Lecture 3: The Science of Oxy-fuel

Transcription

1 Lecture 3: The Science of Oxy-fuel System differences in flames, heat transfer, coal combustion and emissions 4 th APP OFWG Capacity Building Course, Sunday/Monday 2 nd / 3rd September, 2012 Tokyo Institute of Technology, Tokyo JAPAN Rohan Stanger University of Newcastle, Australia

2 Related Literature TEXTBOOK JOURNAL Special Edition WEBSITE Volume 5, Supplement 1, Pages S1- S238 (July 2011) Oxyfuel Combustion Technology - Working Toward Demonstration and Commercialisation Woodhead Publishing February 2011

3 Lecture Outline Flames & heat transfer Coal Behaviour Emissions Impurity Impacts during CO2 compression

4 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

5 Basic Oxy-fuel Circuit Air Separation Unit O2 Heat Transfer N2 Emissions & Impurities Impacts Coal Handling Flame & Coal Behaviour ESP CO2 compression ~30% Transport (pipeline, truck, etc) Sequestration Site Recycled Flue Gas ~70%

6 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 emissivity's The volume of gases flowing through the furnace is reduced (longer residence time) The volume of flue gas (after recycling) is reduced by about 80%. Recycle gases have higher concentrations in the furnace

7 Flames and heat transfer

8 Burner Burner flow comparisons for a retrofit Therefore secondary RFG reduced 27 O 2 % v/v fixed for same HT Fixed velocity ~3% v/v O 2

9 Property/ratio Gas property differences 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

10 AFT of air and oxy cases 2600 Adiabatic flame Temperature (K) air oxy-wet oxy-dry O2 fraction at burner inlet

11 Gas property differences : Emissivity Triatomic gas (H 2 O+CO 2 ) emissivity ~ beam length comparisons Gas Emissivity (-) MW e Oxy-fuel fired furnace Air fired furnace Beam Length (L) (m) Gupta et al (2006)

12 CFD radiative transfer inputs Emissivity (-) a i 0 Oxy-fuel fired furnace 30 MWe 500 MWe Beam length (L) (m), i ( T ) [1 e k i ( p CO 2 ph 2O ) L ] 4 grey gas model, 4-GGM Gupta et al, (2006) 3 grey gas model, 3-GGM Smith et al, (1982)

13 1 MWt test conditions Parameter Full load Partial load Air case Oxy case Air case Oxy case Coal flow rate kg/hr Primary velocity m/s Secondary velocity m/s Secondary swirl number Primary momentum flux kg/s.m 2 /s Secondary momentum flux kg/s.m 2 /s Momentum flux ratio (Pri/Sec) Preheated air/rfg: primary K and secondary K, Wall 1200 K

14 IFRF Flame types from swirl burners Type-0 Low S Type-1 Hi S (S>0.6), Low v 2 Type-2 Hi S, Hi v 2

15 1 MWt Temperature contours at full load X 1.5m Air-case Type-0 flame Oxy-case

16 30 MWe heat transfer results Total surface heat flux (kw/m2) Front wall Rear wall Side wall1 Side wall2 air Furnace wall area (m2) oxy-fuel

17 1 MWt Temperature comparisons for matching furnace heat transfer Flame FEGT

18 Sensitivity analysis full & partial load

19 Effect of momentum flux 21% O 2 /CO 2 Confirms the significance of momentum flux and Gas properties on flame ignition

20 Particle imaging of ignition and devolatilisation of pulverized coal during oxy-fuel combustion. CO2 atmosphere delays ignition and devolatilisation higher Cp of gas Reduced radical pool Lower diffusivity of O2, CxHy Shaddix, C. R. and A. Molina (2009)

21 Coal combustion in Sandia s entrained flow reactor under the intermediate gas temperature conditions. O2% affects flame length and type Controlled through recycle rate Important when matching heat transfer in retrofit applications (Murphy and Shaddix 2006)

22 Recycle Rate Tan, Corragio, Santos, IFRF 2005 Review Normal Air Fired OXY recycle ratio=0.58 (high O2%) O2 Flow is set by stoichiometry Recycle Rate changes O2%, AFT, radiative/convective HT OXY recycle ratio = 0.76 (low O2%)

23 Flame & Heat Transfer Summary Properties of oxy-fuel recycle gas Lowers adiabatic flame temperature (higher Cp) Delays flame ignition (higher Cp, lower O2 diffusion) Affects radiative heat transfer (higher emissivity) Matching heat transfer in a retrofit is a balance of adiabatic flame temperature, recycle rate of flue gas (for O2%) gas emissivity (model validation needed) Managing oxy-fuel flame has more parameters to consider (eg flame type, length) but offers more control (eg O2 Injection)

