Alcoa Gas-Fired Cogeneration Facility, Australia

Stage one of Alinta's natural-gas-fired cogeneration facility at Alcoa's Pinjarra Alumina Refinery in Western Australia is complete. The A$300m facility is being built in two stages, each with a gas turbine generator (GT), heat recovery steam generator (HRSG) and auxiliary equipment. The unit is now generating 140MW of sent out electricity and 240t/h of high-pressure steam. The second, similar, unit started construction in 2005, and was scheduled to come online in 2006.

Alinta owns and operates the facilities which run 24 hours/day. A net annual reduction is expected in greenhouse gas emissions of about 240,000t at current production levels. That will represent roughly a 10% reduction in emissions per tonne of alumina produced when both cogeneration facilities are in operation.

COGENERATION UNITS: GT AND HRSG

The cogeneration units are being built on a 2.5ha site within Alcoa's Pinjarra Alumina Refinery, approx. 6km east of the town of Pinjarra. The site is immediately south of the refinery's existing gas-fired powerhouse and adjacent to the refinery's natural gas supply pipeline and a Western Power 132kV substation.

The cogeneration unit is one of Western Australia's most energy efficient power plants. Waste heat from the gas turbines generates steam for Alcoa's Pinjarra refinery and Alinta supplies power at lower cost to customers within the South-West Interconnected System (SWIS).

Alcoa's Pinjarra refinery now produces 3.4 million tonnes of alumina per annum, representing about 7% of the world's alumina refining capacity. A separately planned A$400m efficiency upgrade at the refinery aimed to improve yield and increase production capacity by 600,000t of alumina per annum by the end of 2005.

ALUMINA PRODUCTION TAKES HEAVY ENERGY INPUT

The bauxite recovered from the Darling Range is of a low quality. For every three tonnes of bauxite mined, only one tonne of alumina is recovered. This means more energy has to be used to remove the alumina from this low-grade ore than at refineries with higher grade ore. The Pinjarra refinery uses three forms of energy in the alumina refining process: natural gas, steam and electricity.

The annual net average CO2 emissions from the cogeneration unit at Pinjarra are estimated to be 474,500t. The efficiency upgrade and the Alinta cogeneration project will result in an overall improvement of about 14% in the greenhouse intensity of the Pinjarra refinery (kilograms of CO2 per tonne of alumina).

COGENERATION ALLOWS THROTTLING BACK OF EXISTING GAS-FIRED BOILERS

Alcoa's existing steam generation facilities at Pinjarra consisted of six gas-fired boilers, which produced an average 910t/h of high-pressure steam. The cogeneration facilities provide steam more efficiently (over 80% efficient compared with 25% to 50% for other power plants operating in WA) and at lower cost. The steam produced by each cogeneration facility replaces the requirement for 240t/h of high-pressure steam previously generated by the refinery's boilers.

The new gas turbines will use dry low-NOx burners to achieve emissions of less than 25ppm(v) NOx (expressed at 0°C, 1,013.25hPa, dry and 15% O2) during normal operations, as required in EPA Guidance Statement No.15 (EPA, 2000).

Heat from the turbine exhaust gases produces high-pressure steam in the HRSG unit. When a gas turbine is operating at loads less than 100% the HRSG is fired using additional supplies of natural gas to maintain 240t/h steam output.

Cogeneration units 1 and 2 supply just under 50% of the refinery's current steam requirements, thus permitting a substantial 'throttling back' of the existing boilers. The improved efficiency is resulting in a net annual reduction in greenhouse gas emissions associated with steam production of some 150,000t CO2 for the two units.

Using refinery 'cold' condensate in the HRSGs instead of the refinery boilers also allows the existing boilers to be supplied with hotter condensate. This reduces the steam needed to heat boiler feedwater and the steam can be used elsewhere in the refinery process, further increasing efficiency.

With two cogeneration units this will reduce steam demand from the existing refinery boilers by 4% to 5% – a further annual reduction of approximately 90,000t of CO2. With two cogeneration units the estimated overall reduction in gas consumption is approximately 13.2PJ/yr at current alumina production rate.

AUTOMATED, INTEGRATED CONTROL

The cogeneration units are being automated and integrated into the refinery powerhouse's existing distributed control system. The units 'load follow' customer requirements and operate at between 85% and 100% capacity on a daily basis.

Power is being generated at 15kV then stepped up to 132kV through an on-site transformer. The annual quantity of power exported by each cogeneration unit is expected to average around 1,100GWh.

NATURAL GAS DISTRIBUTION AND RETAIL

Alinta distributes and retails natural gas in Western Australia, currently delivering natural gas to over 450,000 customers through its network of approx. 11,000km of pipelines. The cleaner and cheaper electricity produced by the cogeneration plant is an important part of Alinta's move into competing for business and industrial customers.

Alcoa produces primary aluminium, fabricated aluminium and alumina. The company is active in all major aspects of the industry including technology, mining, refining, smelting, fabricating and recycling.

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