POWER & ENERGY

Fraser Papers Disposes of Sludge with New Fluid Bed Incinerator

BY KRISTINSCHROEDER, JEFFREY L. McCULLOCH, and JOHN F. MULLEN

In 1990, facing decreased landfill capacity and increased sludge generation from a new waste=paper deinking facility, Fraser Papers' (then Cross Pointe Paper's) Park Falls, Wis., mill began investigating sludge processing and disposal alternatives. After examining such solutions as drying, land spreading, inclusion in lightweight concrete aggregate, landfilling, and combustion in existing mill power boilers, a new fluid bed combustion system was chosen as the environmentally acceptable, economic alternative.

Following pilot scale combustion testing to establish design criteria and estimate emission levels, contracts were awarded to Dorr-Oliver for process equipment supply, to C.R. Meyer for civil engineering and installation, and to the Harris Group for general engineering. The fluid bed combustion system project was implemented on a fast track schedule to insure completion before sludge generation from the deinking facility began.

Design, construction, checkout, and startup of the new fluid bed combustion system occurred in 11 months, with initial sludge combustion in September 1993. Details of the disposal alternatives investigation, pilot testing, and system design were previously reported in P&PÕs September 1994 article titled "Cross Pointe Mill Reduces Sludge Volume with Fluid Bed Combustion."

Major components of Fraser Papers' new fluid bed system are shown in the process flow diagram in Figure 1. The new system has provided the mill with an effective and environmentally sound means of sludge disposal, while preserving the life of an existing landfill. The reduction of sludge to an inert ash has allowed the mill to avoid development of a new landfill site and the perpetual care responsibilities that go with it. Also, by using a proven combustion system, Fraser Papers was able to proceed with the new deinking mill, knowing that a means of waste disposal would exist at startup. In the more than three years of operation since startup, the system has met, and has usually exceeded, expectations, whether technical, environmental, or financial.

PERFORMANCE TESTING. While the new fluid bed combustion system was being designed, an air permit was obtained in March 1993 from the Wisconsin Department of Natural ResourcesÑfour months after initial submission. This rapid turnaround was possible because of coordination by Fraser before application submittal and because of the availability of emission data from the pilot scale testing.

Air emissions compliance testing was conducted in January, February, and June 1994. Results are compared with allowance limits in Table 1. Most actual emissions are an order of magnitude below allowables. Pilot testing had indicated that NOx emissions could be marginal, but were likely to be acceptable. Provisions had been made for future incorporation of a selective noncatalytic reduction system, but this proved unnecessary.

OPERATION AVAILABILITY. Once sludge burning started in September 1993, the unit operated continuously until the mill's annual maintenance shutdown in August 1994. Following this two-week period, the system ran continuously until the next maintenance shutdown. While a normal shutdown occurs annually, the second shutdown was stretched to March 1996 because of the excellent operating experience of the system.

Operational data on system availability for 1995 is shown in Table 2. Of the 8,201 available operating hours, total system availability was 95.5%, and if the feed pump is excluded, incinerator/air pollution control system availability was 96.7%. The majority of downtime for the incinerator and APCD was associated with shutdowns for bed sand removal and for scrubber inspection and cleaning due to scaling, both of which are discussed below.

SHUTDOWN INSPECTION RESULTS. The two maintenance inspections since September 1993 have revealed only minor, easily correctable problems with the new fluid bed system.

August 1994. This initial inspection showed that the refractory and most internals in the reactor, heat exchanger ductwork, and scrubber were in excellent condition. Drooped thermocouples and oil guns within the reactor were replaced. A refractory crack in the windbox floor was also patched. In addition, a light, readily removable dusting was observed in the duct work and heat exchanger plenums. Refractory within the combustor showed no evidence of erosion, spalling, or cracking, and looked like new.

March 1996. As previously indicated, the second maintenance inspection was delayed by more than six months because of excellent operating experience. In March 1996, drooping thermocouples and oil guns were again replaced. The remainder of the internals, including the refractory, were in excellent condition with one exception. Heavy scaling was observed within the heat exchanger tubes, probably due to the high calcium content of the feed. However, high pressure water blasting proved effective in removing this deposit.

During the latter half of 1996, chlorine corrosion was observed in the duct between the heat exchanger and the scrubber. Although a definitive cause has not been established, a possible cause was that free water from the heat exchanger cleaning operation had accumulated in the block insulation adjacent to the steel I.D. Patching permitted continued operation until the next planned shutdown in May 1997, and replacement with ductwork with a mastic coating between shell and insulation is expected to eliminate the problem.

OPERATIONAL EXPERIENCE. As indicated in Figure 1, the scrubber uses treated effluent from the mill's wastewater plant as cooling water, which was apparently the cause of two minor operational problems. The scrubber, particularly the cooling trays, was subjected to lime scaling, and the ash slurry was prone to foaming. The former problem was solved by adding an anti-scale agent with the scrubber water, while the latter problem was solved by the use of a defoamer. Operational experience since these changes were instituted in early l996 has not shown a recurrence of the problems.

The twin-cylinder sludge feed piston pump operating on Fraser Papers' sludge is a high-wear, high-maintenance item, but is still considered the best option available for the difficult application. Occasional pumping problems that do not allow full sludge feed to the combustor have occurred with certain sludge combinations, such as higher dry solids or high fiber content.

Ash from the scrubber is pumped to lagoons for decanting, and operation has in general been satisfactory. However, the nature of the ash does not provide the clear ash basin effluent seen in other applications. Also, the ash does not dewater well and the ash basin cleaning process can be sloppy. Ash for removal is typically in the 20%-40% dry solids range, and water tight trailers need to be used for ash hauling. Fraser is currently investigating further dewatering and alternate ash disposal options.

Operation has shown that the sludge has a higher value of grits and other large inerts than originally anticipated, resulting in increasing bed depth during operation. Draining the bed currently requires shutdown and cooling. Fraser is considering equipment addition to permit bed removal while operating and is experimenting with the use of different bed sands.

To better understand NOx generation, Dorr-Oliver and Fraser conducted a full-scale test program in the first half of 1994. Analysis of this unpublished data showed two things. One was that significantly higher NOx levels result from burning primary sludge than from secondary sludge. The second was that the single variable most effecting NOx production is excess air level, with higher NOx occurring at higher excess air. Figure 2 shows this trend in a computer screen printout obtained during the NOx test program.

SLUDGE DISPOSAL COST. The actual costs associated with sludge disposal using incineration at Fraser Papers in 1995 are shown in Table 3. The unit price for fuel (natural gas) was $2.29/mcf. For electricity, it was 4.5cents/kilowatt-hour, and, for ash landfill (Fraser Papers' site), the unit price was $20/yard. Actual sludge combustion was 16,274 dry tons (65,098 wet tons). The resulting unit cost of $16.87 per wet ton ($67.46/dry ton) compares very favorably with both the original estimates when the project was being planned in 1991/1992, and with the current cost of sludge landfill.

Kristin Schroeder is an environmental engineer and Jeffrey L. McCulloch is environmental manager at Fraser Papers Inc., Park Falls, Wis. John F. Mullen is a program manager at Dorr-Oliver, Milford, Conn.