Three-dimensional modelling of a 300 MWe Flexi-Burn CFB for multifuel combustion in oxygen-fired and air-fired modes

Transcription

1 Lappeenranta University of Technology From the SelectedWorks of Kari Myöhänen September, 2011 Three-dimensional modelling of a 300 MWe Flexi-Burn CFB for multifuel combustion in oxygen-fired and air-fired modes Jouni Ritvanen, Lappeenranta University of Technology Kari Myöhänen, Lappeenranta University of Technology Timo Eriksson Reijo Kuivalainen Timo Hyppänen, Lappeenranta University of Technology Available at:

2 Three-dimensional modelling of a 300 MWe Flexi-Burn CFB for multifuel combustion in oxygen-fired and air-fired modes 2nd Oxyfuel Combustion Conference September 12 16, 2011 Presented by: Jouni Ritvanen a Co-authors: Kari Myöhänen a, Timo Eriksson b, Reijo Kuivalainen b, Timo Hyppänen a a Lappeenranta University of Technology b Foster Wheeler Energia Oy

3 Presentation outline Flexi-Burn CFB process Different scales of experiments and modelling Classification of multiphase model approaches Semi-empirical 3D-model Model results Conclusions References Acknowledgements: The research leading to these results has received funding from the European Community s Seventh Framework Programme (FP7/ ) under grant agreement n

4 Flexi-Burn CFB process Turbine island To cooling tower CPU Fuel Limestone CFB Boiler Secondary gas SH1, RH1, Eco Intrex Fluidizing gas LP EcoA LP EcoB Mixer Filter Wet flue gas recirculation Oxygen Air HRS Condenser H 2 O etc. Vent gases Drying Compression Purification H 2 O etc. CO 2 Transport Storage ASU Preheater Flexi-Burn is a trademark of Foster Wheeler Energia Oy, registered in the US, EU, Finland.

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 BENCH SCALE REACTOR (BFB/CFB) Cooler Cyclone EXPERIMENTAL SCALES To Stack Filter Cooler/heater Secondary air Fuel batch feed Continuous fuel feed O 2, CO2, CO, N2, SO2, NO PC control and data logging system Primary gas heating Air 1D-MODEL n τv = b exp( A/ T)( d / d ref ) MODELS AND DESIGN TOOLS Model analyses Volatile, moisture release CO combustion Mixing Char combustion d d t k = /(1/ Y CO = k Y ef CO + τ ) ef 1 k CO m dm c rc = = kmc X dt n O2 Models for phenomena 1-D process models 3-D process models

6 Classification of multiphase model approaches 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 Semi-empirical 3D-model 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

8 Combustion model Inert Evaporation H 2 O Devolatilization Ash Volatiles Moisture Char Char combustion +O 2 NO, N 2 O HCN, NH 3 H 2 S CO, CO 2 CH 4, C 2 H 4 Char gasification +H 2 O, +CO 2 CO H 2 H 2 S H 2 O CO, CO 2 SO 2 H 2 N 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

9 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

10 Calculation mesh, furnace layout, process data 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 % Lower SA + limestone (8pcs) Lower SA + recirc fly ash (4pcs) Upper SA (9pcs) Fuel chute SH Intrex SUB RH Intrex Image slices

11 Devolatilization and char combustion Air-fired Oxygen-fired Air-fired Oxygen-fired Based on this study, the combustion reactions are fairly similar in oxygen- vs. air-fired mode, if the oxygen content of the inlet gas is close to air-fired mode. The devolatilization and char combustion profiles are similar in shape, but in oxygen-fired mode, the values are higher due to higher boiler load and higher fuel flow rate. The devolatilization rate is high near the fuel inlets. This produces high local concentrations of combustible gases above the fuel inlets. The combustion rate of char is slower, thus, char has time to penetrate to the furnace and flow to the bottom of the furnace, where the maximum char concentrations are found.

12 Heat from reactions and temperature Air-fired Oxygen-fired Air-fired Oxygen-fired Most of the heat originates from combustion of char at the bottom of the furnace. Local higher maximums can be noticed near the secondary air inlets, where the incoming oxygen mixes with combustible material. The temperature is smaller towards the sides due to cooling effect of side walls. The temperature profile of the oxygen-fired case is more uniform, partly due to re-carbonation reactions occurring in colder areas. The heat release and temperature profiles could be adjusted by changing the fuel flow distribution. In oxygen-fired case, modifying the oxygen content in lateral and vertical direction provides more methods to control the combustion process.

