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Resource recovery: turning waste into watts.


Each week, the mid-Connecticut resource recovery facility shreds and separates 12,000 tons of garbage to recover combustible fuel that fires an electric

At the Mid-Connecticut Resource Recovery facility, waste is processed to produce refuse-derived fuel (RDF), which is burned in boiliers to produce steam that is used to generate electrical energy. The waste-processing portion of the facility has a nominal capacity of 2000 tons of acceptable waste per day with a guaranteed capacity of 12,000 tons per week.

The facility uses a municipal solid waste- (MSW) processing scheme that prepares waste for combustion by removing noncombustible materials such as dirt, metals, and glass and then sizing the fuel for improved combustion. The MSW processing system has five main components: inspection/picking ahead of processing; flail shredding; magnetic separation; screening; and secondary shredding. The facility has two parallel identical processing lines, each with a nominal capacity rating of 100 tons per hour.

Construction of the plant began in the spring of 1985, with start-up in the fall of 1987 and full-scale commercial operation in the fall of 1988. The facility was designed and built by Combustion Engineering Inc. Resource Recovery Systems (now ABB Resource Recovery Systems) and is owned by the Connecticut Resources Recovery Authority. It is located at Connecticut Light and Power's South Meadows generating station along the Connecticut River on the south side of Hartford. Coal-fired boilers at the site were removed and the boiler building was rebuilt with new power block facilities. The new waste-processing facilities were built on land adjacent to the existing generating station.

The waste-processing portion of the facility is operated by the Metropolitan District Commission and the power block facility is operated by ABB Resource Recovery Systems. The facility receives residential, commercial, and light industrial waste from four cities and towns in the mid-Connecticut region. Waste is delivered by municipal and private haulers either by direct haul or through transfer stations. The process lines are operated two shifts per day, Monday through Friday, and one shift on Saturday with equipment maintenance performed during off-shift hours.

Waste Processing

Weighed incoming trucks are directed to the MSW receiving area where acceptable waste is discharged onto the receiving building tipping floor. Once on the tipping floor, large wheeled bucket loaders are used to stockpile material in quantities up to 20 feet high and to feed the process lines. This initial waste handling provides an early opportunity for the loader operators to spot and remove nonprocessible, bulky, and hazardous material.

After inspection and sorting on the receiving floor, the first and most important step in processing MSW is metering the waste onto the process line at a closely controlled rate. To accomplish this, bucket loaders are used to remove material from the stockpile and load it onto the horizontal feed conveyor at the beginning of the process line.

Three infeed conveyors are used to progressively reduce the material burden depth so that the waste can be visually inspected for nonprocessible items. The material is then conveyed past a picking station booth where an operator again inspects the waste and removes nonprocessible items.

Primary shredding is the key step in preparing the MSW for further processing. The primary shredder is a flail-type mill. The flail mill consists of a horizontal rotor that is belt-driven by an electric motor. Replaceable swing hammers are arranged on four axial rows on the rotor. The spacing and pattern of the hammers maximize the area over which impact occurs across the infeed. The waste is impacted by the hammers, thrown against the breaker plate, and then discharged onto a rubber belt conveyor.

The primary shredder is located in a blast-resistant bunker for explosion protection. The bunker has thick reinforced-concrete walls that are designed to contain an explosion and direct the energy that is released out through the bunker roof away from plant personnel.

Four noteworthy explosions have been experienced to date, three involving 20-pound propane (gas grill) tanks and one due to a large quantity of volatile fluid from a cosmetics supply house. In all cases, the bunker functioned as designed, safely relieving the overpressure through the roof. Downtime from the explosions was minimal, primarily involving checking for equipment damage and performing repairs to the roof.

The primary shredder has a dedicated dust-control system consisting of a baghouse filter and exhaust fan. The dust system is also isolated from the main process area in a blast-resistant bunker for explosion protection.

