THE AKRON RECYCLE ENERGY SYSTEM: A PERFORMANCE UPDATE

Transcription

1 THE AKRON RECYCLE ENERGY SYSTEM: A PERFORMANCE UPDATE DAVE CHAPMAN Formerly with Department of Public Service The City of Akron, Ohio KENNETH C. O'BRIEN R. W. Beck and Associates Denver, Colorado ABSTRACT Operating and maintenance difficulties were experienced at the Akron Recycle Energy System during initial This paper describes the modifications made to the plant and operating performance subsequent to the completion of the modifications. operation of the plant. The incineration of solid waste was suspended in June of Modifications were made and the plant has been in commercial operation since January A summary of the problems and solutions is presented along with current operating results. ORIGINAL PLANT DESCRIPTION Solid waste is delivered in trucks to the tipping floor where it is weighed on a scale. After weighin, trucks dump the solid waste into a pit which is self unloading by means of two lines of positive displacement multiple rams. INTRODUCTION In July of 1979, construction was completed on a solid waste resource recovery plant for the City of Akron, referred to as the Akron Recycle Energy System. The plant was designed to process 1000 TPD (907 tpd) of solid waste and generate steam for sale in the downtown area. The plant is located south of the central business district, adjacent to the world manufacturing headquarters of B. F. Goodrich. Operating difficulties were experienced during startup and after nearly two years of effort to resolve the problems, performance was still well below the design performance criteria. This was due primarily to operational and maintenance problems with the pneuma tic fuel transport system. The City of Akron and a co'participant in the project, Ohio Water Development Authority studied a number of alternatives and decided to enter into a contract with Tri cu Resources, Inc. to design and construct modifications to the plant to improve performance. TrieU has subse quently been retained to operate the plant. The plant has been operating successfully since it was returned to com mercial operation in January A grapple crane intercepts unsuitable materials as the solid waste reaches the end of the pit. Solid waste is conveyed from the pit to one of two 1500 hp (1119 kw) horizontal hammermill shredders. Each shredder is rated at 60 TPH (54 tph). In the original plant design the shredders were followed by an air classification density separator. The heavy fraction passed through a magnetic separator where ferrous materials were recovered. The remainder of the heavy fraction was landfi1led. The light fraction was pneumatically conveyed to a 1900 ton (1723 t) storage bin. The light fraction was then auger fed to a surge bin from wruch it was pneumatically transported to one of the boilers. One surge bin served all three boilers. The plant design incorporated three Babcock and Wilcox boilers, each rated at 126,000 lb/hr (57,204 kg/h) of steam at 560 psig, (3861 kpa) saturated, when burning refuse having a heating value of 6500 Btu/hr (3611 Call kg) or 95,600.lb/hr (43,402 kg/h) when burning 5450 Btu/ lb (3027 Cal/kg) refuse. The bouers employ the Stirling design, with spreader stokers and traveling grates. Each bouer has an economizer, four stage electrostatic precipi tator and regenerative air heater. The bouers are equipped to burn natural gas or No. 2 oil as alternative fuels. The 248

