rql combustion as an effective strategy to nox reduction

RQL COMBUSTION AS AN EFFECTIVE STRATEGY TO NOX REDUCTION IN GASTURBINE ENGINES

ABSTRACT

The growing need to drastically reduce aircrafts CO2emissions has led engineers and scientists in the last years todevelop a clean, renewable and sustainable energy system.Hydrogen as a fuel in aviation has shown to be a good choice,since it has an energy release much higher than common andlong chain hydrocarbons (119.96 MJ/kg vs ∽42.8 MJ/kg,respectively), a wide flammability limits, a high diffusivity anda very short ignition time. Its thermal conductivity, the highestamong all gases, its high heat capacity and its very lowdynamic viscosity provide superior cooling properties foroperation at high flight speeds and at combustor hightemperatures. Furthermore, its low molecular weight makes itthe fuel with the higher specific impulse (ISP), ∼450 s: thismeans that burning 1 kg/s of hydrogen with oxygen produces athrust of 450 kg-force.

However, for its high combustion temperature, it has beendemonstrated to be disadvantageous in terms of NOxproduction. Although NOx pollution is a relatively small part ofglobal human pollution (less than 4%), a particular feature ofair transport is that pollutants from air traffic are emitted at highaltitudes, in the upper troposphere/lower stratosphere (8 to 12km), where they are of greater influence than those emitted atground level. Moreover, further emission reductions need to beachieved by the air transport community, since air traffic has agrowth (3% to 5% per year) which exceeds the technologyimprovement rate.

Emissions may be controlled by operating at lean or verylean equivalence ratios (thanks to the wider flammability limitsof the hydrogen-air flames compared to kerosene-air flames), orreducing the combustor length (thanks to the higher flamespeed of hydrogen compared to other fuels), or via innovativestrategies. In this paper, the RQL (Rich-Quench-Lean) strategyfor the NOx abatement will be proposed for a high speedhydrogen fuelled vehicle.

INTRODUCTIONNext generation of aviation aircrafts should include long-distance high speed flights. In this context, the EuropeanLAPCAT II project had the goal to develop a commercialvehicle able to fly from Brussels to Sydney (~20,000 km) inless than 4-5 hours [1]. Achieving this goal intrinsicallyrequired the investigation of a new flight regime, a newpropulsion concept, and a high energy content fuel, such asliquid hydrogen [2]. The Reaction Engines Srl conceived theSCIMITAR precooled engine [3-5], capable of sustained Mach5 flight for the A2 LAPCAT vehicle (see Fig. 1).

Antonella IngenitoDepartment of Mechanical and Aerospace

Engineering, University of Rome “Sapienza”Roma, Italy

Antonio AgrestaDepartment of Mechanical and Aerospace


Roberto AndrianiDepartment of Energy

Polytechnic University of MilanMilan, Italy

Fausto GammaDepartment of Mechanical and Aerospace


1 Copyright © 2014 by ASME

Proceedings of the ASME 2014 International Mechanical Engineering Congress and Exposition IMECE2014

November 14-20, 2014, Montreal, Quebec, Canada

IMECE2014-36898

Fig. 1: LAPCAT A2 vehicle with 4 scimitar engines wing-mounted.

This cruise engine has been derived from the ReactionEngines SABRE engine designed to propel the SKYLONSSTO spaceplane. Liquid hydrogen is used for two mainreasons. Firstly, it has a large calorific value (120MJ/kg) whichreflects the LAPCAT project mission requirements. Secondly,although liquid hydrogen is a hard cryogen (having a lowdensity of 68kg/m3 at a boiling point of only 21K), it has a veryhigh thermal capacity, almost 3.5 times that of water. If storedat low enough temperatures to maintain its state, it cantherefore be used to effect the precooling of the air entering thecompressor up to Mach 5, while maintaining an equivalenceratio close to that for optimum performance. A critical issueconcerning the hydrogen choice is the NOx emission index (EI)[6,7]. The emission index of NOx is an important issue sincemuch of the concern regarding the atmospheric effects of highspeed vehicle emissions arises from NOx emissions. The EI isdefined as the grams of total NOx emitted per kilogram of fuelburned. The conventional Scimitar engine released a NOx EI of315 g/kgfuel, i.e., much higher than the ICAO EI constraints(40 g/fuel). In this paper, the RQL strategy has been proposedas possible strategy to reduce the NOx emission of theconventional Scimitar engine [8]. In fact, keeping the ScimitarE.R. of 0.8, this strategy has demonstrated that it is possible todecrease the NOx EI below the ICAO constraints.

