Given the price of fossil fuels and growing awareness of the carbon footprint that many industries leave worldwide, more companies are turning to alternative energy sources, capturing heat that would normally be wasted or used in a low-efficiency application. A new power plant co-located with a steel mill in Brazil will provide 490 MW for consumption at the facility and on the local power grid using waste blast furnace gas and heat scavenged from the iron smelting and coke making process. This type of approach can be applied in a variety of industries that produce waste gasses that are combustible, but of very low heat value.

Brazilian steel producer ThyssenKrupp Companhia Siderúrgica do Atlântico (CSA) is building a new greenfield mill in the industrial district of Santa Cruz at Sepetiba Bay, Rio De Janeiro City. This new operation will begin production in early 2009 and turn out 5 million metric tons of steel slabs annually. The output will be virtually all exported to ThyssenKrupp’s facilities in Europe and North America for rolling and additional processing. “Our company needs more crude steel capacity to supply high-quality carbon steel to the European market, which has grown as a result of EU enlargement, and the NAFTA market,” says Dr. Karl-Ulrich Köhler, executive board chairman of ThyssenKrupp Steel AG.

Construction of the steel mill and power plant is happening in parallel. The power plant is designed by and will use equipment from Alstom Power under a turnkey engineering, procurement and construction contract. Alstom’s plan is to use combined cycle technology, with two gas and one steam turbine to drive two 90 MW and one 320 MW generators, respectively, for 490 MW total useful output. Initially the plant will run with the steam turbine consuming steam from the mill’s boilers and with the gas turbines in simple cycle to bring it on-line as soon as possible, and then move to full production in combined cycle as construction advances.

Combined cycle power plant technology uses two types of turbines to create higher thermal efficiency and energy capture per unit of fuel than is possible using only one type of turbine or a conventional boiler. Typical configurations begin with a gas turbine driving a generator. Since gas turbine exhaust is very hot, it still contains a large amount of heat energy that cannot be captured in the turbine alone. In a combined cycle installation, the exhaust passes through a heat recovery steam generator that feeds a steam turbine and generator combination. Using the two in tandem can achieve thermal efficiency in the 50 to 60% range. By comparison, a typical subcritical coal-fired power plant is usually around 35 to 40%.

The gas turbines in CSA are two Alstom GT11N2 LBTU units, which are specifically designed to burn fuel with very low calorific value. The compressor section has 14 stages, including three with variable guide vanes. These assist in startup and help maintain low emissions while running at partial load. The turbine section has four stages with an exit designed to couple directly to the diverter damper of the by-pass stack and then to the heat recovery steam generator.

The top-mounted “silo” combustor allows the use of fuel carrying a heating value as low as 950 Btu/lb. (2,200 kJ/kg) without blending in higher value fuel. (This compares to heating values of 18,000 to 20,000 Btu/lb. for typical fuel oil.) To control NOx emissions, water or steam can be injected in the combustor as required.

Blast furnace gas will be available in very large volumes, but combustible components are low. In many applications it is regarded as a waste product given the difficulty of capturing its energy value. Typically, is mostly nitrogen and carbon dioxide; however, carbon monoxide and small amounts of methane and hydrogen give it its fuel value. In this application, natural gas is used for startup. Exhaust from the gas turbines goes into two dual-pressure, non-reheat, horizontal type, heat-recovery steam generators (HRSG) attached to each to generate steam for the combined cycle.

The steam turbine is a three-casing configuration, with one high- and two double flow low-pressure sections. It receives scavenged steam from the HRSG and an additional 551 t/h from boilers attached to the coke ovens. These combined sources support its higher output relative to the gas turbines. Although steam from the coke ovens could have run much of the generating plant alone, the steam produced in the HRSGs results in 10% more power than a more conventional steam plant used in similar applications.

“Using gas that would otherwise be wasted to produce power is both of economic and environmental benefit,” says Philippe Joubert, president of Alstom Power Systems. “This project confirms Alstom’s ability to deliver highly efficient, integrated, clean power solutions for the least calorific fuels.”

Exhaust steam is collected in a water-cooled condenser using an adjacent natural source rather than a cooling tower. The gas and steam turbines each drive an air-cooled Topair turbogenerator rather than more complex hydrogen cooled designs.

Keeping this process balanced and running efficiently requires a sophisticated control system given the variety of variables. The distributed control system (DCS) for this application will be Alstom’s Alspa platform. The company considers this a key element if its large plant integration offering, ranging from initial consultations to final turn-key plant construction.

Designed specifically for power generation applications, the Alspa architecture encompasses plant control functions, burner management, boiler control, turbine regulation, flue gas treatment, NOx and SOx control, and all other functions critical to efficient plant operation. The system has been applied to every plant technology approach, from nuclear to hydroelectric. Ethernet Powerlink networking provides high speed and deterministic communication between controllers, field I/O, and safety functions. The platform supports SIL3 (safety integrity level 3) and tri-modular redundant protection for rotating machinery and boilers.

Alstom has built 350 plants around the world, including some unconventional projects using technologies that move out of the mainstream. Examples include the Neurath plant in Germany, which is the world’s largest supercritical steam plant (2 x 1,100 MW) fired by lignite, and the massive 1750 MW turbine for the new EPR nuclear plant in Flamanville, France. The company’s applications of alternative fuels, high efficiency technologies and tight emission controls has earned it a reputation for creativity and environmental awareness.