Dong He*,Weiping Yan

MOE's Key Lab of Condition Monitoring and Control for Power Plant Equipment,North China Electric Power University,Baoding 071003,China

1.Introduction

Syngas(main compositions are H2and CO)has drawn more and more attention recently because of its higher efficiency and clean property used in integrated gasification combined cycle(IGCC).However,syngas,especially with higher hydrogen content,has shorter ignition delay time and is easy to ignite automatically,which affects safe operation of gas turbine.So it's necessary to research influences on ignition delay of syngas with various diluents and provide references for actual combustor design.

Generally,ignition delay of fuel is measured by shocktube[1–5]and rapid compression machine(RCM)[6,7].Shock tube is usually used to measure ignition delay under high temperature and ignition time is defined as the time bet ween arrive of reflected shock ware and maximum OH*gradient.Thiet al.[1]researched ignition delay of multi-component syngas mixtures and they have illustrated the effect of equivalence ratio..And ignition of lean CO/H2in air was conducted by Kalitanet al.[3]atT=890–1300K andp=0.1,0.25and1.5MPa.Ignitiondelay measured by RCM is de fined as the period between end of compression and maximum pressure gradient in ignition process.Mans field and Wooldridge[6]measured ignition delay of syngas at high pressure and low temperature conditions.And Waltonet al.[7]performed ignition of syngas at gas turbine conditions.In general,data measuring process costs lots of time and efforts.In addition,actual syngas compositions used in IGCC as well as operating conditions may not be exactly consistent with experimental measurements.

Kinetic simulation is an important way to calculate ignition delay of fuels[8,9].Usingrelevant H2/CO reaction mechanisms[10–13],ignition delay of syngas can be simply calculated at different temperatures and pressures with various compositions in syngas,which helps to save time as well as experimental cost and provide references to practical designing of combustor.Based on GRI Mech3.0[10]and considering the rate coefficient of H+O2(+M)=HO2(+M)and third body efficiencies published recently,Daviset al.[11]put forward a H2/CO combustion mechanism(Davis Mech)which included 14 species and 30 elementary reactions to simulate high-temperature H2and CO combustion and they compared calculated ignition delay,laminar flame speed and extinction strain rate with experimental results.Petrova and Williams[12]presented a simplified hydrocarbon reaction mechanism(SD Mech)which contains 21 steps among 8 species for H2combustion,30 steps among 11 species for CO combustion,and more than 300 steps for other hydrocarbon combustion which means it takes long time for CO/H2combustion calculation.Kéromnèset al.[13]presented a H2/CO reaction mechanism(NUIG Mech),and further validation was made at different H2/CO molar ratios,pressures and equivalence ratios.In addition,10 steps of OH*Chemiluminescence reaction were added in this mechanism.Besides,accuracy of these reaction mechanisms has drawn more and more attention.Fischer and Jiang[14–16]put forward a numerical optimization method for mechanism evaluation and they have checked the accuracy of several published mechanisms for combustion of biogas and bio-syngas.

Research of lean premixed combustion shows that lowering maximum temperature in combustion process is an important way to reduce NOxemission[17].At present,major diluents used to lower flame temperature are H2O,CO2and N2[18–20].N2could bederived from air separator while high pressure steam could be from heat recovery boiler in IGCC.Though purely adding CO2to syngas will increase CO2emission,MILD combustion technology,which mixes recirculating flue gas with reactors from inlet of com bust or,is a useful way to reduceNOxemission[21].So it's necessary to research ignition process of syngas under CO2-diluted conditions.Das and Sung's[22]research results indicated that when volumetric calorific value of syngas remained the same,5%and 10%H2O could increase reactivity and promote autoignition atp=3MPa.Mathieuet al.[23]pointed out that CO2addition had negligible effects on autoignition of syngas under low CO2content(0.15%).Vasuet al.[24]researched autoignition of syngas under high CO2content,but they didn't compare their results with conditions without CO2dilution.Replacing Ar by N2could promote reactivity for H2/O2/N2/Ar mixtures[25],however,to the authors'know ledge,there have no research and reports for syngas mixtures.

