consulateMild Combustion
Antonio Cavaliere a,*,Mara de Joannon b
a
Dipartimento di Ingegneria Chimica,Universita
`Federico II,Piazzale Tecchio no 80,80125Napoli,Italy b
Istituto di Ricerche sulla Combustione,C.N.R.,Napoli,Italy
Received 12March 2002;accepted 27February 2004
Abstract
The Mild Combustion is characterized by both an elevated temperature of reactants and low temperature increa in the combustion process.The features are the results of veral technological demands coming from different application fields.This review paper aims to collect information which could be uful in understanding the fundamentals and applications of Mild Combust
ion.The information in this field are still spar,becau of the recent identification of the process,so that many speculative considerations have been prented in order to make the whole framework more consistent and rich with potential new applications.
A rigorous definition of Mild Combustion is preliminarily given in order to fix the input variables of the process.Under the constraints the influence of the physical,thermodynamic and chemical variables on the most relevant outlet parameters are analyzed.
The physical aspects taken into account are atomization,evaporation,mixing and radiative heat transfer.In particular,the evolution of the mixing layer for high temperature diluted oxidant is analyzed.It is shown that mass fluxes through the stoichiometric isosurfaces are lower than tho in not diluted conditions and that the annihilation of the isosurfaces is enhanced in the Mild Combustion conditions.Both effects infer low rates of heat relea according to the experimental results reported in the literature.
The thermodynamic aspects are dealt through the comparative analysis of the minimum,maximum and equilibrium temperature profiles versus the mixture fraction in the whole allowable range for the diluted and not-diluted cas.
The chemical aspects have been analyzed in relation to the chemical kinetics rates for different oxidative routes and the temporal evolution of the lf-ignition process.The molecular oxygen addition,the hydroperoxide dissociation and atomic hydrogen oxidation are evaluated in wide pressure and temperature ranges.In such a way lf-ignition regimes which rely on different preferential chemical kinetics routes are identified and comparison between diluted/not diluted conditions are performed for a fixed evolution time.In this ca it is shown that Mild Combustion conditions extend the pressure–temperature range,in which the oxidation is depresd,at relatively low pressure,whereas the ‘ceiling temperature’is shifted to lower temperature for Mild Combustion condition at higher pressure.
The cond part of the review shows the potentialities of the diluted high temperature air combustion in applications related both to efficiency and pollution of thermal generation as well as to abatement of the pollutants along the flue gas stream of a primary combustion system.
Some lected examples in the fields as land-ba gas-turbines,boiler combustion chamber and domestic heating systems are prented.In the,the emphasis,is put preliminarily on aspects related more to efficiency than to pollution reduction,even though this target is implicitly taken into consideration.Then environmental benefits are dealt in relation to the major and minor species,either
organic or inorganic,which can be produced in gas/liquid combustion.They include carbonaceous material,unburned hydrocarbons,nitrogen oxides and sulphur oxides.
Finally,a classification of the possible process relevant along the whole fuel transformation in Mild Combustion is given.In particular ‘clean’,‘cleaning’,‘clearing’combustion process are identified as a convenient categorization in relation toray source
0360-1285/$-e front matter q 2004Elvier Ltd.All rights rerved.
doi:10.1016/j.pecs.2004.02.003
Progress in Energy and Combustion Science 30(2004)329–366
/locate/pecs兴奋用英语怎么说
*Corresponding author.Tel.:þ39-81-768-32-79;fax:þ39-81-593-6936.E-mail address:antonio.cavaliere@unina.it (A.Cavaliere).
the incorporation of pre-combustion or post-combustion units in the main combustion systems.
q2004Elvier Ltd.All rights rerved.
