Applications of a 3-D CFB model on oxycombustion, gasification and calcinator in calcium looping

Transcription

1 Lappeenranta University of Technology From the SelectedWorks of Kari Myöhänen 2011 Applications of a 3-D CFB model on oxycombustion, gasification and calcinator in calcium looping Kari Myöhänen Timo Hyppänen Markku Nikku Matti Koski Jarno Parkkinen Available at:

2 Applications of a 3-D CFB model on oxycombustion, gasification and calcinator in calcium looping Kari Myöhänen, Timo Hyppänen, Markku Nikku, Matti Koski, Jarno Parkkinen Lappeenranta University of Technology 62nd IEA-FBC Meeting in Vienna August 29, 2011

3 Presentation outline Background 3-D model frame and features Validation studies Applications on oxycombustion, gasification and calcium looping Discussion and conclusions Acknowledgements: The presentation contains material, which has been partially produced in co-operation with Foster Wheeler Energia Oy and VTT Technical Research Centre of Finland. The research leading to these results has received funding from the European Community s Seventh Framework Programme (FP7/ ) under grant agreement No , and from the Academy of Finland under grant No

4 Background Combustion in a CFB furnace is 3-D is a three-dimensional process Combustion rate varies locally due to limited mixing rate of fuel, combustible gases and oxygen. The spatial effects affect the combustion efficiency, heat flux distribution and emissions. Comprehensive 3D simulation of the large scale furnaces with fundamental CFD codes is not yet feasible. CFB3D is a semi-empirical code for calculating the 3D CFB process in steady state conditions. Besides CFB air-combustion, the code has been applied for modelling oxycombustion, gasification, calcium looping systems, and bubbling fluidized beds.

5 Secondary air flue gas to sta ck n n+1 n-1 n Prim a ry air Different scales of experiments and modelling Bench scale Pilot scale Boiler scale EXPERIMENTAL SCALES 1D-MODEL n v b exp( A/ T)( d / d ref ) MODELS AND DESIGN TOOLS Model analyses Volatile, moisture release CO combustion Mixing Char combustion dy CO kefyco d t k /(1/ ) ef 1 k CO m dmc rc kmc X dt n O2 Models for phenomena 1-D process models 3-D process models

6 Modelling approaches for fluidized beds Micro-scale Meso-scale Macro-scale Lumped scale 1 h...1 d Time scale 1 year 1 s 1 ms 1 µs Steady state Quasi steady Transient Particle scale DNS,LBM,DEM/DPM 2D/3D Averaged CFD 2D/3D Eulerian-Eulerian continuum models CFD / TFM 2D/3D Lagrangian-Eulerian DEM/DPM-CFD,DSMC 2D/3D Empirical and semi-empirical models 1D/1.5D/3D 1 µm 1 mm 0.1 m 1 m m Space scale Correlation models 0D Global

7 Relationships between CFB modelling fields Heat transfer Temperature field Heat recovery External parameters Reactions Gas and solid sources/sinks Heat generation Comminution Solid sources/sinks Particle size External parameters Fluid dynamics Flow and mixing of solids and gas species

8 CFB3D model frame Flue gas, fly ash Heat transfer to walls and internal surfaces Gas, solids Separator(s) - separation eff. - heat transfer - reactions Combustion, gasification & other reactions Inlet sources - sec. gas - fuel - limestone - sand Fluidization gas Bottom ash Exchange of gas / solids Solids Gas Solids to furnace Solids External heat exchangers - heat transfer - reactions Fluidization gas Recirculation of flue gas / fly ash

9 Model capabilities Flow dynamics of gas and solids Semi-empirical submodels Combustion and gasification of fuel Drying, devolatilization, char combustion, water-gas and Boudouard reactions Comminution of solids Homogeneous combustion and gasification reactions Heat transfer within bed and to surfaces Sorbent reactions and sulfur capture Post-solver for NO x emissions Solid material types: fuel, sand, sorbent (unlimited number of each) combustible fuel = char+volatiles+moisture inert ash handled separately sorbent = CaCO 3 +CaO+CaSO 4 +CaS+inert Gas components: O 2, CO 2, H 2 O, SO 2, CO, H 2, CH 4, C 2 H 4, Cg, H 2 S, NO, N 2 O, HCN, NH 3, Ar, N 2 Oxygen profile of a large scale CFB combustor

