INTEGRATED POLLUTION CONTROL SYSTEM FOR SHREVEPORT, LOUISIANA, MUNICIPAL INCINERATOR

Transcription

1 INTEGRATED POLLUTION CONTROL SYSTEM FOR SHREVEPORT, LOUISIANA, MUNICIPAL INCINERATOR DONALD A. MITCHELL Hydro-Sonic Systems Dallas, Texas ALBERT J. BERGAMINI Demopulos & Ferguson, Inc. Shreveport, Louisiana ABSTRACT An existing municipal waste incinerator with reciprocating grate system was renovated as necessary to improve overall performance and to comply with air and water emissions regulations. The goal of the project was to use the latest technology in a manner which provided the best match between hardware and available resources while meeting a number of system requirements. This paper contains a discussion of the design considerations and describes how the major pieces of equipment were used to satisfy design objectives. INTRODUCTION In 1971 the City of Shreveport began operation of a 200 TPD mass-burn incinerator as a means to reduce haulage and extend the life of its landfill. Designed to standards developed a decade earlier, this facility could not comply with new source standards established under the Clean Air Act. To avoid the prospect of closing the incinerator facility, the City chose to make the modifications necessary to insure that air emissions would not ex ceed any foreseeable state or federal guideline. At the same time, the facility would be upgraded to improve its combustion performance and to reduce the discharge of dirty process water. The improvement program began in 1976 with the selection of a consultant engineering firm, Demopulos & Ferguson, Inc., and was completed upon final acceptance of equipment in THE INCINERATOR AND FLUE GAS EMISSIONS The Shreveport municipal solid waste (MSW) incinerator prior to pollution control modifications is depicted in Fig. 1. MSW is loaded by crane into a hopper located above the water-cooled charging chute. Upon falling by gravity into the furnace, the material is incinerated while traveling along a Detroit Stoker reciprocating grate system. Primary combustion air enters from below the grates, while overfire air enters above the charging chute. Secondary air for improved mixing is injected along the furnace sidewalls above the grates. The flue gas passes through a secondary residence chamber to allow time for more complete combustion. Upon exiting the secondary chamber, the gas is partially quenched by water sprays to a temperature in the range of F ( ). The humidified gas then enters a subsidence chamber where a portion of the entrained fly ash and larger-sized contaminants fall out. The gas is exhausted through a dry cyclonic dust collector into a carbon steel induced-draft fan and is fmally discharged into a 100 ft (30 m) refractory-lined steel stack at a rate of about 129, F (220, K). Stack particulate emissions were measured in 1975 to be approximately 0.55 grains/dscf (1.35 g/m3n) corrected to 12 percent COl. The state emission limit was set at 0.2 grains/dscf (500 mg/m3n), while the new source EPA standard required no more than 0.08 grains/dscf(200 mg/ m3n) corrected to 12 percent COl. In addition to particu lates, the flue gas contains ppmv ofhci (derived from the burning of chlorinated plastics) and approxi mately 50 ppmv SOl. Later analysis of scrubber waste water indicated the presence of substantial quantities of calcium, magnesium, potassium, sodium, iron, and lead and lesser quantities of aluminum, chromium, zinc, and tin. The fact that much of the emitted particulate was submicronic clearly indicated the need for a much more effective air pollution control (APC) system. 412

