Page 245 - Boiler_Operators_Handbook,_Second_Edition
P. 245

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
   240   241   242   243   244   245   246   247   248   249   250