Detailed Chemical Kinetic Modelling of Pollutant Conversion in Flue Gases from Oxycoal Plant

Transcription

1 Detailed Chemical Kinetic Modelling of Pollutant Conversion in Flue Gases from Oxycoal Plant R.K. Robinson and R.P. Lindstedt Thermofluids Section, Department of Mechanical Engineering, Imperial College London, Exhibition Road, London SW7 2AZ

2 Outline Motivation Model Development Calculation of Thermodynamic Data Results, Sensitivities and Product Distributions Conclusions Future Work and Acknowledgements

3 Motivation Carbon Capture and Storage (CCS) aims to capture CO 2 emissions from large scale energy generators. Strongly corrosive impurities such as oxides of nitrogen and sulphur need to be separated from other exhaust gases. Experimental methodologies to remove these impurities have been developed [1,2]. However the chemistry behind these processes is poorly understood. The current work outlines computational methods that attempt to model the relevant conversion processes and the distribution of subsequent products in flue gases. 1. White V. and Allam R.J., Purification of Oxyfuel-Derived CO 2 for Sequestration or EOR, Proceeding of the 8th International Conference on Greenhouse Gas Control Technologies, Trondheim, Norway, (26). 2. Allam R.J., White V. and Miller J., Purification of Carbon Dioxide, US Patent 7,416,716.

4 Background The sulphur chemistry is based on detailed high temperature chemical kinetics obtained from the following studies: F.G. Cerru, A. Kronenburg and R.P. Lindstedt A systematically reduced mechanism for sulphur oxidation Proc. Combust. Inst. 3 (25) F.G. Cerru, A. Kronenburg and R.P. Lindstedt Systematically reduced chemical mechanisms for sulphur oxidation and pyrolysis Combust. Flame 146 (26) The nitrogen chemistry is based on the following studies: Lindstedt, R.P., Lockwood, F.C. and Selim, M. A., Detailed Kinetic Modelling of Chemistry and Temperature Effects on Ammonia Oxidation Combust. Sci. and Technol., 99 (1994), Lindstedt, R.P., Lockwood, F.C. and Selim, M.A., 'Detailed Kinetic Study of Ammonia Oxidation', Combust. Sci. Technol., 18, (1995) In both cases subsequent updates have been performed and validated in combustion applications.

5 Background Work has taken place in 3 key areas: Current kinetic models of combustion involving both sulphur and nitrogen species have been extended to low temperature ranges via the addition of key species and reactions. An aqueous phase mechanism has been developed to model reactions occurring in solution. A mass transfer coefficient has been estimated to allow movement of species between the gaseous and aqueous phases. Accurate quantum mechanical methods have been used to update thermodynamic data for species involved in the model and to calculate new data for aqueous species by taking into account the enthalpy of dissolution.

6 Model Development The original Sulphur mechanism featured 12 sulphur containing species and 7 reversible reactions. The nitrogen mechanism featured 21 species and 95 reversible chemical reactions. The above mechanisms are here combined with hydrocarbon chemistry for C 1 -C 2 species that permit the additional interactions with burnt gas products such as CO, CO 2 and H 2O as well as any remaining hydrocarbon fragments. 16 additional reactions for nitrogen and sulphur gas phase chemistry. 1 mass transfer rates added to allow movement of species between gaseous and aqueous phases. 8 aqueous phase reaction used to create a basic aqueous phase mechanism.

7 Model Development Additions to reaction Mechanism rates taken from NIST Chemical Kinetics Database or CAPRAM Aqueous Mechanism for Troposheric chemistry or extrapolated there from. Gaseous Phase Additions with rates shown in modified Arrhenius form PRODUCTS REACTANTS A n Ea NH3 + NO2 = HNO2 + NH2 ; 2.451E E+ 1.25E+5; NO2 + N3 = N2O + N2O ; 1.24E+8.E+.E+; NO2 + N3 = N2 + NO + NO ; 3.613E+8.E+E+.E+; E+; O2 + N3 = N2O + NO ; 3.613E+1.E+.E+; N3 + O = N2 + NO ; 6.745E+9.E+.E+; N3 + N3 = N2 + N2 + N2 ; 9.33E+8.E+.E+; O2 + NO = NO3 ; 3.43E E+.E+; NO2 + O + M = NO3 + M ; 4.99E E+.E+; E+ NO2 + O = NO3 ; 3.524E+9.24E+.E+; NO2 + NO3 = N2O5 ; 3.73E+7.6E+.E+; NO + NO2 = N2O3 ; 1.65E+6.E+.E+; N2O3 + H2O = HNO2 + HNO2 ; 1.29E+7.E E+4; N2O5 + H2O = HNO3 + HNO3 ; 5.1E-5 5.E+ E.E+; E NO2 + NO2 = N2O4 ; 6.22E+5.E+.E+; H2O + NO + NO2 = HNO2 + HNO2 ; 5.153E-1.E+.E+; SO3 + H2O = H2SO4 ; 7.227E+5.E+.E+;

