Page 377 - Boiler_Operators_Handbook,_Second_Edition
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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
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