it can be used in dual fuel engineoperation to reduce the pollutant emissions mainly due to itshydrogen content. 6,7

The laminar flame speed and the autoignition delay time of the producer gas constitute two of the most relevant thermo-chemical properties affecting the engine performance andpollutant emissions, and our knowledge of them allows us tooptimize the engine efficiency. Laminar flame speed correlationsfor producer gas were proposed in a previous work, 8 showingvalues lower than that of isooctane and higher than that of methane at typical engine conditions. Comparison betweenexperimental and theoretical results proved that the experimentalmeasurement of the laminar flame speed at high pressure wasnot possible due to the wrinkling of flame front, this favoringthe theoretical methods as a more reliable way to obtain laminarflame speed values over a wide range of operating conditions.The autoignition delay time of a fuel/air mixture is defined asthe time interval required for the mixture to spontaneously igniteat some prescribed conditions. Although several research workscan be found in the literature related to the experimental andtheoretical determination of the ignition delay time of conven-tional fuels 9,10 and pure components, 11 - 14 the autoignitionprocess of the producer gas is still not well-known, and itsthermochemical properties cannot be gathered from purecomponent data.

* Corresponding author. Phone: (34)926295300 ext. 3880. E-mail:

JuanJose.Hernandez@uclm.es.(1) Green paper: Towards a European strategy for the security of energysupply ; Commission of the European Communities, Luxembourg, 2002.

(2) Plan de Fomento de las Energ as Reno V ables en Espan a ; Institutopara la Diversificacion y Ahorro de la Energ a: Madrid, Spain, 1999; inSpanish.

(3) Barisano, D.; De Bari, I.; Nanna, F.; Cardinale, F.; Matera, D.;Cavalere, S.; Viggiano, D.; Fanny, Y. Proceedings of the First World Conference on Biomass for Energy and Industry , First World Conferenceand Technology Exhibition for Energy and Industry, Sevilla, Spain, June5- 9, 2000; James & James: London, 2001; Vol. 1, 384 - 389.

(4) Ahrenfeldt, J.; Schramm, J.; Jensen, T. K. SAE Tech. Pap. Ser . 2001 ,2001-01-3681.

(5) Sridhar, G.; Paul, P. J.; Mukunda, H. S. Biomass Bioenergy 2001 ,21 , 61- 72.

(6) Jensen, T. K.; Schramm, J.; Narusawa, K. SAE Tech. Pap. Ser . 2001 ,2001-01-3532.

(7) Tsolakis, A.; Hernandez, J. J.; Megaritis, A.; Crampton, M. EnergyFuels 2005 , 19 , 418- 425.

(8) Hernandez, J. J.; Lapuerta, M.; Serrano, C.; Melgar, A. Energy Fuels2005 , 19 , 2172 - 2178.

(9) Gauthier, B. M.; Davidson, D. F.; Hanson, R. K. Combust. Flame2004 , 139 , 300- 311.

(10) Eckert, P.; Kong, S.; Reitz, R. D. SAE Tech. Pap. Ser . 2003 , 2003-01-0011.

532 Energy & Fuels 2006, 20, 532- 539

10.1021/ef058025c CCC: $33.50 2006 American Chemical SocietyPublished on Web 12/21/2005

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As an extension of the laminar flame speed work and to sup-plement the limited information currently available in the liter-ature, this article presents a computational study of the chemicalautoignition process of producer gas/air mixtures. Correlationsquantifying the influence of the thermodynamic conditions (pres-sure and temperature), the producer gas composition (dependingon the relative biomass/air ratio inside the gasifier), and therelative producer gas/air ratio in the engine have been obtained,and a chemical kinetic analysis of the main reactions involvedin the process has also been carried out. The additivity of theproducer gas delay time values, obtained under stationary con-ditions through the correlations provided, has been analyzed tocheck the Livengood - Wu integral method 15,16 as a way to pre-dict the ignition delay period in an engine, where pressure and

temperature change due to the compression and expansion result-ing from the piston motion and due to the heat release fromfuel.

