Lecture 3: Oxyfuel Combustion Science: Mass and energy balances, heat transfer, coal combustion and emissions Professor Terry Wall and Dr Jianglong Yu University of Newcastle, Australia APP OFWG capacity building course
Oxy- fuel flowsheet for first generation technology, showing additional units for a retrofit in red Air Air Separation unit (ASU) Nitrogen Oxygen Recycled Flue Gas (RFG) Vent Conc. Fuel CO 2 rich Stream Ash removal / Boiler or Flue Gas Purification / cooler / of CO 2 Gas Turbine compression condenser / FGD CO 2 (SO 2 2) Steam Steam Turbine Power
Oxy- fuel flowsheet for first generation technology, showing additional units for a retrofit in red Issues with Oxy-combustion CCS ASU CO 2 handling Purification; compression; transportation; storage Replacement of nitrogen with recycled CO 2 Combustion environment of fuel Fuel reactivity Retrofit Impurity formation and emission ImpurityI impact on CO 2 handling
Lecture context and content Oxyfuel science used here to compare air and oxy-fuel furnace performance, for retrofit of an existing air-fired boiler while maintaining heat transfer, considering Conditions for matched heat transfer Changed burner flows, with flame and heat transfer impacts Coal reactivity and burnout impacts Developments and gaps in knowledge will be suggested
Mass and energy balances and heat transfer
AFT of air and oxy cases 2600 flame Tempe erature (K) Adiabatic 2200 1800 1400 air oxy-wet oxy-dry 1000 0.18 0.22 0.26 0.30 0.34 0.38 O2 fraction at burner inlet
Oxy-fuel: differences of combustion in O 2 /CO 2 compared to air firing To attain a similar AFT the O 2 proportion of the gases through the burner is ~ 30% The high proportions of CO 2 and H 2 O in the furnace gases result in higher gas emissivities The volume of gases flowing through the furnace is reduced The volume of flue gas (after recycling) is reduced by about 80%. Recycle gases have higher concentrations in the furnace
Gas property differences 1: Emissivity Triatomic gas (H 2 O+CO 2 ) emissivity ~ beam length comparisons Gas Em missivity (-) 1 0.8 0.6 0.4 0.2 30 500 1050 MW e Oxy-fuel fired furnace Air fired furnace 0 0 10 20 30 40 50 60 70 80 90 Beam Length (L) (m) Gupta et al (2006)
CFD radiative transfer inputs issivity (-) Em 0.8 0.7 0.6 0.5 04 0.4 0.3 02 0.2 a i 0 Oxy-fuel fired furnace 30 MWe 500 MWe 4 grey gas model, 4-GGM Gupta et al, (2006) 3 grey gas model, 3-GGM Smith et al, (1982) 0 10 20 30 40 50 60 Beam length (L) (m), i ( T ) [1 e k i ( p CO 2 ph 2O ) L ]
Property/ratio Gas property differences 2: Heat capacity etc Impact for air to oxyfuel retrofit Higher O2 thru burner Lower burner velocity, higher coal residence time in furnace Slower flame propagation velocity Gas property ratios for CO 2 and N 2 at 1200 K Properties from Shaddix, 2006
Burner flow comparisons for a retrofit Therefore secondary RFG reduced 27 O 2 % v/v fixed for same HT ~3% v/v Fixed velocity O 2 Burner
1 MWt test conditions Parameter Full load Partial load Air case Oxy case Air case Oxy case Coal flow rate kg/hr 120 120 72 72 Primary velocity m/s 20 23 17 21 Secondary velocity m/s 35 21 18 12 Secondary swirl number - 0.2 0.2 0.2 0.2 Primary momentum flux kg/s.m 2 /s 2 35.7 54.1 20.9 36.8 Secondary momentum flux kg/s.m 2 /s 2 270.2 74.1 38.2 16.4 Momentum flux ratio (Pri/Sec) - 0.13 0.73 0.55 2.25 Preheated air/rfg: primary 350-400K and secondary 450-550 K, Wall 1200 K
IFRF Flame types from swirl burners Type-0 Lo S Type-1 Hi S (S>0.6), Lo v 2 Type-2 Hi S, Hi v 2
1 MWt Temperature contours at full load X 1.5m Air-case Type-0 flame Oxy-case
Coal combustion in Sandia s entrained flow reactor under the intermediate gas temperature conditions. (Murphy and Shaddix 2006)
