United States
Environmental Protection
Agency
Municipal Environmental
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-84-063 Apr. 1984
&ERA Project Summary
Performance and Economic
Feasibility of a Sludge/Wastepaper
Gasifier System
N. W. Sorbo, G. Tchobanoglous, J. R. Goss, and S. A. Vigil
Thermal gasification of densified
sludge/wastepaper fuels has been
shown to be technically feasible for the
co-disposal of sludge and source-
separated wastepaper. The performance
and economic feasibility of a downdraft
packed-bed gasifier system fueled with
densified sludge/wastepaper and
wastepaper fuels has been evaluated.
To assess the performance of the
gasifier, a series of gasifier runs were
conducted at various air input rates with
the use of various densified fuels. In
eight of the runs, a densified fuel con-
sisting of 20 percent lagoon-dried sludge
and 80 percent recycled wastepaper was
used; in five runs, densified wastepaper
cubes were used. Gaseous and particu-
late emissions resulting from the com-
bustion of producer gas were measured.
Char, a by-product of the gasification
process, was evaluated as a substitute
for powdered activated carbon. The per-
formance of a small dual-fuel diesel
engine combusting producer gas gen-
erated from the gasification of densified
wastepaper was also demonstrated.
The cost of sludge disposal in a
sludge-wastepaper gasification system,
using wood chips as an auxiliary fuel,
was found to be competitive with the
cost of sludge disposal by landfilling if
the electricity generated by the gasifica-
tion system can be sold at a cost of
$0.12/kWh, $0.09/kWh, and $0.085/kWh
for communities sizes of 10,000,30,000,
and 50,000 persons, respectively. The
economic feasibility of sludge/waste-
paper gasification is highly sensitive to
the cost of wood chips, the solids con-
tent of the dewatered sludge, the per-
centage of wastepaper collected, and
the resale cost of electricity.
This Project Summary was developed
by EPA's Municipal Environmental Re-
search 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
back).
Introduction
The disposal of sewage sludge and solid
wastes in an economic and environmentally
acceptable manner is a problem common to
many communities. The co-disposal of
sludge and wastepaper in a common facil-
ity is a potential solution to both of these
problems. This research project deals with
the application of the gasification process for
the co-disposal of sludge and wastepaper.
To reduce the volume of sludge, densified
mixtures of sludge and source-separated
wastepaper are gasified in a simple packed-
bed reactor (downdraft gasifier) that uses air
as the oxidant.
Experimental Gasification
System
Gasification results when a carbonaceous
fuel is partially combusted by injecting 20 to
30 percent of the stoichiometric oxygen re-
quirement. The products of the gasification
process are a low-energy gas (producer gas)
and char. Producer gas can be used to fuel
boilers, heaters, engines, or turbines; the
char can, in some cases, be used as a source
of powdered activated carbon.
To evaluate the performance of sludge/
wastepaper gasification, a pilot-scale
gasification system was designed and con-
structed. The complete system consists of
three subsystems: batch-fed downdraft gasi-
fier and producer gas burner, data acquisi-
tion, and shredding/densification system.
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The pilot-scale batch-fed downdraft gasi-
fier used in this experimental program was
designed and constructed for work previ-
ously reported. A producer gas burner was
designed and constructed to provide a facil-
ity from which particulate and gaseous
measurements of both the producer gas and
the flue gas (combined producer gas) could
be taken. A schematic diagram of sludge/
wastepaper gasification system is shown in
Figure 1.
The data acquisition rystem consisted of
an automated temperature measurement
system and a producer gas analysis system.
Temperatures and elapsed time from Type
T, K, and R thermocouples were recorded
automatically and printed on the paper tape
output of a Digitic Model 1000 Datalogger.*
The Papakube Corporation densification
system was used to shred and densify all
fuels. Key features of the Papakube system
include an integral shredder, a metering
system, and a modified John Deere Cuber
equipped with dies developed by the
Papakube Corporation.
Experimental Results
The experimental gasifier runs were con-
ducted at various air input rates with the use
of different densified fuels. A mixture of 20
percent sludge and 80 percent recycled
newspaper was the fuel used for eight gasi-
fier runs; five gasifier runs were conducted
using densified, recycled newspaper fuel
cubes. The gasifier operational data, gaseous
and particulate emissions testing, the fea-
sibility of using gasifier char as a source of
powdered activated carbon, and the opera-
tion of a small diesel engine with producer
gas are summarized below.
