In 1970, Singapore generated just under 500,000 tons/yr of solid waste. The country’s massive growth in population and industrial activity since then has seen this rise to 7.7 million tons/yr, with the figure forecast to grow further in coming years.
However, disposal of this waste continues to pose a serious challenge because the island country has very limited land stock and only one landfill site.
The National Environment Agency (NEA) plans, develops and administers Singapore’s hazardous waste management systems and has implemented a four-point plan to help the country reach its target of generating zero waste. This involves waste minimization/prevention; recycling; waste-to-energy (WTE) technologies/volume reduction; and landfill.
Currently, the island recycles 4.63 million tons/yr of solid waste. Incinerators reduce the rest by up to 90%. The heat recovered from these units creates steam used to generate electricity — meeting up to 3% of Singapore’s needs. Ash and other non-incinerable wastes go to the Tuas marine transfer station for barging to Semakau landfill for final disposal.
Now, the country has taken another important step towards its zero-waste goal with the commissioning of a WTE research facility at Tuas South on the campus of Nanyang Technological University (NTU). NEA contributed S$12 million ($8.8 million) of the overall S$40 million ($29 million) cost of the facility, which is slated to operate for 10 years.
The plant turns municipal waste from the NTU campus into electricity and other useful resources. It’s also a double first for the nation in technology terms as it combines slagging gasification technology with biomass charcoal as auxiliary fuel.
The facility can treat 11.5 tons of waste daily. This waste is sorted, shredded and transported via a conveyor and bucket lift to the top of the furnace tower; biomass charcoal helps maintain the molten slagging layer at the base of the furnace at 1,600°C.
Figure 1. The waste-to-energy (WTE) research facility will be a test bed and demonstrator for new and emerging WTE technologies. Source: NTU.
The waste is dried and gasified as it moves down the furnace. About 85% of the waste converts to syngas — mostly carbon monoxide and hydrogen. This flows to a secondary combustion chamber where it’s burned to heat a boiler to generate steam. The steam then drives a turbine that generates electricity for the campus.
Meanwhile, 12% of the material leaves the bottom of a furnace as slag, a glass-like material that has potential as a construction material, and metal alloy granules that can be recycled.
The exhaust flue gas from the boiler is then treated with slaked lime and activated carbon and passed through a bag filter before being discharged as clean gas to atmosphere. Disposable fly ash makes up the remaining 3%.
In Singapore’s context, slagging gasification technology has potential to complement the current mass burn technology as it can treat diverse mixed-waste streams that can’t be handled by the incinerators because they typically operate at around 800°C.
According to NTU president professor Subra Suresh, the research facility’s use of university waste is well aligned with the NTU smart campus vision and will be a living testbed for advanced technology-enabled solutions aimed at tackling some of the most pressing challenges Singapore and the world are facing. “It will enable our scientists to scale up promising ideas from lab prototypes into practical engineering solutions for sustainable waste management,” he adds.
The plan now is for NTU scientists and engineers from the university’s Nanyang Environment and Water Research Institute (NEWRI) to spend several years collaborating with industry and academic partners on various research projects aimed at developing and testing emerging WTE technologies.
The research facility is designed specifically to simplify test bedding of new technologies in a plug-and-play style. Municipal solid waste also has the capability to process diverse feeds such as incineration bottom ash and sludge.
Provisions exist for evaluating the cleaning and upgrading of syngas to run a gas engine or turbine for higher energy recovery efficiencies, utilization of slag in engineering applications, novel flue-gas-treatment modules for reduced emissions, low-grade heat recovery and using a gas separation membrane to extract oxygen from air. These technologies, if proven successful and implemented, could enable more energy and materials to be recovered from waste, thereby prolonging the lifespan of the Semakau landfill.