United States        Office of Municipal      September 1985
             Environmental Protection    Pollition Control 'WM-546)    430/9-85-002
             Agency    ^ .  -   Washington DC 20460
&EPA       Multiple-Hearth and
             Fluid Bed Sludge Incinerators

             Design and Operational
             Considerations

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MULTIPLE-HEARTH AND FLUID BED SLUDGE INCINERATORS:
      DESIGN AND OPERATIONAL CONSIDERATIONS
                        by
               Metcalf & Eddy,  Inc.
         Wakefield, Massachusetts  01880
                 Project Officer
               Francis  L.  Evans  III
           Wastewater Research Division
      Water Engineering Research Laboratory
             Cincinnati, Ohio  45268
      OFFICE OF MUNICIPAL POLLUTION CONTROL
                 OFFICE OF WATER
      U.S.  ENVIRONMENTAL PROTECTION AGENCY
             WASHINGTON, D.C.   20460

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       This document  is condensed  from an  EPA research  report
entitled "Improving  Design  and Operation of  Multiple-Hearth  and
Fluid Bed  Sludge  Incinerators",  which will be  available  in late
1985.   That  report  has been subjected  to  the  United  States
Environmental Protection Agency's peer review.  This document  has
undergone  an  EPA  administrative  review and has been  found  to be
consistent with  the  EPA research  report  referenced above.   The
information in this document is made available for  the use of  the
technical  community.   The  information contained herein  does  not
constitute EPA policy,  guidance  or  directive.  Design engineers,
municipal officials,  and others are cautioned to exercise care in
applying this general  information  to particular circumstances of
individual  wastewater  treatment  facilities.   EPA  assumes  no
responsibility  for  use  of  this   information  in  a  particular
situation.   Mention  of  trade  names or commercial  products does
not constitute endorsement or recommendation for use.
                                11

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                             FOREWORD


       The construction  of  more wastewater  treatment  facilities
with higher levels of treatment has resulted in the need to treat
and dispose of larger amounts of sewage sludge.  In recent years,
sludge  incinerators  have  been frequently  considered  and  often
installed as  a  final disposal  alternative.   Many of  these were
constructed  using  financial  assistance  from  the  construction
grants program of the U.S. Environmental Protection Agency.

       Many  municipal  sludge  incinerators  have  experienced  a
variety  of  design and  operational problems.   In addition,  the
increased cost  of energy during the  past  decade, as  well  as an
increasing  awareness of  possible  air pollution problems  from
incinerator  emissions,   has  raised  serious  concerns  over  the
suitability  of  incinerators  for  sludge disposal.   EPA's  Water
Engineering Research  Laboratory in Cincinnati, Ohio  has  studied
sludge incinerators  to  identify the nature and extent of design
and operational problems, to identify possible problem solutions,
and to determine  the applicability of the technology  for use as
part of municipal sludge treatment  systems.

       This summary  document is based  on  that EPA study and is
intended to provide a basic understanding  of sludge incineration,
as  well   as   concise  information   on  design   considerations,
operational characteristics,  and  process  and  equipment problems
and possible  solutions.   The  document  will be useful  to design
engineers,  governmental   agency    review  personnel,   municipal
officials, operators, and others who are considering  using sludge
incineration  in  a sludge treatment  train,  or who are concerned
with optimizing  performance of an existing  sludge  incinerator.
The information   in  this summary  supplements detailed guidance
available  elsewhere,  which  should  be  considered  when  making
design or operating  decisions.   Improvements  in  the  technology,
the ability to  integrate  the technology into  the  total treatment
process,   and  the  compatibility of  the process  with  the  plant
environment must  be  considered  along with  associated costs  in
comparing this technology with other treatment alternatives.
                               111

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                             CONTENTS
Foreword	  ill
Acknowledgements	  vi

    1.   Introduction	  1
              Purpose	  1
              Background	  2
              Process Problems 	  3
    2.   Process and Equipment	  5
              Combustion Process Description	  5
              Support System Equipment Description	  15
              Design Improvements	  22
              Process Selection and Application	  24
    3.   Common Problems and Solutions	  32
              Process Design Problems	  32
              Equipment Problems	  39
              Operation and Maintenance Problems	  50
              Administrative Problems	  53
              Technical Investigations of
                Multiple-Hearth Furnace Problems	  54
    4.   Summary of Design
           and Operational Considerations	  56
              Improving System Design	  56
              Improving Existing Systems	  58
              Desirable Operating Characteristics	  59
              Improving Plant Operations and Maintenance..  59

References	  61
English to Metric Units Conversion Table	  62
                                v

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                         ACKNOWLEDGEMENTS
       This  report  was  prepared  for  the  U.S.  Environmental
Protection   Agency   by   Metcalf &   Eddy,   Inc.,   Wakefield,
Massachusetts, under Contract No. 68-03-3208.

       Dr. Charles  F.  von Dreusche of  Chavond-Barry Engineering
Corp. provided special consultant services to this project.

       Mr.  Francis  L.  Evans  III,  EPA  Project  Officer,  was
responsible for  overall  project direction.  Other  EPA staff who
contributed to this work include:

Mr. Howard Wall, Technical Project Monitor, Water Engineering
  Research Laboratory
Dr. Joseph Farrell, Water Engineering Research Laboratory
Mr. Walter Gilbert, Office of Municipal Pollution Control

       Metcalf  &  Eddy   staff   participating   in  this  project
include:

              Allan F.  Goulart,  Project Director
              Thomas K. Walsh, Project Manager
              Francis X.  Reardon, Mechanical Engineer
              Richard Buell,  Mechanical Engineer
              Elizabeth M. Gowen, Project Engineer
                               VI

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                             SECTION  1

                           INTRODUCTION


        Improvements  in  wastewater  treatment  technology and  in  the
 design  and  operation of wastewater treatment plants are  resulting
 in  higher-quality  effluents  with  increased  sludge  production.
 Concurrently,  the  problem  of sludge  disposal  has  become more
 difficult   because   many  disposal  methods  have  been   found   to
 present risks  to public health and safety.   Sludge processing  and
 disposal methods that have  received only cursory attention  in  the
 past  are now being reevaluated.

 PURPOSE

        This  design   information summary  report  presents  data  and
 best  practices relating to the design and operation of  multiple-
 hearth  and  fluid  bed furnace  incineration systems for combustion
 of sludges  in  municipal wastewater treatment plants in the  United
 States.   It  is based  on an  investigation  which  evaluated  the
 causes  of  operations and maintenance problems  experienced with
 multiple-hearth  and fluid  bed   furnace  systems.      The  data
 contained in  the  report were obtained from technical literature,
 discussions with manufacturers, and  telephone inquiries and site
 visits  to municipal  wastewater treatment  plants.   This document
 presents   process   and   equipment    descriptions,   operational
 characteristics, process selection and  application information,
 common  problems  and  solutions,   and  design  and  operational
 considerations related  to incineration of sludge.

        The  emphasis   of this  report  is  on multiple-hearth and
 fluid bed furnace  incineration systems, not  just  on  the furnace
 itself.  In incineration, as  in sludge  handling  in  general, the
 performance and  success of  each  step in  the process  flow  train
 depends  upon   the previous  step.    Cost-effective operation and
 efficient performance of an incinerator depends upon  a  properly
 dewatered and  prepared  feed sludge.   Ease of operation and low
maintenance needs for ash handling systems depend upon the design
and operation  of the  incinerator that  produces  the  ash.   The
performance of the   entire  sludge  handling system, not  just the
 incinerator,  will   determine  the  success  of  the  incineration
process.

       The  purpose of this  report is  to  summarize  concisely the
available design and operational  information on  multiple-hearth

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and fluid bed furnaces,  thereby providing a general understanding
of the incineration process as  well  as  its proper  application in
a  sludge  handling  system.    It  is  not intended  as a  detailed
design  guide  or  as  a  replacement  for   other  design  guides
available  from   manufacturers   or  for   technical   information
contained  in  published  literature.    This  report  should  be
regarded  as  a  summary  of  technical  design  and  operational
information.     Detailed  discussions on  particular  topics  are
contained in  the  original investigation document  (1) and  in the
references cited within the text.

BACKGROUND

       Combustion of  sludge provides  maximum volume  reduction,
destroys  or  reduces   most   toxic  materials,   and  offers  the
potential for energy recovery.   Multiple-hearth furnace  (MHF) and
fluid  bed  furnace  (FBF)  systems  have been  the  most  prominent
types  of  incinerators  used  in  sludge  combustion  in the  United
States for many years.  Multiple-hearth furnace  incineration has
been widely  used  for over  40  years.   Use of fluid  bed  furnace
incineration has increased steadily over the last 15 years.  Most
of the sludge incineration  facilities  currently  operating  in the
United States are MHFs,  which outnumber FBFs about  eight to one.

       Other  types  of  sludge  incinerators include  the  electric
furnace and single hearth  cyclonic furnace.  The  electric furnace
is  relatively new  and  has  been  used  since  1979  in a  limited
number of plants.   The  cyclonic furnace is limited to industrial
applications in the United States.

Multiple-Hearth Furnace

       The MHF  is durable and  simple  to operate  if  sludge feed
quality  and   rate  are  reasonably constant.   This  furnace  can
handle variations in  sludge  characteristics and  loading rates if
such changes  are  experienced over  the  long range,  such  as  month-
to-month,  but  hour-to-hour  variations present  combustion  and
operational difficulties.    The MHF  is  best suited to continuous
operation.   Because of the  time and fuel  required  to  bring the
hearths and  internal  equipment from a  completely  cold  condition
to   operating   temperatures    between   1,400 F   and   1,800 F,
intermittent MHF operation is inadvisable.

       The MHF  was  first used  more than  100 years ago  by the
mining industry  to  dry  and roast  ore  concentrates.   These early
furnaces were constructed of  refractory  brick,  with hearths,  a
central shaft,  and  a rabble  system  like  today's  furnace.   Wood
and coal  were used as  heat  sources.   By   1910,  the  furnace was
being  constructed  with  a  steel  shell,  which  permitted  a  larger
diameter  and  more  hearths,  and  oil and  gas fuel  systems were
added.  The use of stainless steel alloys   for  high  temperature

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applications of  the  furnace,  including sludge incineration, grew
out of technology developed during the 1940s.

       Approximately  350  wastewater  treatment  plants  in  the
United States  have MHF  systems.   Of  these  facilities,  271 were
reported to be operating at least intermittently.  Available data
on operating MHF facilities indicate that the majority use vacuum
filters  to dewater the  sludge  prior  to incineration.   Data on 64
operating  facilities indicate  that  sludge  feed characteristics
range  from 4  to 50  percent  solids  and from  61  to  42,000  dry
pounds per  hour, with averages of 28.2  percent  solids and 3,850
dry pounds per hour  (1).

Fluid Bed Furnace

       The  use  of  the  FBF  for  wastewater  sludge disposal  has
increased in recent years.  These furnaces are characterized by a
combustion  process  taking  place  in  a  fluidized,  sand  bed  and
operating  in  a  temperature  range between  1,400 F and 1,500F.
All combustion gases and ash leave the bed and exit at the top of
the pressurized  furnace.  Heat recovery from furnace off-gases by
means  of a gas-to-air  heat  exchanger is a  desirable  and common
practice.  The  characteristic  feature of the FBF is a constantly
available  heat   sink  in the  sand bed,  which  aids in attaining
steady combustion.

       The first municipal  application of a FBF incinerator  was
in  Lynnwood,  Washington  in  1965.     Today,  approximately  60
municipal  wastewater   treatment  plants  have  FBF  facilities.
Improvements  in  the  FBF  include   the   development   of  an  air
preheating  unit  (a  hot windbox) in the mid-1960s,  and  use  of
waste heat boilers for energy recovery in 1968.

       Approximately  29  FBF  facilities in the United  States  are
reported to be operating. The  data  indicate  that the  majority of
the  facilities   use  vacuum  filters  to  dewater  sludge prior  to
incineration.  Reported sludge feed characteristics range from 21
to 40  percent  solids and from 300 to  6,040  dry  pounds per hour.
Average sludge feed is 2,270 dry pounds per hour at approximately
30 percent solids (1).

PROCESS PROBLEMS

       Although  incineration systems  are a  very effective method
of sludge disposal,  these systems have had  problems in the areas
of design  and  operation that  have  limited successful  and cost-
effective operation.   Design  problems have  related primarily  to
sizing of  the  furnaces  and  to  variable sludge  feed  rates  and
characteristics.  Equipment  problems  have included the selection,
design,  and  layout  of  furnace components  and  support  systems.
Operations  and  maintenance   (O&M)   problems  have  involved  the
handling   of   slag,   clinkers,   screenings,   grit,   and   scum.

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Administrative problems  in  management and training  of  staff and
in  system  optimization  procedures   have   also  plagued  these
facilities.

       In some  cases,  these problems  have  been so  serious that
the process has been abandoned.  In general,  incineration systems
have been shut down as a result of high energy costs making other
sludge disposal  methods more  economically  desirable.   Although
these  costs  are  often attributed   to   improper  design  or  to
improper  operation  of  the  incineration   system,  in  a  number  of
cases  poorly   dewatered  sludge  has  caused   increased   fuel
consumption or  fuel costs  have  exceeded those  anticipated when
the furnace was designed.

       The  potential  for operational problems,  the ability  to
minimize or avoid  these problems through proper  design features
or operational controls, and a careful analysis  of the  operation
and  maintenance  costs  associated with   an  incineration  system
should all  be  considered  before  the process  is selected  over
other sludge  disposal  alternatives.    Both  the benefits  and the
potential problems  attributed  to these systems, as  well  as side
benefits such as the potential  for waste  heat recovery,  should be
included in such considerations.

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                            SECTION 2

                      PROCESS AND EQUIPMENT  ;7,  ;


       The   incineration   process   reduces   sludge   volume   by
evaporating  the  water  and burning the  volatile  matter contained
in  sludge.    The  efficiency  of  this  process  depends upon  the
performance of the preceding dewatering process and the operation
of support systems such as ash handling.  The combustion process,
MHF   and   FBF   incinerators,    support   equipment,   operating
characteristics,  and  guidelines  for  MHF  and  FBF  selection  and
application are discussed in this section.

COMBUSTION PROCESS DESCRIPTION

       Incineration  is a  two-step  oxidation  process  involving
drying  and  then  combustion  or  burning  in  the  presence  of
oxygen.   Drying and combustion  may  be accomplished  in separate
units  or  successively  in  one  unit,  depending  upon  temperature
constraints  and  control parameters.    The steps are  the  same in
both  MHF and  FBF  incinerators.   The temperature  of the  feed
sludge is  raised  to  212F  to  evaporate  water  from  the  sludge.
Then  the  temperature  of the  water  vapor and  air  are increased.
When  the sludge  solids  content  reaches approximately 40 percent,
the temperature of the dried sludge volatiles is increased to the
ignition point,  which  is  less  than 1,000F.   Complete combustion
of all organic  material occurs  at furnace operating temperatures
that  are  in  excess of 1,400F.    The  sludge  solids are converted
to a  relatively  inert  ash.   Moisture, particulates,  and  inert
gases are released through the  furnace exhaust system during the
process.

