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)



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



[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]



Surface Area Evolution at Higher Temperatures

[Source: Tremel (2012)]



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:



[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)]



• 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



Tempera- ture [K]

Volatile Fraction [-]

Reaction Rates [kmol/(m3 s)]

Results at 1200 ºC



Results and Validation – Char and Overall Conversion

1200 ºC 1400 ºC 1600 ºC




Results and Validation – Surface Area




Results – Effectiveness Factor and Reaction Rates



Results – Gas Composition

1200 ºC 1400 ºC 1600 ºC



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



Thank you for your attention!

computational modeling (cfd) of entrained flow...

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