Based on published experimental data and reaction mechanisms,the aim of this paper is to research effects of H2O,CO2and N2as diluents on autoignition of syngas at gas turbine conditions.Firstly,this paper used three reaction mechanisms above to calculate ignition delay time of syngas and compared them with published experimental data to select the most accurate reaction mechanism.And then,the selected mechanism was used to calculate ignition time of syngas at gas turbine conditions with H2O,CO2and N2dilution respectively.Finally,this paper analyzed influences of the three different diluents on ignition character based on chemical and thermodynamic effects at different operating conditions.

Fig.1.Comparison with ignition delay of syngas mixtures from experiments[23,29]and calculating results from three mechanisms at p=1.25 and3.2MPa,syngas mixtures(φ=0.5and O2was oxidizer)were diluted in 98%Ar.

Fig.2.Comparison with ignition delay of syngas mixtures from experiments[1]and calculating results from three mechanisms at p=1 and 2 MPa,syngas mixtures(φ=1.0 and O2was oxidizer)were diluted in 94%Ar.

Table 1Compositions of syngas(mole fraction)

Fig.3.Molar constant-pressure specific heat of H2O,CO2and N2.

2.Kinetic Simulation Method

Fig.4.Ignition delay of syngas mixtures(φ=0.5,with 5%dilution)at 1 and 3 MPa and in temperature range of 900–1400 K.

Fig.5.Temperature pro files at 1MPa and 1200 K(φ=0.5,with 5%dilution).

This paper used closed homogeneous reactor in Chemkin-Pro[26]to calculate ignition time of syngasmixtures. Ignition delay time is defined by temperature inflection in combustion process, similar definition was used by Boivinet al.[27]and their calculating results agreed well with shock tube measurements.Chao and Dryer[28]showed that if constant-internal-energy,constant-volume(constantU,V)was chosen,there will be a distinguishable distinction between calculating results and experimental measurements at low and intermediate temperature conditions because a slight increase of pressure will appear caused by non-ideal facility influences of shock tube.In addition,Thiet al.[1]confirmed similar phenomenon above.So in order to reach more accurate calculating results in mechanism verification part,volume of reactor is defined as a function of time(decrease 2.5%/ms,through isotropic compression transform)rather than constantU,Vconditions.However,the phenomenon above is not due to constantU,Vmodel,so in result and discussion part,constantU,Vwas used to calculate ignition delay of syngas at different conditions.

3.Mechanism Verification

In this section,this paper compared experimental data with calculating results from three mechanisms.Experimental data in Fig.1 were from literatures[23,29],and syngas mixtures(φ=0.5,O2was oxidizer)were diluted in98%Arat12.5and30atm.Datain Fig.2 were from Thiet al.[1],and syng as mixtures(φ=1.0,O2was oxidizer)were diluted in94%Arat 1and2MPa.All the three mechan is mscoul dcaptureexper imentaltrend under these conditions.Comparing experimental data with predictions from mechanisms at elevated pressures,there are little distinctions among results of three mechanisms in high temperature range.SD Mech(dot line)had the highest value while Davis Mech(dash line)had the lowest value.In most case,NUIG Mech(solid line)could predict ignition delay of syngas accurately except Fig.1(b,f)at 12.5 atm and Fig.2(b,d)at 1 MPa in low temperature range.Overall,NUIG Mech was in the closest agreement with measured ignition delay.And Leeet al.[30]had the same conclusion by calculating deviations between prediction and experimental results.Based on former research results,this paper used NUIG Mech as reaction mechanism to present influences of three different diluents on ignition delay of syngas.

4.Mixture Compositions

Fig.6.Mole fraction pro files of(a)CO,H2,and(b)H atom,OH radical at 1MPa and 1200 K(φ=0.5,with 5%dilution).

