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362 Boiler Operator’s Handbook
bines. The first gas turbine powered ship, the Admiral path through the HRSG they pass several “pinch points”
Callahan, was powered by two airplane jet engines where the flue gas temperature approaches the satura-
which exhausted to another turbine that drove the ship’s tion temperature of the boiler or inlet temperature of the
propellers. Gas turbine plants that use that concept heat exchanger.
are now called ‘aero-derivative.’ The growing need for Many of the duct burners simply introduce fuel
improved efficiency, fostered by the deregulation of because the gas turbine at the inlet operates with very
electricity, has seen improvements in common shaft gas high excess air (300% to 400%) so there’s plenty of air for
turbines (basically a jet engine with a shaft sticking out the fuel. Some duct burners have air for the ignitors only
to drive the generator). and some have none, an unusual concept to some of us
A gas turbine consists of three basic parts. A com- old boiler operators.
pressor, burner(s), and turbine. The compressor draws
in atmospheric air and compresses it before supplying Microturbines
it to the burner. The burner mixes the fuel with the Microturbines are very small gas turbine genera-
compressed air and ignites it. The parts containing the tors with some unique differences. Most generators are
burner are protected from the heat of the burning fuel limited to a speed of 3,600 rpm so they can generate
by baffles cooled by some of the compressed air. The 60 Hertz electricity. In Europe the speed limit is 3,000
products of combustion and cooling air mix to provide a to generate 50 Hertz. A gas turbine is more efficient at
cooler product before entering the turbine. The turbine, higher speeds. Microturbines generate direct current
a reaction type, converts the heat energy to shaft power then invert the output with solid state electronics to
to drive the compressor (a large portion of the turbine produce alternating current. That way there’s no link
load) and a load. I have to say load because there are between speed and frequency so the turbine can be
some gas turbine driven pieces of industrial equipment; operated at the most efficient speed for the power it’s
but most of the time they’re used to power electricity generating.
generators. The manufacturers of these small independent
It’s the gas turbine and HRSG combination that power plants, some no larger than a typical desk set up
form the plants we now call ‘combined cycle’ power on one end, provide limited information about them.
plants. The HRSG (Heat Recovery Steam Generator) I’ve seen them sitting in a plant and making a little noise
could be modestly called a waste heat boiler but is much (they’re surprisingly quiet) while generating power but
more than that. It consists of a combination of all ele- that’s the limit of my experience with them. Some of you
ments of a modern boiler plant in a carefully matched may grow to learn a lot more when you have to try to
and packaged combination designed for maximum ef- operate them.
ficiency. With combined cycles utilities have been able Microturbines are an assembly line product with
to increase their efficiency to almost 50%! I should point common sizes being 30 kW and 60 kW. The largest
out that it’s a LEL efficiency so they’re still nowhere near I’m aware of is 250 kW. They also produce hot exhaust
the performance of the common heating plant. The basic which can be directed to a waste heat boiler but many
arrangement of a combined cycle plant is a gas turbine are used as emergency generators with no waste heat
followed by a HRSG which generates steam to power recovery.
a steam turbine with both turbines generating electric
power. Fuel Cells
What exactly is a HRSG? Why is it different than This is a product I haven’t seen in operation. It’s
a waste heat boiler? It’s because it is more than just a relatively new and I know of several plants that use
waste heat boiler. The typical HRSG is a combination them but have no experience with them at all. I do know
of things with the most common arrangement being a a little that I’ll share with you because, if you know
connecting duct for the turbine exhaust with an integral anything about combustion, you’ll discover that they’re
duct burner, superheater, reheater, high pressure boiler, given more credit than they really deserve.
economizer, low pressure boiler for deaerator steam Fuel cells do generate electricity without burning
which flows to the integral deaerator, and low pressure the fuel. That doesn’t mean they run cool. Some of them
economizer. The HRSG is designed to squeeze as much operate at very high temperatures. The concept is one
heat as possible at each section then follow with a lower of hydrogen and oxygen combining to make water by a
temperature boiler or heat exchanger that can absorb sort of reverse electrolysis. If you had chemistry in high
some of the heat that’s left. As the flue gases cool in their school one of the things you did, at least I did, was bury