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Reducing emissions at Alba

In an effort to reduce emissions from its aluminium smelting operations, Aluminium Bahrain (Alba) has undertaken a project to retrofit the gas turbines at its Power Station 3. The main scope of the project is the installation of a new low NOx combustion system on the gas turbines.

Aluminium Bahrain B.S.C (Alba) operates an aluminium smelting plant located in the Kingdom of Bahrain. The plant includes power generation and necessary auxiliary systems to supply power to the electrolytic reduction lines that produce primary aluminium from alumina. Alba was officially commissioned in May 1971 as a 120 000 t/year smelter. Having expanded in 1981, 1990, 1992, and 1997, today the company produces more than 500 000 t/year, making the smelter one of the largest single-site producers of aluminium in the world.

Aluminium production is a very electricity intensive process and consequently the plant is also one of the largest producers of electricity in the Kingdom of Bahrain.

There are four power stations at the site with a combined power output of 1517 MW at ISO conditions. Further expansion, scheduled for completion this year will see the total generating capacity from the power station increased to more than 2000 MW.

Power station 3

The protection of the environment is of considerable importance to Alba. Since 1990, the company has invested more than $300 million plantwide in environmental protection projects. The company is committed to the regular monitoring and reduction of NOx emissions from the plant. In its most recent project, the company is retrofitting its gas turbines at power station 3 to reduce NOx emissions still further.

Figure 1. Electric motor driven centrifugal fuel compressors
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Power station 3 is a combined cycle gas and steam turbine power plant built in 1991. It consists of six large gas turbines and two steam turbines divided into two blocks.

The power station is supplied with sizable air-cooled condensers designed for summer conditions. The gas turbines are capable of delivering 96.5 MW at ISO conditions. The steam turbines are rated to deliver 117 MW and use 480 tons per hour of steam at 40 bar and 470oC. The power station open cycle and combined cycle efficiencies are 31 per cent and 44 per cent, respectively, based on lower heating value of fuel.

The gas turbines are Alstom’s GT13D3 model, these gas turbines operate on natural gas with distillate fuel available for emergency back-up and periodic testing to ensure availability.

The gas turbines currently have a silo-type combustor with single burner as shown in Figure 2. However, the plan is to modify the gas turbines in line with Alba’s commitment to protecting the environment and to reduce nitrogen oxide (NOx) levels below the legislative figure of 50 ppm. The project has no commercial value and offers no return on investment, but was approved because it marks an important environment investment that will help control pollution and provide a cleaner, safer environment for the people of Bahrain.

The project, which was approved in April 2003 by the Alba Board of Directors, involves retrofitting all six of the 13D3 type gas turbines with single burner silo combustors with environmentally friendly (EV) multi-burners silo combustor with low NOx EV-burners.

The EV-burner is a premixing burner which consists of a hollow cone split axially, with its halves displaced cross-wise from one another. The combustion air flows into the combustion zone through the slots that result. The fuel gas flows through two gas-channels, enters into the burner through a row of holes inside the burner cone and mixes with the air.

Figure 2. Single burner silo combustor
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The burner geometry has been optimized to produce a strongly swirled flow with a back-flow zone freely stabilized within the combustion zone. Only in this zone are the flow velocities slow enough to make possible the ignition of the fuel/air mixture, which has, in the meanwhile, become fully homogeneous. The flow and the lean mixture of the air and fuel produce low flame temperatures, which results in low emission levels.

Fuel gas pressure boost: The new EV-burners required a fuel gas pressure of approximately 25 bar[a] compared to 19 bar[a] for the existing burners. The existing fuel gas supply therefore had to be modified to achieve this increased pressure. Alba decided to use electric motor driven centrifugal fuel compressors to boost the fuel gas pressure. Under normal operating conditions, 3×50 per cent compressors will be running to supply 5.1 million m3 (180 million standard cubic feet) to down-stream gas turbines with a fourth machine on standby. Each unit is operating at 67 per cent of its rated maximum capacity of 2.35 million m3 (90 m ft3) and will supply 1.7 million m3 (60 m ft3) at a set pressure between 23 and 29 bar[a].

In the event of a compressor trip to one unit, flow will instantaneously drop from 5.1 million m3 to 3.4 million m3 (120 million ft3) and pressure will start to fall.

Discharge pressure detection systems will relay the falling pressure to the logic controller which will signal the remaining two operating compressor units to increase output to 100 per cent of rated capacity. Meanwhile, the fourth machine will be started up from standby and will reach its programmed discharge pressure within about 10 minutes. The logic controller will equalise the output of each compressor until all three units are again operating at 67 per cent of rated capacity to supply the downstream gas turbines.

Control system upgrade: The gas turbines at power station 3 are controlled by Procontrol P with hardwired Decontic components, I/O bus system and partially connected to a remote bus system. During the integration of the combustion chamber modification for the new EV burners Alba decided to upgrade the complete GT controls with a state-of-the-art Procontrol DCS rather than adding additional control functions to the existing obsolete hardware. This benefitted Alba in terms of spare parts, technology and reliability of the control system for the remaining life of the gas turbines. As the modifications were carried out during the planned annual shutdown (outage) period of the GTs, no additional operating hours were lost.

The new DCS system communication is based on the powerful optical fibre remote bus conforming to the international FDDI (Fibre Distributed Data Interface) standard. The communication of data between Procontrol-P14 stations takes place at 100 Mbits/s and the total distance of the FDDI network can be up to 100 km with 2 km as the maximum distance between two nearest stations. The system is an ISO 9001 certified product.

Project schedule

The agreed completion period of the first machine was 14 months from the order date and the machine shutdown time was three months. Therefore, in order to reduce the machine down time, the erection and commissioning was scheduled during machine major overhaul period. The entire project is scheduled to be completed by 2007.

Figure 3. Multi-burners silo combustor (EV-burners)
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The first of the six gas turbines was successfully retrofitted in June 2004 and performance tests indicate that NOx emissions have been significantly reduced and are now well below even the most demanding international standards. Alba’s power station 3 will become the most environmentally friendly power plant of its kind in Bahrain following the completion of the BD24.5 million ($65 million) project. MEE

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