United States
Environmental Protection
Agency
Water Engineering Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-85/135 Jan. 1986
&EPA Project Summary
Fate of Selected Metals and
Emissions from a
Sludge/Wastepaper Gasifier
N. W. Sorbo, G. Tchobanoglous, and J. D. Lucero
A study was conducted to analyze a
pilot-scale gasification system for sludge
and wastepaper and to quantify its
gaseous, paniculate, and metal emis-
sions. The downdraft. packed-bed, air-
blown gasifier processed up to 1800
MJ/hr of fuel. Four different densified
mixtures of wastewater sludge and
source-separated wastepaper were used
as fuel. Eight experimental gasifier runs
were conducted to quantify operation
characteristics. In addition, particle and
metal size distributions and particulate,
gaseous, and metal emissions were
measured from the combustion of pro-
ducer gas, a product of the gasification
process.
The energy content of the producer
gas varied between 4.83 and 7.04
MJ/m3, with the lowest values associ-
ated with the highest fuel ash contents.
Measured concentrations of gaseous
emissions after combustion of the pro-
ducer gas varied between 15.1 and
16.5 ppm for CO, 1.39 and 12.7 ppm
for total hydrocarbons (THC), 98.6 and
121 ppm for NOx, and 55.5 and 105
ppm for SO2. Particle concentrations
varied between 35.7 and 193.0 mg/dry
standard cubic meter corrected to 12
percent CO2.
Based on metals balance data, volatile
metals like Cd, Pb, and Zn were enriched
on particulate matter, and the matrix
metals like Cu, Cr, and Fe, were en-
riched in the char. Based on particle and
metal size distribution data, the mass
median aerodynamic diameter (MM AD)
for particulate matter varied between
0.31 and 0.95 micrometers. It was also
found that Cd, Pb, and Zn were enriched
on smaller particles, and that Ni, Cr, and
Fe were enriched on larger particles.
This Project Summary was developed
by EPA's Water 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 back).
Introduction
The adverse environmental impacts of
wastewater sludge on both groundwater
and land quality is a growing concern. As
a result, incinerators and other thermal
devices for neutralization and volume
reduction of wastewater sludge will be
used to a greater extent in the future. An
alternative to conventional sludge incin-
eration is the co-disposal of wastewater
sludge and solid waste in a gasifier. The
purpose of this report is to analyze the
feasibility of using a gasifier for process-
ing various sludge and wastepaper fuels
and to quantify its gaseous, particulate,
and metal emissions.
The sludge and wastepaper gasification
process discussed here involves the
gasification of densified mixtures of
sludge and source-separated wastepaper
in a simple packed-bed, batch-fed reactor
using air as the oxidant. The gasification
process involves the partial combustion
of a carbonaceous fuel with about 20 to
30 percent of the stoichiometric oxygen
requirement. The products of gasification
are a low-energy combustible gas (pro-
ducer gas) rich in carbon monoxide,
hydrogen and hydrocarbon gases, and a
solid residue (char). Producer gas can be
used to fuel boilers, heaters, engines, or
turbines. Results from eight experimental
gasifier runs using various mixtures of
sludge and wastepaper were used to
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quantify the gasifier performance, the
particulate and gaseous emissions, the
fate of selected metals, and the distri-
bution of metals emissions with particle
size.
Experimental Apparatus and
Procedures
The pilot-scale, batch-fed, downdraft
gasifier and producer gas burner used in
this experimental program were designed
and constructed for work previously re-
ported. A schematic of the sludge and
wastepaper gasification system is shown
in Figure 1. Temperature, pressure, and
producer gas analysis data were collected
automatically using a dedicated computer
and data collection system. Plots were
produced showing temperature, flow, and
variations in the composition of producer
gas throughout the run. A computer
program was also developed to calculate
mass and energy balances.
Total particulate emissions from the
combustion of producer gas were deter-
mined using U.S. Environmental Protec-
tion Agency (EPA) Method 5. NO*, CO,
02, S02, THC, and O2 were determined
using California Air Resources Board
(CARB) Method 100. Particle size distri-
bution data for the particulate matter in
combustion gases were generated for
three runs using three impactors for each
run—two Anderson Mark III* stack sam-
plers, and one Pilat Mark 3 Source Test
Cascade Impactor.
