Docstoc

Make Every BTU Count

Document Sample
Make Every BTU Count Powered By Docstoc
					Originally appeared in:                                                      October 1990 issue, pgs 217-225.
CHEMICAL ENGINEERING.                                                        Reprinted with publisher’s permission.




                                              Fired heaters
                                            and boilers have
                                            been made more
                                              efficient in the
                                            past decade, but
                                            further gains can
                                               be expected




Ashutosh Garg
                                           MAKE                                       variations to the burners of a fired
                                                                                      heater to + 2% of the mean air flow.
Kinetics Technology International                                                     Such flow modeling of flue ducts can


                                           EVERY
Corp., and                                                                            also help achieve uniform flue gas
H Ghosh                                                                               distribution in air preheaters, avoid-
Engineers India Limited                                                               ing the dead pockets that cause local-
                                                                                      ized dew point corrosion.

Although the world has been awash
with oil, recent events in the Middle
East show how tenuous that condition
                                            BTU                                       Firing Heavier Fuels
                                                                                      With improved methods, petroleum
                                                                                      refiners are extracting more products
                                                                                      from each barrel of crude oil, leaving

                                           COUNT
can be. Forecasters had predicted that
energy prices would be climbing by the                                                a much poorer quality residue to
middle of the decade, if not sooner,                                                  burn in heaters.
even without the spur of Iraq’s invasion
of Kuwait. Energy conservation is ex-                                                 Fuel specifications in one refinery
pected to again become popular and                                                    before and after recent expansions
                                           heat-recovery equipment and how
fuels for fired heaters to become heav-                                               are noted in Table 2. The differences
                                           they can be realized are listed in Table
ier, dirtier and more difficult to burn.                                              are typical of changes in fuel specifi-
                                           1.
Complicating matters, pollution control                                               cations.
laws will further restrict both gaseous    At the Heart of Fired Heaters
and solids emissions.                                                                 In conventional atomizers, fuels of
                                           When fuel was cheap, most fired heat-
                                                                                      inferior quality tend to convert oil
                                           ers were fitted with natural-draft burn-
Heat-recovery equipment is, of course,                                                droplets into small coke particles.
                                           ers, which were operated with high
vital energy conservation. In addition                                                The droplets burn slowly and are not
                                           excess air (30-50%) and long flames.
to seeing larger numbers of air pre-                                                  totally consumed by the flames. En-
heaters and waste-heat boilers, opera-                                                trapped in flue gas, the particles are
                                           They have always been bottom fired,
tions and maintenance supervisors and                                                 carried out through the stack. The oil
                                           with steam atomizing guns. Boilers,
engineers are likely to encounter im-                                                 fuel should be filtered if it contains
                                           on the other hand, were mostly fitted
proved burners and soot blowers in                                                    solids.
                                           with forced-draft mechanically atom-
fired heaters. They will also see many     ized oil burners that worked under
more exchangers for recovering low-                                                   Three things can be done to enable
                                           positive pressure. They have usually
level heat at faster paybacks. Potential                                              fired heaters to burn heavier fuels
                                           accurately scaled transparent model of
improvements in energy efficiency for                                                 more efficiently: (1) raise the pres-
                                           the ducts and windbox. One study
                                                                                      sure and quality of the atomizing
                                           based on such a model reduced flow
                                                                                      steam to improve atomization; (2)



  Furnace Improvements Services
  www.heatflux.com                             1
MAKE EVERY BTU COUNT



                                                                                      of cast-iron tubes, the recuperative
                                                                                      exchanger has been proven in service
                                                                                      as a combustion air preheater. The
                                                                                      cast tubes may have fins inside and
                                                                                      outside, and be located and be placed
                                                                                      at the outlet to avoid dewpoint con-
                                                                                      densation.

