Combustion control and sensors: a review

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1 PERGAMON Progress in Energy and Combustion Science 28 (2002) 107±150 Combustion control and sensors: a review Nicolas Docquier 1,SeÂbastien Candel* Laboratoire EM2C, Ecole Centrale Paris Ð CNRS UPR 288, Grande Voie des Vignes, F-92295ChaÃtenay-Malabry Cedex, France Received 15 September 2000;accepted 18 June 2001 Abstract There is an increased interest in the application of control to combustion. The objective is to optimize combustor operation, monitor the process and alleviate instabilities and their severe consequences. One wishes to improve the system performance, for example by reducing the levels of pollutant emissions or by smoothing the pattern factor at the combustor exhaust. In other cases, the aim is to extend the stability domain by reducing the level of oscillation induced by coupling between resonance modes and combustion. As combustion systems have to meet increasingly more demanding air pollution standards, their design and operation becomes more complex. The trend towards reduced NO x levels has led to new developments in different elds. Automotive engines and gas turbine combustors are considered in this article. In the rst case, complex exhaust aftertreatment is being applied and dedicated engine control schemes are required to ensure and maintain high pollutant conversion ef ciency. For gas turbines, premixed combustors, which operate at lower local temperatures than conventional systems have been designed. In both cases, monitoring and control of the operating point of the process have to be achieved with great precision to obtain the full bene ts of the NO x reduction scheme. For premixed combustors operating near the lean stability limit, the ame is more susceptible to blowout, oscillation or ashback. Research is now carried out to reduce these dynamical problems with passive and active control methods. In addition to a broad range of fundamental problems raised by Active Combustion Control (ACC) and Operating Point Control (OPC), there are important technological issues. This paper contains a review of some facets of combustion control and focuses on the sensors that take or could take part to combustion control solutions. The current status of ACC and OPC is presented together with the associated control concepts. The state of the art in sensors is reviewed and their applicability is evaluated. Research efforts in combustion diagnostics are to a certain extent devoted to the development of sensors for control applications. The objective of such developments differs from that which is pursued when one wishes to perform detailed measurements on a laboratory scale experiment. The sensor system should not necessarily provide quantitative measurements because relative data are already useful for control purposes. This change of orientation will be discussed and illustrated by examples of current interest. It is concluded that development in control will depend critically on the availability of sensors and on their reliability, robustness, immunity to noise and capacity to operate in a harsh environment. Research is needed on the fundamentals of ACC and OPC but it should also address the more technical aspects of the problem. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Combustion control;active control;optical sensors;solid state sensors;gas sensors;gas turbine control;automotive engine control Contents 1. Introduction Combustion control Active control of combustion * Corresponding author. Tel.: ;fax: addresses: (N. Docquier), (S. Candel). 1 Institut FrancËais du PeÂtrole, Techniques d'applications EnergeÂtiques, 1 et 4, Avenue de Bois PreÂau, F Rueil-Malmaison Cedex, France. Tel.: ;fax: /02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S (01)

2 108 N. Docquier, S. Candel / Progress in Energy and Combustion Science 28 (2002) 107± Operating point control Automotive control Current control system Automotive control challenges Gas turbine control Lean premixed combustion Gas turbine control challenges Combustion performances optimization Control concepts Sensing techniques Input parameters for combustion control Operating point control Performance optimization Active combustion control Conclusions Diagnostic techniques Optical diagnostics Optical absorption sensors Fundamentals Absorption data recording Velocity measurements Conclusions Optical emission sensors Experimental setup Chemiluminescence Black body emission IR emission Conclusions Solid-state sensors Gas sensors Zirconia properties Potentiometric Nernst cell for oxygen detection Amperometric cell for oxygen detection Three-phase boundary electrochemistry Non-Nernstian electrodes Mixed potential sensors: CO and NO x sensors Arrays of electrochemical cells: NO x sensor Conclusions Ion current probes Ionic properties of ames Correlations between ion current and operating parameters Practical applications Conclusions Other sensors Resistive temperature detector Micromachined sensors Pressure sensor Flow sensor Viscosimetry and Wobbe index Conclusions Acknowledgements References

3 N. Docquier, S. Candel / Progress in Energy and Combustion Science 28 (2002) 107± Nomenclature A/F air/fuel ratio c speed of light D molecular diffusivity E energy F Faraday's constant f v soot volume fraction g line shape function h Planck's constant I light intensity (Sections 2±4.1.2) I current intensity (Sections 4.2.1±4.2.3) k Boltzmann's constant L optical pathlength L l spectral radiance _m mass ow rate N mole number n electron number p pressure p mean pressure p 0 uctuation of pressure p i soot emissivity parameters q heat release per unit volume q mean heat release q 0 uctuation of heat release R perfect gas constant S cross section S T; n 0 T T T 0 T u T ad T f t U U p v X X X 0 a b e l u K n l n n 0 s t n F transition line strength temperature mean temperature uctuation of temperature fresh gas temperature adiabatic ame temperature ame temperature time voltage drop pumping voltage velocity mole fraction mean mole fraction uctuation of mole fraction molar stoichiometric coef cient number of N 2 moles per unit mole of O 2 in air spectral emissivity angle spectral absorption coef cient air to fuel equivalence ratio (1/F) frequency central frequency leak conductance spectral trasmissivity fuel to air equivalence ratio 1. Introduction Today's standard practice is to control combustion processes in open loop without feedback of information to the injection system. It is well recognized that this does not allow optimization of the process and that the lack of closed loop control may pose serious problems for future developments. It is therefore believed that new combustion technologies will integrate feedback controllers. Automotive engines already use closed loop concepts [1] which allow a ne tuning of operating conditions. This adjustment of operation with sensors monitoring the exhaust gases is considered in many current applications including biomass combustion [2] and gas turbines [3]. In this last area, a notable reduction of NO x emissions has been achieved by adopting a premixed mode of operation in which the ame temperature is reduced and the level of NO x is diminished to a great extent. However, premixed combustion requires a precise determination of the equivalence ratio, which could be deduced from measurements on the ame and in the exhaust gases. Also, the premixed mode of combustion, which is now favored in gas turbine applications leads to instabilities which reach unacceptable levels and may have serious consequences. Efforts are being made to develop active control solutions to this problem, which will require sensors for process monitoring and feedback input. Sensors are critical elements for combustion control and combustion monitoring. Their integration in combustion systems is extensively explored. We try in this article to evaluate research in this area and to analyze progress in sensors in terms of potential for application. To give the proper perspective we begin with an outline of ideas in combustion control. We then present some of the underlying concepts and then survey the state of the art in sensor systems which are, or could be, considered for combustion control. The discussion focuses on principles, characteristics, limitations, and tradeoffs. 2. Combustion control While they share some of their features and objectives, it is convenient to divide combustion control in two categories: Operating Point Control (OPC) and Active Combustion Control (ACC). In the ACC domain, it is convenient to distinguish Active Combustion Enhancement (ACE) from Active Instability Control (AIC). Current developments in ACC will be brie y reviewed while OPC will be discussed in more detail. Table 1 synthesizes