Following primary shredding, the coarsely shredded waste is conveyed to the double-drum magnetic separation system. The purpose of this system is to recover ferrous metal from the process stream for recycling, as well as to reduce wear in the secondary shredder and to minimize the quantity of metals delivered to the boiler.

The ferrous metal is then transported by conveyor to the ferrous metal air classifier, which removes loose, nonferrous material carried over with the magnetically separated material. Primary Separation

With most of the ferrous metal removed, the main waste process stream is divided into two approximately equal streams which are fed into two parallel separation units in each process line. The separation units are completely enclosed trommels (rotary screens) with replaceable screen sections, variable-speed drives, and adjustable pitch angles to allow fine tuning of the separation process. Movement of material through the rotary screens and material separation efficiency is controlled by the slope and rpm of the screen, the lifting and tumbling action caused by the screen and internal lifting baffles, and the sizing of holes in the screen. Three streams are discharged from the primary separators: a residue stream consisting of sand, glass, dirt, and a small quantity of combustible materials; a sized fraction consisting primarily of small combustible products together with some heavy particles of rock, glass, and other materials; and an oversized fraction consisting mainly of paper and cardboard. The sized material fraction discharged from the second stage of the primary separators is con discharged into the secondary separation unit, which is of a design similar to that of the primary units. There are two discharges from this unit: a residue stream and sized combustibles that are transferred to RDF storage.

The oversized fraction from the primary separators is conveyed to the secondary shredder. The function of this shredder is to size-reduce the material for proper feeding to, and complete burning in', the RDF boilers.

The process line dust-control system provides dust collection from the trommels, collection of tramp material from the ferrous air classifier, and negative draft to control dust and assist movement of material through the secondary shredder. A system for each process line is comprised of a cyclone separator, a fabric filter (baghouse), and an induced-draft exhaust fan. Most of the material collected is removed from the airstream in the cyclone. The airstream from the cyclone is cleaned in the fabric filter. Material from the cyclone and baghouse is directed to the RDF storage.

The final step in RDF processing is placement of the RDF into storage. A stationary packer, such as those used in solid waste transfer stations to load transfer trailers, is used to discharge RDF into the storage area from each process line.

The stationary packer outlet penetrates the concrete push wall, which is located between the process area and the RDF storage. The discharge end of the packer is flush with the inside of the storage area wall so that it can't be hit by the front loader in the storage area. RDF conveyed to the packer from the process equipment is pushed through the wall by the continuously cycling ram in the packer. This operation is relatively dust-free and allows the rubber-tired wheel loader freedom to maneuver and stockpile RDF while material is being pushed into the room by the packer.

Power Block Facility

The power block facility consists of three Combustion Engineering VU40 boilers that generate 231,000 pounds of steam per hour while firing 100 percent RDF or 188,000 pounds of steam per hour while firing 100 percent coal. The boilers can also cofire RDF and coal in any combination and generate up to 231,000 pounds of steam per hour. Steam is headered to either of two 45-megawatt 465,000-pound-per-hour turbine generators.

RDF from the storage area is transported about 600 feet from the process plant to the boilers using two parallel conveyor systems. One conveyor system is required for normal operation; the second conveyer is a standby. RDF is fed out of storage by pushing it with a front loader into one of two fuel feed conveying systems.

Each fuel feed system consists of horizontal and an inclined steel pan apron conveyor located along the back wall of the RDF storage room. The inclined conveyor discharges onto a rubber belt conveyor that transports the RDF to the boiler metering bins. At the boiler house, the RDF can be either discharged into a metering bin for the first boiler or diverted onto a second belt to feed the metering bin for the second boiler. If RDF is required at the third boiler, it can be diverted past the second metering bin to feed the third bin.

The RDF surge and metering functions at the boilers are accomplished with the use of a dedicated "live bottom" auger bin for each boiler. The augers meter the RDF into four boiler feed chutes. Each boiler feed chute contains a vibrating metering conveyor to provide a smooth flow of RDF to the boiler.

