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УНИВЕРСИТЕТ ИТМО
НАУЧНО-ТЕХНИЧЕСКИИ ВЕСТНИК ИНФОРМАЦИОННЫХ ТЕХНОЛОГИИ, МЕХАНИКИ И ОПТИКИ март-апрель 2015 Том 15 № 2 ISSN 2226-1494 http://ntv.ifmo.ru/
SCIENTIFIC AND TECHNICAL JOURNAL OF INFORMATION TECHNOLOGIES, MECHANICS AND OPTICS March-April 2015 Vol. 15 No 2 ISSN 2226-1494 http://ntv.ifmo.ru/en
NON-INTRUSIVE GAS-PHASE THERMOMETRY FOR INDUSTRIAL
OXY-FUEL BURNERS
J.W. Trögera,b, Th. Seegera,b
a Institute of Engineering Thermodynamics, University of Siegen, Siegen, 57076, Germany
b Erlangen Graduate School in Advanced Optical Technologies (SAOT), University of Erlangen-Nuremberg, Erlangen, 91052, Germany
Corresponding author: johannes.troeger@uni-siegen.de Article info
Received 23.01.15, accepted 25.02. 15 doi: 10.17586/2226-1494-2015-15-2-211-217 Article in English
For citation: Tröger J.W., Seeger T. Non-intrusive gas-phase thermometry for industrial oxy-fuel burners. Scientific and Technical Journal of Information Technologies, Mechanics and Optics, 2015, vol.15, no. 2, pp. 211-217. (in English)
Abstract. The use of oxy-fuel combustion processes is of large interest for several industrial fields applications since it offers the advantages of low NOx emissions in combination with high combustion temperatures even without additional preheating. For optimization of such processes a detailed understanding based on precise experimental data is necessary. So far there is still a lack of precise experimental data achieved with high spatial and temporal resolution from industrial relevant turbulent oxy-fuel combustion processes. Beside species concentration information the gas phase temperature is of utmost importance for an improved understanding of the basic chemical reactions and the pollutant formation.
The coherent anti-Stokes Raman spectroscopy (CARS) technique is a very well suited laser based tool for a non-intrusive investigation of such turbulent high temperature combustion processes. In this work we analysed an industrial 400 kW oxy-fuel burner with the help of O2 based vibrational CARS system which is integrated in an industrial relevant test furnace. The burner is fed with pure oxygen and natural gas at an equivalence ratio of ф=0.9. At one downstream position temporal and spatial resolved temperatures were measured along a 600 mm line. Additional air sucked in from the environment seems to influence the gas phase temperature significantly.
Keywords: vibrational coherent Anti-Stokes Raman scattering, spectroscopy, temperature measurement, oxy-fuel combustion, combustion diagnostic.
Acknowledgements. The authors gratefully acknowledge the AiF for funding the IGF work 17838 N / 4 of the research association Gas- und Wärme-Institut Essen e.V.- GWI, Hafenstraße 101, 45356 Essen, Germany, in the framework of the industrial common research (IGF) of the Federal Ministry of Economics and Technology based on a resolution of the German Federal Parliament.
ДИСТАНЦИОННЫЙ МЕТОД КОНТРОЛЯ ТЕМПЕРАТУРЫ ГАЗА ДЛЯ ПРОМЫШЛЕННЫХ КАМЕР СГОРАНИЯ И.В. Трёгер^, Т. Зеегер"^
a Институт технологий термодинамики, Университет Зигена, Зиген, 57076, Германия
^Высшая школа передовых оптических технологий (SAOT), Университет Эрлангена-Нюрнберга, Эрланген, 91052, Германия
Адрес для переписки: johannes.troeger@uni-siegen.de Информация о статье
Поступила в редакцию 23.01.15, принята к печати 25.02.15 doi: 10.17586/2226-1494-2015-15-2-211-217 Язык статьи - английский
Ссылка для цитирования: Трёгер И.В., Зеегер Т. Дистанционный метод контроля температуры газа для промышленных камер сгорания // Научно-технический вестник информационных технологий, механики и оптики. 2015. Том 15. № 2. С. 211-217.