4 110 N. Docquier, S. Candel / Progress in Energy and Combustion Science 28 (2002) 107±150 Table 1 Combustion control references Category Solid state sensor combustion control experiments and illustrates important aspects of this discipline. Experiments are gathered according to the sensing technique and to their objectives: combustion enhancement (ACE) for performance optimization, instability control (ACCÐreduction of combustion oscillations) and OPC for ef ciency and emissions level management Active control of combustion Optical sensor Combustion enhancement 2, 4±7 8±15 Instability control 5±7, 16±25 4, 8, 12, 13, 26±28 Operating point control 1, 29±42 11, 13, 15, 43±48 Basic concepts of active control were mainly demonstrated in small scale laboratory experiments (Rijke tubes [26], laminar ame burners [49], turbulent combustors [6,24,50]). Some recent studies have also been carried out on larger power devices [19,20,25]. Control has been achieved in many cases with a variable degree of success. Another set of experiments also indicated that active control had potential applications in combustion enhancement. Typical ACC experiments are shown in Fig. 1. Instability control is one major area of concern. At this point in time, it is the most advanced but has given rise to a limited number of large-scale applications. Active control is speci cally considered for ramjets [53], lean premixed combustors [54], afterburners, segmented solid propellant rocket motors [55]. Many aspects of active control of combustion instabilities are reviewed in [56±58]. Active control is now considered for other applications and more speci cally for the active reduction of pollutant emissions. A rst case of technological interest is that of the lean premixed combustors which are being developed to reduce the exhaust emissions of gas turbines. These modern devices have excellent emission characteristics and bring a signi cant reduction of nitric oxide levels. Unfortunately the lean premixed mode of operation is also quite sensitive to pressure waves and features instabilities and associated side effects like ashback or ame blowout. Active control may then be used to avoid unstable operation or at least augment the stability margin of the combustor [8,16,20]. This is an indirect application of ACC: control is used to prevent instability thus allowing smooth operation of a low emission level combustor. The second case of interest concerns standard combustors operating in the non-premixed mode. The reactants are introduced separately and their mixing is often poor leading to long ames with elevated temperatures in broad regions. Emission levels may be quite high in such circumstances. Active control may be used to enhance mixing thus modifying the conversion rate, combustion ef ciency, ame temperature and pollutant formation and destruction [9,59,60]. Active control may also be used to reorganize the ame structure by redistributing fuel and oxidizer to `stage' the regions of conversion. Combustion enhancement by means of active control is reviewed in Ref. [61]. Such direct applications of active control could be used to retro t existing con gurations in order to comply with more stringent regulations Operating point control Control of the operating point concerns every combustion device designed to date. From the early problem of load control to the somewhat more recent concern of pollutant emissions reduction, any engine or combustion process relies on a more or less complex control system in order to operate in an acceptable way as regards to power, safety or emissions. Various aspects of OPC are shown in Fig. 2. In this section, the major issues of operating point control in combustion are discussed in relation with automotive engines and gas turbine combustors Automotive control It is known from early work that the smog problems affecting large urban areas like Los Angeles or Tokyo since the late 1940s result from sunlight-driven reactions involving oxides of nitrogen and hydrocarbon compounds exhausted by, among other sources, motor vehicles [62]. As a result, emission standards were introduced in the mid-1960s and forced the installation of emission control devices Current control system. Automotive engines now feature a combination of electronic fuel injection, catalytic converters and feedback control [33], for metering air and fuel mixtures (Fig. 3). Control of the engine operating point is driven by the conversion ef ciency (from hydrocarbons, carbon monoxide and oxides of nitrogen to carbon dioxide and nitrogen) of standard three-way catalyst (TWC). It is known that this ef ciency is about 100% close to stoichiometric conditions and dramatically decreases on the lean side for oxides of nitrogen and on the rich side for the other carbon compounds, as sketched in Fig. 4. The control system has to maintain stoichiometric combustion to ensure ef cient catalytic conversion and clean engine operation. This is achieved with the `lambda' probes (l designates the air to fuel equivalence ratio). These sensors respond to the presence of oxygen and they are particularly well suited to the detection of the

5 N. Docquier, S. Candel / Progress in Energy and Combustion Science 28 (2002) 107± Fig. 1. (a) Heavy duty (260 MW) gas turbine: instability control and operating range extension by fuel ow modulation (Direct Drive Valve) and pressure measurement (from Ref. [19]). (b) Model gas- red combustor (15kW): active optimization by fuel ow actuation and CO p 2 measurement (from Ref. [14]). (c) Liquid-fueled dump combustor (270 kw): active instability suppression by fuel modulation and pressure measurement (from Ref. [51]). (d) Swirl-stabilized combustor: pressure oscillation reduction by acoustic air ow modulation and pressure or OH p measurement (from Ref. [52]). (e) Dump combustor (5kW): active combustion enhancement by fuel ow acoustic forcing and laser diode T and X H2 O measurement (from Ref. [12]). (f) Turbulent spray combustor (137 kw): active instability control by fuel ow modulation (DDV) and pressure measurement (from Ref. [17]). stoichiometric point. This condition corresponds to a sizable variation in the sensor signal (Fig. 5) Automotive control challenges. Motor vehicle emission regulations are continuously tightened worldwide. Table 2 presents their evolution for Europe (EU) and United States (LEV). As the main vehicle emissions appear today during the warm-up phase (e.g. <80% of the HC emissions are produced in the rst 40 s after cold start [65]), the strategies to comply with Low Emission Vehicle

6 112 N. Docquier, S. Candel / Progress in Energy and Combustion Science 28 (2002) 107±150 Fig. 2. (a) Premixed gas turbine combustor (30 MW): operating point control by fuel regulation using air ow calculation, hygrometry and fuel properties measurements (from Ref. [32]). (b) Liquid spray ame burner: operating point control by atomizing air regulation and OH p imaging using a CCD camera (from Ref. [43]). (c) Hencken at ame burner: temperature control by methane regulation using laser diode absorption measurements (from Ref. [45]). (d) Gasoline direct injection engine: engine and catalysts management system using O 2 and T sensors (from Ref. [37]). (e) Classical gasoline engine: engine operation at l ˆ 1 for maximum three-way catalyst conversion ef ciency using a lambda probe for stoichiometry measurement (from Ref. [1]. (f) Waste incinerator (20 MW): combustion monitoring and control based on laser diode absorption O 2 and H 2 O measurements (from Ref. [10]). (LEV) standards are mainly supported by A/F control early after engine start-up. Therefore, fast oxygen sensor light-off is required while additional reduction in time delay can be achieved by the relocation of the probe (together with the catalyst) closer to the engine on the exhaust manifold (Fig. 6). These modi cations require temperatures at sensor tip in excess of 10008C and accordingly, new lambda sensors are being designed [66±70]. Furthermore, it appears that the current lambda control strategy, a two-level controller in which combustion is considered as being stoichiometric or non-stoichiometric, is not suf ciently accurate to maintain high catalyst conversion rate at the maximum deviation

