Waste to Engergy (WtE) – Electricity from Municipal Solid Waste
The Guardian newspaper has stated we could provide 20% of the UK electricity requirement by incineration of our rubbish. This seems an incredible claim, however it is totally feasible. The main constraint is from the NIMBY brigade, believing in a worst case scenario where toxins and particles are allowed to escape from the incinerator flue. Actually, this is highly unlikely due to the presence of efficient and strictly controlled exhaust filtration systems before the flue exit.
There are two different types of MSW incineration plants
- Mass Burn
This is the most common type in which all the MSW is supplied to the furnace
- Refuse Derived Fuel (RDF)
In this method the ferrous/non ferrous metals, builders rubble and other non combustible materials, and recyclable materials are removed before incineration.
Operation of a MSW Incineration Plant
The operation starts at the incineration plant with the arrival of the Municipal Solid Waste (MSW). This is tipped into a loading bay from which it is lifted out by an overhead crane that loads a hopper continuously, thus avoiding blowback.
The hopper drops the waste onto a chain conveyor that feeds the waste into the furnace onto the moving grate where it is incinerated, assisted by fossil fuel burners and secondary combustion air blown through the grate.
It is mandatory that the temperature of 850°C is obtained in the first few seconds at start of combustion to ensure destruction of organic toxins.
The bottom ash falls through a water seal into the bottom ash treatment section where it is subjected to vibrating screens and a magnetic drum.
Any ferrous metal will have melted and be recovered from the bottom ash by the drum magnet, with the remainder going to the landfill. This ash is about 25% of the original MSW input to the incinerator.
The hot combustion gasses produced in the furnace pass upwards through water tubes in waste heat boiler which produces superheated steam. This superheated steam is used to drive steam turbine generators as in a normal steam generation plant system and should produce about 2MW for every tonne of MSW. It should be noted that the furnace combustion air is preheated by a heat exchanger positioned in the fume extract ducting, and secondary air is blown into the furnace through the moving grate, to aid separation of waste and also assist combustion.
Extraction of Fume Impurities in a MSW Incineration Plant
The furnace gases exit the boiler and are directed through an air heater, which heats the boiler combustion air increasing the plant efficiency. The fumes then pass through the fume extraction plant, a series of scrubbers and filters.
- Activated carbon scrubbing
This scrubber removes the heavy metals from the gas stream, with the particulates being drawn off at the bottom or passing on to the filtration unit where the are removed
- Ammonia scrubbing (urea)
This scrubber removes the nitric gasses (NOx) from the fumes by catalytic reduction. The residue is either removed at the bottom or when passing onto the filtration unit.
- Lime and water scrubbing
This scrubber uses a mixture or slurry of lime and water to remove the sulphur dioxide from the fumes. Gypsum is a useful by-product of this method of scrubbing
Note: Any water and liquid residue collected in the scrubber hoppers are transported to a waste water plant for further treatment.
- Electrostatic precipitation.
This equipment consists of an enclosure through which the combustion gasses flow. It contains a bank of steel collector plates, interspaced with high voltage wires which hang down between the plates and act as electrodes. The fumes pass between the plates and electrodes.
Electric currents are passed through electrodes causing the dust from the flue gas to adhere to the plates. A rapper knocks against the plates dislodging the dust which falls down from the precipitator into a dust hopper where it is collected for safe disposal.
- Baghouse filters
This is a steel rectangular box structure through which the gasses flow. Dust particles are efficiently removed by passing through the fabric filters. These filters are of circular section, open at the bottom and coming to closure at the top in a conical shape. The gas enters at the bottom of the filter and the particles are removed as it passes upwards and through the fabric of the filter. A layer of dust builds up on the inside of the bags known dust cake which aids the recovery of very microscopic particles, increasing the efficiency. To clean the filters the system is isolated and a shaker mechanism vibrates the bags, with the dust falling off being collected in hoppers underneath.