Total Water Management in Thermal Power Station By Mr. N. Ramachandran - Associate Vice President (Technology) Introduction Large volumes of water are required by thermal power stations for various applications such as boiler feed, condenser cooling, bearing & seal cooling, ash handling and lubrication of pumps besides water for drinking and sanitation. A 210 MW unit typically requires 30,000 to 33,000 m3/h of water. A large part of this water is used for condenser cooling and a small quantity is used for boiler feed makeup and other uses. Total Water Management in thermal power station consists of : 1. Treatment of water for boiler feed 2. Treatment of water for condenser cooling 3. Treatment of wastewater for disposal or recovery of water for reuse. 1.0 Treatment of water for boiler feed 1.1 Boiler feed water treatment for high pressure boilers is almost standard. Raw water is clarified and filtered for removal of undissolved impurities and demineralised for removal of dissolved salts. Dissolved oxygen is removed in a thermal deaerator. Residual dissolved oxygen is removed by hydrazine. Trisodium phosphate is added into the boiler drum for ph conditioning and to precipitate hardness if any introduced inadvertently into the boiler drum due to mal operation. Condensate corrosion is controlled by addition of ammonia or neutralising amine such as morpholine or cyclohexylamine. We shall discuss two contaminants which are of particular interest in high pressure boiler water treatment. 1.2 Presence of silica in boiler feed water is harmful as silica tends to volatalise along with steam and get deposited as glassy and hard deposits on the turbine blades. Silica deposition is usually controlled by maintaining its concentration in the boiler drum within limits specified by boiler manufacturers. It has been established that concentrations of silica in excess of 0.03 mg/l invariably causes problems in turbine operation. Suitable lower silica level should be maintained boiler water to maintain silica less than 0.02 mg/l in steam leaving the drum. Fig 1 shows the maximum permissible concentration of silica in boiler drum water as a function of boiler pressure.
Fig -1 2 2 1.3 Non-reactive Silica 1.3.1 Silica in water is present mostly as reactive or dissolved silica. In surface waters, a small quantity of non-reactive silica (in colloidal dimensions) may also be present during parts of the year especially during the monsoon. A DM plant removes reactive silica almost completely, to less than 0.005 mg/l. However, non-reactive silica is not removed and finds its way into the boiler drum where it gets converted into reactive silica under the operating conditions of high pressure and temperature. The station chemists usually overcome this problem by having increased blow-downs during these periods.
3 1.3.2 Increased blow-down however means loss of valuable condensate and when one considers the cost of condensate at Rs. 100/m 3, even a small increase in blowdown of say, 1 m 3 /h for four months in a year, will mean a loss of Rs. 2.7 lacs annually. In addition, removal of silica scales is very difficult and needs shutdown of the power station disrupting power supply. It is therefore important that nonreactive silica also is removed from boiler feed water. 1.3.3 Estimation of non-reactive silica in water is often difficult as it is present in very small quantities (generally less than 1.0 mg/l) and calls for a high degree of expertise on the part of the chemist. Besides, it requires handling of hazardous chemicals like hydrofluoric acid. 1.3.4 Presence of non-reactive silica is therefore best ascertained by continuous monitoring of silica in boiler drum water. Ingress of non-reactive silica will be seen as an abnormal increase in boiler drum silica values during some parts of the year. 1.4 Removal of Non reactive silica There are atleast two methods for removing non-reactive silica from boiler feed water : - Pretreatment of water ( coagulation and clarification) - Ultrafiltration 1.4.1 Pretreatment of water Difficulties in removing non-reactive silica from the fact that it is not present as a simple colloidal particle and is not amenable for coagulation under normal conditions. It is mostly present as hydrated silica associated with organic matter naturally present in soil and hydrated oxides of iron & aluminium. A multipronged attack is therefore called for when one wants to remove non-reactive silica in pre-treatment plant. Experience has shown that optimum removal of non reactive silica can be achieved if all of the following conditions are met. a) Effective pre-chlorination of water to oxidise organic matter associated with colloidal particles. b) Maintaining optimal ph conditions and dosing of a primary coagulant like alum and a polyelectrolyte for flocculation, in dosages determined by jar tests. c) A solids contact type clarifier, with solids recirculation, provides ideal conditions for coagulation of non reactive silica. As it is present in small quantities, recirculation of solids ensures adequate contact between the colloidal particles and the coagulant. Coagulation at best is a physical process and one can therefore expect to remove upto 80 to 90% of non reactive silica present in water under ideal conditions. Fig. 2 shows cross sectional view of a solids contact type clarifier.
