New developments in spinning fluidised bed incineration technology

At the present time, the sewage treatment plants in the UK produce about 25 million tons of sewage sludge each year at a concentration of 4% solids. New regulations forbid sea dumping and in the near future new incinerators will be required to dispose of about 5 million tons per year. Bubbling fluidised bed incinerators are widely used to burn sewage sludge at a typical consumption rate of about 0.02 kg(dry) • S
^−l
• m
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, and it follows that over 300 conventional fluidised bed incinerators of 3 m diameter could be required to cope — with the increased demand. At Sheffield University Waste Incineration Centre (SUWIC) research work is being carried out to develop a novel spinning fluidised bed incinerator. The key factor to note is that when air flows up through a bed of near mono-sized particles, it fluidises when the pressure drop across the bed is equal to the weight of the bed. Normally, the weight of the bed is determined by gravity. However, if the bed is contained by a cylindrical air distributor ‘plate’ that is rotating rapidly about its axis, then the effective weight of the bed can be increased dramatically. The airflow passing through the bed can be increased proportionally to the “q” level produced by the rotation and it follows that the process has been intensified. In exploratory tests with a spinning fluidised bed we have achieved combustion lntenslties with coal combustion as high as 100 MW/m
³
• A problem with burning coal is that it was difficult to remove the heat and rotating water seals had to be used to transfer cooling water into the bed. In the case of sewage and other sludges, this problem does-not exist since the flue gases can remove the small amount of heat released. The rotating fluidised bed sludge incinerator is a novel device, which is very compact. It is able to solve the turndown problem encountered with conventional fluidised beds by simply changing the rotation speed. Bearing in mind that a centrifugal sludge de-waterinq unit is already used on sludge incineration plants, it is likely that the rotating Iluldlsed bed can be combined with the de-watering unit, resulting in further process integration and intensification. Furthermore, our ash sintering system can be added to render the ash non-toxic and unleachable thus reducing the cost of its disposal.