       The primary combustible  elements  in sludge  and  in  most
supplemental  fuels  are  fixed  carbon,  hydrogen,  and  sulfur.
Because  free   sulfur   is   rarely  present  in  sludge  to  any
significant extent and  is being  limited in fuels,  sulfur  content
can be neglected in determining  the fuel  value  of  a sludge.   The
fuel  value  of  sludge  is  based on   its  carbon  and  hydrogen
(volatile) content.  In conventional  solid fuels, volatile solids
content is determined by heating  the  fuel  in the absence  of air,
and   the  combustible   content   is   determined  by  ignition  at
1,336F.     The  difference  in   weight  loss  between  these  two
procedures is the  fixed carbon content  of  the  fuel.   In sanitary
engineering,  the  volatile content of a fuel,  such  as  sludge,  is
determined  by   heating  the  sludge  in the  presence  of  air  at

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1,021F;  this temperature  is  higher than that  used  for volatile
solids measurement for solid fuels  and  includes a  portion of the
fixed carbon.  The terms volatiles  and  combustibles  will be used
interchangeably   in  this  report  in accordance with  wastewater
industry  practice.

       Solids with a  high percentage of  volatiles,  such as grease
and scum, have-i;hgh fuel values.   Grit  or  chemical  precipitates
do not have 'hidjh ,fu?l values  because of the  large percentage of
inert material in them, and they require auxiliary fuel to burn.
           ,      	.^j
       Incinerator   operations    require   air   in   excess   of
theoretical   requirements  to achieve complete  combustion.   The
excess air  increases  the  opportunity   for  contact  between  the
oxygen contained  in  the air  and the fuel.   To ensure complete
combustion,   air   volumes  of  50 to 150 percent  in  excess  of
theoretical   requirements  must   be   provided  in  the  combustion
zone.  When the  amount of  excess air is inadequate,  only partial
combustion of carbon  occurs,   and   carbon  monoxide,  soot,  and
odorous hydrocarbons  are produced.

       The amount of excess air  required varies with the type of
incinerator,   characteristics of  the sludge,  and  the disposition
of  the  stack gases.    Cost-effective   operation  requires  that
excess air be minimized  to  reduce energy consumption while still
achieving complete combustion.    Energy  will  be consumed  by the
operation of  air  blowers and by  using supplemental fuel to raise
the temperature  of  the combustion  products and excess  air from
ambient to that  of the combustion zone.

       Thej 'amount of  supplemental  fuel  required  is   not  only
dependent upon   the  amount of   excess  air  needed  for  complete
combustion,   but   also  on  the   water   content  of  the  sludge,
radiation losses> and  the  heating of gas streams and sludge feed
solids.   The heat   released   by  the   burning sludge  must  be
sufficient  to   raise  the  temperatures  of   the   air  and  all
substances in the incoming sludge from ambient  levels to those of
the exhaust and  ash  and to compensate  for  any  radiant heat loss
from the incinerator.  If  the available heat  from sludge burning
is  sufficient to  maintain combustion   without the  addition  of
supplemental fuel, the process  is termed autogenous.

       Details of  combustion  theory and procedures  to determine
heat  balances and fuel  requirements  are   presented in  the EPA
publication  entitled  "Process Design Manual for Sludge Treatment
and Disposal" (2).

Multiple-Hearth Furnace

       A  cross-section  of  a  MHF  is   shown  in  Figure 1.   The
furnace  consists of  a  circular steel  shell  with  a  series  of
horizontal hearths made of fire bricks.    MHFs  are  available  in

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                CENTRAL SHAFT
                COOLING STACK

           COOLING AIR
           EXHAUST DAMPER
SLUDGE CAKE,
SCREENINGS,
AND GRIT,
                  RETURN AIR
                  DAMPER
                                    -I    IN HEARTHl
 AUXILIARY
 AIR PORTS
                  GUULJtJ
 RABBLE ARM
 2 OR 4 PER
 HEARTH
                                                            BURNERS
                                                            SUPPLEMENTAL
                                                            FUEL

                                                            COMBUSTION AIR
                                                            FOR BURNER

                                                            SHAFT COOLING
                                                            AIR RETURN
                                                            SOLIDS FLOW
           DROP HOLES
     CLINKER
     BREAKER
             ASH
       DISCHARGE
                                                     COMBUSTION
                                                     AIR
SOURCE: U. S. EPA PROCESS DESIGN MANUAL FOR
      SLUDGE TREATMENT & DISPOSAL
      EPA 625/1-79-011
          FIGURE 1. CROSS SECTION OF A MULTIPLE-HEARTH FURNACE

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diameters  ranging  from 4.5  feet to  29  feet  and  can  have  4 -to
14 hearths.    Two  access   doors   with   observation  ports  are
generally provided at  each hearth.   The rotating central shaft is
a hollow  iron column  cast  in sections.   The shaft is  normally
insulated with castable refractory,  which is a mixture  of heat-
resistant  aggregate  and  cement.    Insulation  renders  the  shaft
suitable for temperatures  of about  800F for continuous operation
and 1,100F $r short  term  operation.   Shaft  speed is  adjustable
between 1/2 arid -2 resolutions per minute (rpm).   Dewatered sludge
is fed  into  the  furnace at the  top hearth  and  proceeds downward
through  the  furnace  from hearth  to  hearth,  moved by  rotating
rabble  arms  with  rabble  teeth  or  plows attached  to the central
shaft.    The  arms  are normally 25  percent  chrome,  12 percent
nickel  alloy castings.    The  rabble  arms  constantly  move  the
sludge  in  the  hearths,  aiding  drying  and  burning.   Ash  is
discharged from the bottom  of the furnace,  and  the exhaust gases
are discharged from the top of the furnace.

       Air  for  combustion  and cooling of  the  shaft  and rabble
arms  is supplied by a  fan.   A cold  air tube from the  fan runs up
the center of  the  shaft;  air lances extend from  the tube to the
end of each rabble arm as seen in Figure 2.  Ambient air is blown
through the  cold  air  tube and  lances.   The cold  air  exits from
the  lance tips,  flowing  back  to the annular space  in the shaft
through  the  space between  the  lances  and the rabble arm wall.
This  flow of air  cools the  shaft and  rabble  arms by  convection.
The central shaft cooling air is returned to the bottom  hearth of
the furnace  to be  used as sludge combustion air.   If all the air
is not  needed for  combustion, it is discharged to the  atmosphere
through  the  central  shaft  cooling stack.   Because  the heated
central  shaft cooling  air   is  not  contaminated  with  combustion
air,  it may also be used for direct  forced warm  air  heating of
the  furnace  area.   An MHF can  also  have  a combustion  air blower
which supplies auxiliary  air  to  the combustion hearth.

        The  functions  of  drying the wet  feed,  combusting sludge
volatiles,  complete burning of fixed carbon,  and cooling ash are
performed in  distinct  zones of the  furnace from top to  bottom as
seen  in Figure 3.   The first  zone  (drying zone)  consists of the
upper  hearths  where  heated  combustion  gases   flow  upward
countercurrent  to  the  descending  sludge,  thereby  drying  and
heating the  sludge.   The  second zone  (combustion zone)  generally
consists  of  the central  hearths.   In this zone,  the  majority of
volatile  organics  are  burned and some of the fixed  carbon in the
sludge  begins combustion; temperatures reach between  1,400F and
1,700F.    In the third  zone  (fixed  carbon burning  zone), the
burning of the fixed  carbon  continues  and is  completed.  Ash is
cooled  and discharged  from  the fourth  zone,  utilizing  returned
central shaft air for  cooling.  The  sequence  of these zones is
always  the same,  but  the  number of hearths employed in  each  zone
 is dependent  on the characteristics  of  the  feed   sludge   and  the

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                                                                            STEEL
                                                                            SHELL
     HOT AIR
     COMPARTMENT
     COLD AIR
     TUBE
                                      AIR LANCE
                                 RABBLE ARM TEETH
                                                        SHAFT COOLING
                                                        AIR FAN
                                                                            FIRE
                                                                            BRICK
AIR HOUSING
   SOURCE: U. S. EPA PROCESS DESIGN MANUAL FOR
          SLUDGE TREATMENT & DISPOSAL
          EPA 625/1-79-011
          FIGURE 2. INTERIOR CUTAWAY VIEW OF A MULTIPLE HEARTH FURNACE

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                         SLUDGE
                          FLOW
           DRYING  ZONE
         COMBUSTION ZONE
           FIXED  CARBON
           BURNING ZONE
           ASH COOLING
               ZONE
      NORMAL
        AIR
    TEMPERATURES
\
\
\
K
\
r\
\
\
\
\\\\
 ^1400 to
 .1700F  . ,
  \\.\\\
                               AIR
                               FLOW
  SOURCE. U. S. EPA PROCESS DESIGN MANUAL FOR
       SLUDGE TREATMENT & DISPOSAL
       EPA 625/1-79-011 (MODIFIED)
FIGURE 3.  PROCESS ZONES IN A MULTIPLE HEARTH FURNANCE
                          10

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design of  the  furnace and burner system.   Drying and combustion
may occur on the same hearth in some instances.

       Furnace  gases  exit from  the MHF at  temperatures ranging
between   600F   and   900F   in   normal   operation   without
afterburning.   At  some  sites,  air emissions standards and sludge
characteristics require  an afterburner to  raise  the temperature
of the exhaust gases to 1,400F or higher to destroy odor-causing
constituents and  to  burn hydrocarbons.  Afterburners  may either
be inside the top of the furnace or outside the furnace.

       Generally,  the heating value of the sludge is insufficient
to sustain autogenous combustion, and additional heat is provided
by adding  supplemental  fossil fuel  to the MHF.   Auxiliary fuel
burners  for  supplemental  fuel   and  combustion  air  ports  are
located at  selected  hearth levels in the furnace, normally below
the combustion zone hearth in the fixed carbon burning zone.  The
position  of  the   combustion  zone  can  be modified or  changed
depending  upon  the sludge  feed  rate,, solids  content,  auxiliary
heat input,  and central  shaft  speed.   Burners  may operate either
continuously or  intermittently on  selected hearths  to maintain
temperatures best  suited to the sludge feed.

Fluid Bed Furnace

       The  FBF, seen in  Figure  4,  is a vertical cylindrically-
shaped,  refractory-lined  steel shell  that contains a  sand  bed
(media),   fluidizing  air  orifices,  and  auxiliary  burners  to
produce and  sustain  combustion.   The  FBF  is  normally available
from 9  to  25 feet  in diameter.   The  sand bed  is  approximately
2.5 feet  thick when quiescent,   resting   on  a  brick  dome  or
refractory-lined  grid.     The  sand bed  support  area  contains
orifices,  commonly  known  as  tuyeres,   through  which  air  is
injected into the  furnace at a pressure between 3 psig and 5 psig
to fluidize  the bed.   The  tuyeres  are installed at an  angle  to
the bed to prevent media from flowing back  into the windbox.  The
structure of the  bed  support varies depending  upon the operating
temperature of fluidizing air.   Dewatered sludge is either pumped
or carried by screw conveyors into the sand bed.   Sludge may also
be pumped  or  conveyed  into  the top  of  the  furnace,  but  this
practice is  not recommended for municipal  sludge.   When the sand
bed  is  active and  at  operating  temperature  it  expands  to
approximately double the at-rest  volume.   Sludge is quickly mixed
within  the  fluid  bed   by the   turbulent  action   of  the  bed.
Evaporation  of  the  water and  combustion  of the  volatile  solids
within the sludge rapidly  takes place.   Combustion  gases and ash
leave the  bed  and are transported through  the  freeboard area  to
the gas outlet at  the top of the furnace.   Combustion  gases  and
entrained ash  are  normally scrubbed  in  a  venturi  scrubber.   In
some  designs,  the  exhaust gases  pass  through  a  gas-to-air  heat
exchanger to preheat the fluidizing air.  A flow  sheet  for  a FBF
system is shown in Figure 5.
                               11

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      THERMOCOUPLE
THERMOCOUPLE
 SLUDGE     
 INLET
                        FLUIDIZED  :../.
                        SAND BED .::::/:
                                          EXHAUST AND ASH
                                              PRESSURE TAP
                                              SIGHT
                                            Y GLASS
                                                  BURNER
                                                TUYERES

                                               FUEL
                                               GUN
                                             PRESSURE TAP
                                              STARTUP
                                           -i  PREHEAT
                                            DBURNER
                                           JFOR HOT
                                              WINDBOX
FLUIDIZING
AIR INLET
 SOURCE. U. S. EPA PROCESS DESIGN MANUAL FOR
       SLUDGE TREATMENT & DISPOSAL
       EPA 625/1-79-011
     FIGURE 4.  CROSS SECTION OF A FLUID BED  FURNACE
                           12

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                                                             LAGOON
SOURCE U. S. EPA PROCESS DESIGN MANUAL FOR
      SLUDGE TREATMENT & DISPOSAL
      EPA 625/1-79-011
FIGURE 5.  FLOW SHEET  FOR SLUDGE INCINERATION IN A
                     FLUID BED FURNACE
                          13

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       The  water  and  volatile   solids  content  of  the  sludge
normally establishes the heat demand in the bed once  air flow is
set.  Fuel is injected  into the sand bed as required  to maintain
bed temperature or to heat the fluidizing  air.   Auxiliary burners
may be  located  either  above  or  below the sand  bed.    In  some
installations, a  water  spray  in  the  freeboard area  or  a  heat-
removal system in the bed controls furnace  temperature.

       Both  drying  and  combustion  of  sludge  occur  primarily
within the fluidized sand bed.  The minimum temperature  needed in
the  sand  bed prior  to  injection  of sludge  is  approximately
1,300F.   The temperature of  the sand bed  is  controlled between
1,400F and 1,500F.  Gas residence time  is between 5  seconds and
10 seconds.

       The  freeboard  space  above  the   expanded  sand  bed  is
designed  to   allow  disengagement  of  entrained  sand  particles.
Sand  or  other bed  media not  disengaged   in  the  freeboard  zone
leaves  the furnace with  the  ash  and  must be replaced period-
ically.  Media  losses are approximately 5 percent  of the design
bed volume for  every  300 hours of operation.    Replacement  media
are introduced to  the vfurnace either  above or  directly into the
bed.

       Combustion  of  gases  and  entrained  sludge  solids  will
continue in  the  freeboard area after  their separation  from the
bed,  and adequate  detention time  and  volume must  be  provided to
complete this combustion  prior  to exhaust.  Freeboard combustion
is  evidenced  by  an increase  in  temperature between  the bed and
freeboard  as  measured  by  thermocouples.   Temperature  increases
across the freeboard section must  be monitored to  control furnace
operation.   The amount of increase that may be expected is unique
to  each   facility  and  must   be  controlled   to  keep  furnace
temperatures  below  1,600F.   Increases in  the  order of  100F are
considered  normal,  and considerably  higher   increases  are  not
uncommon, but must be limited.  Up to 5 percent of the combustion
in the FBF may occur in the freeboard area. Freeboard combustion
in excess of  5 percent may result  in incompletely  burned organics
passing through the exhaust system.

       Effective  destruction  of  organic   substances  that  might
cause odorous exhaust gases occurs when (1) the combustion within
the expanded  sand  bed is 90  to 98  percent complete,  (2) overall
residence time is 5 to 10 seconds  with adequate freeboard volume,
and   (3) a  temperature  range  between 1,400F  and  1,600F  is
maintained.   In  normal  FBF operation, because  the  exhaust  gases
are maintained  at  temperatures   of  1,400F to  1,500F  for  the
stated  time  period,  unburned  hydrocarbon  emissions  are minimal
and strict  hydrocarbon  regulations can be met without  using an
afterburner.   However,  operating  conditions  must  be proper and
steady to ensure a continuous  low level of emissions.
                                14

-------
       In  a FBF,  sufficient air  is  provided for  combustion by
allowing  for 15 percent  excess air.   To account  for imperfect
mixing in  the  combustion  zone and to ensure that adequate oxygen
is available, the FBF is typically designed with 30 to 45 percent
excess air  capacity.   Less  excess  air  capacity  may  result in
incomplete  combustion.

