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Energy from Waste (EfW) continued
Fluidised Bed Combustion Technology
The combustion of MSW using fluidised bed (FB) involves pre-sorting the waste materials to remove heavy and inert objects and non-ferrous metals prior to processing. The incinerator is normally a single stage process and consists of a refractory lined chamber with a granular bubbling bed of an inert material such as coarse sand or similar bed medium. This bed is supported on a distribution plate and fluidised by air or other gas being blown through the plate at a high flow rate. Particles are carried out of the vertical chamber by the flue gas into a cyclone. Most of the particles are sand, which are returned to the fluid bed. FB systems offer superior thermal efficiency.
There are two main types of fluidised bed combustor (FBC), bubbling fluidised bed (BFB) and circulating fluidised bed (CFB). The principal difference is that in the CFB boiler the bed material is entrained in the gas stream above the bed, causing more vertical carryover of particulates, which are separated and returned to the bed in the cyclone. In a BFB combustion takes place in the bed which is held in suspension by an upward flow of combustion air. The airflow and velocity are chosen to give intense mixing of material and minimum carry over of particles out of the furnace. The CFB boiler is taller physically than the BFB. Slightly higher thermal efficiencies can be achieved with the CFB boiler (over 90% compared to 89%). The increased turbulence, which is a feature of the CFB boiler technology also, gives lower CO and NOx emissions for the same fuel than BFB technology. The CFB boiler also has a greater fuel flexibility and turndown capability.
Strengths and Weaknesses of Fluidised Bed Incineration Units
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Strengths
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Weaknesses
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Can be used in conjunction with front end recycling operations
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Requires some degree of pre-treatment
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Increasing perception that new processes are more environmentally responsible
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Limited commercially proven experience in the UK, although there is operating experience elsewhere in Europe (Dundee plant has experienced problems, Allington, Kent, plant due for commissioning in 2005)
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Combustion efficiency is high and temperatures are uniform making residence time calculations more reliable
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Capital costs high (as per mass burn)
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Improved emissions control – lower temperature tends to reduce NOx formation
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Negative public perception (as per mass burn)
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Less ash as non-combustible content is lower
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Debate over dioxin emissions (as per mass burn)
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Relatively low maintenance due to single furnace with no moving parts
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Minimum or guaranteed tonnages may be required by operator to cover costs
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Better combustion conditions and burnout - sand provides continuous attrition of the burning material removing layer of char and exposing fresh material for combustion - thereby increasing overall efficiency
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Less ash, but still an operational issue over ash reuse / disposal
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Other issues
The issues affecting energy from waste using moving grate technology are similar to those facing fluidised bed systems. There is the additional issue of a lack of successful operational experience at commercial scale in the UK (although a substantial new plant is due to be built in Kent). Linked to this are potential operational issues with the pre-treatment front end of the process which is required to be robust to homogenise municipal wastes for delivery into the fluid bed.
Other Kilns
Rotary and Oscillating Kilns have also been used internationally for municipal waste energy from waste plant. The only commercial scale facility being developed in the UK at present is the NewLincs Development using an Oscillating kiln design in Grimsby. This plant uses technology utilised in France, which is smaller scale Energy from Waste, in this instance incorporating district heating. The arguments against smaller scale EfW plant tend to be less vociferous particularly when plant are integrated and provide a more local solution.
New article - recommended insight into incineration benefits:
Incineration and Diversion of Waste away from Landfill
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