Coal consignments reach the plant in river barges. Barge-docking and unloader support facilities at the midConnecticut plant were rebuilt and a new clamshell barge unloader was installed.

Arriving coal is conveyed to a transfer building from which it may be directed to the plant coal silos or sent to a spout discharge in the coal yard. The coal may either be placed in inactive storage or left in the stockout pile as active storage for demand usage.

The coal yard provides storage of up to 3000 tons of coal. The area has a synthetic liner to prevent contamination of the groundwater system by coal leachate. A stormwater-runoff detention pond is provided to collect rainfall from the storage pile.

Washdown water, process overflow, and storm drainage from the entire air-quality control system area drain into this same pond and are recycled for use in the flue gas scrubbing system. This design reduces to zero the discharge of plant water containing ash, coal fines, or chemicals.

The steam generators incorporate spreader stokers as the fuel firing system for RDF. A specially designed refuse combustor stoker is used for refuse firing. It is a catenary grate design that eliminates the need for a tensioning device. The stoker has operator-controlled multiple undergrate air zones to optimize combustion from the front to the rear of the grate.

The RDF feed system uses a pneumatic distributor air fan to inject the RDF fuel into the boiler. Coal is introduced into the furnace by a mechanical distributor within the same combination fuel distributor housing, using a rotating drum to throw the coal toward the rear of the furnace.

Combustion air is introduced as either undergrate air (UGA) or overfire air (OFA). Operation of an RDF system generator normally requires 50 to 70 percent excess combustion air. About 60 percent of the combustion air is introduced as UGA, with the balance as OFA. When burning RDF, the preheated OFA is introduced through four separate tangential wind boxes located above the stoker. The UGA is preheated to aid drying of fuel on the stoker. The refuse combustor undergrate air temperature is limited to about 550' F to maintain structural integrity and minimize clinkering.

Combustion of both RDF and coal on the grate has been a success. Operators must, however, be especially attentive during transition from coal to RDF in order to avoid localized hot spots. Aluminum plugging problems that have occurred at virtually all other RDF grates have not been experienced at the mid-Connecticut facility.

Steam Generators

The three steam generators, which can burn both coal and RDF in any combination, have a total steaming capacity of 650,000 pounds per hour at 880 psig and 825 degrees F when two units are burning RDF and one unit is burning coal.

Waterwall screens are provided to reduce the flue gas temperature entering the superheater. The screens act as a buffer to help maintain the gas temperature entering the superheater. The screen assemblies are placed on wide, transverse spacings with the tubes tangential to the direction of gas flow to minimize ash deposition.

The superheater is located above the furnace nose arch to protect it from the direct radiation of the furnace and excessive metal temperatures. The unit is a two-stage superheater with an interstage desuperheater for final steam temperature control and minimization of metal temperatures. The two stages are arranged with steam flow parallel to flue gas flow. Superheater metallurgy is critical in the design of an RDF steam generator due to the corrosive environment in which the superheater operates. The mid-Connecticut superheater is provided with chrome-moly tubes in the first stage and chromized tubes in the second stage. Experience to date has not indicated any significant corrosion in the superheater.

The removal of acid gases and particulate matter from the flue gas stream at the mid-Connecticut facility is accomplished by a spray dryer absorber followed by a baghouse.

A spray dryer absorber vessel for each boiler removes acid gases using lime. The reaction product is a dry mixture of calcium sulfite/sulfate, calcium carbonate, and fly ash. The absorber vessel is sized to provide intimate contact and sufficient residence time for the absorption and drying processes to occur. This is accomplished by introducing a finely atomized cloud of alkaline slurry into the flue gas stream using a highspeed 12,000-rpm) rotary atomizer. One operating atomizer and one spare are provided for each absorber vessel. The scrubbing system contains an additive preparation system consisting of a lime storage silo, slaker, and lime slurry storage and additive feed tanks with installed mixers and slurry feed pumps.