Keywords:Combustion;Hydrocarbons;Reactor
Contents
1.Introduction and definitions (330)
2.Fundamentals (335)
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2.1.Physical aspects (335)
2.1.1.Atomization and evaporation (335)
2.1.2.Mixing (336)
2.2.Fluid-dynamic aspects (337)
2.3.Thermodynamic and chemical aspects (340)
2.4.Kinetic aspects (341)
2.4.1.Controlling reaction steps (341)
2.5.Features of kinetic controlled process (344)
2.6.Luminous emission and radiative and convective heat exchange (345)
3.Basic aspects related to technological applications with the efficiency,reliability,economic benefit (346)
3.1.High pressure devices (347)
3.1.1.High pressure diluted lf-ignition (347)
3.1.2.Alternative engines(HCCI) (348)
3.1.3.Gas turbine (348)
3.2.Low-pressure devices (350)
3.2.1.Heat preheating classification (350)
3.2.2.Heat mode(recirculation and staging) (351)
3.2.3.Heat and mass mode(recirculation and staging) (352)
4.Basic aspects related to technological application with environmental benefits.Clean–cleaning–clearing
combustion (354)
4.1.Soot depression,destruction (354)
4.2.NO x depression,destruction (357)
4.3.Synthesis (357)
5.Conclusions (358)
Appendix A.Clearing combustion (359)
A.1.SO x depression and removal (359)
the pursuit of happynessA.2.Minor inorganic elements (360)
References (362)
1.Introduction and definitionsdefected
Combustion process are controlled byfluid-dynamic, thermodynamic and composition variables.A
mong them the temperature is the most reprentative in characterizing the process.It is common to distinguish the combustion process in dependence of their temperature:in this n a rough classification of combustion process divides them as occurring at either low or high or intermediate temperature. This is,indeed,a loo way of classifying such complex process,and usually needs at least additional specification of the stage of the process considered.There is further complexity when there is more than one temperature relevant to the combustion process.For instance,process designed to control both the minimum and the maximum temperatures are hard to be described when the two temperatures are changed in opposite directions.It could em an incongruity to refer to a process as developing at low temperature when the reactants are preheated at relatively high value.Nevertheless there are process which satisfy such conditions[1–3].
From a historical point of view systems with so a high reactant temperature were introduced by rearchers who focud their interest on preheating regenerative systems [4–6]applied to air system.Therefore,the most ud acronym is related to air.It is High Temperature Air Combustion(HiTAC)and it appears in veral review lectures[5,7,8]as well as in the title of veral symposia [1–3].HiTAC,previously known as highly preheated air combustion,refers to a rigorous definition reported thefirst time by Katsuki et al.[5],originated from the concept of the ‘large excess e
nthalpy combustion’introduced by Weinberg [9].It is a process in which air temperature is such high that the inlet temperature of reactants is higher than autoignition temperature of the mixture.High temperature combustion
A.Cavaliere,M.de Joannon/Progress in Energy and Combustion Science30(2004)329–366 330
technology(HiCOT)refers in wider n to all the technologies which exploit high temperature reactants.In this n it is not limited to the u of air.Other definitions have also been ud during last years.Mild Combustion is a subt of the HiTAC or HiCOT domain.
Mild Combustion is an unfamiliar term and an unfamiliar subject.In order to clarify the conditions at which Mild Combustion refers let us start by using the following example of a well stirred reactor(WSR)where a stoichiometric mixture of methane/oxygen/nitrogen is fed with a residence time of1s.In Fig.1working temperature ðT wsrÞcomputed in adiabatic conditions for three values of
oxygen molar fractionðX O
2Þare reported as a function of
inlet temperatureðT inÞ:The results of the calculation at
X O
2¼0:2;reprented in Fig.1with the dashed line,show
gigolos>gaddafithe typical trend of T wsr versus T in expected for such a
system,in fact an S-shape curve is partially visible.
According to the classical literature on reactor behavior
[10,11]lf-ignition temperatureðT siÞof a WSR is the inlet temperature at which any differential temperature increa
makes the system reach the higher branch of the S-shape
curve and the chemical process lf-sustains.Therefore,for
T in equal or higher than T si mixture ignites and burns
increasing the temperature of the system.The maximum
temperature increaðD TÞis the difference between the
maximum temperature,which occur in the reactor,and the
temperature of inlet reactants T in:In this ca for T in¼1100K the resulting D T is about1600K and much
of the hydrocarbon fuel is consumed during the considered
residence time.Now if we change the operating conditions
by increasing the dilution level at constant methane/oxygen
ratio,the system responds by reducing the temperature increa during oxidation.For instance,at X O
2
¼0:1and T in¼1100K D T is about1000K and becomes550K for X O
2
¼0:05which is a very low temperature increa compared with the conventional combustion process. The latter condition can be considered to belong to Mild Combustion category.On the bas
is of the indication we define Mild Combustion in the following way:“A combus-tion process is named Mild when the inlet temperature of the reactant mixture is higher than mixture lf-ignition temperature whereas the maximum allowable temperature increa with respect to inlet temperature during combustion is lower than mixture lf-ignition temperature(in Kelvin).”
