Hachioji Waste Disposal Plant Uses NKK Ash Melting Furnace

On April 11, the suburban Tokyo city of Hachioji officially commissioned its new Tobuki waste disposal plant constructed by NKK. The main facilities for the plant were supplied by NKK, including three stoker-type incinerators and two NKK-developed electric-resistance ash melting furnaces, the first such installation in Japan.

NKK received a ¥22 billion full-turnkey order for the waste disposal plant from Hachioji in 1994. The plant employs three 100-ton-per-day stoker-type in cinerators capable of 24-hour continuous combustion and two 18-ton-per-day electric-resistance ash melting furnaces, enabling integrated municipal waste treatment from incineration to ash melting.

The stoker incinerator incorporates a two-way hyper grate furnace with a compact horizontal structure instead of conventional ladder-type grates, thus ensuring highly efficient, uniform combustion. A full-boiler steam turbine uses the incinerator's waste heat to generate 1,990kW of electricity to run the ash melting furnaces and other facilities at the plant. Exhaust gases are treated by means of lime injection, filtration dust collectors and catalyst denitration.

The NKK electric-resistance ash melting furnace relies on molten slag's conductivity and resistance heat (about 1,500°C), thus it does not require the ultra-high temperatures needed by conventional arc or plasma-arc furnaces. The furnace's simple heating mechanism allows complete melting of ash and stable operation. Its fully-sealed operation means less electrode consumption and reduced running costs. With specific-gravity molten metal separation, it is possible to tap uniform-quality metal-free slag, which permits a variety of slag recycling applications such as in high-strength roadbed and building materials.

The installation of the new ash melting furnaces will allow Hachioji to end its conventional residual ash disposal in landfills and promote efficient resource recycling. Following the Hachioji project, NKK has received another order from Yokohama City for a 60-ton-per-day ash melting furnace for its new1,200-ton-per-day waste disposal plant in Kanazawa Ward, which is due to be completed by March 2001.