POLLUTANTS IMPACT ON ENVIRONMENT ANDHUMAN HEALTH

Due to the growth of the aviation transport in the lastdecades, it is essential to consider the environmental impact ofaviation to ensure in advance that such a rate of development issustainable. The risk of climate change linked to the effects ofgreenhouse gases is a major concern: altered properties of theatmosphere affecting local areas include reduced visibility,resulting from the presence of carbon-based particulate matter,sulphates, nitrates, organic compounds, and nitrogen dioxide;

increased fog formation and precipitation, reduced solarradiation, and altered temperature and wind distributions [9].On a larger scale, greenhouse gases may alter global climates.Also, acid rain, produced from NOx emissions (nitric oxide(NO) and nitrogen dioxide (NO2) are the major constituents ofNOx) affects lakes and susceptible soils.The catalytic destruction of stratospheric ozone by NO is also atopic to be investigated [10-11]. In fact, the reaction mechanismshows that the NOx radical catalyzes the destruction of ozonemolecules in the stratosphere[12]:

NO + O3 → NO2 + O2NO2 + O → NO + O2

Net reaction:O + O3 → 2O2

This mechanism points out that O3 is destroyed in the firstreaction by NO, while NO is regenerated in the second reactionto enter again into the O3-destruction step. Removal of O3 fromthe stratosphere allows harmful ultraviolet solar radiation topenetrate to the Earth’s surface.The health of population in relation to air quality is the secondmajor concern. Because of the difficulty of conducting researchon human subjects and the large number of uncontrolledvariables, controversy exists in assessing the effects ofpollution on human health. However, it is well known thatpollutants can aggravate pre-existing respiratory ailments.Nitric oxide and various hydrocarbons formed ozone, organicnitrates, photochemical aerosol which cause some irritations.The ensuing destruction of the Earth’s protective ozone layerleads to an increased exposure of all life forms to ultravioletlight such as UV-B, which can increase the incidence of skincancer.That's why, the European Union and the U.S. EnvironmentalProtection Agency (EPA) are applying pressure on theInternational Civil Aviation Organization (ICAO, this agencypromote the safe and orderly development of international civilaviation throughout the world. It sets standards and regulationsnecessary for aviation safety, security, efficiency and regularity,as well as for aviation environmental protection) that regulatesaircraft emissions for additional nitrogen oxide reductions fromaircraft [6].In Table 1 [3], emissions from various vehicles aresummarized. The Concorde values are measured during flight.The supersonic transport (SST) column represents the input toatmospheric chemistry models and reflects the relative lack ofsophistication of models at that time [13]. The High-SpeedCivil Transport (HSCT) column refers to work in the UnitedStates in the 1990’s and illustrates advances in emissionscharacterization and chemical modeling by that time [9, 14].The Scimitar engine exhaust is as given by finite rate chemicalkinetics calculation.Compared with other high speed vehicles, it can be seen thatthe predicted Scimitar-propelled aircraft would have: no SOxchemistry /NO soot chemistry /NO CO or CO2 , however, NOxemissions are excessive.


Table 1: Emissions from a selection of vehicles (EI valuesgiven in units of grams emission per kilogram of fuel burned)[3].

Concorde SST HSCT Scimitar

Mach number 2 2.4 2.4 5Cruise altitude(km) 18.3 20 20 24-28

EI CO2 3155 * 3155 0EI CO 3.5 * 2.9 0EI H2O 1237 1250 1237 8810EI NOx 18 40 3-15 315EIHydrocarbons 0.2 * 0.3 0

EI Soot 0.03 * 0-0.02 0* Not considered

OXIDES OF NITROGEN FORMATION

Nitric oxide is an important minor species incombustion because of its contribution to air pollution. In thecombustion of fuels that contain no nitrogen, nitric oxide isformed by chemical mechanisms that involve nitrogen from theair: the thermal or Zel’dovich mechanism [15-16], theFennimore or prompt mechanism[17] and the fuel mechanism[18]. The thermal mechanism dominates in high-temperaturecombustion over a fairly wide range of equivalence ratios,while the prompt mechanism is particularly important in richhydrocarbons combustion. It appears that the fuel mechanismplays an important role in the production of NO in very lean,low-temperature combustion processes.

THERMAL MECHANISM

Thermal NO contributes to a large portion of the overallNO levels in most conventional gas turbine combustors. Thismechanism consists of the two chain reactions:

O + N2 ↔ NO + NN + O2 ↔ NO + O

The extended Zel’dovich mechanism includes the additionalreaction:

N + OH ↔NO + Hwhich becomes important in fuel-rich mixtures when O and O2concentrations are low.The first reaction in the extended Zel’dovich mechanism is therate-limiting step.In this step, high activation energy is required to break the triplebond in the N2molecule.The rate of formation of NO via theZel’dovich mechanism can be obtained using chemical kinetics,combined with steady state assumptions for the N-atom

concentration and a partial equilibrium assumption for the O-atom concentration.Using equation about chemical kinetics, this yields thefollowing NO reaction rate expression [15-16]:

[ ]=√

[ ][ ] / exp[−38.370

]

The resultant equation for the rate of NO formation shows anexponential dependence on the combustion gas temperature(Low T(K) →Low NO).In the SCIMITAR engine, the Zel’dovich mechanism is themost important mechanism for the NOx formation. However,for the completeness of this topic, the other two mechanismswill be reported in the next Sections.

