Page 44 - Boiler Operator’s Handbook

Operating Wisely 29

fully, learn from my mistakes and not get as frustrated At either end of the flammable range, which we
as I have trying to get a fuel to burn. There are two rules. also call the explosive range, are the two limits of flam-
First, the fuel and air mixture has to be in the flammable mability. The one where flammability will be lost if we
range and secondly, you need a fuel rich condition to add any more air is called the lower explosive limit, LEL
start. The hard part for those of us designing and build- for short. The one where too much fuel prevents sus-
ing boiler plants is to make certain we have those condi- tained combustion is called the upper explosive limit,
tions. UEL. If you think about it, it’s essential that we have this
What’s the flammable range? It just happens to flammability range. Otherwise the sticks would burn as
be the same thing as the explosive range. It’s a range you carried them back to put on the campfire; actually
of mixtures of fuel and air within which a fire will be everything would burn up. On the other hand, that nar-
self supporting, not requiring added heat to keep the row range of mixtures keeps me humble and could do
process of combustion going. To be perfectly honest the same to you. It isn’t as easy to get a fire going in a
with you, every time we fire a burner we’re producing furnace when you consider that you have to get the fuel
an explosive mixture of fuel and air. It doesn’t explode and air mixture within that narrow range. You get to by-
because it burns as fast as we’re creating it. If it doesn’t pass most of the experiences we engineers have because
burn and we keep creating that mixture the story is a lot we make sure it works before you get your hands on it.
different. Eventually something will produce a spark or Getting the mixture in the flammable range isn’t
add enough heat to start it burning. Then the mixture the only criteria when it comes to combustion in a boiler
burns almost instantly and it’s that rapid burning and furnace. The only way that flame will burn steady and
heating to produce rapidly expanding flue gases that we stable is if it begins at the UEL. In other words, the point
call an explosion. where ignition begins is where the fuel and air mixture
A graphic of a typical fuel’s flammability range is pass from a really fuel rich condition into the explosive
shown in Figure 1-9. At the far left of the graph is where range. I can still recall looking through the rear observa-
we have a mixture that’s all air, no fuel. On the far right tion port into a furnace full of pulverized coal and air,
is where we have all fuel and no air. The quantities of so much that it looked like a fog in there. I could see the
fuel and air in the mixture vary proportionally along the bright flame of the oil ignitor burning through the fog
graph as indicated by the two triangles. The thin line but the darn coal wouldn’t light! Needless to say I was
in the middle of that band is the stoichiometric point, very uncomfortable looking at that mixture of fuel and
the mixture that would produce perfect combustion. air and wondering whether it might suddenly light.
Mixtures to the left of the stoichiometric point are called Many a boiler failed to light because there wasn’t
lean mixtures because they have less fuel than required that fuel rich edge right where the ignitor added the
for perfect combustion. They can also be called air rich. heat to light it off. Usually it’s due to the mixture be-
Mixtures to the right are called fuel rich because there is ing too fuel rich and the ignitor not reaching the point
more fuel in the mixture than that required for perfect where the UEL is to get things started. In other situa-
combustion. Keep in mind that we’re looking at pounds tions the fire is lit and the heat from the fire manages to
of air and pounds of fuel, not volume. The flammability force ignition into the fuel and air entering the furnace
range is the shaded area and it’s only within that narrow until the fire reaches a point that’s way too fuel rich
range of mixtures that a flame will be self sustaining. and the fire goes out. Then, because the furnace has
some heat, the fuel and air mix again to reach the flam-
mable range and the mixture lights again and burns
back toward the burner again. We call it instability, you
typically call it “run like hell.”
Here’s where I always tell boiler operators that you
shouldn’t always do what you see the service engineer
doing. It’s standard practice for service engineers to
manually control the fuel going into the furnace when
lighting a burner they just adjusted. They do it because
they aren’t certain about the mixture and have their
hand on the valve to control it, usually shutting the
burner down faster than the flame safety system would.
Figure 1-9. Flammability range Once they get it right, they usually let it light off the