United States
Environmental Protection
Hazardous Waste Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-85/136  July 1986
Project Summary
Coal/d-RDF  Co-Firing  Project
Milwaukee  County,  Wisconsin
R. S. Hecklinger and F. R. Rehm
  The potential for reduction in fuel
cost impelled Milwaukee  County  to
explore the possibility of supplementing
coal with refuse  derived fuel at the
Milwaukee County Institutions' Power
  A Research and Development Project
was carried out to mix a densified refuse
derived fuel (d-RDF) with coal at the
fuel receiving point and to co-fire the
mixture in a spreader-stoker fired boiler.
  Two basic series of test runs were
conducted. For the first series, coal was
fired to establish a "base line." For the
second series, a mixture of coal and d-
RDF was fired.
  The full report describes the equip-
ment used to density refuse derived
fuel, procedures used  to prepare and
handle the coal and d-RDF mixture and
test results.
  The results include the effect of coal
and d-RDF mixture on plant operations,
boiler efficiency, stack emissions, and
extraction procedure toxicity.
  This Project Summary was developed
by EPA's Hazardous Waste Engineering
Research Laboratory, Cincinnati, OH,
to announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at

  Milwaukee County, Wisconsin owns
and operates an extensive County Insti-
tutions complex. Energy needs are served
by a  co-generation power plant. The
power plant produces electricity, heating
steam, and chilled water.
  In an effort to control inflating energy
costs, the County, in 1977, embarked on a
program to determine the feasibility of
using energy from refuse to supplant
fossil fuels presently fired in the boilers at
the power plant.
  After exploring the viability of several
alternatives, a Research  and Develop-
ment Project was formulated to determine
the feasibility of co-firing coal and dens-
if ied  refuse derived fuel (d-RDF) in one of
the existing spreader-stoker fired boilers.
  Concurrently, the Americology Division
of the American Can Company was
developing and placing in operation a
refuse processing and resource recovery
plant in the City of Milwaukee. The
Americology Plant was designed to  re-
ceive and process municipal refuse from
the City of Milwaukee and surrounding
communities to produce fuel and recy-
clable materials. The refuse derived fuel
(RDF) was intended for use  as a sup-
plementary fuel in a large,  pulverized
coal-fired utility generating station.
  The full report summarizes the coal/d-
RDF co-firing demonstration.

  Various equipment and techniques  for
densifying RDF have been proposed. A
modified John  Deere 390 Stationary
Cuber was installed in the Americology
Plant. A feed chute was provided to divert
a portion of fluff RDF normally produced
to the cubing  machine where it is dens-
if ied and subsequently mixed and co-fired
with  lump coal at the County Institutions'
power plant.
  A trial program was undertaken  to
evaluate the suitability of the 390 Cuber
for producing densified cubes. The fol-
lowing five parameters were examined to
determine impact  on  cube quality and
  1.  RDF particle size
  2.  Cuber feed rate

  3.  Cuber die length
  4.  RDF moisture content
  5.  Use of binder material

  Twenty-seven trials were run during
which the above five  parameters were
adjusted and combined. The best results
were obtained under the following condi-
  1.  RDF particle  size at 90  percent
     passing a three-quarter inch screen
  2.  Cuber feed rate maintained at three
     to four tons per hour
  3.  Use of 13-inch straight sided dies
  4.  No attempt was made to add mois-
     ture  in the cuber. Daily  samples
     were taken and analyzed for mois-
     ture content
  5.  No use of binder material

  More than  380 tons of d-RDF were
produced during the trial program. Of that
quantity, nearly 130 tons were shipped to
the Milwaukee County Institutions'Power
Plant and co-fired with coal. Production of
d-RDF was terminated due to a mechan-
ical failure of the  cuber press wheel.
Cuber component wear was much higher
than anticipated.
  The circumstances  of this trial period
indicate that  cuber maintenance costs
could exceed the value of d-RDF  as a fuel.
However,  metallurgic changes in  the
press wheel and dies could reduce these
costs to an acceptable  level.

