United States
Environmental Protection
Agency
Municipal Environmental Research^
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-82-047  August 1982
Project  Summary
Co-Gasification  of Densified
Sludge and Solid Waste  in  a
Downdraft  Gasifier
S. A. Vigil and G. Tchobanoglous
  Thermal gasification is a new pro-
cess for the co-disposal of densified
sludge and solid waste in a co-current
flow, fixed bed reactor (also called a
downdraft gasifier). The advantages
of this technology include lower costs
than other incineration or pyrolysis
technologies, simple construction and
operation, and the ability to use a var-
iety  of fuels including agricultural
wastes and other biomass materials in
addition to densified sludge and  solid
waste.
  Essentially, the gasification process
involves the partial combustion of a
carbonaceous fuel to generate a low
energy combustible gas and a char.
Operationally, fuel flow is by gravity
with air and fuel moving co-currently
through the reactor. The low energy
gas is composed primarily of carbon
monoxide, hydrogen, and nitrogen
and of trace amounts of methane and
other hydrocarbons.
  Although  fixed  bed  gasifiers are
mechanically simpler than other co-
disposal reactors, such as  multiple
hearth furnaces or mass fired incinera-
tors, they have more exacting fuel
requirements which include: moisture
content, <20 percent;  ash  content,
<6  percent; and relatively  uniform
grain size. Without front end process-
ing, neither municipal solid waste nor
dewatered sludge meet these criteria.
Demonstrating that a suitable gasifier
fuel could be made with a simple front
end system consisting of source sepa-
ration for solid  waste,  a sludge de-
 watering system, and fuel densifica-
 tion system has been one of the
 objectives of this project.
  To demonstrate the gasification
 process, a pilot scale gasifier was con-
 structed. A broad range of fuels have
 been tested with the gasifier including
 an  agricultural residue, densified
 waste  paper,  and densified waste
 paper and sludge mixtures containing
 up  to 25 percent sludge by weight.
 The sludge fuels were made from mix-
 tures of lagoon-dried primary and
 secondary sludge and from recycled
 newsprint (in  full  scale systems a
 mixed paper fraction of solid waste
 could be used). Mixtures were densi-
 fied using commercially available agri-
 cultural cubing equipment.
  The gasifier was operated with each
 fuel, and measurements of the varia-
 bles needed to  characterize the pro-
 cess were made. The results of gas,
 fuel, and char analyses were used to
 compute energy balances. These data
 were also used to calculate efficien-
 cies for each run. Hot gas efficiency.
 which include the sensible heat of the
 gas, ranged from 40.0 to 85.2 per-
 cent. The cold gas efficiency, which
 does not include the gas sensible heat,
 ranged from 37.1 to 80.7 percent. The
 dry low energy gas produced during
 the tests ranged in a higher heating
 value (HHV)  from 4.52 to 6.79
 MJ/m3.
  This Project Summary was devel-
 oped by EPA 's Municipal Environ-
 mental Research Laboratory, Cincin-

-------
nati, OH, to announce key findings of
the research project that is fully docu-
mented in a separate report of the
same title (see Project Report ordering
information at back).

Introduction
  The co-disposal of  sludge (the  solid
residues of wastewater treatment) and
solid waste in a joint facility is accepta-
ble from an environmental,  economic,
and  energy standpoint.  However, the
trend in development of such projects
has been towards very large systems. It
has been assumed that the economics
of scale precludes the use of such tech-
nology by small communities (less than
50,000 population). The ever increasing
costs of energy and disposal of sludge
and solid waste  make small  scale co-
disposal attractive.
  This report presents the development
of a new process for the co-disposal of
sludge and solid waste that, unlike
existing co-disposal technology, can be
implemented on a small scale. This air-
blown  gasification process  has  been
widely applied to coal, wood, and agri-
cultural wastes  but has never before
been used for the co-disposal of sludge
and solid waste. Co-gasification of den-
sified mixtures of  sludge and source
separated solid waste occurs in a simple
fixed bed reactor, also known as a  mov-
ing packed bed reactor. Energy, in the
form of a low energy gas (LEG) produced
by the process, can be used to fuel boil-
ers, heaters, engines, or turbines.

Experimental Gasification
System
  To investigate the co-gasification of
densified sludge and solid waste, a pilot
scale gasification system was designed
and constructed. The complete system
consists of three subsystems: batch fed
downdraft  gasifier, data acquisition,
and solid waste shredding and densifi-
cation.

