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New Developments in Combustion
Technology
Geo. A. Richards, Ph.D.
National Energy Technology Laboratory
U. S. Department of Energy
2014 Princeton-CEFRC Summer School On Combustion
Course Length: 6 hrs
June 23-24, 2014
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‹#›
Energy - Everyday, Everywhere
Healthcare, education, infrastructure, water, transportation, communication, agriculture, recreation………
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‹#›
Energy - Everyday, Everywhere….
….Except……
From the International Energy Agency Web-site:
“….Based on this updated analysis, we estimate that
in 2009 the number of people without access to
electricity was 1.3 billion or almost 20% of the
world’s population…..”
http://www.worldenergyoutlook.org/resources/energ
ydevelopment/accesstoelectricity/
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‹#›
Gas 23%
Nuclear 10%
Renewables 25%
Oil 2%
Coal 40%
Gas 34%
Nuclear 16% Renewables
16%
Oil
-
‹#›
Why not my way?
“It’s all regional. It’s all local. And we just have to descend to that level to
judge it.” – Vaclav Smil, discussing preferable energy resources, Wall Street
Journal, Wed April 9, 2014, pp. R-1, Business and Environment special section.
Coal
Wind
Hydro
Gas
Nuclear
Solar
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‹#›
Why not my way?
One size does not fit all
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Energy and Carbon Dioxide • Carbon dioxide capture and storage – costly, but not explicitly required.
• Carbon dioxide utilization in enhanced oil recovery (EOR) is needed, now.
• Carbon dioxide costs from natural source < anthropogenic sources.
Graphics and information NETL. Reference: DiPietro, J. P., Next generation Enhanced Oil Recovery, Presented at
the Carbon Dioxide Utilization Congress, San Diego, California, Feb. 19, 2014.
Available at http://netl.doe.gov/research/energy-analysis/publications/details?pub=68d576c3-a2ac-4e77-8963-
5ada3b04984f
Domestic EOR CO2 Use*
* A typical 550 MW coal plant emits 3.5 million tonne/ year
CO2;http://www.netl.doe.gov/energyanalyses/refshelf/PubDetails.a
spx?Action=View&PubId=348
Can we develop efficient &
affordable methods to supply CO2 ?
CO2 supply for North America EOR
million tonne/yr
Delivered CO2 prices today ~ $10-40/tonne
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‹#›
Greenhouse Gas New Source Performance Standard (Proposed standard for new sources; comments accepted to May 9, 2014. Differs from proposed emission standards for existing
sources, released June 2, 2014, http://www2.epa.gov/carbon-pollution-standards)
0
500
1,000
1,500
2,000
2,500
Lb
CO
2/M
Wh
Existing
Subcritical PC
New
SC
PC NSPS
Limit
New
Uncontrolled
NGCC New
Supercritical PC
90% CCS New
NGCC
90% CCS
“Cost and Performance Baseline for Fossil Energy Plants, Volume 1: Bituminous Coal and Natural Gas to Electricity”,
Revision 2a, September 2013,
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$70 $60
$21
$18
$32
$27
$0
$20
$40
$60
$80
$100
$120
$140
Subcritical PCRetrofit, 90% CCS
New Supercritical,1,100 Lb CO2/MWh
$/t
on
CO
2
First of a kind
Next of a kind (high range)
Next of a kind (low range)
$123/ton CO2
$105/ton CO2
Cost of CO2 Capture, $/ton CO2
“Cost and Performance Baseline for Fossil Energy Plants, Volume 1: Bituminous Coal and Natural Gas to Electricity”, Revision 2a, September 2013
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Carbon dioxide capture options
(1) Add a flue gas CO2
scrubber (e.g. photo below).
(2) Convert hydrocarbons to
hydrogen and CO2/w
separation/capture.
(3) Separate oxygen from air
and use oxy-fuel
combustion.
• Existing options: costly!
• Next generation options:
depend on thermal
science research:
– Supercritical carbon dioxide
power cycles: 300 bar
combustion?!
– Pressurized oxy-fuel.
– Chemical looping
combustion.
– Pressure-gain combustion
for efficiency.
– “Direct Power Extraction”
via MHD
240 MWe slipstream at NRG Energy’s
W.A. Parish power plant – Note the
size of CCS process area. Photo courtesy Mike Knaggs, NETL
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The role of capture AND generator efficiency
• A simple
heat/energy
balance defines
the overall
efficiency hov with
a carbon
separation unit.
• Reducing the
penalty from
carbon capture
comes from
BOTH:
– Decreasing wCO2
– Increasing hg
Generator Carbon
Separation
Unit
Q = mfDH Fuel Heat
Input
hg Generator
Efficiency
Wo Gross
Generator
Work
W1 Net
Output
Define:
a = (kg CO2 produced) / (kg fuel burned )
wCO2 = (separation work, Joules ) / (kg CO2) CO2
Approx Ranges: (30 – 60%) (6-10%)
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This presentation
Updated, expanded from 2012 CEFRC lecture:
– Inherent carbon capture: chemical looping combustion (Day 1)
– Step-change in generator efficiency: pressure gain combustion (Day 2)
– Frontier approach (?!): making oxy-fuel an efficiency advantage (Day 2)
P-gain rig @ NETL
RDC
Sampling
&
Diagnostics
Flow
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Disclaimer
* This report was prepared as an account of work sponsored by an agency of the United States
Government. Neither the United States Government nor any agency thereof, nor any of their employees,
makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy,
completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that
its use would not infringe privately owned rights. Reference herein to any specific commercial product,
process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute
or imply its endorsement, recommendation, or favoring by the United States Government or any agency
thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the
United States Government or any agency thereof.
*
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Chemical Looping Combustion
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Oxy-fuel background
Oxy- fuel achieves carbon capture very easily:
Air-Combustion:
CH + 5/4(O2 + 3.8N2) CO2 + 1/2H2O +4.7 N2
Oxy-Combustion:
CH + 5/4(O2) CO2 + 1/2H2O
“Usual” oxy-fuel approach: oxygen diluted with
CO2 or H2O added to an existing boiler cycle.
– Dilution used to keep the temperatures same as
existing cycle.
– Efficiency of the plant is penalized by the energy
needed to make oxygen.
Significant oxy-fuel demonstration projects are
occurring around the world.
– See for example: http://www.newcastle.edu.au/project/oxy-fuel-working-group/demonstrations.html
– More than 14 demos >10MWth listed
Costly to extract the CO2 from the N2 with amines
Easy to extract the CO2 from the H2O via condensation
Meridosia Illinois – Future Gen 2.0 planned site
Courtesy University of Utah – oxyfuel burner tests
http://www.newcastle.edu.au/project/oxy-fuel-working-group/demonstrations.htmlhttp://www.newcastle.edu.au/project/oxy-fuel-working-group/demonstrations.htmlhttp://www.newcastle.edu.au/project/oxy-fuel-working-group/demonstrations.htmlhttp://www.newcastle.edu.au/project/oxy-fuel-working-group/demonstrations.htmlhttp://www.newcastle.edu.au/project/oxy-fuel-working-group/demonstrations.htmlhttp://www.newcastle.edu.au/project/oxy-fuel-working-group/demonstrations.htmlhttp://www.newcastle.edu.au/project/oxy-fuel-working-group/demonstrations.html
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Making oxygen for oxy-fuel
• Oxygen can be supplied today by commercial Air Separation Units
(ASU) based on established cryogenic separation.
• The energy needed to separate oxygen from air is significant (see
below).
• In conventional oxy-combustion, we dilute the purified oxygen to
maintain the same boiler flame temperature as in air-combustion.
