THIRTY YEARS OF SATISFACTORY FLUID BED SLUDGE INCINERATION
THE NORTHWEST BERGEN COUNTY UTILITIES AUTHORITY EXPERIENCE
John W. Myer
Northwest Bergen County Utilities Authority
John F. Mullen
Ondeo Degremont, Inc.
Dr. Ky Dangtran
Ondeo Degremont, Inc.
2924 Emerywood Parkway
Richmond, VA 23294, USA
ABSTRACT
Fluid bed incineration has been used as a sludge disposal method at the Northwest Bergen
County Utilities Authority since 1969 when the first incineration unit was installed. In 2000,
the third generation plant became operational. Over this period, environmental regulations
have changed, as have the State mandated air pollution control devices. Since the State
mandated a wet electrostatic precipitator (WESP) for the third unit in addition to the wet
venturi scrubber, a comparison is provided of emissions with and without the WESP. The
operational and repair history of the units is reviewed, concentrating on the secondgeneration unit which operated continuously for twelve years before being placed on
standby when the third unit was installed. Experience with these units has proven that fluid
bed incineration is an economic, environmentally acceptable method of sludge disposal,
satisfactory to the Authority, the permitting agencies and the public it serves.
KEYWORDS
Incineration, Fluid Bed, Sludge Disposal, Thermal Treatment
BACKROUND
The Northwest Bergen County Utilities Authority operates the wastewater treatment
facility in Waldwick, New Jersey, which serves the Borough and seven surrounding towns.
They have been using fluid bed incineration as their sludge disposal option for over thirty
years. The plant has a design capacity of 11.5 mgd on an annual average and a peak thirtyday capacity of 16.8 mgd. It currently operates at an average of approximately 8.5 mgd.
Sludge dewatering is via belt press to about 23% dry solids and the incinerator combusts a
50:50 mix of primary and activated sludge. Sludge is primarily municipal with less than
0.1% from industrial sources. Septage is also accepted but it constitutes a minimal amount
of the total waste combusted.
The initial incinerator at the plant began operation in 1969. In 1988 a new incineration
system was installed and the 1969 incinerator was placed in back-up status. In 2000, an
additional unit replaced the 1969 installation and the 1988 unit is now used for back up.
Installation of the 2000 unit was a tight fit, since it was larger and had additional equipment
compared to the 1969 unit it replaced. All of the units were operated on a basic schedule of
twenty-four hours per day, 5 days per week. Over weekends, they were run for a short
period on oil to maintain minimum reactor temperature and avoid the requirement for a cold
start.
SYSTEM DESCRIPTION
Although the system design has evolved over the years, all three units have been fluid
bed reactors with external heat exchangers for preheating the combustion air and venturi
scrubbers/tray towers as air pollution control devices.
A process flow diagram for the 2000 unit is shown in Fig. 1. Sludge is dewatered using
belt filter presses and pumped to the reactor via piston pumps through two feed ports. No.
2 fuel oil is used as auxiliary fuel during startup and operation as needed. The freeboard
operates at a design temperature of 1550 oF. The reactor offers an expanded freeboard to
allow deceleration of the larger particles to minimize sand carry over and maximize
carbon burnout. The bed make-up sand may be fed to the reactor pneumatically during
operation if required. The reactor is of the hot windbox type equipped with a refractory
arch distributor. To minimize auxiliary fuel usage, fluidizing air is preheated to 1250 oF
in an external tube and shell heat exchanger utilizing the exhaust flue gas of the reactor as
a heat source. The air pollution control system includes a venturi scrubber following by a
tray tower and a wet electrostatic precipitator. Hot air at 530°F is also added to the stack
gas for plume suppression. Plume suppression air is preheated in a secondary heat
exchanger using the exhaust flue gas from the primary heat exchanger. There is
approximately 500 pounds per hour of ash and sand carryover in the flue gas. The solids
are removed in the high-pressure drop venturi scrubber where an ash slurry is produced,
which flows by gravity to an outdoor ash settling lagoon system for dewatering. Dry ash
at approximately 50% total solids is removed from the drying lagoon once a month. In
the early years of operation, this ash was used for roadbed material in and around the
plant, but it is now trucked to landfill since all required plant area has now been paved.
