The Perham Resource Recovery Facility (PRRF) located in Perham, Minnesota, is owned and operated by the Prairie Lakes Municipal Solid Waste Authority (PLMSWA), a joint powers agreement between Becker, Clay, Otter Tail, Todd and Wadena counties. This facility was built in 1986 and originally owned by Otter Tail Power Co. Subsequently, the facility was owned and operated by the city of Perham, which transferred ownership to the PLMSWA in June 2011.
The PRRF receives and processes approximately 60,000 tons per year of MSW from the five counties, and consists of the following components: a material recovery facility (MRF); combustion units; waste heat boilers; steam generators; and air pollution control (APC) equipment. The facility combusts approximately 45,000 tons per year and produces steam. The facility also has the ability to generate electricity as needed by the Otter Tail Power Co.
The facility was expanded and upgraded in 2014 by adding a MRF at the front-end and retrofitting the waste-to-energy (WTE) facility by adding a second boiler and additional APC equipment. The result increased PRRF’s MSW processing capacity from 116 tons per day to 200 tons per day. These changes allowed the PRRF to increase overall recycling rates, produce more steam for its customers and reduce the use of an existing natural-gas fired auxiliary boiler for meeting normal steam demand.
The authority aimed to reduce the amount of material sent to landfills to fulfill its goal of landfilled tonnage representing no more than 15 percent of the total MSW generated in the region by 2030 despite any projected increase in waste generation.
SERVING A DUAL PURPOSE
RRT Design & Construction (RRT), a systems integrator and engineer/constructor for solid waste processing facilities based in Melville, New York, was contracted by the authority to design and construct the MRF. The MRF has two main functions at the facility.
One is to improve the characteristics of the incoming MSW by removing noncombustible materials and undesirable wastes prior to being fed to PRRF combustors. The second function is to recover recyclables in the MSW helping the authority to meet its recycling goals, and reduce the amount of ash the facility sends to the landfill.
The MRF was installed with new equipment enclosed within a new building approximately 100 feet by 275 feet. The major equipment included a trommel, eddy current separator, balers, magnetic belt separators, sorting stations and various conveyors. The approach was to apply automation where possible and use manual sorting techniques limited toward high-value recyclables and removal of undesirable waste for the combustors.
The MRF system was designed to recover ferrous and nonferrous metals, old corrugated containers (OCC) and plastics for recycling. Undesirable waste such as e-scrap and fines, including glass and grit, are removed by the system as well.
The e-scrap is shipped off-site to markets to be recycled. The fines are currently being landfilled but approval is being sought for use as daily cover. The remaining material is conveyed to the existing refuse pit that is then used as the fuel supply for the WTE plant.
The system starts on the tipping floor where MSW is received and bulky materials are initially removed. The rest of the waste is then fed through an in-feed hopper and conveyed to a sorting station where other bulky items (rejects) and clean corrugated paper are removed. Sorters at this generally sized station also remove any unprocessable items such as propane tanks, electronics, batteries and wires, all of which are recycled. OCC recovered at this station is dropped down a chute to a conveyor that transports the material to a baler. This equipment bales OCC and aluminum recovered further downstream in the system.
The waste then enters a trommel, which opens trash bags and also separates materials into three streams: unders, middlings and overs. The unders are further processed by a screen where material larger than one inch is conveyed to the main process line. Material less than one inch enters an air classifier where light materials are recovered and fed to the main process line, and heavy matter is conveyed to a compactor for diversion.
Middlings are transported on a series of conveyors where they are exposed to a belt magnet and an eddy current separator. The oversized materials combine with the middlings discharge to a second sorting station to remove any additional rejects missed in the first sorting station, as well as to recover additional recyclables. This combined stream is then exposed to another magnet to recover additional ferrous metals.
The MRF was able to divert 18 percent of the total amount of waste received in 2015 from being fed to the facility’s combustion units. The number of recyclables recovered by the system amounted to approximately 3 percent from the total waste received. Nearly 40,000 tons of waste were combusted at the facility. While the amount of waste processed is partially related to the MRF addition, the main reason is due to the increased capacity of the WTE.