Air
Separation
Unit
(ASU)
1 mole of air
0.21 moles oxygen pO2
= 0.21 atm
0.79 moles nitrogen
pN2 = 0.79 atm
0.21 moles oxygen
pO2 = 1 atm
0.79 moles nitrogen pN2
= 1 atm
Reversible separation work:
~6 kJ/gmol O2 produced*
Current actual process:
~18kJ/gmol O2 produced**
*e.g, the change in gibbs energy for ideal mixing (Sandler, Chemical Engineering Thermodynamics (1989) pp. 313.
**See Trainier et al., “Air Separation Unit…..” Clearwater Coal Conference, 2010.
C + O2 CO2 DH ~ DG = 394 kJ/gmol (C or O2)
In efficient powerplants we convert
less than ½ of DH to work.
Thus~200kJ/gmol O2 work produced
Roughly ~1/10 of that is needed for ASU.
Dilute again
with CO2 or steam
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Chemical Looping • Shares advantages of oxy-fuel
– Product is just CO2 and H2O
• No separate oxygen production is needed
• Schemes for H2 production, carbon capture…
CO2 + H2O
Ash
Recycle
CO2 + H2OFuel
Air
Seal
Seal
N2 + O2
(vitiated air)
Carbon + metal oxide = CO2 + metal
Metal + air (oxygen) = metal oxide
CO2 + H2O
Avoiding confusion
with nomenclature:
Refer only to the
FUEL REACTOR
AIR REACTOR
Here’s why it can otherwise be confusing:
the air reactor “burns” or oxidizes the metal.
The fuel reactor “reduces” the metal oxide but
oxidizes the fuel.
Thus, you could call the fuel reactor an oxidizer for
the fuel OR a reducer for the metal oxide.
Today’s
discussion:
Focus on
air reactor
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Not quite new
• Chemical looping has been
around – but for different
reasons and applications.
– 1954 patent to manufacture CO2
• Similar process: iron-steam
route to hydrogen (circa 1920)*
Reduce iron with fuel and oxidize it with steam:
F3O4 + 4 CO 3Fe +4 CO2 3 Fe + 4 H2O Fe3O4+ 4H2
“Production of Pure Carbon Dioxide” US Patent 2,665,972 (1954)
Notice the heat exchangers (HX) in BOTH fuel and air reactors.
Should have made it a boiler?
Air Reactor
M + (O2 + 3.8N2)
MO2 + 3.8 N2 Fuel Reactor
MO2 + C
CO2 + M
M
MO2
CO2
HX
• And, before that….respiration.
Hemoglobin “loops”
to carry oxygen
from lungs for
hydrocarbon
oxidation in cells. *Hurst, S. (1939). “Production of Hydrogen by the Iron-Steam Method”, Journal of the American Oil Chemist’s Society, 16 (2), pp. 29-36.
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Basic Thermodynamics
In CL combustion, the overall reaction (1) of fuel with oxygen is split into two steps (2&3) , which add to the overall.
Consider an example of carbon and a metal/metal oxide (M/MO):
1) C + O2 CO2 DH1 Overall fuel oxidation - exothermic DH1 = DH2 + DH3 _______________________________________________________ 2) C+ MO2 CO2 + M DH2 Metal oxide reduction & fuel oxidation – can be endothermic OR exothermic
3) M + O2 MO2 DH3 Metal oxidation - exothermic
Nomenclature used in this talk:
Exothermic carriers Endothermic carriers Neutral carriers
DH20 DH2~0
Fuel reactor
releases heat Fuel reactor
consumes heat
Fuel reactor
does not consume or
release heat
Fuel reactor
Air Reactor
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Chemical Looping Heat Release
1) C + O2 CO2 DH1 Overall fuel oxidation - exothermic _______________________________________________________ (2) C+ MO2 CO2 + M DH2 Metal oxide reduction & fuel oxidation – can be endothermic OR exothermic (3) M + O2 MO2 DH3 Metal oxidation - exothermic
(3) (2)
CO2 + H2O
Ash
Recycle
CO2 + H2OFuel
Air
Seal
Seal
N2 + O2
(vitiated air)
CO2 + H2O
Air reactor: always exothermic; it releases heat.
Exothermic carriers: some heat will also come out in the
fuel reactor
Endothermic carriers: fuel reactor needs heat to reduce
the metal.
• If you don’t put heat into the fuel reactor, the
temperature will drop as reaction (2) proceeds, and the
reactions will stop.
• You add the heat by carrying it with the oxygen
carrier. Thus, the temperature of the carrier drops
some DT from inlet to exit of the fuel reactor.
• The heat flow rate is then (mass flow) x (CpDT ) and
must balance the heat used by reaction (2)
• Note that steam pipes won’t work to transfer heat into
the fuel reactor (>800C input, typically - exceeds
steam piping temperature limits).
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A useful concept: Chemical looping creates a “meta-fuel” for the air reactor
• A useful way to think about chemical looping combustion :
– The fuel reactor takes the hydrocarbon fuel (coal, oil, biomass,
natural gas) and uses that to make a different fuel (Cu, Fe, FeO,
Fe3O4, CaS…etc; the reduced oxygen carrier).
– For convenience, call the reduce oxygen carrier a “meta-fuel”*
*This is not a new phrase; a web-search reveals that the phrase was used to describe (inedible) tablets of fuel sold for camp stoves dating back to the 1920s:
“…..‘Meta Fuel’ is now extensively used to replace methylated spirit for such purposes as are fulfilled by small spirit lamps and stoves. It acts as an efficient substitute in such
circumstances, and has the advantage of being a solid substance, and thus easily portable and specially convenient. It is sold in small lamps and stoves, and refills are dispensed in the
form of white tablets or cakes. Judging from my experience in connection with the first case detailed below, it appears that many who sell this material are ignorant of its composition
and nature. Its poisonous properties on ingestion can hardly be too widely known……” R. Miller (1928). Archives of Diseases in Childhood. 3(18): 292–295.
Coal Iron oxide
“meta-fuel”
Burns w/o
any CO2.
Recyclable!
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Potential oxygen carriers
Many oxygen carriers have been studied to date: Iron: Fe2O3 Hematite = Iron (III), Fe3O4 Magnetite = Iron (II,III), FeO= Iron(II), Wusite, Fe Copper: CuO Copper oxide, Cu2O Cupric Oxide, Cu Nickel: NiO, Ni Manganese: MnO2, MnO, Mn2O3, Mn3O4 , Mn Cobalt: Co3O4, CoO, Co Sulfates-Sulfides: CaSO4-CaS, MnSO4-MnS, FeS-FeSO4 And others: Sb, Pb, Cd…
Fan, L. S., (2010). Chemical Looping Systems for Fossil Energy Conversions , John Wiley and Sons Publishers, see pp. 61 ff
Thermodynamics for iron and copper: Methane overall ½ CH4 + O2 ½ CO2 + H2O DH1000K = -402kJ Exothermic overall reaction Copper carrier 8 CuO + CH4 4Cu2O +CO2 + 2H2O DH1000C = -283kJ Exothermic metal reduction 4 CuO + CH4 4Cu +CO2 + 2H2O DH1000C = -211kJ Exothermic metal reduction 2 Cu + O2 2CuO DH1000K = -274kJ Exothermic metal oxidation Iron carrier 12Fe2O3 + CH4 8Fe3O4 + CO2 + 2H2O DH1000C = +154kJ Endothermic metal reduction 4Fe2O3 + CH4 8FeO + CO2 + 2H2O DH1000C = +303kJ Endothermic metal reduction 4/3Fe2O3 + CH4 8/3Fe + CO2 + 2H2O DH1000C = +154kJ Endothermic metal reduction 4/3Fe + O2 2/3 Fe2O3 DH1000C = -539kJ Exothermic metal oxidation
What large
difference in
system
configuration
must exist for
copper versus
iron carriers?