Sensitivity analysis full & partial load
Effect of momentum flux Confirms the significance of momentum flux and Gas properties on flame ignition
1 MWt Temperature comparisons for matching furnace heat transfer Flame FEGT
Coal combustion in Sandia s entrained flow reactor under the intermediate gas temperature conditions. (Murphy and Shaddix 2006)
Particle imaging of ignition and devolatilization of pulverized coal during oxy-fuel combustion. Shaddix, C. R. and A. Molina (2009)
Devolatilization of coals under fluidized bed conditions in oxygen-enriched air (Borah, Ghosh et al. 2008)
30 MWe Burner plane Temperature contours Air case Oxy case
30 MWe heat transfer results Total surf face heat flux (kw/m m2) 250 230 210 190 170 150 Front wall Rear wall Side wall1 Side wall2 air 45 51 56 56 Furnace wall area (m2) oxy-fuel
425 MWe burner zone temperatures 2200 2000 s temperatur re (K) Peak ga 1800 1600 1400 Air Oxy-0.28-3GGM Oxy-0.28-4GGM 1200 0 2 4 6 8 10 12 Burner horizontal level
Summary plot relating gas emissivity changes to burner oxygen 1 atio of gas emissivity (oxy to air) 2.0 1.8 1.6 1.4 wet recycle 1.2 MWt 30 MWe 425 MWe dry recycle 1.2 MWt 30 MWe 425 MWe Gas Emissivity (-) 0.8 0.6 0.4 02 0.2 0 R 1.2 0.20 0.25 0.30 0.35 0.40 Oxygen fraction at tburner inlet t() (-) 0 10 20 30 40 50 60 70 80 90 Beam Length (L) (m)
Heat transfer: Other relevant study (CFB (Zhejiang 2003)) Through early modelling although CO 2 has a different thermodynamic properties, under the same oxygen content, t oxy-combustion heat transfer does not differ much from air combustion because oxy-combustion has a lower flame temperature; CFD based modelling indicated that when oxygen content in the oxy-combustion increases from 21% to 29%, combustion efficiency and boiler efficiency increase. But further increase in oxygen content does not significantly improve the efficiency and 30% above oxygen causes other problems and safety concerns. The oxygen content in oxy-combustion therefore vary from 21% to 29%.
.. And O 2 fraction at the burner can be determined by flue gas recycle ratio (here, for wet recycle)
Coal combustibility
1 MWt Combustibility comparison 6 Coal A Coal B 5 Coal C Parity, equal C-in-ash air/oxy ode?% Carbon-in n- ash, Oxy m 4 3 2 1 0 0 2 4 6 8 10 Carbon- in- ash,air m ode (%)
Unburnt carbon in ash: oxy vs air combustion Electric Power Development & IHI, Kimura 1995
Illustrative differences in air and oxyfuel which influence burnout For matched furnace heat transfer: Oxyfuel has longer furnace residence time, ~20% Good Oxyfuel has lower temperatures, ~ 50 C In oxyfuel, coal experiences an environment with higher O 2 Bad Good
Pyrolysis and oxidation reactivities of Coal A & Coal B in TGA Reac ctivity (m min -1 ) 0.12 0.1 0.08 0.06 004 0.04 0.02 Coal A N2 Coal A CO2 Coal A Air Coal A Oxy Coal B N2 Coal B CO2 Coal B Air Coal B Oxy Coal B Oxidation Coal A Oxidation Coal B Pyrolysis Coal A Pyrolysis 0 0 200 400 600 800 1000 1200 Temperature ( o C)
Heat flow during pyrolysis & oxidation of Coal A & Coal B in TGA Hea at flow (m mw) 200 150 100 50 0-50 -100 Oxidation Exothermic Pyrolysis & gasification Endothermic Coal B Oxidation Coal A Oxidation Coal B Pyrolysis Coal A Pyrolysis Coal A N2 Coal A CO2 Coal A Air Coal A Oxy Coal B N2 Coal B CO2 Coal B Air Coal B Oxy 0 200 400 600 800 1000 1200 Temperature ( o C)
Volatile yields in DTF at 1400 C Coal B Coal C Coal D V* (N 2 ) 36.7 30.9 53.5 Q factor 1.52 1.43 1.76 (N 2 ) V* (CO 2 ) 43.3 32.2 66.2 Q factor 1.79 1.49 2.18 (CO 2 ) V* - Volatile yield at 1400 o C Q factor Ratio of V* and volatile yield obtained by proximate analysis
Char burnout in DTF taking V*(N 2 ) to estimate char yield Char burn nout daf basis (%) 100 90 80 70 60 50 40 Coal B Air Oxy 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 Oxygen concentration (atm)