Theoretical Calculations

The lignocellulosic biomass used in this work consists of wastefrom felling and pruning of Mediterranean pine ( Pinus pinaster ),dominant in the central areas of Spain (Castilla - Leon, Castilla -La Mancha, and Extremadura). 17 The ultimate composition of thistypical biomass can be directly obtained from the empiricalformula: CH 1.57O0.78N0.0056 S0.0001 . The biomass/air equivalence ratioinside the gasifier, F rg, is defined with respect to the stoichiometricratio as shown in eq 1.

An equilibrium model 18 derived by the authors has been used tocalculate the producer gas composition for different biomass/airratios. The assumption of chemical equilibrium allows very reliableresults to be obtained, due to the long residence time of the gas athigh temperature inside the gasifier. 19 The model is based on theequilibrium constant method and comprises 29 equilibrium reactionsthat, together with the six mass conservation equations for the sixelements considered (C, H, O, N, S, and Ar), allow us to calculatethe equilibrium concentration of 35 chemical species (N 2, O2, CO2,

H2O, N2O, NO2, HO2, S2, SO, SO2, SO3, NH, CO, H2, NO, OH,N, H, O, Ar, S, NH 3, COS, H2S, HS, HCN, CN, NCO, NH 2, CH 4,H2O2, CHO, CH 2O, CH3, and CH3O). Table 1 shows the molefractions of the main species in the producer gas, obtained withdifferent values of the relative biomass/air ratio, F rg. Producer gasconsists of hydrogen and carbon monoxide as the main fuel species,followed by methane in the case of high values of F rg. The modeledproducer gas composition is in agreement with typical experimentalcompositions obtained in other work. 18,20

Additionally, the autoignition delay time depends on the producergas composition and on the producer gas/air ratio in the engine.While F rg represents the relative biomass/air ratio in the gasifier,the relative gas/air ratio in the engine is defined in eq 2 as F r. Thedifference between both ratios is shown in Figure 1. Results havebeen obtained for an F rg ratio ranging from 3 to 5 and for F r valuesranging from lean conditions ( F r ) 0.6) to rich mixtures ( F r )1.8).

The CHEMKIN software 21 and the GRI-Mech 22 chemical reac-tion mechanism have been used to obtain the autoignition delaytime of producer gas. GRI-Mech 3.0 is a detailed kinetic mechanismthat describes the oxidation of methane and natural gas through325 elementary reactions and 53 species. Several works prove thevalidity of this mechanism to obtain the autoignition delay time of some gaseous fuels through comparison with experimental resultsobtained using different devices. Bozhenkov et al. 11 used a shock tube to determine the ignition delay of CH 4 /air and H 2 /air mixturesat wide pressure (3 - 450 atm) and temperature (1200 - 1750 K)ranges, using the OH emission technique. The experimental datawere in agreement with the theoretical data obtained using GRI-Mech at the whole range. Vandebroek et al. 23 measured the auto-ignition limits of CH 4 /air mixtures using a closed spherical vessel,and the results were compared to the computed ones obtained usingdifferent chemical reaction mechanisms, showing a good coinci-dence when the GRI-Mech mechanism was considered. Moreover,the suitability of GRI-Mech to simulate the laminar flame speedof the producer gas has been validated in previous work 8,24 throughcomparison with the experimental speed obtained from the instan-taneous pressure signal measured in a constant volume combustionvessel.

CHEMKIN 4.0 consists of a set of applications for solvingchemical kinetics problems. The homogeneous 0-D reactor model

(11) Bozhenkov, S. A.; Starikovskaia, S. M.; Starikovskii, A. Yu. SpacePlanes and Hypersonic Systems and Technologies , 11th AIAA/AAAF

International Conference, Orleans, France, Sept 29 - Oct 4, 2002; AssociationAeronautique et Astronautique de France: Verneuil-sur-Seine, France, 2002;p 2002-5185.

(12) Petersen, E. L.; Davidson, D. F.; Hanson, R. K. Combust. Flame1999 , 117 , 272- 290.

(13) Oran, E. S.; Boris, J. P.; Young, T.; Flanigan, M.; Burks, T.; Picone,M. Eighteenth Symposium (International) on Combustion , Waterloo, Canada,Aug 17 - 22, 1980; The Combustion Institute: Pittsburgh, PA, 1981; pp1641 - 1647.