To determine the fate of metals in the
gasification process, samples of the fuel,
char, fly ash, slag, and particulate matter
(Method 5 filter) were analyzed for seven
metals. Metals were extracted from each
sample using room temperature HCI-HF-
HaBOs digestion procedures. Atomic ab-
sorption spectroscopy (AAS) was used to
determine the concentration of Cd, Pb,
Zn, Ni, Cu, Cr, and Fe in each of the
gasification process samples for seven
gasifier runs. A parallel metals analysis
was performed for one run using both
AAS and X-ray fluorescence (XRF). In
addition, the distribution of metals in a
sample preparation for metals analysis
was assured by analysis of a National
Bureau of Standards (NBS) certified
sample of coal fly ash.
"Mention of trade names or commercial products
does not constitute endorsement or recommenda-
tion for use.
Summary and Discussion of
Experimental Results
The eight experimental gasifier runs
were conducted at nearly identical air-
flow rates (1.92 to 2.01 mVmin at 20 °C,
and 1 atm) using four different mixtures
of sludge and wastepaper. Data on gasi-
fier operations, particulate and gaseous
emissions, and the fate of metals are
presented in this section.
Gasifier Operational Data
During each gasifier run, data were
collected on fuel, char, fly ash, process
rates, temperatures, pressures, and pro-
ducer gas composition. From these data,
energy balances were calculated.
Fuel Characteristics—Four different
densified mixtures of sludge and waste-
paper were used as gasifier fuel for these
tests. A summary of the fuel composition
is presented in Table 1. The bulk density
of the fuels varied between 334 and 595
kg/m3. The higher heating value (HHV)
fuels ranged from 17.12 to 19.43 MJ/kg
on a dry basis. The fuels with the highest
ash contents, the highest bulk density,
and the lowest energy contents are all
associated with mixtures containing the
greatest fraction of sludge.
Warm-Up Flare
Fuel Hopper
Extension
/ \
Fuel Hopper
Safety Flare
Flexible Pipe
Producer Gas
Sample Pipe
High Efficiency
Cyclone
Sample Port
Fly Ash Bucket
Combustion
A-
•(!••
^- Platform Sea
Sample Ports L
Flue Gas Stack
^
o
Observation Port L
Producer Gas Burner
1 Tr^i
Platform Scale
^Cooling Air
Damper
Figure 1. Schematic diagram of the experimental sludge and wastepaper gasification system.
2
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Table 1. Fuel Composition
Element
Carbon. %
Hydrogen, %
Oxygen. %
Nitrogen. %
Sulfur. %
Ash', %
Cadmium, ppm
Lead, ppm
Zinc, ppm
Nickel, ppm
Copper, ppm
Chromium, ppm
Iron, ppm
Range
44.36
5.60
39.70
0.12
0.04
3.1
2.05
37.2
207.
21.0
109.
118.
2500.
- 47.54
- 5.80
- 44.67
- 0.63
- 0.29
- 9.5
- 5.32
- 106.
- 507.
- 88.5
- 280.
- 355.
- 7210.
'Not an element, obtained from ultimate
analysis.
Char and Fly Ash Characteristics—Char
and fly ash collected from each gasifier
run were analyzed physically and chem-
ically. The energy content of the char
varied between 14.74 and 20.46 MJ/kg
(HHV), and the carbon content of the char
varied from 39.36 to 58.37 percent. The
energy content of the fly ash varied
between 18.42 and 21.99 MJ/kg and the
carbon content of the fly ash ranged from
52.65 to 61.87 percent.
Slag Formation—Slag is formed in the
gasifier when the fuel ash reaches 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. Based on experimental
gasifier runs, it was found that densified
mixtures of sludge and wastepaper with a
fuel ash content less than or equal to 6.9
percent can be gasified without signifi-
cant slag formation.
Process Temperatures—Process tem-
peratures throughout the gasification
system varied widely. A knowledge of
process temperatures at five different
locations in the gasification system was
important in understanding the gasifica-
tion process.