Fig -2 4 1.4.3 Ultrafiltration Ultra-filtration (UF), a pressure activated process and employs a semi permeable membrane with asymmetric structure (see fig 3 ) and can be effectively employed for removal of non-reactive silica. Membranes with a molecular weight cut off of 100,000 remove upto 99% of non-reactive silica present in feed while a tighter membrane with a molecular weight cut off of 10,000 removes upto 99.8% ( see Table-1). A schematic diagram of an installation for removal of colloidal silica is shown in Fig 4 and 5. Fig -3
Table-1 5 Component 10,000 MWCO 100,000 MWCO Colloidal Silica 99.8% 99.0% Colloidal Iron 99.8% 99.0% Colloidal Aluminium 99.8% 99.0% Suspended Solids Virtually all SS Turbidity Less than 0.1 NTU Silt Density Index Less than 1.0 SDI Giardia 6 LRV 5 LRV Crytosporidium 6 LRV 5 LRV Bacteria 6 LRV 5 LRV Viruses 5 LRV 4 LRV Endotoxins 4 LRV 2 LRV Total Organic Carbon Avg. 70% Avg 30% Fig -4 Fig-5 Removes colloidal silcates, aluminum and organics (humic, fluvic acids) Reduce silica deposition on turbine blades Reduce corrosion potential of high molecular weight organics Reduce turbine maintenance down time Reduced boiler feed chemicals Fewer make-up water upsets means more on-line time Less power purchased from other utilities
6 The best way of ensuring maximum removal of non-reactive silica will be to remove the bulk of it in the pretreatment plant and polish it with an ultrafiltration system installed at the outlet of the mixed bed unit. The effectiveness of the system can be seen from the results (Fig 5) obtained from an ultrafiltration unit at New York State Electric & Gas, Millkaen Station in New York, USA. A number of similar installations are in operation in the US and other countries. In a developing country like India, where the plant load factor is an important consideration, the additional investment in ultrafiltration is quite justified since it reduces the number of shut downs caused by turbine failure due to silica deposits. 1.4.4 Organic matter Organic matter present in surface water is mostly of vegetable and animal origin and consists essentially of large molecular weight carboxylic acids collectively termed as humic and fulvic acids. These carry a negative charge and therefore are adsorbed by a strong base resin in a DM plant. Organic matter is harmful if present in boiler feed water as it often breaks down in the boiler drum, depresses the ph and causes corrosion. A lower ph increases the risk of silica carry-over in steam. Recent specifications for cogen boilers require organics in boiler feed water to less than 20 ppb. Removal of organic matter Organic matter can either be in colloidal or dissolved form or both. Here again, removal of organics by proper pretreatment is the most logical and economical way. Pre-chlorination of water oxidises dissolved organic matter and colloidal organic is best removed by proper coagulation and clarification. Ion exchange resins resistant to fouling by organics present in water should be used. Any residual organic matter can be removed by Nano-filtration (NF) membranes. UF membranes are not very effective due to the low molecular weight of organic matter present in surface waters. 2.0 Treatment of water for condenser cooling 2.1 The present generation power plants employ open recirculation cooling water treatment to conserve water. Scaling, corrosion and fouling are the three problems in operating these systems. A few power stations operate at very low cycles of concentration to avoid scaling and do not use any treatment. Most of the power stations in Maharashtra, however,use softened water as CT makeup. Corrosion is minimised by the choice of corrosion resistant material of construction. Bio-fouling is controlled by chlorination of recirculating water. When softened water is used as make-up, disposal of regeneration waste is a major environmental problem. Besides the scheme involve a high capital and operating costs. Cold lime softening would be a better option as this will involve only solid sludge disposal.