2 -- SUBSIDEN< :1': CHAMBER FllJE AIR SPRAYS UNDERFIRE AIR FIG.1 SHREVEPORT INCINERATOR FLOW DIAGRAM PILOT SCRUBBER TESTING At the consultant's recommendation, the City entertained a pilot scrubber test program utilizing an advancedtechnology scrubbing system developed by Lone Star Steel Company of Lone Star, Texas. The proximity of this company afforded test support at a minimum of expense. Utilizing "free-jet" mixing principles well known in aerodynamics, the Hydro-Sonic scrubbers had been shown to be quite effective in the capture of fme particulate and acid-fonning gases [1-3]. Several scrubber configurations were tested and all were found to be quite able to meet air emissions standards. One configuration of particular interest was an ejector-driven device which could be powered by steam injection, thus eliminating the need for a fan exhauster. However, because steam production was not im-, mediately planned, the primary scrubber choice was limited to a two-stage, fan-driven arrangement called the "tandem nozzle hydro" depicted in Fig. 2. The results of testing on this device indicated a required pressure drop of about 35 in. WG (90 cm water). As with other gas-atomized scrubbers, this device follows the "contacting power rule," showing an increase in efficiency with increased pressure drop at a given water injection rate, taken in this case to be about 2 kg of water per kg of gas scrubbed. Depending upon water quality and temperature, an increase in water can to some extent serve to improve cleaning perfonnance just as an increase in energy (pressure drop). FACI LITY IMPROVEMENT PROPOSALS In conjunction with the successful testing of the pilot APC system, the City's consultant made several recommendations including the following major items: (1) Incinerator upgrade, including refractory wall replacement and improvements in combustion air, stoker drive components, and controls. (2) Installation of three parallel scrubbers with the ability to isolate any given unit for maintenance and the controls necessary to automatically shut down the scrubbers and revert to the old flue gas system on an emergency basis. (3) Installation of a sophisticated water treatment system to treat and recycle scrubber liquid as well as contaminated water from the grate siftings hopper, quench pit, and subsidence chamber drain. Based upon projections as to equipment requirements, the City followed a cost-saving recommendation to open bidding for pre-purchase of the required major sub-systems, including the scrubbers, induced-draft fan, motor control system, and water treatment system. Emphasis was placed upon reliability, low maintenance, and automation of controls and safety devices. THE WATER TREATMENT SYSTEM While the requirements for the incinerator upgrade and APC system were thought to be substantially well defined, there was much less certainty concerning the nature of the water handling circuit. The proposed system would have a total water demand of about 1700 gpm (386 m3/h), Ashladen streams from the furnace and subsidence chamber would be alkaline and heavy in solids. The scrubber quench drain would be quite acidic due to HCI absorption, while the scrubber discharge would contain very fme 413

3 'IO I.D. F AN I r- 'IO LANDFILL FIG. 2 SIMPLIFIED DIAGRAM OF THE AIR CLEANING AND WASTEWATER TREATMENT/RECIRCULATION SYSTEM TANDEM NOZZLE "FREE-JET" SCRJBBER QUENCH CYCUNE INCINERA'IDR FliJE GAS L Y i SUMP -... I... 0VERF'I!l'l * I S UMP..- H2 SO4 t f Feso,4.., ACIDIFIER; -.I NEUTRAL t rca (00) 2 [ POLYMER!_.. o..j l FlOC. _".I CIARIFIERJ RECYCLE -f mw, 'IO SEWER 'lhickner [ ::LJ =. POLYMER I I I [?P3, 1 'IO LANDFILL

