Rheological studies of coal fly-Ash slurries

Abstract

Coal based thermal power plants play a major role in current energy policy in India. Indian coal has high ash content up to over 45%. India produced 163.56 million tonne of ash in 2012-13 and is expected to produce about 300 million tonne by 2017 and 900 million tonne by 2031-32. Hence, there is a critical need to handle this large amount of fly ash in the most economical way with minimum environmental damage. A suitable disposal technique is required to utilize this huge amount of fly ash. One major potential usage of fly ash is hydraulic transportation of the fly ash slurry and use it for mine filling. This is going to be an economically and environmentally viable technique for mine site rehabilitation and acid mine drainage control in mine areas. An understanding of the flow properties of fly ash slurry is important for effective transportation and further utilization. This study presents the results of rheological studies conducted on Fly-ash samples collected from Centre for Fly-Ash Research & Management (C-FARM), New Delhi. The main problem associated with hydraulic transportation of fly-ash slurry is the gravitational settling of fly-ash particles in the pipe sooner than desired. Our results show that preparing the fly ash slurries in equimolar aqueous solutions of Cetyltrimethyl Ammonium Bromide (CTAB) and Sodium Salicylate (NaSal) prevent the settling of fly ash particles. Results from rheological studies on fly ash slurries with 10 - 60wt% fly ash loading and at different additive concentration show that all slurries are shear thinning when sheared from 1S-1 – 400S-1. The zero shear viscosity of the slurries increases with the fly ash loading. The rheological data are fitted to a power law model which shows that the shear thinning exponent (n) and consistency factor (k) depend on the loading of fly ash. These results help in understanding the relationship between fly-ash slurry rheology and the flow characteristics in circular pipes. We found minimum power consumption and minimum energy dissipation rate during hydraulic transportation for the highest loading i.e. Cw = 0.6 (60wt %) in tap water at 0.1% additive and 0.5 m of pipe diameter.