Hint: where does the heat
go, above?
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Thermodynamic limits on conversion
How much oxygen, CO, H2 will exist in the products of CL combustion? CO2 Enhanced Oil Recovery specification establishes potential requirements
*
*QUALITY GUIDELINES FOR ENERGY SYSTEM STUDIES -CO2 Impurity Design Parameters, DOE/NETL-341/011212, Jan 2012.
http://www.netl.doe.gov/energy-analyses/pubs/QGESSSec3.pdf
CO2 + H2O
Ash
Recycle
CO2 + H2OFuel
Air
Seal
Seal
N2 + O2
(vitiated air)
Notice there is no
“excess air” to
consume unused fuel.
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‹#›
Understanding equilibrium limits on conversion
MeO2
Me
O2 (g)
The metal/oxide reaction (i) M + O2 MO2 ; DGT(i) = DGT(i)˚ + RTln(1/PO2) At equilibrium, DGT(i) = 0, denote PO2
* ; DGT(i)˚/(2.3RT)= log(PO2*) The gas-phase reactions (ii) 2CH4 + O2 2CO + 4 H2 ; DGT(ii)˚/(2.3RT)= 2log(PCH4
* / PCO*PH2
*2)+ log(PO2*) (iii) 2CO+O2 2CO2 ; DGT(iii)˚/(2.3RT)= 2log(PCO
* / PCO2*)+ log(PO2*)
(iv) 2H2+O2 2H2O ; DGT(iv)˚/(2.3RT)= 2log(PH2* / PH2O
*)+ log(PO2*) If you have the values for DGT˚’s you can solve immediately for PO2*, (PH2
* / PH2O*) and (PCO
* / PCO2*). You can get absolute concentrations of CO and H2
by noting the fuel is mostly converted to CO2 and H2O.
CH4, CO,
CO2, H2,
H2O
Notice that at any temperature
if PO2 < PO2* defined by (i), the
metal oxide (MO2)is reduced to
the Metal (M).
Quiz for grad students:
Your chemical looping
combustor is making 30
ppm CO.
Your professor wants you
to add more metal oxide
to improve CO burnout.
Will it work?
A) Yes because….
B) No because….
C) Maybe because….
D) I just want to
graduate.
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‹#›
Figure shows the reduction of Fe2O3Fe3O4 with H2 or CO. Even at equilibrium, there are ppm levels of residual H2 & CO
making a slightly reducing environment. Note the combination of (ii) and (iii) is water gas shift CO + H2O CO2 +H2.
Residual CO increases with temperature because the CO Fe2O3 reduction reaction is slightly exothermic (H2 Fe2O3 reduction
here is endothermic). Results are based on simulations using HSC Chemistry 7.1. Courtesy Mike Gallagher, NETL.
Fe2O3 reduction to Fe3O4 with H2/CO The metal/oxide reaction Fe2O3 = hematite; higher oxidation, can reduce to magnetite.
(i) 6Fe2O3 O2 + 4Fe3O4
Fe3O4 = magnetite; lower oxidation, can oxidize to hematite, i.e., reverse of (i), or reduce further to FeO (Wustite), not discussed here.
If you fix temperature, (i) will proceed forward or backward depending on the oxygen pressure established with gas-phase fuel reactions, below:
The gas-phase reactions
(ii) 2CO+O2 2CO2
(iii) 2H2+O2 2H2O
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‹#›
Fe2O3 reduction to Fe3O4 with H2/CO The metal/oxide reaction Fe2O3 = hematite; higher oxidation, can reduce to magnetite.
(i) 6Fe2O3 O2 + 4Fe3O4
Fe3O4 = magnetite; lower oxidation, can oxidize to hematite, i.e., reverse of (i), or reduce further to FeO (Wustite), not discussed here.
If you fix temperature, (i) will proceed forward or backward depending on the oxygen pressure established with gas-phase fuel reactions, below:
The gas-phase reactions
(ii) 2CO+O2 2CO2
(iii) 2H2+O2 2H2O
Notice what happens if you add (i) + (ii) :
2CO + 6Fe2O3 4Fe3O4 + 2 CO2
And similar for (i) + (iii):
2H2 + 6Fe2O3 4Fe3O4 + 2 H2O
The oxygen “disappears”.
Why bother referencing oxygen ?
As will be seen, it is easier to understand…
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‹#›
Ellingham diagrams-1 These diagrams help quickly assess what species will
oxidize or reduce. Write any reaction with gaseous
oxygen as the reaction with one mole of O2, e.g.:
4Fe3O4 + O2 6Fe2O3 (rxn 1)
Then, at equilibrium, Drxn1G = 0 implies:
6GoFe2O3 -4GoFe3O4 – G
oO2 –RT ln (po2 ) = 0
ZERO
STD. GIBBS FREE ENERGY CHANGE FOR (rxn1), DGorxn1
LOOK UP IN TABLES, DEPENDS ONLY ON TEMPERATURE
Then, re-arranging the above equation
DGorxn1 = RT ln (po2 ) (eqn 1)
The equilibrium expressed by (eqn 1) is shown in the
graph as the intersection of the lines for different values
of oxygen partial pressure.
If the oxygen pressure is higher than equilibrium, (rxn 1)
will go forward. If it is less, (rxn 1) goes backward.
Fe2O3
Fe3O4
O2 (g)
CH4, CO,
CO2, H2,
H2O
What determines pO2? The gas reactions
DG
o r
xn
or
R
T l
n (
po2 )
k
J/gm
ol
O2
Temperature (K)
RT ln (po2 )
@ po2 = 10-8 atm
10-12 atm
10-16 atm
Note: You want it to go backward in the fuel reactor
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‹#›
Ellingham diagrams-2 These diagrams help quickly assess what species will
oxidize or reduce. Write any reaction with gaseous
oxygen as the reaction with one mole of O2, e.g.:
4Fe3O4 + O2 6Fe2O3 (rxn 1)
Then, at equilibrium, Drxn1G = 0 implies:
6GoFe2O3 -4GoFe3O4 – G
oO2 –RT ln (po2 ) = 0
ZERO
STD. GIBBS FREE ENERGY CHANGE FOR (rxn1), DGorxn1
LOOK UP IN TABLES, DEPENDS ONLY ON TEMPERATURE
Then, re-arranging the above equation
DGorxn1 = RT ln (po2 ) (eqn 1)
The equilibrium expressed by (eqn 1) is shown in the
graph as the intersection of the lines for different values
of oxygen partial pressure.
If the oxygen pressure is higher than equilibrium, (rxn 1)
will go forward. If it is less, (rxn 1) goes backward.
Fe2O3
Fe3O4
O2 (g)
CH4, CO,
CO2, H2,
H2O
What determines pO2? The gas reactions
DG
o r
xn
or
R
T l
n (
po2 )
k
J/gm
ol
O2
Temperature (K)
RT ln (po2 )
@ po2 = 10-8 atm
10-12 atm
10-16 atm
What if you want to consider other
metal/oxide reactions
(e.g. Me + O2 MeO)?
