POWER June 2014 - 46

GAS-FIRED GENERATION
5. Fast starts. NEM USA's DrumPlus HRSG design, shown here at NRG's El Segundo Energy
Center, allows for fast starts and rapid cycling with no reduction in life expectancy. Source:
POWER/Tom Overton
MHPS H-50 turbine was launched in 2010,
and the company plans to commercialize it
this year. The H-50 turbine with AHAT outperformed
the same turbine in combined cycle
mode, achieving 70 MW output at 50.6%
efficiency. MHPS believes efficiencies above
60% are achievable with larger turbines.
MHPS is also developing a related retrofit
product called Smart AHAT, which involves
adding significant steam injection to a combined
cycle arrangement, with AHAT's water
recovery system added to the exhaust.
New HRSG Technology
HRSG manufacturers have also been working
to meet the demand for more flexible
operations. NEM USA's DrumPlus design
is engineered to combine the advantages of
drum-type HRSGs with the responsiveness
of once-through design.
In the DrumPlus, the drum is replaced by a
knock-out vessel with external separator bottles.
The smaller drum has a relatively thin wall
and is thus subject to lower thermal stresses
with changes in output. The reduced volumes
of both steel and water give the DrumPlus the
dynamic capabilities of once-through steam
generation, as well as the increased lifetime.
These lower stresses eliminate the need for
hold points on the gas turbine during startup,
which allows faster startups, more cold starts,
and more rapid load changes. DrumPlus
HRSGs are able to handle 10-minute startups
with no reduction in life. The El Segundo
plant shown in Figure 1 employs a DrumPlus
HRSG design (Figure 5).
Alstom is also offering HRSG designs for
update to its turbine line with the 60-hertz
M501JAC (Figure 4). MHPS's steam-cooled
J-series turbines, which operated at temperatures
of 1,600C, were introduced in 2011 and
have been deployed mostly in Asia, with several
plants coming online in 2013 and 2014.
The M501JAC adds an optimized aircooled
combustor from the M501GAC
model and offers output of up to 450 MW in
combined cycle mode at better than 61% efficiency.
The cooling holes in the turbine are
also optimized for reduced gas temperatures.
The M501JAC offers improved operational
flexibility, such as by shortening the starting
time while maintaining the same level of performance
as the M501J. First shipments are
expected in 2015.
New Approaches to Simple Cycle
Not all of the action is in combined cycle.
46
MHPS is developing an approach to simple
cycle turbine generation that could potentially
equal or exceed combined cycle generation
in efficiency. The technology, which
is currently being commercialized for release
later this year, is called AHAT, or advanced
humid air turbine. AHAT takes a simple cycle
turbine and uses humidified compressed
air for combustion. The combustion air is
cooled by water atomization, compressed in
the compressor, and then passed through a
humidification tower. The humidified air is
then heated in a heat exchanger using the turbine
exhaust before entering the combustor.
The water vapor in the exhaust is then recovered
and returned to the humidifier.
The method is similar to steam injection
but adds far more water to the combustion
process. MHPS has been developing the technology
since 2000. A pilot project using an
www.powermag.com
increased cycling. The Alstom OCC approach
employs reduced header thickness-to-tube
thickness ratio, single-row harps, and finned
tubes with no bends. These changes reduce
thermal stress by reducing areas of temperature
difference and adding thermal flexibility
to areas of the HRSG that will experience rapid
changes in temperature with faster cycling.
Nooter/Eriksen is developing several options
for increased cycling and faster startup.
These include the use of stronger materials,
multi-drum designs, and thinner drum walls.
Looking Ahead
This highly competitive market is sure to
continue evolving. The demands from increased
renewable generation are certain to
increase pressure on the gas turbine fleet to
become even more flexible and responsive,
both in upgrades to existing plants and in
new plants yet to be constructed. Whatever
your role, these are definitely " interesting
times " for natural gas. ■
-Thomas W. Overton, JD is a POWER
associate editor (@thomas_overton, @
POWERmagazine).
POWER | June 2014

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POWER June 2014

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