PROMPT MECHANISM

The prompt mechanism is intimately linked to thecombustion chemistry of hydrocarbons [17]. Fennimorediscovered that some NO was rapidly produced in the flamezone of laminar premixed flames long before there would betime to form NO by the thermal mechanism, where from theprompt NO appellation. The general scheme of the Fennimoremechanism is that hydrocarbon radicals react with molecularnitrogen to form amines or cyano compounds.The amines and cyano compounds are then converted tointermediate compounds that ultimately form NO:

CH + N2↔ HCN + NThe conversion of hydrogen cyanide, HCN, to form NOfollows the chain sequence:

HCN + O ↔NCO + HNCO + H ↔NH + CO

NH + H ↔N + H2N + OH ↔NO + H

Fig. 2 shows the path of the generation of prompt NO fromHCN, described above.

Fig. 2:Prompt NO generation via HCN

NO is formed at rate faster than predicted according to ‘quasi-equilibrium’ calculation, i.e., is formed early in the flame zone.In pre-mixed flames, dominant cause is reaction of hydrocarbonradicals (CH) with N2.


FUEL MECHANISM

This mechanism is important in fuel-lean (Φ<0,8), low-temperature conditions [19].Fuel NO is formed from nitrogen bound in the fuel, and isusually assumed to proceed through formation of HCN and/orNH3 which are oxidized to NO while being competitivelyreduced to N2 according to the overall reactions :

HCN/NH3 +O2 →NO + ...NO +HCN/NH3 → N2 + ...

Nitrogen bound in the coal is released during the destabilizationprocess. A fraction of the nitrogen (α) is rapidly converted toHCN, and the remaining portion of the fuel nitrogen reacts toform NH3. These two species react to form either NO or N2depending on the local conditions.The NO formed can be reduced by heterogeneous reaction withchar particles. This is schematically illustrated in Fig. 3. Infuel-rich regions, these nitrogen-containing species willtypically be reduced to N2, and in fuel-lean regions they aregenerally oxidized to form NO. Controlling the localenvironment in which nitrogen is released from the fuel is aprimary means of controlling NO emissions.Fuel NO is the dominant NO formation mechanism in flameswhich contain nitrogen in the fuel, and typically accounts formore than 80% of the NO formed in air/hydrocarboncombustion systems.Experimental evidence indicates that HCN is normally the firstmeasurable volatile nitrogen species. HCN may consist of onlya small portion of the nitrogen compounds initially evolvedfrom the coal, but the remainder of these nitrogen compounds israpidly converted to HCN in the absence of oxygen. In fuel-richgas flames doped with nitrogen-containing compounds, HCN isthe major product of the fuel-nitrogen/hydrocarbon interactions,and fuel-nitrogen exists mainly as HCN just downstream of thereaction zone. This agrees with the finding that HCN is themost stable nitrogen product in the primary reaction zone ofhigh temperature, premixed hydrocarbon flames.

Fig. 3: Schematic representation of the fuel NO formation andreduction process

High initial concentrations of HCN decay rapidly to low valuesat reaction times on the order of 100 ms for systems withstoichiometric ratios .0.70. In systems with fuel-nitrogenpresent and stoichiometric ratios, 0.70, hydrocarbons recycleNO back to HCN by reactions such as:NO +CXHY → HCN + ...

The HCN reacts further to form NO or N2. The relative yieldsof NO and N2 depend mainly on the local stoichiometric ratio.For sufficient residence times in a very fuel-rich gas, the fuel-nitrogen converts to relatively small amounts of NO and largeamounts of N2.Homogeneous NO reduction partially explains the sensitivity ofthe NO concentration to the local stoichiometric ratio. At fuel-rich conditions, less oxygen is available for NO formation, andmore nitrogen containing species are available forhomogeneous reduction of NO to N2.

Fuel NO has been shown to have a weak temperaturedependence in turbulent, diffusion-type, pulverized-coal flames.This could be attributed to the offsetting effects of volatilesproduction and stoichiometric ratio on NO formation atdifferent temperatures. Homogeneous fuel NO formation is alsosensitive to burner design and various techniques for fuel andair contacting.

NOX EMISSIONS ANALYSISThe goal of this section is to perform a parametric analysis

of the pollutants production as function of temperature,pressure, E.R. and residence time in order to define key issuesfor a NOx reduction strategy.Simulations have been performed by means of the CHEMKINPRO software [20].

Fig. 4 shows the simplified combustor diagram view. Inparticular, there are two internal sources for inlet gas (Air andH2), one perfectly stirred reactor (air and hydrogen aresupposed to be perfectly mixed) and an outlet flow.

Fig. 4: Simplified combustor diagram view

The Konnov’s H2/air kinetic scheme [21] consisting of 1016reactions and 127 species has been chosen.Fig. 5 -Fig. 7 show that the production of NO increases withthe residence time advancement until equilibrium is reachedand keeps constant thereafter. In particular, assuming as initialtemperature Ti=1000 K, at p=10 atm and E.R.=0.5, the NOxproduction always increases in time (from 0 at t=0 s to 0.02 att=0.05s) since the equilibrium for the NOX production is notreached , whereas, assuming Ti=1400 K, the NO mole fractionincreases from 0 to 0.0138 at t=0.04 s and than keeps constant.This means that reducing the residence time, i.e., the combustorlength, in principle, it is possible to reduce the nitrogen oxideproduction.Increasing the initial temperature, the NOx production increaseswhichever the equivalence ratio and pressure are.