Аннотация. Применение процессов сгорания топлива в присутствии чистого кислорода представляет значительный интерес для многих отраслей промышленности благодаря низкому содержанию выделяемых оксидов азота NOx при высоких температурах сгорания даже при отсутствии предварительного подогрева. Для оптимизации подобных процессов требуется ясное понимание их сущности, основанное на точных экспериментальных данных. В настоящее время отсутствуют точные результаты экспериментов с высоким временным разрешением, касающиеся турбулентных процессов сгорания топлива в промышленных установках. Помимо определения концентрации выбрасываемых веществ, огромное значение имеет информация о температуре сгорающего газа, позволяющая лучше понимать протекающие химические реакции и процессы формирования выбросов в атмосферу.
Когерентная антистоксовская рамановская спектроскопия (CARS) - это отличный дистанционный лазерный метод исследования высокотемпературных турбулентных процессов сгорания топлива.
При помощи колебательной CARS системы с применением О2 исследована работа промышленной камеры сгорания на 400 кВт, сопряженной с соответствующей вспомогательной печью. В камеру сгорания подавался чистый кислород и природный газ в эквивалентном соотношении ф=0,9. Пространственно разнесенные температуры в нижнем потоке измерены на протяжении 600 мм. Установлено, что дополнительный подсос атмосферного воздуха может в значительной степени влиять на температуру газа.
Ключевые слова: колебательная когерентная антистоксовская рамановская спектроскопия, измерение температуры газа, сгорание топлива, диагностика процесса сгорания.
Благодарности. Авторы благодарят AiF за финансирование совместной прикладной исследовательской (IGF) работы 17838 N / 4 Исследовательской ассоциации Института газа и теплоты, г. Эссен, e.V.- GWI, HafenstraBe 101, 45356 Essen, Germany, работа проводилась в рамках IGF Федерального министерства экономики и технологии на основе Постановления Федерального Парламента Германии.
Introduction
Combustion processes on an industrial scale in which oxygen instead of air is used as an oxidizer are getting more and more into focus. These so-called oxy-fuel combustion processes are of great interest. They are further developed, for instance, for the steel and the glass industry [1, 2]. In the glass industry of the United States already more than 25% of all glass-melting furnaces are heated by such oxy-fuel combustion processes [3]. The expression "oxy-fuel combustion" is used in two different ways. In the area of power plant technology the aim is to prevent the release of large quantities of CO2 into the atmosphere. One possibility is to combust the fuel in an oxygen-carbon dioxide atmosphere [4, 5]. By introducing an exhaust gas recirculation the exhaust gas, which is mostly consisting of carbon dioxide and water vapour is fed back into the combustion chamber to keep the temperature in the combustion chamber on a low level and to rise the amount of CO2. Thereby a significant increase of the carbon dioxide concentration can be reached which makes its removal from the exhaust gas more effective. This removed CO2 can be stored e. g. in depleted oil and gas fields which is the basic idea of carbon capture and storage (CCS). In the area of power plant technology the oxy-fuel combustion is one potential means of mitigating the contribution of fossil fuel emissions to global warming.
In thermal process engineering however the oxy-fuel combustion process is used to reach high combustion temperatures even without preheating. In this case the combustion of fuel with pure oxygen as an oxidizer is referred as oxy-fuel combustion. The application of such oxy-fuel burners shows some clear benefits compared to air breathing burners. The amount of NOx emissions of a pure oxygen combustion process is significantly lower, which is important in order to fulfil emission regulations. Additionally the combustion with pure oxygen leads to high temperatures without any costs for air preheating. As an example, the calculated adiabatic flame temperature for a stoichiometric methane-air flame (p=1 bar, Tsurrounding=298 K) is approximately 2250 K compared to roughly 3050 K for the corresponding methane-oxygen flame [6]. Although this type of oxy-fuel combustion is increasingly applied in thermal process engineering, there is still a need for optimization. For such an optimization the species concentration information and the gas phase temperature is of utmost importance. These data can be used for a validation of numerical models and they are additionally necessary for an improved understanding of the basic chemical reactions and the pollutant formation.