WTE PLANT UPGRADES
The WTE facility currently consists of two municipal waste combustion (MWC) units, two waste heat boilers and two APC systems. The 2014 facility upgrade, in conjunction with the MRF addition, included an extended boiler/baghouse building that houses a new waste heat boiler, economizer, acid gas removal equipment and induced draft fan for the south unit. New refractory lined ducting was added to direct the flue gas from the existing south unit to the new equipment. A new air emissions stack was required and serves as a combined stack for both combustion units.
The WTE system processes approximately 40,000 tons per year of refuse-derived fuel (RDF) produced by the MRF. In each unit, the process begins by feeding RDF to the furnace hopper. The hopper discharges the material onto a stationary inclined grate where the RDF is combusted in a fluidized bed. The grates for the north and south unit consist of 160 plates and 120 plates, respectively. Combustion temperature in the furnace is maintained around 1,850 degrees Fahrenheit by regulating the hopper discharge rate and the amount of air injected through the grates. The furnaces are lined with 6 to 10 inches of hard face refractory material on top of 4 inches of lightweight insulation.
The resulting flue gas passes through a boiler and heats up water inside the boiler’s coils, producing superheated steam at 610 degrees Fahrenheit and 240 pounds per square inch. The flue gas then passes through the economizer that recovers additional heat from the gas. Superheated steam produced in the boiler is desuperheated in an attemperator to saturated steam before being delivered to PRRF’s customers.
After passing through the economizer, flue gas from each unit enters their respective APC systems before they’re released into the atmosphere. Both APC systems have the same air pollution equipment: dry lime injection system for sulfur dioxide (SO2) and hydrogen chloride (HCI) control, activated carbon injection for mercury control and a fabric filter baghouse for particulate control. The cleaned gas is then discharged to the atmosphere via a single 125-foot tall primary stack. The stacks are monitored continuously and tested periodically according to permit conditions to verify compliance with regulatory standards.
PRRF also has a steam turbine generator used to produce electricity and an auxiliary natural-gas-fired boiler. Approximately 300 million pounds of steam is produced and sold annually by the PRRF using a combination of the two heat boilers and the auxiliary gas fired boiler.
After the RDF has been combusted in the furnace, the grate discharges the contents into a leachate-filled trough. The leachate comes from the landfill. Bottom ash is collected using a submerged drag chain conveyor after being quenched and transported to the Northeast Otter Tail Landfill. Prior to disposal, ferrous metals are recovered from the ash and sold to markets for recycling. The Northeast Otter Tail Landfill has a dedicated lined ash cell where the PRRF ash is disposed of. The leachate at the landfill is collected and transported to the facility to replenish the liquid in the trough where the submerged drag chain lies.
EFFECTS ON MSW QUALITY
The MRF plays a direct role at the facility in manipulating the incoming waste for combustion by removing OCC, ferrous and nonferrous metals, fines and rejects (material of no heating value).
Although the MRF recovers corrugated cardboard that has relatively high British thermal unit (Btu) properties, it also removes a considerable amount of noncombustible matter (such as glass, metals and electronic scrap) and organic waste with high moisture content as part of the fines.
The average heating value of the waste processed after the MRF addition has increased by over 20 percent. Another parameter to note is the amount of steam produced per ton of waste processed. RDF from the MRF produces more than 16 percent more steam at a higher energy density compared with unprocessed MSW.
The RDF at higher heating value allows the WTE plant to produce the same amount of steam at lower feed rates. Previously, 7.7 tons per hour of unprocessed waste would be needed to be combusted previous to the MRF in order to generate 51,500 pounds per hour of steam. Now, the operator only needs to combust 6.4 tons per hour of waste to meet the 51,500 pounds per hour steam output. Notable however is the additional cost to operate the MRF.
Overall, Several operational and cost benefits were realized with the MRF addition at the PRRF while also fulfilling the authority’s goals to increase the region’s recycling rate, produce more of its customer’s steam needs with MSW and have the capacity to service a wider geographic market.