8 Model Development Mass Transfer Rates between Gaseous and Aqueous Phase taken to be.1 kmol m -3 s -1 after sensitivity analysis performed. Basics Aqueous Phase Mechanism with rates shown in modified Arrhenius form PRODUCTS REACTANTS A n Ea SO2 = SO2(A) ;.1E+.E+.E+; NO2 = NO2(A ;.1E+.E+.E+; NO = NO(A) ;.1E+.E+.E+; HNO3 = HNO3(A) ;.1E+.E+.E+; HNO2 = HNO2(A) ;.1E+.E+.E+; NO3 = NO3(A) ;.1E+.E+.E+; N2O5 = N2O5(A) ;.1E+.E+.E+; N2O3 = N2O3(A) ;.1E+.E+.E+; N2O4 = N2O4(A) ;.1E+.E+.E+; H2SO4 = H2SO4(A) ;.1E+ 1E+.E+ E+.E+; E+ SO2(A) + H2O(L) = H2SO3(A) ; 6.27E+4.E+.E+; NO2(A + NO2(A + H2O(L) = HNO2(A) + HNO3(A) ; 1.E+8.E+.E+; HNO2(A) + HNO2(A) + HNO2(A) = HNO3(A) + NO(A) + NO(A) + H2O(L); 6.E+.E+.E+; N2O5(A) + H2O(L) = HNO3(A) + HNO3(A) ; 5.1E-5.E+.E+; N2O4(A) + H2O(L) = HNO2(A) + HNO3(A) ; 5.1E-5 5.E+.E+; HNO2(A) + OH(A) = NO2(A + H2O(L) ; 1.E+9.E+.E+; NO(A) + NO2(A + H2O(L) = HNO2(A) + HNO2(A) ; 1.E+8.E+.E+; NO2(A + NO2(A + NO2(A +H2O(L)= HNO3(A) + HNO3(A) + NO(A) ; 1.E+8.E+.E+;

9 Calculation Method for Thermodynamic data Molecular Mechanics Minimisation i i and Conformational Analysis used to locate starting structure High Accuracy Quantum Mechanics G3B3/G3MPB3 Energy Calculation Atomization Energies, Enthalpies and Vibration Frequencies produced in G3B3 log file Program loctorsion ran to locate all internal rotations and create input files DFT Quantum Mechanics used to scan and analyse Internal Rotations Program scancalc ran to harvest internal rotation data, fit V = ½ V n (1 cos(nθ)) and calculate IR symmetry numbers and Moments of Interia Program polyscript ran to harvest data from G3B3 and scancalc log files, calculate Enthalpies of Formation and Moments of Inertia, and produce input for next stage Statistical Mechanics Package PAC 99 used to calculate thermodynamic values from 2K to 6K 7 Term JANAF Polynomials produced by regression of calculated data

10 Examples of NO x Thermodynamic Data Thermodynamic data calculated for species where data were not available or required updating and fitted to 7 term JANAF polynomials. For aqueous species enthalpy of dissolution were taken into account by modifying the enthalpy of formation at 298K. NO 2 (Aq)Calculated Data N 2 O 3 Calculated Data f H kj/mol f H kj/mol S J/mol/K S J/mol/K C p kj/mol C p kj/mol HNO2(A) 2K-6K REF : R.ROBINSON 3-Dec E E E E E E E E E E E E E E+ N2O3 2K-6K REF : G3B3 R.ROBINSON 16-Dec E E E E E E E E E E E E E E+

11 Examples of NO x + SO x Thermodynamic Data HNO 2 Calculated Data HNO 2 (Aq) Calculated Data f H kj/mol f H kj/mol S J/mol/K S 298 S J/mol/K S 298 C p kj/mol C p kj/mol SO 2 Calculated Data f H kj/mol SO 2 (Aq)Calculated Data f H kj/mol S J/mol/K S J/mol/K C p kj/mol C p kj/mol