7 N. Docquier, S. Candel / Progress in Energy and Combustion Science 28 (2002) 107± Fig. 3. Conventional automotive engine control system (adapted from Ref. [33]). Fig. 4. Three-way catalyst, conversion ef ciency diagram (adapted from Ref. [63]). from l ˆ 1: This is especially a problem with aged catalysts. To reduce these deviations, linear l-control strategies have been devised [36,41,50] which require new linear wide band sensors with high accuracy around l ˆ 1: The more recent concern about global warming due to greenhouse gases like carbon dioxide is now shifting the objective of automotive combustion control. Current efforts are made to decrease the total amount of emissions and to minimize fuel consumption. To address these two requirements, manufacturers are now focusing on lean burn gasoline (Gasoline Direct Injection conceptðgdi) and diesel engines. Unfortunately, the latter feature relatively higher NO x in comparison with conventional gasoline engines equipped with a TWC and the oxygen excess in the exhaust gases of a GDI engine operating in a lean mode also prevents the reduction of NO x with existing TWC aftertreatment systems. A solution can be found in the development of NO x adsorption catalyst [71±74] which adsorbs NO x during lean operation and decomposes the adsorbed NO x with combustible gases such as CO, HC and H 2. These gases are supplied during a short period of rich operation or post injection of HC or urea. In addition to this NO x trap, a pre-catalyst (or oxidation catalyst) is closely mounted to the engine to achieve high HC and CO conversion rate. This state of the art depicted in Fig. 6 complicates the engine control system which has to continuously adapt the fuelling strategy to take into account the status of the exhaust aftertreatment devices. In conclusion, new types of gas sensors are needed. On the one hand, lambda (oxygen) sensors have to be redesigned in Table 2 Present and future automotive emission limits (from Ref. [65]) Regulation HC CO NO x ULEV a (g/mile) SULEV a (2003) (g/mile) EU III b (2000) (g/km) EU IV b (2005) (g/km) Fig. 5. Lambda probe, typical output signal (adapted from Ref. [64]). a b FTP-test. EU III testcycle.

8 114 N. Docquier, S. Candel / Progress in Energy and Combustion Science 28 (2002) 107±150 Fig. 6. Control system for new gasoline direction injection engines. order, to either cope with more dif cult working conditions such as elevated temperatures, or to extend their sensing range in order to supply accurate measurements to the control system. On the other hand, there is a need for monitoring other gaseous species to solve exhaust aftertreatment dif culties encountered with the new automotive combustion concepts like GDI. As an example, reliable and accurate NO x sensors will be the key to the management of adsorption catalysts [65,75], in order to determine the appropriate time to switch from lean to rich or to inject HC or urea. Finally, to comply with on-board diagnostic requirements which come together with the new emissions regulations, the catalysts conversion ef ciency will have to be monitored using new temperature sensors (resistive detectors [37,76]) or HC/CO gas sensors [77,78] Gas turbine control Requirements to limit pollutants emissions also concern the gas turbine industry. Table 3 presents some of the limits which apply worldwide for stationary (ground-based) equipment. Regulations are becoming so stringent (`single-digit' NO x emissions are targeted in California) that they have often resulted in a radical redesign of combustion systems employed on high pressure ratio aero derived gas turbines. Reduction of emissions is also a permanent objective for jet engines manufacturers. Important progress has been obtained with premixed combustors [79±86]. It is known that oxides of nitrogen may be notably reduced if the mixing Table 3 Emission limits for ground based gas turbines Country NO x O 2 ) CO O 2 ) Rates power ECC 25 vppm Not stated. 50 MWth France 40 vppm 80 vppm. 20 MWth Italy 29 vppm 48 vppm. 50 MWth United Kingdom 28 vppm 80 vppm. 50 MWth Japan (Tokyo) 28 vppm No limits Not stated USA (California) 9 ppm Not stated Not stated of reactants takes place before burning. This diminishes the ame temperature and the related production of NO x which depends exponentially on temperature. Moreover, lean premixed combustion under gas turbine conditions does not reduce cycle ef ciency because the latter depends (for a given pressure ratio) on the turbine inlet temperature. This temperature is essentially limited by materials and it has to be well below stoichiometric values. In the classical combustor of the non-premixed type, stoichiometric conditions are maintained in the primary zone to stabilize the ame, the mixture is subsequently diluted with secondary air to obtain the proper mixture ratio and temperature. Therefore, the existence of a near stoichiometric region featuring elevated temperatures results in excessive levels of NO x. Lean premixed combustion is considered for both terrestrial and mobile applications, the rst being more advanced because the safety issue is somewhat less critical. Also, concepts are easier to integrate in ground-based systems where there are less constraints on the combustion chamber geometry and compacity Lean premixed combustion. Fig. 7 shows the evolution of the main pollutants generated by combustion of a lean mixture of methane (main compound of natural gas) and air. It appears that NO x and CO emissions follow opposite trends. Whereas low ame temperature is favorable to NO x reduction, it rapidly prevents the complete oxidation of CO and HC. Control of the operating point of such a combustion process has to de ne a trade-off between both pollutants reduction. In an ideal case, one will try to drive the system towards the optimum point where both CO and NO x levels will be minimum. However, in an industrial context, the objective is to cope with emission limits imposed by the legislation. Therefore, the control system will have to operate the combustor in a range of equivalence ratios avoiding the rapid increase of CO and NO x Gas turbine control challenges. Premixed combustors are quite attractive but pose dif cult practical problems. First, they operate near the lean stability limit and under these conditions, the gas turbine is more susceptible to

9 N. Docquier, S. Candel / Progress in Energy and Combustion Science 28 (2002) 107± Fig. 7. Lean premixed combustion, typical pollutant emissions (adapted from Ref. [44]). serious incidents like ame out, instabilities or ashback induced by pressure waves. Accordingly, the control system will also have to monitor the process so that none of these troubles occur, that is, operate the ame in a safe range, rapidly detect the occurrence of combustion instabilities, or provide information, e.g. to a controller which is dedicated to the reduction of these dynamical problems (ACC). Control of the gas turbine combustor operating point which consists in adjusting the equivalence ratio of the fuel/air mixture is a dif cult task. On the one hand, the quantity of air which ows in the combustor is not known with suf cient accuracy. The mass ow of fuel is well metered but there is no easy way to evaluate the equivalence ratio. Previously, control systems used to (and sometimes still) operate the combustor on the basis of a limited set of information regarding the actual ame properties, and estimate air mass ux by means of a look-up table using the compression system rotational speed, guide vane angle, intake air temperature and pressure as inputs [32,87]. Although effective, this method suffers from the possible deviation of engine properties with degradation or aging and it will probably be discarded as regulations become more stringent. On the other hand, most of the gas turbines used for power generation are supplied with natural gas. This allows a clean operation of the engine. However, gas suppliers use different sources resulting in regional and temporal differences in the fuel properties. Table 4 gives the composition of several natural gas sources supplied to European customers. The control system has to adapt to these changes either by on-line monitoring of the fuel quality [40] or by sensing a combustion parameter allowing to compensate for changes in fuel calori c value. In conclusion, control of the operating point of a gas turbine combustor of the premixed type will have to rely on the development of new strategies allowing to cope with future regulations. Small changes in the operating conditions (a few tens of degrees) might be responsible for ame-out or dramatic increase of the pollutant emissions. A precise adjustment of the equivalence ratio is required to deal with changes in the external conditions are (air moisture, fuel quality, power demand). Unfortunately, due to the harsh environment prevailing in the chamber, limited access to the combustion state is offered to the control engineer. Once again, this lack of information should be overcome by designing new sensors rugged enough to withstand these dif cult conditions over an extended period of time. We describe in the following sections some of the sensors or diagnostic techniques which could be used in this context. Table 4 Composition and heating value of different natural gas sources (from Refs. [40,88]). Source Main components (vol.%) Calori c value (kwh/m 3 ) CH 4 C 2 H 6 C 3 H 8 N 2 CO 2 Others Germany Netherlands Norway Russia