The primary differences in the 1969, 1988 and 2000 systems are in the air pollution
control train and in the system capacity. The first two units had neither the wet
electrostatic precipitator nor the plume suppression system. The former was mandated by
the State as a part of the permitting process as best available control technology for the
third unit, while the plume suppression system was voluntarily added by the Authority to
improve the esthetic aspects of the plant. (The second unit, now on stand-by, will use the
new air pollution train when it is in service.) Another difference is the substitution of
piston pumps for progressive cavity pumps as sludge feeders on the latter two units.
Piston pumps, although higher in initial capital cost, have higher reliability and
significantly lower maintenance costs. Reactor diameter and system sludge combustion
capacity was also increased with each new unit, from 1100 to 1900 to 2200 pounds per
hour at 23% dry solids.
PERFORMANCE AND EMISSION TESTING
All of the units satisfied their performance requirements on the initial test Emission
requirements were met by a wide margin upon initial testing and continued in
conformance during all subsequent testing, except for mercury emissions in the 1988 test,
which is discussed later. As an example, stack testing was performed in February 2000 to
demonstrate compliance for the latest unit. The requirements included:
-
-
The US EPA 503 regulations for carbon Monoxide (CO), Total Hydrocarbons
(THC), Total Suspended Particulate (TSP) and the following Heavy Metals:
arsenic, beryllium, cadmium, chromium, lead, mercury and nickel.
The National Emission Standard for Hazardous Air Pollutants (NESHAPS)
requirements for beryllium and mercury.
The New Source Performance Standards (NSPS) requirements for particulate
matter
The New Jersey Air Pollution Control Regulations codified as N.J.A.C.7: 27-1 et.
seq.
During the test, the plant was operated at a sludge feed of 2,250 pounds dry solids per
hour (slightly over the design loading). The exhaust gas temperature and the excess air
were continuously monitored and maintained at not less than 1500°F and 3% oxygen wet
basis, respectively. Pressure drop across the venturi scrubber was maintained at a
minimum of 26 inches water.
Table 1 shows the average emissions and the compliance status for each constituent. Note
that, with the exception of NOx and carbon monoxide, all emissions, including metals,
are one to two orders of magnitude below the allowable limit.
A comparison of the emission testing of the 1988 (Kawahata, et al, 1989) and 2000
(Daniels, 2000) unit is of interest both because of changing compliance requirements and
because of the addition of the WESP to the air pollution control train in 2000. Table 2
provides such a comparison.
Except for sulfur dioxide and mercury, emission requirements for the 2000 test are at
least equal to, and in some cases (total hydrocarbons, NOx and arsenic), considerably
tighter than for the 1988 test. As previously noted, the mercury emissions in the 1988 test
exceeded the allowable. At that time, the State had recognized the mercury limits were
overly stringent and was in the process of revising them, as is reflected in Table 2.
Northwest Bergen therefore chose to temporarily pay the fines rather than incur the
capital and operating cost of a mercury removal system that at that time was not well
proven.
As previously reported, the addition of a WESP does not have any significant impact on
the removal of carbon monoxide, total hydrocarbons, nitrogen oxides, which are more
dependent of the incinerator operating conditions than on downstream air pollution
control devices (Dangtran et al, 2001).
To further look at the impact of the WESP, additional data for the two units for metal
input and emissions and metal removal efficiency, as well as the Table 2 particulate
emissions is provided in Table 3. Although particulate emission is reduced by an order of
magnitude, the impact on metal removal efficiency is mixed. This may simply be due to
the difficulty of recording accurate low level concentration of these metals in sludge.
Except for mercury, the venturi scrubber or the venturi scrubber followed by WESP
removed over 98% of all tested metals.
Table 4 provides a comparison of the actual metal emissions with the allowable limits. As
may be seen, emissions (with the exception of mercury as previously explained) are one
to two orders of magnitude below the allowables.
OPERATIONAL DATA
From the time the 1988 unit went operational until the time it was placed on standby, the
system ran for 12 years without a shutdown or a requirement for a cold start. Minor
required repairs were performed with the unit maintained in hot standby. These included
replacement of instrumentation, tightening of bolt circles, minor weld repair to ductwork,
etc. Similar operating experience was achieved on the 2000 unit that has been running
continuously except for one three-month period in 2001. A failure of the primary heat
exchanger occurred and the system was out of service for the period while it was repaired
under warranty. During this time, the 1988 unit was operated with no operational
problems.
