POWER January 2011 - 55

CARBON CAPTURE
air separation is a proven technology, oxycombustion
requires much different oxygen
conditions than typical cryogenic air
separation applications. In addition, opportunity
exists to improve overall plant
efficiency by integrating heat from the
ASU and other sources within the process
into the steam cycle (Figure 4).
To maximize value, " peak-shaving " designs
that employ liquid oxygen storage
can be utilized. During peak load periods
when the need for and price of electricity
are high, the ASU can be shut down or
run at low load, and liquid oxygen can be
used to supplement operation of the plant.
The ASU power would then be available
for sale to the grid. In off-peak periods the
ASU could run at full capacity, supplying
oxygen to the combustion process while
the remainder goes to storage.
Partial capture mode can be achieved by
alternating between air and oxy-firing modes
to obtain the desired CO2
capture rate.
Under oxy-firing conditions the CPU's
noncondensable vent is the only source of
air emissions. As a result, the emissions
from an oxy-combustion plant are stellar;
there are essentially no particulate,
mercury, heavy metal, HF, HCl, NOx
SOx emissions (see table). Only a small
amount of NO, CO, and CO2
or
is emitted to
the atmosphere.
B&W and AL have made significant advances
in improving the efficiency and economics
of oxy-combustion. Combining the
advancements made by the two companies
has resulted in improvements to plant efficiency
and economics (Figures 5 and 6).
In sum, the B&W/AL warm flue gas
recycle design promises noticeably higher
efficiency and improved economics compared
with other carbon capture technologies
when using DOE data. When
ultrasupercritical boiler designs become
available, the efficiency and economics
of oxy-combustion will be comparable to
those of today's state-of-the-art air-fired
supercritical power plants.
Oxy-Coal Demonstration Plant
Figure 7 illustrates all of the major components
of the B&W/AL 150-MW gross (about
100 MWe net) oxy-coal demonstration plant
design. The design uses the warm flue gas
recycle process, in which the secondary recycled
gas (shown with green flues) has only
particulates removed, while the remainder of
the flue gas (blue flues) has SOx
, particulates,
and moisture removed. The plant includes a
regenerative air/gas heater with a special internal
arrangement to eliminate oxygen loss
and a unique direct contact cooler that removes
and makes use of the water condensed
54
Comparison of oxy-combustion air emissions to those from air firing
and IGCC. Note that the oxy-combustion process will produce negligible emissions of common
air pollutants. Source: Babcock & Wilcox Power Generation Group and Air Liquide
Air-fired
plant
Fuel type
Steam conditions (psi/F/F)
Plant performance
Power (gross MW)
Power (net MW)
Heat rate (net Btu/kWh)
Capacity factor (%)
Expected emissions
NOx (lb/MBtu)
SOx (lb/MBtu)
Particulate (lb/MBtu)
Hg (lb/TBtu)c
Expected CO2 emissions
CO2 removal efficiency (%)
CO2 produced (mmt/yr)
CO2 captured (mmt/yr)
CO2 released (mmt/yr)
3,600
598
550
8,662
85
0.060
0.040
0.015
0.784
3.26
3.26
/1,100/1,100
745
556
10,505
80
0.048
0.010
0.007
0.571
90
3.64
3.28
0.36
IGCC plant
with CCSb
Oxy-fuel plant
with CCS
3,600/1,100/
1,100
733
550
10,143
85
Note a
Note a
Note a
Note a
92.5
3.82
3.53
0.29
Air-fired
plant
3,600/
604
550
9,250
85
0.060
0.080
0.012
0.820
3.68
3.68
1,100/1,100
Oxy-fuel plant
with CCS
Bituminous Bituminous Bituminous Subbituminous Subbituminous
NA
3,600/1,100/
1,100
733
550
10,831
85
Note a
Note a
Note a
Note a
92.5
4.31
3.99
0.32
Notes: CCS = carbon capture and sequestration, MBtu = thousand Btu, mmt = million metric tons, NA = not applicable.
a. Below practical measurement limits.
b. Integrated gasification combined cycle (IGCC) is based on the General Electric IGCC system with CO2 capture per
DOE/NETL-2007/1281 Report, Case 2.
c. Air-fired emissions based on 90% removal expected.
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POWER | January 2011
4. Reducing auxiliary loads. Compared with the state of the art in 2000, the energy
needed for air separation has been progressively reduced. AL has improved the air separation
unit (ASU) design to reduce its energy consumption. By optimizing the ASU for oxy-combustion
conditions, this " low energy " ASU design produced a 7.5% reduction in power consumption:
from the 200 kWh/ton baseline to185 kWh/ton. Further process and technology improvements
(the " extra low energy " design) reduced separation energy a cumulative 17% from the baseline,
to 166 kWh/ton. Adding heat integration into the plant steam cycle added another 7% improvement
for a cumulative reduction of 24% below the baseline, to about 150 kWh/ton. AL is ready
to demonstrate this design, and additional work is under way to achieve further reductions by
2015. In addition, the CPU is designed to achieve a low specific energy, on the order of 120 kWh/
ton. Power reduction and the benefits of heat integration are major factors enabling the B&W/AL
oxy-combustion design to achieve significant efficiency and cost advantages over other carbon
capture technologies. Source: Babcock & Wilcox Power Generation Group and Air Liquide
260
240
220
200
180
160
140
120
100
238
200
185
7.5%
24%
166
152
140
17%
Pure oxygen
2000
Integrated
gasification
combined cycle
2000
Oxy combustion
" low energy "
2010
Oxy combustion
" extra low energy "
2010
Target
2015
Separation energy (kWh/ton)

POWER January 2011

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