Fig.7.Mole fraction pro files of(a)CO,H2and(b)H atom,OH radical at 3MPa and 900 K(φ=0.5,with 5%dilution).

In this paper,ignition delay of syngas was calculated in temperature rangeof900–1400Kandat pressuresof10and30atmrespectively.Syngas used in this paper is from fluidized bed coal gasification[31]with air and steam supply(specific compositions are shown in Table 1)and its volumetriccalorificvalueis10.013MJ·nm–3.Airwasuse dasoxidant and syngas mixtures were diluted with H2O,CO2and N2respectively.Fig.3 presents molar constant-pressure specific heats of three different diluents in the temperature range of 900–2700 K[32].Triatomic molecules have higher specific heats than diatomic molecules.In addition,when temperatureis increased,triatomic moleculeshavea hig hertem perature dependence than diatomic molecules.Equivalence ratio of syngas mixtures is de fined as

In addition,mole percentage of diluent,xdil.is de fined as

where,Xdil.,Xsyn.,Xair,are the mole content of diluent,syngas and air in syngas mixtures respectively.

5.Results and Discussion

This section mainly presents calculating results based on NUIG Mech.At first,sensitivity analysis method used in this paper was introduced.After that,the fictitious material method was introduced.And then,this paper showed effects of different diluents on ignition delay of syngas at different operating conditions.Finally,this paper presents influences of dilution rate and equivalenceratio on ignition time of syngas.In addition,special phenomena caused by these diluents are discussed and explained in this section.

Fig.9.Sensitivity coefficient of different elementary reactions at 1 MPa and1200K(φ=0.5,with 5%dilution),showing the ten most sensitive reactions.

5.1.Brute-force sensitivity analysis method

In order to analyze effects of main elementary reactions on ignition delay of syngas with different diluents at different operating conditions,this paper used NUIG Mech as reaction mechanism.Sensitivity coefficient is defined as Eq.(3).τ(2ki)is calculated when rate constant ofith reaction is multiplied by a factor of 2 while τ(0.5ki)is calculated when the constant is divided by 2.

Fig.8.Ignition delay of syngas mixtures(φ =0.5,with 5%dilution)at 1 and 3MPa and in temperature range of 900–1400 K((a)H2O,(b)CO2,(c)N2).

where,kiis the rate constant ofith reaction and τ is the ignition delay time.IfSiis positive,it means that this reaction will inhibit reactivity of reactants.

5.2.General analysis of different diluents'influences

Adding diluents to syngas mixtures will lower burnout temperature in combustion process,thus inhibiting reactivity of syngas. In addition,some elementary reactions are influenced by collision efficiencies of differentthird bodies, such as H+O2(+M)=HO2(+M).Besides,some diluents are reactants or products of certain elementary reactions,for example,CO2is product of CO+OH=CO2+H.In order to show specific influencing extent of three conditions above,Liuet al.[33]put forward a fictitious material which has the same thermodynamic and transport properties with original material, but doesn't participate in reactions or just plays as third body role.This section fabricated FH2O,FCO2,FN2(not participate in reactions or play as third body)and TH2O,TCO2,TN2(only play as third body)and analyze specific effects of different diluents.

