Boiler Water Treatment in Palm Oil Mills
Having eight 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 in comparison 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 because of the high amount of unrecoverable sterilizer condensate as well as the application of live steam in certain 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 has lots of impurities, turbidity, dissolved gases, and mineral content. Sometimes algae, mud and oil could possibly be present and silica is especially a problem in water from lakes, and this scenario complicates treatment further. Lack or inadequate water treatment methods are dangerous as it may cause scaling and corrosion which modify the reliability, efficiency, and safe operation from the boiler.
In general, boiler water treatment includes external and internal water treatment. External water treatment describes conditioning boiler feedwater by removing impurities, hardness, oil, oxygen, dissolved and suspended solids, outside the boiler and usually accomplished by mechanical means for example continuous clarifier, pressure sand filter, water softener, and deaerator. Because it's not possible to obtain a perfect boiler feed water by external water treatment, an internal water treatment, which mainly by chemical means, will probably be employed. All boiler engineers must be aware that the external management 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 done by the municipal water treatment system. This really is however a different story in palm oil mills. The external treatment for boiler makeup (feedwater) includes pH adjustment, coagulation, flocculation, sedimentation, filtration, water softening, last but not least, deaeration. Soda ash is added when the pH of the water is low. Soda ash serves as pH adjustment because coagulant functions at pH ranges over 5.5 to 8.0. The coagulation chemicals are dosed to the inlet pipe ultimately causing the bottom of a continuous water clarifier. Coagulation and flocculation would be 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 happen to be 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 for example polyacrylamide (PAM) to attach and bridge between particles to make larger agglomerates.
From the natural resources, water is pumped by booster pumps into the continuous clarifier to allow flocs to settle down. The water rises with decreasing velocity and also the resultant floc forms a sludge blanket in the upper part of the conical section. The clarifier is blowdown periodically to eliminate 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 your clarifier and fine particles are filtered and removed. Normally, the sand filter removes fine particles up to 10 �m. Sand filters normally contain sands and gravels, 100% anthracite, or combination of sand, anthracite, and gravel. Sufficient water reservoir is maintained within the overhead tank and when the level of water within the tank becomes low, the process of boilers could be affected. In water softener, sodium zeolite ion-exchange process can be used to remove hardness. The part of a softener is to remove water hardness (magnesium and calcium) using ion exchange process. Normally sodium zeolite is utilized 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 taken away by replacing sodium ion using resin ion-exchange bed which can be covered with Na+ ions. Here in palm oil mills, the softeners are inclined to bio and iron fouling. Soft water will then be pumped into the deaerator for deaeration. The pressurized deaerator operates by allowing steam to the feed water by way of a pressure control valve to keep the desired operating pressure, thus temperature at a minimum of 105�C. The steam improves the water temperature causing the discharge of O2 and CO2 gases which are then vented from the system. This type can help to eliminate the oxygen content to 0.005 ppm. The lake produced is oxygen-free therefore oxygen corrosion within the boiler can be avoided.
The inner treatment involves the addition of reactant chemicals such as sulfite, sodium phosphate, chelates, polymers, and caustic, which each has different function for boiler water. To stop oxygen corrosion, caffeine known as oxygen scavenger is added. The common oxygen scavengers are sulfite, hydrazine, and DEHA. Hydrazine however needs to be avoided because it is carcinogenic.. Phosphate prevents scaling by precipitating calcium as calcium triphosphate or hydroxylaptite, which may be removed via blowdown.