|Title||Oxy-coal combustion: effects of PO2 on coal jet stability in 02/C02 environnments|
|Creator||Zhang, Jingwei; Kelly, Kerry; Eddings, Eric G.; Wendt, Jost O. L.|
|Publisher||American Flame Research Committee (AFRC)|
|Description||The purpose of this research is to understand and predict the effects of near burner zone environment on ignition characteristics of a turbulent axial pulverized coal jet. In oxy-coal combustion this environment consists of O2 and CO2 (instead of N2). Ultimately the experiment will provide data for simulation validation studies that can be used to predict how air fired combustors may be retrofitted to oxy-coal. Specifically the objective is to explore effects of variations in the partial pressure of O2 and CO2 on coal jet ignition and flame stability. During the oxy coal jet ignition process, the partial pressure of oxygen, PO2, in the transporting fluid, comprises one additional degree of freedom in controlling flame aerodynamics, and enhancing coal ignition, flame attachment/detachment and modifying pollutant formation. The experimental investigations are carried on in a 100 kW laboratory furnace outfitted with an axial burner and arrays of electrical-heated panels in the burner zone in order to control variations in near-burner heat loss. The furnace also has visual access through quartz windows to. Sufficient supply of O2 and fresh CO2 are supplied through special utility infrastructure in the laboratory. A special CMOS sensor based camera is used to capture Type 0 axial turbulent diffusion flame shape for the use of statistical studies of flame length at different operational parameters, with special emphasis on systematic variations of partial oxygen pressure in both transport and secondary oxidant streams. Our experimental data show how flame attachment increased with PO2 in either primary or secondary oxidant streams. These variables were also shown to have a firstorder influence on Fuel NOx. Future work will use these data to validate CFD simulations of axial coal jet ignition in turbulent flows.|
INTERNATIONAL FLAME RESEARCH FOUNDATION 16th IFRF Members’ Conference Boston, MA, June 8 th -10th, 2009 OXY-COAL COMBUSTION: EFFECTS OF P0 2 ON COAL JET STABILITY IN O2/CO2 ENVIRONMENTS Jingwei Zhang, Kerry Kelly, Eric G. Eddings, and Jost O.L. Wendt Department of Chemical Engineering & Institute for Clean and Secure Energy University o f Utah, Salt Lake City, Utah 84112, USA ABSTRACT: The purpose of this research is to understand and predict the effects of near burner zone environment on ignition characteristics of a turbulent axial pulverized coal jet. In oxy-coal combustion this environment consists of O2 and CO2 (instead of N2). Ultimately the experiment will provide data for simulation validation studies that can be used to predict how air fired combustors may be retrofitted to oxy-coal. Specifically the objective is to explore effects of variations in the partial pressure of O2 and CO2 on coal jet ignition and flame stability. During the oxy coal jet ignition process, the partial pressure of oxygen, PO2, in the transporting fluid, comprises one additional degree of freedom in controlling flame aerodynamics, and enhancing coal ignition, flame attachment/detachment and modifying pollutant formation. The experimental investigations are carried on in a 100 kW laboratory furnace outfitted with an axial burner and arrays of electrical-heated panels in the burner zone in order to control variations in near-burner heat loss. The furnace also has visual access through quartz windows to. Sufficient supply of O2 and fresh CO2 are supplied through special utility infrastructure in the laboratory. A special CMOS sensor based camera is used to capture Type 0 axial turbulent diffusion flame shape for the use of statistical studies of flame length at different operational parameters, with special emphasis on systematic variations of partial oxygen pressure in both transport and secondary oxidant streams. Our experimental data show how flame attachment increased with PO2 in either primary or secondary oxidant streams. These variables were also shown to have a firstorder influence on Fuel NOx. Future work will use these data to validate CFD simulations of axial coal jet ignition in turbulent flows. 