POWER August 2010 - 43

ADVANCED MATERIALS
The higher efficiency would also reduce CO2
emissions by more than 30%, compared to
a 35% efficient plant, and the resulting
decrease in CO2 emissions per MWh of electrical
output could be achieved at a cost of
about $20 per avoided metric ton of CO2,
which is far less than the avoided cost of
employing CO2 capture and storage.
countered. Nickel-based superalloys are,
however, quite expensive relative to steel,
and any power plant incorporating these
alloys must limit their use to the most critical
components. Figure 2 illustrates the
stress/temperature tradeoffs for a variety
of materials.
Overall, the scope of work involves developing
a conceptual design and evaluating
aspects of the candidate materials in
seven areas: mechanical properties, steamside
oxidation, fireside corrosion, welding,
fabricability, coatings, and changes to current
design codes.
Preliminary Conceptual Design and
Economic Studies. Working with the four
major boiler manufacturers, the project team
developed two conceptual designs of 750MW
boilers and established heat balance diagrams.
Dimensional requirements have been
estimated for tubing, headers, and steam
pipes. Based on metal temperature calculations,
possible materials were selected for
various sections of the boiler.
An engineering-economic analysis indicated
that a 750-MW plant operating at
1,400F steam conditions could achieve a thermal
efficiency of 45% (HHV basis) for single
reheat and 47% for double reheat. Compared
to the current domestic subcritical fleet operating
at 37% efficiency, the A-USC plant
could save nearly $16.5 million annually in
fuel and balance-of-plant costs, and $330
million over a 20-year plant life. The higher
efficiency would also reduce CO2
emissions
by more than 30%, compared to a 35% efficient
plant, and the resulting decrease in
CO2
than the avoided cost of employing CO2
emissions per MWh of electrical output
could be achieved at a cost of about $20 per
avoided metric ton of CO2
, which is far less
capture
and storage.
In ongoing work, the project team is revisiting
conceptual designs with updated
2. Many material options. Materials selection options for advanced ultrasupercritical
alloys, especially boiler superheater and reheater tubing strength, are a function of operating
pressures and temperatures. Source: EPRI
1,600
Haynes 230
1,400
IN 617
Super 304H
1,200
T91
T23
T22
1,000
3,000
August 2010 | POWER
4,000
Pressure (psig)
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5,000
6,000
304 H
HR3C
T92
Haynes 282
IN 740
material property information and input
from the steam turbine studies. In addition,
the team is evaluating the potential
for a conceptual design employing oxyfuel
combustion technology. This technology,
also known as oxycombustion,
involves burning fuel in the presence of
essentially pure (95% to 99.5%) oxygen,
rather than air, and recycling flue gas to
the combustion chamber. The advantages
are that, compared to a conventional power
plant, the carbon dioxide can be captured
at much lower cost and the amount of nitrogen
oxides is significantly lower.
Mechanical Properties. Following the
conceptual design, six alloys were initially
selected, based on creep strength:
■ One ferritic steel (SAVE12) for furnace
tubing and piping.
■ Two austenitic steels (HR6W and Super
304H) specifically for superheater/reheater
tubes.
■ Three nickel-base alloys (Haynes 230,
CCA 617, and Inconel 740) for tubing as
well as heavy-wall piping.
Haynes 282 was added at a later stage.
The relative strength of the various candidate
alloys has been measured based on
creep-rupture tests, with some tests extending
to as long as 30,000 hours (3.4 years).
Following testing, two alloys (SAVE12
and HR6W) were eliminated when they
did not meet initial strength projections.
Creep-rupture tests also showed that
Inconel 740 is the strongest alloy capable
of operating at temperatures approaching
1,400F, followed by Haynes 230 and CCA
617. A new nickel-base alloy material,
Haynes 282, has also shown the requisite
strength to meet the 14.5 ksi target. However,
because Inconel 740 and Haynes 282
are not code approved, extensive creeprupture
studies are being conducted to
evaluate heat-to-heat variations in strength,
product form variation, stability with time,
and heat treatment. Rupture data, aimed at
durations up to 50,000 hours, is currently
being generated for two commercial heats.
Testing of a third heat of Inconel 740 has
been initiated, because a minimum of three
heats are required for code acceptance.
Steamside Oxidation. These studies
were conducted to determine the steamside
oxidation behavior and temperature limits
of the materials and to understand the effect
of variables such as alloying content
and environmental conditions. Testing has
been completed at 1,200F for durations up
to 4,000 hours. Results show that, among
the ferritic steels, two new steels (MARB2
and VM12) display the best oxidation be43
Maximum
midwall temperature (F)

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POWER August 2010

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