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Oxy-fuel Combustion: Near, Intermediate & Long term
Adel F. Sarofim
University of Utah
Reaction Engineering International
Seventh Annual MIT Carbon Sequestration Forum
Cambridge, MA
October 31 & November 1
Outline
• Oxy-fuel combustion principles• External Recycle for Retrofit Applications• Internal Recycle for New Plants• New Concept• Concluding comments
Oxy-fuel Combustion Options (adapted from Wall, 06)
Hot RFG
Cold RFG
Furnace Heat Extraction
Gas Cleanup
Coal + O2
Stack
Compression / Sequestration
CO2
Nitrogen is eliminated from air to produce CO2 stream for sequestration. Oxygen is diluted with recirculated flue gas. Several options exist for location of recirculation stream. Cold RFG or external RFG is option best suited to retrofit. Hot RFG or internal RFG is option available for new plants.
Cold RFG: Retrofit of Existing Boilers (adapted from Kobayashi, ’05)
The basis for oxy-fuel combustion is to have oxygen mimic air by mixing 1 mole of oxygen with R moles of recirculated flue gases
R is the volume (molar) ratio of recirculated flue gas to oxygen1
R1 + R
Issues: What are the impacts on gas composition, heat transfer, NOx & SOx emission?
Retrofit of boiler (Stromberg, 2004)
Compositions and volume for bituminous coal (CH1.1O0.2N0.017S0.015) fired with air and oxygen (0% excess air)
Air Firing Oxy-Firing
CO2 17 % by volume 64%
H2O 8.9% 34%
NOx 2770xCR* ppm 10,700xCR* ppm
SOx 2470 ppm 9400 ppm
Moles 1 0.26
CR* = fractional conversion of coal nitrogen to NOx
The ratio R of recirculated CO2 to Oxygen is between 2 and 3 if the adiabatic temperature or maximum heat flux
for air combustion is to be matchedCase O2, eff TAF
K pc pw ε, T=1500 K
L=15 mqmax, kW/m2
Air 21% 2302 0.16 0.089 0.51
0.68
0.68
0.68
812
O2R=1
51% 3176 0.64 0.34 3,946
O2
R =235% 2330 0.64 0.34 1,140
O2
R=327% 1891 0.64 0.34 496
Detailed analyis for a 50 MWe plant shows a match for the furnace at a value of ~ 3.3 (Payne et al., ’89)
= R
NO concentration relative to unstaged combustion in air. Results based on Air Liquide/B &W 1.5MWt study for oxy-firing staged and unstaged (US
regulations are 0.14 lbs/million Btu ~ 84 ppm for air firing) (Wall et al., ’06)
0
20
40
60
80
100
120
Air-case Oxy-case 1 Oxy-case 2
Unstaged Staged US Regulation100
63
47
29 3124
100
x N
Ox/(
NO
xfo
r uns
tage
dai
r com
bust
ion)
Primary reason for the reduction is the partial destruction of the NO recycled through the flame. Some enhanced capture of SO2 by ash was also observed as a result of the higher SO2 driving force.
Options for Control of trace gas contaminants
Oxy-fuel flow chart by Okawa et al., 1999
Combustion
Modification for NOx
Disposal with CO2?
Separation during CO2condensation?
SOx, Hg Control?
Constraints on composition for piping CO2
•CO2 > 95%•H2O < 100 ppm•H2S (or SO2) < 1450 ppm•N2 < 4%•HC < 5%Concentration limits for on-site sequestration are uncertain, both technical and regulatory.The nitrogen limit is determined by the difficulties of separating non-condensables from CO2 during compression and imposes severe constraints on furnace inleakage for retrofits.