24 Coal Behaviour

25 Pyrolysis of Coal D in N2 & CO2 TGA Experiments Reactivity, R m,p (s -1 ) Coal 100% N2 Coal 100% CO2 devolatilisation reactivities are similar CO 2 reactivity with char Char gasification begins CO Temperature, T p (K) Devolatilisation reactivities are similar in both N 2 and CO 2 atmospheres. Char-CO 2 gasification reaction is clearly evident in a CO 2 atmosphere. N 2 Rathnam et al, OCC1, 2009

26 Comparison of Apparent Volatile Yields in N2 & CO2 DTF Experiments at 1400 o C 26 Apparent volatile yield (wt. % daf basis) N2 CO2 Proximate Analysis C in Coal (wt. %, daf basis) Higher apparent volatile yield at higher temperatures and heating rates in DTF at 1400 o C compared to the proximate analysis volatile matter. Higher apparent volatile yield in CO 2 atmosphere - attributed to the char-co 2 gasification reaction. Rathnam et al, OCC1, 2009

27 Char Swelling in N2 & CO2 27 Chars were formed in the DTF at 1400 o C Cumulative volume percentage Coal A Coal N2 Char CO2 Char Similar N 2 and CO 2 chars Particle diameter (μm) Coal C Rathnam et al, OCC1, 2009 Cumulative volume percentage Coal A 0 Coal N2 Char CO2 Char Cumulative volume percentage Coal N2 Char CO2 Char Particle diameter (μm) Particle diameter (μm) Coal C Coal B Coal B Swollen chars Larger CO 2 chars

28 Char Micropore Surface Areas of Chars 28 Chars were formed in the DTF at 1400 o C Micropore surface area (m 2 /g) N2 Char CO2 Char C content in coal (wt. % daf basis) Rathnam et al, OCC1, 2009

29 Coal A Reactivity in O2/N2 & O2/CO2 TGA Experiments Constant Heating Rate 25 C/min - Lower reactivities & heat flows at low temperatures in O 2 /CO 2 combustion - Endothermic gasification reduces heat flows and increases reactivities at high temperatures Reactivity (min -1 ) Coal A REACTIVITY 21% O 2 10% O 2 3% O 2 N 2 CO Coal A HEAT FLOW 21% O Temperature ( o C) 0 Heat Flow (mw) % O 2 3% O CO 2 N 2 Endothermic gasification -100 Temperature ( o C) Rathnam et al, OCC1, 2009

30 Coal A Burnout & Reactivity 30 DTF EXPERIMENTS ISOTHERMAL TGA EXPERIMENTS Coal burnout (wt. %, daf basis) Coal A O2/N2 O2/CO Reactivity (min -1 ) COAL A 1400N2 CHAR 1000 O C, 50% CONVERSION Air - O2/N2 Oxy - O2/CO Oxygen concentration (atm) Oxygen Concentration (atm) Coal burnout (wt. %, daf basis) Furnace temperature ( o C) Coal A O2/N2 O2/CO2 Reactivity (min -1 ) Air - O2/N2 Oxy - O2/CO2 COAL A 1400N2 CHAR 10% O 2, 50% CONVERSION Temperature ( o C) Rathnam et al, OCC1, 2009

31 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 Reaction rate Low temperatures 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 )

32 32 SUMMARY OF COAL RESULTS Variable Technique Difference in O 2 /CO 2 or CO 2 atmosphere Apparent Volatile Yield DTF Higher in CO2 Pyrolysis Rate TGA Similar peak devolatilisation Enhanced at higher temperature (due to gasification) Char Swelling DTF/PSD Variable Micropore Surface DTF/BET Different results in CO2 to N2 Area Coal Burnout DTF Higher in O 2 /CO 2 conditions in some coals Char Reactivity TGA Higher in O 2 /CO 2 conditions

33 Coal Behaviour Conclusions 33 There are significant differences in pulverised coal reactivity in air and oxy fuel conditions. The extent of the differences depend on the coal type (rank). The devolatilisation reactivity of coal in N 2 and CO 2 is similar as shown by TGA results. The apparent volatile yield appears to be higher in CO 2 due to mass loss during the char CO 2 gasification reaction. Depending on the coal type, there are also significant differences in the characteristics of char formed in N 2 and CO 2 atmospheres. Some coals exhibit greater swelling in a CO 2 atmosphere. Higher char reactivity in O 2 /CO 2 conditions, especially at high temperatures and low O 2 levels, is attributed to the char CO 2 gasification reaction. Reactivity parameters for char combustion need to be estimated separately for oxyfuel conditions including the char CO 2 gasification reaction and other relevant differences need to be accounted for when modelling pulverised coal combustion in O 2 /CO 2 conditions.