Fig. 5: X[NO] history vs T at p=1atm and E.R.=0.5

Fig. 6: X[NO] history vs T at p=1atm and E.R.=1

Fig. 7: X[NO] history vs T at p=1atm and E.R.=1.5

The NOx highest production after a residence time of 10 ms isin proximity of the stoichiometric equivalence ratio anddecreases going to leaner or richer mixtures.In fact, assuming a residence time of 0.01s, the increase of theequivalence ratio from E.R.=0.5 to E.R.=1 at a fixedtemperature of Ti=1000 K leads to an increase of X[NO] from0.0008 to 0.006 (compare Fig. 5 and Fig. 6). Still increasing theequivalence ratio from stoichiometric to rich mixtures, thenitrogen oxide molar fraction starts decreasing up to 0.002 atE.R.=0.4(see Fig. 6 and Fig. 7).The same behavior is found at higher temperatures (i.e.,Ti=1400 K): the nitrogen molar fraction initially increases from0.008 to 0.009 and then decreases to 0.004 at the E.R.respectively of 0.5, 1 and 1.4 (compare Fig. 5- Fig. 7).As for the effect of pressure, Fig. 8 shows that for lean mixtures(i.e. E.R.= 0.5), the increase of pressure increases the NOx molefraction at the combustor exit; for stoichiometric or rich

mixtures instead, the NOx production increases with pressure inthe range of 1-2 atm and then stays approximately constant (seeFig. 20).

Fig. 8:NO Emissions vs p and E.R. at Ti=1400 K

The greenest equivalence ratio, i.e. the equivalence ratio thatallows the lower NOx production depends on the residence timewithin the reactor (see Fig. 8): in fact, for an initialtemperature T=1400 K, at p=2 atm, and assuming a residencetime of 0.01 s, the NOx mole fraction produced is XNO=0.002 atE.R.=0.5, XNO=0.004 at E.R.=1.4 and XNO=0.009 at E.R.=1,while, after 0.01s the NOx mole fraction produced isXNO=0.004 at E.R.=1.4, XNO=0.01 at E.R.=1 and XNO=0.012 atE.R.=0.5. Therefore in order to control the NOx emissions, anappropriate choice of the equivalence ratio, pressure andtemperature is critical.

Fig. 9:Ignition delay time vs pressure

The combustor initial temperature and pressure also affects theignition delay time, calculated as the time spent to reach atemperature of 400K above the ignition one; as known theincrease of temperature always has the effect to decrease theignition delay time. As for the pressure, Fig. 9 shows a “kneeshape” ignition delay vs pressure behavior. In particular, goingfrom 1 atm to 2 atm, no variation are predicted while,increasing the pressure from 2 atm to 7 atm, the ignition delayincreases from 0.2 ms to 7.4 ms, and start decreasing when thepressure increases beyond 7 atm.

Analyzing the NOx production against the time, Fig. 10 showsthat the choice of the equivalence ratio is highly affected by theexhausts residence time.In fact, assuming a nominal combustor pressure of 1 atm and ainitial temperature of T=1400 K, Fig. 10 shows that the threecurves of the X[NO] production against the residence timecross each other at t=0.0035s and at t=0.009 s.

E.R.=0.5 p=1 atm

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0 0.02 0.04t [s]

X[N

O]

Ti=1000 K

Ti=1400 K

Ti=1100 K

Ti=1200 K

Ti=1300 K

E.R.=1 p=1atm

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0 0.005 0.01t[s]

X[N

O] Ti=1000 K

Ti=1100 K

Ti=1200 KTi=1300 K

Ti=1400 K

E.R.=1.4 p=1atm

00.0020.0040.0060.0080.01

0.0120.014

0 0.005 0.01t[s]

X[N

O] Ti=1000 K

Ti=1400 KTi=1100 KTi=1200 KTi=1300 K

t=0.001s

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0 2 4 6 8 10p[atm]

X[N

O]

E.R.=1

E.R.=0.5

E.R:=1.4

t =0.01s

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0 2 4 6 8 10p[atm]

X[N

O]

E.R.=1

E.R.=0.5

E.R:=1.4


Therefore, before t=0.0035s, an equivalence ratio of 0.5 resultsin the greenest, whilst it is the worst after 0.009 s.This means that the NOx reduction strategy is sensitive to thecombustor operative conditions and the exhausts residencetime.

Fig. 10: X[NO] history vs E.R. and combustor inlet T atp=1atm

Fig. 11: X[NO] history vs E.R. and combustor inlet T atp=5atm

Fig. 12: X[NO] history vs E.R. and combustor inlet T at p=10atm

Assuming instead a lower temperature, i.e. Ti=1100 K, thethree curves (at p=1 atm) for the three different equivalenceratio, don’t cross in the time. This suggests that the strategy tolimit NO seems simple: burn lean, E.R. = 0.5 in this case is thebest choice.At combustor pressures higher than 1.5 atm (e.g. p = 5, 10 atm)the three E.R. curves always cross.

Fig. 13: Zoom on NO emissions vs time: Burn via RQL

By zooming on the Fig. 12 (Fig. 13), the history of NOxemissions as a function of E.R. shows that, in order to reducethe NOx emission, it is convenient previously burn a richmixture and than a lean mixture. This suggests theimplementation of the RQL strategy to reduce NOx emissions[19, 20]. This strategy, in fact, consists of burning with a richequivalence ratio, mix instantaneously the burned mixture andthen burn with a lean equivalence ratio.

