Anders Ahnger of Wärtsilä explains how – at a time of increasingly stringent environmental limits and rising fuel prices – combined-cycle techniques can increase the power output and efficiency of plants that use medium-speed gas and diesel engines

Anders Ahnger, Wärtsilä, Finland

The energy world is experiencing big changes. Environmental requirements are becoming more and more stringent and the Kyoto protocol is being implemented country-by-country, step-by-step. Authorities and governments around the world are enacting measures to fulfill carbon dioxide (CO2)and other emission targets and limits. Directives on combined heat and power (CHP) that strive for better use of primary energy, better economy and greater savings are being published and implemented to improve general plant efficiencies. Emission targets for CO2 are making renewable fuels preferable to fossil fuels, while fuel prices are high and slowly but steadily increasing. Electricity prices are also increasing, although not in direct relation to, nor as fast as, fuel prices.

To meet these demands, modern power plant technologies need to be further developed for higher efficiencies. Technical solutions will require, among other things, greater sophistication. All waste heat from the prime movers must be better used to produce secondary energy, either to generate heat directly or for secondary power generation. Governments are ensuring that such developments are heading in the right direction. With this situation very much in mind, Wärtsilä is continuously studying different types of combined-cycle solutions and developing them further. Wärtsilä also has several combined-cycle plants under delivery.

Interest in engine combined-cycles (ECCs) among power producers and plant owners is growing, and there is clearly an increasing awareness of using energy that would otherwise be wasted to create additional electricity, for example.

Engines in combined-cycles

Diesel and gas medium-speed internal combustion engines are already efficient in the way they produce power – by design and in their construction. Today, we talk about a gross electrical efficiency of up to 47 per cent for engines with outputs above about 3 MWe. This is a remarkable figure, even compared with large power plants that employ other technologies. The energy in the fuel that is not used to produce power emerges in the form of heat in the exhaust gases and engine coolant. In a typical CHP application, this waste heat is valuable. But in many applications there is no heat sink into which this energy can be dumped, so secondary power generation is preferred, as in an engine combined-cycle configuration.

The steam generation system used with an internal combustion engine
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The most common technique for secondary power generation is based on a conventional steam bottoming cycle. This can be defined as a post-engine ‘hang-on’ type of cycle because it does not directly affect the performance or the running of the engine. Prerequisites for a modern and efficient steam bottoming cycle are high steam pressures and temperatures. As previously pointed out, the reciprocating engine is, by virtue of its basic design, already efficient, which means that the temperature of the waste heat is low for a steam bottoming cycle. Nevertheless, very high total plant efficiencies are easily obtainable.

The main equipment in the system comprises the heat recovery steam generator (HRSG), the steam turbine set and alternator, the condenser and cooling, and the feed water and water treatment systems. The main heat source from the engine is the exhaust gas, where the temperature plays a more important role in the efficiency of the steam cycle than the flow of the exhaust gas does. Taking a normal medium-speed engine directly off the production line, the exhaust gas temperature is about 340-370 °C for diesel engines and 370-410 °C for gas engines. The factors that determine the design of a steam generation system are engine exhaust gas temperatures, ambient conditions, steam condenser cooling, engine type and the type of fuel. Usually, a single-pressure steam system operates economically at 1200-2000 kPa. Engine cooling water can preheat the feed water and condensate flows before feeding into the feed water treatment system.

More sophisticated steam systems employ exhaust gas boilers that have several pressure levels. Also, multi-stage steam turbines are possible. Generally, an ECC based on a single-pressure steam system achieves a total gross efficiency of 49.0-51.5 per cent. With a sophisticated multi-pressure system, efficiencies of up to 53.5 per cent are possible.

Organic Rankine cycles

The Organic Rankine Cycle (ORC) is a closed thermodynamic Rankine cycle process in which heat is added to an organic fluid at a constant pressure to vaporize it. It is then expanded through a turbine, condensed under very low pressure and fed back into the system for heating and evaporation.