4 particles compqsed of heavy metals, requiring treatment beyond what is conventionally required for fly ash removal. Several companies were contacted in an effort to find an acceptable proposal for water treatment. While a number of companies were ready to supply equipment, very few seemed able to actually design a complete system entailing a number of uncertainties concerning liquid stream contaminants and potential scaling and corrosion problems inherent in a tightly recirculated liquid stream_ After further investigation, Lancy International of Zelienople, Pennsylvania, offered a system capable of handling the problems. A simplified schematic of the wastewater treatment and recirculation system is shown in Fig. 2. The objective is to counter-flow the water through the scrubbing train so as to maximize the efficient use of water while minimizing the quantity of liquid ultimately treated. The system contains three major counter-flow recycle loops, finally treating only 537 gpm (122 m3/h). Acids from the scrubber quench are neutralized by lime addition at a level not so high as to induce scaling. Heavy metals in the incinerator effluent are prepared for subsequent precipitation by addition of ferrous sulfate at reduced ph. Lime neutralization initiates precipitation, aided by flocculation with a polyelectrolyte. A tube settler clarifier separates the solids from the water. A net blow-down of 80 gpm (18 m3/h) containing about 6 mg/l of total suspended solids is discharged to the sewer system (compared to 350 gpm (80 m3/h) of once-through water for the old incinerator facility). Sludge from the tube settler clarifier is discharged into an existing sludge thickener, and the stabilized sludge is trucked to the landfill. cent weight reduction upon combustion. The stack gas contained 3.5 percent CO2 (dry), 16.7 percent O2 (dry), 24.6 percent H20, 0.7 percent ppmv HCI and 15 ppmv S02 at 166 F (348 K). Uncorrected particulate emissions average about 0.02 grains/dscf (50 mg/mgn). Nuisance problems associated with piping, nozzle plugging, controls, and ducting were noted and corrected. The one remaining deviation of the system from design concerns the quantity of flue gas treated. Although designed for a flow rate of 50,500 dscfm (80,000 m3/hn) plus moisture, the high excess air rate of approximately 380 percent in the stack suggests appreciable overdrafting, as indicated by a measured stack flow of about 75,000 dscfm (118,000 m3/hn) plus moisture. Part of this excess can be attributed to in-leakage through dampers, but a significant portion must be attributed to a high rate of air entering the furnace. While this overdrafting does apparently extend the life of furnace grating, it also imposes a penalty on the APC system, resulting in a reduced water injection ratio and increased CO2 correction factor which must be applied to the measured emission level. It also increases the fan energy requirement. Nevertheless, the system is able to accommodate this level of drafting without exceeding state or EPA codes and without limiting incinerator throughput. Based upon maintenance and operating cost data furnished by the City of Shreveport for the 6-month period from January 1, 1983 to June 31, 1983, the City incinerated 25,064 tons at a total cost of $495, giving a total cost per ton of garbage incinerated of $19.78 and showed $0.97 is directly attributable to the APC system. Table 1 provides a summary of the added pollution control system operating cost on a projected annual basis. Project costs are summarized in Table 2. THE COMPLETED SYSTEM AND ITS PERFORMANCE Renovation of the Shreveport MSW incineration facility was completed without major changes in the spring of Down time of the incinerator was minimized by erecting the new equipment adjacent to the old. New steel ducting carried the flue gases from the subsidence chamber to the new APC system. The scrubbers and downstream ducting were constructed of fiberglass-reinforced plastic to eliminate corrosion. The induced-draft fan, supplied by Champion Blower and Forge, Inc. of Roselle, Illinois, has an Inconel 625 wheel and 316L stainless steel housing. It was specified for an inlet of 91,000 acfm (155,000 m3/h) at a draft of about 40 in. WG (100 cm water). Compliance testing of air [4] and water emissions [5] were completed in July All environmental regulations were satisfied. The incinerator was operating at a feed capacity of tons/hr with a nominal 76 per- TABLE 1 ADDED OPERATING COST Projected Item Annual Cost Previous Cost Added Cost Electricity $ 158,740 $ 77,912 $ 80,828 Chemicals 16, ,686 Water 14,515 63,504 (-48,989) TOTAL $ 189,941 $ 141,416 $48,525 TABLE 2 SUMMARY OF PROJECT COST Item Hydro-Sonic Air Cleaning System Wastewater Treatment and Recirculation System Induced-Draft Fan and Motor Erectidn and Installation Engineering Services TOTAL Cost $ 281, , ,153 1,049, ,098 $2,158,

5 No maintenance costs were attributable to the APe system and no new personnel were added to the labor force to operate the system. After more than a year of operation, it appears this facility will operate successfully for years to come. Experience gained will aid in improved designs for any similar facility. REFERENCES [1) McCain, J. D. and Smith, W. B., "Lone Star Steel Steam- Hydro Air Cleaning System Evaluation," EPA-650/ , April [2) Mitchell, D. A., "lrtjproving the Efficiency of Free-Jet Scrubbers," Environment International, Vol. 6, pp , [3) Choung, Y., et 01., "Simultaneous Fly Ash and Sulfur Dioxide Collection in a Hydro Sonic Systems Scrubber," 75th Annual Meeting of the Air Pollution Control Association, APCA Publication , June [4) "Source Emissions Survey of City of Shreveport Incinerator," Mullins Environmental Testing Co., Inc., July [5) "Analysis Report," Lancy Laboratories, July 26, Key Words: Incinerator Pollution Rehabilitation Scrubber. Shreveport. Treatment 416