34 Emissions PM, SOx, NOx, Hg Concentration in the flue gas and in furnace

35 Particulates Dust concentration higher due to lower gas flow 35 Lower gas flow due out of Furnace (before recycle) due to higher O2% at burner 1000 Dust Oxy, g/nm Coal A Coal B Coal C Gas Flow Oxy, Nm3/h Coal A Coal B Coal C Dust Air, g/nm3 Gas Flow Air, Nm3/h Higher concentration of dust & longer residence time for flue gas/particulates

36 NO x Concentration & Emission 1MW t NOx concentration higher due to lower gas flow & recycled flue gas Coal A Coal B Coal C NOx emission much lower due to reburning effect in furnace and lack of N2 Coal A Coal B Coal C Wall, T., et al., An overview on oxyfuel coal combustion--state of the art research and technology development. Chemical Engineering Research and Design, (8): p

37 SO 2 Concentration & Emission 1MW t SO2 concentration higher due to lower gas flow & recycled flue gas Coal A Coal B Coal C SO2 emission lower due to enhanced sulphur retention in fly ash & possible SO3 deposition Coal A Coal B Coal C Wall, T., et al., An overview on oxyfuel coal combustion--state of the art research and technology development. Chemical Engineering Research and Design, (8): p

38 SO 3 Concentration & Emission- 1MW t SO3 concentration higher due to lower gas flow & recycled flue gas SO3 emission lower due to enhanced sulphur retention in fly ash & possible SO3 deposition Coal A Coal B Coal C Coal A Coal B Coal C Wall, T., et al., An overview on oxyfuel coal combustion--state of the art research and technology development. Chemical Engineering Research and Design, (8): p

39 SO3, ppm SO3 Pilot Comparison Uncertainty in SO3 due to difficulty of measurement Controlled condensation method with inertial separation of fly ash prevents gas-solid interactions Results are approaching consensus, but understanding? Low range SO3 SO3/SO2 Conversion- Oxyfuel Firing, % 5 4 Stuttgart ANL 3 CANMET IHI/Callide Utah 2 E.on Alstom SO3/SO2 Conversion -Air Firing, % SO3, ppm 200 High range SO IHI/Callide Coal A-Air IHI/Callide Coal A-Oxy IHI/Callide Coal B-Air IHI/Callide Coal B-Oxy IHI/Callide Coal C-Air IHI/Callide Coal C-Oxy ANL-Air [17] ANL-Oxy [17] CANMET-Air [13,30] CANMET-Oxy [13,30] IVD Stuttgart-Air [31] IVD Stuttgart-Oxy [31] Utah-Air Utah-Oxy E.on-Air E.on-Oxy Als tom-air Als tom-oxy SO 2, ppm SO 2, ppm

40 SO3 Behaviour Marier and Dibbs, Thermochimica Acta, MW PC Firing Eddings, Ahn, Okerlund, Fry, IEA SO2/SO3/Hg Workshop 2011 TEMPERATURE Cooling rate 1.0 FLY ASH CATALYSIS & CAPTURE Fly Ash species + dust load Residence time Equilibrium H 2 SO 4 SO 2 SO EQUILIBRIUM Gas Species Temperature, o C

41 Hg in oxyfuel Becoming more important due to downstream corrosion of aluminium heat exchangers in inerts (O2, N2, Ar) separation in cold box Difficult to measure due to low concentration and manual methods (similar to SO3) Removal depends on species of Hg Popular Science website Hg 0 (elemental Hg) not water soluble, but absorbed on unburned carbon in fly ash (Hg P ), potentially captured as Hg(NO 3 ) 2 (aq) in compression Amalgam Catalytic Oxidation Hg 0 (g) AuHg AgHg Hg 2+ X(g) Species Hg 2+ (ionic Hg, mainly HgCl 2 ) is water soluble Hg(NO 3 ) 2 HgO Hg tends to coat walls and piping, taking a long time to reach equilibrium due to low concentrations Measurements in large pilot systems may never reach this point Cl/HCl/Cl 2 NO/NO 2 SO 2 /SO 3 Hg 0 Vaporization Halogenation Sorption HgCl 2 (g) HgBr 2 (g) HgF 2 (g) Hg(p) Species HgCl 2 HgO HgSO 4 HgS HgSe Pavlish, J. Understanding the fate of Hg during oxyfuel combustion. IEA Workshop on SO2/SO3/Hg/Corrosion in Oxyfuel Fuel Combustion Ash Formation and Particle Growth Postcombustion Hg Species

