Page 43 - Boiler Operator’s Handbook

28 Boiler Operator’s Handbook

re-refined by different processes to make more gasoline molecules of oxygen consist of four atoms so its weight
to satisfy our love for driving around in automobiles. is 32 (4 × 8). The CO is 12 + 2 × 8 and the two water mol-
2
The basic process of separating the different compo- ecules are twice (2 × 1 + 8). The law of conservation of
nents from crude oil is distillation where the oil is heated mass means that we should have as much as we started
until the lighter portions including naphtha, gasoline, with and, sure enough, 16 + 32 is 48, the same as 28 +20.
and others evaporate. We engineering types use this business about
A good portion of our kerosene and light fuel oil weights to get an idea of the amount of energy in the
(Number 2) is produced by distillation. Some of that and fuel. Remember earlier we said we could make 14,096
heavier parts of the crude oil are heated further and ex- Btu for every pound of carbon we burned? Well, in the
posed to catalysts (materials that help a reaction occur) case of methane 12/16ths of it is carbon, and that will
to “crack” them, breaking more complex hydrocarbons provide 10,572 Btu per pound of CH (12 ÷ 16 × 14,096).
4
down into lighter, less complex ones. That’s what hap- Similarly, the 4/16ths of hydrogen will produce 15,257
pens when the fuel is exposed to the heat of the fire, Btu (4 ÷ 16 × 61,031). Add the two values to get a higher
it’s distilled and cracked until it becomes very simple heating value of methane of 25,829 Btu per pound. Now
hydrocarbons that readily react with air to burn. It’s I know that doesn’t meet with your understanding of
argued, with some degree of accuracy, that only gases how we normally measure the heating value of natural
burn and the heat has to convert the fuel to a gas before gas. We say natural gas produces about 1,000 Btu per
it will burn. All that distillation and cracking takes some cubic foot, right? That’s because it’s always measured by
time and that’s why a fuel doesn’t burn instantly once volume, in cubic feet. However, the measurement is also
it’s exposed to air. always corrected for the actual weight of the gas because
Now let’s try something just a little more compli- it’s the mass that determines the heating value, not the
cated. Let’s burn the major portion of our natural gas. volume.
It’s mostly methane, which is represented by the for- Whenever an engineer wants to know exactly how
mula CH . The same rules for formulas apply. To burn much flue gas will be produced by a fuel, precisely what
4
the methane we need a couple of oxygen molecules, O the air to fuel ratio is for that fuel, and how much energy
2
from the air. One molecule of the O combines with the we’ll get from the fuel we ask for an “ultimate analysis”
2
carbon to form CO and the other combines with the of the fuel. That analysis tells us precisely how much
2
four hydrogen atoms to make two molecules of H O. carbon, hydrogen, sulfur, etc. is in the fuel. An ultimate
2
The equation is: analysis also includes a measure of the actual heating
value. The worksheet in the appendix on page 382 is
CH + 2 O => CO + 2H O used to determine the amount of air required to burn
4 2 2 2
16 32 28 20 a pound of fuel and some other information we use as
engineers.
The numbers under the groups of molecules in the I still haven’t really explained why the big sticks
equation represent the atomic weights of the different on that campfire didn’t start burning right away. In
molecules. I’m sure you know that metals have different addition to the fact the big heavy stick sucks up all the
weights, aluminum being a lot lighter than steel so you heat from the match without its temperature going high
can easily agree that carbon, hydrogen, and oxygen have enough for it to burn it has to do with something we call
different weights. You’ll also be pleased to know that flammability limits. If you add enough heat to any mix-
even I don’t remember the atomic weights, it’s not nec- ture of air and fuel some of it will burn. What we really
essary to, so you can relax, you don’t have to remember have to do is come up with a mixture of air and fuel that
the numbers, only the concepts. Atomic weights have will not only burn, but will produce enough heat in that
no units, they’re all relative with oxygen assigned an process that it will continue to burn. I really wonder if
atomic weight of 8 as the reference because it’s the stan- I’ll ever stop finding situations where I can’t get a fuel
dard we use to measure molecular weights. Hydrogen and air mixture to burn. After fifty years in the business
has an atomic weight just slightly more than one and you would think I could always get a fire going, not just
we use one because it’s close enough for what we’re do- campfires, fires in a boiler furnace. Throw in enough
ing. Carbon has an atomic weight of twelve and that’s heat and some fuel and air and it should burn, right?
all we need to see the total balance of the combustion Well, I can honestly say “no” because I’ve been through
equation for methane. One carbon plus four hydrogens several bad times trying to get a fire going with no suc-
gives methane a molecular weight of 16 (12 + 4). The two cess. This is one of those situations when you can, hope-