10 116 N. Docquier, S. Candel / Progress in Energy and Combustion Science 28 (2002) 107± Combustion performances optimization The pollutant emissions pattern shown in Fig. 7 paves the way to combustion performance optimization procedures. One would like as an example to minimize both emission levels (NO x and CO) and/or maximize combustion ef ciency. In some laboratory experiments [5,60] as well as in industrial applications [10,15,89], on-line optimization strategies have been successfully applied simply by adjusting the operating point of the process. References to such experiments are included in Table Control concepts Control of a combustion process relies on several elements which are essentially similar for the automotive and the gas turbine applications. In these two cases, combustion takes place between fresh reactants supplied (at least for the fuel) by ow metering injectors. Exhaust gases are produced and pass downstream of the combustor through a manifold. ² In ACC, the controller output is used to modulate the ow properties (e.g., fuel ow rate modulation) to avoid or limit pressure oscillations, or to improve the combustion characteristics. ² In OPC, the injection of fuel is regulated in order to maintain certain ame parameters like the equivalence ratio F in a prescribed range of values. The timescales differ somewhat from one case to the other but control strategies typically operate from 20 Hz to a few khz for ACC and from 1 to 100 Hz for OPC applications. The fundamental question is to ask to what extent sensors may provide the information needed for control. Is current sensor technology capable of supplying a suitable description of the state of the combustion region? This section will go through some of the issues related to combustion control, the role of sensors and diagnostics in particular Sensing techniques Fig. 8 shows a generic combustion process with available or promising sensing techniques. We also consider in this discussion diagnostics, which might evolve into sensors. Sensors may be placed at various locations. In some cases, it might be useful to sense the ow properties in the upstream manifold, in the fresh reactants, e.g. to evaluate the mixing quality [90±92] and improve it if necessary, determine fuel or air composition [34,40], or estimate the ow velocity [93,94]. Most of the sensors however are located further downstream to observe the ame or to analyze the ue gases. The number of ame parameters, which can be measured is relatively small and the sensing techniques differ in terms of time and spatial resolution as well as in accuracy. Table 5 classi es the diagnostic techniques sketched in Fig. 8 according to their time response, principle of operation and sensed parameter. The selection of a diagnostic technique not only depends on the type of controller one would like to design, but also and mainly on practical aspects. As an example, optical techniques generally feature a good spatial and temporal resolution and are non-intrusive, but require at least one optical access, which is dif cult to incorporate in a practical device. Indeed, windows and window mounts have to sustain the pressure and temperature of the ow. For in-situ measurements, lm cooling with purge gas might also be necessary to cool the windows and keep soot and particles from impacting and depositing on the surface. Although future combustors might be designed to provide parts for optical access, the retro tting of existing systems will require clever engineering. In this context, the development of optical bers opens new possibilities to the application of optical diagnostics to industrial systems. Fibers are available for UV to IR light channeling and feature a reduced cross section. Their mechanical resistance may be augmented by a protective envelope. The sensor and its equipment may be installed at a distance from the combustor thus simplifying the set-up (see e.g. Refs. [119,120] for in-cylinder and Fig. 8. Typical combustion process, sensors and diagnostic techniques for combustion control (LDÐlight detector, LSÐlight source).

11 N. Docquier, S. Candel / Progress in Energy and Combustion Science 28 (2002) 107± Table 5 Optical and solid-state sensors for combustion control Optical sensors Frequency Technique a Detector b Parameter References Low UV±Vis EM CCD camera OH p,ch p,c2,co p 2, p T 8 (soot) [43,95] WE SPEC [11,47,96±100] IR EM CCD camera T 8 15 High UV±Vis EM PMT and lter OH p,ch p,c2,co p 2 p [27,54,101±103] PD and lter [4,13,44,48,104,105] Vis±IR EM PD and lter T 8 [106,107] NB SPEC [3,108] IR ABS LD and PD H 2 O, CO 2, T 8, p [10,12,89,109±111] Low Electrochemical O 2, CO, HC, NO x [31,41,78,112±114] Catalytic CO, NO x [115,116] Thermocouple, RTD T 8 [2,5,30,32,37] Viscosimeter Wobbe index (W 0 ) [40] High Spark plug Ion current [29,35,39,117,118] Electric, resistive Pressure ( p) [20,21,23,25] a EMÐEmission, ABSÐabsorption. b NB/WB SPECÐNarrow/wide-band spectrometer, PMTÐphoto multiplier tube, PDÐphoto diode, LDÐlaser diode, RTDÐresistive temperature detector. Refs. [44,106] for gas turbine measurements). One practical problem which needs to be addressed from the start is that of obscuration of the light collecting ber by deposition of soot or solid particles. Indeed, obscuration degrades the signal to noise ratio in absorption measurements, and it directly affects methods relying on absolute values of the light intensity. Whereas spatial and time resolution might be essential for controller design, it is important to note that performance will be measured in terms of global emissions of the combustion process. In this respect, the line of sight methods might lead to errors in the evaluation of the system output if the ow is highly non-homogeneous. In this case, the global performance could be evaluated with, for example, solid-state gas sensors exposed to combustion products in the exhaust stream (forthcoming regulations for automotive enginesðtable 2Ðinclude such on-board diagnosis requirements). This will induce however a time lag due to gas transport and it will slow down the controller response to changes. Finally, due to the hostile conditions prevailing in combustors, it is of paramount importance to show that the sensor will withstand exposure to this environment. The sensor not only has to be accurate enough for control but also has to operate safely for long periods of time. Automobile sensors have to be reliable for more than 160,000 km and the mean time between failure for gas turbine sensors should be about 25,000 h. Attention should be paid to sensor and process aging which are likely to modify the controlled system behavior. While techniques presented in Table 5 can be implemented simultaneously, the controller may also rely on a complex modeling effort of the combustion process (see e.g. Refs. [36,38,50,121±123]). This allows predictions of the system response upto a certain limit and makes use of sensor data to correct the model and take decisions for process optimization. Whereas a complete controller design also requires ef cient actuators and control algorithms, this article is intended as a review of the diagnostic techniques which are available or should be developed to allow precise and robust control. First of all, a central issue is to identify parameters which are best suited for this duty Input parameters for combustion control In this section, we aim at de ning parameters which could be used as input to the control system. Of course, the selected parameter has to be detectable but it also has to be closely related to the property one wishes to control Operating point control In most cases, the control of the equivalence ratio F of the mixture is crucial for maintaining emissions at a low level. It has also been shown in the case of lean premixed combustion that one of the physical parameters responsible for the rapid change of NO x and CO with F is the ame temperature T f. Therefore, sensing of F or T f would be directly useful for control. However, for some reasons (lack of physical or optical access, low measurement accuracy, lack of sensing technique) it is not always possible to

