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Wastewater treatment systems, both industrial and municipal, produce large quantities of sludge that require disposal. And sludge disposal poses significant environmental liabilities, logistical problems, and economic burdens.
Minergy Corp. has developed and implemented—working with several Wisconsin pulp and paper operations—several new technologies for recycling high volume wastes such as sludge, ash, and foundry sand. These technologies perform mineral recovery from the waste material, converting them into construction material and industrial feed stocks that are inert, marketable products.
With a continuously operating processing system installed at the waste source, these technologies reduce handling, eliminate downstream liabilities, and reduce disposal costs. Glass aggregate technology, discussed in this article, provides a solution for recycling sludge and establishing a true low-cost, long-term beneficial reuse.
INTEGRATED ENVIRONMENTAL APPROACH. The production of construction aggregates from sludge and/or ash is an alternative to landfilling, which is the current option for the majority of this type of waste material. Landfilling sludge consumes significant land resources and permanently removes land from future productive use. Landfills are also a potential long-term environmental liability due to the threat of ground water and soil contamination. The conversion of sludge to aggregates not only conserves landfill space, but also results in marketable products that have received exemptions from solid waste regulations.
The glass aggregate process has been developed to meet all applicable government regulations for air quality and solid waste management. Organic compounds in the sludge are destroyed in the high-temperature, high-retention mixing environment. Trace metals present in the sludge are permanently stabilized in the product, and water leach test results meet primary and secondary drinking water standards.
Sludge plays two beneficial roles when it is processed in this way. First, the organic component of the sludge provides a significant portion of the energy required. These organics are essentially a biomass fuel that is renewable through the cycle of water use and wastewater treatment. Second, the mineral content (ash, clays, and mineral fillers) found in the sludge are put to beneficial use.
The glass aggregate process offers many environmental and economic benefits to municipal wastewater treatment systems, paper mills, and surrounding communities. These benefits include reducing long-term dependence on landfill disposal, providing residents and local industries with a cost-effective alternative for managing sludge, and providing public agencies with a more comprehensive and integrated approach to solid waste management.
Glass aggregate technology converts sludge into a glass aggregate product. The markets for this industrial material include sand blasting grit, abrasives, roofing shingle granules, asphalt aggregate, chip seal aggregate, and decorative landscaping.
In addition to manufacturing a marketable product, the glass aggregate technology provides a complete solution to the disposal problem of sludge. Many other disposal technologies, including incineration, have significant quantities of ash or other waste that continue to pose disposal problems.
Because the process incorporates very high combustion temperatures with excellent fuel and air mixing, high destruction efficiencies of organic compounds are achieved, and trace metals contained in the sludge are permanently stabilized in the glass aggregate matrix.
FOX VALLEY GLASS AGGREGATE PLANT. Minergy Corp. has developed the world’s first facility, in Neenah, Wis., to recycle sludge into a glass aggregate product. Eight paper mills in the area—owned by Wisconsin Tissue Mills, Kimberly-Clark, P.H. Glatfelter, Ponderosa Fibres, and Gilbert Paper—supply the plant with paper sludge. The facility was designed and built by Atlanta, Ga.-based ESI Inc. of Tennessee.
The Fox River Valley glass aggregate plant, with P.H. Glatfelter mill shown in the background, is designed to process up to 1,300 tpd of paper mill sludge.
The Fox Valley Glass Aggregate Plant is designed to receive and process up to 1,300 tpd (450,000 tpy on an as-received basis) of paper mill sludge with a moisture content of up to 60%. Other facility design parameters include:
• Generate up to 235,000 pph of steam, initially at 350 psig/ 575ºF
• Export up to 165,000 pph of steam to a steam host 2,500 ft. away
• Convert more than 80% of the mineral component of sludge into glass aggregate product
• Extremely low noise and odor affecting neighboring properties
• Operate at high thermal efficiency to reduce operating cost.
The operations of this complex facility can be subdivided into six major categories that include:
• Sludge material handling and storage
• Sludge drying and wastewater recovery
• Dry sludge firing
• Steam production and distribution
• Glass aggregate processing
• Emissions controls.
Each of these areas presented unique challenges for ESI due to the fact it is a first-of-its-kind facility. The site itself created several challenges that had to be overcome in both the design and construction phases.