Once the 2000 unit had become operational, minor repairs were performed on the 1988
unit in its shut down condition prior to its being placed on standby after 12 years of
operation. These included replacement of ductwork, tightening of bolt circles and the
addition of an expansion joint between the primary heat exchanger and the venturi
scrubber. All sand was removed from the bed and the refractory was subjected to a
complete inspection. It was found to be in excellent condition and only minor patching
was required.
One change in operational procedures between the 1988 and the 2000 unit was the use of
olivine rather than silica sand in the latter. The increased hardness of olivine sand makes
it less sensitive to decrepitation than silica sand and results in a significant reduction in
sand carryover with the flue gas. At Northwest Bergen, however, as has occurred at some
other operating units, the presence of small amounts of small size grit in the input sludge
results in an increase in bed depth. Although bed material can be removed while
operational, this is more labor intensive than the addition of sand while operational and
the utility is considering a return to silica sand.
The operation of the incineration system is monitored by a single operator, who also is
responsible for the sludge belt press operation. There is an average of 0.5 mechanic per
shift for the solids handling building doing repairs and preventive maintenance for the
complete dewatering and incineration operation.
PUBLIC PERCEPTION OF THE PLANT
Despite the fact that the plant site is in a valley surrounded by expensive private homes,
the permitting process for all units went smoothly with no public opposition. The
Authority had a public outreach program and the actual operation of the systems had not
been a source of concern to the public. In the early 1980s because of a shut-down of the
1970 unit for required repairs, landfill was used as a temporary disposal method. This
resulted in the only instance of significant public complaint about the facility, due to the
truck traffic.
REFERENCES
Kawahata, M. and Bailey, G. (1989) Environment One Report Source Test for
Wastewater Sludge Incinerator at the NW Bergen County Utility Authority, Waldwick,
NJ.
Daniels, T. (2000) Adirondack Final Emission Test Report for the Emissions Testing at
the NW Bergen County Utility Authority Located in Waldwick, NJ.
Dangtran, K. and Holst T. (2001) Minimization of Major Air Pollutants from Sewage
Sludge Fluid Bed Incinerators, Presented at the WEFTEC 74th Annual Conference,
Atlanta, October 2001.
Table 1
Emissions Results and Compliance Status
2000 Fluid Bed Incineration System
Test
Parameter
Carbon
Monoxide
Total
Hydrocarbons
Nitrogen
Oxides
Sulfur Dioxide
Hydrogen
Chloride
Opacity
Particulate
Compliance
Basis
Avg. 3 runs
Any 1 test run
Reporting
Unitsa
Lb./hr
ppmvd @7%O2
Emission
Limit
0.55
45
Emission Test
Result
0.19
9.72
Avg. 3 runs
Lb./hr
0.30
0.05
Avg. 3 runs
Lb./hr
2.79
1.83
Avg. 3 runs
Avg. 3 runs
Lb./hr
Lb./hr
2.42
0.93
0.03
0.01
< 3 min. in any
30 min. period
Any 1 min.
reading
Any 1 run
Avg. 3 runs
%
10
0
20b
0
1.30b
1.0
0.04
0.07
0.07
0.002
0.0011
0.0008
0.416c
0.0021
0.0081
N/A
0.04
0.010
133.33d
0.015
N/A
N/A
0.00003
0.00003
0.01
0.00003
0.00003
0.0001
0.0009
0.0019
0.85
0.0003
0.0001
0.0013
Arsenic
Beryllium
Avg. 3 runs
Avg. 3 runs
Cadmium
Total Chromium
Copper
Lead
Mercury
Avg. 3 runs
Avg. 3 runs
Avg. 3 runs
Avg. 3 runs
Avg. 3 runs
Nickel
Selenium
Zinc
Avg. 3 runs
N/A
N/A
a
b
c
d
Lb./dry ton
Lb./hr
Gr./dscf
@7%O2
Lb./hr
Lb./hr
G/hr
Lb./hr
Lb./hr
Lb./hr
Lb./hr
Lb./hr
G/hr
Lb./hr
Lb./hr
Lb./hr
Reporting units are also the units of the emission limits.
Based on NSPS requirements for particulate matter
This is based on the emission limit of 10 grams per 24 hour period as specified in
NESHAPS Title 40 CFR, Part 61, Subpart C.