Batch Fed Downdraft Gasifier
  A pilot scale batch fed downdraft gasi-
fier was designed and constructed for
the experiments. The design of the gasi-
fier is  based on laboratory and  pilot
scale gasifiers built by the Department
of Agricultural Engineering at the Uni-
versity of California, Davis.
  As shown in Figure 1, the gasifier is
built in three main  assemblies,  fuel
hopper,  firebox,  and  ashpit. The  fuel
hopper is a double walled cylinder. The
inner wall is in the form of a truncated
cone to reduce the tendency for fuel
     Air inlet
     pipe
Gas dispersion
zone
                        Air inlet
                        pipe


                        Gas outlet
                       pipe
Figure 1.   Schematic of a downdraft gasifier.
bridging. The double wall acts as a con-
denser to remove water vapor from the
fuel before gasification.  Condensed
vapor is collected in a condensate gutter
and drained off after each run. The fuel
hopper is mounted on the firebox with
quick-release  clamps to  allow easy
inspection after experimental runs.
  The firebox  is also  a double walled
cylinder. The inner cylinder is the actual
firebox. Air is supplied by four air tubes
to the annular  space between the walls
that acts as an air plenum to distribute
air evenly to the sixtuyeres(air nozzles),
which supply air for partial combustion
of the fuel. A choke plate acts as a large
orifice,  replacing  the  Venturi section
previously used in earlier gasifier
designs. The firebox assembly is flange
mounted to the ashpit.
  Char is collected in the ashpit during
an experimental run. A rotating eccen-
tric grate is located in the ashpit imme-
diately below the choke plate. The grate
supports the fuel bed, and allows pas-
sage of char and gas into the ashpit. Gas
isdrawn off continuously through a pif
on the side of the ashpit.
  The choke plates and tuyeres  we
constructed  from Type 304 stainle
steel. A temperature resistant allc
ASTM Type A515*, was  used for tl
firebox and rotating grate. The remai
der of the gasifier was constructed fro
Type 1040 mild steel.
  The rolled cylindrical sections,  inn
and outer walls of the firebox, ashp
and inner and outer walls of the fi
hopper were fabricated by commerc
machine  shops.  All other cutting, 
welding, and assembly were done int
College of Engineering shops. Full siz
gasifiers could easily be constructed
relatively unsophisticated machi
shops since exotic materials orcomp
machining are not required.

Data Acquisition
  The data acquisition subsystem is
automated temperature measuremi
system. Temperatures are measui
with Type K thermocouples located

-------
 shown in Figure 2. A Type Tthermocou-
 ple is used in the air inlet line and a Type
 K thermocouple is installed in the gas
 outlet pipe. Provision is made for three
 magnetically mounted Type K thermo-
 couples for surface  temperature mea-
 surements. Thermocouple number,
 temperature, and elapsed  time  are
 printed on the paper tape output of a
 Digitec Model 1000  Datalogger.

 Solid Waste Shredding
 and Densificat/on
   Based on the successful cubing test
 with the John Deere cubing machine at
 the University, the Papakube Corpora-
 tion was contracted to prepare sludge/
 solid waste cubes. Key features of the
 Papakube system include an integral
 shredder, a metering system that main-
 tains optimum moisture content of the
 newspaper, and a modified John Deere
 Cuber.  The  extrusion dies of the
 machine have been  modified with a
 proprietary coating and finishing treat-
 ment that is said to allow the densifica-
 tion of many materials without binding
 agents.

 Experimental Results
  In the experimental phase of the proj-
 ect the gasifier was operated  at a con-
 stant  air flow rate but fueled with five
 different types  of fuel: wood chips,
 almond shells, densified source sepa-
 rated sol id waste (two types), a nd densi -
 fied mixtures of sludge and solid waste
 (10, 15,  20, and 25 percent sludge  by
 weight). The characteristics of the fuels,
 operational data from the test runs, and
 energy  balances for two  typical runs
 (RUNS 11 and 12) are  presented and
 discussed below.

 Fuel Characteristics
  All  fuels were tested for proximate
 analysis, ultimate analysis, and energy
 content (Table 1). In general, the gasifier
 fuels tested were all relatively high  in
 volatile combustible matter (VCM), low
 in carbon content, and  low in energy
 content as compared with coal, but sim-
 ilar to most woods. Both bulk and indi-
 vidual  particle densities of the fuels
were also measured (see Table 1). Bulk
density  as it  relates to storage  and
transportation is a  significant parame-
ter, and  the bulk density  of densified
fuels is twice that most natural fuels
(e.g., wood chips).
  Operational Data

    The results  of the gasification  test
  series are given in Table 2. All test runs
  were conducted at the same air flow
rate, 0.41 mVmin (1 atmosphere, 0C).
Thus, the flow rate of fuel through the
gasifier, the efficiency, and gas quality
are a function of the gasification charac-
teristics of the fuel.
                                                               Fuel
                                                               hopper
                                                               Condensate
                                                               gutter
         Tuyere
                     Choke plate
 "Mention of trade names or commercial products
 does not constitute endorsement or recommenda-
 tion for use
         Rotating
         grate


Thermocouple
Locations

      Tuyere

      Reduction zone

 T3\  Ashpit

uSj  Fuel hopper

[76]  Air plenum
                                       Figure 2.  Cross section  UCD sludge/solid waste gasifier.

