CFD Model for Optimisation of an Entrained Flow Gasifier for Black Liquor

Transcription

1 CFD Model for Optimisation of an Entrained Flow Gasifier for Black Liquor Magnus Marklund 1, Rikard Gebart 1, David Fletcher 1 Energy Technology Centre in Piteå, Sweden Department of Chemical engineering, University of Sydney, Australia

2 Introduction Development plant for pressurised black liquor gasification under construction Underpinning research program with 4 subprojects at ETC, LTU, UmU and Chalmers Aim of current project: development of a CFD model for optimisation of the reactor.

3 Introduction Three main parts, a hot reactor, a quench cooler and a counter current condenser. Atomisation to fine droplets (~ µm) Partial combustion of fuel spray with oxygen

4 The CFD paradigm Reality Numerical model Conceptual model with sub models The conceptual model is an approximation of reality The numerical model is an approximation of the conceptual model Verification of the numerical model must be done before validation. (AIAA, ERCOFTAC)

5 Current model Based on CFX4 CFD code Distribution of non interacting discrete droplets forming a spray Droplet conversion by customised subroutines in the solver k-ε and Reynolds stress turbulence models Gas combustion modelled by Eddy Break Up/Kinetic model Discrete transfer model for thermal radiation Coupled solver for heat conduction in the walls

6 Model status Included gas species: O, H O, CO, H, CO, CH 4 Sulphur and sodium reactions neglected Combustion is controlled by mixing and kinetics Gas phase combustion reactions: CH CH H H + O + 0.5O CO + 0.5O CO + H O O CO CO H CO CO O + 4H + H + H O Temporarily neglected

7 Model verification Unexpectedly high flame temperature predicted by the model. Input parameters, model, numerical or programming errors? Simplified model of a long rectangular duct to verify the results. Mass flux of O (λ=1.1) and droplets at inlet Droplets consist only of CO (volatile matter) and smelt Constant specific heat Endothermic reactions from pyrolysis neglected Resulting outlet temperature can be computed by hand

8 Model verification Reactions run to completion well before the outlet from the plug flow reactor CFD simulation predicts an outlet temperature of 163 K. Hand calculation yields 171 K. Overall error less than 1% likely to come from numerical errors

9 Uncertainty from input data Pyrolysis rate parameters (λ (O ) = 0.4 in test case) : Case I: Standard values used for coal (Ubhayakar et. al.) Case II: 90 % of the activation energies in Case I (Case II results in a faster pyrolysis compared to Case I, see graph) Pyrolysis rate Rate (1/s) T (K) Case I Case II

10 Case I Uncertainty from input data Case II U (m/s) Axial velocity and streamlines Note: Recirculating flow outside of flame Peak axial velocity slightly lower for case II Overall solution very similar U (m/s)

11 Case I Uncertainty from input data Case II T (K) Temperatures Note: High flame temperatures predicted Shorter flame and higher flame temperature for case II Temperature at outlet close to adiabatic flame temperature T (K)

12 Predicted flame temperature (Case I) CO O T (K)

13 Predicted flame temperature Recirculation brings hot reactive gases (e.g. CO) in contact with oxygen in the flame region The preheating of the fuel gas explains the high temperatures Conclusion: Flame temperature reasonable after all Addition of the water/gas shift reaction will lower the flame temperature Additional reactions (e.g. CO dissociation) may have to be included in the model if local temperatures becomes too high

14 Summary Partial verification of gasifier model for a plug flow reactor with an error less than 1% Relatively low sensitivity in the model from different devolatilization rates Higher flame temperature and shorter flame length was detected for the faster devolatilization rate Locally high flame temperatures can be explained with recirculation of hot, partially burnt fuel gas that gets in contact with oxygen Water/gas shift reaction creates numerical instabilities but needs to be included for accurate predictions of the gas composition and temperature

15 Future work Continue work on verification and sensitivity to input parameters Perform optimisation studies on flame shapes in the DP1 reactor Construct a pressurised test rig for qualitative (high speed photography and flow visualisation) and quantitative (PDA) measurements of atomisation Candidate nozzles for the Chemrec reactor will be tested in the rig Refine conjugate heat transfer model for prediction of temperature in refractory lining and pressure vessel Add sodium sulphate reduction chemistry to CFD model

16 PDA equipment at ETC Simultaneous measurement of velocity (up to 3 components) and size of spherical particles as well as mass flux, concentration High accuracy and high spatial resolution (small measurement volume) Particle sizes between ~1 µm and several millimetres