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1 AIR CLASSIFIER PERFORMANCE AND OPERATING PRINCIPLES DONALD L. MURRAY All is-chal mers Appleton, Wisconsin ABSTRACT Allis-Chalmers involvement in solid waste processing began in the late 1960's. An air classifier was developed in the mid 1970's. Test data is presented showing how performance was evaluated and how performance can be predicted. Emission characteristics for the process air were measured and are presented for comparison to state standards. Finally, a recirculation network was designed and tested and design parameters are presented. GENERAL DUAL VORTE X AIR CLASSIFIER t upp r turbulent zone Allis-Chalmers involvement with solid waste processing began in the late 1960's when its Swedish subsidiary, Svedala Arbra sold a refuse processing system at Kovik, Sweden. Involvement in the United States began in the early 1970's with the formation of the Solid Waste Processing Operation. Shortly thereafter the directive was given to develop an air classifica tion system. After 3 years of intensive research and data generation, three technical reports, and the submittal and granting of two patents, the result obtained was the unique Allis Chalmers Dual Vortex Air Classifier. wall l_---r--divergence zone \ t l_---t---iower turbulent FIG. 1 zone THE DUAL VORTEX AIR CLASSIFIER The basic design philosophy used in developing the Dual Vortex Air Classifier and also the key to successful application of the air classification principle is the concept of distribution of individual particles in the air stream. Several features incorporated in the Dual Vortex design functionally affect this distribution of materials. A proftle of 50S

2 the Dual Vortex Air Classifier is shown on Fig. 1. Distribution begins with the passage of refuse feed through the rotary gate and deposition on a very broad and long feed chute. The long feed chute aids in giving velocity of entry to the material for penetration of the air column and distribution across the classification chamber. The area required for the air classifier chamber, defined by the throat dimensions, was found through extensive testing to be with slight variation 0.6 ft2 (0.056 m2) per ton (metric) of refuse feed. This is 0.45 fe (0.042 m2) per American short ton. Increasing the width of the air classifier decreases the lateral dimension of the chamber, making distribution across the chamber easier. Width increases at a faster rate than depth in the Dual Vortex Design. As the refuse leaves the feed chute it encounters the first turbulent zone, which again acts as a distributor in that the material is agitated throughout the upper chamber filling it. Some of the material, such as small paper and film plastic, reaches a critical flying velocity at this point and becomes entrained in the upward flowing air stream. The material whose inertia has not been overcome begins to settle in the air stream and enters the zone of wall divergence. Here the cross-sectional area of the chamber gradually is reduced, subjecting the downward moving mass to increasingly higher velocities. Again, as a critical flying velocity is reached, material becomes entrained and peels off to move upward in the air column. Finally, the remaining material encounters the lower turbulent zone. This zone is caused by and controlled by an air blade located at the throat entry. Here all but the most resistant clumps of refuse particles are redistributed and broken up, allowing recovery of the previously trapped light material to be made. RECOVERY PROJECTIONS Through the mid-1970's a multitude of tests were performed utilizing many tons of shredded refuse from the Outagamie County Refuse Shredding and Transfer Station located in Appleton, Wisconsin. For each of these tests the entire collected fractions, which have become commonly referred to as "lights," or that material captured by the air stream, and "heavies," the material not captured, were carefully hand separated to determine the efficiency of separation. Variables tested included chamber shape, velocity, size variation of the shred material, moisture variation, effects of increasing or decreasing the volume of material feed and others. A characterization of the feed material (i.e., how much paper or how much ferrous, etc.) was never attempted. Rather, a quantity of refuse, typically 1000 to 1500 Ib (453.6 to kg) was collected from material which had been freshly shred at the Outagamie County Refuse Shredding and Transfer Station. The nominal or 90 percent passing size at this facility was approximately 3 in. (76.2 mm). Data previously presented (''The Dynamics, Operation and Evaluation of an Air Classifier," Waste Age, March 1977) indicate that nominal particle sizes of 2 through 7.5 in. (50.8 to mm) had no effect on performance. No specific care was taken to include or exclude any specific materials or particles other than turning off the magnet allowing the ferrous to remain mixed in the shredded refuse being collected. While the material was being collected, random samples of approximately 3 to 5 Ib (1.36 to 2.27 kg) were taken for moisture determination. About 10 samples were drawn per refuse lot tested. It should be noted that the material gathered for testing was collected in the bucket of a front end loader as it cascaded into the ram chamber of a refuse compactor. Thus, the refuse tested received no compaction pressure prior to being introduced into the air classifier. After completing the test run, the entire "light" and "heavy" fractions were gathered and hand separated into tubs and weighed to determine actual separation effectiveness by general category of material. The data used in developing the category method of performance evaluation presented in this paper was the result of nearly 200 tests of the type just described. Air velocity for all these tests was held constant at approximately 1800 fpm (548.6 m/min). The general categories selected were paper including corrugated, plastic, wood, textile, nonferrous, and "grit". Grit represents that portion of the refuse stream generated by the shredding process and made up of minu te [defined as being minus 0.25 in. (6.35 mm)] pieces of all the categories. Typically, the grit was about 60 percent combustible in nature and was between 10 and 15 percent of the total material fed to the air classifier. Test data from two of the collection efficiency rests is presented in Tables I and 2. These tests 506