Summary of Gasifier
Operational Data
During each gasifier run, data were col-
lected on fuel and char characteristics
(physical and chemical), process rates, tem-
peratures and pressures, and producer gas
composition. From these data, energy bal-
ances were calculated.
Fuel Characteristics
The higher heating values and wastepaper
fuel for the sludge/wastepaper fuel were
18.78 and 19.42 MJ/kg. The bulk density of
these fuels varied between 284 to 595 kg/m3,
with the bulk density of the sludge/waste-
paper fuel being significantly higher than that
of the wastepaper fuels. The fuel ash con-
tent is one of the most critical parameters
in any downdraft gasifier application. The
percentage of fuel ash (dry basis) was 4.6
Mention of trade names or commercial products does
not constitute endorsement or recommendation for use.
percent in the sludge/wastepaper fuels and
1.0 percent in the wastepaper fuels.
Char Characteristics
The major constituents of gasifier char,
the major waste product of the gasification
process, are carbon and ash. Char produc-
tion should be minimized it represents an
energy loss as well as a waste product that
must be disposed. Reducing the carbon con-
tent of the char improves the gasification
process efficiency as well as lowers disposal
costs. The physical and chemical properties
of the gasifier char are presented in Table 1.
Slag Formation
Slag is formed in the gasifier when the fuel
ash, upon reaching its melting point, flows
together and cools. Excessive slag formation
in downdraft gasifiers can block the flow of
fuel and char through the gasifier, and thus
cannot be permitted. Because of the rela-
tionship between the fuel ash content and
reactor temperature, there has been concern
about the technical feasibility of gasifying
sludge in downdraft gasifiers. Based on a
series of eight experimental gasifier runs, it
can be concluded that fuel mixtures of 20
percent sludge and 8C percent wastepaper
can be gasified without prohibitive slag for-
mation.
Distribution of Gaseous
Constituents
Producer gas is the desired product of the
gasification process. Three gaseous consti-
tuents make up the combustible fraction of
producer gas: CO, H2, and total hydrocar-
bons. The concentration of each gaseous
constituent did not vary much despite a large
change in the air input rate (Figure 2).
Energy Balances
Temperature, process rate, and gas
analysis were used to calculate energy
balances. Gasifier efficiencies (obtained from
the energy balance calculations) as a func-
tion of air input rate are graphically sum-
marized in Figure 3. If the producer gas can
be used hot (in a boiler), the gasifier process
efficiency is the sum of the cold gas energy
and the sensible heat.
Feasibility of Using Gasifier
Char as a Source of Powdered
Activated Carbon
Char samples from eight gasifier runs were
analyzed for the effectiveness as a replace-
ment for powdered activated carbon. The
following tests were conducted on ground
and classified char samples: proximate
analysis, ultimate analysis, apparent density,
methylene blue number, iodine number, rate
of adsorption of TOC (total organic carbon).
relative TOC reduction factor (TOC RF). set-
tling tests, and scanning electron micro-
scopy. The most important char charac
terization test is the relative TOC RF, which
is used as an indicator of the relative effi-
ciency of removal of TOC with respect to a
reference activated carbon.
In most cases, the adsorptivity of the char
varies inversely with the percentage of
sludge in the fuel used to generate the char
and directly with the carbon content of the
char. Because high gasifier efficiencies are
attained when the carbon content of the char
is minimized and high char adsorptivity is at-
tained when the carbon content of the char
is maximized, the production of char that has
good adsorption properties is not feasible
when the gasifier is operated to maximize the
production of high quality gas.
Gaseous and Particle Emissions
Four tests were done to determine the
gaseous and particle emissions from the
combustion of producer gas. The producer
gas, generated by the gasification of den-
sified sludge/wastepaper cubes, was com-
busted in the producer gas burner. Both
gaseous and particle emissions data were
collected from the flue gas sample ports
shown in Figure 1.
