|Swiss Combi – drum drying
|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.
|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%
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
|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 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
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