2 original plant incorporated a manual dry ash removal system. The steam generated by the plant is delivered via a pipeline system to steam customers or is used inhouse to generate electricity in one of two 2MW turbine generators. The 10 psig (69 kpa) exhaust steam is used for feedwater heating or can be vented. Deaerating steam can also be supplied directly from the 560 psig (3861 kpa) header. Condensate is returned from certain customers, amounting to about 30 percent of the steam supplied. PLANT MODIFICATIONS Construction of the plant modifications began in July of The work was substantially complete by October of Figure 1 shows the plant as it looks with the modifications completed. Figure 2 shows a simplified flow diagram for the material handling systems after modifications were made. The major changes to the plant are as follows: (1) Fuel Preparation and Handling. The air classification density separator was removed. Refuse is conveyed, by belt conveyor, directly from the discharge of the shredders to the magnetic separator. The original magnetic separator was replaced with a stronger duty double drum unit, with an air scrubber. The light fraction storage bin and pneumatic fuel transport system were deactivated and replaced with a belt conveyor and chute system which conveys fuel directly from the magnetic separator to the boilers. A series of proportioning devices are used to deliver the proper quantity of fuel to each boiler with the excess being conveyed back to the receiving/storage pit. (2) Ash Handling. The manual dry ash handling system was replaced with a wet system. Bottom ash from a boiler drops from the grates directly into a water quench tank, which also acts as an air seal. Ash recovered in the economizer section is also conveyed to the quench tanle Fly ash from the precipitator is transported to a silo and then to a pug mill. Ash from the quench tank along with fly ash is conveyed to a truck loading station for disposal. (3) Process Control. A microprocessor based control system was installed in place of the original pneumatic system. Other changes included; shredder in feed surge bin modifications, installation of shredder baffles, and in stallation of a shredder blast protection wall. In addition to the modifications made to improve plant performance, a hot water heating system was in stalled which uses 10 psig (58.95 kpa) exhaust steam from the turbine generators to produce 210 F (99 C) water for sale to nearby businesses and residences. PLANT PERFORMANCE The modified plant was first returned to operation using solid waste for fuel in late October of Performance tests were run during the period from November 15, 1982 to December 14, Acceptance criteria were as follows: (1) 800 ton (726 t) average daily throughput. (2) Excess air at or reasonably close to 50 percent. (3) Ash with carbon content less than 3 percent. The average daily throughput during the test was 656 tons (595 t). The 800 TPD (726 tpd) level was not reached primarily due to. difficulties with the delivery of solid waste to the plant. The plant processed more than 800 tons (726 t) on 13 days, more than 1000 tons (907 t) on 3 days with a daily high oq 141 tons (1035 t). Excess air was measured at percent and carbon content of the ash was measured at less than 1 percent. Plant performance was judged to be satisfactory and Trlcil began operation of the plant on a commercial basis under con tract with Akron on January 1,1983. Since that time the plant has been operating successfully. For the period January 1 October 2, 1983, the following performance was recorded: (1) Average daily throughput of 599 tons (543 t). (2) Total steam production of 1.07 X 109 lb (0.486 X 109 kg) using solid waste. (3) 84 percent of steam generated with solid waste. The remainder was generated with natural gas. (4) 100 percent of steam customers requirements served without interruption. (5) 11,200 hr of boiler operation using solid waste fuel. During the month of May, average daily throughput was 804 tons (729 t) with 91 percent of the steam generated with solid waste. The plant processed more than 1000 tons (907 t) on 4 days during that period. Performance data for the months January through August are shown in Table 1. The difference between steam produced (which is measured at the bouer outlet header) and steam sales is accounted for in distribution line losses, deaerator makeup requirements, steam used for electric power generation and in some cases steam vented to atmosphere. The quantities of ferrous metals, ash and rejects (material extracted by the grapple crane in the raw refuse area) are measured in truck loads. Trucks are weighed period ically to provide a means of estimating weight. The following conversion factors were provided: Ferrous 15 tons (13.6 t)/truckload Ash 17 tons (15.4 t)/truckload Rejects 9 tons ( 8.2 t)/truckload Using these factors, the weight of ferrous metals recov ered was calculated to be approximately 5 percent of 249