3 TABLE 1 TEST RUN 153 DATA Lights Heavies % of Material in Category Ib kg Ib kg Lights Stream Paper & Corrugated Plastic Wood Textile Nonferrous Ferrous Glass Yard Waste Putrescibles Rubble Rubber & Leather Grit TOTALS 787 (357.0) 26 (1 1.8) 32 (14.5) 26 ( 11.8) 0.2 (0.1 ) 3 (1.4) 1 1 (5.0) 29 ( 13.2) 1 (0.5) 0.2 (0.1) 0 (0) 197 (89.3) (504.7) 32 (14.5) (0.9) (4.5) (0.9) (6.0) (24.5) (46.3) (5.9) (5.9) (6.0) (0) 0 30 (1 3.6) (129.0) Refuse Run Moisture Nominal Particle Size Chamber Velocity Total Mass Balance 1397 Ib (633.7 kg) before 23.3%... after 21 % 2.47 in. (6.27 cm) 1800 ft/min (548.6 m/min) Lights: 79.6 Heavies: 20.4 TABLE 2 TEST RUN 141 DATA Lights Category Ib (kg) Paper & Corrugated (251.5) Plastic 22 (10.0) Wood 45.5 (20.6) Textile 24.5 (1 1.1) Nonferrous 0.1 (0.11) Ferrous 0 (0) Glass 3 ( 1.4) Yard Waste (60.6) Putrescibles 13 (5.9) Rubble 0.8 (0.4) Rubber & Leather 0.2 (0.1 ) Grit 119 (54.0) Total (41 5.7) Efficiency of Heavies Recovery Ib (kg) (% Flown) 21.5 (9.8) (2.3) (10.7) (1.1 ) (1 1.9) ( 18.6) 0 93 (42.2) (32.9) (44.0) (18.2) (6.1 ) (8.2) (206.0) Refuse Run Moisture Nominal Particle Size Chamber Velocity Total Mass Balance 1370 lb (621.7 kg) Before: 23%... After: 21% 2.34 in. (5.94 cm) 1800 ft/min (548.6 m/min) Lights: 66.9 Heavies: 33.1 were from a series used to determine the collection efficiency of the air classification unit under a fixed set of conditions. The shape and dimension of the chamber was fixed, a velocity was set and an attempt was made to minimize the variation in moisture content and particle size. The tables in comparison reflect quite uniform collection efficiencies, yet exhibit a rather wide variance in total collection due to the inherent differences in the initial makeup of the feed stream. Over a series of tests it was determined that collection efficiency by categories remained rather constant. This allowed the generation of the general recovery table shown in Table 3. The general recovery table is actually a summary of all the tests conducted and can be applied to any given refuse characterization or analysis to determine a projected collection efficiency and average mass transfer ratio. An example is shown in Table 4. Here the higher general recovery multipliers were applied to obtain an average recovery esti- 507