(14) Oran, E. S.; Boris, J. P. Combust. Flame 1982 , 48 , 149- 161.(15) Heywood, J. B. Internal Combustion Engine Fundamentals ; McGraw-

Hill: New York, 1988.(16) Livengood, J. C.; Wu, P. C. Proceedings of the 5th International

Symposium on Combustion 1955 , 347- 356.(17) Melgar, A.; Dez, A.; Lapuerta, M.; Hernandez, J. J.; Alkassir, A.

Energa 2003 , 171 , 50.(18) Lapuerta, M.; Hernandez, J. J.; Tinaut, F.; Horrillo, A. SAE Tech.

Pap. Ser . 2001 , 2001-01-3586.

(19) Li, X.; Grace, J. R.; Watkinson, A. P.; Lim, C. J.; Ergudenler, A.Fuel 2001 , 80 , 196.

(20) Sridhar, G.; Paul, P.; Mukunda, H. Proc. Inst. Mech. Eng., Part A: J. Power Energy 2005 , 219 , 195- 201.(21) Kee, R. J.; Rupley, F. M.; Miller, J. A.; Coltrin, M. E.; Grcar, J. F.;

Meeks, E.; Moffat, H. K.; Lutz, A. E.; Dixon-Lewis, G.; Smooke, M. D.;Warnatz, J.; Evans, G. H.; Larson, R. S.; Mitchell, R. E.; Petzold, L. R.;Reynolds, W. C.; Caracotsios, M.; Stewart, W. E.; Glarborg, P.; Wang, C.;Adigun, O.; Houf, W. G.; Chou, C. P.; Miller, S. F.; Ho, P.; Young, D. J.CHEMKIN Software , release 4.0; Reaction Design, Inc.: San Diego, CA,2004.

(22) Smith, G. P.; Golden, D. M.; Frenklach, M.; Moriarty, N. W.;Eiteneer, B.; Goldenberg, M.; Bowman, C. T.; Hanson, R. K.; Song, S.;Gardiner, W. C., Jr.; Lissianski, V. V.; Qin, Z. GRI-Mech home page. http:// www.me.berkeley.edu/gri_mech/ (accessed June 2005).

(23) Vandebroek, L.; Winter, H.; Berghmans, J. Second Internet Confer-ence on Process Safety, March 20 - 24, 2000. http://www.safetynet.de/seiten/ 10.html.

(24) Hernandez, J. J.; Serrano, C.; Perez, J.; Horrillo, A. Inf. Tecnol.2004 , 15 , 19- 22.

Table 1. Mole Fractions of the Producer Gas Components forDifferent F rg Values

F rg ) 3 F rg ) 3.5 F rg ) 4 F rg ) 4.5 F rg ) 5

XH2O 0.05857 0.04220 0.03745 0.03418 0.03127XN2 0 .41195 0.38699 0.37195 0.35901 0.34719XCO2 0.09003 0.10166 0.11458 0.12719 0.13928XCH4 0.00049 0.01613 0.04022 0.06402 0.08663XH2 0 .19937 0.20917 0.19355 0.17584 0.15861XCO 0 .23958 0.24385 0.24226 0.23976 0.23701

F rg ) mbiomass / mair

(mbiomass / mair)st(1)

Figure 1. Definition of the fuel/air ratios in the gasifier - engine system.

F r )mproducer gas / mair

(mproducer gas / mair)st(2)

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compensated by the lack of oxygen, which is necessary topromote the reaction H + O2 ) OH + O. As has been shownin the literature for other fuels, 9 the influence of varying F r is

more significant when the mixture moves from lean conditions(F r ) 0.6) to the stoichiometric ones ( F r ) 1).

Three different correlations for the calculation of the producergas autoignition delay time (eqs 4, 5, and 6), corresponding tothe temperature ranges considered, have been obtained. In thoseequations, is the autoignition delay time (seconds), T is theinitial temperature (K), and p is the initial pressure (bar). Ashas been mentioned before, the effect of pressure has not been

considered in the middle temperature range, while the effect of F r has been only considered at temperatures lower than 1200K. The delay time data fit the Arrhenius expression with acorrelation coefficient ( R2) equal to 99.9% for eq 4 and 97%for eqs 5 and 6.