Particulate Control

The particulate-removing fabric filter for each boiler consists of two rows of six modules, each with an air-to-cloth ratio of 2:1 with two modules removed from service for cleaning or maintenance. The particulate matter collects on the inside of cylindrical fiberglass bags forming a cake-like coating. This coating is made up of the residue from the dry-scrubbing system and contains some unreacted lime. As the gases pass through this cake, additional acid gas neutralization takes place. The cake is not removed from the bag until the flue gas pressure drop across the bag reaches a preset limit. When the limit is reached, the bags are cleaned by blowing cleaned flue gas over the bags to dislodge the caked particulate matter, which then falls into the collection hoppers below the bags. The hoppers direct the particulate into the fly ash removal system.

The use of large heavy-duty submerged scraper conveyors (SSC) was adopted for the mid-Connecticut facility. The SSC provides for the continuous removal and dewatering of bottom ash. Steel flights consisting of 4-inch-by-4-inch-by-36-inch angles transport the ash along the bottom of a water-filled trough and up the dewatering slope.

Because the mid-Connecticut facility was built by retrofitting an existing plant, the only suitable location for the ash storage bunker was adjacent to the air quality control system. This resulted in an extensive bottom ash conveying system that required elevating the bottom ash up and over an existing warehouse. A single train of rubber belt conveyors transports the ash to the ash loadout area.

The fly ash conveying system consists of single- and double-chain conveyors that transport the dry fly ash to the fly ash conditioning system. Redundant ash conditioning systems consisting of surge bins, rotary feeders, and pug mills are provided to wet the fly ash for dust control prior to its being deposited on the conveyor that transports the combined ash to storage.

The waste-processing facility passed the facility capacity test by processing more than I 1,000 tons in five days and easily handling more than 12,000 tons in less than six 16hour operating days. The power block also easily passed the seven-day facility capacity test.

The waste-processing facility and power plant had a combined total combustible loss of 6.7 percent in the process residue and power plant ash.

The waste-processing facility passed the process-line capacity test, achieving an average of 1412 tons per day for process line 100 and an average of 1433 tons per day for process line 200 for three consecutive 16-hour operating days.

Each process plant ferrous recovery system, which currently uses a double-drum single-stage magnet system to remove ferrous metal from more than 100 tons per hour of coarsely shredded MSW, did not achieve the 90 percent guarantee and was low by about 10 percent. The Connecticut Resources Recovery Authority decided not to add additional equipment that would allow the 90 percent level to be achieved.

The RDF resulting from he process was approximately 83 (by weight) of the MSW processed and had an ash content between 10 and 15 percent by weight. Each steam generator was tested for its thermal efficiency while firing 100 percent RDF. The steam generator's thermal efficiency averaged 77.05 percent for the three units. The power block flue gas emission control system met all its guarantees, in many cases by wide margins.

Commercial Operation

The mid-Connecticut facility commenced commercial operation in October 1988. During precommercial operation of the boilers, tube corrosion rates in the lower furnace area were monitored. Industry experience suggested that the most significant corrosion would likely occur in this area. During that period, there was no significant metal loss.

Shortly after commercial acceptance, Boiler 12 experienced a rupture of the waterwall in an area above the monitored zone. Metallurgical inspection of the other boilers found excessive tube wastage from lead chloride corrosion in the same relative areas. The boilers were subsequently taken down one at a time to correct the problem. The ruptured unit was retubed and an Inconel overlay was added to the waterwall tubing of all three boilers to reduce future corrosion to acceptable levels.

As confirmed by subsequent monitoring, the problems relating to the accelerated boiler tube corrosion observed after commercial acceptance were accurately identified and corrected.

The plant is now running well. Through October 1990, the facility had received and processed over 1,470,000 tons of waste. The RDF has averaged 82.2 percent of the processed waste, the residue 10.97 percent, and the ferrous metal recovered 4.01 percent.
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Title Annotation:ABB Resource Recovery Systems' mid-Connecticut facility
Author:Boley, Gary L.
Publication:Mechanical Engineering-CIME
Date:Dec 1, 1990
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