| 英文摘要 |
Fly ashes from municipal solid waste (MSW) incinerators containing harmful constituents such as heavy metals and dioxins are classified as hazardous wastes. Twenty-seven MSW incinerators with total design capacity of 25,400 tons/day to be installed in Taiwan shall generate fly ashes and bottom ashes daily with a quantity about 1,000 tons/day and 4,000 tons/day respectively. Due to space limitation and high population density, the treatment and disposal of MSW fly ash has become an issue of the public concerns in Taiwan. Currently, MSW fly ashes are treated by cement solidification method and then sent to disposal sites. In order to provide a better solution, Taiwan EPA and the Institute of Nuclear Energy Research (INER) joined together to study the plasma melting process for recycling the ash residues through a cooperated project between November 2003 and September 2005.
Bottom and fly ashes were sampled from five MSW incinerators (i.e. Beitou, Neihu incinerators in Taipei city, Shulin incinerator in Taipei county, Nanlao incinerator in Shin-Chu city and the central area incinerator in Kaohsiung city) for this study. The chemical compositions, dioxins contents, radioactivity and leaching rates of the regulated heavy metals etc. were analyzed. Plasma vitrification of bottom ashes and fly ashes from five operating MSW incinerators in Taiwan was studied. Experimental results from lab-scale testing show that all the bottom ashes (BA) and fly ashes from boilers and economizers (FA) can be converted into high-quality slag at 1,450°C，but fly ashes from semi-dry air pollution control device (i.e. reaction ashes, RA) can not be melt even at 1,650°C due to high basicity (CaO/SiO2) and high chloride contents. To effectively vitrify the reaction ashes, bottom ashes or boiler ashes were added with proper weight ratios to control the basicity of the ash mixtures in between one and three.
Pilot-scale plasma furnace with drum feeder was used to process the pre-compressed ash residues into barreled slag in batch operation mode to verify reliability of the whole system. Successful plasma vitrification testing encourages Taiwan EPA and INER to build a demonstrative plasma melting system to vitrify the ash residues into water-quenched slag in continuous operation mode for further recycling study. The plasma demo-system have been operated in 19 performance test runs and accumulated more than 1,000 operation hours. One continuous operation last for seven days was completed to prove the availability and reliability of the plasma vitrification technology developed by INER. Sixty-six tons of bottom ash and fly ash were vitrified during the testing and forty-seven tons of water-quenched slag was generated. Several eco-products such as permeable bricks, glass ceramic, perfume bottles and statues etc. were made from water-quenched slag for recycling practice. The water-quenched slag was also mixed with bitumen to form asphalt concrete for road pavement.
To estimate the operation cost, a plasma vitrification plant with a capacity of 50 tons per day to recycle the MSW ash residues was evaluated based on the conditions that the mixing ratio of fly ash and bottom ash is 1:1 and the water-quenched slag are used to produce permeable bricks. The process cost is about 8.91 NT dollars per kilogram ash mixture. By counting the revenue of the permeable bricks, the net operation cost is reduced to 4.90 NT dollars per kilogram ash mixture. To increase the feeding ratio of fly ash, waste glasses can be used as flux instead of bottom ash and the operation cost to vitrify the fly ash becomes to 5.55 NT$/kg. For comparison, the disposal cost of fly ash by using cement solidification process with chemical stabilizer is given as 5.7 NT$/kg.
From this study, one can conclude that fly ashes from MSW incinerators could be vitrified into high quality slag and recycled as different eco-products with considerable value by plasma torch. Domestic plasma vitrification technology to recycle the MSW ash residues has been established through the execution of this project. Installation of commercial plasma melting plants to recycle MSW ash residues using domestic plasma technology should be considered and promoted by government for creating an everlasting environment.
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