Plot the DGo versus T for the metal of
interest; examples shown above (a)
and below (b) the Fe3O4 / Fe2O3 line.
b
a
-
‹#›
Ellingham diagrams - 3 4Fe3O4 + O2 6Fe2O3 (rxn 1)
Follow the exact same procedure to write the
Ellingham diagram for CO or H2
2CO + O2 2CO2 (rxn 2)
2H2 + O2 2H2O (rxn 3)
Consider (rxn 2); you can treat (rxn 3) in exactly the
same manner (not covered here). Then, at
equilibrium, Drxn2G = 0 implies:
2GoCO2 - 2GoCO - G
oO2 - RT ln (po2 pco
2 / pco22 ) = 0
DGorxn2 – 2RT ln(pco/pco2) = RT ln (po2 )
Notice you need to know the ratio of CO to CO2 partial
pressure to plot the left side versus temperature.
Assume some values for the ratio, and plot the left side
(10-2 , 3*10-5 and 10-7 ). The intersection with the
oxygen lines represents an equilibrium condition.
The plots match your chemical intuition – higher CO
levels have lower O2 at equilibrium
DG
o r
xn
or
R
T l
n (
po2 )
k
J/gm
ol
O2
Temperature (K)
RT ln (po2 )
@ po2 = 10-8 atm
10-12 atm
10-16 atm
10-16 atm
10-12 atm
RT ln (po2 )
@ po2 =
10-8 atm
500 1000 1500
Higher CO
Lower
O2
DG
o r
xn
or
R
T l
n (
po
2 )
k
J/gm
ol
O2
-
‹#›
Ellingham diagrams - 4 The last step is to add the equilibrium for the metal
and oxide reaction to the CO reaction, i.e. combine
rxn1 and rxn2 plots as shown in the bottom graph
4Fe3O4 + O2 6Fe2O3 (rxn 1)
2CO + O2 2CO2 (rxn 2)
The intersection of lines at (1000K, -225kj/gmol),
circled, represents equilibrium. • What is the oxygen partial pressure?
• The ratio of CO to CO2?
The graph provides insight into how changing
parameters affects the equilibrium of (rxn 1): • What happens if you raise the temperature?
• Add more CO?
• What happens if you consider a Me/MeO rxn that
“sits” above or below rxn 1 on the plot?
DG
o r
xn
or
R
T l
n (
po2 )
k
J/gm
ol
O2
Temperature (K)
RT ln (po2 )
@ po2 = 10-8 atm
10-12 atm
10-16 atm
10-16 atm
10-12 atm
500 1000 1500
P
10-12 atm
Lower
O2
Higher
CO/CO2
DG
o r
xn
or
R
T l
n (
po
2 )
k
J/gm
ol
O2
b
a
You can use published Ellingham diagrams with CO/CO2,
H2/H2O, O2 nomographs and various A +O2 AO2
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‹#›
Quiz
• Your classmate wants to make an oxygen carrier from
aluminum because it is very energetic when it burns!
• How would you argue from experience if this is a good idea?
• How would you use an Ellingham diagram to figure out if that is
a good idea?
~1014
-
‹#›
Solid Carbon Formation • What happens if any solid carbon is left on the oxygen carrier
when it leaves the fuel reactor? (Red arrow, below)
• Carbon formation via equilibrium (chart, right) and also
hydrocarbon cracking.
• Notice that solid carbon on a metal oxide may not be a problem!
*Gaskell, D. R. (2008) Introduction to the Thermodynamics of Materials, 5th ed, Taylor and Francis, pp. 365-366
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
0 200 400 600 800 1000 1200
Vo
lum
e F
racti
on
CO
Temperature (C)
Boudard Reaction Equilibrium
C (s) +CO2(g) --> CO(g), 1atm, only CO/CO2 gases*
Carbon
Forms
Carbon
Gasified
to CO
CO2 + H2O
Ash
Recycle
CO2 + H2OFuel
Air
Seal
Seal
N2 + O2
(vitiated air)
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‹#›
OXYGEN CARRIER CAPACITY AND
CIRCULATION RATES
Inert
support,
mass: minrt
Fully reduced
active species
(e.g., Cu, Fe, etc.)
Partially oxidized
active species
Fully oxidized
active species
(e.g., CuO, FeO, etc.)
active mass: mred
X = 0
active mass: mox
X=1
active mass: m
X = m-mred
mox-mred
Define
conversion
X for a
“supported”
metal oxide
carrier
-
‹#›
Mas
s of
carr
ier
(1 k
g o
xid
ized
sta
te)
Nomenclature
0.90
0.92
0.94
0.96
0.98
1.00
0 0.2 0.4 0.6 0.8 1
0.00
0.20
0.40
0.60
0.80
1.00
0 0.2 0.4 0.6 0.8 1
B
A C
C
DX X2 X1
A= Active mass, oxidized
B = Inert mass
C = Working oxygen capacity
co = A/(A+B)
Ro = C/A
co Ro = C/(A+B) co =
active
mass: m
mred
-
‹#›
Values for Oxygen Transport Capability (Ro) for Some Metal/Oxide Pairs
Table 1. Values of Ro for some potential oxygen carrier reactions, arranged small to large.
Inexpensive
Good capacity
Very inexpensive.
Throw-away option
Fe2O3 / Fe3O4 0.034
Mn2O3 / MnO 0.100
Mn2O3 / Mn3O4 0.034 Cu2O/ Cu 0.110
CuAl2O4 / CuAlO2 0.044 CuO / Cu 0.200
Fe2O3Al2O3 / FeAl2O4 0.045 CoO / Co 0.210
Co3O4 /CoO 0.067 NiO / Ni 0.210
Mn3O4 / MnO 0.070 Co3O4 /Co 0.270
CuAl2O4 /CuAl2O3 0.089 ZnSO4 / ZnS 0.396
NiAl2O4 / Ni Al2O3 0.091 CuS04 / CuS 0.401
CuO / Cu2O 0.100 MnSO4 / MnS 0.424
Fe2O3 / FeO 0.100 FeSO4 / FeS 0.425
CaSO4 / CaS 0.470
-
‹#›
Petrochemical
Fluid Catalytic Cracking (FCC)
Establishing Carrier Requirements
from FCC Experience Some FCC units operate with 41,000 kg/min solid circulation rate.
Proven Process Technology
Flue
Gas
Catalyst
Regenerator
Stripping
Stream
Air
Air Heater
Cyclone
Vessel
Dispersion Steam
Lower Feed Injection
Stripper
Stripper
Standpipe
Regenerator
Standpipe
Riser
Reactor
-
‹#›
Properties of the Oxygen Carrier
• Assuming a solids circulation rate ~ operating fluid
catalytic crackers (41,000 kg/min)
• Calculate the thermal output possible for a chemical
looping system for different carriers/conversions
• This only accounts for supplying oxygen, not thermal
balance (next slide).
0
200
400
600
800
1000
1200
1400
1600
1800
2000
0.5 0.7 0.9
Th
erm
al O
utp
ut
[MW
]
Conversion
[Pure Fe: Fe2O3→Fe3O4]
[Ilmenite: Fe2O3→Fe3O4]
[Ilmenite: Fe2O3→FeO]
[Cu, 60%Al: CuO→Cu]
[Pure Fe: Fe2O3→FeO]
[Pure Cu: CuO→Cu]
Acce
pta
ble
Op
era
ting
Ra
ng
e?
0 500 1000 1500 2000 2500
1
2
3
4
5
6
Thermal Output [MW]
[Pure Fe: Fe2O3→Fe3O4]
[Pure Fe: Fe2O3→FeO]
[Ilmenite: Fe2O3→Fe3O4]
[Ilmenite: Fe2O3→FeO] [Pure Cu: CuO→Cu]
[40% Cu, 60%Al2O3: CuO→Cu]
-
‹#›
Determining the solids circulation rate
• The oxygen carrier circulation rate is determined by:
1) The oxygen needed for the fuel flow rate
2) Endothermic carriers: the heat needed to drive the fuel reactor
• How are requirements (1) and (2) compatible?