       Air  supplied to a FBF may either be at ambient temperature
(cold windbox)  or  heated  (hot windbox).   In cold windbox design,
the  support beneath the sand bed  serves  as  the  air distribution
plate.  Because this plate is not subject to the same temperature
conditions  as  that of the hot  windbox,  it  can be  constructed of
metal.  This metal  plate  is  air cooled,  which prevents excessive
expansion,   and  designed  for  temperatures  up  to  1,OQOF  or
slightly higher.   Provisions for plate  expansion  at much higher
temperatures require greater design attention and cost.

       In hot windbox design, seen in Figure 4, the air is heated
by  burners  within  the windbox  or  by  a  heat  exchanger   that
captures heat  from the high-temperature  exhaust of the furnace.
The hot windbox unit utilizes a refractory brick dome bed beneath
the  sand.     This   construction   is  somewhat  similar  to  the
refractory  hearths  of  MHFs.   The brick  domes  are  often twice as
thick as MHF hearth cross-sections, however,  since they support
the  sand   bed  and  the  air  pressure for  f luidization.   Large,
specially  shaped  bricks in  the dome are pierced  by  holes  1 to
3 inches in  diameter, through which metal air nozzles are placed,
forming the  tuyeres for injecting the fluidizing air.

       The  fluid bed acts as a  thermal sink providing substantial
heat  storage   capacity.     This   capacity  dampens  temperature
fluctuations  (thermal  cycling)  that  may result from  short   term
variations  in sludge feed properties and feed rates.  To indicate
the  heat  storage  characteristics of  a  FBF, a sand bed suitable
for  combustion  at  a rate of  6,000,000 Btu per  hour would absorb
or  release  about   1,000 Btu to  change  the  expanded bed  area
temperature  by  20F.   This  heat  storage capacity  also  enables
relatively quick startups if the furnace shutdown period has been
short, e.g., overnight,  and  protects  the  refractory dome  and
support  arches  from   cracking  by   dampening  out  temperature
fluctuations.

SUPPORT SYSTEM EQUIPMENT DESCRIPTION

       The  performance  of  either a  MHF  or a  FBF  incinerator is
dependent    upon   the   provision   of   proper  support   system
equipment.    This includes  ash handling  equipment,  scrubbers,  and
other equipment directly associated  with  the furnace.   A typical
MHF  system,   including  most  of  the   support   equipment,   is
illustrated  in Figure 6.
                                15

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EXHAUST
STACK
            -CENTRAL SHAFT COOLING STACK

           -COOLING AiR EXHAUST DAMPER
          -RETURN AIR DAMPER

         HOT AIR RETURN
 SCRUBBER
                                         ROTARY ASH
                                         CONDITIONER
   RABBLE
   ARM DRIVE
          ^M	
      NNXT ^^^
      COMBUSTION
      AIR BLOWER
SHAFT COOLING
AIR FAN
     SOURCE: U.S. EPA OPERATIONS MANUAL
            SLUDGE HANDLING AND CONDITIONING
            EPA 430/9-78-002.
             FIGURE 6.  SCHEMATIC OF MHF INCINERATION SYSTEM
                                       16

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Ash Handling System

       There are  two  types  of ash  handling systems,  hydraulic
(wet) and mechanical (dry).  The MHF can use either the hydraulic
or mechanical  type;  the  FBF  can only use a  hydraulic  ash system
because  wet ash   is   discharged  from  the  FBF  scrubber.    The
hydraulic ash  system,  seen in Figure 7, has a  steel  ash hopper,
pump, discharge pipeline,  and water supply.  The  ash  drops into
the hopper, which  is  filled  with water.  The wetted  ash settles
and the  resultant  ash  slurry  is pumped to a lagoon or  fill area
for further settling.

       The mechanical  ash  system,  shown in  Figure 8,  consists of
screw conveyors, a bucket  elevator,  an  ash  bin,  and a  rotary ash
conditioner.   The  dry ash is discharged  from  the furnace  to a
bucket elevator, which lifts  the ash to a screw conveyor and into
a  storage  bin.   From the bin,  the dry ash  is  conditioned  or
wetted by a conditioning  screw or rotary drum mixer with internal
water sprays prior  to disposal to reduce dust.   The  conditioned
ash is normally disposed  by truck at a landfill.

Scrubber System

       The venturi  scrubber with an impingement tray separator is
the  most   commonly   used  exhaust   gas  scrubber  in  municipal
incinerator facilities.    As  seen in  Figure 9,  the exhaust gas
leaves the  furnace,  passing  into a  precooler  section  with water
sprays,  and then into  a  quench section  in  which water  flows over
the metal walls, forming a water  seal.   After  quenching, the gas
passes into a venturi section where its velocity increases.  This
increases particle collisions,  promoting droplet formation.  The
gas and  liquid pass  into a flooded  elbow,  after  which  the clean
gas passes  through an  impingement tray  separator that  disengages
the liquid  from the  gas.  Following this,  any  remaining mist is
separated from the gas  in the demister section.  The  clean gas
then passes through an induced draft (ID)  fan  and out  through an
exhaust  stack.   Scrubbing  water  can be recycled and/or supplied
by make-up water.  Waste scrubbing water from  an MHF  is normally
recycled into  the  main  process  train  in a  wastewater  treatment
plant.  With a FBF, the scrubbing water contains the ash from the
furnace and is normally treated and sent to an ash lagoon.

Other Support Equipment

       Other equipment directly related to efficient operation of
the  MHF  includes  a  well-sealed  ash discharge  outlet  from the
furnace  to  prevent  the infiltration of ash  into  the  furnace and
lance  or poke holes  at perimeter  drop  hole  locations  in the
hearths to allow access for control of slagging.

       Additional support system equipment  for a MHF includes the
following:


                                17

-------
oo
                                                                              FURNACE
                               TO ASH LAGOON
                              DISCHARGE PIPE-

                            GRID WASH NOZZLE
          CENTRIFUGAL ASH PUMP
             & V BELT DRIVE
                                                 \
                                                                      ASH
                                                                                   I
                                                                                 ASH INLET
                             WATER INLETS
                             FLOAT VALVE
OVERFLOW WATER
     LEVEL
                                                            NORMAL WATER
                                                                LEVEL-
                                 -FLOAT
                                      f    '  \ 1
                                                      L SUCTION
                                                          PIPE
                                     ,'<:'l::*!^:.:,'
                                 7 ,A;.' <-.-. i'& .  .- ;  >^r \ -'A-.-.?; a  .7. A ,.o -^/.  t> .-  <:. &.>.-^ * -.*  |
                                                   ASH SLURRY
                                       --EMERGENCY
                                       OVERFLOW PIPE

                                       - RAKE OUT DOOR
                                                                                                    AGITATOR NOZZLE
                                                                                    C
          SOURCE BEAUMONT BIRCH COMPANY, NJ
                 (MODIFIED)
                                  FIGURE 7.  EXAMPLE OF HYDRAULIC ASH HANDLING SYSTEM

-------
                                                 SCREW CONVEYOR
                                         COUNTERWEIGHT
                                            TAKE-UP
                                                        ASH BIN
                                                          ROTARY
                                                          ASH
                                                          CONDITIONER
                                                              ASH DISCHARGE
                                                              TO LANDFILL
SOURCE: BEAUMONT BIRCH COMPANY, NJ
       (MODIFIED)
             FIGURE 8. EXAMPLE OF MECHANICAL ASH HANDLING SYSTEM

-------
to
                                 PRECOOL
                               WATER SPRAY
                         ADJUSTABLE
                         THROAT
                                                   IMPINGEMENT
                                                      TRAY
                                                    SEPARATOR
                              RECYCLE PUMP
                                  FIGURE 9.  EXAMPLE OF A VENTURI SCRURItER

-------
    A sludge  cake feed  conveyor  system  and  feeder  that
    provides a steady,  nonvariable input  to   the  furnace.

    A live bottom bin  that  regulates  the  dewatered  sludge
    feed  to  the  incinerator.    This  type  of  bin  has  a
    series of  augers  or  screws at the  sludge  discharge
    point to  facilitate  sludge discharge  and  to  prevent
    sludge from bridging.

    Auxiliary fuel burners  that are sealed  at the furnace
    entry to prevent  air  infiltration  into the furnace.

    Burner system  blowers and  central  shaft cooling  air
    fan.

    Central  shaft  cooling  air  return  ductwork  to hearths
    below the combustion  zone.

    An  induced  draft  fan  designed with  an  adequate
    capacity  range  to satisfy  the  desired  excess  air
    levels in  the furnace.   The  fan sizing should  make
    allowances for anticipated  infiltration and variations
    in air requirements.

    An oxygen analyzer  to sample flue  gases in the exhaust
    gas outlet from the top hearth of  the furnace.

    Temperature  measurement  in  all   hearths  and  draft
    measurement in the  top hearth and  at  selected points.

    Automatic damper-operated air and  gas ducts.

    A  heat   recovery  system  consisting   of  a  convective
    waste heat boiler ahead of  the venturi scrubber.

Additional support system equipment for a FBF includes:

    Progressive  cavity  pumps,   piston  pumps,   or  screw
    feeders for feeding sludge  beneath the  surface of the
    bed.

    A multi-staged fluidizing air blower.

    A preheat  burner mounted in the  furnace  to raise the
    temperature of the  inert bed for ignition of auxiliary
    fuel and sludge during furnace startup.

    Fuel  injectors  for  feeding  auxiliary  fuel  directly
    into the bed.
                         21

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           An auxiliary fuel system incorporating flame safeguard
           controls  for   burners   and  a  separate  furnace  fuel
           injector system.

           A  furnace   freeboard   temperature   control   system,
           consisting of  a high pressure  water pump and sprays.

           A gas-to-air heat exchanger  for  hot windbox FBFs.

           A  heat  recovery  system  consisting  of  a  convective
           waste heat boiler ahead of the venturi scrubber.

DESIGN IMPROVEMENTS

       Contemporary design and  operation of MHF and  FBF  systems
incorporates  procedures   that  conserve  auxiliary  fuel.    These
diverse  fuel-saving   procedures   range   from   improved   sludge
dewatering to modifications  of  the furnace itself.   Because the
focus of this document is on improvements to incinerator systems,
this discussion  includes  a  brief  summary of  operating variables
that   affect   fuel   consumption   and    a   description  of   two
installations where fuel-saving designs  and procedures have  been
undertaken.

Multiple-Hearth Furnace

       The primary  factors  that  affect  fuel  consumption  in the
MHF are the sludge feed rate and  its fuel value.   Although these
factors are often  not subject  to  control,  the  sludge combustion
air flowrate, the  auxiliary  fuel  and sludge combustion rate, and
the rotational  speed  of  the rabble arms are  operating variables
that can  be  controlled.   Manipulation  of  these  variables  will
directly affect the following:

       -   Exhaust  temperature  (temperature   of  the  uppermost
           hearth)

           Excess air in the exhaust gas

           Temperature of the gas  in the  combustion zone.

In order to conserve auxiliary  fuel consumption in the MHF, these
variables should be  controlled.   If  not, the  following operating
conditions can result, leading  to  high  fuel  consumption:

           High incinerator  exhaust temperatures

           High combustion zone location  in the incinerator

           Greater draft than necessary
                               22

-------
           Underutilization of heated cooling return air

           Unsatisfactory burner use patterns.

       A new MHF system at San Mateo, California has incorporated
fuel-efficient  design  features  for  that type  of  furnace.   The
feed sludge  to  the MHF  has  a dry solids content  of 25 percent.
The  furnace  itself has  an  oversized  combustion  hearth  located
approximately in  the  middle  of  the  furnace.   The exhaust gases
exit the furnace from this combustion hearth, not from the top of
the  furnace,  at temperatures  between  1,400F and 1,600F.   The
furnace does  not  have  auxiliary burners mounted  on  the  walls;
rather there  is  a  separate combustion  chamber mounted externally
on  the  furnace.    This  chamber contains the  only  auxiliary fuel
burner in  the system.   All fuel burning occurs in it, providing
more precise control of fuel and air combustion'.  Flue gases from
the drying zone  of the furnace are recirculated into this chamber
and  then   pass  into  the fixed  carbon zone.   Excellent  design
features include:

           Use of moisture in the sludge to absorb excess heat in
           the combustion zone and oxygen in the dry recirculated
           off-gases.

           A  variable  gas recirculation  rate.   For  example,  to
           accommodate variations in sludge  feed  solids content,
           a  decrease  in  sludge   solids  can  be offset  by  a
           corresponding increase in gas  flow through the drying
           zone, ensuring  that  the  sludge  reaches  the combustion
           zone  with the solids content required for combustion.

           Control of excess air.

           An external combustion chamber that can burn a variety
           of available  fuels,  including waste   fuels,  without
           flame impingement on the rabble arms or on the central
           shaft because no burners are mounted in the furnace.

           A  suitable  residence  time  and   temperature  (between
           1,400F and  1,600F)  for   exhaust   gases  to  achieve
           deodorizat^on, making afterburning unnecessary.

           Return  air  temperature  is maintained without  the  use
           of fossil fuel.

Fluid Bed Furnace

       As  with  the MHF,  the sludge feed rate and  its fuel value
are  the  primary  factors affecting fuel  consumption in  an FBF.
Because  the  FBF   is   designed  with   specific  fluidizing  air
requirements, the air  flowrate is more or less fixed.  The sludge
feed rate is matched to the air flowrate and is therefore limited
                                23

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 to  a   narrow   design   range.    Auxiliary   fuel   consumption  is
 dependent  upon the fuel value of the sludge and the  required bed
 temperature.

        While   bed  temperature  is  a  function   of   the   sludge
 characteristics and feed  rate,  the  furnace exhaust  temperature
 can  be  directly  controlled   by  a  temperature control  system,
 consisting  of  a  series  of high pressure water sprays  that  cool
 the  exhaust gases.

        Improvements  in fuel efficiency of  FBF systems  primarily
 depend  on  revisions to  support  systems and  changes  in modes  of
 operation.  This is illustrated  by modifications  to a FBF  system
 at a wastewater treatment  plant  in Norwalk, Connecticut, as  seen
 in Figure 10.   The FBF system, installed in  1973,  utilized  a  cold
 windbox  without  heat  recovery.     Sludge  was   dewatered   by
 centrifuge prior  to incineration.

       The system was  revised  by replacing  the centrifuge  with  a
 belt filter press for dewatering and installing a  FBF  to act  as  a
 dryer in  series with a  FBF acting as a combustor.   Sludge  from
 the  belt filter  press, at approximately 25 percent solids,  is  fed
 directly to the dryer FBF.  The dried  sludge  and sand from  the
 dryer FBF  flow  by  gravity down to  the combustor  FBF where  the
 sludge  is  burned.   A sand lift  blower circulates the  hot  sand
 from the  combustor  FBF to the dryer  FBF, providing a constantly
 hot  bed  for  drying.  Hot  exhaust  gas  (approximately  1,500F)  is
 routed  from  the  combustor FBF  through two heat  exchangers  in
 series  and  then  through  a  scrubber.    These heat  exchangers
 preheat fluidizing air to approximately  1,200F for the  combustor
 and  230F for  the dryer FBF.  The key to fuel saving operation  is
 the  use  of a  low temperature FBF as a dryer.   With less fuel
 required for evaporating moisture, more air is available to burn
 sludge.

       These   improvements   have  resulted   in   reductions   in
 operating crew,  hours of daily operation, and fuel oil use while
 doubling  sludge  capacity.    Annual   cost  savings   have  been
 estimated at between $300,000  and $400,000  (1984 dollars).