3 FIG.1 AKRON RECYCLE ENERGY SYSTEM plant throughput while rejects amounted to approximately 3 percent. A similar comparison was not useful for the ash since it is weighed wet. Electrical consumption at the plant has averaged 90 to 100 kwh per ton (99110 kwh/t) of refuse. This high value was due to a low power factor, approximately 0.7. The power factor is being raised to 0.9 and electrical consumption is expected to be in the range of 60 to 70 kwh/ton (6677 kwh/t). Figure 3 graphically presents the monthly performance data. The steam produced with gas cannot be measured directly. Tricil indicated that the plant uses a conversion factor of 0.79 lb (0.36 kg) of steam per 1 SCF (0.028 m 3 ) of gas. Weekly performance reports are prepared at the plant which show important operating data for each day. A typical weekly report is reproduced in Table 2. The quantity of waste processed cannot be directly measured but is estimated based on the weighing of the incoming refuse trucks and on the amount of waste remaining in the receiving/storage pit at the end of each day. This may produce some error fn the daily statistics but is not significant in the long run since the pit is emptied periodically. The number of hours each boiler is operated using solid waste is recorded as indicated in the table. The remainder of the time the boiler is either out of service or is being fueled with natural gas. Boilers are not normally fueled with solid waste and natural gas simultaneously. The boiler operating data shown in Table 2 are typical in that there are relatively few days where boilers are operated with solid waste the full 24 hr. Tricil reports that this is largely related to the availability of solid waste fuel from the fuel supply system. This condition can be detected from Table 2 where it can be seen that the maximum run times on solid waste are identical for 2 boilers on several days. The quantity of steam generated using waste is calculated by subtracting the amount generated using natural gas from the total quantity of steam generated by all three boilers, which is metered. The steam generated by natural gas is calculated by applying the 0.79 factor mentioned above to the metered gas consumption. Figure 4 shows the steam generated by solid waste as a percent of total steam generated for the period from October 1, 1982 through October 2, This curve was generated from weekly performance data. Natural gas is only used to supply steam to uninterruptible customers when solid waste cannot be supplied to the boilers. Therefore, this figure provides an indication of total plant availability to process waste. However, owing to the fact that the number of boilers and total boiler capacity are large compared to current firm steam customer loads, it is believed that this curve is also a reasonable representation of solid waste fuel supply system availability. Tricil indicated that plant performance during the first. three months of 1983 was affected to some extent by "debugging" activities with the modified plant. SOLID WASTE SUPPLY AND ENERGY SALES The Akron plant has the dual purpose of disposing of 250

4 SCALE RECEIVING AND STORAGE SHREDD ER REJECT GRAPPLE CRANE RECEIVING AND STORAGE SHREDDER MAGNET IC SEPARAT OR PREPARED FUEL RECYCLE FERROUS RECOVERY BOILER BOILER BOILER!;:i t' V> 0 a: I U I.LJ a: «l Il. V> 0 a: t I.LJ ' Il. u I.LJ V> 0 a: Il. t I.LJ c..) (..) G.J ASH DISPOSAL STACK FIG.2 SIMPLIFIED FLOW DIAGRAM 251

5 _ AUg!!st 15, , , , TABLE 1 PERFORMANCE DATA 1983 January February March Aprll May June July Solid Waste tons 15,899 15,239 16,444 19,617 23,978 20,671 Received Steaa Produced 10 6 Ibs Ibs tv Vl tv Natural Gas Purchased ":F 32,617 57,232 42,292 18,528 17,806 14,817 Electricity: Purchased Generated b Total Ferrous Recovery Loads Ash Loads Rejects Loads

6 (f) (f) Z <t 0 a:: (f) c (!) z 9 u a:: (J) z w 0 CD CD O I 0 40, ,000 GR OSS STE AM PR ODUCTI ON 75 c w > LLI U W a:: 20,000 w (f) J c oj g 10,000 10,000 SOLI D WASTE RECEIVED STEAM SALES eo to STE AM PRO UCED WITH GAS o ot._..._j.._""""_l_lo J f M A M J A o MONTH FIG.3 PERFORMANCE DATA 1983 solid waste and providing energy to customers in the form of steam and hot water. In general, the plant has been operated at less than optimum conditions due to difficulties with both the solid waste supply and energy sales. Specifically, steam and hot water sales' have not been great enough to properly utilize the plant. Also, availability of solid waste during January through March was limited due to outside causes, thus affecting the plant throughput during that period. This factor must be taken into account in an evaluation of the plant performance to date. Efforts are currently under way to improve this situation to allow better utilization of the facuity. DISCUSSION OF OPERATIONS Solid waste is currently delivered to the plant within the following schedule: Monday Friday 7:00 a.m. 4:30 p.m. Saturday 7 :00 a.m. 2 :30 p.m. Sunday Closed This schedule combined with the existing storage capacity of approximately 1200 tons (1088 t) places a limit on the rate at which solid waste can be processed between Saturday afternoon and Monday morning. Solid waste tends to bridge in the storage pit. This problem has been solved by use of a special crane to resettle the waste. The shredder' has experienced one explosion since January. This caused damage to sheet metal parts but not to the shredder itself. The shredders are operated 12 hr each on a rotating basis. Performance has been satisfactory according to TricU. 253