1 . in tro d u c tio n Oxy-coal combustion is one of the three major CO2 capture and sequestration technologies , to fight global warming. The whole idea is to fire coal with pure O2 mixed with recycled flue gas, which is majorly CO2 stream, to produce a nearly pure CO2 stream in the exhaust, to be captured and sequestrated. Because of this nature of oxy-coal combustion, one issue of interest is to study the effect of partial pressure of oxygen, which is another degree of freedom during coal jet ignition. Because of the differences in thermodynamics and transport properties of N 2 and CO2, some changes with respect to traditional air combustion can be expected. One such effect might be the variation of coal ignition and flame stability . The partial pressure of O2 may influence flame attachment/detachment and coal ignition as shown by the oxygen enrichment studies of Ogden and Wendt . Although in the past a few decades, numerous studies have been conducted on coal particle ignition behaviors [4-19], only Molina and Shaddix [3,4], have considered the effect of elevated levels of CO2, as present in oxy-coal combustion systems, on the ignition of coal particles. They found the temperature and size of the diffusion flame of soot cloud that surrounds the particle are lower and larger respectively when CO2 is used instead of N2, due to the reason that N 2 has higher heat thermal diffusivity than CO2. They also quantified the characteristic devolatilization time and ignition time and found ignitions occurs at higher oxygen concentration, and the presence of CO2 retards coal and char ignition, but has a negligible effect on the duration of devolatilization. Therefore coal particles ignition mechanisms under oxy-coal combustion conditions are still inadequate. Another interesting issue is related to near burner phenomena. Previous studies [20-23] show that the experimental measurements and CFD simulations only agree well far downstream of the near burner zone. There are some reasons on both sides of experiments and simulations. It is believed that new models/submodels are needed to quantify the near-bumer phenomena. Therefore novel accurate measurements are required for the model validation under this interested. The purpose of this research is to understand and predict the effects of near burner zone environment on ignition characteristics of a turbulent axial pulverized coal jet. Flame stability is quantified by standoff distance, which is the distance from the burner tip to the base of a down-fired flame. A novel photo-imaging technique is also developed to quantify the flame stability and flame shape. Ultimately the experiment will provide data for simulation validation studies that can be used to predict how air fired combustors may be retrofitted to oxy-coal. 2. EXPERIMENTAL SETUP A new 100kW down-fired, oxy-coal combustion furnace was designed and constructed to allow for the systematic investigation of near-burner aerodynamics of axial diffusion flames using a mixture of oxygen/carbon dioxide to replace the combustion air. It has heated walls and quartz windows for optical access that permit flame detachment studies and future optical diagnostics (shown on Figure 1). The new furnace will simulate the environments experienced by pulverized coal jet flames in certain boilers (such as tangentially-fired units) and will provide for the systematic control of burner momentum and velocity variables, as well as wall temperatures. The furnace consists of an oxy-fuel combustion chamber, followed by downstream section with controlled temperature cooling to simulate practical furnace conditions. It allows stabilization of axial Type 0 (no swirl) pulverized coal, diffusion flames, through the use of heated walls, and variations of oxygen content of transport and secondary air streams. It also represents those typical of existing tangentially-fired boilers or cement kilns. The time-temperature history, for combustion with air, represents that of existing boilers. The burner design parameters are: 1) primary sleeve: O.D. = 21.34 mm, S = 2.769 mm; 2) secondary sleeve: I.D. = 42.16 mm, S = 3.556 mm; 