4 TABLE 3 GENERAL RECOVERY LEVELS OF MIXED MUNICIPAL REFUSE BY MAJOR CATEGORY Recovery Level % by Weight of Category Paper & Corrugated Plastic Wood Textile Nonferrous Ferrous Glass Yard Waste Putrescibles Rubble Rubber and Leather -1/4 In. Grit Category in the Feed 90-95% BO " " s5 Very dependent upon nom inal particle size mate of 69.3 percent. This value can be applied as a long term estimate by a client in determining mass balance and process flow and in estimating pot ntial RDF income, if the preprocessing system operates within the parameters defined previously. In addition, an indicator of the RDF quality is obtained. It should be noted that better quality (less contaminants such as glass, etc.) can be obtained by adjusting to the lower multipliers, but RDF quantity would be reduced in doing so. Actual reduction in quantity would be from 69.3 percent to 62 percent though it would be comprised of more than 99 percent light material categories as opposed to 96 percent purity from the higher recovery level. EMISSION EVALUATION Nearly every process developed by industry has a waste stream. In the case of air classification the waste stream is the process air used to separate and convey the light product for ultimate capture as RDF. In an induced draft system, such as the Dual Vortex Air Classifier, the waste stream is discharged from the blower. Air emission quality and discharge policy was one of the first areas to receive critical attention from government agencies. Encouraged by federal prompting, state agencies imposed stringent regulations on processes discharging into the atmosphere. A typical method of controlling discharge quality is to base emissions on process rate. Table 5 summarizes several state policies. Other states use methods basing discharge emission standards on the concentration of particulates in an emission gas such as gr/ft3. The emissions level of an air classification unit is actually a measure of capture efficiency of the suspended particle de-entrainment device. The Dual Vortex Air Classifier shown in Fig. 2 uses a cyclone module for particle de-entrainment. It is of the high efficiency design and is rated at a capture efficiency of 95 percent removal of particles 10 11m (1 X 10-2 mm) in diameter or larger. The emissions levels from the pilot unit shown in Fig. 2 were tested both by Allis-Chalmers personnel and by an independent testing agency. In each case the EPA 5 method of measuring particulate emissions was applied. The results of the TABLE 4 PLOT OF GENERAL RECOVERY Category Paper & Corrugated Plastic Wood Textile Nonferrous Ferrous Glass Yard Waste Putrescibles Rubble Rubber & Leather Totals % of Category in Clients Refuse Stream Ib/l00 Ib (kg/l00 kg) of Refuse Actual Recovered General Ib/l00 Ib Recovery (kg/l00 kg) Level (%) of Refuse Fed or % ,

5 TABLE 5 PERMISSIBLE EMISSIONS STANDARDS Process Rate Tons Short (Metric) Wisconsin 5 (4.5) 9.7 (4.4) 10 (9.1) 15 (6.8) 25 (22.7) 26 (11.8) 50 (45.4) 32.2 (14.6) Permissible Discharge in Ib/hr (kg/h) by State Texas Colorado Connecticut 15.2 (6.9) 9.7 (4.4) 9.7 (4.4) 30.1 (13.7) 15 (6.8) 15 (6.8) 64 (29.0) 26 (11.8) 26 (11.8) 78.1 (35.4) 32.2 (14.6) 32.2 (14.6) TABLE 6 EMISSION TEST RESULTS Parameters Stack Velocity ft/min (m/s) Stack Flow ACFM (m3/s) Emission Rate gf/scf (g/m3) Emission Rate Ib/tons Short (kg/t) Extrapolated Emission Ib/50 tons Short (kg/50 t) Test (18.47) 6072 (2.87) (0.011) (0.027) 3.3 (1.36) Test 2 Test 3 Test (21.39) (20.7?) (21.89) (3.79) (3.73) (3.94) (0.006) (0.004) (0.008) (0.018) (0.014) (0.027) (0.91) (0.68) (1.36) PILOT MODEL tests are shown in Table 6. In addition to reporting the test data obtained on the pilot unit, an extrapolation of the data was made to demonstrate emissions expected at a 50 tons/hr process rate assuming similar capture efficiencies. This is also shown in Table 6 and can be compared with the state standards in Table 5. Comparison obviously shows favorable results. The samples collected during the emissions tests were microscopically analyzed to determine capture efficiency and to identify the material which characterized the typical particle load. The majority of captured particles were paper fibers with some crystalline particles indicating glass or glass-like materials were also present percent of the particles captured were smaller than 10 J..Lm with the largest captured particle measuring 20 J..Lm (2X 10-2 mm). RECIRCULATION CONSIDERATIONS FIG. 2 The last step in the development of an air classification system is providing adaptability in the installation of the system to satisfy all process requirements in the most economical manner. The 509

