Computational Modeling (CFD) of Entrained Flow...
Transcript of Computational Modeling (CFD) of Entrained Flow...
Computational Modeling (CFD) of Entrained Flow Gasification Kinetics with focus on the Structural
Evolution of Char Particles
Dipl.-Ing. Stefan Halama1, Prof. Dr.-Ing. Hartmut Spliethoff1,2
1LES, TU München, Germany 2ZAE Bayern, Germany
20.05.2014
6th International Freiberg Conference on IGCC & XtL Technologies, Coal Conversion and Syngas, 19-22 May 2014, Dresden/Radebeul
Modeling of Entrained Flow Gasification
Particle Structure Modeling
Geometry and Boundary Conditions
Results and Validation
Conclusion and Outlook
Content
2/15 6th International Freiberg Conference on IGCC & XtL Technologies, Coal Conversion and Syngas,
19-22 May 2014, Dresden/Radebeul
Modeling of Entrained Flow Gasification
• The design of entrained flow gasifiers plays an important role in the development of IGCC power plants
• Large-scale gasifier = High temperatures and high pressures
Goal: Prediction of conversion rates and gas composition in an entrained flow gasifier
• Validation against experiments for various coals, operating pressures and operating temperatures
• Mechanistic modeling approach Better understanding of reaction kinetics, more flexible model (e.g. temperature range)
3/15 6th International Freiberg Conference on IGCC & XtL Technologies, Coal Conversion and Syngas,
19-22 May 2014, Dresden/Radebeul
Modeling Approach
• Software: ICEM CFD and ANSYS Fluent with User Defined Functions (UDF)
• Devolatilization: Two-Competing-Rates Model, Volatiles = CxHyOzNa (x = 1)
• Homogeneous Kinetics: Jones-Lindstedt Mechanism for Hydrocarbon Combustion
• Heterogeneous Kinetics: Intrinsic rates (O2, CO2, H2O), nth order effectiveness factor approach (Regime I-III), Thermal annealing model
• Particle Structure Evolution: Surface area, diameter/density, porosity, pore diameter
4/15 6th International Freiberg Conference on IGCC & XtL Technologies, Coal Conversion and Syngas,
19-22 May 2014, Dresden/Radebeul
[Source: HotVeGas I]
• State-of-the-art surface area evolution model for Regime I (& II)
• Does not account for pore opening/closing or particle diameter changes Important at higher temperatures (in particular above the ash fusion temperature)
Random Pore Model
[Source: Tremel (2012), 750 ºC, TGA]
5/15 6th International Freiberg Conference on IGCC & XtL Technologies, Coal Conversion and Syngas,
19-22 May 2014, Dresden/Radebeul
Surface Area Evolution at Higher Temperatures
[Source: Tremel (2012)]
6/15 6th International Freiberg Conference on IGCC & XtL Technologies, Coal Conversion and Syngas,
19-22 May 2014, Dresden/Radebeul
Modeling of Pore Closing and Diameter Evolution
• Based on average pore size model [Petersen (1957), Wheeler (1951)]
• Diameter evolution: Power law approach, Exponent β = f(reaction regime)
• Pore closing (e.g. by melting of mineral matter) is implemented by a time- and temperature-dependent factor that reduces the total length and void volume of the pore system, based on pyrolysis experiments:
7/15 6th International Freiberg Conference on IGCC & XtL Technologies, Coal Conversion and Syngas,
19-22 May 2014, Dresden/Radebeul
[Source of exp. values: Tremel (2012)]
Geometry
• Pressurized High Temperature Entrained Flow Reactor (PiTER)
• Operated at the Institute for Energy Systems (TU München)
CFD Mesh:
• 1.3 mio. nodes
• Min. orthogonal quality: 0.22
• Max. aspect ratio: 19
Calculation:
• Approx. 24 h for one simulation
• 8 x 3.4 GHz Intel Xeon, 64 GB RAM [Source: Tremel (2012)]
8/15 6th International Freiberg Conference on IGCC & XtL Technologies, Coal Conversion and Syngas,
19-22 May 2014, Dresden/Radebeul
• Coal and char diameter distributions and densities
• Proximate / ultimate analysis, volatile yield (f(p,T)), heating value
• Standard reactivities of pyrolysis chars (annealing) + surface areas (pore closing)
• Intrinsic rates of annealed char (O2, H2O, CO2)
Test case:
• T = 1200 ºC / 1400 ºC / 1600 ºC, p = 5 bar, O/C = 1
• Coal = Rhenish lignite (ca.: 11% moisture, 5% ash, 57% volatiles), 1.25 kg/h
• Diameter distribution: 1 to 180 µm (Rosin-Rammler)
• Residence time in the reaction tube: ca. 2 s
Measured Input Values and Operating Conditions
9/15 6th International Freiberg Conference on IGCC & XtL Technologies, Coal Conversion and Syngas,
19-22 May 2014, Dresden/Radebeul
Tempera- ture [K]
Volatile Fraction [-]
Reaction Rates [kmol/(m3 s)]
Results at 1200 ºC
10/15 6th International Freiberg Conference on IGCC & XtL Technologies, Coal Conversion and Syngas,
19-22 May 2014, Dresden/Radebeul
Results and Validation – Char and Overall Conversion
1200 ºC 1400 ºC 1600 ºC
11/15 6th International Freiberg Conference on IGCC & XtL Technologies, Coal Conversion and Syngas,
19-22 May 2014, Dresden/Radebeul
[Source of exp. values: Tremel (2012)]
Results and Validation – Surface Area
12/15 6th International Freiberg Conference on IGCC & XtL Technologies, Coal Conversion and Syngas,
19-22 May 2014, Dresden/Radebeul
[Source of exp. values: Tremel (2012)]
Results – Effectiveness Factor and Reaction Rates
13/15 6th International Freiberg Conference on IGCC & XtL Technologies, Coal Conversion and Syngas,
19-22 May 2014, Dresden/Radebeul
Results – Gas Composition
1200 ºC 1400 ºC 1600 ºC
14/15 6th International Freiberg Conference on IGCC & XtL Technologies, Coal Conversion and Syngas,
19-22 May 2014, Dresden/Radebeul
Conclusion and Outlook
State-of-the-art surface models cannot capture all effects of surface area evolution in entrained flow gasification (ash melting, carbon softening, diffusion effects)
A new modeling approach has been proposed and validated for a Rhenish lignite
The new approach predicts structural parameters (such as particle diameter, density, surface area, porosity) that are important for the modeling of pore diffusion
Outlook:
Validation of predicted diameter distributions
Validation of the model for other fuels
Application to large-scale geometries
15/15 6th International Freiberg Conference on IGCC & XtL Technologies, Coal Conversion and Syngas,
19-22 May 2014, Dresden/Radebeul
Thank you for your attention!