3) primary velocity is always fixed as 6.16 m/s, secondary velocity to primary velocity ratio varies from 2.5 to 6 .2 . A brand new K-Tron twin-screw Loss-In-Weight coal feeder a modified eductor was installed to provide steady pulverized coal feeding. The whole system is controlled by Opto22 commercial software automatically. The data, including measurements of temperature, pressure, and gas components, etc. can be also acquired by the Opto22 instantaneously and automatically. The top section (Figure 2, Figure 3), is the burner zone, with dimensions of 0.610 m I.D., 0.914 m O.D. and is 1.219 m in height. It is insulated by 2600 Fiberboard with a thickness of 76 mm. The burner zone is heated electrically by 24 x 840 W flanged ceramic plate heaters (3 rows and 8 heaters per row, as shown in Figure 2). This control of wall temperature enables the 100 kW system to simulate larger boilers in the field. Each row of heaters is embedded with K-type thermocouples to control or monitor the temperature. The independent control of wall temperature provides another degree of freedom, because wall temperature may impact pulverized coal ignition or flame attachment/detachment during the combustion process. There are 3 layers of insulation in the radiant zone downstream: 1900 Fiberboard (thickness of 51 mm), 2600 Fiberboard (thickness of 76 mm), and 700 Ultra Green castable refractory. And there are 2 layers in the convective zone: 2600 Fiberboard (thickness of 76 mm), and 700 Ultra Green castable refractory. There are also 8 heat exchangers to cool down the flue gas before entering the exhaust system. A preheater is also installed to preheat the inlet oxidant to temperatures as high as 640 K. The experiments will focus on the effect of O2 partial pressure (PO2) in the transport stream, on flame stability, ignition stand-off distance, and NOx in the exhaust. The relationship between PO2 in the transport stream and in the secondary oxidant stream, on the near-burner aerodynamics, as well as flame stability and detachment, will be investigated and quantified. The experimental conditions will include those expected for retrofit oxy-coal combustion configurations: a normal pulverized-coal particle size distribution, a representative transport fluid stream, and O2/CO 2 mixtures for the secondary oxidant stream. The secondary oxidant stream PO2 will remain constant at “typical values,” expected for oxy-coal retrofit applications. r -24" ID . x 30" O.D. Burner Zone View/Ash w._ j : Blowout Port---- J ' l T T Figure 1. Design sketch of the new 100kW oxy-coal combustion furnace the top section A special CMOS sensor based camera, which is more sensitive to the near infrared wavelength (responsivity: 1.4 V/lux-sec (550nm)), is applied to capture Type 0 axial turbulent diffusion flame shape for the use of statistical studies of stand-off distance at different operational parameters, such as systematic variations of partial oxygen pressure in both transport and secondary oxidant stream. Statistical analysis is used to scale the effect of oxygen partial pressure in both transport and secondary stream on coal jet ignition and flame stability. Ultimately the statistics help to generate probability density function (PDF), which can be used to evaluate the experiment accuracy, reproducibility and to do model validation. Due to the reason that inserting a probe in the near burner zone influences our Type 0 pulverized coal flame and coal jet ignition, detailed 2D temperature profile and gas components profile is difficult be measured in our tests. Therefore, to quantify the flame stability, we developed our own optical measurements and image processing technique. During the pulverized coal ignition process, the sequences of images of flames are taken by a CMOS sensor based camera with at least 24 frames per second. Then all the images are analyzed in a MatLab script developed by our group. This code can automatically analyze the following parameters: 1) 2) 3) 4) 5) 6) 7) 8) average intensity of the whole image average intensity within the flame envelope visible flame length(luminous zone) mean standoff distance standoff distance in the centerline total area of the flame number of blobs