12 118 N. Docquier, S. Candel / Progress in Energy and Combustion Science 28 (2002) 107±150 Fig. 9. Diagram of a premixed combustion process. detect the appropriate ame characteristic and one has to combine several parameters. As an example, the equivalence ratio F, ame temperature T f and oxygen mole fraction in the ue gas X O2 can easily be linked together. If one considers combustion of a fuel lean mixture of methane and air (see Fig. 9), the chemistry can simply be modeled using a single step in nitely fast global reaction (1) where a ˆ 2 and b ˆ 3:76: FCH 4 1 a O 2 1 bn 2!FCO 2 1 a FH 2 O 1 bn 2 1 a 1 2 F O 2 1 For lean mixtures F, 1 ; the equivalence ratio of the mixture can directly be correlated to the excess of oxygen present in the burnt gas stream, no oxygen indicating stoichiometric or rich combustion. The identi cation of the oxygen mole fraction X O2 in the reacted mixture yields: F ˆ a b X O 2 a 1 X O2 2 Flue gas analysis is commonly used to determine the equivalence ratio of the ame [124]. This analysis can now be carried out on line with new solid-state gas sensors. A relation like Eq. (2) can be obtained for any type of fuel but in many cases of interest, the fuel composition is not known precisely or may vary with time. Also, in cases where atmospheric air is employed, the variation of air humidity may affect the determination of F. As an example, Fig. 10 shows the in uence of air moisture on the adiabatic ame temperature of a methane/air mixture computed with the equilibrium code included in Chemkin [125]. In this simulation, the mixture is prepared as if perfectly dry air were supplied. One notes that temperature decreases with increasing humidity content for all F. This effect is known and put to use when steam is injected into the fuel for power augmentation or NO x abatement. In the present case, Fig. 10 indicates that using air and fuel mass ow rates to adjust F and achieve a given temperature may lead to errors because of the non-monotonic behavior of the temperature curves for X H2 O;air $ 8%: Apart from this humidity effect, using Fig. 10. In uence of moisture (expressed in vol.% of air ow) and equivalence ratio F on the adiabatic ame temperature T ad of a methane/air mixture F does not account for the presence of water in the air. Fresh reactants temperature of 650 K, pressure of 1 bar.

13 N. Docquier, S. Candel / Progress in Energy and Combustion Science 28 (2002) 107± Fig. 11. Adiabatic ame temperature of a lean premixed methane/air ame as a function of F and unburnt gas temperature T u (adapted from Ref. [126]). Eq. (2) gives a good indication of the equivalence ratio of the ame provided that one is able to measure X O2 in the burnt gases. In addition, sensing of any of the stable combustion products (CO 2 or H 2 O) should theoretically provide estimates of F. However, if one burns unknown fuel or wet air, measuring the three reaction products might be necessary. Otherwise, the choice of the molecule depends on the available measuring technique or on its reliability and accuracy. The equivalence ratio of the ame can also be deduced from ame temperature. Relations exist between the adiabatic ame temperature T ad and F. As an example, correlations have been derived for fuel lean methane/air mixtures which indicate that T ad is almost independent of the pressure and can simply be expressed as a function of F and the unburnt (or fresh) gas temperature T u (see Fig. 11). The adiabatic ame temperature is only an indication of the real ame temperature but it represents its uppermost value and certainly follows the evolution of the real temperature pro le with F. Thus, a temperature-based controller could use oxygen as an input to drive the combustor or to correct other ame temperature measurements, depending, e.g. on the time response of the various techniques. Remark. If one uses an estimation of the equivalence ratio as controller input to maintain the pollutant emissions at a low level, the controller has to rely on a preliminary study of the combustor performances. Indeed, this control method will not provide information with regard to the pollutants themselves. Their mapping as a function of the controller input is necessary. This might be suf cient for present regulations but new NO x targets will probably be dif cult to achieve with a controller relying on a table lookup likely to drift with engine components aging. Therefore, where pollution matters, new controller strategies should include, in addition to equivalence ratio or temperature monitoring, information about pollutants, namely NO x and CO Performance optimization The optimization of combustion performance is often associated to pollutant emissions reduction or combustion ef ciency improvement. In each case, the observation of combustion parameters directly related to the parameter one would like to improve should preferably be performed. As an example, major pollutants such as NO x, CO and soot should be sensed to achieve pollutant emissions reduction. Successful experiments have already been carried out using classical gas analyzers [5,127], calibrated optical sensors [14], or optical sensors delivering uncalibrated signals [4]. Combustion ef ciency optimization has also been performed using optical measurements of H 2 O and temperature [12] or gas analyzers [7] Active combustion control Active control generally requires an observation of the dynamical state of the system and a performance index. In many applications, this index is deduced from the signal(s) describing the state. In some cases, the performance index is obtained from separate measurements. In closed loop active control the state information is fed back by the controller to an actuator or a set of actuators, and