– It turns out the required circulation rate is sometimes entirely
dominated by the need to supply heat to the fuel reactor.
– In that case, a higher capacity oxygen carrier is not needed.
-
‹#›
Circulation rate analysis
• Solids circulation shown
by the double curved
arrows, A and B.
• There is only ONE
circulation rate – why
show as two arrows?
– Emphasize that you
must satisfy two
conditions:
A. Supply enough oxygen to
react with all the fuel.
B. Endothermic carriers:
supply enough heat to
keep the carrier “hot”.
• Note that the mass flows
that satisfy A has a
definite minimum for a
fixed fuel flow rate, but
no maximum.
CO2 + H2O
Ash
Recycle
CO2 + H2OFuel
Air
Seal
Seal
N2 + O2
(vitiated air)
CO2 + H2O
Fuel flow
for 1MWth
(assume CH4)
A
B
CO2 H2O
Air in
Vitiated Air Out
Heat
Out
(steam)
Which is bigger? A or B ?
-
‹#›
Circulation rate analysis, continued
A – the mass flow needed to supply
enough oxygen to react with the fuel.
B- the mass flow needed to carry enough
heat to support the endothermic reaction
via a 50°C temperature drop.
A
B
CO2 H2O
Air in
Vitiated Air Out
Heat
Out
(steam) Fuel flow
for 1MWth
Fuel
Reactor Air
Reactor
A B B/A
Carrier Pair Ro Min Circ Rate O2 supply
(kg/sec)
Circ rate to supply
heat (kg/sec)
Ratio: Heat/O2
Rate
Fe2O3 / Fe3O4 0.034 2.35 3.36 1.4
Fe2O3 / FeO 0.1 0.80 6.75 8.4
Fe2O3/Fe 0.3 0.27 6.10 22.9
CuO / Cu 0.2 0.40 n/a n/a
CaSO4 / CaS 0.47 0.17 3.07 18.1
DT = - 50°C
Analysis is for methane fuel; could be repeated for coal, etc. Note that the exothermic carrier CuO has a much lower circulation rate than all the others.
-
‹#›
KINETIC RATES AND REACTOR SIZE
-
‹#›
KINETIC RATES AND REACTOR SIZE
Air
Fluid Bed
Carrier
State X1
Carrier
State X2
Inputs
moc,iXi mair,i
Ti
Reactor
Volume
VR
Outputs
moc,oX
mair,o T
Reactor
Temperature
T
A “bubbling” fluid bed (BFB) is one possible reactor configuration (left).
BFB is approximately like a stirred reactor (right).
The usual combustor design concepts apply: 1) heat release rate balances the incoming rate of cold reactants or it will blow out
2) for a given throughput, the reactor volume is inversely proportional to the reaction rate.
-
‹#›
thermocouple
Experimental measurement of kinetics Thermo gravimetric Analysis (TGA)
For gaseous reactions, sample weight describes the conversion:
Cycling the fuel and oxidizer over the sample pan will give
many “cycles” to analyze.
Solid fuels (coal and biomass): must re-load the pan every cycle.
-
‹#›
Example of kinetics measurement (CH4)
Typical mass and temperature measurement for CuO/bentonite
particle and 100% CH4 for reduction and air for oxidation
reactions
Effect of reaction temperature on CuO/bentonite
particle and CH4 reaction
0
0.2
0.4
0.6
0.8
1
0 0.4 0.8 1.2 1.6 2
Conv
ersi
on (
X)
Time (min)
750
800
850
900
T (oC)
0
200
400
600
800
1000
30
32
34
36
38
40
42
44
0 500 1000 1500 2000
Tem
per
atu
re (
oC
)
Mas
s (m
g)
Time (min)
1 rrox r
m mX
m m
=
dp=150-250 mm
TGA Data for Cu-based carrier 4 2 24 4 2CuO CH Cu CO H O
100% CH4, 800 oC
100% CH4
Monazam et al. (2012) “Kinetics of the Reduction of CuO/Bentonite by Methane (CH4) During Chemical Looping Combustion”, to appear Energy and Fuels.
4
11
(1 )[ ln(1 )]m nCHdX
ky n X Xdt
= Results fit to Jonson-Mehl-Avarmi (JMA) rate equation
-
‹#›
A visual representation of reaction rates
• An informative/interesting way to see the metal-metal oxide cycle.
• Combustion Quiz
– What type of flame does a propane torch use (e.g., diffusion, premixed,
partially premixed?)
– What is the partial pressure of oxygen inside the flame?
Movie of copper reduction and
oxidation
-
‹#›
Kinetics in a stirred reactor bed (1/2)
• With kinetic rates, write
energy and mass balances
with an “efficiency” of
conversion hI (defined below).
• Does the bed have “light-off”
behavior?
Green line depicts efficiency as a function of output
temp (from mass balance using kinetic rates), dashed
line shows same using energy balance equations.
Point of intersection is the desired steady-state
solution.
0
200
400
600
800
1000
1200
1400
1600
1800
2000
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Rea
cto
r In
tern
al T
emp
erat
ure
(C)
Efficiency (N-I)
Air Reactor (Cu2O-->CuO)
10 sec
800C - Input Temp
Tout at steady-
state
-
‹#›
Kinetics in a stirred reactor bed (2/2)
• With kinetic rates, write
energy and mass balances
with an “efficiency” of
conversion hI (defined below).
• Does the bed have “light-off”
behavior?
0
200
400
600
800
1000
1200
1400
1600
1800
2000
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Rea
cto
r In
tern
al T
emp
erat
ure
(C)
Efficiency (N-I)
Air Reactor (Cu2O-->CuO)
10 sec
800C - Input Temp
Tout at steady-
state
Different inlet
temperature, mass
flow, etc.
Green line depicts efficiency as a function of output temp
(from mass balance using kinetic rates), dashed line
shows same using energy balance equations. Point of
intersection is the desired steady-state solution.
Detailed approach: kinetics inserted
into CFD models (described later).
-
‹#›
How quickly do the reactions need to occur?
Answer: it depends on several parameters Simple estimate:
The extent of conversion is X.
Assume constant rate (dX/dt)
Residence time is t.
t (dX/dt) = DX (2) Assume we want DX ~ 1
and combine (1) and (2):
m = mass flow rate
“active” species*
M = “active” mass , raV
t = M/m, res. time
DH = heat release/mass
m DH Thermal output
V Volume =
ram DH
raV = ra DH / t
or, multiplying by ra/ra:
(1)
Thermal output
Volume =
(3)
raDH (dX/dT)
*In fluid bed boilers, the solids flow will include significant amounts of inert material like ash , limestone, sand .
[1] Steam, 41st edition, Babcock and Wilcox, see Figures 4, 12 in chapter 17 for bed density; (2% )discussed later.
Using bubbling fluid bed boilers as an example, you
can “check” this with reasonable parameters:
ra = (2%) rbed; rbed ~ 500kg/m3 [1]
t ~ 60 sec, Basu pp. 123
DH = 23MJ/kg, coal HHV Leads to volume heat release 3.8MJ/m3; “reasonable” see
Basu pp. 236
Note: (1) you can adjust ra
(2) t is tens of seconds, or more
-
‹#›
Solid fuel combustion
-
‹#›
TGA Profile of Coal +CuO in N2
Can We Use Coal Directly?