10 Combustion model Inert Evaporation H 2 O Ash Moisture Char combustion Devolatilization Volatiles Char +O 2 NO, N 2 O HCN, NH 3 H 2 S Char gasification +H 2 O, +CO 2 CO H 2 H 2 O CO, CO 2 CO, CO 2 CH 4, C 2 H 4 H 2 S SO 2 N 2 H 2 Gas combustion reactions Shift conversion CO + 0.5O 2 CO 2 CO + H 2 O CO 2 + H 2 H O 2 H 2 O CH 4 + 2O 2 CO 2 + 2H 2 O C 2 H 4 + 3O 2 2CO 2 + 2H 2 O H 2 S + 1.5O 2 H 2 O + SO 2

11 Sorbent model Calcination CaCO 3 CaO + CO 2 CaSO 4 Carbonation CaO + CO 2 CaCO 3 CaCO 3 Sulphation CaO + SO 2 + ½O 2 CaSO 4 Direct sulphation CaCO 3 + SO 2 + ½O 2 CaSO 4 + CO 2 Desulphation CaSO 4 + CO CaO + SO 2 + CO 2 CaO

12 Particle size fractions and comminution of solids Solid materials (combustible fuel, ash, sand, limestone) are typically divided into six particle size fractions. The figure below illustrates comminution paths from coarser to finer fractions. Agglomeration of particles can be simulated as well, but normally the particle size is decreasing (mechanical wear, temperature shocks and effects of chemical reactions). Fraction 6 Fraction 5 Fraction 4 Fraction 3 Fraction 2 Fraction 1 q m,c61 Fragmentation Chipping Attrition Abrasion q m,c63 q m,c52 q m,c41 q m,c65 q m,c54 q m,c43 q m,c32 q m,c21 Agglomeration q m,c64 q m,c53 q m,c42 q m,c31 q m,c62 q m,c51

13 Features under development: fuel convection model The dispersion model: The fuel is spreading from the inlet zones by dispersion towards an empirical target profile. In the new model, a momentum equation is defined for fuel particles: The main affecting forces: gravity and momentum exchange terms. The new model is better suited for reactive and elutriative fuels, such as many biofuels. Initial results of a model: Model domain Target-dispersion, coarse d p Fuel convection, coarse d p Fuel convection, very fine d p Outlet Fuel feed (1 m/s) Grid air

14 Features under development: Lagrangian modelling of sorbent particles Target: combination of Eulerian 3D-CFB model and a separate particle model for limestone Eulerian flow field of solid phase Sorbent particle Mean velocity u i u i Fluctuation ' Total velocity u i u i u ' i Particle model for limestone T, CO 2, O 2, SO 2,... q ce q q de cw q W P E dw r e rw Particle tracks by random walk model (eddy dissipation model) Particle environment during tracking

15 Validation case: air-fired 15 MWe CFB burning recycled wood Gas Concentration (%-dry). Modelled vs. measured profiles Relative Width O2 (measured) CO (measured) CH4 (measured) T (measured) O2 (calculated) CO (calculated) CH4 (calculated) T (calculated) Mesh O2 (0-16%) CO (0-10%) T ( C) Temperature T ( C) Max Measurement probe Min

16 Validation case: air-fired 235 MWe CFB burning lignite Modelled vs. measured profiles Oxygen Concentration (%-dry) Furnace Width (m) Meas. 2.5 m Meas. 2.0 m Meas. 1.1 m Meas. 0.7 m Model 2.5 m Model 2.1 m Model 1.0 m Model 0.6 m Heat flux Oxygen CO Uneven fuel feed distribution

17 Oxycombustion case study: Lagisza 460 MWe Furnace layout Model geometry Air-fired Oxygen-fired (inlet O 2 30%) Hot loop share of total heat duty of boiler (%) Existing CFB units Oxy-CFB designs O2 content of oxidant normal air combustion O 2 =21% O 2 =40% 100 O 2 =100% O 2 =80% O 2 =60% O2 contente of oxidatnt (%) Adiabatic combustion temperature ( C)

18 Oxycombustion case study: high inlet O 2 AirDesign OxyDesign (inlet O 2 59%) AirDesign Model geometry coloured by heat flux OxyDesign Temperature Oxygen concentration

19 Oxycombustion case study: Compostilla 300 MWe Turbine island To cooling tower CPU Layout from top Fuel Limestone CFB Boiler Secondary gas SH1, RH1, Eco Intrex Fluidizing gas LP EcoA LP EcoB Mixer Filter Wet flue gas recirculation Air Oxygen HRS Preheater Condenser H 2 O etc. Vent gases Drying Compression Purification H 2 O etc. CO 2 Transport Storage ASU Image slices Outlets to separators (4) Parameter Units Air-fired Oxygen-fired Fuel flow Anthracite Petcoke kg/s 18.1 (70%) 7.8 (30%) Limestone flow kg/s Inlet gas flow to furnace Ambient air Oxygen Recirculation gas kg/s (70%) 9.4 (30%) Oxygen content of inlet gas % Primary gas ratio % Fly ash recirculation share % 25 25