flame width at different locations For the thresholding, instead of manually selecting the threshold to convert the grayscale image to the black and white image, the edges of the flame are detected using the Sobel method in Matlab. The Sobel method finds edges using the Sobel approximation to the derivative (See Figure 4 c). It returns edges at those points where the gradient of the grayscale intensity is maximum. The above methodology is shown in Figure 4. (a) (b) (c) (d) (e) Figure 4. OFC flame image processing. (a) original image, (b) image converted to grayscale, (c) edge detection using the Sobel method, (d) image converted to black and white using the threshold calculated from the Sobel method, (e) measurement of image statistics: standoff distance (if any), flame length, and intensity within flame envelope. Table 1, cases selected for the comparison between O2/N 2 flame and O2/CO 2 flame O 2/N 2 mixture Case No. Tadb (K) secondary PO2 overall PO2 2394 0.209 0.209 2580 0.240 2701 2798 O2/CO 2 mixture Tadb (K) secondary PO2 overall PO2 I 2289 0.330 0.313 0.236 II 2389 0.380 0.352 0.260 0.254 III 2446 0.415 0.379 0.280 0.270 IV 2490 0.445 0.402 2.1, PRELIMINARY EXPERIMENTS TO UNDERSTAND EFFECT OF Po 2 IN SECONDARY STREAM Air-fired cases, oxy-enriched cases and oxy-fired cases have been tested and analyzed. The whole idea is to compare the luminosity, NOx formation, and flame stability under O2/N 2 environment and O2/CO 2 environment by varying the partial pressure of O2 in secondary stream to match the same adiabatic flame temperature. The fixed parameters were: Overall S.R. = 1.15 = 0.11 (primary) + 1.04 (secondary); Tprimary stream= room temperature, Tsecondarystream= 561 K, Twall = 1255 K, Primary PO2 = 0.21. Coal feed rate = 10 lb/hr = 4.54 kg/hr, Coal type: Utah bituminous (Ultimate(wt%, daf): C 77.75%, H 4.63%, N 1.44%, S 0.45%, O 12.17%; Proximate: Moisture 3.03%, Volatile Matter 38.81%, Fixed Carbon 46.44%, Ash 11.72%; HHV = 11731 BTU/hr). The testing matrix is shown in Table 1. 2.2, EXPERIMENTS TO UNDERSTAND THE EFFECT OF PO2 IN THE PRIMARY (TRANSPORTING) STREAM ON THE AXIAL FLAME STAND-OFF DISTANCE The primary focus of this research is when overall PO2 in the total mixture and both of primary and secondary velocities are fixed, and PO 2 in the transport stream are varied, the effect of O2 partial pressure (PO2) in the transport stream, on flame stability, ignition stand-off distance, and NOx in the exhaust, shall be studied under oxy-coal combustion conditions. The fixed parameters are: Overall S.R. = 1.15= 0.11 (primary) + 1.04 (secondary); Tprimarystream= room temperature, Tsecondary stream= 489 K, Twall = 1255 K, Overall P O2 = , Secondary stream velocity : primary velocity = 2:1, Coal feed rate = 1 0 lb/hr = 4.54 kg/hr, Coal type: Utah bituminous (Ultimate(wt%, daf): C 77.75%, H 4.63%, N 1.44%, S 0.45%, O 12.17%; Proximate: Moisture 3.03%, Volatile Matter 38.81%, Fixed Carbon 46.44%, Ash 11.72%; HHV = 11731 BTU/hr). In table I, oxy-combustion case IV is chosen as the base one to carry one the study. Utah bituminous coal is chose as the base coal. Standoff distance is measured by applying image capture and processing technique. 3. RESULTS AND DISCUSSIONS 3.1 Comparison between O 2/CO 2 flame and O 2/N 2 flame in the prelim inary experiments This comparison is conducted under conditions listed in Table 1, when matching adiabatic flame temperatures of a O2/N 2 mixture flame and a O2/CO 2 mixture flame. Figure 5 shows the NOx formation, as a byproduct of this research, when varying partial pressure of O2 in the secondary stream. NOx was measured at the exit of the exhaust close to the entrance of the stack. Top curve represents O2/N 2 flame (both air flame and oxygen enriched flame), which shows the tendency that increasing PO2 leads to a higher NOx formation due to higher temperature. Once the flame gets attached to the burner, when P O2 in the secondary stream is higher than 25.4% in our operation conditions, NOx formation is slightly lower, due to the different mixing pattern and fluid mechanics because of the flame attachment. However, the NOx formation curve, the bottom one, does not show that under oxy-coal combustion conditions, NOx