The reduction zone temperature repre-
sents the temperature at which most of
the producer gas is made. The reduction
zone temperature varied betwee 950 and
1000°C. The partial combustion zone
temperature represents the temperature
at which exothermic combustion reac-
tions occur. Slag formation is initiated in
this region. Though not measured in this
study, partial combustion zone tempera-
tures are 50 to 100°C higher than those
in the reduction zone.
The lower reduction temperature rep-
resents the temperature at which the
producer gas and char leave the reactor
bed. The lower reduction zone tempera-
tures varied between 780 and 830°C.
The cyclone and stack gas temperatures
represent those at which fly ash and
particulate matter are collected, respec-
tively. The cyclone temperature varied
between 550 and 610°C, whereas the
stack gas temperature varied between
380 and 420°C.
Producer Gas Composition—Producer
gas is the desired product of the gasifica-
tion process. Carbon monoxide, hydrogen,
and total hydrocarbons make up the
combustible fraction of producer gas.
Results of gas analyses for producer gas
are summarized in Table 2. Though the
air input rate was relatively constant for
all runs reported, a wide variation occur-
red in the energy content of the producer
gas. The lowest producer gas energy
contents were generally associated with
fuels having high ash contents.
Energy Balances—Energy balances and
gasifier efficiencies were calculated using
the data described above. The cold gas
efficiency of the gasification system (i.e.,
the efficiency of converting fuel to pro-
ducer gas at 25°C) varied between 52 and
85 percent. However, because of experi-
mental errors, efficiencies above 75 per-
cent should be considered suspect. If the
sensible energy of the producer gas can
be used in a boiler, then the conversion
efficiency is the sum of the cold gas
efficiency and the sensible heat (about 12
percent).
Particulate and Gaseous
Emissions
Particulate matter emissions from the
combustion of producer gas were meas-
ured over a series of six runs. Both
gaseous emissions and particle size dis-
tributions from the combustion of the
producer gas were measured over a series
of three runs. Particulate matter emis-
sions samples, particle size distribution
samples, and gaseous emissions samples
were taken from the sample ports shown
in Figure 1.
Based on data from EPA Method 5
tests, concentrations of particulate matter
(corrected to 12 percent CO2) ranged
between 36 and 193 mg/DSCM (dry
standard cubic meter). The stack velocity
was 6.19 to 7.25 m/sec, and the iso-
kinetic ratio varied from 88.6 to 97.4
percent. Based on these measurements,
the producer gas burner system met
federal standards for particle emissions
from incinerators (189 mg/DSCM) for all
runs except Run 36.
A summary of gaseous emissions is
presented in Table 3 and the particle size
distribution results are discussed later.
Fate of Metals
The fate of metals in the gasification
process is presented and discussed using
enrichment factors that are useful pa-
rameters for comparing the metal com-
positions of char, fly ash, and particle
emissions with that of fuel. Enrichment
factors correct for increases in metal
concentration resulting from carbon
losses by normalizing the sample and the
fuel with an ash matrix constituent.
Enrichment factors presented in Figure 2
were normalized with respect to iron
according to the following formula:
EF =
[M]/[Fe] sample
[M]/[Fe] fuel
where [M] = concentration of
metal in ppm
[Fe] = concentration of
iron in ppm
The metals in the samples have been
normalized against Fe because it is a
reasonably good tracer for the ash matrix
of the fuel. It is not volatilized to any
significant extent at the thermal condi-
tions experienced in these tests. Any
metal that behaves just like Fe will yield
an EF equal to 1 in the char, fly ash, and
particulate matter.
If EF < 1 for the char, then metal has
been lost from the matrix, most likely by
volatilizaton. If EF > 1 for the char, then
Fe has been lost from the matrix and the
initial assumption of an association of the
metals in a common matrix is not a good
one. EF > 1 implies particles containing a
preponderance of Fe are selectively being
removed from the char by the producer
gas stream.
For the fly ash and particulate matter, if
EF > 1, then condensation or absorption
of volatile metals on the particles has
occurred. If EF < 1, then metals are
leaving the particles. Because the fly ash
and particulate matter are in gas streams
that are cooling, volatilization is unlikely.
