OXY-COAL COMBUSTION: EFFECTS OF P O2 ON COAL JET STABILITY IN O 2 /CO 2 ENVIRONMENTS

Transcription

1 OXY-COAL COMBUSTION: EFFECTS OF P O2 ON COAL JET STABILITY IN O 2 /CO 2 ENVIRONMENTS Jingwei Zhang, Kerry Kelly, Eric G. Eddings, Jost O.L. Wendt Department of Chemical Engineering & Institute for Clean and Secure Energy University of Utah, Salt Lake City, Utah st Oxyfuel Combustion Symposium, Cottbus, Germany September 7 th -10 th, 2009

2 The problem: Coal jet ignition Small particles Large particles Ignition behavior Flame stability Flame length Standoff ignition distance depends on primary jet velocities, wall T, and P O2, which becomes an independent variable under oxy-coal combustion Sub-model should capture observations that smaller particles preferentially migrate to jet edge. [Sinclair Curtis group Purdue University,2003]. Implications on effects of secondary P O2, also an independent variable. Pyrolysis y behavior. (Naredi and Pisupati, 2007, Penn State University) Particle ignition. (Shaddix and Molina, 2005, 2006, Sandia Labs) Influence of gas properties which vary heat transfer to coal particle.

3 This work Determine, in a systematic manner, how burner operating parameters and oxygen partial pressure influence flame stability and coal ignition. Explore effects of variations in the partial pressure of O 2 and CO 2 on coal jet ignition and flame stability(specific objective of this work). Systematically investigate near-burner aerodynamics and ignition zone for Type 0 axial diffusion flames (no swirl) Develop technique to quantify coal flame length or stand-off distance from photo-images to allow quantitative comparison with simulations, together with uncertainty quantification.

4 Design and construction of an oxy-fuel combustion furnace A 100 kw, down-fired, oxy-coal combustion furnace, once-through CO 2 (no recirculation) Heated walls and quartz windows for optical access that permit flame detachment/attachment studies and optical diagnostics Simulate the environments experienced by pulverized coal jet flames in boilers Systematic control of burner momentum and velocity variables, and wall temperatures

5 Design details Top section: m I.D., m O.D., m in height; 2600 Fiberboard (δ = 76 mm) W flanged ceramic plate heaters with k thermocouples controlling or monitoring i the temperaturet 3 layers of insulation in radiant zone and 2 layers insulation in convection zone 8 heater exchangers to cool down flue gas A preheater (room temperature - 640K) in secondary stream

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7 Coal Analysis (Utah Bituminous coal) HHV: BTU/lb ( 27.3 kj/kg)

8 Outline Background and Motivation Objectives Preliminary shakedown experiments Experimental setup and system validation Methodology of flame stability measurements Preliminary Data Results of studying effects of primary P O2 on flame stability Future work Acknowledgements

9 Methodology of Stand-off distance and flame length measurement We use: CMOS sensor-based camera (EPIX SV5C10) 2592 x 10 fps; 1280 x 30 fps; 640 x 89 fps; 8 or 12 bits per pixel. This work used 200 x 32 fps. Software: XCAP-std, MatLab Image processing procedures to get standoff length: Capture sequences of images Normalized image intensity Using EPIX to segment (blob analysis) the normalized images Using MatLab

10 An example of flame length(luminous zone) measurements: live animation (enriched O 2 /N 2 flame, P O2(pri) =0.21, P O2(overall) =0.27, P O2(sec) =0.28, attached) Date Length (inches) STD(inches) Threshold Aperture 29 Oct Oct Oct Oct Nov Nov Average Tentative ti conclusion: Results depend on selection of threshold value. Therefore must use Sobel (maximum gradient) method which is less subjective, and widely accepted.

11 OFC flame image processing methodology: (a) original i 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

12 Outline Background and Motivation Objectives Preliminary shakedown experiments Experimental setup and system validation Methodology of flame stability measurements Preliminary data Results of studying effects of primary P O2 on flame stability Future work Acknowledgements

13 Comparison between O 2 /N 2 flame and O 2 /CO 2 flame Fix the following parameters: Overall S.R. = 1.15 = 0.11 (primary) (secondary) T pri = room temperature, T sec = 561 K, T wall = 1255 K Primary P O2 = 0.21 Coal feeding = 10 lb/hr = 4.54 kg/hr, Utah bituminous (HHV = BTU/hr, 27.3 kj/kg) O 2 /N 2 mixture No. O 2 /CO 2 mixture Tadb (K) secondary P O2 overall P O2 Tadb (K) secondary P O2 overall P O I II III IV