is influenced by partial pressure of oxygen or flame temperature that much because theoretically there is no thermal NOx generated without the presence of N2. Almost all the NOx comes from the nitrogen compound in the fuel under oxy-coal combustions. Unit: mg/MJ Unit: Ib/MMBTU Adiabatic flame temperature (K) Figure 5, Comparison of NOx formation under O 2/N 2 environment and O 2/CO 2 environment (red and blue numbers show P O2 in percentage in secondary stream of each case, P O2 in transport stream is always 20.9%) Figure 6 shows the comparison of flame luminosity between an O2/CO 2 flame and an O2/N 2 flame. Flame luminosity, often related to soot formation in the flame, is of interest to be quantified by assuming flame luminosity is proportional to the flame intensity. Figure 6 shows the results of the comparison from the image processing technique, when calculating the average flame intensity within the whole image. It turns into the conclusion that when matching adiabatic flame temperature, an O2/CO 2 flame is more luminous than an O2/N 2 flame mostly, which is contrary to the observations from the experiments. To improve it, a new concept of “average intensity within a flame envelope” has been developed. Figure 7 and 8 show the histograms of average intensity within the flame envelope for an O2/N 2 flame and an O2/CO 2 flame. The results show that for case IV in table, when an O2/N 2 flame and an O2/CO 2 flame match almost the same adiabatic flame temperature and both are attached, an oxy-coal flame is less luminous than an oxygen-enriched flame. This work is still under the way. Adiabatic(lametemperature(K) Figure 6 , Comparison of relative flame luminosity between O2/N2 environment and O2/CO2 environment 120 100 I N o v 4 Frequency I Nov3 Frequency 80 O tc 2 9 Frequency IO tc31-l Frequency iO ct3 1 -ll F requency 60 40 20 ■ il I O U D r \ J 0 0 ,^ ' O V D r M 0 0 ^ f O U D r \ l 0 0 «Nj-C LT) LO <_D r— OOOOCTi Cr i CDT- HT- Hr Nj r Nj r O^ ~' Average intensity^i/vifiiin'the'^lame envelope ( 6 ^- ^ 5 ) ^ Figure 7, Histograms of average intensity within flame envelope for an oxygen enriched ( 0 2/N2) case IV(see Table 1) attached flame 120 100 </> QJ tu n>O E o oL /l c 3 o 29-Oct i 4-Nov Oct31a I Cct31b 80 60 40 20 i Ui i i r 0 ^ ^ ^ ^ ^ ^ ^ ^ ^ Average Intensity within Envelope (0-255) Figure 8 , Histograms of average intensity within flame envelope for an oxy-coal ( 0 2/C 0 2) case IV (see Table 1) attached flame 3.2 The effect of P 02 in secondary stream on flame stability under oxy-coal combustion conditions P 0 2 in s e c o n d a r y s t r e a m (%) Figure 9, Flame length vs PO2 in secondary stream under oxy-coal combustion conditions Figure 9 shows how PO2 in secondary stream influences flame stability (attachment/detachment) and visible flame length (luminous zone) under oxy-coal combustion conditions. The primary stream partial pressure of O2 is always kept as 20.9%. Increasing PO2 in secondary stream helped flame turning from unstable and detached to stable and attached. It also reduced the standoff distance, which also means the coal particles are getting faster ignited. The total flame length is defined here as the sum of luminous zone length and standoff distance. The results may also suggest that when a flame is attached, PO2 may have minor effects on the flame length, when the change of burner operation parameters, such as velocity and momentum, can also influence the flame length. The results in figure 9 are expressed in average values with error bars. With the concern of LES model development in CFD simulation, it is of more interest to quantify the precision of the LES model under different applications. Since ultimately the data acquired in the experiments are going to be used for model validation purpose, simply providing the trends made by mean values will not be sufficient in this application. Therefore, statistical analysis must be provided to quantify the accuracy, the error and the reproducibility of the experiments. For instance, figure 10 shows the histograms of visible flame length of oxy-coal case IV (see Table 1), which has an attached flame. All the histograms always yield Gaussian distributions according to