Up to now there are only a few experimental investigations of such industrial relevant oxy-fuel processes available [7-9]. Nevertheless, there is still a need for precise experimental data achieved with high spatial and temporal resolution. These data could then also be used for a validation of numerical models. Such precise measurements in industrial relevant oxy-fuel processes like, e.g. in glass melting furnaces are challenging due to the high combustion temperature in combination with high turbulence. Here laser-based non-invasive measurement methods are in principle well suited to deliver the necessary information (see e.g. [10, 11]). Nevertheless, the presence of dust particles, high temperatures near the furnace due to radiation and the availability of only small optical accesses within a large furnace are still limiting the application of most laser based measurement techniques.
Coherent anti-Stokes Raman scattering technique (CARS) offers the possibility for precise temperature measurements in flames (see e.g. [12-14]). It has been developed and used now for some decades and is currently one of the best established non-intrusive thermometry tools in the field of combustion analysis. Several studies of different types of flames have proven its high capability [15-18]. Today, the CARS technique using the N2 molecule as a temperature indicator is routinely used even in harsh combustion environments such as IC engines, high-pressure burners and gas turbines [19-24]. In order to probe temperatures in a technical relevant combustion chamber, optical access is always restricted by the need for very small windows. Since CARS is a coherent technique, it has the advantage that only two, small, line-of sight windows are required to focus the laser beams into the probe volume and to collect the generated signal.
Therefore, in this work we present the application of a vibrational-CARS system for gas phase temperature measurements in large scale furnace equipped with a typical industrial oxy-fuel burner. Since in the oxy-fuel process no nitrogen is present, the O2 molecule was used as a temperature indicator.
Experimental
1.CARS setup
Generally, in CARS the Raman transition of interest is excited resonantly by the frequency difference of two laser beams irradiating the sample, one narrowband e.g. a frequency doubled Nd:YAG (raO and the other (ra2), a broadband source, usually a dye laser or an optical parametric oscillator (OPO). The third laser beam (ra3), normally from the same laser source as the first pump beam, is scattered from the excited Raman transitions and the signal is emitted in a coherent, laser-like beam at the anti-Stokes frequency of the probed species. This is schematically shown in Fig. 1. The species dependent CARS signal is generated at frequency racARS=rai-ra2+ra3. The frequency difference between the pump beam o>i and the Stokes beam ra2 has to match a Raman resonance of the molecule of interest which is used for the temperature determination. In air breathing flames N2 is usually used as a temperature indicator in the CARS process since it is inert. In an oxy-fuel process, however, there has to be used either O2, CO or CO2 as a temperature indicator. We used oxygen for the temperature determination since a lean oxy-fuel flame is investigated. Additionally, the vibrational CARS process for diatomic molecules is still easier to model and therefore such CARS signals are simpler to evaluate [25]. A detailed description of the CARS process can be found, e.g. in [10, 26].
