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Swiss Combi – drum drying
 
Process Schematic
The Swiss Combi® sewage sludge dryer is a convective drum dryer. Using a patented Closed Loop design the plant offers the best thermal and environmental efficiencies to be found anywhere in the global sludge drying market.
 
Sludge path
Before being fed into the drying drum, mechanically dewatered sludge is mixed in a twin shaft mixer with recycled dry sludge from the recycle silo to form a homogenous, free flowing product with a DS-content in the range 55 - 75%.

Pre mixing the dewatered sludge with dry granules eliminates the ‘adhesive’ phase of the dewatered sludge and avoids the accumulation of sludge deposits in the drying drum and also assists in forming the granular structure. The material is then transferred to the drum by a feed screw.

The drying drum is specifically designed for sludge. The rotation and internal drum geometry lifts the granular material and progressively transports the material along the drum by the pneumatic properties of the hot air stream.

The process is self regulating and ensures even drying and minimal damage to the product. Hot air circulating in a closed loop enters the drum at a temperature of 400 - 450°C and passes through the drying drum evaporating the water content of the sludge. The drum outlet temperature is maintained at a set point value in the region of 120 - 125°C to ensure drying to a level of > 90% dry solids.

After passing through the drying drum the dried product is now at a temperature of approximately 90°C. Separation of the granulate from the drying air stream is achieved in a combined filter and cyclone. The dried product is discharged through a rotary valve to a discharge screw for transfer to a cooling screw where partial cooling of the product takes place. After the product has cooled down to approximately 70°C, it is gently transported to a vibrating screen sizer via a slow speed bucket elevator.

The vibrating screen sizer divides the dried product into three fractions (coarse, medium and fine). The medium product is conveyed as end product through a final cooling screw where the temperature is reduced to below 50°C, before being transferred to a final product bulk storage silo or bagged storage system.

The coarse and fine material is transferred to the recycling silo to be used for mixing with the incoming dewatered sludge.
 
Aspiration air, heat generation and exhaust air
The system operates under slight negative pressure ensuring that odour and dust remain within the operating system. This system generates a number of benefits including the ability to maintain a clean and pleasant working environment, avoiding local nuisance complaints and reducing condensation problems with the plant.

The aspirated air is cleaned in the aspiration bag filter and then redirected to the combustion air (primary air) of the dryer combustion chamber. The dust separated in the filter is transported, via discharge screw, to the twin shaft mixer.

The heat energy required for the drying process is generated in a combustion chamber fired by natural gas, biogas and or fuel oil.

The combustion air (primary air) comprises ambient air together with filtered aspiration air. The non-condensable gases from the condenser are also blown into the combustion chamber as secondary dilution air. With the combustion chamber operating at approximately 800°C this ensures the complete destruction of any unpleasant odours.

The hot flue gas flows through the heat exchanger where most of its thermal energy is transferred to the air/vapour mixture circulating in the closed drying air loop.

The cooled flue gas (exhaust air) leaves the heat exchanger and, via the exhaust air blower, is released into the atmosphere through the exhaust stack at a temperature of 140 –180°C.
 
Drying air circuit
The heat energy needed for drying the sludge transfers through an air-to-air heat exchanger. The drying air is contained within a closed loop system giving the benefit of substantial heat recovery and therefore making the process more efficient than other systems.

Additionally, because the source of primary energy generation is separate and not in direct contact with the sludge, there is no risk of sparks or hot particles generated in the combustion chamber, being carried directly to where potentially high levels of explosive or combustible material are present.

As the air passes through the drum, it evaporates the water from the sludge. This air/vapour mixture is then separated from the dried sludge in the combined cyclone/filter unit and is returned to the heat exchanger for reheating where its temperature is raised from 120°C to about 400 – 450°C.

Part of the air/vapour mixture is continuously tapped off from the drying air loop and passed through a mixing condenser where the vapour is condensed by means of injected cooling water. After condensation, the air tapped from the drying circuit and the non-condensable gases pass through the secondary air blower and are returned to the combustion chamber for thermal oxidation at c. 800°C. The condensate, approximately equivalent to the volume of water evaporated from the dryer, is discharged from the condenser together with the injected cooling water.

The closed loop system results in a low operating oxygen content in the drying stage, in the range 5 – 10% by volume. This gives the system a high margin of safety.

A spray water module (inertisation) is included in the drying air circuit between the heat exchanger and drum inlet to ensure that an inert atmosphere is maintained under all operating conditions. This device allows the oxygen level within the drying air circuit to be reduced on start-up prior to introduction of sludge. It can also be used to reduce drying air temperatures and maintain safe oxygen levels during plant shutdown.
 
A detailed schematic of the process can be found here
Schematic