Page 376 - Boiler_Operators_Handbook,_Second_Edition

Plants and Equipment 361

GAS TURBINES, ENGINES AND HRSGs two or more engines so steam generation can be main-
tained. Frequently there’s an auxiliary burner installed
This is just a small amount of information about somewhere to provide additional heat or fire the boiler
the other types of cogeneration plants you may encoun- when the engines are shut down.
ter. Once again, it’s the manufacturer’s instructions that The auxiliary burner in those applications
are going to be most valuable in developing your op- shouldn’t be a conventional boiler burner. I saw such an
erating knowledge of these plants. I have to admit I’ve application a few years ago with the conventional burn-
never operated or even studied one of the land based er’s inlet fitted with another fan to produce the static
plants that cogenerate, all I’ve done is visit some of pressure that matches the engine exhaust pressures and
them. Each is a little unique so once again the instruction overcomes the pressure drop through the boiler, econo-
manuals provide critical guidance. mizer, and stack. That little burner looked somewhat ri-
Just because the fuel isn’t burned in a conventional diculous with the fan blowing into it and I’m not certain
boiler furnace doesn’t mean a boiler operator can’t it won’t blow apart under the pressures it is subjected to.
handle it. The combustion chemistry doesn’t change, The principle difference in engines and turbines,
all the formulas stay the same, we’re still simply burn- as far as combustion is concerned, is that engines are
ing hydrocarbons to release heat. Gas turbines and gas typically fired fuel rich to keep them cool and turbines
engines burn the fuel and some of the thermal energy is are fired air rich. There’s a catalytic converter on your
extracted from it to generate power. They’re not a lot, if car because the engine would get too hot if the fuel was
any, more efficient than a utility steam plant so there’s burned completely. By running the engine fuel rich the
heat left over to make steam. There are many engine combustion products are much cooler, you don’t get
generator plants with waste heat boilers in this coun- that extra 10,000 Btu from complete burning of the car-
try that have been operating for more than thirty years bon. The catalytic converter combines the exhaust with
and gas turbines aren’t as new as some people think. In the air added by the air pump to burn off the carbon
many cases all that’s new is a way of putting equipment monoxide later, after the energy produced by the initial
together and a HRSG is a prime example of that. combustion has been converted to mechanical power to
Unless you’re a rare individual you have general drive the car. The catalyst simply provides a source of
knowledge of how your automobile engine works. If certain ignition of the lower temperature exhaust gas
you’re like me you also know that modern technology and air mixture to ensure more complete combustion.
has limited working on it to someone with computers It raises the gas temperature in the process to waste the
that can talk to the computer in the car but that doesn’t heat out the tailpipe. It isn’t perfect or complete because
mean the combustion process is any different. Most elec- a little CO manages to sneak by a converter but it does a
tric power generating engines work the same, using ei- pretty good job. You will find some engine cogeneration
ther the Otto or Diesel cycle to convert energy in fuel to plants operating with catalytic converters, more for NO
x
output power at the engine shaft which drives the gen- reduction than CO reduction.
erator. Otto is the guy that came up with the four cycle Gas turbines, on the other hand, used to use lots
engine, the scheme of intake, compression, ignition and of excess air to absorb the heat of combustion and lower
exhaust. A diesel engine can be two cycle or four cycle the operating temperatures so the turbine blades don’t
but most are four cycle with the only difference being melt. The high volumes of excess air make it difficult to
Otto used a spark to ignite the fuel. Fuel is injected into a get complete combustion but providing cooling water
diesel engine right before ignition and ignites spontane- to spinning turbine blades is virtually impossible. New
ously in the hot compressed air. techniques and construction are changing the form of
The water jacket surrounding the engine’s cylin- gas turbines to allow lower excess air. Once scheme now
ders don’t absorb much heat compared to a boiler so used is to bleed air or steam through holes in the lead-
there isn’t much energy recovered by the water jacket. ing edge of the first row of turbine blades to create a film
I’ll admit I’ve heard some of them called cogenerators of cooling air or steam flowing back over the blades.
but I really don’t consider them as such. Some plants use Designs of power generating gas turbines are evolving
the heat of the jacket for building heat and other purpos- rapidly so you will have to read to keep up with what’s
es but most of the energy remains in the exhaust gases. happening with them.
Cogeneration plants using engines have a waste heat Gas turbines are not a new thing. I said that a bit
boiler that recovers the energy in the engine exhaust ago didn’t I? Anyway, gas turbines are at least sixty
gases. The boiler or boilers are commonly manifolded to years old. Every jet plane flying is powered by gas tur-