PRINCIPLES OF RQL COMBUSTION

GENERAL DESCRIPTION

The Rich-Burn, Quick-Mix, Lean-Burn (RQL) combustorconcept is introduced as strategy to reduce nitrogen oxidesemissions [23, 24]. Rich-burn, Quick-quench, Lean-burncombustor zones are characterized by the presence of twoseparate reaction zones, operating respectively in rich and leanconditions. The idea is to take advantage of both of the goodstability and low NOx emissions associated with the richcombustion zone and, subsequently, to complete thecombustion of the unburned H2 in a lean stage where additionalNOx production is also low.The RQL concept is predicated on the premise that the primaryzone of a gas turbine combustor operates most effectively withrich mixture ratios (Fig. 14).

Fig. 14:Outline of the RQL combustor

For staging to be effective, the mixing of rich products and airmust be very rapid. Fig. 15 gives an illustration of thiscombustion.

T=1000K T=1100K p=1 atm

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0.009

0.01

0 0.002 0.004 0.006 0.008 0.01

t[s]

X[N

O] E.R.= 0.5 T=1000K

E.R.= 0.5 T=1100KE.R.= 1 T= 1000KE.R.= 1 T=1100KE.R.= 1.4 T=1000KE.R.= 1.4 T=1100K

T=1200K T=1400K p=1 atm

00.001

0.0020.003

0.0040.0050.006

0.0070.008

0.0090.01

0 0.002 0.004 0.006 0.008 0.01

t[s]

X[N

O]

E.R.= 0.5 T=1200KE.R.= 0.5 T=1400KE.R.= 1 T=1200KE.R.= 1 T=1400KE.R.= 1.4 T=1200KE.R.= 1.4 T=1400K

T=1000K T=1100K p=5 atm

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0.009

0.01

0 0.002 0.004 0.006 0.008 0.01

t[s]

X[N

O]

E.R.= 0.5 T=1000KE.R.= 0.5 T=1100KE.R.= 1 T= 1000KE.R.= 1 T=1100KE.R.= 1.4 T=1000KE.R.= 1.4 T=1100K

T=1200K T=1400K p=5 atm

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0.009

0.01

0 0.002 0.004 0.006 0.008 0.01

t[s]

X[N

O]

E.R.= 0.5 T=1200KE.R.= 0.5 T=1400KE.R.= 1 T=1200KE.R.= 1 T=1400KE.R.= 1.4 T=1200KE.R.= 1.4 T=1400K

T=1000K T=1100K p=10 atm

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0.009

0.01

0 0.002 0.004 0.006 0.008 0.01

t[s]

X[N

O]

E.R.= 0.5 T=1000K

E.R.= 0.5 T=1100K

E.R.= 1 T= 1000K

E.R.= 1 T=1100K

E.R.= 1.4 T=1000K

E.R.= 1.4 T=1100K

T=1200K T=1400K p=10 atm

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0.009

0.01

0 0.002 0.004 0.006 0.008 0.01

t[s]

X[N

O]

E.R.= 0.5 T=1200K

E.R.= 0.5 T=1400K

E.R.= 1 T=1200K

E.R.= 1 T=1400K

E.R.= 1.4 T=1200K

E.R.= 1.4 T=1400K


Fig. 15:Schematic representation of staged combustion on NOx

The basic and ideal concept is illustrated in Fig. 15 for a rich-lean sequence. Consider the ideal staged-combustion processin Fig. 15 represented by the path 0-1-2-3 where the bell-shaped curve represents the NOx yield for a fixed residencetime, (Δt= Δtrich). In the rich stage, the amount of NOx formedin the time Δtrichis represented by the segment 0-1. Secondaryair is then instantaneously mixed (Δtmix = 0) with the rich(segment 1-2) with no additional NOx formed. In the lean stage,the H2 is oxidized and an additional amount of NOx is formed(segment 2-3) in the time associated with the lean stage (Δtlean).The burned fuel from the rich primary zone will be high in theconcentration of partially oxidized hydrogen, therefore, theaddition of a secondary air is needed to oxidize the highconcentrations of hydrogen, and intermediates. This isaccomplished by injecting a substantial amount of air throughwall jets to mix with the primary zone effluent and create a“lean-burn” condition prior to the exit plane of the combustor.

DETERMINATION OF MIXING CHARACTERISTICS

In the RQL combustors, the jet mixing plays an importantrole in minimizing all pollutant emissions and maximizingcombustion efficiency. The rich and lean combustion zones ofan RQL combustor are established by dividing the totalcombustor airflow.Since all of the fuel is injected into the first combustion zone,the rich and lean zone equivalence ratios (Φrich and Φlean,respectively) are related by the airflow split (S) defined to bethe ratio of the rich zone airflow (WAR) to the total combustorairflow (WAT) (Fig. 16) :

lean

rich

WARSWAT

(1)

At high power conditions, with a nominal value of Φlean= 0.4,23% of the total combustor airflow is admitted to the rich zoneto achieve Φrich = 1.8. The airflow not admitted into the richcombustor is termed the quench airflow, and is rapidly mixedinto the rich combustor effluent to achieve the overall leancombustor equivalence ratio. Since values for S are typically 20to 30%, the quench airflow (WAQ) represents the majority ofthe combustor airflow, values including between 70 and 80%.This energy must be used efficiently since the effectiveness ofthe quench mixing process is critical to achieving the RQLcombustor NOx emissions control. If a rapid transition from therich to lean conditions is not achieved, near-stoichiometric

mixtures will exist for unacceptably long times and hightemperatures and high NOx formation rates will be experienced.

