From Process Lines
Incineration of sewage sludge in cement factories is becoming a more and more interesting issue worldwide. Besides that municipalities can solve the disposal problem of dewatered or dried sewage sludge, cement factories offer a perfect match for the treatment necessities of sewage sludge. The dried sludge can be used as fuel and, depending on its contents, replace a significant part of costly fossile energy sources such as coal. Sewage sludge, once dried to >90% dry solids normally shows caloric values of 10 to 12 MJ/kg.
The benefit of combining a sewage sludge drying installation and a cement factory is the perfect environment a cement process offers for the sewage sludge drying line. Cement processes normally cause a lot of waste heat that can be used to run the sewage sludge drying line. Hence, the sewage sludge drying installation does not require external heating sources which minimizes the heat-energy costs, which is the most costly part of operational costs of drying lines installed on e.g. municipal WWTPs. By using a ANDRITZ Paddle Dryer for the sludge-drying line, the off gas volumes are so small that they can be easily treated thermally in the cement process itself, eliminating any biological or other multi-stage filter for the off-gas treatment. The produced condensate should be either sent to the sewage system or can be treated locally.
- Compact, safe and simple
- Closed Loop implementation
- No Off-gases to be treated
- Most Flexible on all different sewage sludges
One essential requirement for treating sewage sludge at a cement factory is the flexibility of the sewage sludge drying system to cope with many different kinds of sewage sludge. Typically cement factories take sludge from various stations, where the sludges can also have different pre-treatment. In other words, the sludge-consistency at intake of cement factories varies much more than the sludge at only one wastewater treatment plant.
Nevertheless, the sludge taken by a cement factory is normally dewatered by means of a decanter, centrifuge or belt filter press. After dewatering the sludge has a typical dry solids content of approx. 18 - 25%, and sometimes up to 35% in case of lime stabilisation, which is all very suitable for thermal treatment in a ANDRITZ paddle dryer.
More and more sludges are digested before dewatering to generate biogas as a renewable energy. Even though the behaviour of digested and undigested sludge can vary a lot, a sludge drying installation at a cement factory must be able to handle all types and mixes.
By means of heat exchangers, the waste heat from the cement factory is used to create a thermal oil of 200 to 230°C, which flows continuously through the jacket, hollow shafts and the paddles. Basically there are several points from where the heat can be generated: outlet gas of klinker cooler or outlet of pre-heat column. The klinker cooler offers the benefit of an inert dust in the gasflow. The outlet of the pre-heat column offers the benefit of higher gas-temperatures and more available heat, but the dust taken will contain some fresh 'green' material. Systems to prevent the heat-exchangers from fouling are available to cope with this circumstance.
Due to the indirect ('contact') concept of drying the sludge, the exhaust from the drying installation mainly consists of vapour and a very small amount of non-condensable gases. The vapour is normally condensed in a direct (spray-type) or indirect condenser (heat-exchanger) and the condensate must be discharged back to the sewage or, if necessary, must be treated first. The remaining non-condensables leave the condenser at the top and must be treated to prevent smells and odour emissions. Due to the very small amount, they can easily be treated thermally in the cement process. Again, there are several options to inject the non-condensables, for example at the cement kiln directly or at the calciner.
The heart of the installation is the ANDRITZ Paddle Dryer. It consists of a trough containing two counter rotating shafts, arrayed with paddles. The sludge is fed at one side and flowing through the dryer, while being agitated/mixed by the paddles on the shafts. The heat is transferred from the thermal oil/steam inside the trough jacket, the shafts and the paddles through the metal wall into the sludge. This indirect heat transfer avoids air flows while the fully enclosed operation enables the safe treatment of sludge due to an absence of oxygen.
The ANDRITZ Paddle Dryer does not require any backmixing to overcome the possible plastic phase of sludge, occurring typically between 45 and 55% dry solids. Hence, the necessary equipment to dry the sludge from 20% to >90% dry solids stays simple and is reduced to a minimum.