  Coal is normally fed through  a screen
into  a  hopper  where a reciprocating
feeder discharges the coal onto a moving
belt. As the coal was conveyed upward to
a moveable tripper and into selected coal
bunkers, it readily became apparent that
the tendency  of the d-RDF material to
"bridge"  in the hopper and retract onto
the reciprocating feeder made it imprac-
tical to co-feed coal and d-RDF without
some means of preventing retraction of
  A number of modifications were made
to produce a more positive feed  of d-RDF
including adjustment of the length  and
rate of stroke of the  pneumatic ram to
provide control of the d-RDF feed rate.
  With such modifications, a coal/d-RDF
mixture in which the energy from refuse
approached 15 percent of the total boiler
energy input was achieved. This was not
as high as anticipated for the project, but
it  was the  maximum that could  be
achieved without major modifications to
the power plant coal feed equipment.
  Although the coal and d-RDF appeared
to be well mixed on the conveyer belts,
there was a tendency for the two fuels to
demix in a layered manner as it fell into
the bunkers. This was, perhaps, due to
the difference in density of the two fuels.
Thus, throughout the test program sub-
stantial variations in coal to d-RDF ratios
were experienced at the bunker outlets.

Combustion and Steam
  Fuel  from the bunkers is fed through
two  gravimetric  weigh  scales to  two
conical chutes. Each conical chute feeds
two Hoffman spreader stokers. There are
four spreaders for each of three identical
coal fired furnaces  and boilers in the
Institutions' Power Plant. Boiler #3  was
selected for the coal/d-RDF  cube firing
  The generated steam is  used to drive
two  back-pressure turbine generators,
one condensing turbine generator, chiller
system auxiliaries, and other plant aux-
iliaries.  Thus, the County Institutions'
Power  Plant is a co-generation system
producing electrical energy, high and low
pressure heating steam, and  chilled wa-
  Gas cleaning for each coal-fired boiler
is accomplished by a mechanical collector
followed by a three-field electrostatic
precipitator. The standard for particulate
emissions as established by the Wiscon-
sin Department of Natural Resources for
the Institutions' boilers is 0.15 pounds of
particulate emission per million Btus of
heat input.

Boiler Operation
  In operation, the coal/d-RDF mixture
fed reasonably well  through  the conical
chutes and the spreader stoker feeders.
On occasion,  however, manual rodding
was required when higher percentages of
d-RDF were experienced. By the time fuel
reached the boilers very little of the refuse
could  be characterized  as "cubed."
Throughout the fuel-handling system
there was progressive breakdown of the
densified material. Once in the furnace,
the d-RDF burned well. The coal/d-RDF
mixture did tend to clinker more than coal
alone.  This entailed constant operator
  Steaming ability was limited when d-
RDF was fired along with coal. The plant
operators had devised  a system  to in-
crease the feeder  stroke in order to
maximize steam generation rates  with
poor quality fuel. Even with the maximum
extended stroke, the test boiler was not
able to achieve steam generation rates
that had been easily attained when firing
coal alone. This was due to the volumetric
limitations of the spreader feeder equip-
ment when firing the less dense coal/d-
RDF mixture.
  When d-RDF fuel was bunkered with
coal, the refuse material could be handled
without incurring housekeeping problems
in the boiler plant.  Some  minor odor
problems were reported by operating

Test Program
  A  comprehensive  test program was
developed to establish  the impact of
burning d-RDF  on boiler performance,
stack emissions, and the  leachate poten-
tial of the ash residues.
  The test program was accomplished in
two  phases. First, base-line data were
established when  firing coal alone. Then
a mixture of coal and d-RDF was fired
continuously for a period of eight days. A
schematic  depicting the  parameters
measured for coal and the coal/d-RDF
mixture is shown in Figure 1.
  At all times during the test program two
coal fired boilers were operated contin-
uously. Test boiler. Unit #3, was held at
constant  load while  the second boiler.
Unit #2, automatically adjusted for vary-
ing load requirements of the Institutions'
co-generation system.
  The performance test showed a dif-
ference in efficiency  between coal runs
and coal/d-RDF mixture  runs of 2.0 per-
cent, largely due to the higher  moisture
and hydrogen content of the refuse.
  A summary of stack emissions is pre-
sented in Table 1.
  The quantity of particulate entering the
precipitator was somewhat less from the
coal/d-RDF mixture than when  coal was
fired alone. This is partly due to reduced
rate of steam  generation  during the
period when the coal/d-RDF mixture was
fired. In all measures, the performance
met applicable emission standards.
  As a whole, emission levels of various
trace metals are  low.  There  was  ar
approximate 10-fold increase in cadmiurr
emissions when burning a coal/d-RDF
mixture. Since the average weight of d
RDF in the fuel mixture was just over 2C
percent, more tests are needed  at highe
d-RDF fuel mixtures in order to draw an'
specific conclusions.
  Opacity measured  by  visual  observa
tions was reported to be less than fiv<
percent in all cases. However, visua
emissions from boiler Unit #3 when firini
the coal/d-RDF mixture were virtually un
detectable,  whereas visual emission

