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, 0°C).
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
-------
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
(0°C, 1 atm)
Gas output rate, rrP/min
(0°C. 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, 0°C, 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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