What A Load of Rubbish!

Have you ever wondered what happens to your domestic waste? Philip Murrell did wonder and he arranged the February monthly meeting of the Science and Technology group to find out more at a disused rag stone quarry next to the M20 motorway! Read on to find out more.

Over the past 15 years the Spanish firm FCC Environment have been constructing and operating a huge plant that uses a form of alchemy to turn half a million tons of domestic waste into enough electricity to meet the needs of 70,000 homes.  Our household waste is compressed and sent on large tipper lorries to the plant, where it passes over a weigh bridge and our council is charged £100 per ton.  The lorries then move on to the tipping hall, a vast space served by three entrances, with perfume sprayers above the doors to neutralise any smells. The hall is also under negative pressure which sucks all smells into the plant’s furnace. The waste is pushed by tractors into a vast pit.

The operation from here on is all controlled by computer. The waste is taken from the pit by two giant grabs and deposited on conveyors where metal is removed by magnets and then on to shredders that increase the surface area to assist burning. The processed waste now falls into a pair of hoppers which at their base have screw augers that feed the waste to the furnace at the desired rate for the furnace conditions. When the furnace is started up there are two opposing oil burners just above the bed of the furnace. The bed is composed of a layer of sand held in suspension which boils like water by a network of primary air jets beneath the sand.

When conditions are right the waste falls through oil burner flames and completes combustion on the red-hot sand. The sand is constrained to the central portion of the bed by channels with an air up blast that is just sufficient to contain the sand but lets large particles of clinker or ash fall into the channels and onto a conveyor for removal.

The hot gasses now start a long journey to heat exchangers to extract heat to raise steam which is fed into a turbine coupled to a generator and thus produce the electricity. The gasses contain an unhealthy cocktail of noxious gasses that need to be removed. Urea sprayed in deals with sulpher and nitrate based chemicals and also cools the gasses. After further cooling in a pair of cooling towers, a scaled up version of the car radiator and fan, the gasses pass through long pipe bags made of filtration fabric thus performing the same function as the domestic vacuum cleaner and removing all dust particles. The gasses then start up the 80 metres high smokestack where again more chemicals are introduced to ensure that output from the top of the stack is far less than that produced by the M20.

Some statistics:-

  • 40 percent of Kent’s waste is processed the rest comes from parts of Surrey
  • 100 people are employed on site, 56% on operations, 28% on maintenance, 16% on administration
  • 45 Mega Watts per hour produced
  • 24 Giga watt hours produced in 2015
  • 200 Lorries per day
  • 300 tonnes of ferrous metals recovered each week

Visit report by John Vale Taylor