0 50 100 150 200 250
0
5
10
15
20
25
30
35
40
Reaction time (min)
weig
ht
(mg
)
0
200
400
600
800
1000
Reactio
n te
mp
era
ture
(oC
)
44.88%
55.12%
N2 O2
TGA Profile of Coal in N2
Volatiles &
mositure out
combustion
combustion
0 20 40 60 80 100 120 140 160 180
160
170
180
190
200
210
Reaction Temperature (oC)
Weig
ht
(mg
)
200
400
600
800
1000
Tem
pera
ture
(oC
)
Nitrogen Oxygen
0 20 40 60 80 100 120 140 160 180
160
170
180
190
200
210
Reaction Temperature (oC)
Weig
ht
(mg
)
200
400
600
800
1000
Tem
pera
ture
(oC
)
Nitrogen Oxygen
Reaction time (min)
Rates were higher than expected.
Why?
-
‹#›
Reaction Pathways for Solid Fuel CLC
• Coal CLC with metal oxides via gaseous intermediates:
In N2:
Coal Coal pyrolysis
CO/H2 + CuO Cu +CO2 /H2O
CO2 + C 2CO
In CO2:
C+CO2 2CO
CO+CuO Cu+CO2
• CLOU mechanism (Chemical Looping Oxygen Uncoupled):
CuO Cu/Cu2O +O2
Coal + O2 CO2
• Solid-solid interaction: MeO+C MeO +CO2
Discussed
next
-
‹#›
Possible Reasons for Rapid, Low-Temperature reaction
of solid carbon with CuO
2CuO = Cu2O + 1/2O2
At 500 oC, PO2 is 1.1*10-9
Will removal of oxygen (1.1*10-9)
continuously by carbon, facilitate the
CuO decomposition?
• Reacted C with various oxygen partial
pressures
– With air: modest reaction at 500-600oC.
– No reaction at 500-600 oC with oxygen at
low O2 partial pressure ( At 500oC
Significant
Reaction
at 500oC vitiated air
-
‹#›
Combustion Rates of Coal (100 micron) with
Various Particle Sizes of CuO in TGA
Higher combustion temperature with increasing particle size
0 20 40 60 80 100
0.00
0.02
0.04
0.06
0.08
0.10
Re
actio
n te
mp
era
ture
(oC
)~5 micron
713 oC
63-177 micron
780 oC
354-595 micron
874 oC
Re
actio
n r
ate
(m
in-1)
Reaction time (min)
0
200
400
600
800
1000
-
‹#›
Effect of Dilution by Quartz Powder on the TGA
Combustion Performance of Carbon and CuO
• Mixed CuO, Quartz and C
powders
• CuO/C ratio was kept
constant
• Reaction T increased with
increased dilution
These data and others
(flow tests + DFT
calculations)* suggest a
solid-phase reaction
between carbon and the
oxygen carrier (Fe, too).
Reduces coking automatically!
* Siriwardane, R. Tian, H., .Miller, D., Richards, G., Simonyi, T., Poston, J. (2010). Evaluation of reaction mechanism of coal-metal oxide interactions in
chemical-looping combustion, Combustion and Flame, Combustion and Flame, Volume 157, Issue 11, November 2010, Pages 2198-2208
-
‹#›
Reactor and System Design
-
‹#›
An introduction to fluidization
• Fluidization is used
widely in the
chemical industry – Fluid Catalytic Cracking
(Hydrocarbons)
– Catalytic reactions.
– Drying and calcining.
– Many reactions.
• Coal, biomass, and
waste combustion
or gasification.
• Chemical looping.
Main topics
• Physical description of
fluidization.
• Key velocity parameters:
minimum fluidization,
bubbling, and terminal.
• Fluidization regimes,
application, and
components.
• Effect of particle
morphology
• Reactor configurations.
-
‹#›
References
Basu, P.(2006), Combustion and Gasification in Fluidized Beds , CRC Press,
Taylor and Francis Group, Boca Raton, FL.
Excellent treatment of practical issues in design of fluid bed combustors and
gasifiers.
Kunii D., Levenspiel, O. (1991). Fluidization Engineering, 2nd ed.,
Butterworth-Heinemann, Newton MA.
A classic text on fluidization – get a copy if you work on multi-phase flows.
-
‹#›
Physical Description of Fluidization
• A granular material
does not typically
flow like a fluid; it can
form a “pile”; (a).
• But, if you supply
fluidizing gas,
granular material can
behave as a liquid:
– Horizontal surface (b)
– Flow from holes (c)
– Equalizes levels (d)
– Floats light objects (next slide)
gas
gas
mesh
No gas
(a) (b)
(c) (d)
No gas gas
-
‹#›
Physical description of fluidized beds (cont.) – Lighter objects float.
– The bed volume is larger in a
fluid state:
void fraction e = Vgas/(Vgas + Vsolid)*
– The gas flow rate is typically
described by the superficial
velocity* U: (Gas volume flow rate)
(Cross-section area, no solids present)
– The bed pressure drop
balances the overhead weight
gas
mesh
No gas
* Typical e values( - ), U values [ m/s]: Packed Bed (0.4-0.5) [1-3], Bubbling Bed (0.5-0.85)[0.5-2.5], Circulating Bed (0.85-0.99) [4-6], Transport Reactor(0.98-0.998)[15-30], Basu pp. 22
Larger
Volume
DP Axc = Axc Lb (1-emf) (rsolid-rgas)
DP
Lb
The mf subscript: voidage at minimum fluidization, explained next
gas
Floats!
Sinks!
-
‹#›
Velocity parameters
• Minimum Fluidization Velocity Umf:
– The superficial velocity that “just” fluidizes;
the point where the bed weight balances the
pressure drop.
– Typically you measure Umf to get an
accurate value for a new granular material.
• The minimum bubbling velocity Umb:
– The velocity where bubbles first appear.
– Can be equal or greater than Umf.
• The particle terminal velocity Ut:
– Note the velocity is NOT a uniform profile.
– What happens if U> Ut ?
gas
gas
gas
Shown smaller
to with more height
above the bed
Fluidization regimes: smooth, bubbling, turbulent,
fast, pneumatic transport
-
‹#›
Application to
chemical looping
• Air reactor (right), fuel
reactor (left).
• Solids are transported from
the air reactor where U>Ut.
• Solids are fluidized in the
fuel reactor where Umf
-
‹#›
Loop seal and L-valve components
Loop seal – Isolates process gas
above (A) from process gas below (B). L-Valve- controls the solid flow
delivered to the right.
Fluidizing gas
Fluidizing gas
(A)
(B)
Solid flow
-
‹#›
The particle properties: effect on fluidization Fluidization regimes depend on the
particle morphology, size, density.
Geldart Classification A, B, C, D
A = Aeratable. Can achieve smooth
fluidization, low density (
-
‹#›
Types of gas-solid reactors
• Fixed bed: Not fluidized, UUtr, the particles are carried
out of the bed, and are re-cycled.
Ex: CFB Boiler, FCC for
hydrocarbon cracking.
• Moving bed: Not neccesarily
fluidized, but the solid moves
countercurrent to the process
gas. Ex: Lurgi gasifier.
• Entrained: no “bed”, dilute
phase. Ex: Pulverized coal boiler
Circulating fluid bed combustor
-
‹#›
Discussion-
• The preceding fluidization introduction has covered
just hydrodynamics. Also need to consider reactions,
conversion, heat transfer.
• If you want to design a solid fuel chemical looping
boiler, what combination of gas-solid reactors is the
best?
• What are the tradeoffs for small or large oxygen
carrier particles?
• Some of these issues are best addressed with
validated CFD simulations and system models.