Sensitivity Analysis of the Producer Gas AutoignitionDelay Time. To study the main species and reactions affectingthe producer gas autoignition process, a sensitivity analysis hasbeen performed at different pressures (20 and 80 bar) andtemperature values (900, 1400, and 1800 K). The relativegasifier biomass/air ratio and the relative producer gas/air ratioin the engine have been set at F rg ) 3.5 and F r ) 1, respectively.This analysis has been carried out by using the homogeneous0-D reactor model of CHEMKIN. Due to the large number of reactions constituting the GRI-Mech reaction mechanism, a rate-of-production analysis has been previously made to determinethe contribution of each reaction to the net production ordestruction rate of the main propagating species (HO 2, H2O2,H, OH, O). Through this rate-of-production analysis, carriedout at each of the temperature and pressure values considered,34 of the 325 reactions that constitute the GRI-Mech wereselected as the most important ones that may affect theautoignition process of the producer gas. After that, a manualsensitivity analysis was performed over this set of 34 reactions,and a sensitivity coefficient ( C ) was calculated through theexpression

where k 0i is the pre-exponential factor of the reaction rate

Figure 4. Autoignition delay time (in microseconds) of a stoichiometricproducer gas/air mixture vs F rg at different pressure and temperaturevalues.

Table 2. Mole Fractions of the Components of a Producer Gas/AirStoichiometric Mixture for Different F rg Values

F rg ) 3 F rg ) 3.5 F rg ) 4 F rg ) 4.5 F rg ) 5

XO2 0 .10715 0.11551 0.12283 0.12879 0.13368XH2O 0.02847 0.01884 0.01542 0.01311 0.01127XN2 0 .60208 0.60549 0.61291 0.61945 0.62495XAr 0.00479 0.00517 0.00549 0.00576 0.00598XCO2 0.04393 0.04556 0.04737 0.04898 0.05039XCH4 0.00024 0.00720 0.01656 0.02455 0.03121XH2 0 .09690 0.09338 0.07969 0.06743 0.05714XCO 0 .11644 0.10885 0.09974 0.09194 0.08539

Figure 5. Autoignition delay time (in microseconds) of producer gas/air mixtures ( F rg ) 3.5) vs temperature, at 80 bar and different F r values.

(s) ) 1.189 E - 11 exp(21 277T ) p- 0.64F rg

0.42F r- 0.51

900 e T e 1200 K (4)

(s) ) 5.177 E - 13 exp(21 279T )F rg0.971200 < T e 1500 K (5)

(s) ) 3.980 E - 11 exp(13 552T ) p- 0.50F rg

2.85

1500 < T e 1800 K (6)

C )

p,i -

2k 0i - k 0ik 0i

) p,i -

(7)

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constant corresponding to reaction i in the GRI-Mech mecha-

nism, is the nonperturbed delay time of the producer gas ateach pressure and temperature value, and p,i is the relatedperturbed delay time, resulting from multiplying the pre-exponential factor of reaction i by a factor of 2. Results for thenine reactions with a C value greater than 0.01 (this meaning a1% change in ) are shown in Figures 6 and 7. If the sensitivitycoefficient exhibits a negative value, that would indicate anincrease in the overall reactivity of the chemical system (a lowerdelay time), and a positive value would imply a decrease in theoverall reactivity. A large sensitivity coefficient value indicatesa strong influence of the reaction on the producer gas auto-ignition process.

The first conclusion that can be obtained from Figures 6 and

7 is that, at temperatures around 900 K, the radical HO 2 becomesthe main propagating species, which reacts as follows:

The temperatures at which this particular sequence isimportant are in the range 900 - 1200 K. As Figures 6 and 7also show, the main reactions predicted as promoting the

autoignition process at high temperatures (1800 K) are those

involving the very reactive H, O, and OH radicals:

At intermediate temperature values (around 1400 K), kineticsdetermining the propagating chain and thus the autoignitionprocess involve both sets of reactions. When pressure increasesfrom 20 to 80 bar, the HO 2 reactions become more importantcompared with the H kinetics, which would retard the auto-ignition process. However, the much lower sensitivity coefficientof the reaction H + O2 + N2 ) HO2 + N2 at 80 bar, which

inhibits that process, compensates for such a retarding effect.This is the reason the producer gas delay time remains roughlyconstant with pressure at temperatures around 1400 K (Figure3).