Fig. 16: Airflow distribution into RQL

The quench mixer configurations being considered for the RQLcombustor are generally fixed geometry devices. As shown inFig. 16, jets are typically injected in crossflow.One parameter of importance to the mixer performance is themomentum flux ratio (J) of the quench jets to the crossflow.Generally, an optimal value of J exists which maximizes themixing.For a fixed geometry jet mixer, J depends primarily on thedensity and velocity of the air entering the mixing region(WAQ) and the reach mixture already burned in the first reactor(FMR):

2

2

.

.WAQ WAQ

FRM FRM

VJ

V

(2)

The velocities VWAQ and VFRM are not given in practical cases,but the volumetric flow rates under standard conditions ofpressure and given temperature.The velocities can be calculated using:

2

2

. .4

.4

W AQ W AQ

FRM FRM

dn V

D V

(3)

where FRM is the actual volumetric flow rate of themainstream, D is the chamber diameter, WAQ is the actualvolumetric flow rate for all orifices (n), and d is secondary airorifice diameter. The actual density and the actual volumetricflow rate were replaced by the reference values under standardconditions (with T, the absolute mixing temperature), using thefollowing equation:

00

00

TT

TT

(4)

Thus, equations give a new momentum flux ratio expression:


0

0

0

0

2 22

2WAQ WAQFRM

WAQ FRM FRM

T ndJT D

(5)

The momentum flux ratio can now be calculated directly basedon the process conditions and geometric configurations used inthe chemical processing industries (see Eq. (5)).This correlation shows that mixing is much more efficient asthe air mass flow rate in the mixing region is high. This meansthat mixing is favoured by imposing a very rich conditions inthe first reactor, in order to ensure a high J in the mixing region.The effect of this choice on the NOx emission index shouldtherefore be investigated.In particular, writing J as function of the rich and totalequivalence ratio, it can be noted that J increases increasing therich equivalence ratio, and decreasing the total equivalenceratio.

2 2

2 2

1 1WAQ WAQ

FRM FRM

R T

T

V VD DJV nd S V nd

(6)

Thus, Eq. (6) shows the geometric and thermodynamicparameters that should be accounted to maximize the mixing. Agood mixing separates clearly rich and lean combustion, asshown inFig. 17.

Fig. 17: General survey of a RQL combustion in a cylindricalduct

THE RQL STRATEGY FOR THE SCIMITARCOMBUSTOR

The previous section has shown that, looking at theimpact of the E.R., temperature, pressure and residence time onNO mole fraction, it is possible to derive a strategy to reduceNOx emissions. In particular, at combustor pressures higherthan 1.5-2 atm, the three E.R. curves always cross with the timeadvancing.Since the Scimitar combustor pressure is 7.6 atm, the RQLstrategy has been proposed and investigated.In particular, assuming the Scimitar Engine conditions asreference conditions at the combustor inlet [5]:

1. Air initial temperature = 989K

2. Hydrogen initial temperature = 920K

3. P =7.6 atm

4. tr= 1 ms

the NOx production has been estimated.Reference conditions at the combustor inlet are summarized inTable 2 [5]:

Table 2:combustor inlet conditions

In order to verify the effect of the initial temperature on theScimitar engine NOx EI and on the ignition delay, andassuming the nominal pressure of 7.6 atm, three differenttemperatures from 920 K to 985 K have been accountedfor.

Fig. 18:Ignition delay time vs E.R. (p=7.6 atm)

Fig. 18 shows that depending on the initial temperature, theequivalence ratio affects more or less significantly the ignitiondelay. In particular, at 920 K, increasing the equivalence ratiofrom 0.5 to 1.75, the ignition delay decreases of about 19%.Assuming an initial combustor temperature higher than 920 K,i.e., Tinit=956 K, for the same equivalence ratio interval, theignition delay decreases of about 8%.Increasing the exhaust residence, the NOx ppm peak (at theoperative conditions of the Scimitar engine) shifts from E.R.=1to E.R.=0.8 (see Fig. 19).These figures confirm that reducing the residence time, thepollutant emissions significantly decrease: this means that thecombustor length has a significant impact on the NOx ppm.

total pressure 10 bartotal temperature 920 K

173.6592 kg/s

total pressure 10 bartotal temperature 989 K

4.048 kg/s

total pressure 7.6 bar

SCIMITAR COMBUSTOR

mass flow rate

mass flow rate

H2

AIR

Combustion chamber

0.01

0.03

0.05

0.07

0.09

0.5 1 1.5E.R.

t [s]

Tinit=920KTinit=956KTinit=985K


Fig. 19: NOxppm at different residence times

As for the NOx EI, since hydrogen and kerosene have differentreaction heat, i.e.Hrker ≈ 43.2 MJ/kgHrhyd ≈ 119.7 MJ/kgand the ICAO normative are generally referred tohydrocarbons, the SCIMITAR EI have been calculated both interms of g/kJ and of g/kg. The EI expressed in g/kJ gives anidea of the NOx produced with respect to the reaction heatreleased.