This is based on the emission limit of 3,200 grams per 24 hour period as specified
in NESHAPS Title 40 CFR, Part 61, Subpart E.
Table 2
Emissions Results and Compliance Status
Comparison of 1988 and 2000 Systems
1988 Unit (Kawahata, et al, 1989)
Venturi Scrubber
Test
Parameter
2000 Unit (Daniels, 2000)
Venturi Scrubber & WESP
Limit
Lb./hr
Emitted
Lb./hr
Limit
LB/hr
Emitted
LB./hr
Carbon
Monoxide
Total
Hydrocarbons
Nitrogen
Oxides
Sulfur Dioxide
0.55
0.55
Hydrogen
Chloride
Opacity
1.36
0.16
(9.9 ppmv/dscf)
0.02
(2.8 ppmv/dscf)
1.30
(39 ppmv/dscf)
0.017
(3.6 ppmv/dscf)
N/A
N/A
N/A
Particulate
1
0.385
(0.011 gr/dscf)
0.00000126
N/A
0.00008
0.00005
0.19
(9.7 ppmv/dscf)
0.05
(1.5 ppmv/dscf)
1.83
(53 ppmv/dscf)
0.03
(0.6 ppmv/dscf)
0.01
(0.37 ppmv/dscf)
0%
0%
0.07
(0.002 gr/dscf)
0.00003
0.00003
0.00003
0.00003
1
5.3
1
0.3
2.79
2.42
0.93
10%a
20%b
1
Arsenic
0.0039
0.0011
Beryllium
N/A
0.0008
Cadmium
0.0021
0.0021
Total
0.0081
0.0081
Chromium
Lead
0.040
0.0010
0.040
0.0009
Mercury
0.00017
0.0068
0.010
0.0019
Nickel
0.015
0.00005
0.015
0.0003
a
No longer than 3 min in any 30 min period
b
Any 1 min reading
All concentrations in parenthesis are volumetric concentration, based on dry gas
corrected to 7% O2.
Table 3
Particulate and Metals Removal
Comparison of 1988 (Without WESP) and 2000 (With WESP) Units
1988 Unit (Kawahata, et al, 1989)
Venturi Scrubber
Test
Parameter
Particulate
Gr./dscf
@7%O2
Arsenic
Beryllium
Cadmium
Total
Chromium
Lead
Mercury
Nickel
Feed
Lb./hr
Emitted
Lb./hr
2000 Unit (Daniels, 2000)
Venturi Scrubber & WESP
Removal
%
Feed
Lb./hr
0.385
0.011
Emitted
Lb./hr
Removal %
0.07
0.002
0.0026
N/A
0.0050
0.0270
0.000001
N/A
0.00008
0.00005
99.95
N/A
98.38
99.80
0.00203
0.00203
0.00217
0.02167
0.00003
0.00003
0.00003
0.00003
98.52
98.52
98.62
99.86
0.1740
0.0070
0.0040
0.0010
0.0068
0.00005
99.40
2.86
98.65
0.04644
0.00119
0.02061
0.0009
0.0019
0.0003
98.06
N/A
98.54
Table 4
Particulate and Metal Emissions
Actual Emissions as a Percent of Allowable Limits
Component
Percent of Allowable
1988 Test
Percent of Allowable
2000 Test
Particulate
38.5
7.0
Arsenic
Beryllium
Cadmium
Total Chromium
Lead
Mercury
Nickel
0.03
N/A
3.81
0.62
2.50
4000
0.33
2.73
3.75
1.43
0.37
2.25
19.0
2.0
Figure 1: North West Bergen County - Unit 2000
Compressed Air
Exhaust
Gas Duct
Sand
Storage
High Pressure
Water Pump
Continuous Emissions
Monitoring System
Service Water
Stack
Sand
Heat Exchangers
Primary Secondary
Sludge
Feed
Purge Blower
Sludge
Dewatering
Sludge Feed Pump
Water
Spray
Pump
Purge Air Blower
Fluidizing
Air Blower
Venturi
Scrubber
Auxiliary
Fuel
Oil
Preheat Burner
Combustion Air
Preheat
Gas
Feed
Preheat
Auxiliary
Reheat Air
Blower
Tray
Scrubber
Wet
Electrostatic
Precipitator
Ash Treatment