computational modeling (cfd) of entrained flow...
TRANSCRIPT
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
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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)
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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
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[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]
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Surface Area Evolution at Higher Temperatures
[Source: Tremel (2012)]
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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:
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[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)]
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• 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
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Tempera- ture [K]
Volatile Fraction [-]
Reaction Rates [kmol/(m3 s)]
Results at 1200 ºC
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Results and Validation – Char and Overall Conversion
1200 ºC 1400 ºC 1600 ºC
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[Source of exp. values: Tremel (2012)]
Results and Validation – Surface Area
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[Source of exp. values: Tremel (2012)]
Results – Effectiveness Factor and Reaction Rates
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Results – Gas Composition
1200 ºC 1400 ºC 1600 ºC
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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
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Thank you for your attention!