Fig. 20:E.I. [g/kJ] compared with RB211engine (t=0.001s,t=0.02s)

Fig. 20 shows that assuming at the exhausts residence time of0.001 s after ignition, the NOx EI peak decreases from about3800 g/kJ to about 3000 g/kJ, by decreasing the initialtemperature from 985 K to 920 K. Increasing the residencetime to 0.02 s, the NOx EI peak increases to roughly 5400 g/kJfor the different initial temperatures.

In order to estimate the acceptable level of NOx EI, these valueshave been compared with those the RB211 engine that isamong those engines with the highest NOx emissions in theICAO data bank [21]: this engine has been chosen as the worstreference limit.The RB211 EI has been calculated both in terms of both g/kJand of g/kg: E.I (see Table 3).

Table 3: RB211 engine from ICAO data bank

Using the E.I. (in g/kJ) of the RB211 as the maximumacceptable value, Fig. 20 shows that the Scimitar engineemissions became acceptable only for E.R.<0.6 and E.R>1.2.Actually, being the Scimitar nominal equivalence ratio of 0.8,the maximum NOx EI is achieved, confirming the necessity toimplement a EI NOx reduction strategy. Note also thatincreasing the residence time, from t=0.001s to t=0.02s, thelean mixture always overcomes the E.I. limits and therefore theonly possible strategy became to burn rich.In the next section the RQL strategy for the Scimitar conditionsis reported.

MODELING APPROACH OF THE RQL STRATEGYAND RESULTS

In this Section, the effect of the equivalence ratio in therich stage on the Scimitar combustor NOx emissions has beenanalyzed. In order to verify a geometry strategy, also the effectof the residence time in the rich combustor has been examined.Simulations have been performed by means of the CHEMKIN-PRO SW.

Fig. 21: RQL combustor diagram view

In the RQL combustor diagram view, there are three sources forinlet flow (Primary_Air,H2, and Secondary Air), two perfectlystirred reactors (Mix reactors), two plug-flow reactors (RichReactor and Lean Reactor) and an outlet flow(reactor products).

In the first reactor, (mix to rich reactor), air and H2 areperfectly mixed. Once mixed, the mixture enters the fuel richreactor where the reactions occur (the residence time is 0.005s). The burned mixture is therefore instantaneously mixed withthe fresh air in the mixing reactor, and, once mixed, this entersthe lean reactor (the residence time in the lean reactor is about0.05 s).The equivalence ratio in the rich reactor has been variedfrom 1.29 to 19.36, corresponding to the hydrogen and air massflow rate reported in Table 4.

Table 4: RQL Scimitar conditions

The residence time in the rich reactor is 0.001s in the firstsimulation, 0.02 s, 0.05 s, 0.08 s and 0.125 s respectively in the

T=920 K

0.0

1000.0

2000.0

3000.0

4000.0

5000.0

6000.0

0.5 1 1.5

equivalence ratio

NO

ppm

t=0= t delayt=0.0001st=0.001st=0.01s

T =956 K

0.0

2000.0

4000.0

6000.0

8000.0

10000.0

12000.0

0.5 1 1.5E.R.

t=0= t del ay

t=0.0001s

t=0.01s

t=0.001s

Type RatioRatioOutput T/O C/O

RB211-524H MTF 4.2 264.4 65.84 10.26

RB211-524H MTF 4.2 264.4 1524 2381072

46.31

(kN) ---------------g/kJ--------------

App

(kN) ---------------g/kg--------------

B/P Rated ---------------EI NOx--------------Engine Eng

Air Mass flow rate inthe Rich Reactor

Equivalence Ratio in theRich reactor

Air Mass flow rate inthe Lean Reactor Overall E.R.

7.18 19.36500133 166.620886917.18 8.093172848 156.620886937.18 3.739664054 136.620886947.18 2.947026484 126.620886977.18 1.801512173 96.62088691

107.18 1.297263571 66.62088691

RQL SCIMITAR CONDITION ANALYZED

0.8


other four simulations. Downstream the rich stage, the flowhas been mixed with the secondary air and entered in the leanreactor for 0.05 s therefore, having supposed the mixing of thetwo streams (main and secondary flow) being instantaneous,the initial time in the lean reactor corresponds to the final timeof the rich reactor.The EI and Temperature in the lean reactor (second stage) asfunction of the total residence time, i.e., from 10-3 to 10-1s isshown in Fig. 22.

Fig. 22: T and EI as function of the RQL geometry strategy forE.R. =8.1 in the rich stage

Fig. 22 shows that increasing the residence time within the richreactor increases the final rich reactor temperature, i.e. the leanreactor entrance temperature, without affecting the NOxemissions.