Additionally, the time history of the representative species,H and HO2, is shown in Figures 8 and 9. In each figure, separatecharts have been made for 20 and 80 bar, with the delay timeindicated by a dotted line. As can be observed, the H molefraction is nearly zero before the delay time at low temperature(900 K), and only when the delay time is reached does the HO 2species vanish and the H radical appear. However, increases intemperature provoke the H kinetics to be more important, theH radical appears earlier, and a higher H peak value is alsoreached. The effect of pressure on the above-mentioned switch

Figure 6. Sensitivity coefficient of the delay time with respect to the main reactions, at 20 bar and different temperature values ( F rg ) 3.5, F r )1).

Figure 7. Sensitivity coefficient of the delay time with respect to the main reactions, at 80 bar and different temperature values ( F rg ) 3.5, F r )1).

H + O2 ) O + OH

OH + H2 ) H + H2O

O + H2 ) H + OH

HO2 + CO ) OH + CO2

HO2 + H2 ) H + H2O2

H2O2 + M ) 2OH + M

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in the propagating species from H to HO 2 is the opposite of that observed for temperature. While increases on temperaturecause the H radical to become more significant, higher pressuresfavor the slower HO 2 kinetics.

Comparison with Conventional Fuels. To analyze thepotential of the producer gas to be used as a fuel in internal

combustion engines, a comparison between the predictedautoignition delay time of producer gas, methane (as representa-tive of natural gas), and isooctane (representative of gasoline-like fuels) is shown in Figure 10. The isooctane delay timecorrelation has been taken from the literature, 15 whereas thepredicted methane values have been obtained from CHEMKINcomputations. As Figure 10 shows, the autoignition delay timeof isooctane is more sensitive to pressure variations than thosepredicted for methane and producer gas, but it shows lowersensitivity to cylinder temperature variations. At a constantpressure of 20 bar, autoignition time with producer gas is muchlonger than that with isooctane in the low-temperature range(below around 950 K), but it becomes much shorter in the high-temperature range. However, as pressure increases, the inversion

temperature (where autoignition time for producer gas andisooctane are equal) also increases (about 1100 K at 50 bar),and the differences between the producer gas and the isooctaneautoignition time are less significant in the high-temperaturerange. However, the autoignition time of methane is higher thanthat of producer gas in the whole thermodynamic conditions

range.Although the laminar flame speed of producer gas is lowerthan that of isooctane and higher than that of methane, 8 the useof producer gas from biomass gasification has been proved 4,5to reduce the knock tendency with respect to gasoline or naturalgas under typical pressure and temperature conditions in an SIengine (unburned temperature below 1100 K and high pressure).Thus, a producer gas-fuelled engine would permit someoptimization of the thermal efficiency through a certain increaseof the compression ratio, with similar knock tendency and NO xemission level (due to the lower combustion temperature of theproducer gas 18).

Additionally, and due to the shorter delay time of producergas compared with methane, the use of producer gas as a fuel

Figure 8. Time evolution of the HO 2 and H concentrations at 900 K ( F rg ) 3.5, F r ) 1). Delay time: 56 ms at 20 bar, 23 ms at 80 bar.

Figure 9. Time evolution of the HO 2 and H concentrations at 1800 K ( F rg ) 3.5, F r ) 1). Delay time: 5.88 10 - 4 ms at 20 bar, 2.94 10 - 4ms at 80 bar.

Figure 10. Autoignition delay time (in microseconds) of stoichiometric mixtures of methane, isooctane and producer gas ( F rg ) 3.5) with air atdifferent temperature and pressure values.

