HIGH CONCENTRATION SLURRY DENSITY SYSTEM – AN EFFICIENT AND ECONOMICAL WAY OF FLY ASH HANDLINGCoal fired power stations are main source of power generation in INDIA contributing almost 54 % of total power generating capacity of 1,32,300 MW as on date with 76,000 MW to be added by the end of 11th Plan. The bulk of this additional capacity shall be catered through coal based thermal power station, which means more ash to be handled as coal available in INDIA is of inferior quality having high ash content (in the range of 40-45%.). The coal combustion results in both bottom ash (20%) and fly ash (80%), which are disposed by various combinations of method. In majority of thermal plants ash is being disposed in wet slurry form which requires huge land for dumping ash & huge quantity of water to act as carrier and in turn danger of polluting soil due seepage of water. In recent past, fly ash has been found to be of usage and is being converted into useful products such as bricks, cement mix etc. However fly ash utilization as on date is only 43% and still huge quantity of fly ash is required to be stored in ash ponds. As a solution to this nagging problem of disposal of huge quantity of ash, High Concentration Slurry Density (HCSD) has come up as the efficient and economical disposal mode of handling fly ash. The Fly ash –water slurry mixture is prepared at a high solid concentration (by weight) of 65-70% resulting in paste like homogeneous mixture, which is pumped using positive displacement pumps and this mixture is self-settling type. Mechanism: In conventional (lean) slurry disposal system, the concentration of solid in the slurry generally remains at 25-28% by weight. At low concentration of solids, slurry behaves like Newtonian fluid with pumping at high velocity to keep solids in suspension. As the concentration of solids increases, the viscosity also increases and slurry starts behaving more and more like a non-Newtonian fluid practically above concentration of 50% by weight. At this stage, velocity at which solid settles reduces considerably and slurry behaves as self limiting property at discharge point. System Description (Fig -1): The fly ash from silo/bin is fed to pre-mixing feeder (paddle feeder) through variable speed rotary vane feeder at controlled rate and mass flow meter to measure ash flow. The pre-mixing unit (ash conditioner) is provided to moisten the fly ash 1 A control loop with centrifugal pump is provided for recirculating the slurry back to mixing tank where water/fly ash is added as required. The control loop provided with density meter and pressure transmitter.Fig 1: Flow Scheme for High Concentration Slurry Density System before being lead to mixing tank for proper mixing. continuously monitors the density of the slurry and pressure 2 . The moisten fly ash then drops into mixing tank where further mixing of ash – water mixture takes place to attain the right rheology of slurry. for an equivalent annual tonnage of ash. Self Limiting and self setting property: The slurry is self-limiting and in normal weather condition it dries up to a dense compact deposit hard enough to walk on within a day or two. which provides a mechanical separation between the pumped liquid and the clean propelling liquid in which the moving parts are running. Slurry is pumped to ash dump area through seamless Mild Steel pipes and fittings with HDPE pipe over dyke. No mechanical spreading or operational intervention is required as slurry spreads over area due to gravity. Saving in power on ash pond recycle of water will be additional. Specific Energy Consumption (SEC): HCSD requires much less specific power (Kwh per tonne –km) than the conventional lean slurry disposal of boiler ash.drop for pumping thickened slurry and automatically provides for correction if required through the addition of water. 3. a water return/recirculation system is not required. Since this water is not released at disposal. The screens ensure any dried lumps of ash or trap material do not enter the mainline pump and pipeline. Centrifugal booster pump is provided to feed main positive displacement pump through screens. Pump design is such that pumped slurry does not come in contact with moving parts. 2. 300 Nb lean phase line would be equivalent to 100 Nb HCSD. This is accomplished by means of a rubber diaphragm.25 Kwh per tonne-km (at concentration of 55-60% by weight) compared to 0. 3 . The slurry is transferred to the installed mainline Positive displacement (piston diaphragm) pumps. Less requirement of area and versatile siting: HCSD uses less than half area required for lean phase slurry disposal and does not require any ash dyke development/retaining structure. Specific energy consumption for HCSD system is in the tune of 0. 4. Emergency flushing pumps are provided to ensure flushing of line in case of any malfunctioning with main piston pumps.75 Kwh per tonne-km for conventional slurry (at concentration of 28% by weight). 5. Special Features and Advantages of High Concentration Slurry Density System 1. Slurry pipeline diameter: For comparison sake. Ash can be stacked up to height of 25-30 meters at an angle of 60 Deg to base. such as piston and piston rods. Low water consumption: Water consumed is approximately half the weight of the ash transported. Material for Slurry pipeline: Slurry velocity in HCSD being very low (less than 1. Permeability and Leaching: For an in-situ deposit density in the range of 1. 8. Mild Steel is a sufficient material for the slurry pipeline and life expectancy is high. This prevents rainfall from accessing in the deposit and prevents leaching. Girish Bhagchandani Dy. 9. 2 x 250 MW CSES.4 t/cum (on dry basis). Significant cost saving in disposal area. Manager (MAUX) 4 .5 m/s). 2 x 210 MW MPSEB. Khaparkheda TPS (under operation) 3. Very strict quality control of all process parameters is required to ensure trouble free working of the system. it is evident that the High Concentration Slurry Density system will provide efficient & economical solution to problem of disposing high volume of ash being generated in INDIA and an easy to operate and environment friendly system. Installations in INDIA 1. 4 x 120 NALCO TPS (under execution) From the above. 4 x 135 MW BALCO TPS (under operation) 4. liner and dyke construction of upto 60% (approximation. the ash permeability has been measured to 10-8 to 10-9 m/s. may vary from soil to soil condition) Some disadvantages of High Concentration Slurry Density system 1. Budge-Budge TPS (under operation) 2. Generation of fugitive dust does not occur due to dense nature of deposit. wear is not problem in HCSD system. 2.6. 7. Experience with bottom and fly ash mixture (in Indian scenario) being pumped together is still not proven. High Capital cost due to special purpose pumps (imported) and instrumentation. 2 x 210 MW APEGENCO. which will result into benefit in longer period. However overall operational cost is less compared to conventional slurry pumping system.3 – 1.Rayalseema TPS (under execution) 5. 3.