14 120 N. Docquier, S. Candel / Progress in Energy and Combustion Science 28 (2002) 107±150 related variables like mixture fractions may also be used to observe the state of the system. These variables may be used in AIC but they are less directly related to the mechanism of instability. On the other hand, such variables are probably more suitable in ACE applications where mixing, temperature and temperature uctuations are a central issue. Fig. 12. Active combustion control scheme. the performance index is used to adjust the controller parameters in order to optimize the combustor operation (see [57] for a review). Fig. 12 illustrates this basic principle. It is convenient at this point to distinguish active instability control (AIC) from active combustion enhancement (ACE). In AIC, the objective is to diminish the level of oscillation in the system while in ACE, the aim is to improve the operation of the combustion process, for example by decreasing pollutant levels and maintaining a constant ef ciency (see Ref. [61] for a review). The requirements in terms of sensing for these two classes of applications are synthesized in Table 6. In AIC, the observation of the dynamical state may rely on a range of variables. The pressure uctuation at a single point or at an array of points is often used to characterize the state of oscillation in the system. The pressure eld can only be measured at the system walls but it is often possible to infer an acoustic wave eld from a discrete set of detection points. For example, if the wavelength is much larger than the transverse dimension of the system, waves propagate as plane modes and two pressure transducers are in principle suf cient to determine the modal amplitudes. The light emitted by free radicals like CH p ; C p 2; OH p has also been used extensively to observe the dynamics of free or con ned ames. It was shown in some fundamental experiments that under certain conditions the radiation intensity from free radicals could be interpreted in terms of heat release uctuations [128±130]. For premixed ames, it is even possible to establish an essentially linear relation between I CH p; I C p 2 ; and q 0. For non-premixed or partially premixed ames, the light emitted from free radicals is less directly correlated with heat release but the level of uctuation in light intensity constitutes a suitable indication of the ame motion. While pressure and heat release uctuations p 0 and q 0 are directly involved in the instability process which one wishes to control, other variables may also be considered. Time resolved measurements of temperature, mole fractions and Table 6 State variables and performance index for AIC and ACE Active control application State variables Performance index Combustion instability p 0 ; q 0 p 02 ; q 02 ; p 0 q 0 Combustion enhancement X 0, T 0 X; T; X 02 ; T Conclusions Combustion control strategies are needed to ful ll future objectives in pollutant emissions reduction and instability alleviation. This will require monitoring of a broad range of parameters. Detection should be carried out with fast, reliable and preferably low cost sensors. At present, only some of the combustion parameters of interest can be detected easily (e.g. pressure and oxygen content of the ue gases) but promising techniques are now being developed for this purpose. Some of them, together with wellestablished techniques will be presented in Section 4. Table 7 gathers most of the combustion parameters which can serve as inputs to combustion controllers. References to the corresponding diagnostic techniques are also included in this table. 4. Diagnostic techniques Basic challenges of combustion control as well as guidelines for selecting appropriate diagnostic techniques were discussed in Section 3. Diagnostics and sensors are now surveyed. It is out of the scope of this article to present an extensive review of the sensors available for combustion control. While classical devices such as thermocouples, pressure sensors and optical detectors (photodiodes, photomultiplier tubes and spectrometers) certainly deserve the interest of the control engineer, they will not be covered in this survey which rst considers optical diagnostic techniques (A) and then some of the most promising solid-state sensors (B) Optical diagnostics Optical diagnostics operate in a broad spectral range, from UV (<200 nm) to IR (<10,000 nm), most sensors operating in the UV to near IR (<2000 nm) range. In each spectral domain, the optical sensors can either monitor emission, absorption, scattered light or uorescence. Qualitative or quantitative data may be collected. Sensors detecting the light emission essentially provide qualitative information. A calibration procedure may be used to extract an indirect information on the ame parameters (equivalence ratio or heat release). Absorption sensors on the other hand may be used to determine the mole fraction of combustion intermediates products as well as the temperature and sometimes even the pressure or the velocity of the probed medium. Only these two types of sensors will be discussed in this section.

15 Table 7 Flame parameters and diagnostic technique N. Docquier, S. Candel / Progress in Energy and Combustion Science 28 (2002) 107± Parameter Principle a Location b References A/F ratio Ion current Flame [29,35,39,118,131] UV±Vis EM Flame [11,44,96,98,99,101,103,119,132] Heat release UV±Vis EM Flame [8,13,22,26,27,133,134] Mass Flux Vis±IR ABS Fresh [94,135±138] MEM Fresh [93] Mixing LIF Fresh [90] IR ABS Fresh ue [91,92] Pressure MEM Fresh, ame [120,139] Microphone Flame [16,17,20,21,23,25,28] Temperature IR ABS Flame, ue [12,89,109,111,137,140,141] Vis±IR EM Flame, ue [15,95,106±108,142] SS sensor Flame, ue [2,5,30,32,37] CO IR ABS Flame, ue [110,111,143±145] SS sensor Flue [77,78,113,115,146±148] CO 2 IR ABS Flame, ue [89,110,111,149,150] IR EM Flame, ue [108] SS sensor Flue [151] H 2 O IR ABS Flame, ue [10,12,109,110,135,141,149] HC SS sensor Fresh, ue [40,64,152±154] NO x IR ABS Flue [143,155±158] SS sensor Flue [112,114,159±163] O 2 NIR ABS Fresh, ame [10,89,164] SS sensor Flue [66,67,70,165±168] a b ABSÐabsorption, EMÐemission, MEMÐmicromachined electromechanical, SSÐsolid state. Diagnostics locations in the referenced experiments Optical absorption sensors A notable effort has been expended to devise practical sensors based on near IR and visible absorption. These devices have been mainly tested in laboratory scale facilities [12,111,140,141,149, 158,169] but experiments in a 20 MW waste incinerator [10] and a 1 GW gas- red power plant [89] have been reported. Tests of an optical mass ux sensor on a full-scale gas turbine engine have also been carried out [136] and ight quali cation is under way [94]. Equivalence ratio uctuations have also been measured using probes based on the absorption of the 3.39 mm He±Ne laser line [91,92] by gaseous hydrocarbon fuel molecules. More references concerning the detection of combustion products and temperature are presented in Table 7 (`ABS technique'). As pointed out in Ref. [170], the development of near-ir absorption techniques is closely related to the progress in room temperature, tunable and low cost diode lasers in the telecommunications industry. Commercially available units range from 0.63 to 2.0 mm (AlGaAs, InGaAsP, InGaAs/InP) and allow exploitation of absorption bands of species like H 2 O, NO x,coorco 2. Processing of the absorption data collected at several wavelengths may be used to calculate the temperature. The light is provided by diode lasers and the detection system includes optical isolators, bers and photodetectors which are most of the time photodiode elements (InGaAs, Si). Taking advantage of ber components for wavelength division multiplexing, multiple lasers may be combined into common signal and reference bers. Each laser may be swept at different phases of a common ramp function. This provides a time-domain multiplexing where a single photodetector observes the sequential absorption features as each laser is swept across its corresponding lineshape [140,171]. Wavelength-domain multiplexing has also been used in diode-laser combustion sensors [12,111,172]. In this con guration, the lasers can be ramped across their respective absorption lines simultaneously. Multiple wavelengths appearing in the transmitted beam are separated onto multiple detectors using a frequency-dispersive unit such as a diffraction grating. Fig. 13 presents such a con guration Fundamentals. Absorption techniques rely on the Beer±Lambert law which relates the transmissivity t n of narrow-linewidth radiation at frequency n through a uniform medium of length L (cm) to the spectral absorbance k n L: t n ˆ In ˆ exp 2k I n L 3 0 n In this expression, I n is the monochromatic laser intensity at frequency n measured after propagating a pathlength L, I 0 designates the incident intensity and k n the spectral

