MSW

EfW

Advanced Thermal Treatment of MSW

Operation

Delivered waste (typically unsorted MSW) is weighed in over the weighbridge
The delivered waste is tipped onto the Tipping Floor and then visually inspected for hazardous or oversize items before being loaded into one of two loading hoppers by front-end loading shovel. Any oversized wastes are shredded to a nominal 400mm prior to loading.
From the hoppers the waste is conveyed up an inclined conveyor fitted with belt weighers to one of two drag-link conveyors to a set of Ram Loaders at the front end of each of the large aerobic BioDigesters.
Each BioDigester can accept up to 150 tonnes per day of MSW.
The BioDigesters each have a full-load capacity of 300 tonnes of waste and rotate at a rate of 0.7 - 1.0 rpm.
The material residence time in the BioDigester is 2 days after which any bags will have been opened and the biodegradable content aerobically composted whilst the non-biodegradable fraction (metals, plastics, glass, textiles, rags) remain undigested.
Optimum composting conditions (moisture content and oxygenation) are maintained by a forced airflow up through the digester and by the recirculation of leachate from the sump to maintain moisture content.
After two days in the BioDigesters the digested MSW is unloaded and conveyed to the Primary Trommel screen in the Materials Recovery Facility (MRF).
The Primary Trommel is fitted with 25mm screens that very efficiently separate the <25mm biodegradable Trommel Unders from the non-biodegradable >25mm Trommel Overs.
The biodegradable Trommel Unders are further screened by overband magnets, eddy-current separators, and density separators to remove ferrous and non-ferrous metals and any pieces of glass larger than circa 10mm that may have found their way through the Primary Trommel.
The prepared biomass is weighed and then dried in a drum dryer before being pelletised to produce fuel pellets for the Power Island.
The separated non-biodegradable Trommel Overs are further screened through magnetic and eddy-current separators to recover ferrous and non-ferrous metals for baling ready for shipment off site.
Density separators remove any heavy items for selective picking to ensure that no stone or glass reaches the granulators.
The remaining, mainly plastic and textile materials are then granulated and pelletised into fuel pellets for the Power Island.
The Tipping Hall, BioDigester Unloading Hall and MRF are maintained under negative air pressure by large fans that extract air from the halls and blow it through scrubbers and a biofilter to eliminate process odours prior to release to atmosphere.
The prepared fuel pellets are bunkered prior to being fed into the input hopper of an advanced thermal treatment system that indirectly heats the pellets to a temperature of approximately 650ºC (dependent on pellet composition) in a starved oxygen environment.
This causes the pellets to decompose into a mixture of mainly short chain hydrocarbon gases including methane, hydrogen, carbon monoxide and nitrogen (Pyro-gas). Longer chain hydrocarbon gases are condensed into a liquid fuel (Pyro-oil).
Once cleaned the Overs and Unders derived Pyro-gas and Pyro-oil is fired in dual fuel engines powering electrical generators. The Unders derived electricity is over 90% renewable "green" power and is exported to the national grid. The Overs derived electricity is mainly from non-renewable sources and is therefore considered "brown" power but it can be used to supply the facility and therefore displaces other fossil derived electricity that would otherwise have to be imported from the grid. Typically there will be an excess of the "brown" power that can also be exported to the national grid.
The advanced thermal treatment process also leaves a carbon rich char that is conveyed to a bunker ready for briquetting as a high calorific fuel for other industrial processes, for use as a carbon rich soil enhancer or as carbon black.
The high grade waste heat from the generators is fed back to the drum dryers to dry the Trommel outputs prior to gasification.
Any residuals and all recyclables are weighed back out over the weighbridge.