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228 Boiler Operator’s Handbook
to block valves and regulators resulting in dangerous North Africa which has an air-fuel ratio ten percent
operating pressures. higher than domestic natural gas. LNG is compressed
When water separates from the oil in storage tanks and cooled until it becomes a liquid; is loaded into tanks
it settles to the bottom. It will eventually accumulate aboard ships built exclusively for the purpose; then
until, all of a sudden, you find yourself trying to burn transported across the Atlantic Ocean to special port
water. Water in fuel oil also provides a medium for cor- facilities near Boston and Baltimore among others.
rosion of the fuel tank and piping. It’s one of the reasons Ash in the fuel, whether it’s coal, oil, or biomass
for leakage of underground storage tanks (USTs) with can create problems with firing. The ash fusion point is
some serious consequences. Water can be emulsified (a the temperature at which the ash melts. If furnace condi-
process that mixes the fuel and water distributing water tions produce higher temperatures the ash will melt then
throughout the oil) but it can still produce corrosion and solidify again when it cools, usually forming large accu-
will always require the addition of latent heat to vapor- mulations of solidified ash that can block air or gas flow
ize it in the furnace. passages or grow in the upper portions of the furnace.
Small and controlled quantities of water emulsi- They grow until they get too heavy to maintain their ad-
fied in oil can help reduce soot formation which can hesion to the tubes or refractory and fall crashing to the
improve heat transfer to the degree it compensates for bottom of the furnace doing damage to tubes, grates, etc.
the latent heat loss. When I was sailing for Moore Mc- When firing fuels with a low (less than 1800°F) ash fu-
Cormack Lines in the 1960’s we were conducting an ex- sion temperature the operator has to monitor the furnace
periment with injecting small quantities of superheated conditions inspecting it and recording draft readings to
steam into the fire to reduce sooting. I never did find out detect hardened ash accumulations early.
what the results of that were. One of my projects included burning dust from a
Water in fuel gas systems can be a considerable laminate sanding operation where portions of the ash
problem when the gas pressures are low because it can had very low fusion temperatures. We operated that
collect and produce blockages in the piping as well as boiler at very high excess air just to keep the furnace
promote corrosion. When you have wet fuel gas you’ll temperatures down to prevent the ash melting and
have additional requirements for handling the liquids sticking to the tubes. Someone advised that customer
that settle in your piping because there can be liquid they could save a lot by decreasing excess air (true in
fuels as well as water. Water draining from a coal pile is other situations) so they did; and ended up with huge
highly corrosive and must be discharged to a sanitary globs of solid ash stuck to the furnace walls and tubes.
sewer after it is neutralized. You should always know what the vanadium con-
The discussion in the chapter on combustion tent of your fuel is because that material produces a lot
helps explain why firing conditions change when the of low melting point ash. Just last week I spent a Satur-
fuel changes. Most of the time the air-fuel ratio is close day evening crawling into a boiler to see the result of
enough to ignore the variations. When a service techni- blockage due to low melting point ash. The customer’s
cian uses a portable analyzer to calculate combustion fuel oil only had about 30 ppm of vanadium in it but was
efficiency that analyzer contains a “typical” fuel analysis enough to completely block up the first pass of the boiler
for the fuel and determines efficiency based on that typi- with ash that took about two days to clean with a high
cal analysis. I’ve always wondered if those analyzers pressure washer.
are calibrated for the area because the carbon content
of natural gas can vary from 20.3% to 23.5% between
the east and west of the country. That amounts to a 15% FUEL GASES
variation in higher heating value of the fuel and it’s one
reason I refuse to believe the efficiency on one of those Natural gas is mostly methane (CH ) with portions
4
machine’s printouts. It’s only important that you know of other flammable gases, oxygen, carbon dioxide, and
that the analysis can change and have an equal distrust nitrogen. A typical volumetric analysis is 96.53% meth-
of those electronic analyzers’ efficiency indications. ane, 2.38% ethane, 0.18% propane, 0.02% iso-butane,
In the Baltimore area we can experience changes 0.77% carbon dioxide, and 0.12% nitrogen. That’s east
in natural gas depending on the source of the gas in coast gas. Gas constituents will vary depending on the
Pennsylvania, Texas or Louisiana and the blending of well the gas came from. When a boiler is fired with oxy-
gases from those sources. We also have a chance to burn gen trim controls to achieve very small quantities of ex-
some of the LNG (liquefied natural gas) imported from cess air those controls accommodate the varying air-fuel