16 122 N. Docquier, S. Candel / Progress in Energy and Combustion Science 28 (2002) 107±150 Fig. 13. Laser diode based near-ir absorption spectroscopy. I cal is used to convert signals from time to frequency domain in I ref is used as a reference signal. Laser beams are multiplexed using a ber coupler and wavelengths in the transmitted beam are separated onto multiple detectors using a diffraction grating (adapted from Ref. [172]). absorption coef cient (cm 21 ). Near a spectral line this coef cient can be expressed as: k n ˆ S T; n 0 px abs g n 2 n 0 where S(T, n 0 ) (cm 22 atm 21 ) is the temperature-dependent transition line-strength centered at n 0, X abs is the mole fraction of the absorbing species, p is the total pressure and g n 2 n 0 (cm) is the frequency-dependent lineshape function normalized such that R g n dn ˆ 1: This function depends on temperature through Doppler broadening and on both pressure and temperature through collisional (pressure) broadening. The temperature-dependence of the line-strength arises from the Boltzmann population statistics governing the internal energy-level population distribution of the absorbing species. The line-strength is a fundamental spectroscopic property of these species which may be found in spectroscopic databases such as HITRAN [173] and HITEMP. In many cases however, the line strength must be speci cally determined for the particular temperature and pressure conditions of interest. A short review of the spectroscopic literature dedicated to this aspect is presented in Ref. [174] together with the main features of the diode laser absorption sensors for gas-dynamics and combustion ows Absorption data recording. Two techniques described in Ref. [172] have been developed to acquire and process the absorption data, the scanned-wavelength approach and the more recent xed-wavelength technique. In the rst approach, a narrow-line width laser scans a speci c spectral range and the resultant transmission signal is integrated. If the gas temperature, line-strength and absorption path are known, the measured transmission may be directly related to the absorbing species partial pressure. It is usually possible to select a transition such that the temperature variation of the line-strength can be neglected at least over some limited range (several hundred K). Corrections using separate temperature measurements may also be performed. Alternatively, two 4 absorption transitions may be scanned and the ratio of the integrated absorbance of each transition is only a function of temperature [175]. With the temperature determined, one or both absorbances can be used to calculate the partial pressure. The scanned-wavelength technique has been mainly used under atmospheric pressure conditions [109,140,144,175, 176]. Indeed, the broadening of spectral lines with pressure (Fig. 14) prevents measurement of the zero-absorption baseline. This problem may be solved with a collection of laser sources operating at xed wavelengths [141]. The absorbances recorded at these wavelengths are used to determine the temperature and the mole fraction of the probed species. This attractive technique relies on the accurate modeling of the spectral absorption of a given species under standard conditions of operation. One dif cult problem is the determination of the baseline (see Fig. 14). The feasibility of the method is however demonstrated in Refs. [141,172]. It is interesting to note that for the xed wavelength technique, the tuning requirements are signi cantly relaxed and therefore, the measurement acquisition rate is essentially limited by the detection system (photodiode and data acquisition electronics) whereas for the scanned wavelength technique, the measurement acquisition rate is limited by the tuning rate of the lasers as well Velocity measurements. An interesting feature of the diode laser absorption sensors is the possibility to determine the velocity of a ducted ow. The principle of the setup is shown in Fig. 15. The velocity of the ow can be deduced from the Doppler frequency shift Dn 12 measured with a pair of pro les simultaneously acquired in two different directions u 1 and u 2 with respect to the ow. The velocity is given by: v ˆ cdn 12 n 0 cosu 1 2 cosu 2 5

17 N. Docquier, S. Candel / Progress in Energy and Combustion Science 28 (2002) 107± Fig. 14. HITEMP calculation of H 2 O absorbance at various pressures for a mixture of N 2 and CO 2 corresponding to the products of a stoichiometric methane/air combustion. T ˆ 1000 K; 10 cm pathlength (adapted from Ref. [141]). where c is the speed of light and n 0 designates the unshifted centerline laser frequency. Moreover the absorption measurement itself may be used to obtain the density of the gas. When combined with the velocity deduced from the Doppler shift, it allows a direct estimation of the mass ow rate. Such measurements have been performed in air ows using O 2 absorption near 763 nm (wind tunnel and full-scale engine tests [94,136]) and in high speed ows containing water vapor using H 2 O absorption bands near 1.31 mm [137] and 1.38 mm [135]. The air mass ow rate estimation is particularly interesting for combustor control schemes based on the regulation of the equivalence ratio since the fuel ow rate is generally known with good accuracy and intake air mass ow can be measured directly using this absorption and Doppler-shift Fig. 15. Sketch of a basic diode laser mass ux sensor (adapted from Ref. [136]). combined method. Provided one can estimate the amount of air bleeding, which may take place between intake and combustor, it should be possible to directly adjust the equivalence ratio. One may note that air bleeding is exactly the parameter most affected by aging Conclusions. Diode laser based absorption sensors are quite promising. In-situ measurements in the ame region or in the exhaust gases of major combustion species such as H 2 O, CO 2 and O 2, gas temperature, velocity and pressure are well demonstrated. Although the detection of minor species such as NO x [157] and CO [111] have been carried out in combustion environments, strong interferences with high temperature water vapour limits the sensitivity (e.g. <140 ppm for NO [157]). Sampling the combustion gases into a low-pressure, low-temperature multipass absorption cell [144,177] eliminates many of the interference problems associated with in-situ measurements. Better sensitivity is achieved (e.g. <10 ppm for CO [144]) but the time response notably increases. A typical cell response time is <1 s [144] which is still better than conventional gas analyzers (a few seconds) but far slower than in-situ measurements (less than 1 ms). Under laboratory conditions, diode laser based absorption data are in good agreement with reference measurements. Relative errors on T and major combustion products (H 2 O and CO 2 ) of about 5% have been achieved [109,137]. Similar precision has been obtained for velocity measurements [135,137]. Practical problems arise however. Weak absorption strengths in the visible±near-ir range require advanced detection schemes such as frequency or

18 124 N. Docquier, S. Candel / Progress in Energy and Combustion Science 28 (2002) 107±150 wavelength modulation [178] and balanced ratiometric detection [179]. Also, the absorption database in near IR is incomplete especially for CO 2 and further work is needed on the spectroscopic level, in particular under high temperature and high pressure conditions which correspond to typical engine operation. Furthermore, ow turbulence or mechanical vibrations can introduce pointing-instabilities in the incident beam, which may induce amplitude instabilities in the transmitted radiation and can severely degrade the sensor response. This might require tight alignment tolerances in some cases. This problem may be solved with an active alignment control loop developed in Ref. [89] to cope with wall deformations in a gas- red power-plant. Despite these technical challenges, fast, simultaneous and in-situ (path-averaged) measurements of important combustion parameters have been demonstrated. Spatially resolved information could also be obtained from multiple beam absorption measurements as explored for example in Refs. [164,180,181]. Progress in fundamental spectroscopic studies as well as in the spectral range covered by laser diodes (mid-ir) should lead to a widespread application of this type of sensor (see Ref. [174]) Optical emission sensors The light emitted by the ame may be used to monitor and control combustion. Information may be gathered from UV to IR wavelengths. Several phenomena contribute to the light emission process. Discrete emission spectra correspond to the chemical reactions producing electronically excited radicals in the ame front in the UV±visible range (chemiluminescence). Combustion products in thermal equilibrium such as H 2 O, CO 2 as well as CO are also responsible for rotation±vibration emission bands in the near-ir and IR range (IR emission). Continuum emission may also be considered whenever soot is produced and radiates in the combustion process (black body emission). A broad review of the spectroscopy of ames may be found in Ref. [182] and references concerning the detection of combustion products and temperature are presented in Table 7 (`EM technique') Experimental setup. Sensing emission is simpler than measuring absorption. A single optical access to the process is required and the association of a lter, a collecting lens and a photo-detector (photo-multiplier tube or photodiode) is suf cient to observe the light emitted by the ame (from UV to IR) or the burnt gases (IR). Fig. 16 presents a typical con guration used to investigate light emission. A grating spectrometer may also replace the lter and the photo-detector. The time resolution of emission sensors is only limited by the detector response, the acquisition electronics and the data processing algorithms. In its simplest form, the method only allows line-of-sight detection and provides mainly qualitative information with Fig. 16. Typical setup used to collect the light emitted by a ame. A single optical access is needed. Photomultiplier tubes, photodiodes (associated to band-pass lters) or spectrometers may be used to record information. little spatial resolution. We will see however that under some circumstances, quantitative or spatial information may be derived from light emission measurements Chemiluminescence. Among the contributions to the natural light emitted by a ame, the chemiluminescence is related to the generation of electronically excited species (i.e. in thermal non-equilibrium) D p during chemical reactions such as: A 1 B! C 1 D p This chemically excited molecule D p may be destroyed by spontaneous emission D p! D 1 hn or collisional quenching D p 1 M! D 1 M : Chemiluminescence corresponds to the spontaneous emission of photons (hn ˆ E 2 2 E 1 with E 2 the energy of the excited state and E 1 the energy of the nal state). This emission generally takes place in the UV or visible range. Each radical produced in an excited state is responsible for a particular spectrum, which is related to its quantum properties, and can be easily identi ed. Table 8 gathers the most luminous excited radicals observed in typical ames together with the corresponding spectral transitions and bandhead wavelengths. Other radicals such as NO p or CN p also radiate light but their luminosity is in general much lower in typical industrial ames (by three orders of magnitude lower for Table 8 Excited radicals observed in typical industrial ames [183] Radical Transition l (nm) OH p A 2 S 1! X 2 P Dn ˆ (Q2) OH p A 2 S 1! X 2 P Dn ˆ (Q2) CH p B 2 S 2! X 2 P CH p A 2 D! X 2 P C2 p A 3 P g! X 3 P u Swan CO2 p Continuum 350! 500 6

