Page 40 - Boiler_Operators_Handbook,_Second_Edition

Operating Wisely 25

carbon in the fuel and lighter as hydrogen increases. We use different adjectives for combustion in-
There are other factors but let’s just discuss simple com- cluding partial, perfect, complete, and incomplete to
bustion first. describe different results when burning fuels. Partial
If you were ever in the Boy Scouts, you were combustion means we burned part, but not all, of the
taught the fire triangle. To create a fire you need three fuel. Incomplete combustion is basically the same but
things, a fuel, air, and enough heat to get the fire going. the difference is we intentionally have partial combus-
You also probably discovered that you can stack up a tion and incomplete combustion is undesirable. Perfect
campfire (you’ll discover I love campfires) using pieces combustion is an ideal condition that is almost never
of wood about four inches in diameter and over a foot achieved. It’s when we burn all the fuel with the precise
long and even though you have a lot of fuel there with amount of air necessary to do so. Of course we engineers
air all around it you can’t start the darn thing with a have to use a fancy word to describe that condition, and
match. Obviously there’s fuel and air so the problem is it’s “stoichiometric” combustion. Complete combustion
not enough heat. To get that fire going you have to have burns all the fuel but we always have some air left over.
some kindling, smaller and lighter pieces of fuel that Every fuel has its air-fuel ratio. That’s the number
will continue to burn once you heat them with a match of pounds of air required to perfectly burn one pound
and they produce more heat to light those big sticks you of fuel. The air-fuel ratio of a fuel is principally depen-
put on the campfire. dent on the ratio of carbon to hydrogen in the fuel, the
Once the fire gets going, the heat generated by amount of hydrocarbon in the fuel, and, to a lesser de-
those big sticks burning is enough to keep them going gree, the air required to combine with other elements in
and light more big sticks as you stack them on the fire. If the fuel. Note that this is a mass ratio, not related to vol-
you pull the fire apart, isolating the big sticks from each umes, but it can be converted to a volumetric ratio (cubic
other, the fire will go out. Now we have a very good les- feet of air per cubic foot of fuel) provided we specify the
son on the relationship of fuel and heat in a fire. As the conditions of pressure and temperature to define the
fuel burns it generates heat and some of that heat is used density of the fuel and air. The air-fuel ratio for a fuel
to keep the fuel burning and some is used to start added can be determined from an ultimate analysis of the fuel
fuel burning. When the fire is compact, where a good (Appendix L, page 380).
portion of the heat it generates is only exposed to the fire The air required for the fuel is not consumed com-
and more fuel the fire will be self supporting. If the fire is pletely, only part of the air is used, the oxygen. I’m sure
spread out where all its heat radiates out to cold objects you know that atmospheric air, the stuff we breathe,
the fire will go out. contains about 21% oxygen by volume. We engineers
The fuel in the furnace of a boiler burns at tem- get more precise and say it’s 20.9% but for all practical
peratures in the range of 1200 to 3200°F which is usu- purposes 21% is close enough. What’s in the other 79%?
ally more than enough to keep it burning and heat up It’s all nitrogen, what we call an “inert” gas because it
any new fuel that’s added to the fire. Modern furnaces, doesn’t do much of anything except hang around in the
however, are almost entirely composed of water-cooled atmosphere. When we get to talking about the air pollu-
walls which absorb most of the radiant heat of the fire. tion we create when operating a boiler you’ll discover
Despite that high temperature a fire in a modern boiler it isn’t entirely inert. That little tenth of a percent we
is barely holding on and it doesn’t take much to put it engineers consider contains a lot of gases, mostly carbon
out. That’s why we need flame detectors, which are cov- dioxide, that don’t really do anything in the process of
ered in a later chapter. combustion either so we can say they’re inert.
All of our fuels are principally hydrocarbons, It’s a good thing that air has that 79% nitrogen
material containing atoms of hydrogen and carbon in because it absorbs a lot of the heat generated in the fire
various combinations with varying amounts of other el- and limits how fast that oxygen can get to the fuel. It’s
ements. The reason hydrocarbons are important is they considered a moderator in the process of combustion
release energy in the form of heat when they burn. We because it keeps the fuel and oxygen from going wild;
call the burning of the fuel the “process of combustion.” without it everything would burn to a crisp in an awful
That’s because we engineers have to use big words, we big hurry.
say combustion instead of burning to give the action a You should recall an incident in the early days of
name, burn is a verb, combustion is a noun. It really isn’t the manned space flight program where three astro-
that complicated a word and most operators have no nauts were burned to death in a capsule during a test
problem using it. while sitting on the ground. At that time they were us-