The site is a closed sludge landfill, so the piling and foundation work required careful management of excavated materials to make sure no landfilled material was removed from the site and potential run-off contained during construction. All excavated materials were replaced as backfill under the facility foundations and roadways. A methane gas collection, monitoring, and alarm system was designed and installed beneath all occupied buildings.
SLUDGE MATERIAL HANDLING AND STORAGE. Wet sludge is transported to the facility in large tandem self dumping trailers. Two truck scales at the plant entrance allow actual sludge deliveries to be measured. The sludge is dumped in a receiving building, either onto a tipping floor or directly into a live bottom pit. From this pit, wet sludge is transported using mechanical conveyors into a 2,600 wet ton capacity storage silo.
Sludge received at the recycling facility is either dumped onto a tipping floor or directly into a live bottom reclaim pit (shown).
The sludge is reclaimed at the bottom of the wet storage silo using a live bottom sweep auger that discharges onto a series of screw and belt conveyors to transport the wet sludge to the drying system. The feed rate of auger is controlled by dryer outlet moisture. The sludge feed to the dryers must be divided equally to provide equal loading to the two rotary steam tube dryers.
The system was provided can transport sludge directly from the receiving building unloading pit to the dryers, bypassing the wet sludge storage silo. However, operating experience has shown that when feeding the dryers directly from the silo reclaimer, a more consistent and homogeneous moisture content in the sludge feedstock to the dryer can be maintained, resulting in smoother dryer operation, more consistent dry sludge moisture content, and, ultimately, better cyclone furnace operation.
From the dryers, sludge is again mechanically conveyed through a series of screws and belts to the dry sludge storage silo. This silo has approximately eight hours of storage at full rated capacity.
Being a first-of-a-kind design and installation, a pelletizing system was installed between the dryers and the dry sludge storage silo. Any quantity of the dry sludge stream from the dryers to the silo can be pelletized. The pelletizing option was installed to allow sizing flexibility in the sludge stream to the cyclone furnaces. The concerns focused primarily on the inability to control excessive small particle sizing and subsequent potential ash and unburned carbon carry-over.
SLUDGE DRYING AND WASTEWATER RECOVERY. In the initial conceptual design stage, a cascade drying system was to be used that would utilize the sensible heat in the exhaust flue gas to dry the sludge. However, due to the nature and location of the facility, Minergy chose to permit the facility with essentially zero volatile organic compound (VOC) emissions.
Subsequently, a closed loop drying system was required that would allow recovery of the water vapor from the wet sludge driven off in the dryers. This water vapor is condensed in a packed tower wet scrubber, while the remaining non-condensable gases are directed to the secondary air fan inlet making up combustion air to the cyclone furnaces. Therefore, all potential VOC emissions generated in the drying process are introduced into the high temperature cyclone furnace to ensure destruction.
To maximize the facility thermal efficiency, steam is used for sludge drying. The facility has two 85-ft.-long, rotary steam tube dryers that are designed to dry 1,300 tpd of 60% moisture sludge down to 20% moisture. An initial visit to a previous sludge drying installation revealed that at these high moistures, particularly when high clay content sludge is being dried, the steam dryer tubes have a tendency to foul, requiring periodic shutdown and water washing. The dryer system was designed with future dry sludge recycle capability to reduce the propensity for fouling.
Two 85-ft.-long, rotary steam tube dryers are designed to dry 1,300 tpd of 60% moisture sludge down to 20% moisture.
Operating experience has revealed that the dryers do not foul to the point of requiring periodic shutdown for cleaning. However, downstream firing operations were initially hampered due to the periodic shedding of dry sludge buildup from the tubes. A retrofit installation of a rotary disc screen has essentially corrected this problem.
The dryer system is designed to remove 1.3 million lb/day of water from the wet sludge stream. The challenge with the closed loop drying system design focused around condensing this water from the saturated vapor stream while simultaneously avoiding fouling of the heat exchanger due to particulate carry over. The final design includes a wet scrubber that condenses all the water vapor from the dryer air closed loop and simultaneously removes the majority of any fine particulate that would potentially foul downstream equipment.
The waste water recovered from the sludge in the dryer system is discharged as a scrubber/condenser bleed stream to the local municipal waste water treatment plant. Pumps, heat exchangers, and a cooling tower are used to reject the excess heat absorbed by the scrubber water utilizing separate closed loop systems.