                                                                                      The heat transfer fluid in the circulat-
                                                                                      ing liquid preheater is warmed in the
                                                                                      fired heater’s convection section and
                                                                                      passes through a liquid to air ex-
                                                                                      changer, which heats the combustion
                                                                                      air. Such a unit is the preheater of
                                                                                      choice when space is limited (because
                                                                                      it does not need an induced-draft fan
                                                                                      and flue gas ducts) or when a heat
                                                                                      transfer fluid is available in the plant.
                                                                                      This preheater can also be used to
                                                                                      preheat process and utility streams.

mix the fuel oil and atomizing fluid      circulating-liquid system.                  Exchangers having a transfer surface
thoroughly; (3) and reduce the size                                                   of Pyrex are gaining acceptance. This
of the spray droplets so that they will   Frequent in steam-generation service,       borosilicate glass makes an ideal sur-
burn quickly without producing un-        the regenerative exchanger has been         face for exchangers that are designed
burned carbon. These changes are          successfully adopted for fired heaters.     for cooling a flue gas below its dew-
limiting particle emissions to less       Seals have been improved in design          point. Chemically inert and resistant
than 50 mg/m3, even when heavy            and in material to reduce leakage of air    to scaling, the glass is also inexpen-
fuel oils and vacuum residues are         to flue gas, a hazard.                      sive. Glass tubes will be increasing
burned.                                                                               installed in recuperative exchangers
A recent trend is to use fuel gas, if     A newer approach to leakage control in      for recovering energy now vented out
available at a high enough pressure,      large preheaters involves automatically     of stacks.
as the atomizing fluid. Not only is       activated deflecting sector plates at the
gas inexpensive, it also enriches the     hot end.                                    Plate heat exchangers, which can op-
flame.                                                                                erate below the flue gas dewpoint, can
                                          Different heat-transfer shapes have also    lower flue gas temperatures to about
Preheat Combustion Air                    been adopted to suit alternative fuels.     65oC because condensation occurs
Heat is recovered from the stack of       Other improvements pertain to modular       below 65oC, this exchanger recovers
fired heaters and used to warm up         motors, support bearings and soot           both sensible and latent heat.
combustion air. The most common           blowers.
air preheaters are the regenerative       A static exchanger consisting of circu-     Protected by an acid-resistant coating,
and recuperative exchangers and the       lar tubes, or a rectangular cross-section   the heat transfer metal surface is not
                                                                                      contacted by condensate. Even when
                                                                                      the flue gas temperature drops to
                                                                                      55oC, the flue gas leaves the air pre-
                                                                                      heater superheated by about 20oC.
                                                                                      This prevents downstream condensa-
                                                                                      tion and corrosion.

                                                                                      Waster-heat boilers represent a good
                                                                                      choice for raising fired-heater effi-
                                                                                      ciency if steam is needed, because
                                                                                      heat recovered as steam is more valu-
                                                                                      able than heat saved as fuel. A boiler
                                                                                      can be installed in convection section
                                                                                      of a fired heater. The stack tempera-
                                                                                      ture depends on the boiler steam pres-
                                                                                      sure and feed water temperature. The



  Furnace Improvements Services
  www.heatflux.com                               2
MAKE EVERY BTU COUNT



      inlet temperature must be controlled to
      avoid dewpoint corrosion.

      Direct-contact recovery of condensate
      heat holds promise for recovering low-
      level heat. In this new system, hot flue
      gas is passed through a cold water stream.
      This cools the flue gases to between 32 oC
      -43oC, and heats the water stream to
      about 55oC, recovering both latent and
      sensible heat from the flue gas, and boost-
      ing the efficiency of the fired heater from
      5% to 8%, one side benefit of the scrub-
      bing is a reduction of SO2 and SO3.

      The limitation of this equipment is that
      low-level heat is useful mostly for warm-
      ing water.

      Sootblower Advances
      Soot deposits on tube surfaces can reduce
      heat-transfer efficiency by as much as
      10%. Conventionally, high pressure
      steam blown across tube surfaces,
      (usually, about once per shift) and re-
      moves ash deposits. Steam blowers re-
      quire continuous maintenance. Micro-

                                                                             processor-based control has boosted
       Heat recovered from fluegas can be used to warm up the combus-        blower flexibility, making it possible
       tion air (below); Convection section of fired heater can be adapted   to blow areas where heat transfer has
       to generate steam (right).                                            dropped below a critical level and to
                                                                             operate the blowers in varying se-
                                                                             quences.

                                                                             A new method of keeping a heat
                                                                             transfer surface clean is to suspend the
                                                                             ash and soot in a gas stream by means
                                                                             of nearly continuous sound pressure
                                                                             waves. These sonic sootblowers gen-
                                                                             erate sound by means of a flexible
                                                                             diaphragm or a motor connected to a
                                                                             frequency converter.