 PROCESS SELECTION AND APPLICATION

       Selection of the incineration  process for municipal  sludge
 is generally based on  the  results of  a  technical and  economic
 evaluation  and  a   comparison   with  other  sludge  handling
alternatives.   Because the  incineration  process can handle  nearly
all  types  of   sludge,  the  primary  factors   in  the selection  of
 incineration   are  plant   size   and   economics.    An  economic
comparison  of   incineration    with   other   sludge   handling
alternatives  is beyond the  intent of  this  report and is discussed
 in other  technical  literature  (2).    The  major   advantages  and
disadvantages of incineration  are summarized in Table  1.
                               24

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               EXHAUST
               TO SCRUBBER
(Jl
                                                              EXHAUST GAS
                                                                         HEAT
                                                                      EXCHANGERS
                                                         FLUIDIZING
                                            COMBUSTOR FBF  AIR


                                                   FLUIDIZING AIR
BLOWERS
                                                                                            EXHAUST TO
                                                                                            SCRUBBER AND
                                                                                            ASH DISPOSAL
                          FIGURE 10.  SCHEMATIC OF FBF SYSTEM AT NORWALK, CONNECTICUT

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     TABLE 1.  ADVANTAGES AND DISADVANTAGES OF INCINERATION


Advantages

   Reduction  of volume and weight of wet sludge cake by
       approximately 95 percent,  reducing volume  for disposal

   Destruction or reduction of  toxics

   Potential  for recovery of energy from waste heat

Disadvantages

   Generally  higher capital and  O&M costs than for alternative
       disposal methods,  especially if energy  recovery and fuel
       efficient operation are  not considered

   High maintenance requirements  due to high temperature
       operations

   Highly skilled and experienced operators required

   Discharges to atmosphere that  may require extensive treatment
       Factors that  affect  selection  of  both the  incineration
process and  the  type of  incinerator  are plant size  and  process
flow train,  furnace  design  differences,  emissions levels,  fuel
requirements,  maintenance,   power,  labor,  and  chemical  sludge
conditioning.

Plant Size

       Practical  wastewater treatment  plant size for incineration
of  sludge is  primarily  dictated  by  the  feasibility  of  other
volume  reduction  and  stabilization  processes at the site,  and the
availability  of   other,  lower-cost  disposal  methods   such  as
landfilling,  land application,  or composting.   Generally,  smaller
plants  (less  than  10 million gallons  per  day,  mgd) have  options
such as these  that are  viable  and  less costly  for  the  volume of
sludge  they   produce.    Other   factors   influencing  this  choice
include the  location of  the plant and  the availability  of the
land for   alternative, more  economical  disposal methods.   Smaller
plants  in urban   areas  may  find  incineration the best  method,
whereas plants of equal  size in less industrial, more  rural areas
may  not.     Sludge  characteristics,   such  as  volatile  solids
contents,  also affect the decision  to  incinerate.
                               26

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       In  general,  incineration  is  cost-effective  in  larger
installations  where  the  support  facilities discussed  below are
readily  available  and alternative disposal  options  are limited.
An approximate plant  size at which incineration becomes feasible
is about 20 mgd  for a  MHF system and  10 mgd  for a  FBF system.
Other factors that dictate practical plant size are:

           The opportunity to  incorporate energy saving features
           into  the  overall  treatment  process by  waste  heat
           recovery, steam generation, use of the incinerator for
           odor  control  of  off-gases  from other processes,  and
           the use of digester gas for auxiliary fuel.

           The   availability   of   O&M   personnel    with   the
           comparatively   sophisticated  training   required  to
           operate and maintain  incineration units.   Maintenance
           and   instrumentation   personnel   with  the  necessary
           skills are not normally present at smaller plants, and
           service contracts are usually quite costly.

           The ability  to finance  the considerable  spare  parts
           inventories and preventive maintenance programs needed
           to  minimize  downtime,  which may not be   feasible  in
           small  plants.

       For  smaller  facilities, the  FBF is  available  in smaller
units  than  the  MHF,  and  the FBF  has  the  flexibility to  be
shutdown for  short  periods without using  significant  amounts  of
auxiliary  fuel to  restart or  maintain heat.   This  allows  small
plants to operate with one shift per day.

Design Comparison

       Differences  in  design  between  the  MHF  and  the  FBF are
primarily in the  areas of ash handling, incinerator  construction,
feed solids content, and level of instrumentation.

           The MHF can use a dry ash handling system; its dry ash
           is lighter and  therfore cheaper to  haul  and landfill
           than is wet ash from a FBF.

       -   The MHF does not require a regular supply  of bed media
           (sand) and  is  not  likely  to   incur  as  much  erosion
           damage in the exhaust gas system as  is the FBF.

           The FBF system  requires  gas  tight construction.   MHFs
           are under  negative  pressure and must be  designed  to
           minimize   air  infiltration   to   maintain   thermal
           efficiency,  but they are not necessarily gas tight.

       -   Both incinerator  systems  must  have  properly designed
           furnace refractories and flues.
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           Slagging and clinkering may be more  easily handled in
           a MHF because of the accessibility provided by furnace
           doors.   Although this advantage  is marginal,  slagging
           in a  FBF may  cause defluidization  or shutdowns  for
           long  periods to remove  the slag.

           FBFs  can better accommodate grease and scum,  using it
           as auxiliary fuel in sludge burning.   These materials
           can burn  in the  fluidized bed,  which supplies  good
           contact  with combustion  air.

           The FBF  is more easily  controlled than the MHF because
           it has a simpler burning  process.  MHF control can be
           simplified by  low  excess  air  operation and  a longer
           sludge drying period.
Emissions
       Both types of  furnaces  can meet present  federal  emission
standards  when   fitted   with   appropriate   emission   control
devices.     In  many   cases,  pollutant  emission  levels  may  be
decreased through  improved combustion control.   In  the  future,
increasingly stringent  requirements on  emissions may  influence
furnace selection.

       Under normal  operation,  FBFs have  exhaust  temperatures
between   1,400F  and    1,500F,   which   destroy   odors   and
hydrocarbons,   and  reduce particulates.    Because  MHF  exhaust
temperatures are  below  this   range,  it  may  be  necessary  to
incorporate afterburning  in some MHF  installations to  destroy
odors and to burn out hydrocarbons.   Present  afterburning methods
use  considerable  amounts  of  auxiliary  fuel,  raising  operating
costs.   Given  the  present  use  of  the  MHF,   the   FBF  has  an
advantage  where   afterburning   is  necessary.     If   an  exhaust
temperature of  1,600F  is  required  for emission  control,  the FBF
can be operated at  this  level  without afterburning.

Operational Flexibility

       Based on  current  use  in  municipal plants and  available
manufacturers'  products,  the  FBF provides  more flexibility  in
furnace operation  than  does  the  MHF.   The primary  advantage  of
the FBF  is  its  ability  to be  operated  for less  than 24  hours a
day.   The  FBF  can  accommodate  shorter periods of sludge  feed  by
operating  fewer  hours   a day  and  can  be  placed  on  standby
overnight  without  experiencing  appreciable  heat   loss  in  the
bed.  The MHF cannot  be  operated intermittently for  short periods
without  maintaining   furnace   temperatures  at  the   expense  of
auxiliary fuel.   The  FBF is also more responsive to  variations  in
feed characteristics  and  rate than  is the  MHF.   In addition, the
FBF is better able to handle  scum,  grit, and  screenings  than  is
the MHF.   The main  disadvantage of the FBF is that it has minimum
                                28

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air  requirements  to  fluidize   the  bed  media  that  cannot  be
reduced.    Therefore,  the  FBF  should  be  operated  near  design
sludge loading rates even for short periods.

Fuel Requirements

       A comparison  of  the fuel requirements of the MHF and the
FBF must be based on  the  characteristics  and type  of  sludge feed
to the furnaces.   These characteristics include volatile content
and odor  potential.   FBFs perform better  than MHFs  where high
volatile content sludges  are  incinerated.   The  typical 50/50 mix
of polymer conditioned  raw primary/waste activated sludge  is in
this category.

       The  FBF  has  better  fuel   efficiency  than  the  MHF  if
afterburning  is  required.   In  the FBF,  all exhaust  gases pass
through a  zone between  1,400F and 1,500F  before exiting from
the furnace.   Odor  causing components  of  the exhaust  gas usually
have  ignition  temperatures  under  1,420F  and  are  therefore
destroyed.

       In  the MHF,  furnace  exhaust gas  temperatures  range from
600F to 900F when maintaining correct operating temperatures in
the combustion  zone without afterburning.   For  sludge with odor
causing exhaust gas constituents, afterburning may be required in
the   MHF.     However,  MHFs   may   operate   with   reduced  fuel
requirements  with  less  odorous  sludges,  such as  primary  and
trickling  filter   sludges  conditioned  with  lime  and   ferric
chloride.  With these types of  sludges, odor  causing constituents
are minimal and afterburning would not  normally be required.

       Conventional auxiliary fuels for both  furnaces are oil and
gas.  In some cases, digester gas can replace or supplement these
fuels.  The FBF can also  operate on nonconventional types of fuel
such  as coal  or refuse derived  fuel.

Maintenance

       Maintenance   and   replacement   requirements   for   major
components of  the MHF and  FBF systems show  no distinct advantages
of one system over  the other.

           Both MHF rabble arms and FBF air  distribution systems
           are durable, and neither requires  replacement for many
           years with ordinary  maintenance.   Although  rabble arms
           are more exposed to abrasion and  differential heating
           than  are FBF orifices,   long service  is attainable if
           they are properly designed and maintained.

           Hearths  in the  MHF and  refractory  domes in  the FBF are
           subject  to cyclical  temperature  swings which  can cause
           damage.   The  MHF  has a greater  potential  for  damage


                                29

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           because it has  more  frequent  temperature fluctuations
           than  the  FBF and more  hearths  that must  be repaired
           and replaced.   However,  with  a  steady  sludge feed and
           a  minimum  of  shutdowns  and  startups,  maintenance
           requirements in these refractory areas  are reduced.

           If both  types  of furnaces use  hydraulic ash handling
           systems,  the  general  opinion  is  that  there  is  no
           outstanding  difference  in  maintenance  between  the
           two.     Dry  ash  handling  for   the  MHF  requires  a
           considerable degree of maintenance.

           Because the  exhaust  gases of the FBF   carry  both ash
           and  bed  media,  there  is  a  significant  problem  of
           erosion in flues from the point  at which gas exits the
           furnace to the scrubber entrance.

           The exhaust  gas systems  for  the  FBF,  including the
           heat   exchanger  and  expansion  joints,   often  require
           serious maintenance  and must normally be  given  more
           maintenance  attention  than  the  MHF  exhaust  system.
           Corrosive attack  is  reported more often for  the FBF
           exhaust system.  However, the  induced draft fan in the
           MHF  has  high  maintenance  requirements  that  do not
           exist for the FBF.

Electric Power

       Under  normal   operating    conditions,   electric   power
requirements are not significantly  different  for  the MHF and the
FBF systems.  In the MHF, the major electrical equipment  is air
blowers for the  auxiliary fuel system,  the  induced draft fan, and
the furnace drive.   Fluidizing blowers and burners are the major
electrical  equipment in  the  FBFs.   Despite  the  considerably
higher pressure  required  for  the FBF fluidizing  blower,  it  must
supply only half the air volume  of a comparably sized MHF.

Labor Cost

       For  either   type  of   furnace,  a   full-time  operator  is
required  when one furnace  is operating.    If  two  furnaces are
operating, operator  attention will  increase  similarly for  either
type  of   furnace,  requiring  supplemental  part-time  help.    The
required skills  for operators  of  either  furnace are considered to
be equal.

       The FBF is  more  advantageous where operating  with  one or
two shifts  per  day  is  preferred  to 24-hour  operation.  The MHF
performs  more  economically  under  continuous operation.     If
furnace capacity exceeds  sludge  production or  sludge  dewatering
is not  continuous,   labor  costs  are more  favorable for the FBF
system.
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       Maintenance labor for MHF and FBF systems is approximately
equal.

Inorganic Chemical Conditioning

       Lime, metal  salts,  and  polymers are  frequently  used  as
chemical  aids  in wastewater  treatment processes  and  in sludge
conditioning.  The burning of sludges containing metal salts from
these processes has caused  severe  slagging  and clinkering.   Some
FBFs that have burned municipal sludge containing ferric chloride
have had  slagging  so  severe as  to  plug the gas outlet  from the
furnace.     Burning  sludge   containing   polymers   has   caused
clinkering in some MHFs.

       In  both  types  of  furnaces,  sludges  containing  lime  or
metal   salts  increase   O&M   costs,  fuel   consumption,   and
corrosion.   Higher  operating  and  maintenance costs  result  from
the added  ash and  clinkers produced  when using  these chemicals.
The  additional   inert  materials  from chemicals  fed  into  the
furnace   also   increase   fuel   consumption.      If   calcining
temperatures are reached,  the  endothermic  reaction of  lime  will
require additional  energy.   At  high  temperatures the  chlorides
carried in  ferric chloride  sludges will  result  in  accelerated
corrosion of metal parts.
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                            SECTION 3

                  COMMON PROBLEMS AND SOLUTIONS
       Problems  involving  design,   equipment,   operation,   and
administration of MHF and FBF systems, summarized in Table 2, are
common  at  many  municipal wastewater treatment  plants.    These
problems increase the unit cost  of  incinerator  operation, reduce
its  operational  efficiency,  and/or  cause equipment  or  system
failures.    Solutions  to  these  problems  exist  and  have  been
successfully implemented.  The solutions presented can be used as
guidelines  for  designing  and  operating  MHF  and  FBF  systems,
keeping  in mind  that specific solutions may have to  be  modified
to suit a particular plant.

       This  information,  gathered as  part of  a research  study
conducted for  EPA,  is  drawn  from a number  of sources,  including
discussions  with manufacturers  and  consultants,  and  telephone
inquires and site visits  to municipal  wastewater treatment plants
with MHF and FBF facilities.

PROCESS DESIGN PROBLEMS

       Process design  problems  relate  to  selection  and  appli-
cation of the  type  of  incinerator,  as well as  the  equipment for
the incinerator  and  its  support  systems.   These  support  systems
include  sludge dewatering and  sludge  feed  facilities.   Problems
that  relate  more specifically  to the  design  and  use of  other
equipment components are  discussed as  equipment  problems.