7 2.2 A few plants use zinc and ortho-phosphate treatment and operate the cooling tower at high cycles of concentration. While zinc phosphate treatment gives maximum corrosion and scale control, the treatment scheme is expensive and requires stringent ph control. In addition, discharge of orthophosphate containing effluent has become an environmental concern. The presence of ortho-phosphate in effluent is known to cause algal blooms in receiving water bodies. 2.2 Ion Exchange (India) Ltd. has developed an alternate treatment that overcomes these limitations namely the Zinc Organic treatment. This treatment programme operates at alkaline and scaling conditions. The use of high efficiency dispersants and phosphonate controls scaling even upto Ryzner Saturation Index (RSI) of 4.0. Corrosion control is achieved by controlled use of zinc only and this treatment avoids Ortho- Phosphate completely. As a result, not only is the effluent more environment friendly, but algae growth in cooling water is also lesser. 2.3 This was tried at a power plant in Andhra Pradesh. The power station was commissioned in the middle of 1997. Initially, no treatment was employed and instead the system was operated at low COC ( < 1.3). 2.4 The plant is located in an area with water scarcity. As a result, the plant management decided to go in for chemical treatment that would blow down and reduce water consumption. 2.5 Basic advantage of operating the cooling tower at high cycles of concentration are reduced quantity of make-up water to minimal treatment cost. 3.0 Treatment of wastewater and its disposal or recovery and reuse of water. 3.1 Water is a scarce resource and Thermal Power stations are today being compelled to minimise consumption of water to the extent possible. It is possible to recover and reuse water from most of the waste streams generated in a thermal power station. The main waste streams are : Gravity filter backwash water Wastewater generated from the DM plant Ash pond overflow water Boiler blow down and turbine drains. Recovery of water from treated sewage 3.2 Gravity filter backwash water Gravity sand filters require to be backwashed once a day and in the process generate a large volume of wastewater during backwash. The entire quantity of backwash water can be collected in a sump and pumped back to the clarifier.
3.3 Wastewater generated from the DM plant 8 Regeneration of DM plant produces two streams - a lean stream with low TDS and a concentrated stream with high TDS. The lean stream consists of water generated during backwash and fast rinse stages and can be recovered and recycled back to the clarifier. The strong stream consists of water generated during regenerant injection and slow rinse stages and can be routed to the solar pond. 3.4 Ash pond overflow water A large quantity of water is required to pump ash slurry to the ash pond. It is not uncommon for power stations to deliberately operate the cooling tower at low cycles of concentration and pump water from the cooling tower sump to the ash handling system. Water from ash pond overflow can be recovered and reused after treatment through a solids recirculation type clarifier. This step will ensure that there is no discharge from the ash handling system. 3.5 Boiler blow down and turbine drains These are fairly clearly streams and can be recovered and reused. 3.6 Recovery of water from treated sewage Conclusion Water can be recovered from treated sewage for reuse for low end uses. The tertiary treatment plant will generally consists of a chlorination step followed by filtration through a sand filter and carbon filter. The treated water may be used as CT make-up or for gardening. Industries in the recent past have realised that water a precious natural resource that sustain the world is fast becoming a scarce resource and that it needs to be treated with respect and managed efficiently. A thermal power station consumes a large quantities of water and therefore needs to adopt modern technologies to conserve, recover and reuse water wherever possible and approach zero discharge. The objective of this presentation is to persuade everyone in the industry to do their bit in conserving water - our earth s most precious resource.