Implementation of a reheat steam cycle to maximise electrical power from a circulating fluidised bed multi-fuel cogeneration facility at an alumina refinery expansion

Abstract

The expansion of the Worsley Alumina refinery in Western Australia from an annual output of 3.5–4.6 million tonnes of alumina per annum provided the incentive for increased efficiencies in both the refinery process and the steam raising plant. The multi-fuel cogeneration (MFC) power station was installed to meet the additional steam requirement and to improve the use of energy per unit of alumina produced. The power station is capable of providing 605 t/h of process steam at two pressure levels while developing 113.6 MW of electrical power from two boiler/turbine units. A reheat steam cycle was implemented by integrating the high-pressure cylinder of a utility steam turbine with an industrial back-pressure extraction machine. When compared to the capability of the older coal-fired (CF) power station, the cycle and plant configuration selected provides a 37% increase in the electrical power generated per tonne of process steam supplied to the refinery. The main source of fuel is a local semi-bituminous coal that prescribes the installation of efficient plant to limit carbon dioxide emissions. Fuel flexibility is provided by circulating fluidised bed boiler units that enable the provision of up to 30% of the heat input from biomass. Greenhouse gas emissions from the MFC power station are some 10% less per unit of process steam than those from the older CF power station. Firing biomass in the MFC power station further reduces the carbon foot print. Boiler feed water is provided by condensate returned from the process that has total organic carbon levels some two orders of magnitude greater than that permitted by international boiler water standards for the 16.0 MPa(g) units. Boiler water chemistry is based on caustic treatment. Amine dosing is applied to the feed water obtained from selected condensate streams.