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or as a fuel component in a natural gas HCCI engine wouldpermit us to achieve stable temperature-controlled autoignitionat lower charge intake temperature values, thus reducing the

need for the preheating methods (residual gas trapping, electricalheating, etc.) typically used in this kind of engine. 26,27 Figure11 shows the ignition delay time ( t i), calculated by using theclose internal combustion engine simulator of CHEMKIN andassuming an adiabatic compression process of a lean producergas/air and methane/air mixture ( F r ) 0.6) under engineconditions (pressure and temperature changing instantaneously)at two different volumetric compression ratios ( r c ) 12 and r c) 16) and different intake temperature values ( T in). Thevolumetric compression ratio has been defined as eq 8 shows,where V d is the swept volume and V c is the clearance volume.The engine characteristics used for the calculations have beenthe following: supercharging up to an absolute intake pressure

) 1.5 bar, bore ) 82.7 mm, stroke ) 93 mm, connecting rodlength ) 144 mm, swept volume ) 499.5 cm3, and speed )3000 rpm. As can be observed, the predicted ignition delay timeof a producer gas fuelled engine is lower than that for methanefor the whole intake temperature range, and the minimum intaketemperature to achieve autoignition is also lower when producergas is used (around 460 K for producer gas and 520 K formethane at r c ) 12, and around 400 K for producer gas and460 K for methane at r c ) 16).

Comparison with the Livengood - Wu Integral Method.In this paragraph, the reliability of the widely used Livengood -Wu integral method 16 as a way to predict the ignition timing orthe knock tendency of a fuel/air mixture under engine conditionshas been described. The method assumes that autoignition takesplace when

where t i is the ignition delay period, is the ignition delay atthe pressure and temperature conditions pertaining at time t (calculated assuming stationary conditions by using eq 4, 5, or6), and t 0 is the initial time used for calculations (usuallycorresponding to the start of injection in a diesel engine or tothe end of the gas compression stroke, around the TDC, in a SIengine). Equation 9 assumes the following hypothesis: theoverall rate of production of the critical species in the delayperiod chemistry, for a given mixture, depends only on the gas

state, the concentration of the critical species during the delayperiod is conservative and additive, and the concentrationrequired to initiate autoignition is fixed.

Figure 12 shows the predicted ignition delay time of aproducer gas/air mixture for different gas composition, differentF r values, and different engine compression ratios ( r c). Theengine characteristics considered have been the same as thoseused in the previous section, and all the calculations have beencarried out for an intake temperature equal to 480 K. The delaytime values have been obtained both by using the Livengood -Wu method and by using the close internal combustion enginesimulator of CHEMKIN, the latter considering the variationson the species concentration caused by the chemical reactionstaking place during the compression process (pressure andtemperature changing instantaneously). No significant discrep-ancies between the CHEMKIN procedure and the Livengood -Wu method have been observed, although the latter seems tobe less sensitive to the producer gas composition (correspondingto different F rg values) and more sensitive to F r variations. Whenthe compression ratio increases, the maximum error resultingfrom the use of the Livengood - Wu method decreases due tothe shorter period to achieve autoignition (6 and 4% for F rg )5 when r c equals to 12 and 16, respectively).

Figure 11. Ignition delay time (in milliseconds) of a producer gas/airand a methane/air mixture ( F r ) 0.6) under engine conditions (intakepressure ) 1.5 bar).

Figure 12. Ignition delay time (in milliseconds) of producer gas/air mixtures under engine conditions (intake pressure ) 1.5 bar). Solid lines:Livengood - Wu values. Dashed lines: CHEMKIN values.

r c )V d + V c

V c(8)

t 0t i dt

( p,T )) 1 (9)

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Conclusions

The effect of the main parameters (producer gas composition,temperature, pressure, and relative producer gas/air ratio in theengine) affecting the autoignition delay time of producer gascoming from forestry biomass gasification under typical engineconditions has been studied theoretically. Results predicted bythe CHEMKIN software have been fitted to an Arrheniusexpression to be used in combustion models. While the predicted

autoignition delay time decreases exponentially when temper-ature increases, the influence of pressure is not so clear, due tothe pressure dependency balance between the reactions involvingthe HO2 radical and those involving the H atom. While in alow (T < 1200 K) and in a high temperature ( T > 1500 K)range the producer gas autoignition delay time decreases whenpressure increases, at intermediate temperature values (1200