Domain

Explanation

CGC visit on 15Feb2003

• NUS Campus Green Committee site visit to:
• Tuas South Incineration Plant 3000tons/day (~800 full truckloads of incinerable solid wastes), 2000
• Singapore's 4^th & largest incineration plant to date
• Others:
• Ulu Pandan Incineration Plant: 1600tons/day, 1979
• Senoko Incineration Plant: 2400tons/day, 1986
• Tuas Incineration Plant: 1800tons/day, 1996
• Charges: $77-$79/ton of incinerable wastes (if too high, people dump), real cost of incineration & disposal $88/ton
• Tuas South Plant design:

1. Modular construction by United Engineers Pte Ltd (Designers & Checkers) & Chew Eu Hock Construction Co. Pte Ltd (Main Contractor)
2. Mechanical, Electrical & Chemical Process & Control (central digital control system) Engineering: Mitsubishi Corporation/Mitsubishi heavy Industries Ltd
3. Central digital control centre for real-time monitoring, faults, control; signals: green ok, blue warning, red alert
4. Bulky waste crushing: cranes on steel girder of varying linear depths with stiffeners under dynamic, cyclic-fatigue & impact loads
5. Fire protection system with infra-red heat detection system: water nozzles at 5m c/c to catch fires at any angle & depth in bunkers & boilers (1000deg flue gas paths lined with refractory tiles: SIC ceramics) --> under Factory Act, each boiler checked every single year
6. Flue gas treatment system: electrostatic precipitator to clear 95% dust particles (fly ash); special fabric bag filter system to filter out the rest of 5% particles & hydrated lime powder is injected into the (~170deg) acidic flue gas to reduce acidity & increase pH, so as not to disrupt surroundings by acid rain
7. Cleaned flue gas at 130deg is blown high out through two 150m chimneys: temperature kept high enough so that no rapid condensation takes place for acidic rain in vicinity & warm to rise higher into the clear atmosphere

Wastes

• Types of wastes:

1. Solid wastes: in bulk, heterogeneous form
2. Sewage: fluid waste discharges

• Originating sources:

1. Residential: mostly East & North
2. Industrial: mostly West

• Types of solid wastes:

1. Incinerable wastes: able to be burnt to reduce bulk weight by 85% and reduce bulk volume by 90% --> incineration plants
2. Non-incinerable wastes: not able to be combusted for reasons like hazardous, high melting points (uneconomical), suitable as fill materials or recycling --> landfill, dumping & other uses

• Incinerable solid wastes:

1. Bulky: large chunks, wooden boards --> crushed into homogeneous pieces for distribution into refuse bunker before incineration
2. Non-bulky: dumped straight into refuse bunker

• Waste reclamation:

1. Scrap metal: only ferrous metals presently, using magnetic separators --> least revenue, sold to NatSteel for construction rebars
2. Fly ash & blast furnance slag: the left-over product of incineration; can be used for construction (pozzolanic materials to replace expensive cement in concrete), pavements subgrade layering --> more revenue, others dumped in Semakau Landfill to settle properly in order to develop into niche island lifestyle future
3. Generate electricity: using the flue gas from the refuse combustion in the special boiler (total 6 in Tuas South) to heat water pipes for super-heated steam (~370deg) to drive electric turbines to generate electricity (80MW capacity) --> most profitable, 25% used by plant itself, 75% sold to Singapore Power
4. Water reclamation plant: for replacement of water loss in the heated-cooled pipes by recycling industrial water (pilot project), rain water (2 water buffer basins of 7000m³), and finally PUB water if other sources insufficient to meet demand; using processes like microfiltration for pretreatment, reverse osmosis for final treatment with membrane technology

• Important checks:

1. Gas laws: V, P, T
2. Fire prevention, detection & control: boilers need to shut down slowly, else serious damages
3. De-clogging of bulk wastes: due to drivers not declaring bulk wastes (deep) inside trucks, especially paths between refuse bunkers & boilers
4. Control systems: fault detection, identification & controls, SOP & scheduling of operations

Waste Incineration Plant Processes

• Container trucks empty collected solid wastes into 100m(l)*30m(w)*34m(h) bin centre
• Large solids are shredded first
• Overhead cranes lowers 34m to pick up 8~10 ton of assumed homogeneously-mixed rubbish from refuse bunkers to dispose into the adjacent furnace; heavily reinforced & short steel girders
• Refuse is deliberately moved by perpendicular sliding & rolling systems to ensure more thorough combustion of the refuse
• Rubbish is burned at 1000 deg. C into ashes, but scrap metals have not melted
• Heat generated is used to repeated convert water to steam to drive turbines for power generation
• The furnace exhaust is specially filtered and released into the atmosphere through two 150m high chutes
• Scrap metals in the ashes are removed using electromagnetism for recycling
• Effluent ashes are only ~10% of influent solids
• The ashes are dumped offshore on Semakau Landfill (~2035)

Overall impressions

• Safety measures intact; corridors free & clean
• Places shown are clean, swept, little dust
• Swallows seen at bulky wastes refuse collection
• Cobwebs mark places of little air flow
• Steel beam girders of varying linear depths with stiffeners as crane gantry girders
• Supporting columns with corbels: design for dynamic, cyclic & impact loads
• Roof: steel lattice girders as primary beams, SHS trusses as secondary beams and purlins & ties as braces & intermediate ties to anchor to external columns (separate crane gantry columns)