Status•Existing Pilot Scale (< 5 MWt)
•EER (CA), 3.2 MW; IFRF (Neth.), 2.5 MW; IHI (Japan); Air Liquide, B&W (OH), 1.5 MW; CANMET (Canada), 0.3 MW; Alstom (CT), 3.0 MW CFB
•Planned Pilot Demonstration (>20 MWt)•Vattenfall 30 MWt Schwarze Pumpe Germany. Groundbreaking 5/06Japan (IHI) –Australia (Queensland) Oxy-Fired Retrofit with oxygen plant, CO2 compression and sequestration (including combustion and heat transfer evaluation); PF boiler (Callide A 30 MWe Unit owned by CS Energy).•Hamilton (OH) B&W 24 MWe retrofit
•Economic Assessmets•Many
Outline
• Oxy-fuel combustion principles• External Recycle for Retrofit Applications• Internal Recycle for New Plants• New Concepts • Concluding Comments
New Plant Design with Internal Flue Gas Recycle (Kobayashi, ’05)
Aspirating (A) burner for having oxygen designed to mimic air at fuel jet (Kobayashi, 2005)
BOC burner
The A burner is one of a number of burners using internal gas recirculation widely used in industry (30% of glass making furnaces)
New concept (DOC) being introduced in steel industry (Kobayashi, 2005)
(DOC)
Possible Applications of DOC Concept to Utility Boilers(N. B. flue gas stream leaving furnace has 26% of volume of air blown
furnace)
Five partial division walls
Tangential firing, oxygen and fuel alternating with height
Six front wall and five rear
NOx ports
Four rows front and rear wall
burners
Opposed wall firing = Fuel injection
= Oxygen injection
Outline
• Background• Oxy-fuel combustion principles• External Recycle for Retrofit Applications• Internal Recycle for New Plants• New Concepts• Concluding Comments
Energy flows for Conventional Lignite-Fired Boiler (Stromberg, 2004)
Energy Flows for the case of Oxy-Fuel Firing Showing Losses with Air Separation Unit and CO2 Compression
(Stromberg, ’04)
vs. 865 MW - 42.7%
Cost of Producing Oxygen (Kobayashi, 2005)
• Current technologies to produce 95% purity oxygen require ~ 200 Kwh/ton O2
• Theoretical energy required to compress oxygen from 0.21 to 1 atmosphere is about 30 KWh/ton O2
• There is potential for enormous savings with innovative designs– Chemical looping combustion being pursued by several
groups [Chalmers (Sweden), GE-EER (USA), Zaragosa(Spain), KIER (Korea), NNTU (Norway),…) using Fe, Mn, Cu, Ni based oxygen carriers.
– CO2 in-furnace capture and recovery using solid sorbents (e.g., CO2 wheel and lithium sulfate or hydrotalcite)
– New concept by Praxair using oxygen transport membranes appears to be exciting alternative.
Oxygen Transport Membranes Integrated into Boiler Offer Potential for Major Cost Reduction
(Kobayashi, 2005)
O2 Flux = C⋅ln(P1/P2)
High fluxes can be obtained with P1/P2 > 3 achievable by compressing air to 14.3 atmospheres to produce pure O2 corresponding to an ideal compression power of 250 kwh/ton O2 at 80°F
OR by using air at 1 atmand dilute oxygen combustion at 7% O2
(P1)
(P2)
P is oxygen partial pressure
Conceptual sketch of OTM-Dilute oxygen combustion (Kobayashi, 2005)
Conceptual OTM furnace design (Kobayashi, 2005)
•OTM boiler reduces air separation power by 90%
•High purity CO2 product reduces cost of capture
One Praxair Concept for OTM FurnaceBecause of the nitrogen elimination greater fraction of energy is
transferred in radiative section (Kobayashi, ’05)
Concluding Comments
• Near Term Prospects– Commitment to oxy-fuel evident from investments of € 50 M
($63.6 M) for Vattenfall pilot and A$ 180 M ($138 M) for Callide retrofit, SaskPower $1.33M for 300MWe ?
– Enhanced oil recover potential niche market for retrofit?• Intermediate and Long Term Prospects depend on
costs vs IGCC or other emerging technologies– Reduction in cost: CFBC or Internal Gas Recycle for capital
costs, ITM for operating costs– In-furnace OTM shows potential for major costs reductions
but faces major technical hurdles
Acknowledgements
Milind Deo, Sho Kobayashi, Lars Stromberg, Terry Wall, Jost Wendt