Dewatering

       Regardless of the  type of  incinerator, the extent  to which
feed  sludge  is  dewatered has  a  major  impact  on  incineration
efficiency  and  costs.     The  low  solids  content  of   sludges
dewatered by vacuum filters or centrifuges is a  common problem at
plants completed prior  to 1979.   Sludge having a  high  moisture
content  reduces  the  equivalent dry  solids  throughput  capacity of
furnaces  and  requires  larger  amounts  of auxiliary  fuel  to
evaporate the water prior to  or during combustion.
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   TABLE  2.   SUMMARY  OF  MAJOR  PROBLEMS  WITH MHF AND FBF SYSTEMS
Process Design Problems

 Inefficient sludge dewatering
 Oversizing of incinerators
 Variable sludge composition and feed rate

Equipment Problems

 Failure of hearths
 Overheating and failure of rabble arms
 Cracks in central shafts
 Design, draft control, vibration, noise, and corrosion of
   induced draft fans
 Failure of poorly fitted bypass dampers
 Failure of thermocouples
 Failure of refractory domes
 Sand leakage into air distribution piping
 Corrosion in flues
 Wear, dust control, and abrasion in ash handling systems
 Wear and maintenance of conveyors
 Misalignment of off-gas system
 Improper sizing, design, and corrosion of boilers
 Improper design and corrosion of scrubbers

Operation and Maintenance Problems

 Slag and clinkers
 Improperly adjusted burners
 Screenings, grit, and scum handling

Administrative Problems

 Lack of system understanding by senior management personnel
 Poorly qualified O&M staff
 Insufficient operator training
 Lack of process optimization
       Drier sludge  cakes  are achieved  at  plants  that  use more
efficient belt presses or recessed plate filter presses.  In some
cases, incinerator fuel consumption has been reduced by more than
50 percent  as  a  result.  In multiple  incinerator systems,  drier
feed  sludge  has  also  eliminated  the use   of  one  or   more
incinerators.      Other   savings   occur  in  labor,   power,  and
maintenance costs due  to  the  higher   solids  content  and  lower
volume of  sludge  to  be incinerated.   The cost for supplementing
or  replacing   dewatering  equipment  would   be  expected   to  be
recovered from O&M savings  in three months to two years.


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Equipment Sizing

       Incineration systems are often oversized.  Their selection
and  sizing  is  frequently  based only  on projected  design loads
that  either  are never  realized or are  so  much higher  than the
sludge volumes  provided  in the initial  years  of  plant operation
that  furnace  operation  cannot  be efficiently  reduced  to match
production.      An   oversized   incinerator   must   be   operated
intermittently, which  leads to  very  high  auxiliary  fuel costs
associated with reheating  the  furnace to operating temperature
after cooling down or maintaining  the  furnace  in  a "hot" standby
condition.   Oversizing  is  the result  of a  compounding  of  the
following factors:

           Use  of  peaking  factors  of  1.5  to  2.0  times  average
           sludge  production  to  define  maximum  weekly  loading
           conditions

           Use of  overly conservative  criteria  (i.e.,  design for
           the highest  probable moisture content of the dewatered
           cake at  the  highest  solids loading)

           Adoption   of  excessive  safety   factors  for   such
           parameters as sludge cake  loading  on  MHF  hearths  or
           FBF fluidizing and freeboard velocities.

       Excess  MHF  capacity increases capital and  sludge  disposal
costs.   The  unit  costs  of MHF operation  and maintenance  will
increase  for  the following  reasons:

           Total labor  costs will  remain  nearly the same whether
           operating  at  full  or  partial  incinerator  capacity
           because  staffing for both cases will be  approximately
           the same.

           Total electric  power  costs  for  units  operating  at
           partial capacity will be nearly equal  to costs at  full
           capacity   due  to operation   of electrical  motors  at
           inefficient  levels.

          Unit  fuel  costs  will   be   higher   due   to  frequent
           startups   or   a  need  for  standby  heating  for  long
          periods.   Additionally,  extended sludge storage  can
           reduce dewaterability,  increasing  sludge  cake  moisture
          and the fuel  required for  evaporation.

          Total maintenance costs will increase because  cycling
          of  the  incinerator  for  intermittent   use decreases
           refractory life, requiring  more  frequent  replacement
          of  brickwork.
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       The impacts of oversizing the FBP are similar to those for
the MHF.   The  most significant impact is the added capital cost
for larger FBF unit(s).   Labor costs for operation  of  a  FBF are
not significantly  affected  by  oversizing,  and may  be  reduced if
sludge can be  processed during one or two  shifts  per day.   As
with  the  MHF,  unit  electric  power costs will  be higher  due to
inefficient  operation  of  motors.    Maintenance  costs  for  an
oversized FBF will be increased by cyclic operation and increased
needs for repair  or replacement of the refractory.

       The first  and most  important step  for  the  design  of  a
municipal sludge  incineration system is to define  the sludge feed
characteristics  and  to   establish  the   operating  parameters
required for  the  furnace and support systems.  Two basic criteria
are required:

           Sludge feed rate:  pounds of wet  cake per hour

           Properties of the sludge feed:

               Percent solids (preferable to moisture content)

               Percent   combustibles   in    the   solids   (total
               volatiles)

               Gross heating value of combustibles

               Analysis  of total,  or ultimate,  combustibles

               Presence  of chemicals that react  endothermically.

       The softening and  fusion points  of the ash,  determined by
ASTM Method D-1857-68, are also highly desirable.   These criteria
can be determined if a valid specimen of the sludge or  ash can be
obtained.

       In many instances,  the  above criteria are  not  known with
precision  before  specifications   are  prepared.    The  designer
depends  upon  the  accumulation  of  in-house  data  on sludge  feed
characteristics.    Ranges  of  expected  values  are  inspected  to
ensure that  the  furnace  will  meet  the  needs  of  the  wastewater
treatment plant.

       An  effective  alternative  to  specifying wide   ranges  in
these  criteria  is  to  develop plant  operating mode  scenarios.
This  will not  only  facilitate  the  development   of   the  above
criteria and heat  and material balances, but will  also provide a
more  realistic  picture  of operation  of   the  solids  handling
train.   Once  these criteria are  developed, minimum and  maximum
furnace exhaust temperatures and  minimum  and maximum percentages
of oxygen (excess air)  in  the exhaust gas can be determined.
                               35

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        Before  specifications  are finalized,  heat  and material
 balances  should  be  revised  and  finalized  for  each  operating
 scenario  and a table  prepared  indicating  the following:

            Sludge  combustion air  required,  as both mass flowrate
            and volume rate,  in pounds per hour and standard  cubic
            feet per minute,  respectively

            Shaft cooling air recycle  (if a MHF is considered),  in
            pounds  per hour

            Ambient air temperature, in F

            Auxiliary  fuel  required,  in Btu per  hour,  or in fuel
            volume    terms,    including    fuel    analysis    and
            characteristics

            Auxiliary  fuel  combustion  air  required,  in pounds per
            hour and standard cubic feet per minute

            Furnace exhaust  flue gas  volume,  in actual cubic feet
            per minute.

       After this  table  of values is  completed,  a summary table
 indicating  the  minimum  and  maximum values  for each parameter
 should be prepared.  The summary table should then be examined  to
 determine   whether  the  desired  capacity  range  of  individual
 equipment items is within the useful and feasible operating range
 of available equipment.

       The  problem of  excess  furnace  capacity  in  the  initial
 years  of  operation may  be  addressed  by  two  design  approaches.
 One  approach  is  to  install furnaces  that  can  be  incrementally
 modified  to activate  the use  of  more hearths and/or  combustion
 volume as sludge  quantities increase.   An  alternative approach  is
 to use smaller  multiple  units to achieve incremental  increases  in
 plant  furnace  capacity.    Although multiple  units increase  the
 capital cost of the plant,  they provide considerable  flexibility
 and  increased  reliability.    If  all  of  the  projected  furnace
 capacity  is  not   initially  installed,   benefits  derived  from
 advanced  technology  and  onsite  system   improvements may  be
 realized when the  additional units are installed  in  the future.
 The  applicability  of  either  approach  is   dependent on   the
 projected growth  rate  of  the sludge load as  well  as other factors
 unique to each  individual  treatment facility.

       In existing MHFs,  modifications  are possible to minimize
 the impact  of oversizing.   Modifications  can be made  to the MHF
 to  reduce  the  number of  sludge  processing  hearths   by cutting
holes in the upper  hearths  to allow sludge to  be  fed  two or  three
hearths lower.   The  burners on the  upper  hearths may  be  sealed
off or  used as  afterburners if required.   Reduced gas  flows  to


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scrubbers  can  be  appropriately  handled  by  variable  venturi
throats and/or variable speed fans.

       The MHF  can also  be  modified to  handle low  sludge  feed
rates  by  allowing  the  sludge  to  burn  on  a  higher  hearth  than
normal  and permitting  the  lower  hearths  to  simply  transport
ash.  For prolonged low solids throughput, revising the operation
to  the higher  hearths  and  discontinuing  heating  of  the  lower
hearths could  be practiced.   The entry  of excess air  could  be
sealed off in the lower part  of  the  furnace.   Combustion air and
gas would also be proportionately reduced.

Variable Sludge Composition and Feed Rate

       Variations in feed rate, heating  value, and solids content
of  sludges result  in  unsteady conditions in  MHFs  and difficulty
in  maintaining regular operation at  low  excess  air  in FBFs.   The
inability  to  operate  at  planned  excess  air  levels  seriously
affects fuel  economy,  capacity, and power consumption.

       The combustion air  requirements of a furnace are directly
related to the  combustible solids feed rate  of  the feed sludge.
The combustible  content of the  sludge is  directly  related to its
dry  solids content and  to  the particular  constituents of  the
sludge, such  as  volatile  solids, scum,  grease,  and inert solids
content.   For  a  particular sludge, a 50  percent  increase in dry
solids  content  will equal a  50 percent   increase  in  combustible
material.  Under steady sludge feed conditions, this represents a
50 percent increase in combustion air requirements.   Similarly, a
decrease in the  inert solids  content of  feed  sludge will produce
a  proportionate  increase  in  the  combustible  content  of  the
sludge.    Finally,  an   increase  in  sludge  feed  rate  to  the
incinerator  will  increase  the  rate   of  feed  of  combustible
material.  If all of  these changes occur  simultaneously, as  they
frequently do in  municipal  installations,  they  can  produce  a
radical change in the  heat input to the  furnace and in combustion
air  requirements,  resulting  in  extreme  demands  on air  and  fuel
supply  systems.   Where  those demands are beyond  the  capacity  of
installed equipment, excessive temperatures  will result.

       An  increase  in  the solids content of  the  sludge feed can
also place additional demands  on the MHF.   Increasing the solids
content  decreases  the  quantity  of  water  to  be  evaporated,
decreasing the  hearth  drying  area  required.    If  the  solids
content of feed  sludge  is increased suddenly,  it could be  dried
to  the point  where it  could ignite while  still on an upper drying
hearth.  The  preceding lower  solids content  sludge,  still burning
at  its normal  rate on  the lower hearths,  would contribute heat  to
further raise  the temperature  on  the  upper  hearths,  increasing
the  combustion  rate on the upper  hearths.   This burning  in two
zones  of  the  furnace  is  termed  double  burning and results  in a
very  large increase  in  combustion air  needs.    Once  the  upper


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hearth ignites, temperatures on that hearth and all hearths above
may rise, causing burning on yet a third hearth.

       These  conditions  result in  unstable combustion  in  a MHF
due  to  high  temperatures  and  a  lack  of  sufficient  air  for
complete combustion.   Without this air,  the  furnace may produce
smoke  or  exceed  safe  temperature  limits,  depending  on furnace
control  setting  and operating conditions  prior  to  the change.
When combustion occurs on  the upper hearths,  combustion space is
inadequate,  and  the lower hearths  are  rendered useless.   These
conditions are  extremely serious and result  in flue  damage and
severe smoking,  in  addition  to limiting MHF  capacity  to a level
far below the design rating.

       On  a   long  term  basis,  if  feed  sludge   is  dryer  than
anticipated  during  furnace design,  the  upper  hearths of existing
MHFs may be  bypassed by  cutting drop holes  in the  hearths.   This
would improve control of  the furnace to  eliminate double burning,
reducing  fuel  requirements.     However,  the  previously  noted
increase  in   combustion   air   needs would  not  be  changed  and
sufficient air would have to be supplied.

       Because dewatering  and incineration  operations  within  a
treatment facility  are  often  separate,   the  incinerator  operator
may not  be  aware  that  short  term sludge  feed rate  or quality
changes  have  occurred  until  temperature  readouts  indicate  a
markedly  changed  furnace  condition.    This  effect may not  be
apparent in  the  MHF until the sludge has passed  through several
hearths.   MHF  response  to^ new burner settings  and  central  shaft
speed changes  is  very  slow,  and  these  changes usually  are best
made slowly.    Because  of  this sensitivity to  changes  in  sludge
characteristics,   the ability   to measure temperature  and  excess
air in the burning zone of a MHF is the  key to furnace control.

       The effects of variable sludge  feed rates on the operation
of FBF units  are different from those  in the MHF.   Generally, the
FBF requires a  forced  feed   injection  system to  overcome  the
pressure in  the  furnace.   Feed rate surges do  not  occur because
the screw or  the progressive cavity pump used  to feed the furnace
operates at a set speed or rate.   A sludge with an  unusually high
solids content may  cause "over pressure" stoppage  of  the  feeder
or plugging due to the  increased viscosity of  the  sludge.

       The effects  of varying  the  solids content of  feed  sludge
on a FBF are similar to  those for a MHF.   If  the  solids content
of feed sludge is increased from 27 to  30 percent  and  the  sludge
feed rate and combustible content  remain unchanged,  the feed rate
of  combustibles  is  increased  by  10  percent.   If  the FBF  was
operating at 25  percent excess air  originally,  this  same  air
flowrate  would be  equivalent  to 15  percent excess air  with the
higher solids content  sludge.   At  this lower  excess  air  level,
the chance for smoking is  greater.   In  the FBF, the  time  needed
                               38

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to  sense  increasing bed temperature is  shorter  than in the MHF,
which  permits the  operator  to  reduce  auxiliary fuel  flow more
quickly,  restoring  the  excess  air  to  the  safer  25 percent
level.   Under autogenous burning  conditions,  water  sprays  or a
reduction of  the sludge feed  rate would be necessary  to reduce
bed temperature.

       If sludge solids content were  to change  in  the opposite
direction,  from  30 to  27  percent solids,  combustibles  would
decrease by 12 percent.  If original operation were at 25 percent
excess  air,   the new  condition would  be at  37 percent  excess
air.   This  condition could be  maintained until  dryer sludge was
available or  adjustments to fuel  or sludge feed  rates  could be
made.

       Ideally,   sludge  storage   should  be   provided  before
dewatering  to   level   out   variations   in  daily   sludge  feed
quantities and characteristics.  The use of sludge blending tanks
will help minimize  variations   in moisture,  chemical,  and grease
content  in  the  liquid sludge feed  to  dewatering.    Multiple
dewatering  units will  also even  out   variations in  the  sludge
dewatering process and provide  a constant sludge feed rate to the
incinerator.    Minimizing  these  variations over short periods
(minute-to-minute and  hour-to-hour) will help  establish  steady
and fuel-efficient incineration.

EQUIPMENT PROBLEMS

       Major  equipment  problems  in MHFs   occur  with  hearths,
rabble  arms,  central  shafts,   induced  draft fans,  dampers,  and
thermocouples.   FBF  components  that experience frequent problems
include  refractory  domes,  air  distribution  piping, and  flues.
Components common to  both  types of  incinerators  that experience
problems include ash handling  systems,  sludge  conveyors, off-gas
systems, waste heat boilers, and scrubbers.

Failure of Hearths

       MHF  hearth  failures  can result  from  lack  of  feed  rate
control  as  well as  frequent,  rapid   temperature   cycling;  but
hearths  primarily  fail  as   a  result  of 0 sudden  changes  in
temperature.  Frequent cycling between 1,300 F and 1,800 F can be
worse  than  controlled  shutdowns.    In  normal  operation  where
sludge  feed  is  steady,  properly constructed hearths  should not
fail in less than ten years, even with  weekend  or nightly standby
cooling^ as  long as temperature  changes are controlled to less
than 50 F per hour.

       Proper record keeping can be helpful  in predicting hearth
failure.  During internal  inspections  of the  furnace,  the high
point on each hearth should be  measured from a common benchmark,
such as  the  bottom of  an  inspection  door,  and recorded.   An
                                39

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increase in  the  rate of  arch  settlement indicates  an impending
problem.    Refractory  behavior   is   relatively  unpredictable,-
however,  and  hearths  that  started   out  almost  flat  or  even
slightly negative  in slope  from perimeter  to center  have  been
known to last  a  year or  more.    If  the hearths rise  on  initial
heating, they will generally have a normal service  life.

