Page 245 - Boiler_Operators_Handbook,_Second_Edition
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230 Boiler Operator’s Handbook
If you’re firing digester gas you usually will have time information to a control system on a boiler to burn
a responsibility to monitor the digester itself. A little those gases. Here again, you’ll need to read the instruc-
training on how those anaerobic bugs work and you’re tion manuals and will more than likely receive special
a wastewater plant operator as well. You’ll quickly learn training for operating a boiler burning those gases.
that if you don’t burn the gas in the boilers and allow All too often gas is taken for granted. You just as-
it to escape to the atmosphere everyone in the neigh- sume it will continue flowing out of the pipeline. The
borhood will be complaining about the odor. When a gas flow can stop if a line ruptures, a compressor station
boiler plant can’t burn all the digester gas or the boiler breaks down or has a fire or other emergency, or some-
plant is temporarily shut down for maintenance the gas one burning gas near you has a failure. We also have to
is usually burned off using a flare (Figure 7-1). You’ll stop burning gas when we’re on an interruptible gas
find yourself responsible for the flare too, but it’s only a service. If we don’t the owner will pay a serious fine for
burner without a furnace and boiler around it so it isn’t burning gas.
that difficult to handle. Some older plants had “gas holders” expandable
Landfill gas is very much like digester gas. The tanks that used the tank weight to pressurize the gas in
anaerobic bacteria work on the garbage in the dump (a storage. You probably can recall seeing one on some city
landfill is, after all, nothing more than a well maintained skyline in the past. Those gas holders provided a source
garbage dump) to generate the gas. There are some of gas in case of an emergency. Utilities use mines where
potential problems with landfill gas that are not encoun- they compress the gas for storage and there’s liquefied
tered with digester gas. The carbon dioxide content can natural gas storage facilities in a few spots in the coun-
vary more (over extended periods of time) and air can try. Regardless of all these provisions most of us have to
leak in through breaks in the cover of the landfill. The be prepared for an interruption in the gas supply.
gas will also vary in mix of fuel gases because the gar- Being able to burn one of the LPG choices is one
bage in the landfill is not consistent. way to have a standby provision in the event the gas
Refineries produce a variety of gases with various supply fails. LPG is expensive and a storage facility
blends which have different heating values and air fuel capable of providing any extensive operation of a boiler
ratios. I remember the familiar sight of flares burning off plant is very expensive so few plants use that option.
those gases but problems with hydrocarbon emissions Most of the time we use fuel oil as a backup to loss of our
from those flares and the waste of energy combined natural gas supply. Either LPG or fuel oil will be stored
with modern technology that allows us to burn them on site for interruptions to a natural gas supply regard-
efficiently has reduced their numbers and use. Control less of the reason for the interruption.
systems that continuously measure the heating value and
combustion air requirements of the gases can provide real
FUEL OIL
Fuel oils are identified by ASTM specification D-
396-62T which replaced the Pacific Specifications (now
obsolete) that originally identify the oils by a grade
number. Number 1 is basically kerosene and is seldom
used in boilers. The common fuel oils are grades 2, 4,
and 6. The term “grade” was dropped so now they’re
normally identified by the number alone.
Number 2 is called “light fuel oil” which is not as
dense as the others. Light fuel oil is basically the same
as diesel engine fuel. It has a typical heating value of
141,000 Btu per gallon, weighs about 7.2 pounds per
gallon and has an air-fuel ratio requirement of 16.394
pounds of air per pound of fuel that is approximately
equal to 218 cubic feet of air per gallon, 189 cubic feet
of air per minute per million Btuh. It is relatively clean
burning and has almost no ash. There is one common
Figure 7-1. Flare myth about Number 2 fuel oil, it is not a low sulfur oil. It