19 N. Docquier, S. Candel / Progress in Energy and Combustion Science 28 (2002) 107± NO p in rich premixed propane/air ames [184]). Fig. 17 shows three spectral bands corresponding to OH p, CH p and C p 2 taken from the emission spectrum of a premixed methane/air ame F ˆ 1:1 : The formation mechanisms of these excited radicals are not yet completely understood but some key reactions have been identi ed. The formation of OH p and CO2 p are well documented in Refs. [185,186]. Reactions leading to CH p have also been investigated [187] but more work is still needed for C p 2: As an example for methane/air ames, the following kinetic reactions are put forward to describe the production of OH p,ch p and CO p 2 CH 1 O 2! OH p 1 CO C 2 H 1 O 2! CH p 1 CO 2 C 2 H 1 O! CH p 1 CO CO 1 O 1 M! CO p 2 1 M Additional paths have been identi ed for CH p and C p 2 but the rate constants are not well known C 2 1 OH! CH p 1 CO C 1 CH! C p 2 1 H Numerical simulations using these kinetics and experimental measurements of OH p and CH p are in satisfactory agreement in the case of methane/air ames under atmospheric [185,187] and elevated pressures [96]. One interesting feature of chemiluminescence for combustion control is that it may be related to several important ame parameters. Correlations between chemiluminescence and equivalence ratio have been reported [11,46,47,96,101,103,119,132]. Fig. 18 presents the evolution of the normalized OH p,ch p and C p 2 emission as a function of the equivalence ratio for a premixed methane/air ame. One notes the strong in uence of F on the signals. The light emitted by OH p is the most intense under lean to stoichiometric conditions whereas the CH p and C p 2 chemiluminescence signals are stronger in stoichiometric to rich ames. Depending on the targeted F range, a controller could use one or several of these lines to estimate the equivalence ratio of the process and adapt the injection of fresh gases to achieve a given F setpoint. In active control applications, chemiluminescence signals have also been widely used to characterize heat release uctuations [17,18,26,28,54,57] or de ne the ame front motion [22,23,27,59,188,189]. As an example, Fig. 19 shows simultaneous measurements of velocity and CH p chemiluminescence in an acoustically pulsated premixed methane/air ame. While chemiluminescence measurements are essentially integrated along the line of sight, spatially resolved chemiluminescence measurements have also been achieved using Cassegrain optics [98,102,190,191], CCD cameras [43,96] and tomographic techniques [142,192,193]. Information concerning tomographic techniques may be found in Refs. [194±196]. Fig. 20 shows some of the techniques used to obtain spatially resolved information: (a) CCD camera associated with deconvolution algorithms such as the Abel Fig. 17. Premixed methane/air ame, F ˆ 1:1 and p ˆ 1 bar. Emission spectra: OH p (transition A 2 S 1! X 2 P (0±0)). From left to right, R1, R2 and Q2 bandheads), CH p (transition A 2 D! X 2 P: From left to right, Q(0±0), and Q(2±2) bandheads) and C p 2 (Swan band A 3 P g! X 3 P u;, Dv ˆ 0: From left to right, (1±1), and (0±0) bandheads) excited radicals. Spectra recorded with a CHROMEX grating spectrograph (1800 lines/ mm, nm resolution) (from Ref. [96]).

20 126 N. Docquier, S. Candel / Progress in Energy and Combustion Science 28 (2002) 107±150 Fig. 18. Premixed methane/air ame at atmospheric pressure. Normalized OH p,ch p and C p 2 emission as a function of F. Signals are recorded with a 600 lines/mm grating spectrometer using a 10 nm wide top hat lter centered around 310, 431 and 516 nm respectively (from Ref. [96]). transform [197,198], (b) tomography [196], (c,d) Cassegrain optics [98]. Chemiluminescence sensors have already been designed for operating point and active control purposes. In active control applications, chemiluminescence has been mainly used to track heat release uctuations [4,8,13,26±28,57] but control schemes have also been developed to perform operating point control of boilers [11,43] as well as for combustion performance optimization [14,59]. In-cylinder measurements of fuel/air mixing have also been carried out [119]. Equivalence ratio and power measurements have been obtained using a bered miniaturized PC-based spectrometer in a 2 MW furnace [47]. Chemiluminescence has also been investigated for gas turbine operating point control [44,96,101] Black body emission. It is possible to estimate the ame temperature whenever soot is produced during the combustion process. Indeed, it is generally accepted that soot particles are in thermal equilibrium with the local combustion products (although a recent study [106] indicates that soot could achieve radiative equilibrium with the dominant source in the combustor). As a result, determining the soot particles temperature distribution may give indications on the temperature pattern in the combustor. The principle of the method relies on the characterization of the light emitted by the particles as they radiate at elevated temperatures. For a radiating body at temperature T, the monochromatic spectral radiance L s l (Wm 22 m 21 sr 21 ) can be obtained by multiplying the spectral emissivity e l with the black body spectral Fig. 19. Premixed CH 4 /air conical ame burner (F ˆ 0:95; mean ow velocity of 1.2 ms 21 ). Simultaneous measurement of axial velocity component measured at 1.5 mm above the exit of the burner and global spontaneous emission of CH p in the whole ame. The axial ow is acoustically modulated at 10 Hz (left) and 30 Hz (right). The ame acts like a low-pass lter (from Ref. [133]).

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