Rabble Arms,  Teeth, and  Central  Shaft

       Rabble arms  are  subject  to  overheating  and  accumulating
sludge on top of  them.  A steady sludge feed  and adequate  cooling
air flow can  prevent overheating of the rabble arms and are vital
to rabble arm life.  Sufficient  cooling air  is dependent upon the
design  and  size  of   the   furnace   and central   shaft.     The
temperature of the  cooling  air  at the  central  shafQt cooling air
stack should  be  maintained  between  250 F and 350 F.   Castable
refractory    insulation   can  also  protect   rabble   arms   from
overheating and appears  to  be  effective.  By  placing  the  rabble
arms for each  hearth at  an  angle to  those on  adjacent hearths,
accumulation  of sludge on the rabble arms of  the lower hearths is
avoided.

       Rabble  teeth  may  bend  if  exposed   to  short  term  high
temperatures.    The  solution   is  to  control  temperatures  by
maintaining a  steady sludge  feed.    Corrosion  of  the teeth  is
generally  caused  by  chlorides  and  can  be  minimized  by  careful
control  of ferric  chloride  use  in  liquid and  solids  treatment.
In addition,  use  of a  two-part  tooth avoids  replacement  of the
tooth holder.

       Central  shaft  problems  include  insulation  anchoring  and
cracks.   Various  means  have been used to  anchor  the  castable
refractory  insulation  to the  shaft.     None  have  been  entirely
effective due to the  difference  in coefficients of expansion for
the cast iron shaft  and the insulation material.  Cracks  in the
insulation  can  be  repaired with  patches, but  proper selection and
application of  the patch material are  required.

       Minor  expansion cracks in  the  cast iron  shaft  should not
be a major  concern.  The  cast iron can  be exposed  to the  fire as
long  as the  area  receiving  the  radiation  is  small  and  the
surrounding cooling wall area is  large.

Induced  Draft Fans

       Serious  problems  with improperly  selected  induced  draft
fans may be  correctable,  but often  at  considerable cost.   The
long  term  effect  of  operation  with  a deficient  fan  can  be
extremely  costly  in  terms of reduced sludge  disposal  capacity,
limited     furnace   operating    time,    and    excessive    power
consumption.   Major problems  with ID fans are mainly  attributable
                               40

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to the physical  layout  and orientation of the  fan  and ductwork,
draft control, vibration, sizing,  noise,  corrosion,  and drainage.

       Fan inlet conditions may not permit the ID fan to meet its
rated capacity  and  pressure.    Sharp,  short  radius  bends  in the
inlet duct and the resulting change in direction of  the inlet gas
stream upstream of  the fan  impeller can  cause an unbalanced,
nonuniform flow condition that reduces fan performance.  Adequate
space for  fan  and duct  layout  is  the solution.  Where  space is
limited,  modification of inlet  ductwork  and  use of  straightening
vanes with short radii can  avoid many flow problems.

       Draft  control and   scrubber   pressure   drop  are  usually
obtained  by  modulating  the flow  using  an automated damper.   In
almost all cases, an  inlet  damper to the fan is  preferable  to a
discharge damper.  Either parallel or  radial  leaf dampers  may be
used.  Parallel  leaf  dampers  resemble louvers or Venetian  blinds
mounted either  vertically  or  horizontally.   The  louver  segments
of radial  dampers are mounted radially to the  centerline  of the
fan  shaft.   Butterfly-type  dampers mounted too  close  to the fan
inlet can  cause flow unbalance and  a higher pressure  loss  than
radial or parallel leaf  dampers.

       Induced  draft   fan  vibration  can   be   minimized   by  a
structural analysis of  the  fan  foundation block and by provision
of vibration  isolators  on  the  foundation.   Where the ID  fan is
located high  in a steel frame  structure, a careful analysis is
required  to  avoid harmful  vibration.  Impeller  imbalance  caused
by  the  accumulation  of particulates,  tars, soot, grease,  and
water with subsequent loss of part of  the build-up,  is common in
wastewater  treatment  plants.    Vibration switches that  permit
quick fan shutdown are mandatory in a fan subject to accumulation
of   grease   and  solids   to   prevent   damage    from   excessive
vibrations.

       In  many  instances,   improper   sizing  of  the ID  fan  has
considerably  limited  furnace  capacity  and   resulted  in  severe
noise generation.  An ID fan must  be  sized in accordance with air
flow and pressure requirements,  and  fan  tip speed.  Selecting a
fan that  is too small and operating it at a  speed higher  than its
normal operating range will cause  high noise  levels.  If properly
sized and  selected,  the fan  capacity will  be  adequate for the
furnace and  the  noise level can  be  maintained  within  acceptable
limits.

       ID fan  corrosion is  a result  of  using improper materials
of construction.   Because  scrubbers  do  not  absorb  all  the  acid
gases produced  in combustion,  these  gases and  the  mist  droplets
carried through the  scrubber can produce  some high concentrations
of acid.    Chloride  corrosion of  ID  fans  following  wet scrubbers
can be avoided by selection of special stainless steel  alloys for
fan  parts.   Depending  on chloride  concentrations,   a  variety of
                               41

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materials including 316L stainless  steel,  Iconel  600  series,  and
Hastelloy  C276,   increasing  in  corrosion  resistance,   can  be
used.  Corrosion  can also occur on exhaust stacks  and  other parts
following induced  draft  fans.   The  ID  fan drains and the stack
drains must be kept clear to allow the corrosive liquids  to drain
away.  Drains of corrosion-resistant  material  have  been  found to
be necessary in many plants.
       The combustible materials  built-up in  the  ID  fan  can be
ignited  under  certain circumstances.   With a loss  of  scrubber
water, the bypass dampers in a  MHF  system should  open to protect
the  ID  fan  and  scrubber  from excessive temperature.    If  the
bypass does  not  open,  the hot  gases  passing through  the exhaust
system can   ignite  the  fan  deposits.   A  combination  of  water
sprays in the fan and mechanical cleaning are mandatory to reduce
the  rate of  material build-up  in  the  fans and  to  protect against
fan fires.

Emergency Bypass  Damper

       In a MHF system, one  of the most  common  problems is poorly
fitted bypass dampers.   Seating  of  the damper disc  is  normally
imperfect in the  large  refractory-lined  stacks.   As  seat areas
deteriorate  and  become worn,  gases  that  have  not  been  scrubbed
can  leak  from  the  stack.  Although  an  inflow  of air  to a lower
pressure  area below the damper disc should occur,  untreated gases
can  flow from  this area.   Alternatively, failure of  the bypass
stack  damper  to  close reasonably  tightly at  high  furnace draft
can result in major in-leakage of  air adding to the gas flow that
the  scrubber and  the  ID  fan  must  handle.    The solutions  are
provision of a refractory seat with closer seating tolerances and
routine checking  by maintenance personnel.

       Damper bearings  installed  on  the  bypass stack can over-
heat  and seize,  rendering the  damper inoperable.  The  bearings
should be offset  from the stack with an intervening heat  shield.

Thermocouples

       Ceramic protective shields  for thermocouples are  prone to
shattering.  They have had satisfactory service life in MHFs when
special  refractory shapes  are  provided  above  the  shields  to
protect  the  thermocouple  from  falling sludge.   However,  when the
ceramic  type shield  is  placed unprotected  and  in  the   path of
dropping sludge,  the thermocouple  tends to shatter very easily.

       Most  ceramic shields  for thermocouples  in MHFs have been
replaced with Inconel 601  shields,  eliminating  the   shattering
problem.    Thermocouples  with  304 stainless  steel  shields  are
usually not  suitable for service in MHF combustion hearths.

       Ceramic type  thermocouples are  used  in FBFs  because the
frequency  of  shatter   is  much  lower.     The  thermocouples  are


                                42

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suspended from  the  furnace wall in  the  freeboard and fluidizing
sand bed areas and are less subject to physical impact,

Refractory Dome

       In FBFs,  failure  of the refractory  dome  beneath the sand
bed has been as severe a problem as hearth failure in MHFs.  Just
as  some  MHFs  have  had  long  hearth  life  and  others  have  had
frequent failures,  the  FBF refractory dome experiences  are also
varied.  Maintaining constant temperatures or  heating and cooling
slowly are normally the key factors in avoiding problems.

       An  improved  practice   for   FBFs  is   to  introduce  the
fluidizing air  into a  recessed chamber  with  a  burner that heats
the  air  to prevent localized  hot  and  cold  areas in  the brick
arch.  This practice is  probably more  important  when oil is used
as  the  auxiliary   fuel  than  with   natural  gas  because  of  the
luminosity of oil flames.   In  all  cases, consideration should be
given  to uniform temperatures  on brick  domes  or  arches  during
heating.

       During startup of a hot windbox  FBF,  a  preheat burner is
used to  heat  fluidizing  air and bring the  sand  bed to operating
temperatures.   Once  operating  temperatures  are  reached,  they are
maintained  by  auxiliary    burners  in  the   furnace,  and  the
f luirlizing air  is  heated using waste  heat.   At  this point,  the
preheat  burner  is  turned  off.   When this is  done,  the  burner
should be shut  down slowly, allowing  the windbox temperature to
gradually reach equilibrium with that of the fluidizing air.  Tlje
windbox temperature  should  not  be allowed to  drop more than 50 F
per hour to avoid damage to the refractory dome.

FBF Air Distribution Pipes

       Maintenance   of   FBF  air   distribution   pipes  can   be
difficult.   Keeping the pipes  locked  in place  in  brick hearths
and  sufficiently  tight  to prevent  sand from  leaking  into  the
windbox during  shutdown  periods is  a  problem because  the metal
pipes  have  a  coefficient  of  expansion  three  times  that  of  the
brick  hearth.   Proper design  of  the  arch  in the  hearth,  which
results  in  air  pipes  of  different bends  and  lengths,  permits
controlled expansion of  piping without allowing sand to leak.

Exhaust Flue Corrosion

       Chlorides  and  sulfur   in  dewatered  sludge  cake  are
transformed   during   the   combustion   process    into  corrosive
constituents that damage  metal components in the furnace and flue
gas system.

       At incinerator  temperatures,  most  chloride   salts  react
with  water  vapor   to  produce  hydrochloric  acid  (HC1).    Many


                                43

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metallic chlorides first vaporize, then react with water vapor  in
the gas phase, leaving an extremely fine metallic oxide dust plus
hydrochloric  acid  vapor.   Hydrochloric acid  is corrosive in the
hot   dry   state,   as   well   as  upon  condensation.     At  metal
temperatures  above  800F,  alloys of  the  type  used  in MHF rabble
arms  and  teeth  are  subject  to  dry corrosion  by  HC1.    This
corrosion   is   not   generally  a  noticeable   problem  but  can
contribute  to shortened rabble tooth  life.

       Sources  of  chloride   include  sludge   conditioning  with
ferric   chloride,    industrial   contributions,  and   saltwater
intrusion  into  the  sewer  system.  Where  ferric chloride is used
to coagulate  sludge  solids,  the  ferric chloride  will contribute
chloride to the water.  The  water retained in the dewatered cake
contains the  same  chloride  concentration.   The  chloride  in the
wet cake will be present in the exhaust gas.

       Unlike HC1,  SC^/SO-^   is corrosive  only  upon  condensation
with  water  or  absorption  in water.   The  most  critical corrosion
conditions  occur when the temperature of the steel scrubber shell
and  flue  drop momentarily  below  the dew  point  of  the  furnace
combustion  gas, causing condensation  to occur.   This absorbs HC1
and  S02/S02 to create  acidic solutions.   When  the temperature
rises or the partial pressure of water diminishes within the flue
duct  or vessel shell,  the water  evaporates, leaving concentrated
acid  on the steel.   Repeated occurrences of these conditions will
produce concentrations of hydrochloric, sulphuric, and sulphurous
acids,   which  attack   the   metal   surface   on   which   they
concentrate.  These conditions are considerably lessened in areas
where  in-leakage  of  air  occurs,  diluting  the concentration  of
corrosive gases near the steel surface.

       FBFs are constructed  to operate under  positive pressure.
The higher  pressure reduces  the temperature at which the acids in
the exhaust gases  condense,  causing  corrosion in exhaust  ducts
and at  expansion  joints.    Although  MHFs  are  not immune  to  the
same  phenomenon,  the  situation is mitigated because  MHFs  are not
pressurized and are subject  to air infiltration.

       Heat exchangers  and  waste  heat boilers located ahead  of
the scrubber frequently have not  been designed with  consideration
for local cold spots (e.g.,  first tubes contacted by the entering
cold  fluid) where  metal temperatures are below the acid vapor dew
points.  A general rule  for  waste heat  boilers is  to  generate
saturated  steam  greater than  300 psi  to maintain  boiler  metal
temperatures  safely  above  400F  and  to  raise steam  generation
pressures before acid bearing  gases are allowed into the  boiler,
unless other specific  anti-corrosion measures are taken.

       If  the  boiler   is  to  be   bypassed,  recognizing that  the
dampers cannot make positive gas-tight closure,  the  boiler  casing
                               44

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can be pressurized  such that  sufficient clean air forces leakage
across the dampers from the boiler into the bypass duct.

       The  solutions  to  the  problems raised  by chlorides  and
sulfur in  sludge  begin with recognition and consideration of the
problem.  Improved dewatering will help chloride removal.  Twenty
percent  solids  sludge  contains  4 pounds  of  water per  pound of
solids;  33 percent  solids  sludge,  contains 2  pounds  of water per
pound of  solids.   This increase  in  solids reduces the inorganic
chloride content  by  half  since all inorganic  chlorides are water
soluble.   Using  polymer  or   ferric  sulfate  instead  of  ferric
chloride  in  solids   coagulation  may  be  preferable   to  further
reduce the chloride content of sludge.

       Proper  handling  of   exhaust   gas  for  either  type  of
incinerator  will  minimize  corrosion  problems.   Transition  from
hot  dry  gas  conditions  to   wet  scrubber   conditions  must  be
accomplished  without  an  interfacial  zone subject to alternate
wetting  and  drying  conditions.   The exhaust  gas  should  be  kept
hot enough to keep all metal  parts well above the temperature at
which  sulfuric  acid  condenses.    The gas  should  pass  into  the
throat of  the venturi  scrubber,  where the flow  of water covers
metal parts  and is  sufficient to  absorb the acidic gases without
a significant change in water pH.

       Corrosion  problems  for all  equipment in  contact  with
incinerator  off-gases  can  be minimized  by  process  temperature
control,  insulation   for  temperature  control,  and selection  of
proper materials.   Materials selection can be based on historical
data   or  on   actual  testing   of   samples  in  an  existing
installation.   The  equipment  involved  includes  heat  exchangers,
waste heat boilers,  ID fans, stacks, duct work, and drains.

Ash Handling

       Ash  handling   problems have  been  experienced  with  both
mechanical and hydraulic ash systems.

       Mechanical  Type.  The  mechanical (dry) ash system is  only
applicable  to MHF  systems.   The  dust  from non-tight  dry  ash
systems   and  the   abrasive   ash   create   severe   maintenance
problems.  The  inability to keep  the plant clean  causes operator
morale problems  that  further  detract from performance.   Common
problems include the following:

           The ash drop hole  and  chute at the bottom of  the MHF
           can  become  plugged  with  large  clinkers  or  loose
           brick.     This  problem  can  be  reduced by adding  a
           diversion chute and coarse screen. The fine ash  will
           pass through the screen, but larger  objects  will  be
                                45

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           diverted  to  the  other  chute  for  removal.    This
           solution  helps  to  protect  ash  conveying  screws  and
           bucket elevators.

