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