                                                                                           Sulfur Dioxide
                                                                                          Nitrogen Dioxide
                                                                                         Hydrogen Chloride
                                                                                            Trace Metals
                                                                                            Gas Analysis
      Coal and
   d-RDF Samples
                                         ..    ,  Forced
                                   Super Heater I Draft Fan
                                                                                                        Mercury Vapor
                                                                                                          Key tones
                                                                                                        Acid Dew Point
                                  Draft Fan
                                                13 Fields)
      Reciprocating Feeder
         Fuel Flow
Scales (2)
 Feeders (41
Figure 1.    Schematic of test program.
Table 1.    Stack Emissions

Sulfur Dioxide*
Nitrogen Dioxide*
Hydrogen Chloride*
Mercury Vapor*
Acid Dew Point, F
Trace Metals^

     Inlet paniculate  to  the  precipitators
   was analyzed for resistivity and break-
   down  voltage. No  conclusions can  be
   drawn from the results obtained.
     Combined ash samples from the ash
   silo were analyzed for extraction proce-
   dure (EP) toxicity. The results suggest that
   concentrations of contaminants for  EP
   toxicity  characteristics, measured  in
   mgs/l, decrease when d-RDF is fired with
   coal. It also appears that EP toxicity is
   likely to be less than  EPA's maximum
   allowable concentrations for boilers firing
   a coal/d-RDF mixture.


    1. The modified John Deere 390 Sta-
       tionary Cuber was not able to make
       cubes of refuse derived fuel of suf-
       ficient density  and durability  to
       remain intact during transportation
       and handling.
    2. The Cuber experienced a wear rate
       and deterioration of parts that ren-
       dered it unsuitable for extended use
       to produce d-RDF. Improved metal-
        lurgy  is needed to reduce wear
       rates to an acceptable level.
    3. The d-RDF/coal mixture presented
       difficult handling and mixing prob-
        lems with the existing power plant
       equipment and resulted in a  non-
       uniform  mixture reaching the stok-
       er. This  caused erratic feeding of
       the refuse mixture with  adverse
        impact on the  boiler steaming rate.
       An improved  design  is needed to
        achieve  and  maintain  a  uniform
       coal/d-RDF mixture.
    4. The lowdensityd-RDF, when mixed
        with coal at 20 to 25 percent d-RDF
        by weight, increased the specific
            volume of fuel to the point that the
            feeders on the test boiler had dif-
            ficulty maintaining 70  percent of
            rated boiler load.
        5.  The coal/d-RDF mixture developed
            clinkers on the  stoker grate more
            readily than coal.
        6.  At the firing rate tested (20 to 25
            percent  d-RDF by weight), the d-
            RDF produced no adverse effect on
            stack emissions or EP toxicity when
            compared to coal alone.
        7.  Additional developmental work is
            required to reduce the high cost of
            d-RDF cube production to a level
            competitive with the cost of coal.
        8.  Additional developmental work is
            necessary to improve d-RDF density
            for enhanced handling and firing of
            a coal/d-RDF mixture  in  conven-
            tional coal-handling   and  firing
         R. S. Hecklinger is with Charles R. Ve/zyAssociates, Inc., Armonk. NY 10504; and
           F. R. Rehm is with County of Milwaukee, Milwaukee, Wl 53233.
         Michael I. Black is the EPA Project Officer (see below).
         The complete report, entitled "Coal/d-RDF Co-Firing Project, Milwaukee County,
           Wisconsin," (Order No. PB 86-135 381 /A S; Cost: $11.95, subject to change)
           will be available only from:
                 National Technical Information Service
                 5285 Port Royal Road
                 Springfield, VA 22161
                 Telephone: 703-487-4650
         The EPA Project Officer can be contacted at:
                 Hazardous Waste Engineering Research Laboratory
                 U.S. Environmental Protection Agency
                 Cincinnati, OH 45268
United States
Environmental Protection
Center for Environmental Research
Cincinnati OH 45268
Official Business
Penalty for Private Use $300

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