Industrial and Municipal sludge

Drying

Wet materials like municipal and industrial sewage sludge are dried suspended in superheated steam generated from the water in the sludge. The drying takes place in a closed loop pneumatic conveying system, with the result that no dust or odour is discharged from the system. Simultaneously, a continuously generated steam surplus can be used for heating purposes.

The mechanically dewatered sludge is first mixed with dried sludge and then suspended in a superheated transport steam. The steam is indirectly heated via a tubular heat exchanger using heating media such as medium pressure steam, flue gases or thermal oil, or electrical heat. The dried sludge is then separated from dry steam in a cyclone for continued processing.

Low energy consumption

Using this indirect heating means that every type of fuel can be used; sludge digestion gas, natural gas, biomass (sludge) etc. The closed drying system results in extremely small or no heat losses to the surroundings. Re-using generated steam in another process can recover 70-90 % of the energy from excess steam.

By drying and incinerating sludge, the calorific fuel value of the sludge exceeds the requirement of drying; the main part of the energy is available as product steam, thus giving positive cash flow.

Free from emissions

Dust and odour emissions are easily minimised by the closed system and are then handled by the furnace. [Waste] water from condensing excess steam is preferably treated in the sewage plant.

Flexibility

All types of sludge and variations in feed moisture can be easily handled by internal backmixing of dry material. The dryness of the final product is controlled by a heat exchanger. Capacity can be widely varied (30-100 %), with the same high efficiency still present.

High availability and safety

The Superheated Steam Dryer (SSD™) has few movable parts making it easy to operate. This affords a high availability of the dryer and low maintenance costs. The steam in the system safeguards against fire and explosions.

End product

For use as landfill, farming, or for combustion, dried sludge can be compacted for easier handling. A more defined product for agricultural use can be produced via pelletization or granulation. The product, completely sterile, has been confirmed by a number of independent analyses.

Incineration

If no existing source of heat can be used, dried sludge contains more combustion heat than the drying process needs. A combination of drying and incineration is therefore more than self-supporting and provides a massive excess of heat. Dried sludge has a heat value of 10-20 MJ/kg (2.5-5 MWh/ton dried sludge). Experiences from incineration on grates and in fluid bed exist. The type of incineration equipment chosen depends on capacity, the possibility of co-firing with other fuels, as well as the characteristics of the ash (melting point). Varying legislation from state and/or federal authorities on discharge of flue gases to atmosphere is another issue that must be considered.

Flue gas treatment

Dry as well as wet methods can be used to clean flue gases to levels acceptable to authorities in the EU and other countries. An example from a fluid bed combustion with lime absorption flue gas cleaning and a suppression filter is shown in the table below.

Ash treatment

The ash is slaked by adding water and becomes less dusty. It can be used as landfill, fertilizer or alternatively stored for recycling of phosphorus and precipitation chemicals. These techniques are under development.

Energy utilisation

With an integrated system of drying and combustion, the excess energy of the sludge is about 2,6 MWh per ton dry sludge and can, for example, be used to supply heat for sludge digestion and/or for district heating. Electric energy (about 200 k Wh per ton dry sludge) would be the largest operating cost. Refer to the example below:

Energy diagram for integrated drying and incineration

Mass & Energy balance for complete system with integrated drying and incineration. Example with 1 ton dry sludge or 4 ton wet at 75% moisture.