Sludge Utilization

 


Thermal utilization of sewage sludge

In this example a fluidized bed plant for the thermal utilization of sewage sludge comprises the following main components:


  • sludge dewatering
  • fluidized bed incinerator
  • waste heat steam boiler for heat recovery
  • flue gas cleaning


The figure below shows the main components which will be described in the following.




First the sewage sludge is pre-dewatered mechanically e.g. by means of centrifuges, chamber filter presses or belt presses.

The incineration of the sewage sludge in the fluidized bed incinerator is determined by the calorific value of the pre-dewatered sludge. Mostly the water content of the mechanically pre-dewatered sludge is too high (and thus the calorific value too low) for an incineration without additional measures.

There are some possible additional measures which will be necessary as a single measure or which will be combined according to the requirements: preheating of the combustion air, additional fuel (e.g. fuel oil, gas, coal, secondary fuel, biomass) and the thermal drying (partly) of the sludge.

The sludge is combusted in the fluidized bed incinerator at a temperature of approx. 870 °C. Therefore the combustion is absolutely odourless. In the downstream waste heat steam boiler the flue gas is cooled down and steam is produced. The steam can be used for current generation or for heating purposes, thus for a thermal sludge drying and/or for combustion air preheating.

Depending on the calorific value of the sludge and the plant composition an autothermal process can be realized and additionally electric current can be fed into the grid or steam can be delivered for process- or heating purposes.

The flue gas cooled down in the steam boiler is cleaned in the flue gas cleaning plant from dusty and gaseous harmful substances to keep the emission limits. The clean gas is released through the stack into the atmosphere.

The dusty ashes and residues from the flue gas cleaning plant are recycled or disposed according to the relevant regulations.

The entire incineration plant may be erected as an outdoor plant or partly respectively completely inside a building. The processes in the plant are largely automated. The operator in the control room only interferes in case of failures, change-overs and during start-up and shut-down procedures.