42 Hg in oxyfuel - results Little differences observed in Hg speciation between Air and Oxy firing Mainly attributed to differences in firing systems Further trials necessary before conclusions made 45% 100% 40% 90% Carbon in Ash 35% 30% 25% 20% 15% 80% 70% 60% 50% 40% 30% HgP Hg2+ Hg0 HgTG 10% 20% 5% 10% 0% CFB Air CFB Oxy L1500 Air L1500 Oxy 0% CFB Air CFB Oxy L1500 Air L1500 Oxy Fry, A., et al. Mercury Speciation & Emission from Pilot Scale PC Furnaces under Air & Oxyfired Conditions. IEA Workshop on SO2/SO3/Hg/Corrosion in Oxyfuel. 2011

43 Impact of Emissions/Impurities on CO2 processing PM, SOx, NOx, Inerts, Hg

44 Impurity impacts on the oxyfuel process (A generic diagram) CRR=92.15% Li, H., J. Yan, et al. (2009)

45 Impurity impacts on the oxyfuel process -Particulates Fly Ash removed prior to recycle to avoid excessive fouling Particulates can damage compressors Natural SO2 capture by CaO & MgO fly ash species Fabric Filter/ESP Absorbed SO3 can improve ESP performance Natural Hg capture by unburned carbon in fly ash Li, H., J. Yan, et al. (2009)

46 Impurity impacts on the oxyfuel process -SOx Furnace corrosion enhanced by high SO2/H2S FGD placement before/after recycle depends on furnace corrosion (fuel-s<1%) Remaining SOx removed as H2SO4 in corrosive condensate SO3 forms H2SO4 which condenses <160 C in colder sections (eg recycle, mill) FGD & FGC SO3 absorbs on active Hg sites in activated carbon reducing efficacy Li, H., J. Yan, et al. (2009) If not removed SO2 would report with CO2 product, corroding transport materials and acidifying storage media

47 Impurity impacts on the oxyfuel process -NOx Direct O2 injection can lower NOx NOx recycled to furnace is re-burned and reduced to N2 in furnace NO2 forms at higher pressure and oxidises SO2 to H2SO4 SCR Remaining NO would be vented as a stack gas SCR catalyst may enhance SO3 formation Remaining NOx removed as HNO3 in corrosive condensate Li, H., J. Yan, et al. (2009) If not removed NO2 would report with CO2 product, corroding transport materials and acidifying storage media

48 Impurity impacts on the oxyfuel process -Inerts N2 & Ar IN O2 purity Air ingress N2 & Ar OUT O2 in CO2 product can corrode pipelines & support biological growth, lowering injection rates Dilutes CO2 + affects CO2 VLE & capture rate Reduces storage media capacity Li, H., J. Yan, et al. (2009)

49 Impurity impacts on the oxyfuel process -Hg Activated carbon guard beds are affected by SO3 absorption at low pressure and may be explosive under high pressure with O2 Hg can corrode brazed aluminium HEX used in the cold box during CO2 distillation Hg is expected to be removed as Hg(NO3)2 with NOx condensates Experience with Hg comes from natural gas industry where high pressure activated carbon beds are commonly used without O2 Li, H., J. Yan, et al. (2009)

50 Impacts Summary Particulates SOx Can capture extra SOx and Hg in fly ash Can increase the load on ESP x Can be abrasive to compressor if not removed x Can be corrosive in coal mill, boiler and convective pass Can be corrosive if condensed as H2SO4 (in recycle lines & compression) Higher acid dew point in oxy-fuel ( SO2, SO3, H2O) Higher SO3 on fly ash can enhance ESP H2O Involved in corrosion (H2SO4, HNO3) Can form solid ice-like hydrates with compressed CO2, causing blockages

51 Impacts Summary NOx Hg Less of a problem as lower emission than with air firing (re-burning effect) Environmental limit of NO in stack (as ppm and/or mg/mj) Could form HNO3 in compression (corrosive to plant, pipeline) Hazardous to health, emissions becoming limited by regulation Can be extremely corrosive to cryogenic aluminium heat exchangers (for separation of inerts O2/N2) Inerts Lowers CO2 purity & reduces pipeline/storage capacity (affects economics) O2 corrosive to pipeline, and some storage media O2 supports biological growth, Affects compression energy

52 Additional research required Heat transfer and combustion Hot spot evaluation, and burner impacts (cfd based) Validate emissivity modelling at large scale Gas quality and furnace Sulphur gases and corrosion Fly ash-sox interactions (coal specific?) NOx/SOx interaction at higher pressure Mercury levels and form, and impact/control in CO2 handling Gas quality for transport, compression and storage Plant impacts, regulation and safety issues Pipeline corrosion and effect in storage media

53 Thank you for your attention!