5.3.Effects of different diluents on ignition delay of syngas

Fig.4 presents ignition delay of syngas mixtures(φ=0.5,with 5%dilution)at 1 and 3MPa and in temperature range of 900–1400 K.There are significant differences among three different diluents atp=1MPa andT=1150–1250 K.For example,comparing with 5%N2dilution at 1200 K,ignition time of adding H2O is increased by 264.70%and adding CO2is increased by 67.01%.Fig.5 shows calculated temperature pro files at 1MPa and 1200 K with different diluents.Adding CO2has the lowest burnout temperature while adding N2has the highest one.This phenomenon is caused by specific heats of different diluents.Fig.6 shows mole fraction pro files of CO,H2,H atom and OH radical.In combustion process,oxidation of H2is in advance of CO because H2has higher reactivity.In addition,adding H2O causes the highest concentration of OH radical while these diluents have little differences on H atom.However,under low temperature conditions,there are inconspicuous differences caused by these diluents,even at 3MPa,adding H2O presents slightly shorter ignition time than CO2and N2.For example,comparing with 5%N2dilution at 900 K and 30 atm,ignition time of adding H2O is decreased by 12.08%and adding CO2is increased by 2.77%.Fig.7 presents the mole fraction pro files of CO,H2,H atom and OH radical under condition above.Being different with information in Fig.6,adding H2O produces higher concentration of H atom and OH radical than CO2and N2.

Fig.11.Ignition delay of syngas mixtures with 5%and 15%dilution(φ=0.5,(a)H2O,(b)CO2and(c)N2as diluents).

In order to analyze specific effects of different diluents,this section used method in Section5.2 to cal culate ignition delay of syngas(calculating results showing in Fig.8).Comparing with FH2O,there are indistinguishable differences between H2O and TH2O dilution,which means that as reactants or products,H2O has little influences on autoignition of syngas,however,as third body,it has significant influence.In addition,in high temperature range,addingCO2has similar phenomena.Fornitrogen,there are little differences among N2,FN2and TN2.But comparing with H2O or CO2,the differences above are negligible,which means adding nitrogen mainly affects temperature rise in combustion process.

In order to illustrate effects of different elementary reactions on ignition delay of syngas,this paper used NUIG Mech to do sensitivity analysis.Fig.9 shows analysis results at 1MPa and 1200 K.Under conditions above,chain-branching reaction H+O2=O+OH(R1)and chain propagating reaction H+O2(+M)=HO2(+M)(R9)are the two dominant reactions that control ignition delay of syngas.In addition,R1 and R9 compete for same reactants.Research of fictitious material shows that in high temperature range,H2O and CO2mainly influence ignition of syngas through being as third body in certain elementary reactions.So for reaction R9 in NUIG Mech,H2O and CO2have higher collision efficiencies than N2,thus promoting forward reaction of R9 and inhibiting reactivity of syngas.

Similarly,Fig.10 presents sensitivity analysis results at 30 atm and 900K.Underconditionsabove,H2O2(+M)=2OH(+M)(R18,including R19:H2O2(+H2O)=2OH(+H2O))and H2O2+H=H2+HO2(R21)are the two dominant reactions that control reactivity of syngas mixtures while R1 and R9 are no longer the primary reactions.In Fig.8(a),H2O as third body will in crease reactivity of syngas in low temperature range.So H2O will promote forward reaction of H2O2(+M)=2OH(+M),thus causing shorter ignition delay.

Researching ignition characteristics of syngas with different diluents can provide references for designing actual combustor.For instance,adding H2O in high and intermediate temperature can lower burnout temperature as well as inhibit autoignition of syngas;however,in low temperature it's necessary to pay attention to promoting effect of H2O,especially at elevated pressure.In addition,adding CO2can evidently lower temperature because of its high specific heat,and have relatively little influences on ignition delay of syngas.

5.4.Effects of dilution rate on ignition delay of syngas

Fig.11 shows ignition delay of syngas(φ=0.5)with5%and15%dilution.With increase of dilution rate,adding H2O has little influences in low temperature field while it causes longer ignition delay in high temperature conditions;for CO2dilution,ignition time of syngas is increased under theres ear ched conditions;for N2dilution,there are insignificant differences comparing with other two diluents.Figs.12 and 13 present sensitivity analysis results atT=1200 K andp=1MPa as well asT=900 K andp=3MPa.For different dilution rates,R1 and R9 are still dominant elementary reactions in high temperature range while R18(including R19)and R21 dominate in low temperature range.For H2O and CO2dilution,deviations between varied dilution rates are more significant at high and intermediate temperature range(in Fig.11(a,b)),especially atp=1MPa,and this is caused by higher collision efficiency of H2O and CO2in R9.Adding 15%H2O makes R1 and R9 not ass ignific ant as adding5%H2Oin Fig.12(a).In addition,addingN2have little effects on main elementary reactions.So,for syngas as well as combustion conditions researched in this paper,adding N2mainly influence temperature rising process,thus inhibiting reactivity of syngas.

