Steam Ejector Reduces Exhaust-plume at a Waste-to-energy Plant
By Kevin Stimpfl,Ogden Martin Systems
Ogden Energy Group, Inc. (OEG) designs, builds, and operates waste-to-energy facilities, as well as various power and steam generating facilities domestically and abroad. In the waste-to-energy process municipal solid waste is combusted at high temperatures and saleable steam or electricity is recovered. OEG operates a total of 27 waste-to-energy facilities across the United States.
In Huntington, NY, OEG's subsidiary, Ogden Martin Systems of Huntington, L.P., processes approximately 900 ton/d of municipal solid waste, from which it recovers approximately 25 MW of electricity. Most of the electricity is sold to the Long Island Lighting Co., with the remaining 3 MW used to run the facility itself.
Why waste to energy?
Waste to energy is an environmentally sound and economically competitive means of waste disposal, which is preferred to landfilling. Landfills use precious space and leachate contamination of ground water is always a possibility. In a small area of land such as Long Island, space preservation is a serious issue. Waste that enters an Ogden facility is reduced by 90 percent of its original volume. The remaining 10% is an inert ash. This reduction of the original volume of waste saves annually hundreds of thousands of cubic feet of landfill space.
Long Island, NY is a 100-mile stretch of land. The island, which begins with bedroom communities for greater New York and ends in truck farms, supports a tremendous beachfront tourism industry. Some residents still depend upon well water as a source of drinking water. Consequently, with the threat of waste contaminating its groundwater supplies, Long Island closed and capped its landfills some years ago.
Communities had the option of hauling their waste many miles by truck, barging it, or finding a local solution. The towns of Huntington and Babylon elected to dispose of trash through Ogden's waste-to-energy facilities.
The Huntington Facility
Ogden Martin Systems of Huntington, L.P. accepts waste from licensed haulers. Haulers tip their load into a refuse pit from which it is transferred by crane to one of three stoker/boiler feed chutes. Each chute feeds the waste fuel into its respective stoker boiler unit, by hydraulic ram feeders. Waste burned on the reverse reciprocating stoker grate of the furnace heats the boiler water, producing steam. Steam from three boilers enters a common steam header and travels to a common turbine generator to produce electricity. The turbine exhaust steam is condensed in a large air-cooled condenser.
Air-Cooled Condenser Served by Ejectors
Turbine exhaust steam is condensed in either a water- or air-cooled heat exchanger. Waste heat from a cooling tower (in water-cooled applications) can result in the formation of a large water plume. Ogden has always been sensitive to plume formation. As such, Ogden uses an air- rather than a water-cooled condenser for the Huntington facility, and thereby avoided a water plume formation. The air-cooled system does the job and is more acceptable to the surrounding community.
The Huntington facility is equipped with steam-jet air ejectors from Croll Reynolds (Westfield, NJ). The twin-engine, two-stage ejector removes air and noncondensable gases from the exhaust of the turbine and air-cooled condenser.
Housed on the same skid is a single-stage "hogging" air ejector to evacuate large quantities of noncondensables from the system at start-up, before the higher vacuum, lower capacity, two-stage ejector takes over. The ejectors utilize steam at approximately 90 psig from the plant steam supply.
Selecting the ejectors
The Croll-Reynolds ejector at the Huntington facility, consists of two 6×5-ft "Y" stages and a 3×3-ft "Z" stage, constructed of carbon steel with 303 stainless steel steam nozzles. A 12-in.-dia. intercondenser and an 8-in.-dia. aftercondenser have carbon steel shells and 304 stainless steel tubes, tubesheets and channels. The unit is rated for 55 lb/h of air and 121 lb/h of water vapor at 1 in. Hg (abs) discharging to 14.7 psig.
The single-stage hogging ejector is made of carbon steel with stainless steel nozzle and carbon steel silencer. It is designed to evacuate 14,250 ft3 of air from atmosphere to 10 in. Hg (abs), in 30 minutes.
Continuous operation
Ejectors are the customary selection for power-plant vacuum needs. They are inherently simpler than vacuum pumps with no moving parts. They also provide a great advantage from a maintenance point of view. In more than six years of operation, little or no maintenance has been required of the Croll-Reynolds ejectors installed at the Huntington facility.
The Huntington facility operates continuously, 24 h/d, 365 d/yr. With three boilers installed, one may be taken out of service for maintenance calls while still operating the turbine generator.
Every two years, however, the facility is shut down for a cold iron outage for preventive maintenance.
In planning these outages, a need developed to perform vacuum leak testing on turbine exhaust valves to the air-cooled condenser. In the "cold iron" condition of the facility, there would be no steam to power the ejector.
Several solutions were considered, including obtaining temporary, portable steam generation or vacuum capacity. In an attempt to find a simpler solution, the manufacturer was contacted. Croll-Reynolds worked with Ogden engineers to set parameters for operating the hogging ejector with motive air. Air is less efficient than steam as a motive force, but was believed to be adequate for this short-term need. Croll-Reynolds' engineers supplied complete data for the volume and pressure of compressed air required for satisfactory operation of the ejector, and the vacuum leak test was successfully completed.
About the author Kevin Stimpfl holds a U. S. Coast Guard engineer's license in addition to his BSEE. He joined Ogden Energy Group, Inc. in 1988, the year the Babylon, NY, facility commenced operations. He moved to the Huntington, NY, facility in 1991 as the chief engineer.