14 Comparison of NO x formation under O 2 /N 2 environment and O 2 /CO 2 environment 2 2

15 Effect of P O2 in secondary stream on flame length and attachment under oxy-coal combustion

16 Comparison of flame luminosity between O 2 /N 2 flame and O 2 /CO 2 flame when matching T adiabatic Co ount/frenqu uncy Enriched Case IV, average intensity within the flame envelope histograms Nov 4 Frequency Nov3 Frequency Otc 29 Frequency Otc 31-I Frequency Oct 31-II Frequency Count (500 imag ges) OxyCase IV, histograms of average intensity with flame envelope 29-Oct 4-Nov Oct31a Oct31b Average intensity within the flame envelope ( 0-255) Average Intensity within Envelope (0-255) Visual observations, and rough measurements (above, using 500 images each run) suggest that the O 2 /N 2 coal flame is more luminous than the O 2 /CO 2 flame at the same adiabatic flame temperature.

17 O2/CO2 (left) O2/N2 (right) Primary P O Secondary P O S.R T adiabatic K 2490 K Pri. velocity 4.5 m/s 5.4 m/s Sec. velocity 18.4 m/s 20.9 m/s Pri. &Sec Sec. matching matching momentum When matching momenta to match turbulent mixing of co-axial coal jet, O 2 /N 2 flame and O 2 /CO 2 flame show different flame shape/structure in the near burner zone. Why? 1) Coal ignition affects internal recirculation zone inside the chamber 2) Influence of gas properties on heat transfer to coal particles.

18 Outline Background and Motivation Objectives Preliminary shakedown experiments Experimental setup and system validation Methodology of flame stability measurements Preliminary Data Results of studying effects of primary P O2 on flame stability Future work Acknowledgements

19 Primary P O2 : 0.21 => 0 => Flame: Attach => Detach => Attach O 2 /CO 2, Overall P O2 =40%

20 P O2(pri) =

21 Probab bility Dens sity (1/cm) Primary P O Probab bility Densi ity (1/cm) Composite Plot Stand-off distance PDFs from 6000 images per run. 5 replicates (left); 2 replicates (rt) Coal type LEFT RIGHT Utah Bituminous Overall 40% PO2 Pri. Stream O 2 /CO 2 mixture Sec. O 2 /CO 2 mixture Stream Preheat ea 489 K 544 K Stand-off Distance (cm) Stand-off Distance (cm)

22 Probab bility Dens sity (1/cm) Primary P O Probab bility Densi ity (1/cm) Composite Plot Stand-off distance PDFs from 6000 images per run. 5 replicates (left); 2 replicates (rt) Coal type LEFT RIGHT Utah Bituminous Overall 40% PO2 Pri. Stream O 2 /CO 2 mixture Sec. O 2 /CO 2 mixture Stream Unsteady attachment Preheat ea 489 K 544 K Stand-off Distance (cm) Stand-off Distance (cm)

23 Probab bility Dens sity (1/cm) Primary P O Probab bility Densi ity (1/cm) Composite Plot Stand-off distance PDFs from 6000 images per run. 5 replicates (left); 2 replicates (rt) Coal type LEFT RIGHT Utah Bituminous Overall 40% PO2 Pri. Stream O 2 /CO 2 mixture Sec. O 2 /CO 2 mixture Stream Preheat ea 489 K 544 K Stand-off Distance (cm) Stand-off Distance (cm)

24 Conclusions Systematic measurements of lift off distance versus primary O 2 concentration (P O2 ) have been obtained, together with uncertainty quantification. Flame lift-off distance is not a continuous variable and attachment/detachment pass through a sudden transition. Secondary stream preheat plays an important role on the coal ignition. Primary P O2 has the first order effect on flame stability and axial coal jet ignition. Data will assist in validation of flame stability sub- models for simulations that can predict effects of conversion from air-firing to oxy-coal firing in existing units

25 Future work Study effects of systematic variations of p O2 in primary flow vs flame stability Analyze collected data on effect of changes of wall temperatures secondary stream preheat temperatures coal composition 32% overall P O2 Conduct special experiments to provide insight into underlying mechanisms (effect of CO 2 rather than N 2, in either primary or secondary streams individually) Present additional statistical analysis to quantify uncertainties for simulation validation. Expand research to investigate how to minimize CO 2 recycle. Expand to flames with swirl.

26 Acknowledgments This material is based upon work supported by the Department of Energy under Award Number FC26-08NT Praxair Inc. for providing O 2 and CO 2 at no cost to the project. Lawrence E. Bool, III, Praxair, for technical input. University of Utah for initial financial support. Technical Staff: Ryan Okerlund, Brian Nelson, David Wagner Undergraduate assistants: Dallin Call, Raphael Ericson, Charles German