-
‹#›
Modeling of Fluidized Beds
(Courtesy: F. Shaffer, NETL)
DEM LBM DNS MP-PIC Multi-Fluids Filtered-Eqs ROM
www.mfix.netl.doe.gov
Movie
http://www.mfix.netl.doe.gov/
-
‹#›
Comparison of CFD and Cold Flow Rig
Oxygen
carrier
Solid fuel
into this bed
Lighter ash
carried out with
fluidizing steam
or CO2
Oxygen
carrier
SIMULATION EXPERIMENT
Air
reactor
Movie
-
‹#›
Design consideration – the air reactor
(fuel reactor discussed later)
-
‹#›
Where do you take the heat out?
Endothermic carriers: you don’t need need
to remove heat from the fuel side.*
Exothermic carriers, you may need to
remove heat from the fuel side to keep from
overheating the carrier. Potentially use the
recycle fluidizing gas as a heat exchange
media.
All carriers: must manage the air reactor
exotherm.
CO2 H2O
Air in
Vitiated Air Out
Heat
Out
(steam) Fuel flow
for 1MWth
Fuel
Reactor Air
Reactor
Endo or
Exo-
thermic?
-
‹#›
(Cont) where do you take the heat out – endothermic carrier?
Must allow equal temperature rise in the air reactor DTar as
temperature drop in the fuel reactor DTfr
CO2 H2O
Air in
Vitiated Air Out
Fuel flow
for 1MWth
Fuel
Reactor Air
Reactor
DTfr = - 50°C DTar = + 50°C
Heat
Out
(steam)
A simple enthalpy balance will
show that the air needed to
transport the carrier has a minor
effect on the carrier temperature -
even w/o exothermic reactions.
You will need to take a lot of heat
out as the reactions occur. How?
-
‹#›
Conventional Circulating Fluid Bed Combustion (CFB)
• Fuel is added to the “riser” and reacts with
air.
• Unburned fuel may
circulate around the
“loop” several times.
• The circulating “bed” of
material is mostly inert,
and provides a large
thermal mass.*
• Heat can be removed in
several places.
• Note the similarity to
chemical looping
combustion.
* “…..the mass flow rate of recycled solids is many times the mass flow rate of incoming air, fuel, and limestone….the bed solid temperature
remains relatively uniform” Steam, Edition 41, pp. 17-9, The Babcock and Wilcox Company.
-
‹#›
Circulating Fluid Bed (CFB) Combustion
versus Chemical Looping Air Reactor
• The CFB case uses
significant inert flow
that moderates the
temperature rise.
• The CLC case
appears more like
pulverized coal –
potential for
significant
temperature rise?
• Compare reaction
enthalpies (i.e., HHV)
for meta-fuel versus
carbon (next slide).
Solid
Fuel
Particles
Inert bed
Particles
(ash, sand)
Meta
Fuel
Particles
CFB
Combustion
CLC
Air reactor
Air Air
-
‹#›
Circulating Fluid Bed (CFB) Combustion versus Chemical Looping
Solid
Fuel
Particles
Inert bed
Particles
(ash, sand)
Meta
Fuel
Particles
CFB
Combustion
CLC
Air reactor
Air Air
Reactions 1 – 7 compare DH for meta-fuel (CLC) and
carbon fuel (CFB).
Per mass basis because CFB circulation is
expressed as mass flux (kg/s/m2).
The meta-fuel particles produce 3-60 times less heat
per mass than carbon fuel.
But, there can be significantly more mass flux of
meta- fuel versus carbon fuel (how much more?).
"meta" fuel or carbon fuel Reaction
Reaction DH per kg
'fuel', MJ/kg *
Iron carriers
1 Fe3O4 + 1/4O2 --> 3/2 Fe2O3 -0.51
2 FeO+1/4O2-->1/2Fe2O3 -1.96
3 Fe+3/4O2 --> 1/2Fe2O3 -7.39
Copper carriers 4 Cu+1/4O2 --> 1/2Cu2O -1.34
5 Cu+1/2O2 --> CuO -2.46
Calcium sulphide carrier 6 CaS + 2O2 --> CaSO4 -13.24
Carbon fuel 7 C+O2 --> CO2 -32.76
0
5
10
15
20
25
30
35
1 2 3 4 5 6 7
Reaction
DH
, J/k
g
* Standard reaction enthalpy at 298K; data calculated from NIST web book and Lange’s Handbook of Chemistry
Iron Copper
CaS
Carbon
-
‹#›
CFB Typical Flow Parameters
Parameter* Boiler FCC Reactor
External Circulation Rate (kg/m2/s) 10-50 500-1000
Superficial Gas Velocity (m/s) < 6 < 25
Suspension Density, upper region (kg/m3) 1-10 10-100
Particle Size (microns) 200 70
• The chart below provides typical parameters for
CFB operation in boiler and Fluidized Catalytic
Cracking applications.
• Using the superficial gas velocity for the boiler ~6
m/s and multiplying by the air density (800°C, 1
atm) provides a mass flux of air : Ga= 2 kg/m2/s.
• With the smallest circulation rate, from chart below,
Gs = 10 kg/m2/s. Solids to air mass ratio is 10/2 = 5.
*Basu, P.(2006), Combustion and Gasification in Fluidized Beds , pp. 42, CRC Press, Taylor and Francis Group, Boca Raton, FL.
** This ratio can cover a wide range of values; this value uses the parameters indicated.
Solid
Fuel
Particles
Inert bed
Particles
(ash, sand)
Air Mass Flux Ga
Solid Mass Flux Gs
Thus, Gs/Ga ~ 5 typical** solid/air mass ratio
-
‹#›
The relation between fuel/air ratio and Gs/Ga If the solids flow is meta-fuel (or, pure solid fuel):
fuel/air mass ratio = Gs/Ga
If the solids flow has a mass fraction of fuel Yf:
Yf = (fuel mass flux)/(solid mass flux)
fuel/air mass ratio = Yf ·Gs/Ga
The chart below lists the stoichiometric fuel-air
ratios for several meta-fuels and carbon fuel:
Most of the f/a ratios are less than Gs/Ga = 5
(except reaction 1).
Thus, you need to operate very dilute OR don’t
convert much of the material.
Coal CFBs accomplish dilute fuel operation with
the addition of lots of inert materials (~ 3/2 Fe2O3 + 0.94N2 6.73
2 FeO+¼(O2+3.76N2) -->1/2Fe2O3 + + 0.94N2 2.09
3 Fe+3/4(O2+3.76N2) --> 1/2Fe2O3 +2.28N2 0.542
Copper carriers 4 Cu+1/4(O2+3.76N2) --> 1/2Cu2O + 0.94N2 1.85
5 Cu+1/2(O2+3.76N2) --> CuO + 1.88N2 0.926
Calcium sulphide carrier
6 CaS + 2(O2 +3.76N2) --> CaSO4 + 7.52N2 0.266
Carbon fuel 7 C+(O2 +3.76N2) --> CO2 +3.76N2 0.0874
* For carbon fuel example, f/a= Yf·Gs/Ga is 0.0874 = Yf ·5. Thus, for stoichiometric conditions, Yf = .0175. Actual plants operate with excess air, lowering this value further.
-
‹#›
Practical issues
The air reactor residence time is not necessarily
long enough to allow full oxidation. • CFB riser residence time ~ 5 seconds, max.
• Full oxidation takes ~ >>10 seconds for some carriers.
Options: A. Supply approximately 10/5 = ~ 2x more carrier than
stoichiometric; convert part of the carrier (figure A, left) and use
external heat exchanger.
B. Add a bubbling bed at the bottom with in-bed heat removal
(figure B, below).
