Waste gasification and melting technology

Written by: RWW | Published:

The direct melting system is a shaft-furnace gasification and melting process that offers the advantage that no pre-treatment of municipal solid waste is required. Nobuhiro Tanigaki reports.

Today waste gasification is attracting great interest in several countries. Gasification is the conversion of any carbonaceous fuel to a gaseous product with a useable heating value and is widely used for energy conversion from coal and biomass. Gasification of municipal solid waste (MSW) and biomass as an energy recovery method has been widely researched all over the world. Considerable research on biomass gasification technologies has been conducted in the EU and US and some is in the lab- or demonstration phase. 

The direct melting system (DMS) is a shaft-furnace type gasification and melting process and is classified as an atmospheric moving bed gasifier. The DMS has been in commercial operation at more than 40 plants in Japan and South Korea since 1979. 

Process description  

The plant mainly consists of an MSW charging system, a gasifier, a combustion chamber, a boiler and a flue gas cleaning system. 

One of the advantages of the DMS process is that no pre-treatment of MSW is required, which differs from other gasification technologies, such as a fluidised bed gasifier. 

MSW is directly charged into a gasifier from the top, together with coke and limestone, which function as a reducing agent and a viscosity regulator, respectively. 

Due to the addition of the limestone, the viscosity of the melt is adjusted properly and the melt is discharged smoothly from the bottom of the gasifier without any clogging. Oxygen-rich air is blown at the bottom of the furnace via tuyere. 

The gasification and melting furnace consists of three main parts: a drying and preheating zone, a thermal decomposition zone and a combustion and melting zone. 

MSW is gradually dried and preheated in the upper section (the drying and preheating zone). 

Combustible waste is thermally decomposed in the second zone and syngas is discharged from the top of the gasifier. 

The syngas, which mainly contains carbon monoxide, carbon dioxide, hydrogen, methane, hydrocarbons and nitrogen, is transferred to the combustion chamber in the downstream of the gasifier and then completely burned. Incombustible waste descends to the combustion and melting zone (1,000-1,800ºC) at the bottom and is melted with the heat generated by coke burning. 

Gasification reactions such as water-gas-shift, water-gas and boudouard reactions mainly take place in this zone. Molten materials are intermittently discharged from a tap hole, quenched with water, and magnetically separated into slag and metal, which are recycled completely. 

Operating and environmental impact 

The DMS has several advantages, which can minimise environmental impact and maximise resource recycling from waste. 

Firstly, the DMS shows the waste flexibility and stable operation which can be achieved by high temperature gasification. 

MSW with various kinds of waste can be processed without any further pre-treatment, which is different from other gasification technologies. 

C&I waste, sewage sludge, incombustible and combustible residues, clinical waste, asbestos, automobile shredder residues (ASR) and landfill waste can be processed and converted into power and materials. 

The high temperature gasification also leads to stable operation and syngas with higher net heating values (NCVs). The NCVs of the syngas ranged from 4.4 MJ/m3N,db and 5.9 MJ/m3N,db in the case of processing waste with NCVs of 6.5 and 9.1 MJ/kg. 

Gasification reactions with a low equivalence ratio lead to production of syngas with higher NCVs. Combustion conditions are well controlled due to the syngas having high NCVs. The syngas with high NCVs is combusted by air and achieves homogeneous. This leads to stable operation and can minimise PCDD/DFs emissions. Secondly, material recovery from waste can be conducted as well as energy recovery from waste. 

The inert materials in waste processed can be recycled as slag and metal, using the DMS technology. 

Due to the high temperature reducing atmosphere at the bottom of the gasifier, toxic heavy metals are completely volatilised and distributed in fly ash. 

The slag produced contains few hazardous heavy metals and is significantly stable, and can be recycled in wide applications without additional treatment. 

Specifically, it can be used not only as secondary materials such as interlocking block aggregate, concrete aggregate, or civil engineering materials with no additional treatment, but also for agricultural utilisation (fertiliser or soil) without any further post-treatment such as ageing. 

The metal produced is also recycled. Iron and copper are mainly distributed in metal, which can be recycled in a steel factory, and heavy metals with a low boiling point, such as lead and zinc, are mostly distributed in fly ash. From this point of view, the DMS can contribute to environmental conservation and resource recycling. 

Thirdly, low environmental impact can be achieved using this technology. DMS has the potential to reduce total emissions. In tests we have demonstrated that HCl at the inlet of BF is less than 180mg/m3N due to the de-acidification by limestone injection to the gasifier. 97.8% of HCl is removed by BF SO2 concentrations at the inlet of the BF are less than 12.7mg/m3N. PCDD/DFs generations are also significantly low, even at the inlet of the BF, which is caused by homogeneous combustion of the syngas with high NCVs. This indicates that the DMS has possibilities for reducing environmental impact. 

Ensuring reliability

Lastly, the long term operating and maintenance results show its reliability. The first DMS plant was constructed in 1979. It was operated for 31 years and replaced with a new facility. 

Due to its stable and safe operation for 31 years, the municipality decided to employ the same technology for its replacement. 

In addition to this track record, R&D is further contributing to the stable operation of the DMS. Breakthroughs via O&M activity can improve on and develop the DMS technology. 

Given the above findings, the DMS offers great possibilities for contributing to the development of an ideal approach to environmental conservation and resource recycling. 

For more details, visit www.eng.nssmc.com/english/

Nippon Steel & Sumikin Engineering will be exhibiting at the the 10th International Energy from Waste Conference that is being held in London on February 26-27. For further details on the conference, visit www.efwlondon.eu or call 01722 717024



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