The following values from the gaseous
emissions measurements were obtained:
NOX concentrations varied between 60 and
115 ppm, noncondensible hydrocarbon con-
centrations (based on hexane) were generally
less than 1 ppm, S02 concentrations (cor-
rected to 12 percent C02) ranged from 0.091
to 0.227 grams per dry standard cubic meter
(g/dscm). When the total particle emission
rate was measured (by the EPA Method 5
protocol), the particle concentration in the
flue gas (corrected to 12 percent CO2) range
from 0.068 to 0.164 g/dscm. Based on these
measurements, the producer gas burner
system meets federal standards for particle
emissions from incinerators (0.189 g/dscm)
without any cleanup equipment. From an im-
pactor study, it can be concluded that the
cut diameter for particles in the flue gas is
approximately 8 microns.
Operation of a Small Diesel
Engine with Producer Gas
The performance of a small diesel engine
operated in a dual-fuel mode with producer
gas was evaluated in a series of three gasifier
runs. The producer gas, generated from the
gasification of densified wastepaper, was
cleaned by a fiber glass filter and a con-
denser. The gas was fed to the engine with
a modified natural gas engine carburetor.
The practical output of the dual-fuel engine
operated with producer gas was 71 percent
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Warm-Up Flare
Fuel Hopper
Extension
Fuel Hopper
Inlet Air (
Gasifier
(D
Safety Flare
Flexible Pipe
... . _„. . Producer Gas
H.ghEff.dency Sample Pipe
Cyclone
Sample Port r
Fly Ash Bucket
Sample Ports L
Flue Gas Stack
(Combustion
\ Air
oi
f Observation Port
o
" Producer Gas Burner
I I
Platform Scale
Figure 1. Schematic diagram of the experimental sludge/wastepaper gasification system.
• Cooling Air
Damper
Table 1. Properties of Gasifier (
Run It Fuel Type
14 wastepaper
IS wastepaper
16 wastepaper
17 20% sludge/
wastepaper
18 20% sludge/
wastepaper
J9 20% sludge/
wastepaper
20 20% sludge/
wastepaper
21-A 20% sludge/
wastepaper
21 -B 20% sludge/
wastepaper
26 20% sludge/
wastepaper
27 20% sludge/
wastepaper
28-A, 28-B 20% sludge/
wastepaper
*MC = Moisture content, VCM =
iNA = Not available.
lhar
Energy Content
(Dry Basis. HHV) •
(MJ/Kg)
25.31
26.19
30.44
24.62
25.50
26.20
21.82
25.69
26.60
15.88
17.73
15.62
Volatile combustible
Ultimate Analysis
(% by weight)
C
70.93
73.29
86.62
70.53
71.97
76.27
62.98
68.71
73.46
50.68
51.98
48.18
matter.
H
0.59
0.69
0.92
0.51
0.76
0.65
0.27
1.59
2.59
0.40
0.30
0.30
0
2.78
2.69
2.64
1.06
0.94
1.52
1.70
6.79
9.70
0.0
0.34
0.0
N
0.18
0.19
0.09
0.18
0.25
0.27
0.13
0.36
0.53
0.18
0.17
0.20
S
0.02
0.04
0.03
0.12
0.08
0.09
0.02
0.05
0.12
0.01
0.01
0.01
Residue
25.5
23.1
9.7
27.6
26.0
21.2
34.9
22.5
13.6
49.3*
47.2
51.4*
Proximate Analysis'"
(% by weight)
MC
0.3
0.2
0.6
0.3
0.7
0.6
NA*
NA
NA
NA
NA
NA
VCM
5.4
4.4
4.0
2.9
3.2
3.3
NA
NA
NA
NA
NA
NA
FC
70.8
74.7
84.0
71.0
71.6
74.1
NA
NA
NA
NA
NA
NA
Ash
23.5
20.7
11.4
25.8
24.5
22.0
NA
NA
NA
NA
NA
NA
FC = Fixed carbon.
*As oxides, therefore total is greater than 100%.
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N. W. Sorbo, G. Tchobanoglous, andJ. R. Goss are with University of California,
Davis, CA 95616; S. A. Vigil is with California Polytechnic State University, San
Luis Obispo, CA 93407.
Howard O. Wall is the EPA Project Officer (see below).
The complete report, entitled "Performance and Economic Feasibility of a
Sludge/Wastepaper Gasifier System," (Order No. PB 84-169 317; Cost:
$ 17.50, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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