12 Results : Experiment 1 - Conversion Evolution Conditions : Pressure Atmospheres Temperature - 3 K Species ppm % 1 SO2 9.81E+2.1%.9 NO 3.22E+2.3%.8 NO2 3.58E+1.%.7 O2 5.5E+4 5.5% CO 2.E+2.2% Conversion.6.5 CO2 8.34E %.4 NH3 1.E+4 1.% %.3 N2 9.E+4 9.%.2 H2O 1.E+4 1.% H2O Liquid.1 1.E+4 1.% SO2 NOx Experimental SO2 Experimental NOx

13 Results : Experiment 1 Mass Transfer Sensitivity % Conve ersion % Conversion SO2 Mass Transfer x5 SO2 Mass Transfer.1 kmol3 m3 s-1 SO2 Mass Transfer x NOx Mass Transfer x5 NOx Mass Transfer.1 kmol3 m3 s-1 NOx Mass Transfer x

14 Results : Experiment 1 NO x Ratio Evolution The Ratio of NO to NO 2 is known to change from approximately 9:1 to 3:1 after the compressor/receiver the current model reproduces this affect Conversion % NO NO2 Experimental NO Experimental NO

15 Results : Experiment 1 NO x Ratio Sensitivity NO 2 is more readily absorbed into the aqueous phase due its larger negative enthalpy of dissolution, therefore the ratio of NO to NO 2 influences conversion time for NO x % Conversion %-NO 1%-NO2 75%-NO 25%-NO2 5%-NO 5%-NO

16 Results : Experiment 1 Pressure Sensitivity NO x conversion also pressure dependent as higher pressures lead to greater conversion from NO to NO 2 in the gaseous phase % Conversion n SO2 All Pressures NOx 1 Bar NOx 3 Bar NOx 7 bar

17 Results : Experiment 2 - Conversion Evolution Conditions : Pressure - 5 Atmospheres Temperature - 3 K Species ppm % SO2 7.61E+2.8% NO 2.99E+2.3% NO2 3.32E+1.% O2 5.5E+45E+4 5.5% 5% CO 2.E+2.2% CO2 8.34E % NH3.E+.% N2 9.E+4 9.% % Conversion H2O 9.98E+3 1.% 1.1 H2O Liquid 9.98E+3 1.% SO2 Nox Experimental SO2 Experimental NOx

18 Results : Experiment 2 NO x Ratio Evolution The Ratio of NO to NO 2 is known to change from approximately 9:1 to 3:1 after the compressor/receiver,the current model reproduces this affect Conversion % NO NO2 Experimental NO Experimental NO

19 Results : Experiment 2 Product Distribution NO NO2 SO NO(A) NO2(A) SO2(A) ppm ppm HNO2(A) HNO3(A) H2SO3(A) H2SO4(A) ppm ppm N2O4 N2O4(A) N2O3 N2O3(A)

20 Conclusions The current work shows that detailed models based on chemical kinetics can be of significant help in interpreting experimental data. The approach of not heuristically fitting individual rate constants allows the separation of validation and simulation. Key sensitivities are also identified as part of the modelling process. SO 2 and NO x conversion predominately occurs in the aqueous phase. NO x conversion residence times are highly dependent on the initial ratio of NO/NO 2 and the pressure of system while the SO 2 conversion is less pressure dependent. Any model needs to simulate both gaseous and aqueous phases and the differing conditions of both the compressor and the receiver.

21 Future Work and Acknowledgements Mass transfer rates need to be considered in more detail, and are likely to vary throughout the apparatus. Rates may be estimated from interfacial areas. Currently ionic species are modelled as molecules. A detailed aqueous phase mechanism would include ionic species and the ph of the system. Expansion of the aqueous phase mechanism in line with current findings. We would like to thank Air Products and Doosan Babcock for their support. Also Dr. L.Torrente-Murciano and Prof. D.Chadwick, Imperial College, for the experimental data.

22 Results : Experiment 3 - Conversion Evolution Conditions : Pressure - 5 Atmosphere Temperature - 3 K Species ppm % SO2 6.22E+2.6% NO 1.7E+1.% NO2 1.89E+.% O2 5.5E+4 5.5% CO 2.E+2.2% CO2 8.34E % NH3.E+.% N2 9.E+4 9.% H2O 9.98E+3 1.% H2O Liquid 9.98E+3 1.% % Conversion SO2 NOx

23 Results : Experiment 3 Product Distribution ppm ppm 7 NO 5 6 NO2 45 SO ppm.. HNO2(A) HNO3(A). H2SO3(A) H2SO4(A) ppm NO(A) NO2(A) SO2(A) N2O4.. N2O4(A) N2O3 N2O3(A)