DRY SLUDGE FIRING. From the dry sludge storage silo, a system of screws and mechanical conveyors transports the dry sludge to a dry sludge feed bin. This feed bin is a live bottom storage design with approximately one hour capacity of dry sludge storage. This bin is equipped with eight screws, each controlled by individual variable speed drives. The dry sludge discharge of each screw goes through a rotary valve and is discharged into eight separate pneumatic transport lines that convey dry sludge to the cyclone secant ports. Each transport line has its own mechanical blower, which promotes self cleaning if line pluggage begins to form.
The glass aggregate facility’s boiler is equipped with two 7-ft-dia cyclone furnaces in which the dry sludge is co-fired with natural gas.
The boiler has two 7-ft-dia cyclone furnaces where dry sludge is co-fired with natural gas. These cyclone furnaces are a variation of older coal fired cyclone technology developed and installed for many years by Babcock & Wilcox Co. One major variation is the introduction of fuel into the cyclone using four tangential secant ports on each cyclone.
The key to dry sludge firing in cyclone furnaces is to ensure adequate temperatures to facilitate molten slag formation and tapping. The lower furnace of the steam generator is specially designed to maintain high temperatures to allow the molten slag to run down the lower furnace walls and through the discharge opening in the lower furnace floor.
To lower the melting point of the slag, the firing system was designed to add limestone as a fluxing agent. The flux is blended with each individual dry sludge feed line upstream of each rotary air lock. Proper sludge/flux mixing is attained while the material is transported in the blow lines to the cyclones.
Initial operation has indicated that limestone supplemental fluxing is not necessary because the sludge feedstock contains sufficient calcium and lime to allow the material to an acceptable melting point. Currently, the system is being fired without a fluxing agent. However, continued testing is being performed using other types of fluxing agents to promote slag tapping.
STEAM PRODUCTION AND DISTRIBUTION. The steam generator is a field erected, top supported, two drum boiler of single pass design. The complete furnace enclosure is of membrane welded wall construction. The boiler is capable of generating 235,000 pph of steam at 350 psig/600ºF superheat steam temperature. The actual design of the boiler is for future 800 psig/800ºF superheated steam operation.
This design flexibility afforded Minergy future options regarding higher pressure and temperature operation should a cogeneration retrofit be desired. The thermal efficiency of the facility has been better than predicted.
The steam generator is equipped with an economizer and air heater to utilize high temperature primary and secondary air to the cyclone burners. The facility also includes waterside auxiliaries including a deaerator, boiler feed water pumps, chemical feed systems, continuous and bottom blowdown systems, etc. The original design considerations were for the steam host to provide all feedwater to the facility. However, a small sodium zeolite water softening system has been added to provide backup and supplemental feedwater capacity.
Up to 165,000 pph of 350 psig/575ºF steam is exported to the host paper mill—P.H. Glatfelter—through a 2,500-ft-long steam distribution piping system. The exported steam is used to drive an existing paper mill back-pressure steam turbine-generator producing approximately 4 MW of power.
GLASS AGGREGATE PROCESSING. Once the molten slag discharges through the lower furnace opening, it falls into a quench tank filled with water that is maintained at a temperature of 140ºF. The quench tank water is pumped through heat exchangers in a closed loop equipped with hydroclones to prevent fouling of the heat exchangers. The cooling tower common to the scrubber cooling water loop is used to provide closed loop cooling water to the quench tank.
The molten slag generally fractures into tiny glass aggregate pieces approximately 1/2-in. and smaller. The bottom of the quench tank is equipped with a wet drag conveyor designed specifically to remove the glass aggregate while simultaneously dewatering the glass aggregate product on the wet drag upslope. The glass aggregate from the drag conveyor is discharged into a loadout area that is formed by concrete retaining walls. The glass aggregate is loaded onto trucks through the use of a front end loader.
EMISSION CONTROLS. During the conceptual and final design of the facility, ESI gave careful consideration to control of all plant emissions. The process and/or systems were designed and installed to control the following emissions:
• Particulate
• Nitrogen oxides (NOx)
• VOC
• Carbon monoxide (CO)
• Solids in wastewater
• Fugitive dust
• Odor
• Noise.
Particulate emissions are controlled through the use of a mechanical cyclone collector and fabric filter baghouse. The baghouse is a six-module pulse jet unit designed for offline cleaning. The baghouse design gross and net air to cloth ratios are very conservative. The required maximum stack particulate emission rate of 0.02 lb/mmbtu heat input has been verified.