statistics. Ultimately the histograms are going to be used to generate probability density function (PDF), which will be used for LES model validation to quantify the model precision. This is still under the way. 140 l Oct. 29 120 I Oct311 >. < j c OJ 3 CT 100 Oct31 80 Nov. 4 <11 40 20 jbli “i ILi l i i j i Ln0 Ln0 Ln0 Ln0 Ln0 Ln0 Ln0 cu rMrMnom^^inm<x><x>r^r^oooocn ^ I 0 Visibleflame length (unit: cm) ^ Figure 10, Histograms of flame length of oxy-coal case IV (see Table 1), attached 3.3 The effect of P O2 in transport stream on flame stability Currently this work is still under the way. Photo-imaging data is still being analyzed by our code. Figure 11 shows the effect of PO2 in transport (primary) stream on the flame stability, which is quantified by standoff distance. With overall PO2, primary stream velocity, and secondary stream velocity, total stoichiometric ratio, wall temperature, preheat temperature, coal feeding rate, and camera setting fixed, increasing PO2 in transport stream from 0 to 20.9%, as shown by the numbers underneath each group of picture, can change flame stability. According to the pictures taken from the experiments, standoff distance is decreased when PO2 in transport stream is increased. The bottom pictures show flame shapes in several circular zones, which represent the near burner zone under different cases. The effect of PO2 in transport stream is more significant than the effect of PO2 in secondary stream, because oxygen in transport stream is premixed with pulverized and react with coal rapidly under high temperatures, while secondary oxygen needs the help of either diffusion or small/large turbulent eddies to mix with coal to react. Figure 12 shows an example of histograms of flame standoff distances of an oxy-coal case IV when PO2 in transport stream is 0, when 11,000 images are taken at 30 fps sampling rate. The result shows a very standard Gaussian distribution, which may be used to generate a probability density function (PDF) in the future. This work is still being conducted in our lab. f JiT PO2 in transport stream = 0 0.099 0.144 0.207 Figure 11, the effect of P 0 2 in transport stream on flame stability and near burner flame structure 700 600 >u 500 c <D 3 o- 400 c <u LL. 300 c 3 o 200 u 100 0 ..ill “I I I I I r o<Hr\ino^Ln<x>r-*oocnc><— k r N r N r s i r N i r N i r N r s j r s j r s j r s i m m m m m m m Stanoff distance (cm) Figure 12, Histogram of standoff distance of a detached oxy-coal flame with 0 P 0 2 in transport stream 4. CONCLUSIONS AND FUTURE W ORK A downfired oxy-coal test com bustion facility was used to determine coal je t ignition and stability issues under oxy-coal com bustion conditions. A new m ethodology was developed to quantify flame length, flame stability, ignition behaviour in near burner zone. This involved an im age processing technique and statistical analysis, w hich allowed quantification o f the precision in experiments. Current results show how flame attachm ent increased w ith P O2 either in secondary oxidant streams or in transport streams. The experiments regarding the effect of PO2 in transport stream and the corresponding image processing and statistical analysis are still under the way. More results are expected in the near future. 5. ACKNOWLEDGEMENT The authors are grateful to the US D epartm ent o f Energy for its financial support under Award N um ber FC26-08NT0005015 through U tah Clean Coal Center(UC3) and to Praxair Inc for their contribution o f the oxygen and the carbon dioxide supply and the associated infrastructure. Initial financial support from U niversity o f U tah Institute o f Clean and Secure Energy is also gratefully acknowledged. The authors also acknowledge the technical assistance o f Dr. Lawrence E. Bool, III o f Praxair. 6 . REFERENCE  B. M etz, et al. IPCC Special Report on Carbon dioxide Capture and Storage. Cam bridge University Press, UK, 2007.  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|Relation has part||Zhang, J., Kelly, K., Eddings, E. G., & Wendt, J. O. L. (2009). Oxy-coal combustion: effects of PO2 on coal jet stability in 02/C02 environnments. American Flame Research Committee (AFRC).|
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