                                                                             The major advantages of sonic soot-
                                                                             blowers are that they can be mounted
                                                                             anywhere, cover a large area, provide
                                                                             continuous cleaning and be fully auto-
                                                                             mated. They also need little mainte-
                                                                             nance, because they have only one
                                                                             moving part.

                                                                             Their chief limitation is that they can
                                                                             remove only light, friable deposits of
                                                                             slag or soot. If the fuel oil being
                                                                             burned contains sodium and vana-
                                                                             dium, the deposits in the convection
                                                                             section will be sticky because of their
                                                                             low melting point. Such deposits are



        Furnace Improvements Services
        www.heatflux.com                            3
MAKE EVERY BTU COUNT



      difficult to remove by steam or          ide provides a sensitive, reliable guide    fouls the convection section, boosting
      sound. Fuel treatment can make the       to combustion efficiency. With oil-fired    the emission of particles. The fouling
      deposits friable.                        furnaces, CO contents range from 150        of the convection section lowers the
                                               to 200 ppm. Flue gas analyzers now          draft, and this progressively decreases
      Closer Combustion Control                measure both oxygen, carbon monox-          the air flow, increasing the quantity of
      Little attention was paid to efficient   ide and other combustible gases. With       particles. At turndown operation, heat-
      combustion control when fuel was         the cost of microprocessor based con-       ers emit greater quantities of solids
      cheap. With the development of           trollers continuing to decline, even        because of poor mixing of air and
      forced draft firing and air pre-         small heaters will be equipped with         fuel.
      heaters, controlling excess air be-      advanced control systems.                   Also influential is fuel quality, par-
      came important. Microprocessors                                                      ticularly the content of asphaltenes in
      have made efficient control easy.        Nitrogen oxides, major air polluters,       liquid fuels. Particle emissions from
                                               range from 100 to 500 ppm in flue gas,      modern forced-draft burners being
      Onstream oxygen measurement              predominantly as nitric dioxide (NOx).      fed, preheater air, can be controlled to
      provides the dynamic feedback            Factors that influence the quantity of      50 mg/m3 even when heavy fuels are
      needed for closely controlling com-      NOx in flue gas are: quantity of nitro-     fired with 5% excess air.
      bustion in fired heaters. The oxygen     gen in the fuel, furnace temperature,
      content in exhaust gases represents      excess air and the temperature of the air
      a direct measure of combustion           entering the heater.                               The authors
      efficiency.
                                               The formation of NOx can be reduced                Ashutosh Garg
      When the oxygen content is low,          by re-circulating the flue gas, conduct-
      the measurement of carbon monox-         ing staged combustion, decreasing the              Senior thermal
                                               quantity of excess air, and lowering the           engineer in Ki-
                                               temperature of the combustion air.                 netics Technol-
                                                                                                  ogy International
                                               Reducing the excess air to a heater                Corp. Formerly,
                                                                                                  he was a deputy
                                                                                                  manager with the
                                         Heat can be recovered from fluegas via direct            Heat & Mass Transfer Div. of
                                         -contact spraying (left); sonic sootblowers              Engineers India Ltd. His career
                                         prevent tube deposits by means of pressure               responsibilities have included
                                         waves (below)                                            the design, sales and commis-
                                                                                                  sioning of fired heaters and com-
                                                                                                  bustion systems. He holds a
                                                                                                  degree in chemical engineering
                                                                                                  from the Indian Institute of
                                                                                                  Technology.



                                                                                                  H. Ghosh

                                                                                                   Chief consultant
                                                                                                  with Engineers
                                                                                                  Indian ltd. He is
                                                                                                  responsible    for
                                                                                                  the design of all
                                                                                                  heat-transfer
                                                                                                  equipment.     He
                                                                                                  received a degree in chemical
                                                                                                  engineering from Jadavpur Uni-
                                                                                                  versity, and worked for Fertilizer
                                                                                                  Corp. of India and A.P.V. Cal-
                                                                                                  cutta, before joining Engineers
                                                                                                  India Ltd.




        Furnace Improvements Services
        www.heatflux.com                               4
                                                           *Reproduced with the permission of Chemical Engineering.

				
DOCUMENT INFO
Shared By:
Categories:
Stats:
views:24
posted:5/23/2012
language:English
pages:4