C. Arrange two interconnected loops (figure C, below):
D. Your idea? How about Moving Bed?
CLC
Air reactor
Air, Inlet
Fe3O4 or FeO
+ Fe2O3
Outlet
Fe3O4 or FeO
Fuel reactor
carrier circulation
Air reactor
carrier circulation
Exchange between reactors
Secondary
Transport Air
Steam
Generation
Tbed
Texit
Air A B C
External HX
External HX
-
‹#›
Advantages of case C (last slide)
Follow the mole flow rate of reduced species (subscript r) at
stations 1 – 5 shown in the diagram. Some definitions:
F = single pass fractional conversion; Nr3=(1-F)Nr2.
R = recycle rate; R = Nr5/Nr4 (see note, below)*.
F = 0 implies no oxidation occurs in the reactor.
F = 1 implies no reduced material emerges at station 3 – full oxidation.
R = 2 (for example) means the molar flow in recycle is twice the outflow.
Fuel reactor
carrier circulation
Air reactor
carrier circulation
Nr1
Nr4
Nr1 = moles/sec reduced state input
Nr4 = moles/sec reduced state output
Nr1
Nr4
F R
1
2
3
4
5
* Note that since the flow splits at 3 to 4-5, the recycle/output mole flow ratio is the same for reduced or total
molar flow (oxidized + reduced + inert, tot subscript); Nr5/Nr4 = (Yr5*Ntot5)/(Yr4*Ntot4). The mole fractions Yr5
= Yr4 because they are split from the same stream at station 3. Thus, Nr5/Nr4 = Ntot5/Ntot4 = R.
Nr4 = (1-F)
(1-RF) Nr1 Mole balances imply:
Air reactor process flow Equivalent topology & nomenclature
0
0.2
0.4
0.6
0.8
1
0 0.2 0.4 0.6 0.8 1
Nr4
/ N
r1R
ed
ue
d s
tate
flo
w o
ut/
in
F Single pass fractional conversion
Recycle
RR=0
R=1
R=2
R=4
R=8
Main point: Modest single-pass fractional
conversion can be offset by recirculation; e.g.:
F = 0.2, R = 8 implies Nr4/Nr1 = 0.3
( Only 30% is not reduced, 70% is oxidized)
Disadvantage: large R promotes attrition.
-
‹#›
Recent Research Results
-
Chemical Looping – work in progress at NETL
Cold flow validation rig, 50kWth Chemical Looping Reactor, Single fluid bed, attrition test, microwave solids flow sensor.
– Experiments: Designed, built, and producing validation data.
• Lab-scale to fully-integrated 50kWth loop.
– Simulations: Developed models, simulation tools . • Zero-D to full-loop 3-D.
– Developed novel performance/low-cost carriers. • ”Promoted” iron-ore, and “thermally neutral” Cu-Fe.
– Techno-economic results: results for NG steam. • Confirms low-cost & guides research (shown later).
Fuel Reactor
Separation cyclone
Riser
Crossover
Air Reactor
Loop Seal
L-Valve
Solids Flow
Models: zero-D, 3-D loop, fuel reactor
NETL O2 Carriers Maximal O2 capacity. “Thermally neutral”.
Std. materials & manufacturing
-
‹#›
Key Findings- Chemical Looping – A few more details
• Thermally neutral carrier development:
– Allows independent control of circulation rate.
– Large O2 capacity means low circulation rate
– reduced attrition?
• Hydrodynamic predictions versus
measurements:
– Cold flow predictions significantly affected by
chosen model parameters.
– Emphasizes need for model validation and
diagnostics in hot flow.
– Microwave sensor being developed for hot
flow diagnostic.
• Lessons learned and progress made
• Multiple solids flow issues resolved –
see (1) and (2) - recent test had smooth
solids circulation.
• Batch mode oxidation/reduction on
baseline (raw) hematite; follows
expected model results.
• Bubbles matter! See next slide
Cyclone
C-1200
Test Section
C-1250
Loop Seal
R-1300
Upper
Riser
R-1150
Lower
Riser
R-1100
Air
Reactor
R-1000
Air Pre-heater and Tee
H-1800 & H-1850Air Pre-heater and Tee
H-1800 & H-1850
L-Valve
Housing
R-1450
Fuel
Reactor
R-1400
(1)
(2)
Industrial Carbon Management Initiative
-
‹#›
Cyclone
C-1200
Test Section
C-1250
Loop Seal
R-1300
Upper
Riser
R-1150
Lower
Riser
R-1100
Air
Reactor
R-1000
Air Pre-heater and Tee
H-1800 & H-1850Air Pre-heater and Tee
H-1800 & H-1850
L-Valve
Housing
R-1450
Fuel
Reactor
R-1400
a a’ a a’
Movie (cold flow) a-a cut
Fuel Conversion and Bubbles
Porous plate different injectors Video and CFD courtesy Doug Straub, A. Konan, NETL
-
‹#›
Fuel conversion: CFB versus chemical looping fuel reactor
CO2 + H2O
Ash
Recycle
CO2 + H2OFuel
Air
Seal
Seal
N2 + O2
(vitiated air)
Circulating fluid bed combustor
• Gas-phase combustion for CO &
H2 has ~ 3% oxygen.
• Solid char is recycled.
• Gas-phase combustion for CO & H2
requires oxygen carrier (gaseous
oxygen PO2
-
Techno Economic Studies : Parameter Potential Impacts Expected
Relative to a CLC Base case (assumes changes in parameters are within operating range)
Vessel Height
Vessel Diameter
Circ. Rate
Boiler Eff. Auxiliary
Power CO2
Capture Equip. Cost
Cost of Steam
Carrier Reactivity (literature)
Large + = -
Small + = -
Small + = -
Small + = -
Carrier Loss (0 %) and Price ($0/lb)
Medium + = +
Carrier Size (0.28mm) and Density (203 lb/ft3)
Small + = -
Small + = -
Small + = -
Small + = -
Carrier Conversion (from reducer 47%; from oxidizer 95%)
Medium + = +
Large + = -
Small + = +
Small + = +
Small + = +
Reactor Temperature (1700 F)
Small + = -
Small + = +
Small + = -
Small + = -
Small + =-
Reactor Velocities (reducer outlet 33.6 fps; oxidizer outlet 29.4 fps)
Large + = +
Large + = -
Small + = +
Small + = +
Small + = +
Natural Gas Conversion (97.5%)
Medium + = +
Small + = +
Small + = +
Large + = +
Small + = +
Small + = +
Oxidizer XS O2 (3.8mol % in off-gas)
Small + = -
Small + = +
Small + = -
Small + = +
Small + = +
Small + = +
This is why we want to focus on this issue going forward
Industrial Carbon Management Initiative
-
‹#›
Chemical Looping – Discussion/ Thinking Question
• Relative to the
earlier discussion
of the figure at
the left:
– Does chemical
looping “fit” this
description?
– What are the
potential benefits
and drawbacks
of chemical
looping w/r to
efficiency?
• What applications
of chemical
looping are most
attractive?
Generator Carbon
Separation
Unit
Q = mfDH Fuel Heat
Input
hg Generator
Efficiency
Wo Gross
Generator
Work
W1 Net
Output
Define:
a = (kg CO2 produced) / (kg fuel burned )
wCO2 = (separation work, Joules ) / (kg CO2) CO2
Approx Ranges: (30 – 60%) (6-10%)
-
‹#›
Chemical Looping Summary
• Not completely new, but new interest because of CO2 capture.
• Various metal/metal oxide pairs are candidate oxygen carriers.
• For a given heat output, reactor circulation rates depend on the
oxygen capacity and the thermal balance.
• Practical experience from CFB and FCC applications suggest
the range of application.
• In progress:
– kinetic improvements
– reactor design and optimization
– model validation
– attrition
CO2 + H2O
Ash
Recycle
CO2 + H2OFuel
Air
Seal
Seal
N2 + O2
(vitiated air)
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