Fig.12.Sensitivity coefficient of different elementary reactions with 5%and 15%dilution(φ=0.5,p=1MPa and T=1200 K),showing the ten most sensitive reactions.

Fig.13.Sensitivity coefficient of different elementary reactions with 5%and 15%dilution(φ=0.5,p=3MPa and T=900 K),showing the ten most sensitive reactions.

5.5.Effects of equivalence ratio on ignition delay of syngas

In actual combustion process,equivalence ratio of air and fuel is frequently not a constant,especially under variable working conditions.In order to show influences of equivalence ratio,this section calculated the ignition delay of syngas at different equivalence ratios(φ=0.5,1.0),and calculating results are presented in Fig.14.With the increase of equivalence ratio,ignition delay becomes shorter.For example,curve at φ=1.0(black line)can be approximately derived through shifting curve at φ=0.5(red line)downwards for a certain distance,especially when temperature is lower than 1100 K.Mole fraction of H and OH are higher at φ=1.0(in Fig.15(a))because there is more H2content in syngas mixtures under that conditions.And Fig.15(b)shows temperature pro files atT=900 K with 5%H2O dilution.Comparing with conditions at φ =0.5,burnout temperatures of syngas mixtures at φ =1.0 are significantly increased(over 400 K).Higher equivalence ratio(φ ≤1.0)means that air concentration(especially nitrogen)in syngas mixtures is lower,thus less materials need to be heated.So,under lean premixed conditions,equivalence ratio mainly influences temperature rise in combustion process,thus affecting reactivity of syngas.However,this paper didn't observe opposite influences of equivalence ratio on ignition delay mentioned by Thiet al.[1]because O2was used as oxidizer and syngas was diluted in 94%Ar in that paper.

Fig.14.Ignition delay of syngas at φ=0.5 and 1.0 with 5%H2O,CO2and N2dilution((a)p=1MPa,(b)p=3MPa).

Fig.15.(a)Mole fraction of H,OH and(b)temperature pro files of syngas mixtures at φ=0.5 and 1.0(T=900 K)with 5%H2O dilution.

6.Conclusions

This paper used NUIG Mech to illustrate influences of H2O,CO2and N2as diluents on ignition delay of syngas atT=900–1400 K andp=1 and 3MPa respectively.Some important conclusions were summarized here.In high temperature range,comparing with N2dilution,adding H2O and CO2significantly inhibit autoignition of syngas.As for low temperature conditions,adding H2O can promote reactivity of syngas,especially under high pressure.For syngas and combustion conditions in this paper,adding N2mainly influences temperature rising process of syngas combustion.Syngas mixtures with φ=1.0 has shorter ignition delay time than mixtures with φ=0.5,especially in low and intermediate temperature range,because high equivalence ratio(φ ≤1.0)means less air(especially nitrogen)in syngas mixtures needs to be heated.

Nomenclature

ppressure

Ttemperature

Xair.mole content of air in syngas mixtures

Xdil.mole content of diluent in syngas mixtures

Xsyn.mole content of syngas in syngas mixtures

xdil.mole fraction of diluent

φ equivalence ratio

Acknowledgments

The authors are grateful to Combustion Chemistry Centre in NationalUniversity of Ireland, Galway for the H2/CO reaction mechanism, andNorth China Electric Power University for supporting this research project.The authors are appreciated for Dr. Xiong Xiaohe's useful proposalsduring the shock tube simulation.

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