           Chains and  sprockets must  be  made of  materials  with
           high  hardness  to  protect  against  the abrasive  ash.
           The  sprockets  should  be  harder  than the  chain  to
           prolong sprocket  life.

           Bucket elevators  should  never  be  loaded  greater  than
           80   percent   of   capacity   and   loading   should   be
           considerably less.

           Most  bucket  elevators  have  two  chains,  one on  each
           side  of  the bucket.   One  chain  is  driven, and  the
           other chain is on idler sprockets and used to maintain
           bucket alignment.  Both chains  should be used to avoid
           misalignment.

           Helical screws  in covered  troughs  move  ash horizon-
           tally or  on  an incline.   Hard iron  bearings  support
           the  screw  and  increase  the  bearing  life  against  the
           abrasive ash.   Hard  iron bearings  can  be very  noisy,
           generating a screeching sound.  Grease lubrication may
           reduce the noise  problem, but will  reduce  bearing  life
           as  ash  becomes   entrained   in  the   grease.    Wooden
           bearings can be used to solve the problem.

           The  covers  on  screw conveyors are very  difficult  to
           keep dust  tight.   If  all  the gaskets,  screws and clips
           on  the  cover  are used,  the dust  is  reduced.   After
           maintenance,  the  covers  should be  put back with  all
           fasteners  in place.

       -   Ash   conditioning   screws   and   rotary   drum   ash
           conditioners require  proper  maintenance to wet the ash
           sufficiently to minimize landfill  dust problems,  and
           generally  are  only moderately successful.

           Conditioning screws  must  be equipped  with  adequate
           horsepower drives for moving the  wet ash.

           Control of  the  water  for  wetting  requires  frequent
           operator  attention due  to changing  ash characteristics
           and feed  rates.

           The  ash conveyors should have a  vacuum  dust  removal
           system at  the transfer  points  to  minimize  fugitive
           dust.

       Hydraulic Ash Handling.   Wet ash systems  can  be  used  with
either the MHF  or FBF.   Problems  with hydraulic ash systems  are


                                46

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abrasion and wear  in  the  pump and piping due  to  the  ash slurry.
The slurry, consisting of bottom ash and fly ash,  is abrasive and
accelerates  wear   in  the  piping  and  pump,  especially in  pipe
elbows.   Abrasion-resistant heavy  walled  pipe and rubber-lined
pumps should be used for this service.

       The ash  slurry is generally  disposed  in a  lagoon,  which
will require cleanup  and  disposal after a period of  time.   This
clean-up and long-term  disposal  is  difficult  because  of abrasion
and  accelerated  wear on  cleanup equipment  due to the ash  and
retention of water by the ash slurry.

Sludge Conveyors

       Sludge cake conveyor  systems, normally belts that deliver
sludge to  more  than  one  incinerator,  may restrict operation of
the  incinerator at full capacity.  Common practice is  to  divide
the  feed on  a  conveyor  belt with a plow, which is  a  metal plate
that diverts part of the  sludge to  an  incinerator  or  another
conveyor.   However,   precise division   cannot  be  obtained.   The
result  is  that incinerators may not  be equally  loaded.       A
uniform feed rate  to  more  than  one  incinerator may  be maintained
by  dedication  of   a  sludge  hopper  or   bunker  with a  controlled
discharge system to each incinerator.  To obtain a fairly uniform
sludge distribution,  the bunkers  can be fed  by a  conveyor  system
in  which  automatically-operated  plows divide  the  sludge  feed.
These plows alternately divide all sludge on the conveyor to each
bunker at  frequently  timed intervals.   The  bunkers would  either
be placed  directly below  the main feed conveyor or would  be fed
by individual feed conveyors.

       There are two  ways to feed sludge into a MHF.   The  sludge
cake can  be directly dropped  into  the top  of the  MHF  from  a
conveyor over the  furnace or from a  chute or  screw  conveyor that
receives  sludge  from a conveyor on  the  side of  the  furnace.
Conveyors  located  over  the  top  of  the incinerator  have  more
maintenance   problems   because   of    the    high    temperature
environment.  Sludge cake should be cleaned  off these  belts prior
to shutdown because the sludge will become baked on and difficult
to remove.   Elastomer and rubber conveyors have  reduced life in
this hot environment.

       Although conveyors  located to  the  side rather  than over
the  top  of  the  incinerator have  reduced  maintenance,  the side
conveyors require  a  chute  or screw conveyor  to feed  the sludge.
To avoid  bridging  of  the  sludge at the  transfer  point,  and to
ensure free flow of the sludge,  the chute angle should be between
70 degrees and 80  degrees.   Water sprays to  help  keep the  sticky
sludge moving at the  transfer points are  not  recommended because
of the increased evaporative load to the furnace.
                                47

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Off-Gas System

       Air leakage  into the  furnace  off-gas system  may prevent
furnace  operation  at  design  conditions  and reduce  efficiency.
For example, misalignment of  the  precooler  to a venturi scru'',oer
may not  permit closing  of  the water  seal.   The  air introduced
into  the  off-gas   system   at  this  point  will  reduce  furnace
capacity.   Air in-leakage  from dampers in  interconnecting duct
work may also  be sufficient to  reduce  furnace capacity.   A check
for misaligned equipment  should  be  performed   in  both  new  and
older plants,  and  equipment should be  properly aligned to avoid
this problem.

Waste Heat Recovery Boilers

       The  design   loading  factors  for  dust,   which are  solid
particles in the exhaust gas,  are critical in sizing a waste heat
recovery boiler.   The  values in Table  3  are  typically  used by
manufacturers  in  the  waste heat recovery  boiler  industry.   As
dust  loads  increase,  gas  velocity  must  decrease  to  reduce
erosion.   Reduction  of  gas  velocity  increases boiler  size  and
cost.    Tube  wall  thickness  should  be  increased   for  erosion
protection where required.


               TABLE 3.  TYPICAL DUST LOAD FACTORS

         Percent Dust by WeightMaximum Gas Velocity
            in Flue Gas	     (ft/sec)	

                 10.2                           32
                  0.32                          54
                  0                             66.5
       Control dampers  must be  carefully located  to  avoid dead
spots  where  dust can  build up  when  the dampers  are  modulated.
Modulation changes the velocity distribution profile.

       Unburned carbon  in  the  exhaust  gas can also be  a problem,
coating waste  heat  boilers with soot  and reducing heat transfer
efficiency.   The soot  is  removed by  steam cleaning.    If steam
availability  is  a problem, a  high pressure  air  cleaning system
may be used.

       An auxiliary heating system may be required to protect the
boiler  against condensation  and  corrosion  during  long standby
periods.
                                48

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Air Pollution Control/Scrubbers

       Approximately 20  pounds of uncontrolled  particulates  per
ton of  sludge and  0.0038  pounds  of  particulates per  gallon of
fuel oil fired (3) are generated in MHFs.  The Federal New Source
Performance Standard of 1.3 pounds of particulates per dry ton of
sludge can be met with modern  high energy wet scrubbers,  such as
the venturi.

       Removal efficiency  is  a direct function  of  particle size
and the  amount  of  energy  (or  pressure  drop) designed  into  the
scrubber  system.     Typical   published  values   (3)  for  percent
removal and size distribution data from literature (3) for sludge
incineration of  different particle sizes are in Table 4.


             TABLE 4.   SIZE DISTRIBUTION  AND REMOVAL
                  EFFICIENCIES OF  PARTICULATES
Particulate Size
Range (microns)
<0 . 5
0.5-1.0
1.0-5.0
>5.0
Distribution Percent
by Weight
1.
2.
8.
88
6
2
2
.0
Removal Efficiency
(percent)
75
96
99
100
.9
       Problems  in  meeting   air   pollution  requirements  have
resulted   from  excessive  submicron   particles  with   a  high
percentage of  unburned  organics,  primarily hydrocarbons.Improved
combustion  control  of  the  incinerator  usually  corrects  the
problem of unburned organics.

       A  constant  flow of  water  over  the walls of  the venturi
scrubber must be maintained to ensure scrubber performance and to
minimize  corrosion in  the exhaust gas  system.   Variation  in
scrubber  water  flow may be caused  by  a plugged  strainer  in the
supply line or inadequate flow and pressure control in the supply
system.

       Venturi  scrubbers  are  generally   constructed  of  high
hardness  stainless  steel.     However,   should   abrasion  of  the
venturi throat become a problem, greater wear-resistant material,
such as silicon carbide, can be used.

       The  impingement  tray  separator  section  of  a  venturi  is
also usually  constructed  of  stainless  steel.    Corrosion  at the
liquid gas  interface  above the  tray sections can be  avoided by
making the wall thicker at these points or  by  constructing  the
                                49

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separator in a more corrosion-resistant material, such as silicon
carbide.

       Because the  control  actuator on variable  throat  Venturis
may tend to stick and  cause  erratic operation,  the actuator must
be properly sized and an adequate control  system provided.

       Excessive water  carryover  from the exhaust  stack results
from an  inadequately  designed demister section  in  the scrubber.
Proper  baffling  and  drains  within  the  scrubber  avoid  this
problem.

OPERATION AND MAINTENANCE PROBLEMS

       Improved  operation  of  an  incinerator  system  is  heavily
dependent  on   the   ability  to  deal  with common  operation  and
maintenance problems  of slag  and clinkers;   improperly  adjusted
burners; and screenings, grit, and scum disposal.

Slag and Clinkers

       Clinkers are  free  lumps occurring  in  the burning  bed  of
MHFs that  are carried to the  cooling hearths.   Slag, which can
occur in a MHF  or  FBF, is the accumulation of  fused material  on
walls and dropholes, rabble  arms,  etc.   Failure to recognize the
ash  melting  temperature  under  both oxidizing   and  reducing
conditions and  failure  to   make  proper  tests   has   resulted  in
severe operational  problems  and increased maintenance costs due
to slag and clinker formation.

     0 Slag and  clinkers result  from  high   temperatures  (above
1,650 F) in the  combustion  zone.    These  temperatures are caused
primarily by  variable sludge  feed  and,   to  a lesser  extent,  by
poor  burner   control.   Observations  indicate  that   the  highest
temperatures occur  in  the upper portion of the  combustion hearth
in a MHF where  the sludge volatiles  mix  with air when  they are
passing  through  drop  holes   in  a  MHF.    The greatest  slagging
potential is observed to occur at the drop holes, particularly in
the outer drop hole area.  The intense heat from combustion heats
the refractory at the  drop hole;  when fine ash  particles contact
the drop hole,  they adhere  and  build up.  Eventually,  the drop
holes can plug  entirely.   To minimize this problem,  lance ports
should be located in this area.

       The problem  of  clinkers  deserves more  extensive treatment
than  can be  addressed in  this  report.   Operators with  these
problems should seek expert  advice to:

           Determine where and  how  much air   enters  the  furnace,
           and quantify and  adjust burner  excess air as necessary
                               50

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           Readjust rabble teeth to break up  lumps  and to retain
           lumps longer in the fixed carbon  hearths

           Change chemical conditioning,  if  necessary

           Shred very dry sludge cakes.

       Slagging occurs in several forms.   Oil  and dust can create
slag on burner tiles.  Grease burned on  a hearth may create local
hot spots or  a  local area of ash melting, causing  slag.   Sludge
left on rabble arms  can  be carried  through  the  burner flame zone
and held  at  the  junction with  the central  shaft,  producing  a
roaring volatile  flame  that  turns sludge to  slag.   Lastly, dust
carried in  the gases  from  the  lower  hearths  can   pass  through
burner or  volatile flames, become molten while in suspension, and
then  stick  to the  first contacted surface.  -If the  surface  is
cool  enough,  the  molten droplets freeze  and  drop away.   If the
surface is  sufficiently  hot,  alternating  strata  of  cinder and
glass  deposit  on   the  surface.    At  still  hotter  surface
temperatures, the molten droplets become glass-like  and penetrate
the refractory until  the deposit  can only be  removed by breaking
away the brick surface.  The slag is taffy-like while hot, and if
heated  another  100  to  400F,   will  become  fluid  and  slowly
dissolve the brick.

       In general,  if  the  combustion  zone temperatures are below
1,650F, slag and clinkers are not  a problem  in a MHF.  Problems
occur, however, because bed temperatures and hot spots frequently
exceed this value in localized areas.

       Solutions  to  slagging,  as  well  as  clinkering,  involve
control of  air  infiltration and  oxygen supply.    The slagging
problem is  usually  better  understood  and correctable  where the
analysis  is  not  only  based on  oxygen  measurement  in  the top
hearth, but also on excess air leakage into the furnace above the
fixed carbon burning hearth.

       In the MHF,  one  solution  to  slagging may be  to reduce the
fixed carbon  burning rate  by decreasing  the oxygen  concentration
over  the   bed.     This  solution  reduces   overheating  on  the
combustion hearth above.  Another solution may be to  increase the
air flow  into the volatile  burning  hearth after reducing the air
on  the fixed carbon  burning hearth.    Slagging  problems with
thermally conditioned  sludges  can be  minimized by sizing burners
for 100 percent excess  air.   Slagging caused  by ash melting when
passing through  the burner  flame  can  be eliminated by reducing
flame temperature through use of higher burner excess air.   Other
alternatives  to alleviate slagging problems include:

           Providing  lance  or  poke holes at  perimeter drop hole
           locations
                                51

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            Enlarging drop holes in the combustion zone

            Increasing  turbulent  mixing  of  gases  to  provide a
            uniformity of the fire from one side of the  furnace to
            the other

            Adding  air  below  the  fire,  not  above  the volatile
            burning hearths.
 Burners
        Carbon build-up around burner tiles is an indication of an
 improperly adjusted burner.  Other problems caused by maladjusted
 burners are slag,  clinkers,  refractory  damage,  and furnace shell
 damage from buildup  of  molten material around  burners.   Typical
 reasons for  lack  of  proper adjustment  are failure  to use  the
 proper orifice  plates  in  making  adjustments  and  setting  the
 excess air so low  that  high flame temperatures cause  the  ash to
 slag.   Excess air  in MHFs must be high  enough  to  reduce slaqginq
 around the burner.                                           yy  y

 Screenings,  Grit,  and Scum Disposal

        Screenings,  grit,  and  scum  demand special  handling  if
 processed   in  an  incinerator   system.     The  impact  on   the
 incinerator,   particularly  the   MHF,   has  been  adverse,   and
 relatively  few   such furnaces   have performed  satisfactorily.
 Screenings  can   clog   feed  mechanisms  and   require   constant
 attention.    Operation  costs  are  expensive  due  to  the high
 moisture content  of the  screenings.   Grit is abrasive and  may  be
 high in organic matter and  relatively dry.   Burning grit  causes a
 deficiency in  combustion  air  and  generation  of black  smoke
 Whether  grease  and  scum are  burned  separately  or  in  a high
 proportioned   scum-sludge  mixture,  burning   them  results   in
 uncontrolled  burning,  slag, and  unburned  carbon due  to lack of
 combustion  air.    Grease  and  scum  have a  high heat  value and
 require combustion  air commensurate  with heat  release.   The air
 volume  for  a  high  proportioned  scum-sludge  mixture  is  not
 normally available  in the air distribution system of a MHF.

