Development of a Kinetic Fluidized Bed Gasifier Model for Application in Flowsheet Simulation

Transcription

1 Die Ressourcenuniversität. Seit Department of Energy Process Engineering and Chemical Engineering Development of a Kinetic Fluidized Bed Gasifier Model for Application in Flowsheet Simulation Matthias Gootz, Supervision: Prof. Dr.-Ing. Bernd Meyer Lars- Erik Gärtner Prof. Dr.-Ing. Christian Hasse Leipzig, TU Bergakademie Freiberg I Department of Energy Process Engineering and Chemical Engineering Reiche Zeche I 9596 Freiberg I Germany I Phone +49()3731/ I Fax +49()3731/ evt@iec.tu-freiberg.de I Web

2 Content I I. Introduction II. Model development III. Model validation IV. Simulation results V. Conclusion 2

3 Introduction I Gasification Coal for power generation and chemicals production Commercially available technology Flowsheet simulation with Aspen Plus Implementation of gasifier model into larger flowsheet simulation Coal gasification kinetics Determined by physico- chemical effects Implementation with user- defined subroutine in FORTRAN77 Kinetic entrained flow gasifier model with subroutine already developped at TUBAF 3

4 Introduction I Main objectives Development of fluidized bed gasifier model using the continiously stirred tank reactor model (RCSTR) Comparison of two kinetic rate equation types: Langmuir- Hinshelwood (LH) N- th order Reasons RCSTR suitable for representation of fluidized bed gasifier Few literature on gasifier simulation with RCSTR and LH kinetics available LH more suitable than n- th order equations over broad operating ranges 4

5 Model development: Kinetics II Heterogeneous reactions C + O 2 CO 2 2C + O 2 2CO C + H 2 O CO+ H 2 C + CO 2 2CO Physico- chemical effects Pore diffusion Surface area evolution N- th order Langmuir- Hinshelwood Homogeneous reactions 2CO + O 2 2CO 2 2H 2 + O 2 2H 2 O CH 4 + 2O 2 CO 2 + 2H 2 O CO + H 2 O CO 2 + H 2 5

6 Model development: Aspen Plus II High- Temperature- Winkler- Gasifier Pilot plant Wesseling Separation Ssplit Solids Unconverted char, fines, ash Rawgas Rawgas, fines Freeboard zone Gasification RCSTR Air/ O 2 Steam Separation Generation rate of components Freeboard zone Properties, user input Calculator Kinetics Subroutine Solids Gas Air/ O 2 Steam Coal Air/ O 2 Steam Fluidized bed Recycle Air/ O 2 Fluidized bed Combustion of volatiles Recycled solids RGibbs Generation rate of components Gasification RCSTR Air/ O 2 Steam Volatiles Products of combustion Char Unconverted char, ash Coal Drying, Devolatilization RYield Char Char decomposition RStoic 6

7 Model validation: Case setup and results III Validation case setup Pressurized air/ steam gasification of Rhenish brown coal [1] Gasifier temperatures and synthesis gas composition given Simulation validation LH: T X C N-th order: T X C Observation Low X C at high reactor temperatures for LH simulation T in C T Densebed T Freeboard XC X C LH N- th order Literature X C [1] Hamel et. al. Modeling of pressurized fluidized bed gasification in comparison with experimental data from a commercial scale and pilot scale HTW-gasification plant. Proceedings of 4th International Symposium on Coal Combustion, Beijing(1999), pages , Cases 7

8 Model validation: Results III Simulation results LH: N-th order: CH 4 CO 2 H 2 O CO H 2 CH 4 CO 2 H 2 O CO Syngas components in m³(stp)/h H 2 CH4 4 CO2 2 H2O H 2 O CO H2 H 2 Interpretation LH N- th order Literature Cases Energy balance is solved for lower temperatures (Aspen Plus: Standard Enthalpy of Formation is used) CO H R, = -111 kj/mol [2] CO 2 H R, = -393 kj/mol [2] [2] Chemgapedia: Standardbildungsenthalpien einiger Verbindungen. amik_thermochemie.vlu/page/vsc/de/ch/11/aac/vorlesung/kap_8/kap8_4/kap8_4a.vscml/fragment/faed7b 2f1d262b91a3e662626ed88c-34.html. Accessed

9 Model validation: Langmuir- Hinshelwood III Reactor height in m CH 4 H 2 O CH4 H2O H2 H 2 CO CO2 O2 CO 2 O C- O 2 C- H 2 O C- CO 2 H 2 -O 2 C- O2 C- H2O C- CO2 H2- O2 Freeboard 2 2 Densebed Concentration in mol/mol R h in kmol/( m³ s) Observations: Homogeneous reactions should proceed faster C- CO 2 and C- H 2 O reactions too slow Inhibition by CO Low conversion due to inhibition, but high temperatures due to fast combustion 9

10 Model validation: N- th order III Reactor height in m CH 4 H 2 O CH4 H2O H2 H 2 CO CO2 O2 CO 2 O C- O 2 C- H 2 O C- CO 2 H 2 -O 2 C- O2 C- H2O C- CO2 H2- O2 Freeboard 2 2 Densebed Concentration in mol/mol R h in kmol/( m³ s) Observations: Char oxidation slower than with LH kinetics C- CO 2 and C- H 2 O reactions are faster Faster endothermic reactions cause high conversion at low temperatures 1

11 Simulation results: Variation of air feed stream IV T in C X C T in C X C 5 4 T Densebed T Freeboard XC X C T Densebed T Freeboard XCX C % -5% % 5% 1% 3-1% -5% % 5% 1% Change from original air feed stream Change from original air feed stream LH N- th order Observations: Higher air feed stream raises gasifier output results, but output parameters are still underpredicted Conclusion: No advantages of LH equations over n- th order equations in present simulation 11

12 Simulation results: Range of application (LH) IV T in C T Densebed.2-2 T Freeboard.1 XCX C Fraction of air fed to combustion x Comb 1 X C Combustion of volatiles in separate Gibbs reactor Less air used for coal gasification than in reality Unrealistic distribution of gasification agents between zones 12

13 Conclusion V Gasifier model Use with different reaction kinetics data and rate equations possible Failure to satisfyingly predict gasifier output parameters LH and n- th order simulation No recommendation can be made for present simulation Improvements necessary Suitable rate parameters (from orginial coal) for heterogeneous reaction kinetics Revision of homogeneous reaction equations External volatiles combustion needs to be transferred to RCSTR 13

14 Conclusion V Future developments Adjustment to different fluidization regimes [3] Combination RCSTRs with Plug Flow Reactor Extension for use of fuel blends [3] D. Kunii and O. Levenspiel, Fluidization Engineering, second ed., Butterworth- Heinemann,

15 Conclusion Thank you for your attention. 15