Pump Laser
Ol
Stokes Laser
Ю2
Pump Laser
Ю3
I
CARS Signal
OCARS
Raman resonance
Fig. 1. Energy level diagram of the vibrational CARS process
Fig. 2. Experimental CARS setup: M, mirror; T, telescope; HM, hole mirror; DC, dichroic mirror; L, lense; BD,
beam dump; CCD, charge coupled devise camera
A schematic drawing of the experimental vibrational CARS setup is shown in Fig. 2. The laser system is based on a frequency doubled, injection-seeded Nd:YAG laser. The output power was 800 mJ at 532 nm. The pulse duration is 8 ns and the repetition rate is 10 Hz. This Nd:YAG laser provides in the CARS process the first and second pump laser beam (ra1, ra3). Part of the 532 nm Nd:YAG laser output is used to pump a commercial broadband dye laser at a central wavelength of 580 nm. The dye laser was equipped with a mixture of Rhodamine 6G and Rhodamine B in ethanol. The dye laser acts in the CARS process as the Stokes laser (ra2) and
ю
1,2
these two dyes were mixed to match the vibrational Raman resonance of oxygen. The laser beams were overlapped in a USED CARS geometry [27]. Here the dye laser beam is centred inside the donut shaped Nd:YAG laser beam. The donut shaped Nd:YAG laser beam is generated by a high reflecting mirror (HM) with a central hole (see Fig. 2). In this configuration both laser beams will face the same density changes inside the large furnace and the influence of beam steering is minimized. The laser beams are focused into the probe volume and collimated afterwards by lenses with a focal length of 2500 mm. The resulting measurement volume was determined to be approximately 3.5 mm in length and 150 ^m in diameter. For the dye laser a power of 10 mJ and for the Nd:YAG laser 25 mJ was measured in the probe volume. The O2 CARS signal occurring at 491 nm was separated from the laser beams by dichroic mirrors (DC) and focussed on the entrance slit of a spectrometer. The detection system consists of a spectrometer with a focal length of 550 mm and a grating consisting of 2400 lines/mm. The CARS signal was detected by a charge coupled device (CCD) camera. At each measurement position 1000 single shot spectra were recorded and evaluated using in house developed CARS code [28]. To take line narrowing into account, the theoretical vibrational CARS spectra were calculated by use of the modified exponential gap law [29]. The parameters for O2 were taken from Huber at al. and Rouille at al. [30, 31]. 2.Burner setup
The oxy-fuel burner was mounted inside a large test furnace which is shown schematically in Fig. 2. The furnace has a width of 1000 mm and a length of 5000 mm. Temperature measurements were performed at different positions along a horizontal line as marked in Fig. 3 1500 mm downstream the burner exit. Due to the axial symmetry the probe volume positions have to cover only half of the combustion chamber. For these CARS measurements two opposite holes in the wall of the combustion chamber were used for the incoming laser beams and for the signal detection. The diameter of the hole for the incoming laser beam was 60 mm. The hole on the opposite site was concentric with an inner diameter of 250 mm facing the combustion chamber. The outer diameter was 60 mm. The measurements were performed without optical windows. For these first measurements a downstream position of 1500 mm was chosen in order to investigate a region where chemical reactions take place.
The industrial diffusion type oxy-fuel burner had a maximum available thermal power of 400 kW. Two independent gas supplies, one for natural gas and one for oxygen were connected to this diffusion burner. Mass flow controllers were used to adjust the flow rates for both gases. A flow rate of 40 Nm3/h was set for natural gas and an equivalence ratio of ^ =0.9 was used in the experiments shown in this work.
6000
Fig
Probe volume Burner
position position
3. Sketch of the furnace equipped with the oxy-fuel burner. The probe volume positions of the CARS
measurements are marked (x)
Results and Discussion
The temperature results achieved along the line 1500 mm downstream of the burner are shown in Fig. 4. Measurements were taken each 40 mm. The mean temperature and the standard deviation for 1000 single shot measurements are shown in Fig. 4, a. It should be noted that nearly at all measurement positions a large number of spectra could be evaluated. In maximum only less than 21% of the CARS spectra have to be sorted out probably due to beam steering effects or dust particles.
The points at x = - 50 mm and x = -30 mm were taken in the concentric hole within the chamber wall and a mean temperature of about 1000 K was evaluated. The highest mean temperature of 1787 K was measured at x = 270 mm. The temperature is decreasing towards the centre. At x = 510 mm a mean temperature of 1293 K was achieved. Obviously in this part of the furnace the flame is slightly shifted to one side. This is also confirmed by the standard deviation. As an example in Fig. 4, b-d, scatter plots of the single shot temperatures are shown. The corresponding probably density distributions are displayed in Fig. 4, e-g. Due to the high turbulence of this oxy-fuel combustion process a large shot to shot variation of the temperature can be observed.