As would be expected, the cyclone furnace is an inherently significant generator of NOx. The uncontrolled NOx emissions from the cyclone furnaces were predicted and have been verified to be no greater than 0.8 lb/mmbtu heat input. The facility is equipped with a selective non-catalytic reduction (SNCR) system that uses urea as the reagent to control NOx emissions. The stack NOx emission rate of 0.3 lb/mmbtu can be maintained by using 50% less urea than originally anticipated.
The potential VOC emissions were addressed in the design phase of the project by installing a closed loop drying system using steam as opposed to contact drying with the sensible heat remaining in the exhaust flue gas. Any potential VOC emissions generated in the drying process are carried to the cyclone furnaces in the combustion makeup air and are destroyed in the cyclone furnaces which have operating temperatures that are typically in excess of 2600ºF. The facility is well below its maximum stack VOC emission rate of 17.5 pph.
Like VOC emissions, whatever trace CO emissions generated in the process are destroyed in the high temperature cyclone furnaces. The CO measurements at the stack with approximately 2% oxygen in the flue gas have been less than 5 ppm, which are significantly below the allowable level of 200 ppm.
The wastewater generated in the facility, which is dewatered from the incoming wet sludge, was required to have limited suspended solids content. Good engineering practice relating to sizing of gas ducts and transitions, dryer cyclones, and the wet scrubber have resulted in an acceptable solids loading to the local municipal wastewater treatment plant.
Even though considerations to control fugitive dust were part of the original plant design, the control of fugitive dust has been a continuous learning process as the system has been started up and put into operation. Many retrofit systems have been installed on the drying system at numerous points, whereby fugitive dust emissions of “dry fluff” were a problem. This “dry fluff” is not only a housekeeping and airborne fugitive dust problem, but small piles of this hot material have a tendency to smolder.
One of Minergy’s primary concerns in the design and construction of this facility was to control odor and noise emissions so that there would be no noticeable impact on surrounding properties. Located in downtown Neenah, it is easy to understand why this was such an important criteria.
The fuel receiving building was designed so that the three truck doors could be closed when the trucks are inside and unloading sludge. The makeup air for the dryer system, which is ultimately the combustion air, is pulled from the sludge receiving building by a large odor control air duct system. This odor control duct system also has other pickup points in critical areas of the facility. Odor levels have actually decreased in the area because sludge does not accumulate at the local paper mills, but instead is being delivered to the facility 24 hrs/day.
The facility was also designed to be a very quiet operating facility. Essentially, all equipment is enclosed in buildings or galleries which effectively reduces any fugitive noise emissions. During the initial startup, an abnormally and extremely high noise level emanating from the combination ID fan and stack was experienced. Subsequent testing by sound experts and the design and installation of a silencer in the stack eliminated this problem, resulting in a much quieter operation than is typically experienced in similar type installations.
CONSTRUCTION AND OPERATIONS. ESI began construction of the Fox Valley glass aggregate facility in July 1996. Boil-out was performed on approximately December 1, 1997. Startup and initial operation was commenced shortly after January 1, 1998. Beneficial operation defined as firing the unit on sludge producing steam was attained on March 15, 1998. Commercial operation of the facility was attained on May 21, 1998.
As would be expected in a complex, first-of-its-kind facility, numerous problems have been encountered and overcome. Some of these problems included:
• Sludge handling and storage problems resulting from changing physical properties of wet sludge under high compression loads
• Corrosive attack associated with unexpected constituents in the incoming wet sludge
• Dry sludge and flux handling problems created by high humidity
• Dry sludge firing pneumatic transport line pluggage from debris introduced with wet sludge or from dryer system deposit self cleaning
• Fouling and hard rock formation in the cyclone furnaces and furnace slag discharge opening due to low operating temperatures and experimentation with sludge/flux ratios
• Fugitive “dry fluff” emissions control, handling, and containment.
Continued improvements are being made to improve the facility throughput, efficiency, and systems reliability. Currently, the plant receives and processes more than 1,000 tpd (350,000 tpy on an as-received basis) of sludge from eight area paper mills. Energy from the process is recovered and converted into steam, which is sold to the P.H. Glatfelter mill.
TERRENCE W. CARROLL, P.E., is regional manager, Minergy Corp., Neenah, Wis., and WILLIAM L. REEVES, P.E., is president and CEO, ESI Inc. of Tennessee, Kennesaw, Ga.
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