Feasibility of using Sitalakhya river water as makeup water in Siddhirganj power plant cooling system

Abstract

Water quality is a very important factor in management of cooling systems within power plants and it is related to effective electricity generation. A significant amount of water is needed in powerplants for cooling operation. In order to meet the increasing electricity demand, a new 335MW Combined Cycle Power Plant is being establised at Siddhirganj, a key industrial hub of Bangladesh located 20 km aways from the Capital. The proposed power plant is designed to include a closed-loop recirculating cooling system (CRCS) for cooling operations. The CRCS utilizes the same cooling water several times and only takes up a relatively small amount of makeup water to compensate the water loss through evaporation. Due to evaporation loss the cooling water is operated at several cycles of concentration (CoC) in the cooling system. The proposed power plant at Siddhirganj will use Sitalakhya River Water (SRW) at 2.5 CoC. SRW is polluted near Siddhirganj area due to very high pollution load coming from the industries located on both sides of the river. The degraded water quality of SRW may pose a threat for its efficient use in the cooling system at 2.5 CoC in the proposed power plant. This study focused on feasibility of using SRW as a makeup water in Siddhirganj Power plant Cooling system in terms of corrosion and scaling management. A synthetic recipe of SRW has been formulated in the laboratory to imitate the water quality of actual SRW at 2.5 Cycle of concentration (CoC). Two index values (Langelier Saturation Index & Ryznar Stability Index) was calculated to determine the corrosion and scaling potential of the water. A bench scale reciculating system was desinged and constructed to study the scaling of SRW and corrosion of different metal alloys (Mild steel, Copper and Cupronickel) immersed in SRW at 2.5 CoC. Corrosion rates were measured by Weight Loss method using 2.5 CoC synthetic water without and with addition of a nitrite based inhibitor. Exposure of metal alloys to synthetic SRW at 2.5 CoC resulted in very high corrosion rate according to the corrosion criteria set for power plant cooling system. A chemical regimen program using sodium nitrite corrosion inhibitor reduced the corrosion rate of different metal alloys (mild steel, copper, and cupronickel) by more than 90% and within acceptable limits. Surface analysis of metal alloy (mild steel and copper) specimens immersed in cooling water indicated no presence of any scale on the surface of the metal alloy specimens. Elemental percentage of oxygen on the metal alloy surface was significantly reduced on the surface of the metal alloy specimens while using sodium nitrite corrosion inhibitor. This indicate a reduction in the formation of corrosion products on the surface of metal alloys while using corrosion inhibitors. Hence dosing of sodium nitrite can be employed to manage corrosion related issues in the cooling systems while using SRW at 2.5 CoC.