If EF < 1 for fly ash and particulate matter,
it is more likely that the model is faulty
and that particles leaving the bed do not
have some common ash matrix, but have
been classified aerodynamically into a
fraction that has a lower metal to Fe ratio
than the average ash matrix.
The expectation that Cd, Pb, and Zn
might be volatilized is confirmed by
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Table 2. Composition and Energy Content of Producer Gas
Item
Run 36
Run 37
Run 38
Run 40
Run 41
Run 42
Run 44 Run 44 GC
Dry gas composition
(by volume), %
CO
Hz
CHS
CiH**
CO 2
/V2f
Gas moisture content
(by wet volume), %
Gas energy content, MJ/m3
(dry gas, LHV, 25°C. 1 atm)
20.56
15.44
5.38
2.31
13.83
42.48
15.47
7.04
19.89
15.36
3.68
1.58
13.78
45.71
16.10
5.99
17.93
14.52
2.21
0.95
13.16
51.23
16.22
4.83
20.89
15.50
4.26
1.83
13.50*
44.02
14.31*
6.45
19.68
15.85
3.49
1.50
13.50*
45.98
14.12
5.90
19.83
16.14
2.79
1.19
13.50
46.55
14.42
5.55
20.04
16.59
3.09
1.33
14.29
44.66
13.69
5.73
19.88
15.79
2.69
0.82
10.79
50.03%
13.69
5.28
*Measured as total hydrocarbons (THC); C//4 is assumed to be 70% of THC, and C?H* is assumed to be 30% of the THC.
4 Assumed typical value; experimental data not available.
t/V2 includes nitrogen, argon, and trace amounts of nitrogen oxides. /V2 is determined by difference. A/a = 100% - (CO +
^Includes 0.84% Oz.
THC + COt + On).
Table 3.
Gas
Gaseous Emission Results
Gasifier Run Number
36
37
38
NO*, ppm
SOz, ppm
THC. ppm
CO. ppm
COz.%
Oz.%
98.6
55.5
12.7
16.5
4.33
16.0
99.2
62.6
4.20
16.8
3.19
17.3
121
105
1.39
15.1
3.95
16.9
consideration of the EF values shown in
Figure 2. These metals are removed from
the char (EF < 1). Both the fly ash and the
particulate matter show evidence of
condensation (EF > 1).
The situation is less clear for the non-
volatile metals. Grand Cu behave similarly
and all EF's are slightly less than unity.
The behavior of Cr and Cu reflects experi-
mental inaccuracies probably related to
sampling. Material balances for non-
volatile metals can account for 80 percent
of the metal, whereas over 100 percent of
the iron was found. This discrepancy
probably accounts for the low EF's. It
appears that these elements are matrix
elements that behave much like Fe. The
results for Ni are unclear because of poor
material balances. Based on fly ash and
char results, Ni appears to be a matrix
element, but some enrichment of the
particulate matter also occurs (EF ~ 5).
Material balances were good, and con-
sistent, for all metals except Ni and ranged
fromSOto! 10 percent. Total losses of the
various metals in the particulate stream
are interesting. Average percent values
are: Cd - 60; Pb - 40, Zn - 20, Cu - 2, Cr -
2, Fe - 2. Ni losses were between Zn and
Cu but a range of values cannot be
assigned for this metal.
X-Ray Fluorescence Analysis
To compare XRF with AAS analyses,
both procedures were used to determine
I
I
1.5
1.0
0.5
Ni
Char
Cd
I
_. Zn
ft
Cr
Cu
4.0
3.0
I
I
.5
2.0
1.0
Fly Ash c
Ni
\\
,11
P
d
2
b
n
0.1
Figure 2. Enrichment factors for char, fly ash, and particulate matter. Bar represents all values: horizontal line is mean value.
4
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metal concentrations in gasifier samples
during one run and in an NBS-certified
sample of coal fly ash. Though only seven
elements were analyzed by AAS, 17 metal
concentrations were available with XRF
from only one sample. However, in com-
paring the XRF and AAS results, it appears
that the latter produced much more
accurate results. The lack of XRF accuracy
is probably due to difficulties in sample
mounting. For example, only 2 mg of
sample can be attached to the XRF slide; it
is therefore difficult to get a representa-
tive sample. In addition, it was impossible
to confirm that the sample thickness on
the XRF slide was uniform. Because the
XRF analysis detects metal mass per
square centimeter, it is critical that the
sample layer be of a uniform and known
thickness. Based on these results, XRF is
an excellent method to determine which
metals are present, but it should not be
used to quantify concentrations of metals.