6 previous discussion already indicated process control at an acceptable level of emissions. An air classification unit by nature of its process flow requires large volumes of air. Typically in process design the classification chamber is housed indoors. The cyclones and main blower might be either indoors or outdoors. In temperate or cold weather climates this presents a problem. Generally, the operating floor is provided with sufficient heat for operation comfort. If the air classifier operates indoors and discharges to the atmosphere, tremendous quantities of air must be heated and are then wasted creating an energy demand well above what current conservation practice would find acceptable. This energy wastage problem can be overcome by addition of a baghouse. The baghouse prevents the accumulation of even minor dust loads such as suggested earlier. A baghouse, however, is expensive and requires a large area for operation raising both capital and operating costs. Of course, the maintenance cost also increases. Another answer is to install a recirculating air system, thus reusing the blower discharge air. The result would be obvious cost and maintenance advantages as compared to a once through system with a baghouse. A recirculating test unit was erected in the laboratory. A profile view of its layout is shown on Fig. 3. It contained adjustment to alter the flow balance between re-entry ducts allowing changes in re-entry velocity to be set. It had not only an air AIR RECIRCULATION TEST MODEL classification chamber model but the heavies offloading belt as well. Air flow within the return duct was found to follow traditional flow patterns. Flow and thus velocity was found to be highest on the outside edge of an elbow and lowest on the inside edge. Directional vanes were added to provide more equal distribution in the return duct orifice. Standard duct engineering practice was used in establishing the distance the surface of the return duct orifice is from the induced draft entry point at the throat of the air classifier. A series of velocities was examined and it was found that velocities of 500 ft/min (I 52.4 m/min) and less in the return duct did not exhibit any discernible variation in the air flow patterns at the air classifier intake point. Velocities above 500 ft/min did alter flow patterns. Thus 500 ft/min (I 52.4 m/min) was selected as the maximum design velocity for return duct entry ports. Once air flow was balanced in the return ducts the "heavies" conveyor was added to determine appropriate design defmition. The initial tests quickly demonstrated a very positive dampening effect on velocity distribution imposed by the conveyor, including reduced abnormalities in return duct flow velocity and distribution. Less control was required in flow distribution within the return ducts to achieve desired intake profiles at the air classifier throat. The conveyor height was varied through a test series. This defined an open area through which the air, demanded at the throat of the air classifier, must pass. A division of the air demanded by the area available indicated that velocities in excess of 750 ft/min (228.6 m/min) began to show disruptions in intake pattern while those below 750 ft/min (228.6 m/min) were satisfactory. Thus 750 ft/min (228.6 m/min) was selected as a second design maximum dimension in setting conveyor height. FINAL PROOF FIG. 3 Completion of the recirculation zone experiments completed the collection of initial design data on the Dual Vortex Air Classification system. A 70 ton (6.35 t) per hour unit is being installed at this time in Lane County, Oregon, which will provide full scale operating data. This unit will be carefully monitored and tested to provide the broad data base needed by this industry to confirm good application practice. Key Words: Classification, Emission, Pneumatic, Refuse Derived Fuel, Sampling Methods, Separator 510

PB389 Integrated Solid Waste Management

PB389 Integrated Solid Waste Management Numfon Eaktasang, Ph.D. Thammasat University Solid Waste Management Waste generation Waste reduction and separation at the source Collection Transportation Separation,