Analysis of an alumina refinery steam system

Abstract

The Rio Tinto Alcan - Yarwun (RTA Y) alumina refinery is currently undergoing a major expansion, with the possibility of further expansion in the future. The current expansion sees the addition of a Gas Turbine (GT) and Heat Recovery Steam Generator (HRSG) to the utility system, resulting in significant excess high pressure (HP) steam generating capacity in the utility system. Additionally, an excess of low pressure (LP) flash steam is generated from the HP condensate and is currently vented to atmosphere. The aim of this thesis was to determine if it is worthwhile utilising any of this excess LP and HP steam for additional power generation. In order to achieve this, a process model was developed to determine process steam demands under various refinery flow configurations allowing calculation of available excess HP steam generating capacity. Models for steam turbines, gas turbines and HRSGs were also developed and in combination with the process model allowed an overall utility system model to be created to study the impact on efficiency, C02-e emissions and operating costs of a number of different turbine configurations for varying refinery production rates. The impacts of a carbon tax as well as seasonal variations in electricity prices were also studied. It was shown that the installation of a HP turbine is detrimental to the operating cost of the utility system when operated primarily as a condensing (as opposed to a backpressure) turbine due to the poor overall efficiency of the power generation when not being run as part of a cogeneration system. On the other hand, the installation of an LP turbine would be quite beneficial to the refinery as it is utilising what is effectively waste energy to generate power. At current coal and gas prices, utilisation of supplementary firing in the HRSG is preferred to coal firing of the boiler as not only are C02-e emissions significantly reduced, but the overall efficiency is increased with a corresponding reduction in operating costs. The variation in electricity prices between summer and winter has a significant impact on utility operating costs and can even result in negative returns on power generation. Interestingly, the introduction of a carbon tax may in fact be of benefit to the operation due to the expected increase in grid-generated (predominantly coal fired) power prices exceeding the increased cost of the refinery's gas fired power generation. This is of course highly dependent upon what form a carbon tax would take and how it would be implemented.