Rusal Aughinish is based at Aughinish Island on the Shannon Estuary in Limerick. Aughinish Alumina Ltd (AAL) commenced operations in 1983 and is Europe’s largest and most modern alumina refinery.
AAL produces 20% of the EU’s alumina, 100% of which is exported to smelters in Europe and is used to produce aluminium metal. AAL is considered a benchmark alumina refinery worldwide for its operational efficiency. The plant cost €890 million to construct and a further €580 million has been spent upgrading it with a 160MW natural gas-fired combined heat and power (CHP) facility in 2006 and with two new 150tph gas-fired boilers to replace the heavy fuel oil (HFO) boilers in 2014.
AAL employs 450 permanent workers with a further 150 contractors employed on a daily basis. It is one of a small number of major industries in the mid-west and contributes over €130 million annually to the local and Irish economy.
Owing to the nature of the principal industrial process used at AAL (for alumina manufacturing and post extraction processing), energy represents the most economically significant impact to the process. Significantly at AAL, energy use accounts for approximately 30% of the plant’s operating costs. As such, energy efficiency in such an energy intensive industry is of paramount importance to remain competitive and has always been a top priority for the business.
The main areas where thermal energy is used at AAL are:
- Steam generation for the process
- CHP to generate power for export and the process (steam also produced for the process)
- Calcination to convert hydrate to alumina product
Three key energy indicators are monitored to ensure optimum plant performance; steam efficiency, calciner efficiency and power efficiency.
Digestion consumes approximately 96% of the total steam produced onsite and so is a key indicator for monitoring operational efficiency. The key to reducing digestion steam use is to ensure effective energy transfer between hot and cold process streams and minimised volume inputs to the digestion process.
Thermal Energy Use
From plant start up in 1983 up to 2006, AAL operated as a 100% heavy fuel oil (HFO) site with electricity purchased from the Irish national grid. Over €7million was invested to convert the calcination area of the plant from HFO to natural gas and in 2014, a further €14 million investment allowed AAL to make the final changeover, commissioning two new gas boilers and fully converting the site to natural gas. Figure2 below shows the reduction in HFO usage as a result of efficiency gains following boiler conversion to natural gas.
The full conversion to natural gas presented AAL with the opportunity to integrate high efficiency CHP into the energy generation mix for the site. A 160MW CHP plant was built on site to meet the refinery’s ideal high and constant heat demand and simultaneous electricity power load. The CHP is most beneficial when operated continuously at base load; hence overall efficiency is maintained at 80% or above. It is a true ‘must-run’ CHP and operates at full output for over 8,500 hours per year. The plant is unusual for CHP in Ireland in that it can be dispatched (controlled up or down) by the grid system operator. The importance of retaining priority dispatch is critical for both AAL and Ireland for the long term viability of the largest alumina refinery in Europe.
The impact of the high efficiency CHP and the associated efficiency gains cannot be underestimated and has resulted in improved security of supply, reduced grid losses, primary energy saving and CO2 emission reductions for both AAL and the country as a whole.