20 Compostilla: oxygen and carbon dioxide Air-fired Oxygen-fired Air-fired Oxygen-fired The concentration profiles of O 2 and CO 2 are similar, but due to recycling of flue gas, the CO 2 level is higher in oxygen-fired mode. The location of secondary air inlets is shown by local high O 2 concentrations. The CO 2 level increases towards upper furnace due to combustion reactions. In oxygen-fired case, the CO 2 profile is affected by carbonation, e.g. in the corner of the roof, where the temperature is below the calcination temperature.

21 Gasifier study Thermal capacity: 50 MWth Air-blown, atmospheric gasification Fuel: wood based biomass 49.8% volatiles, 9.0% char 40.0% moisture, 1.2% ash Sorbent: calcitic limestone Geometry approximated by hexahedral control volumes, mesh size cells Outlet to cyclone Fuel, limestone secondary air Fuel, sand secondary air Return leg Grid air Surface mesh colored by solid velocity magnitude

22 Gasification reactions CO from char combustion C + ½O 2 CO Water-gas reaction C + H 2 O H 2 + CO Boudouard reaction C + CO 2 2 CO Shift conversion CO + H 2 O CO 2 + H 2

23 Average gas composition profile of a gasifier 35 Gas concentration (vol-%,wet) O 2 CO H 2 CH4 H 2 O CO 2 O2 CO2 H2O CO H2 CH Relative height

24 Caoling-process Carbon dioxide capture and storage process CaO+CO 2 CaCO 3 Interconnected dual fluidised beds Carbonator as absorber Calciner as regenerator CO 2 depleted flue gas CO 2 rich gas to compression Carbonator Calciner Coal CaCO 3 Flue gas from a combustor unit Oxygen from ASU and fluidizing gas Purge of ash and sintered solids

25 3D-modelling of a pilot calcinator for Caoling Gas and solids to separator Fuel and make-up sorbent feed Return leg Feed from carbonator Bottom ash Fluidization gas

26 Discussion and conclusions A validated semi-empirical 3-D CFB furnace model has its values: Location of the feeding points and heat transfer surfaces can be designed optimally. The model can be applied for various process performance and configuration studies. Helps to understand process phenomena (reactions, heat transfer, emissions). To be used for the further development and testing of new sub-models and theories. Concentrates the knowledge gathered from experimental and computational studies carried out at bench scale, pilot scale, and industrial scale apparatus. A special role in scaling up of new processes from pilot scale to industrial scale. Main challenges in model predictions: Characterization of the feed materials. Validation of empirical correlations at different scales. Flow modelling (multiphase, multimaterial, size fractions). Simulation of transient effects in a steady-state model. The development of the model is a continuous process: Different sub-models are improved as more knowledge is achieved.

27 References Myöhänen, K. and Hyppänen, T. (2011). A three-dimensional model frame for modelling combustion and gasification in circulating fluidized bed furnaces. International Journal of Chemical Reactor Engineering, 9. Article A25, 55 p. url: Kuivalainen, R., et al. (2010). Development and demonstration of oxy-fuel CFB technology. In: The 35th International Technical Conference on Clean Coal & Fuel Systems. Clearwater, Florida, June 6-11, url: Myöhänen, K., Hyppänen, T., and Loschkin, M. (2005). Converting measurement data to process knowledge by using three-dimensional CFB furnace model. In: Cen, K., ed., Proceedings of the 8th International Conference on Circulating Fluidized Beds, pp Beijing: International Academic Publishers. Myöhänen, K., et al. (2009). Near zero CO 2 emissions in coal firing with oxy-fuel CFB boiler. Chemical Engineering & Technology, 32(3), pp url: Myöhänen, K., et al. (2011). Three-dimensional modelling of a 300 MWe Flexi-Burn CFB for multifuel combustion in oxygen-fired and air-fired modes. In: Proceedings of the 2nd Oxyfuel Combustion Conference. Accepted for publication. Koski, M., et al. (2011). Three-dimensional modelling study of a circulating fluidized bed gasifier. In: International Conference on Polygeneration Strategies. Poster presentation. Saastamoinen, J. et al. (2006). Fluidized bed combustion in high concentrations of O 2 and CO 2. In: Proceedings of the 19th International Conference on Fluidized Bed Combustion. Vienna, Austria.