Having several years of experience working with biomass-fired
boilers in palm oil mills, I can say that boiler water treatment in palm
oil mills is a challenging task if compared with industrial boilers.
Unlike industrial boilers which rely heavily on condensate return and
industrial water for boiler feed water makeup, the source of raw water
for boiler feed water in palm oil mills comes from lake, river, or well.
Relatively not much condensate can be recovered in palm oil mills due
to the high amount of unrecoverable sterilizer condensate and the
application of live steam in some of the process heating. In palm oil
mills, low cycle of concentration is not uncommon which indicates more
chemicals are wasted through blowdowns.
The
raw water from natural resources is high in impurities, turbidity,
dissolved gases, and mineral content. Sometimes algae, mud and oil could
be present and silica is particularly a problem in water from lakes,
and this scenario complicates treatment further. Lack or inadequate
water treatment is dangerous as it may cause scaling and corrosion which
affect the reliability, efficiency, and safe operation of the boiler.
In
general, boiler water treatment consists of external and internal water
treatment. External water treatment refers to conditioning boiler
feedwater by removing impurities, hardness, oil, oxygen, dissolved and
suspended solids, outside the boiler and usually accomplished by
mechanical means such as continuous clarifier, pressure sand filter,
water softener, and deaerator. Because it is not possible to obtain a
perfect boiler feed water by external water treatment, an internal water
treatment, which mainly by chemical means, shall be employed. All
boiler engineers must be aware that the external treatment of boiler
feedwater must be emphasized since more chemical addition in the boiler
is undesirable.
In industrial boilers, the removal of turbidity
and suspended matters is already done by the municipal water treatment
system. This is however a different story in palm oil mills. The
external treatment for boiler makeup (feedwater) consists of pH
adjustment, coagulation, flocculation, sedimentation, filtration, water
softening, and finally, deaeration. Soda ash is added if the pH of the
water is low. Soda ash acts as pH adjustment because coagulant functions
at pH ranges over 5.5 to 8.0. The coagulation chemicals are dosed into
the inlet pipe leading to the bottom of a continuous water clarifier.
Coagulation and flocculation are the basic steps in boiler water
treatment to reduce turbidity, organic substances, and color of raw
water. Coagulation neutralizing the negative charges on colloid
surfaces, allowing the particles to agglomerate to form floc, which is
slow settling. The most common coagulants are aluminum sulfate (alum),
sodium aluminate, and polyaluminum chloride (PAC). Synthetic polymers
called polyelectrolites have been developed for coagulation process.
Flocculation is further agglomeration of slowly-settling coagulated
particles into large rapidly-settling floc with the addition flocculant
such as organic polymer (starch) or synthetic polymer such as
polyacrylamide (PAM) to attach and bridge between particles to form
larger agglomerates.
From the natural resources, the water is
pumped by booster pumps into the continuous clarifier to allow flocs to
settle down. The water rises with decreasing velocity and the resultant
floc forms a sludge blanket at the upper part of the conical section.
The clarifier is blowdown periodically to remove the heavy sludge, which
settled at the bottom while clear water is transported to a clear water
tank before passing through a pressure sand filter where solids escaped
from the clarifier and fine particles are filtered and removed.
Normally, the sand filter removes fine particles up to 10 μm. Sand
filters normally contain sands and gravels, 100% anthracite, or
combination of sand, anthracite, and gravel. Sufficient water reservoir
is maintained in the overhead tank and if the level of water in the tank
becomes low, the operation of boilers could be affected. In water
softener, sodium zeolite ion-exchange process is used to remove
hardness. The function of a softener is to remove water hardness
(calcium and magnesium) using ion exchange process. Normally sodium
zeolite is used and a softener would contain 30~36" of synthetic ion
exchange resin. For example, calcium bicarbonate, Ca(HCO3)2 dissolves in
water and split up to Ca2+ and (HCO3)-. In ion-exchange process, Ca2+
is removed by replacing sodium ion using resin ion-exchange bed which is
covered with Na+ ions. Here in palm oil mills, the softeners are prone
to bio and iron fouling. Soft water is then pumped into the deaerator
for deaeration. The pressurized deaerator operates by allowing steam
into the feed water through a pressure control valve to maintain the
desired operating pressure, and hence temperature at a minimum of 105°C.
The steam raises the water temperature causing the release of O2 and
CO2 gases that are then vented from the system. This type can reduce the
oxygen content to 0.005 ppm. The water produced is oxygen-free
therefore oxygen corrosion in the boiler can be avoided.
The
internal treatment involves the addition of reactant chemicals such as
sulfite, sodium phosphate, chelates, polymers, and caustic, which each
has different function in treating boiler water. To prevent oxygen
corrosion, the chemical known as oxygen scavenger is added. The common
oxygen scavengers are sulfite, hydrazine, and DEHA. Hydrazine however
should be avoided because it is carcinogenic.. Phosphate prevents
scaling by precipitating calcium as calcium triphosphate or
hydroxylaptite, which can be removed via blowdown.
