ANOTHER VIEW OF THE CROSSNESS DESTRUCTOR
A new European regulation came into force in December 1998 prohibiting the dumping of sewage sludge into the North Sea. Thames Water were therefore disposing of their daily fleet of boats and now despatch the most harmful sludge from throughout their area, some 40% of the total, to two new incinerator installations, one at Crossness, and the other, of similar design but slightly larger in size at Beckton. These two sites will handle more than 100,000 tonnes of sludge annually.
The remaining 60% of less harmful sludge will continue to be dealt with at local plants such as Mogden.
The Crossness incinerator and ancillary equipment is housed in a modern building of striking design some 12 storeys high, soundproofed to ensure that noise levels at the site boundaries remain at previous levels, and was officially opened in November 1998.
At the time of the Newcomen Society visit, because of the extremely empirical nature of the technology and the advanced concepts employed, detailed experimentation was still taking place to identify, rank and optimise the critical operating parameters and techniques.
Upon receipt at Crossness, the sludge is pumped into reservoir tanks of sufficient capacity to provide up to 10 days buffer storage to cater for possible emergency shutdowns. A poly electrolyte is then added to convert the sludge into a solid suspension in free liquid.
Two Dorr Oliver filter presses which operate singly allowing maintenance on the standby unit, expel the liquid, leaving a damp cake-like material. As the optimum press technique had not, at the time of our visit, yet been established an attendant has for the moment to physically check that each filter bag has completely emptied at the completion of the cycle, and to rake the bag clear if needed. A throughput of 3 ½ tons/hour is currently being achieved.
This damp precipitate is then conveyed, via a holding silo, to the incinerator, which operates on a fluidised bed suspended sand principle. The sludge cake is in the combustion zone for 3 seconds at a temperature of 950oC, sufficiently high to ensure vaporisation of the heavy metal content.
Natural gas firing is employed for start up and to augment as approximate the natural combustion process. Depending on the prevalent dryness factor of the incoming cake, self combustion alone may provide sufficient heat input once the process has been successfully initiated.
The dust laden exhaust gas then passes through a Waste Heat Boiler; the steam thereby produced at 42 bar drives a 5.9 Mega watt turbo alternator operating at 6.6KV.
Depending again on the quality and condition of the sludge cake at any given time, sufficient power can br produced to drive all the auxiliary plant on the site, whilst in the extremes some additional power may be needed from the National Grid or there could be a small surplus of power for export.
From the waste heat boiler the gases flow through an ash and heavy metal removal system in which activated lignite coke is employed to trap the heavy metal content, the ash then being stored prior to removal by road transport to a licensed landfill site. Average rated throughput, 30 tons of ash is produced per day.
After then passing through gas cleaning and scrubbing towers the gas is reheated to ensure a sufficiently high temperature to void acidic condensation, analysed in detail to provide records for environmental control and overall plant optimisation and then discharged throughout the station chimney which is approximately one third as high again as the 12 storey incinerator building
The whole plant is remotely controlled from a single control room with the incinerator functioning automatically.
The Crossness and Beckton stations were both constructed by the AMEC-Luigi consortium, the overall contract being worth £125m. Although these plants represent the very latest ‘state of the art’ practice similar equipment exists abroad, Incineration disposes of about 50% of sludge in Japan, 25% in the USA and 15% in Germany
This appeared in the May 2000 GIHS Newsletter