Particle and Metal Size
Distribution
The "metal size distribution" repre-
sents the fraction of the total metal mass
found in particulate matter of a given size
range. The MMAO of a metal represents
the particle diameter at which 50 percent
of the total metal mass is found on
particles with a diameter less than the
MMAD. A summary of the MMAD's for
particulate matter in the flue gas is
presented in Table 4. The MMAD for
particles in the flue gas varied between
0.31 and 0.95 micrometers based on
impactor studies.
Metal enrichment on small particles
can be determined by comparing the
MMAD's of particulate matter and metals.
Because the MMAD for Cd and Pb is less
than or equal to that for particulate matter.
metal enrichment on small particles is
indicated. Small particle enrichment of
Zn is shown on all runs except PT1 and
AN6. Based on these analyses, the en-
richment of Cd, Pb, and Zn on smaller
particles is consistent with the relatively
high vapor pressure of these metals. By
contrast, it can be concluded that Ni, Cr,
and Fe are concentrated in the larger
particles (see Table 4). Based on the data
presented, Cu seems to behave like a
volatile metal, which is not consistent
with vapor pressure data.
Conclusions
1. Four different mixtures of sludge
and wastepaper with fuel ash con-
tents varying between 3.1 and 9.5
percent were gasified. The fuel
mixture with the highest ash con-
tent that did not cause significant
slag formation is considered to be
the maximum feasible fuel ash
content. For these tests, the maxi-
mum feasible fuel ash content was
6.9 percent.
2. The producer gas generated during
eight gasifier runs had an energy
content that varied between 4.83
and 7.04 MJ/m3 (dry, LHV, 25°C).
The lowest producer gas energy
contents were associated with high
fuel ash contents.
3. The cold gas efficiency of the gasi-
fier varied between 52.35 and
84.58 percent. Because of experi-
mental errors, however, efficiencies
above 75 percent should be con-
sidered suspect.
4. The measured concentrations of
gaseous emissions after the pro-
ducer gas was burned varied be-
tween 15.1 and 16.8 ppm for CO,
1.39 and 12.7 ppm for THC, 98.6
and 121 ppm for N0>, and 55.5 and
105 ppm for S02. Particle concen-
tration values varied between 35.7
and 193.0 mg/DSCM corrected to
12 percent C02.
5. Based on the enrichment factor
data, volatile metals like Cd, Pb, and
Zn are enriched on particulate
matter, and the matrix metals like
Cu, Cr, and Fe are enriched in the
char. Ni cannot be classified either
as a volatile or a matrix metal.
6. Based on the data from one run, the
AAS results are more accurate than
the XRF results.
7. Based on particle and metal size
distribution data, (a) the MMAD for
particulate matter varied between
0.31 and 0.95 micrometers; (b) Cd,
Pb, and Zn were enriched on smaller
particles; and(c) Ni, Cr, and Fe were
enriched on larger particles.
The full report was submitted in fulfill-
ment of agreement CR 809747-01 -0 by
the University of California under the
sponsorship of the U.S. Environmental
Protection Agency.
Table 4. Mass Median Aerodynamic Diameter
uasirier
Run
Number
40
41
42
impacior
Run
Number
PT1
AN1
AN2
PT2
AN3
AN4
PT3
AN5
AN6
Particulate
Matter
NA
NA
NA
0.95
0.31
0.31
0.92
<0.32
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N. W. Sorbo and G. Tchobanoglous are with the University of California. Davis, CA
95616; andJ. D. Lucero is with CH2M-HHI, Denver, CO 80222.
Howard Wall is the EPA Project Officer (see below).
The complete report, entitled "Fate of Selected Metals and Emissions from a
S/udge/Wastepaper Gasifier," (Order No. PB 86-131 026/AS; Cost: $16.95,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, v'A 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Water Engineering 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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