       The experience of plants  utilizing  MHFs adapted  to burn
 screenings, grit,  and scum demonstrate  that  these  furnaces have
 been largely inadequate  for  this  service.   In addition, there is
 considerable risk  to  furnaces  and to emission  violations caused
 by incomplete combustion.

       The  FBF is  much more  adaptable  to burning  grease,  oils,
and  other   materials   of  high  calorific   value.    Pumping  and
extruding   fuels   directly  into  the  media  bed  are  the  normal
feeding methods in  the FBF.  Within  the last 10 years,  10 FBFs
have been  designed  and put  into service to  burn both  sludge and
grease.                                                    y
                               52

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ADMINISTRATIVE PROBLEMS

       Problems in the  administration  of  MHF  and FBF systems are
in the areas of operations management,  staffing,  and  training.

       Management  personnel  should  be  aware  of the  technical
complexities of MHF  and FBF  systems and  should have a knowledge
of  system  O&M needs,  including  expected results,  methods  to
evaluate performance, and a system of personnel accountability.

       Many of  the problems  discussed in  this section,  although
related  to  the design of the system and  the  equipment provided,
can  often  be  eliminated   or  at  least  mitigated  by  proper
operations  managment.    In  many  instances,   properly  designed
systems  have  functioned poorly due to  lack of proper management
procedures.  Areas in which operations management can be improved
to enhance  incinerator system performance  include:

            Control of sludge  dewatering  operation  to maintain a
            steady, uniform  sludge  feed to  the  incinerator

            Control of energy  consumption  through  efficient sludge
            dewatering

            Control   of  sludge feed   and   ash  disposal  through
            scheduled, thorough maintenance of  these  systems.

       Because of the  complexity  of  the incineration process,
having an O&M staff  with the  right qualifications is an absolute
necessity.    This staff  must have  a  working  knowledge  of  the
process,  the  ability to troubleshoot  process  problems,  and  be
motivated  to   learn.      Salaries   commensurate   with    these
qualifications are  also  needed   to  attract  and  retain  highly
skilled  personnel.

       Quality training  is  required  for  personnel  who operate,
maintain,   and manage  these systems.    The  training normally
provided by the  manufacturer during  startup  is, in most cases,
 insufficient   to  ensure that  long-term  training  needs  are  met.
 Specialized training is required  and should include:

            Basic  knowledge  of routine  system  startup, shutdown
            and standby  procedures

            Preventive maintenance requirements and procedures

            System process control theory and  testing procedures

            Process troubleshooting techniques

            System economic  considerations


                                53

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           Process data collection, documentation, and evaluation

           Support   system  O&M,   control,   fine   tuning,   and
           troubleshooting.

TECHNICAL INVESTIGATIONS OF MULTIPLE-HEARTH FURNACE PROBLEMS

       Although the  problems  and solutions in  this  section have
been  catagorized  into  design,  operation,  and  administrative
areas, problems overlap  and system improvements  must  be made in
all  three  categories.   Successful operation  of  an  incinerator
system is  dependent upon  the integration and  implementation of
all   necessary    and   required   improvements.       Technical
investigations of  conventional  MHF  systems  in  three  major cities
illustrate this approach.

       The investigation  of conventional MHF operating  modes in
Indianapolis,  Indiana   (4);    Nashville,   Tennessee   (5);   and
Hartford, Connecticut (6);  and  consequent  changes in these modes
are fully documented  in  EPA reports on  the  respective municipal
facilities.     The  operational  modes   recommended  and  adopted
resulted  in  remarkable savings  in  auxiliary  fuel consumption for
the plants   in these  cities.    In  the  case  of  one plant,  the
operational   changes resulted  in a  large decrease  in  particulate
loading  to the scrubbers  and  the avoidance of  retrofitting with
new, energy-intensive scrubbers at  high capital  cost.

       The  changes  in operation modes for all  of these  plants
include:

          Maximum use of  central  shaft cooling  air  return  to
          minimize hot air heat loss

          Reduced draft  to minimize air  in-leakage

          Slow central  shaft  speed  to  increase sludge  drying
          time concurrent  with  sludge combustion at a  selected
          lower hearth

          Elimination of   air   flow   to  and  operation  of  top
          burners resulting  in provision  of  more  hearths  for
          drying  at  reduced temperatures

          Burner  operation under the combustion  zone  hearths

          Maintenance   of  excess   air   at  no  greater   than
          50 percent

          Instrumentation  and,  in  some cases, control  changes to
          measure and regulate  sludge,  air, and fuel flows  and
          to control  fuel  and  air  flow remotely.


                               54

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       The main  thrust  of  the  work performed  at the  plants in
Indianapolis,  Nashville,  and Hartford  is  the establishment of
operating procedures  suited to  the  sludge  feed  rate  and sludge
volatiles, which  utilize the MHF volume  for  combustion  in  the
most appropriate  zone and  for  drying  in a  far more  efficient
manner  than has  been practiced.   To  achieve  fuel  savings,  the
operation has to  be performed  with  greatly reduced  excess  air.
The  incinerator  upgrading  work,  resulting from pilot  project
findings  and   subsequent  recommendations  by   the  Indianapolis
Center  for Advanced Research (ICFAR)  (4),  is  estimated  to  have
cost $20,000  per  incinerator  in 1980  to  1981  dollars,  which
includes operator  training.   This cost is  normally  recovered in
fuel savings within a 3- to 6-month operating  period,  since the
average savings  per furnace is estimated at $180>f000  per  year.

       This approach  to incinerator upgrading relies  mainly on
well trained,  informed  operators.  The inherent  strength of the
approach  is  transmitting  knowledge  of  the   full  performance
capabilities of  the furnace to the staff.
                                55

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                            SECTION 4

         SUMMARY OF DESIGN AND OPERATIONAL CONSIDERATIONS


       The  selection,  design,  and  operation  of  a  MHF  or  FBF
incineration system must take into account  the  integration of the
system into the overall treatment process and the complexities of
the system.  Failure  to  do  this has led to  many of the past and
current  problems  that  have  plagued MHF  and  FBF  incineration
processes.   Under  proper conditions, these  processes can  be an
effective part  of  the sludge processing  train.

       This  section  summarizes  the  more  important  design  and
operational  factors   to  review  when  considering  MHF  or  FBF
incineration as part of a new or  upgraded  treatment facility, or
when optimizing the performance  of an existing  facility.

IMPROVING SYSTEM DESIGN

       When planning a MHF  or  FBF system, the  following  process
and support system equipment design factors should be considered:

           The   selection  of  an  incineration  system should  be
           carefully  considered  in recognition of  plant size and
           anticipated  sludge   production.      As   a   general
           guideline,  FBF systems  are most applicable  at  plants
           of 10 mgd  or more,  and MHF systems at  plants  of 20 mgd
           or more.

           The  design of  an  incineration process  should  integrate
           the  incineration and  support systems into one  design
           by  developing  operating  scenarios,  calculating  heat
           and   material    balances,   and   defining   operating
           ranges.   One  overall system  safety  factor  should be
           applied  to the  estimated quantities of sludge.

           Incineration and  support   system  equipment  selection
           and  sizing  should be  carefully matched to the rate and
           characteristics of  sludge  production  expected from the
           mainstream  process.     Either  units  that   can   be
           incrementally  modified  or multiple  units  should  be
           provided,  where needed,  to avoid inefficient  operation
           of oversized component  parts.
                                 56

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Incinerator  feed  characteristics  should  include  a
uniform  sludge  mixture with a  high  volatile content,
low grit content, and be greater than 20 to 25 percent
solids to maintain steady operation of the incinerator
and to conserve auxiliary fuel.

Blending tanks and multiple dewatering units should be
provided to minimize  variations in solids content and
feed rate to the incinerator.

The  ferric chloride  dosage  for  sludge  conditioning
should be optimized or eliminated to prevent corrosion
problems.   If  necessary,   corrosion-resistant  metals
should be  considered  for  ID fans,  drains, and exhaust
flues.
An energy  recovery system  to reduce  plant-wide energy
consumption  should  be  incorporated  into the  solids
handling design.

MHF support systems should include:

   sludge storage system with live bottom bins

   an ash  handling system  with a sealed ash discharge
   outlet from the MHF

   an ID fan with adequate capacity range

   central shaft cooling air fan and ductwork

   a venturi scrubber with an impingement tray section

   instrumentation to measure  oxygen  in  the  flue gas,
   temperatures on all  hearths, and draft in  the top
   hearth.

FBF support systems should include:

   sludge   feed  pumps   or   screw   conveyors   with
   adjustable speed drives

   an  external  firebox with   a  windbox  burner  and
   fluidizing air entry for heating the air

   an auxiliary fuel  system  with  flame safeguards and
   direct feed of fuel into the bed

   fluidizing air blowers  with  multiple impellers

   a venturi  scrubber  with  an  impingement  tray  and
   demister section
                      57

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              a freeboard  temperature control  system with  high
              pressure water pump and sprays.

IMPROVING EXISTING SYSTEMS

       Although recent  technical  investigations  have  primarily
focused on operational improvements  to MHFs,  improvements to the
support systems,  greater  efficiency in sludge  dewatering,  and the
principle of more efficient  sludge drying are  applicable  to  both
types of  furnaces.   The  design  factors  discussed above  and the
following key  factors from technical  investigations should  be
used as  guides for determining  needed  modifications  to  improve
operation and  increase the  efficiency of an existing  MHF or FBF
incinerator  system.

           Incorporation  of  sludge  dewatering  systems that  will
           produce dryer  sludge with  an  increased  volatile solids
           content and a  more homogeneous product.

           Development of sludge  feed systems  that  will provide a
           more constant, continuous, and controllable  feed  rate
           to furnaces.

           Utilization of sludge conditioning chemicals,  such  as
           polymers,   that  will  not  produce  the  slagging   and
           corrosion  problems  associated  with  the use of ferric
           chloride.

           Installation  and proper maintenance of  sludge  and ash
           conveyance  systems  to  reduce  abrasion  and  to  improve
           performance.

           Maintenance of  design  furnace capacity  by repair  of
           all  equipment-related  air-leakage  into  an  MHF   and
           minimization  of  the draft in  the  top  hearth of  the
           furnace.

           Minimization  of hot air heat  loss  by maximization  of
           central shaft  cooling  air  return.

           Improvements   in   sludge  drying  by  reducing  central
           shaft   speed   or   by  reducing  or  eliminating  active
           burners above  the  combustion  zone  in  a  MHF  and  by
           using   one  FBF  for  sludge drying  in  a multiple  FBF
           installation.

           Control of  excess air  rates and reduction  of heat  loss
           by using an oxygen  analyzer  to  indicate  the  type  of
           combustion  in  a MHF  (high/low  excess  air).
                                 58

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DESIRABLE OPERATING CHARACTERISTICS

       The key  parameters for  efficient  performance of  MHF and
FBF   systems   include   feed   sludge  characteristics,   furnace
operation, air   rates,  and,  for  the  MHF  system,  temperature.
These parameters are  important  considerations  in designing a new
system or in  optimizing the performance of an operating facility.

           A   uniform  and  constant  sludge  feed  is  needed  to
           maintain steady furnace temperatures and air rates and
           to minimize energy consumption.  Screenings, grit, and
           scum  should  not be  fed directly  into a MHF  to avoid
           uncontrolled burning,  slagging, and air deficiencies.
           Grit and scum  should only be burned in  a  FBF when the
           feed  is  under  manual  control  and  with the  required
           excess air.

           Furnace startups and shutdowns should be  minimized to
           avoid temperature  fluctuations,  which  result  in high
           auxiliary  fuel   consumption.     Continuous   24-hour
           operation of both types of furnaces  is preferable.

           The  rotational speed  of  the  rabble  arms  in  a  MHF
           should be  less than 2  rpm  to  increase sludge drying
           time and to maximize the use of the  hot exhaust air to
           dry the sludge.

           Sludge loading  rates  to the FBF should be maintained
           at  design   values   at  all  times   due   to   minimum
           fluidizing air  requirements that must be  met  in order
           to conserve  auxiliary  fuel  and energy  consumption by
           the fluidizing air blowers.

           The temperature ino the MHF  combustion  zone  should be
           kept  below  1,650 F   to  prevent   slag  and  clinker
           formation.   This  is  accomplished  either by decreasing
           the oxygen  in  the  fixed carbon  burning  zone  or  by
           increasing the air flow into the  combustion zone.

IMPROVING PLANT OPERATIONS AND MAINTENANCE

       The efficient,  safe,  and  cost-effective operation  of  an
incineration   system  is   dependent  upon  having  well-qualified,
trained  personnel,  working  within  a  well-managed  system  with
adequate  budget  support.    The  complexity  of  the  incineration
process and specialized equipment  of MHF  and FBF systems require
that special  attention be  given to the management, training, and
staffing  for  these  systems   to  ensure  safe   and  efficient
operation.
                                 59

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Each  incinerator  facility  should  have  a  detailed
management plan  that includes procedures  to collect,
review,  and  analyze process  data for  trend analyses
and  process    optimization;   communication   channels
between  operators  of solids  handling  processes;  and
procedures   to    evaluate    system    and   personnel
performance.

Training   should    be    provided   for   operations,
maintenance,  and management personnel with emphasis on
"hands-on"   training.     This   training  should   be
routinely  updated   to   reinforce   process   control,
troubleshooting techniques,  and preventive maintenance
procedures.

A  skilled and qualified  staff  must   be  present  to
monitor   and  optimize   the   incineration   process.
Management must  support  this  staff  by  maintaining
adequate  spare parts and supplies  and by  providing
training and  commensurate pay.
                      60

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                       REFERENCES
 U.S.  Environmental  Protection Agency,  "Improving  Design and
Operation   of   Multiple-Hearth   and   Fluid   Bed   Sludge
Incinerators," EPA Contract No. 68-03-3208, in preparation.

 Environmental Protection Agency,   "Process Design Manual for
Sludge Treatment and Disposal,"  EPA 625/1-79-011,  September
1979.

Brown and Caldwell,  "Central Contra Costa Sanitary District -
Solid  Waste  Resource  Recovery,   Full  Scale  Test  Report,"
March 1977.

U.S. Environmental Protection Agency, Indianapolis Center for
Advanced  Research,   "Plant  Scale  Demonstration  of  Sludge
Incinerator Fuel Reduction," EPA 600/2-83-083,  March 1983.

 U.S.  Environmental  Protection Agency,  "Sewage  Sludge Incin-
erator Fuel  Reduction  at  Nashville, Tennessee,"   EPA 600/2-
83-105, December 1981.

 U.S.  Environmental  Protection Agency,  "Sewage  Sludge Incin-
erator Fuel  Reduction,  Hartford,  Connecticut,"  EPA 600/2-84-
146, March 1982.
                              61

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             ENGLISH TO METRIC UNITS CONVERSION  TABLE
       English
Conversion
Factor
Metric Equivalent
British Thermal  Unit,  Btu    1.055

Cubic feet per                0.472
  minute, cfm

Degrees Fahrenheit,F     0.555  (F-32)
Feet per second,
  ft/sec
0.305
Kilojoule,  kJ

Liters per  second,
  L/s

Degrees        0
  Centigrade,   C

Meters per  second,
  m/s
Foot, ft
Million gallons per
day, mgd
Pounds per gallon,
Ib/gal
Pound per pound, Ib/lb
Pounds per
square inch, psi
Pounds per ton, Ib/T
0.305
43,800
0.120
1000
6895
0.500
Meter, m
Milliliters per
second, mL/s
Kilograms per liter,
Kg/L
Gram per kilogram,
g/kg
Pascals, Pa
Gram per kilogram,
g/kg
  * U.S. Government Printing Office: 1986151-098/42548
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