7 TABLE 2 TYPICAL WEEKLY PERFORMANCE REPORT Waste (Tons) Residue No. Hrs of Solid Waste Operation Steam Generated Gas Gas Consumed M 1bs M Cu Ft Date Ash Rejects Ferro us B1r B1r B1r Waste Delivered Processed Loads Loads Loads M 1bs 16 M N N N VI lj"\ 17 T 18 W 4< 0.><: (1) (1) 19 TH 20 F 21 S S WEEK ,088

8 SPECIAL TEST 60 FIRE 4 DEC JAN FE8 M APR MAY JUN JUL AUG SEP MONTH FIG.4 PERCENT OF STEAM GENERATED BY SOLID WASTE The magnetic separator has had four outages since January. When this happens, the ferrous materials are run through the boilers which significantly increases boiler outage time due to jams in the grate system. Also, the excess air must be increased with ferrous materials included in the fuel. The boilers were originally rated at 126,000 lb/hr (57,204 kg/h) of steam at 560 psig (3861 ka), saturated when using air classified 6500 Btu/lb (361 1 Cal/) fuel and 95,600 lb/hr (43,402 kg/h) when burning 5450 Btu/ lb (3027 Cal/kg) fuel. Tricil claims their modified system produces a fuel closer in heating value to the 5450 Btu/lb (3027 Cal/kg) figure and with this fuel considers the maximum continuous steam capacity to be about 100,000 lb/ hr (45,400 kg/h). The boilers have been operated at this rating with no apparent problems. Tricil reports having operated the boilers as high as 120,000 lb/hr (54,480 kg/ h) with a fuel feed rate of 18 tons/hr (16.3 tph). Grate loading is reported to be the limiting factor. The tubes in one boiler were recently inspected ultrasonically and found to have no Significant wastage. The grate system is reported to be the most frequent cause of down time for the boilers. Aluminum cans melt onto or into the grates and are a particular problem. Grate life is presently estimated at 1 year. Tricil believes modifications to the grates could be made which would significantly reduce grate system problems. All three boilers are operated online during the winter months. Two boilers are operated during the summer months with scheduled outages planned during this period. The solid waste fuel supply system, taken as a whole, appears to be a limiting factor in minimizing natural gas consumption. Figure 4 suggests that a solid waste fuel system availability of 90 percent is reasonable. Tricil has indicated they have a target of 90 to 92 percent. They further stated that a redundancy in some of the solid waste fuel supply system components would be desirable but was not considered feasible for Akron due to physical limitations with the existing plant. Tricil reports that other parts of the plant such as the controls, ash system and electrostatic precipitators are performing well. SUMMARY The modification program for the Akron Recycle Energy System has improved the performance of the plant significantly. The plant has demonstrated the ability to provide a reliable supply of energy to customers using solid waste for more than 80 percent of the fuel requirements. Calculations indicate that more than 1100 TPD (998 tpd) of solid waste can be processed through the plant assuming a solid waste fuel system availability of 90 percent. This has been confirmed by actual operation, and compares favorably with the original plant average design capacity of 1000 TPD (907 tpd). 255

9 ACKNOWLEDGMENTS The authors wish to thank Mr. Thomas Smith, Engineer for the City of Akron for providing data used in this report. Special thanks also go to Mr. Ralph Iacono of Tricil Resources, Inc. for providing plant records and operating information. 256