Water minimisation at the power station using process integration

Abstract

The primary objective of this study is to determine the reduction of the raw water intake of an existing power station by applying process integration techniques to optimise the use of water available in the system. The secondary objective is to reduce the waste water produced within the process, hence reducing the cost of water, reducing the amount of chemicals and reducing the energy needed to treat water. This will be achieved by considering a system as a whole (i.e. integrated or holistic approach) to improve its design and/or operation which exploit the interactions between different units to employ resources effectively and minimise costs. Process integration as technique for water minimization is initiated by identifying the water sources (providers) and sinks (users) in the water network, thereafter matching appropriate sources and sinks as water quality allows. The water network therefore first must be compiled and flow and quality data can subsequently be allocated to process units in the network. Based on preliminary runs of the model, three role players in the Kriel water utilisation network were identified: - Wastewater treatment plant water re-use - The possibility of blow down water re-use due to different water chemistry in the respective cooling towers - The ability to use any water to wash floors Three different objective functions were set for each of these scenarios and the objective functions to be minimized are: - Freshwater intake into the station - The sum of freshwater intake and wastewater produced - Cost associated with water intake and waste handling All the scenarios and objective functions were evaluated both with a model utilising a desalination plant and one without a desalination plant. Savings of between 4% and 13% may be possible by changing the way water is currently utilised and re-used at the station. These figures translate to L/kWh sent out values of 2.23 to 2.04 respectively. These savings still do not achieve the design water consumption target of 1.8 L/kWh sent out. The same objective function values are achieved by minimizing freshwater consumption or the sum of freshwater consumption and wastewater produced. Reuse of the wastewater treatment plant effluent has a direct impact on water consumption and investment in infrastructure to enable the introduction of good quality sewage effluent into the cooling towers shows savings in the order of R 2.2 million per year. Optimisation of the stations water network still brings 3% savings without implementation of any of the three preliminary findings mentioned.