                                                                                3
                      Grate drive
                      sprocket

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Table 1     Summary of Fuel Characteristics
Item
                                                RUN 1 1,
                                              20% Sludge
                                                 Cubes
                        RUN 12.
                       25% Sludge
                         Cubes
Proximate analyses
    Volatile combustible matter, %
    Fixed carbon, %
    Ash, %
    Moisture, %
Ultimate analyses
  (Dry basis)
    C, %
    H, %

    S.%
    0. %
    Residue
Energy content, MJ/kg
  (Dry basis. HHV)
Densities
    Bulk kg/m3
    Unit,  kg/m3
Table 2.    Operational Summary
          74.54
          13.05
           3.07
           9.34
          45.24
           5.81
           0.13
           0.11
          46.81
           1.90

          18.93

           536
           486
                                                                73.66
                                                                13.70
                                                                 4.08
                                                                 8.56
                                                                45.27
                                                                 5.77
                                                                 0.42
                                                                 0.16
                                                                44.18
                                                                 4.20

                                                                18.49

                                                                 row
                                                                 1014
Item
Fuel consumption rate, kg/hr
Char production rate, kg/hr
Condensate production rate, kg/hr
Net run time, min
Gas flare ignition time, min
Air input rate, m3/min
(0C, 1 atm)
Gas output rate, rrP/min
(0C. 1 atm)
Average reduction zone temperature,  C
Average gas outlet temperature, C
Volume reduction, %
Weight reduction, %
RUN 1 1,
20% Sludge
Cubes
17.5
2.47
0.50
265
24
.407

.749

779.8
197.6
64
82
RUN 12,
25% Sludge
Cubes
16.3
1.71
0.73
262
44
.415

.735

734.7
180.6
74
83
Gas Analyses
  Gas samples were collected foranaly-
sis in Tedlar gas sampling bags and ana-
lyzed off-line with a Leeds and Northrup
multicomponent gas  analyzer system.
Gas moisture content was determined
by the condensation  method.  Dry  gas
composition, gas moisture content,  and
gas energy content are summarized in
Table 3. The dry gas compositions mea-
sured during RUNS  11  and 12 were
within the normal range expectedforair
blown gasifiers.

Energy Balances -
RUNS  11 and  12
  Energy balances  were calculated
using computer  programs "GASEN,"
"GASHEAT," and "ENERGY." The out-
put from the programs "GASEN" and
"GASHEAT," the fuel and char charac-
teristics, and the operational data from
each run are used as input to the pro-
gram "ENERGY," which, in turn, is used
to compute energy balances. Listings of
the programs and printouts for each run
are attached as Appendixes A, B, and C
to the report. A summary of the energy
balances is shown in Table 4.
  In Table 4, energy balances for each
run are given  both in energy units
(MJ/hr) and percentages, assuming the
fuel net energy as  100 percent.  Gas
chemical energy is the most significant
energy output, ranging from  72 to 81
percent of the input energy. Gas sensi-
ble heat is relatively minor, contributing
only 5 percent to the energy output. The
gas sensible heat could  probably be
increased  in insulating the ashpit and
gas piping to the flare. Afar more signif-
icant energy output is the char energy,
which ranges from 16 to 25 percent of
the input net energy. As char generation
is sensitive to fuel residence time and
air flow rate, char energy could be mini-
mized by optimizing operation. Conden-
sate energy  is very minor  varying from
0.9 to 1.4  percent of the  input  net
energy.
  Energy losses for most runs ranged
from 9 to  49 percent, with 20 percent
being typical. Hot and cold gas efficien-
cies were 40 and 37 percent, respec-
tively,  for RUN 08,  and 85 and 81
percent, respectively, for  RUN 12.  Hot
gas efficiencies in the upper 60 percent
range are  typical for the runs.
  The negative energy losses shown in
Table 4 in  RUNS 11 and 12 are likely the
result of errors made in determining the
amount of char generated during each
run. Because of the relatively large stor-
age volume for char in the gasifier above
the grate, it  was difficult  to determine
accurately the amount of char gener-
ated during a short (2 to 3 hour) run.