Fig. 23:EI as function of the RQL geometry strategy for E.R.=3.73 in the rich stage

At the combustor exit, instead, there are no differences in thefinal T and in the final NO mass fraction produced are shown.This means that the EI is almost indifferent to the secondary airinjection schedule (3rd stage).In fact, the EI depends on the global equivalence ratio of theengine and on the equivalence ratio in the rich stage.

Fig. 24:EI as function of the RQL geometry strategy assumingE.R.=19.36 in the rich stage

This means that the R.Q.L. strategy makes the engine E.I.independent on total residence time. The same behavior hasbeen found for all other equivalence ratio in the rich region,(see Fig. 23 and Fig. 24).

Fig. 25:EI vs E.R. in the Rich Stage

Fig. 25 shows that the E.R. in the rich stage is a key parameterfor the NOx reduction.In fact, keeping the nominal Scimitar E.R.=0.8, and assumingan E.R. higher than 10 in the rich stage, the NOx EI lowersfrom 576 gNOx/kgfuel to 60NOx/kgfuel. A mathematicalcorrelation between the EI and the ER in the rich stage has beenfound:

EI=426.73 ER-0.8563(7)

Therefore, assuming acceptable an E.I. of 20, from:

0.8563426.73ER

EI (8)

it comes out that for the Scimitar pressure, temperature andE.R. conditions , the equivalence ratio in the rich stage shouldbe higher than 35.65.This fact has another positive drawback also on the RQLstrategy since the increase of the equivalence ratio in the richstage increases mixing efficiency.

0

10

20

30

40

50

60

70

80

0 0.05 0.1 0.15

Total residence time [s]

Tota

l EI [

gNO

x/kg

H2]

900

1100

1300

1500

1700

1900

2100

2300

2500

2700

2900

T [K

]

EI 1 EI 2 EI 3 EI 4 EI 5 T 1 T 2 T 3 T4 T5

0

20

40

60

80

100

120

140

0 0.02 0.04 0.06 0.08 0.1 0.12

total residence time [s]

Tota

l EI [

gNO

x/kg

H2]

EI 1

EI 2

EI 3

EI 4

EI 5

0

5

10

15

20

25

30

35

40

0 0.05 0.1 0.15 0.2

t [s]

Tota

l EI [

gNO

x/kg

H2]

EI 1EI 2EI 3EI 4EI 5

y = 426.73x-0.8563

1060

110160210260310360410

0 5 10 15 20

E. R. in the Rich Stage

Tota

l EI g

NO

x/kg

fuel

0.001s

0.08 s

0.02 s

0.05 s

0.125s


CONCLUSION

The goal of this work was to individuate key parametersfor the NOx reduction in order to define a technology for thesepollutants abatement, keeping the engine performance.The strategy proposed is the Rich-Quick-Lean engine, whosecharacteristics, based on the NOx behavior at the differentequivalence ratios, permit to reduce the nitrogen oxidesformation.This combustor is namely divided in two stages, a rich and leanstage.The analysis of the effect of the residence time within eachstage of the combustor has shown that the RQL strategy makesthe combustor NOx production independent on the overallresidence time.As for the equivalence ratio in the rich stage, this is a keyparameter, affecting the goodness of the mixing between thefirst stage exhausts and the secondary air and, as well, adramatic reduction of the NOx formation.In fact, keeping the nominal Scimitar E.R.=0.8, and assumingan E.R. higher than 10 in the rich stage, the NOx EI lower from576 gNOx/kgfuel to 60NOx/kgfuel.A mathematical correlation between the NOx EI and the “richstage” E.R.has identified in 35.65 the equivalence ratio in therich stage that will ensure a acceptable E.I. of 20.

NOMENCLATUREAcronymsICAO International Civil Aviation OrganizationRQL Rich Quench LeanHSCT High-Speed Civil TransportSST supersonic transportEPA Environmental Protection AgencyRoman SymbolsA: Arrhenius constantD: chamber diameter [m]G: standard-state Gibbs free energy [kJ/mol]P: pressure [atm]S: airflow splitT: temperature [K]V: velocity [m/s]E.R.: equivalence ratioE.I.: emission indexT/O: take offC/O: climb outApp: approachJ: momentum flux ratio of the quench jets to thecrossflowWAQ: air entering the mixing regionFMR: air entering the reach reactor: actual volumetric flow rate of the mainstream [m3/s]n: number of orifices [m]d: secondary air orifice diameter [m]k: equilibrium constantm, n: stoichiometric coefficients

Greek SymbolsΦ fuel air ratioρ density [kg m-3] molar concentrations changeSuperscripts and subscripts0: standard conditionsi: condition of i-th speciesp: pressure

ACKNOWLEDGMENTSThe development part of the work was performed within

the ‘Long-Term Advanced Propulsion Concepts andTechnologies II’ project investigating high-speed transport.LAPCAT II, coordinated by ESA-ESTEC, is supported by theEU within the 7th Framework Programme Theme 7TRANSPORT, Contract no.: ACP7-GA-2008-21 1485.Further info on LAPCAT II can be found onhttp://www.esa.int/techresources/lapcat_II.

REFERENCES

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rql combustion as an effective strategy to nox reduction

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