Rheological investigation of fly-ash water slurry using cationic, anionic and non-ionic additives

Abstract

The problems faced by Thermal power plants during the hydraulic transportation of residual fly ash is well known, commonly a lean mixture of 20 to 25% by weight of fly ash is mixed with water to form a slurry and is pumped through pipelines to ash ponds. Rheological properties of fly ash water slurry play a crucial role in estimating the power requirements for such process, fluid properties of such complex fluids such as viscosity and shear stress are strongly related to the pumping power. Clearly low values of such properties demand less power requirement. The idea of using a surfactant to reduce drag properties of such slurries have been established long since. In this work the effect of 3 different drag reducing additives is studied through experimentation. The cost, availability and effectiveness of additives are important parameters to be considered before using them in power plants. Additives used are Cetylpyridinium chloride (reffered to as CPC) which is cationic in nature, non-ionic Triton x-100 (reffered to as triton), and anionic Sodium Dodecyl sulfate (reffered to as SDS). These 3 additives bear hydrophobic tail, which increase the hydrophobic activity of fly ash by reacting on surface. The action of these additives on flow behaviour of slurry is studied at different dosages using a rheometer. The effect of cmc (critical micelle formation) plays an important role in determining viscosity at higher shear rates. It is found that effect of cmc can be used to determine the optimal amount of dosage of additive required for drag reduction. Also in general, composition or particle size of fly ash is not fixed, in this work the particles of different size range are extracted through mechanical sieving and flow behaviour of slurries made from these particles is studied. Large particle size adversely affects good flow behaviour. Another way to study good flow behaviour is by matching actual data with the modelled values. Rheological models are excellent mathematical tools available, which help to analyse good flow behaviour. In this work, extensive amount of data has been analysed with ‘Oswald de Waele’ model, which is basically a power law model. During this process some advantages and shortcomings of this model are discovered, which ultimately conformed its range of reliability and area of application. In this work settling characteristics of fly ash slurry is studied by the use of gravitational settling technique. The effect of additives on settling rate is studied, it is observed that action of these large molecular size additives increase the mass and inertial properties of fly ash particles, thus leading to increase in settling rate. An entirely new approach is adopted to compare the effect of different dosages of additives on settling behaviour of slurry. The process involves the analyses of data points through regression and comparing the slopes of these regression lines. The iii coefficient of co-relation ‘r’ (a parameter which indicates strength of regression) is adopted as a tool to measure the consistency and reliability of the process of regression.