Limitations to the
Co-gasification Process
  Although gasification itself is an ol<
technology,  the application of gasifica
tion to municipal usesisa relatively nev
concept.   Hardware  needed  to imple
ment the  concept is manufactured b
several firms, but the equipment  sti
must be considered to be in the develop
mental stage. Questions on the environ
mental effects of gasification still nee
to be resolved.  Finally, the  limitation
inherent in the prooduction of LEG mus
be recognized. The gas should be use
onsite, most efficiently in a  boiler, a
though it can also be used, with a
acceptable loss in efficiency, in a ga
turbine or internal combustion engine

Conclusions and
Recommendations
  The technical feasibility of operating
fixed bed gasifier with densified sludge
solid waste  mixtures has been demoi
strated. Densified sludge/solid was
mixtures were successfully prepared
a full scale pilot facility, and a pilot sea
downdraft gasifier was designed ar
constructed.
  The gasifier was operated with va
ious  fuels  including an agricultur
waste (almond shells), wood chips.de
sif fed source separated solid waste, ai

-------
densified mixtures of sludge and source
separated solid waste (10, 15, 20, and
25 percent sludge by wet weight). LEG
was produced during the tests with an
energy content ranging from 4.19 to
6.26 MJ/m3at hot gas efficiencies from
40 to 85 percent.
  The co-gasification of densified sludge
and source separated solid waste may
be a new approach to co-disposal that
could be used by smaller communities.
  Before the co-gasification process
can be  considered operational, how-
ever,  several key  issues  must be
addressed in future work:
  1. The optimum conditions for gasi-
    fier operations  in  terms  of fuel
    consumption, air flow, gas quality,
    and efficiency need to be defined.
    These parameters  can be used to
    develop loading  factors and speci-
    fications for the design of full scale
    systems.
  2. Conditions causing  slagging should
    be  determined. Slag control mea-
    sures such as  steam  or  water
    injection, or continuous grate
    rotation should be  investigated.
  3. The fate of heavy metals during the
    gasification  process should be
    determined.
  4. Mass emission rates and particle
    size distributions for particulates
    in the LEG should be measured to
    provide data for the design of gas
    cleaning equipment.
  5. Emission data from engines, burn-
    ers, and boilers fueled  with LEG
    should  be measured. Emissions
    should also be analyzed for poten-
    tially toxic compounds.
  6. Manufacturers of system compo-
    nents should be identified.
  The full report was submitted in fulfill-
ment of  Grant No. 805-70-3010 by the
University of California at Davis, under
the sponsorship of the  U.S. Environ-
mental Protection Agency.
Table 3.    Composition and Energy Content of Low Energy Gas
Item
                                                 RUN J1.
                                               20% Sludge
                                                  Cubes
              RUN 12.
            25% Sludge
               Cubes
Dry gas composition
  fby volume)
                 CO.%
                 Hz.%
                CHS, %
                  Oz. %
                 /V2b, %
Gas moisture content
  (by volume), %
Gas energy content MJ/M3
  (dry gas, LHV, 0C, 762 mm Hg)
20.9
14.5
 2.3
 0.1
11.9
 0.3
50.0

14.15

 5.11
21.5
13.7
 2.5
 0.1
11.0
 0.3
50.9

12.31

 5.17
a Measured as Total Hydrocarbons, (THC),  CH* assumed to be 95% of THC, C2H6
 assumed to be 5% of THC.
b/V2 includes nitrogen, argon, and trace amounts ofnitrcgen oxides. Nz is determined
 by difference, N2 = 100% - (CO + H2 +  THC + C02 + Oz).
Table 4.    Energy Balances
Item
                                      RUN 11,
                                  20% Sludge Cubes
                                  MJ/hr       %
          RUN 12,
     25% Sludge Cubes
     MJ/hr       %
Gross energy, dry fuel
Latent heat, combined water
Latent heat, fuel moisture
Net energy, fuel
Gas chemical energy
Gas sensible heat
Heat loss condenser
Char energy
Condensate energy
Energy losses
Hot gas efficiency
Cold gas efficiency
269.49
18.48
4.15
273.86
197.15
12.37
21.16
69.00
2.38
-28. 19





100.00
71.99
4.52
7.73
25.20
0.87
-10.30
76.51
71.99
268.08
16.26
4.07
247.75
199.93
11.03
19.27
41.45
3.33
-27.25





100.00
80.70
4.45
7.78
16.73
1.34
-11.00
85.15
80.70
   S. A. Vigil and G. Tchobanoglous are with the Department of Civil Engineering,
     University of California, Davis. CA 95616.
   Howard Wall is the EPA Project Officer (see below).
   The complete report, entitled "Co-Gasification of Densified Sludge and Solid
     Waste in a Downdraft Gasifier," (Order No. PB 82-23O 293; Cost: $13.50.
   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:
          Municipal Environmental Research Laboratory
          U.S. Environmental Protection Agency
          Cincinnati, OH 45268
                                                                U. S. GOVERNMENT PRINTING OFFICE: 198^559 -092/0463

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