- The integration
of an evaporation
plant
offers the advantage, that no waste water will be released to
the environment. The evaporation plant uses the surplus heat
out of the energy production for the concentration of the digester
effluent up to a dry matter content where it is suited for drying
(drier
plant)
or composting (about 30%DM).
- Major components
of the evaporation system are:
-
- Degassing
and acidification system
From the screw presses the liquid fraction is pumped into a degassing
and acidification system. The fermentation process generates
substantial amounts of CO2
which partly is dissolved in the liquid phase. During the concentration
process in the evaporators this CO2 is set free rapidly causing foam problems inside
the evaporators.
- It has to be
removed anyway by adjusting the pH-value of the liquid to a value
of about 6 in order to expel the diluted CO2. This is done by adding specific amounts of sulphuric
acid into the acidification system. The liquid inside the buffer
is moved and the automatic dosage is controlled by a pH sensor.
The fermented substrate shows a reasonable concentration of ammonium
(2-4%) as all nitrogen, originally contained in the biowaste,
is converted into this form by the fermentation process. The
low pH is also needed to avoid gaseous emission of ammonia during
evaporation. Since ammonium/ammoniac is a strong pH-buffer system,
the amount of sulphuric acid needed to lower the pH is directly
determined by the ammonium content of the substrate. The consumption
of acid equals - as a rough estimation - is three times the weight
of the ammonium in the effluent.
.
-
- Evaporation
plant
The evaporation system for the decanter water is a forced circulation
apparatus with for effects. The effects are connected in a partial
counter flow system.
While the energy flows from effect 1 to 4 in a cascade, the substrate
is fed to effect 3 and subsequently flows from 3 to 4, 2 and
1. This substrate flow is necessary since the temperature of
the pre-heated substrate will be about 80°C, which is close
to the operating temperature of effect 3 but already too warm
for effect 4.
From effect 1 the concentrated substrate is withdrawn with a
temperature of about 106 °C and pumped to the input buffer
of the dryer.
The first effect is heated by the steam from the falling film
evaporator. Since it works under atmospheric pressure the produced
vapour has a temperature of 100 °C and is used for he heating
of effect 2. Effect 2-4 operate under reduced pressure with a
minimum of 120 mbar (abs.) at effect 4.
The vapour of the final stage (effect 4) is condensed by a surface
condenser which is cooled by a cooling tower.
The condensate of each effect flows through the cascade and leaves
the plant together with the condensate from the cooling tower
with a mixed temperature of 55 °C. It is passed through a
process filter and a heat exchanger where it is cooled down to
35 °C before it is gathered in the condensate buffer. The
heat will be used for digester heating.
The four effects of the evaporator are designed nearly identically
and differ only in the diameter of the cyclone separators which
is adapted to the pressure of each stage and the resulting vapour
volumes. The effects mainly consist in:
- Heat exchangers
Cyclone separator
Circulation pump
Steering and control devices
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