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