This means that process evolves in a rather narrow temperature range,which could be placed in an intermediate region between the very fast kinetics of the
oxidative
undiluted conditions and the relatively slow kinetics linked to low temperature lf-ignition regimes.
Parameters relevant for the process are inletflow percentage of fuel,oxidizer and diluent), pressure and the minimum residence time of reactants. The are the conditions which determine the lf-ignition temperature of the homogeneous fuel/air/diluent mixture.
D T varies according to system configuration.For instance,if reactants are not premixed,the maximum temperature corresponds to the adiabaticflame temperature referred to the stoichiometric condition which may also occur when feed ratio is different from the stoichiometric one.On the other hand,the maximum temperature of a premixed stream isfixed by the feed ratio between fuel, oxidizer and diluent.It is noteworthy that the maximum temperature is related to the maximum oxidation level, which may be different from both the equilibrium and real temperature reached in the reactor.For instance a CH4/O2/N2system evolving in rich,diluted conditions may attain different temperatures according to different kinetic routes followed in the chemical process that,in turn, do not necessary lead to the maximum oxidation level.The diagrams shown in Figs.2and3are examples for supporting this statement.Fig.2pertains to a WSR temperature
increa D T plotted versus inlet temperature for X O
2¼0:05
and X CH
4¼0:1(carbon/oxygen ratio¼1)with a dilution
level corresponding to X N
2¼0:85;with a residence time of
1s(solid line).Details related to this analysis are fully described by de Joannon et al.[12].The dotted and the dashed lines are D T computed for the equilibriumðD T eqÞ; and theoretical adiabaticflame temperatureðD T afÞof the system respectively.In the ca here considered D T af is always higher than D T becau in rich diluted condition the main oxidation product is CO.
The shape of solid line is due to different product distributions obtained by changing inlet temperature.The three arrows related to the curve refer to different outlet compositions according to the different oxidation channels.
The ufulness of the Mild Combustion definition may be appreciated by some considerations in relation to the map shown in Fig.3obtained for the same chemical system prented CH4/O2/N2with0.1/0.05/0.85molar fractions.It defines all possible inlet temperature(abscis
sa) and temperature increa(ordinate)for a residence time of 1s and atmospheric pressure.In this ca the lf-ignition temperature of reactant mixture is1000K according to an evaluation bad on a numerical computation[13]and shown in Fig.2.
The map of Fig.3is divided in three regions by the straight lines intercepting the lf-ignition temperature on both axes.The regions are named Feedback,High Temperature and Mild Combustion respectively.Accord-ing to the definition given
D T,T in,T si)Mild Combustion is placed in the lower-right quadrant.The other two combustion modes are placed in the upper part of the map where the condition D T.T si is satisfied.Although it could be appear pleonastic,the conditions corresponding to the three combustion modes have been also schematically reported in Table1in order to enhance the graphical reprentation of Fig.3.
The meaning of Mild Combustion in comparison to the other twofields is quite straightforward.It differs from the other two regimes becau in Mild Combustion the process cannot be sustained without preheating the reactants.In contrast,Feedback and High Temperature
Combustion Fig. 3.Tin-D T locus of different combustion modes for a methane/oxygen/nitrogen
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mixture.
equipTable1
Summary of conditions identifying the different combustion modes
reported in Fig.3
Combustion mode Inlet conditions Working conditions
Feedback combustion T in,T si D T.T si
High temperature
air combustion
T in.T si D T.T si
Mild combustion T in.T si D T,T si
A.Cavaliere,M.de Joannon/Progress in Energy and Combustion Science30(2004)329–366
332
satisfy the necessary condition for which a traditional combustion process may occur,namely the heat relea is sufficient to sustain the minimum temperature required for process evolution.