Summary of reactivity data for 63-90 micron size cuts of four coals from DTF experiments at 1400 C Volatile yield Coal Burnout Volatile yiel ld at 1400 o C in f basis), % DTF (da 100 90 80 70 60 50 40 30 20 10 0 100 98 96 94 92 90 88 86 84 N2 82 CO2 80 65 70 75 80 85 Fuel Carbon (daf basis), wt.% Coal burno out at 1400 o C in DTF (da af basis), % Air, 10% O2 Oxy, 10% O2 8% O2 65 70 75 80 85 Fuel Carbon (daf basis), wt.% o Coal burnout at 1400 C in DT TF (daf basis), % 100 99 98 97 96 95 Air, 21% O2 Oxy, 30% O2 21% %O2 65 70 75 80 85 Fuel Carbon (daf basis), wt.%
DTF-char reactivity in TGA ity (s -1 ) Reactiv 0.005 0.004 0.003003 0.002 Coal B 50% O 2 21% O 2 10% O 2 Air 2% O2 Oxy 2% O2 Air 5% O2 Oxy 5% O2 Air 10% O2 Oxy 10% O2 Air 21% O2 Oxy 21% O2 Air 50% O2 Oxy 50%O2 Char 0.001 5% O 2 2% O 2 0 300 500 700 900 1100 1300 Temperature ( o C)
Char reactivity comparison for air and oxyfuel conditions at the same O 2 level Air (O 2 /N 2 ) combustion Oxy-fuel (O 2 /CO 2 ) combustion Rea action rate Low temperaturest Reaction kinetics controlled (Regime I) Very high temperatures Bulk diffusion controlled (Regime III) Moderately high temperatures Reaction kinetics & internal diffusion limited (Regime II) 1/T p (K -1 )
Combustion efficiency (cfb) Combustio on efficien cy Boi ler efficiency O 2 O 2
Emissions and impurity impacts PM, SOx, NOx, Hg PM, SOx, NOx, Hg Concentration in the flue gas and in furnace and Impacts on CO 2 handling
Impurity impacts on the purification process in oxy-fuel combustion based CO 2 capture and storage system Li, H., J. Yan, et al. (2009) CRR=92.15%
Size distribution (mass fraction) of the particles collected using DLPI under oxy and air combustion conditions Sheng, 2008
Size distribution (mass fraction) of the particles collected using DLPI under oxy and air combustion conditions Sheng, 2008
Size distribution (mass fraction) of the particles collected using DLPI under oxy and air combustion conditions Sheng, 2008
Formation of Submicron Particulates (PM1) from the Oxygen-Enriched Combustion of biomass Zhang, 2007
Impurity impacts on the purification process in oxy-fuel combustion Li, H., J. Yan, et al. (2009).
Impurity impacts on the purification process in oxy-fuel combustion Li, H., J. Yan, et al. (2009).
Impurity impacts on the purification process in oxy-fuel combustion Li, H., J. Yan, et al. (2009).
Impurity impacts on the purification process in oxy-fuel combustion Li, H., J. Yan, et al. (2009).
Impurity impacts on the purification process in oxy-fuel combustion 2-stage-flash Li, H., J. Yan, et al. (2009).
Impurity impacts on the purification process in oxy-fuel combustion 2-stage-flash Li, H., J. Yan, et al. (2009).
Impurity impacts on the purification process in oxy-fuel combustion distillation Li, H., J. Yan, et al. (2009).
SO 2 formation (CFB ) O 2 O 2 SO 2 emission (CFD, Zhejiang 2003) Normalized SO 2 emission 2 2
NOx emission (PF ) Electric Power Development & IHI, Kimura 1995
Fuel-N conversion into NOx (PF ) IHI, T.Kiga, 1997
NOx emission (CFB ) O 2 SO 2 emission O 2 Normalized SO 2 emission (CFD, Zhejiang 2003)
Additional research required Heat transfer and combustion Hot spot evaluation, and burner impacts (cfd based) Quantify potential O 2 reduction Gas quality and furnace Sulfur gases and corrosion (using pilot-scale data and calculation) Mercury levels and form, and impact on CO 2 handling Gas quality for transport, compression and storage Plant impacts, regulation and safety issues
Thank you for your attention!
Phase Diagram of CO2
Flame propagation speed (mm/s) vs oxygen content in different atmosphere (T. Kiga, 1997)