At x=270 mm a gas temperature between 727 K and 2697 K and a corresponding large standard deviation of 385 K was measured. Only a small part of the temperature fluctuation is caused by the sensor system. In prior laboratory tests with a McKenna burner in a laminar premixed CO flame a standard deviation of around 100 K at a mean flame temperature of 2000 K was observed. This is typical for a vibrational CARS sensor and comparable values have been found for CARS systems using N2 as temperature indicator [32]. Nevertheless the achieved gas phase temperatures are lower than the adiabatic flame temperature of 3051 K for the oxy-fuel flame investigated here. This can be explained by additional air sucked into the combustion chamber due to a pressure difference of -0.08 bar to the environment.
Furnace wall Temperature profile
2500 2000 J 1500 1 1000
£ 500
• • i
\
< \ ■
• • 1
V
\
-50 30 110 190 270 350 430 510 \
x Position [mm] a
utera e
t e
H
n u o
о
3000 2000 1000
0
150
100
50 0
3000 2000 1000
N ♦ + • г
♦*' Л.»* ••У-1 . . * ,
т® • Л
' * *
200 400 600 800 1000 Shot number [-] b
0
200 400 600 800 Shot number [-]
400 1200 2000 Temperature [K]
2800
120 100 80 60 40 20 0 400
3000 2000 1000
0
150 100
.ir "( t^íl^'Ví-
200 400 600 800 1000 Shot number [-]
d
1200 2000 2800 Temperature [K]
f
50 0 400
1200 2000 2800 Temperature [K]
g
Fig. 4. Temperature profile taken along a line 1500 mm downstream of the burner position (a);
Scatter plots for selected measurement positions (b-d);
Probably density distributions for the same selected probe volume positions (e-g)
As an example in Fig. 5 evaluated single shot spectra are shown together with the best fitting theoretical spectrum. The normalized experimental spectra were compared against a library of theoretically calculated vibrational CARS spectra using a least-squares contour-fitting method. The evaluation procedure interpolates between the precalculated spectra of varying temperatures in the library by means of cubic splines using the Levenberg-Marquardt algorithm. It can be seen from Fig. 5 that the fit quality is quite good for the complete temperature range of interest.
I 0,25
S3 '
1520
1540
Raman shift [cm-1]
1560
1520
1540
1560 1520 1540 1560
Raman shift [cm ]
Raman shift [cm ] ■ theory ■ experiment
Fig. 5. Selected single shot spectra taken at x=550 mm shown together with the best fitting theoretical spectrum
c
e
Summary
For the first time temporally and spatially resolved O2 vibrational CARS temperature measurements in a furnace equipped with an industrial relevant oxy-fuel burner were performed successfully. The results show that the developed O2 vibrational CARS sensor is well suitable for a non-invasive investigation of such combustion processes. A temperature profile along a line for a fixed downstream position was evaluated from single shot temperature measurements. The corresponding scatter plots show the turbulent behaviour of this oxy-fuel diffusion flame.
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Johannes W. Tröger - scientific assistant, Institute of Engineering Thermodynamics, University
of Siegen, 57076, Siegen, Germany; scientific assistant, Erlangen Graduate School in Advanced Optical Technologies (SAOT), University of Erlangen-Nuremberg, Erlangen, 91052, Germany, johannes.troeger@uni-siegen.de
Thomas Seeger - D.Sc., Professor, Mentor, Institute of Engineering Thermodynamics,
University of Siegen, Siegen, 57076, Germany; Professor, Mentor, Erlangen Graduate School in Advanced Optical Technologies (SAOT), University of Erlangen-Nuremberg, Erlangen, 91052, Germany, Thomas.Seeger@uni-siegen.de
Трёгер Иоганнес В. — научный сотрудник, Институт технологий термодинамики,
Университет Зигена, Зиген, 57076, Германия; научный сотрудник, Высшая школа передовых оптических технологий ^АОТ), Университет Эрлангена-Нюрнберга, Эрланген, 91052, Германия, johannes.troeger@uni-siegen.de Зеегер Томас — доктор технических наук, профессор, руководитель института,
Институт технологий термодинамики, Университет Зигена, Зиген, 57076, Германия; профессор, руководитель, Высшая школа передовых оптических технологий ^АОТ), Университет Эрлангена-Нюрнберга, Эрланген, 91052, Германия, Thomas.Seeger@uni-siegen.de