13 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.

14 Hydrogen and carbon monoxide Air-fired Oxygen-fired Air-fired Oxygen-fired Hydrogen concentration is high near the fuel inlets because most of the hydrogen originates from devolatilization. CO is found more uniformly across the bottom of the furnace, because the CO originates from burning of char as well.

15 Calcination and carbonation Air-fired Oxygen-fired Air-fired Oxygen-fired In air-fired mode, calcination of fresh limestone occurs quickly near the feed points of limestone. In oxygen-fired mode, re-carbonation can happen at locations, where the local temperature is below the calcination temperature. The re-carbonated limestone is re-calcined at areas with higher temperature. The cycling calcination-carbonation reactions affect the local gas composition, temperature, and gas velocities.

16 Sulphation and direct sulphation Air-fired Oxygen-fired Air-fired Oxygen-fired Highest sulphation rate at the bottom of the furnace, where the concentration of SO2 is high due to combustion reactions. In air-fired case, the amount of direct sulphation is practically zero. In oxygen-fired case, direct sulphation occurs near the side walls, where the temperature is lower and the concentration of CaCO3 is higher due to recarbonation.

17 Desulphation and sulphur dioxide concentration Air-fired Oxygen-fired Air-fired Oxygen-fired In this model, the CaSO 4 can decompose in reducing conditions. The desulphation rate is highest at the bottom of the furnace and near the centerline, where the local concentration of CO is high. The SO 2 profile is a result of different sources and sinks, which are mainly due to combustion reactions and sulphation reactions. The SO 2 concentration is higher at the centre of the furnace, where the temperature and CO concentration are higher, which promote the desulphation.

18 Molar balance of sulphur dioxide for furnace Air-fired (sources) Air-fired (sinks) Oxygen-fired (sources) Oxygen-fired (sinks) Gas feed Char combustion H2S combustion Desulphation Sulphation Direct sulphation Recirc. gas Flue gas Molar flow of SO 2 (mol/s) At this temperature level, sulphur capture is mostly by normal sulphation. In oxygen-fired case, the sulphur capture is higher due to higher SO 2 -content in the furnace, which is due to recirculated gas.

19 Molar balance of carbon dioxide for furnace Air-fired (sources) Air-fired (sinks) Oxygen-fired (sources) Oxygen-fired (sinks) Gas feed Devolatilization Char combustion CO combustion Shift conversion Calcination Carbonation Recirc. gas Flue gas Molar flow of CO 2 (mol/s) In air-fired case, most of the CO 2 originates from combustion reactions added by a small proportion from the calcination of fresh limestone and a very small amount due to shift conversion. In oxygen-fired case, in addition to above, the inlet gas contains a large proportion of CO 2, which results in high molar flow of CO 2 through the system. A small proportion of CO 2 is consumed by carbonation, but this is again released by re-calcination.

20 Conclusions The combustion reactions are fairly similar in air-fired and oxygen-fired combustion, if the oxygen content of the input gas is moderate. Large differences may occur due to changing limestone reaction mechanisms when operating at high partial pressure of carbon dioxide. These differences have to be considered in the furnace design and operations in order to optimize the performance and the emission control, and to avoid operational problems. The findings of this study can be used to support the further design of the OXY-CFB-300 Compostilla demonstration plant. For future improvement, the different empirical correlations describing the essential phenomena need to be further validated based on experimental studies in bench-scale and pilot-scale test equipment.

21 References Kuivalainen, R., et al. (2009). Oxyfuel-CFB boiler scale-up based on integrated experimental and modeling work. In: Proceedings of the 1st International Oxyfuel Combustion Conference. Cottbus, Germany, September 8-11, url: CFB_boiler_scale-up_based_on_integrated_experimental_and_modeling_work.pdf 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., et al. (2009). Near zero CO2 emissions in coal firing with oxy-fuel CFB boiler. Chemical Engineering & Technology, 32(3), pp url: 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: Other semi-empirical, three-dimensional models: Pallarès, D., Johnsson, F., and Palonen, M. (2008). A comprehensive model of CFB combustion. In: Werther, J., Nowak, W., Wirth, K.-E., and Hartge, E.-U., eds, Proceedings of the 9th International Conference on Circulating Fluidized Beds, pp Hamburg: TuTech Innovation. Wischnewski, R., Ratschow, L., Hartge, E.-U., and Werther, J. (2010). Reactive gas-solids flows in large volumes - 3D modeling of industrial circulating fluidized bed combustors. Particuology, 8, pp