It is worth stressing that lf-ignition temperature,to which the definition refers,is that of fuel/air/diluent mixture.Different lf-ignition temperatures may be obtained for different compositions,so that each map such as Fig.3corresponds to a fixed composition.In other words,different points on the map are working conditions of a single reactor with the same pressure,the same types of fuel,oxidizer and diluent and the same residence time.
Another definition of Mild Combustion is given by Oberlack et al.[14]and Peters [15].In relation to the WSR behavior they identify as Mild the conditions at which the ignition and extinction points no longer exist and a monotonic shift from unburned to burned conditions occurs.The curve reported in Fig.1at X O 2¼0:05is reprentative of mild condition also according to such definition.It is clear that also in this ca the inlet temperature has to be higher than lf-ignition temperature of the mixture.By imposing the condition to the steady state mass and heat balances,according to the authors Mild conditions corresponds to the following relation:
E =R
T in #41þc p W f T in QY f
ð1Þwhere R is the universal gas constant,c p the specific heat at constant pressure,W f the molecular weight of the fuel,Q the heat of combustion of the mixture,Y f the inlet fuel mass fraction and E the activation energy of an overall one-step reaction.The term ððQY f Þ=ðc p W f T in ÞÞcan be approximate to ððT 2T in Þ=T in Þ¼ðD T =T in Þwhere D T is the temperature increa during combustion.By substituting this relation in Eq.(1)it is possible to obtain a relation between T in and D T that has been plotted in Fig.4where ðD T =T in Þhas been reported as a function of T in for an activation energy of 40kcal/mol.The region delimited by the curve reprents
the conditions where Mild Combustion occurs.In this region temperature increa is always lower than inlet temperature.For instance,for T in ¼1000K the temperature increa due to heat relea should not exceed 25%of the inlet temperature if Mild Combustion mode is to be maintained.The definition of Mild Combustion given in the prent review paper is unambiguous becau criteria which should be fulfilled to include a process in Mild Combustion are well defined in univocal way.The need and ufulness to define this process parately from the others and why its name is
appropriate derve further specifications.In other words why is important to consider Mild Combustion regime,is there any practical u of this mode of combustion?The need of such a definition is related to the simultaneous occurrence both of ‘higher temperature than lf-ignition one’and of ‘temperature increa lower than a prefixed value’that is not a trivial coincidence and it ems apparently contradicting.Reactants pre-heating can be obtained in different way according to the choice to have a premixed system or a fuel/oxidizer parated system.In the last ca the high temperature of reactant mixture can be ensured by heating or fuel or oxidizer or both.The reasons why we have chon the name Mild are two.The first one is that this word is in contrast with the characteristic of all the other combustion process.The latter evolve in a very wide temperature range in which whatever temperature-dependent process may chaotically (related to turbulence)pass through veral regimes.Kinetic can change during the completion of the process from low to intermediate or high temperature regimes,the physical parameter,like diffusion,surface tension,can also change abruptly from one to the other.In contrast,Mild Combustion mode is characterized by ‘mild’changes and ensures a more gradual evolution during the process.The cond reason is that mild is the acronym of ‘moderate or inten low-oxygen dilution’which is exactly one of the most typical conditions for which the process can be obtained.The relevance of this condition is due to its relatively simple feasibility and that it may be tuned in such a way that it pr
events from soot and NO x formation.This point,which will be discusd and supported by literature in Section 4,is also related to the last question on Mild Combustion.Is there any practical u of this mode of combustion?The first answer is that any new concept in combustion has to be explored independently on its immediate application.Ufulness of a combustion process has to be shown along the years and economic constrains sometime obscure long-time convenience.The cond answer is that this mode has great potentials.This is linked to the fact that combustion process can be restricted to relatively low maximum temperature and temperature increa when Mild Combustion is adopted.The limitation of the maximum temperature can be exploited to limit soot and NO x production as it has been just mentioned.Furthermore,the maximum temperature can be adjusted in such a way that it is lower than that a high temperature metallic material can resist.In the field of
combustion
Fig.4.Identification of Mild Combustion conditions according to Oberlack et al.[14].
A.Cavaliere,M.de Joannon /Progress in Energy and Combustion Science 30(2004)329–366333