A conceptual study of a 140 MWe engine-based combined-cycle power plant for hot climates that uses eight Wärtsilä 18V50DF engines and a steam turbine with a direct air-cooled condenser
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ORC manufacturers typically focus on recovering energy from low-grade or low-volume waste heat sources. An example is geothermal heat, for which an ordinary steam turbine cycle is unnecessarily inefficient and expensive, and for which the investment in a comprehensive steam turbine system cannot be justified. ORC makers produce their equipment as small compact, factory-made modules that are delivered to site as functional, pre-tested units. The markets for such units are generally applications that produce less than 5 MWe. From the point of view of design and construction, gas and diesel engines suit the ORC cycle well.

Selection of the organic fluid in the closed Rankine cycle depends on the application and the temperature of the waste heat source. Organic fluids are seen as environmentally hazardous, and their use raises the question of fluid stability and toxicity. Because the organic fluid remains in the primary cycle, an intermediate thermal oil circuit transfers the heat from the waste heat source to the ORC module. Today’s ORC products give between 8-10 per cent more power, even with small engines. Even though the ORC has been on the market for more than 20 years, there is still potential to develop it further to increase efficiency and output.

Research continues

A comparison of investment and performance shows that the ordinary steam turbine cycle is a good option for engine plants of sizes above 30 MW, whereas the ORC is well suited for plants below 30 MW and especially those installations that employ only one engine. ORC still has the potential to be developed further concerning optimization of the cycles and techniques for reciprocating engines, and for the better use of engine cooling water. Both of these ‘hang-on’ combined-cycle technologies have spawned numerous research activities, developments and studies that aim to develop much more efficient bottoming cycles. These activities are also looking into integrating these technologies with other combustion methods and boilers, such as ordinary supplementary-fired heat recovery steam generators and with different kinds of ‘hybrid’ plants. At the same time, much development work is targeting areas such as the engine and prime mover itself, combustion and efficient turbo-charging with direct use of excess exhaust gas. Many of these are, however, not yet fully commercialized. It is likely to take some years before they can be applied to combined cycle technologies and ordinary power plants.

Power and energy demand is growing hand-in-hand with economic growth and is an important global concern. We have many interesting years ahead of us as we strive to meet the increasing demand for better and more efficient energy generation while complying with environmental limits.

1. Miroslav Petrov, Steam bottoming cycles for the W20V34SG gas engine. KTH 2006.

2. Seminar work by students at the Technical University of Helsinki and the Royal Institute of Technology, Stockholm, 2007.

Attock Refinery, Rawalpindi, Pakistan

A power plant at the Attock oil refinery uses a combined-cycle configuration to provide a potential net electrical efficiency of 45 per cent over a lifetime of 25 years. Heavy fuel oil fires its nine Wärtsilä 18V46 gensets, each with an HRSG, to output 160 MW. It employs one 12 MWe condensing steam turbine plant and a 132 kV switchyard. The plant will connect to the national grid.

ItalGreen II, Monopoli, Italy

This 100 MW combined-cycle plant in Monopoli in Puglia (pictured) burns palm oil as fuel. It has a gross electrical efficiency of 50 per cent and employs six Wärtsilä 18V46 engines, each with an HRSG. Natural gas-driven duct burners improve the steam parameters before it is fed to the condensing 12 MWe steam turbine.

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Conditions in Italy favour the economics of a biofuel plant. Bio-oil prices are competitive, and the government provides considerable incentives to generate power using renewables. Fuel for the plant comes mainly from Southeast Asia.

ItalGreen, a supplier of household and commercial food oils, owns the plant, which is one of the largest in the world for the production and refining of vegetable oils for food and has four packaging lines.

Green Energy, Italy

Heat from this plant’s single liquid biofuel-fired Wärtsilä 18V46 unit is recovered only from the exhaust gases. A thermo-oil intermediate heat circuit feeds the heat to an ORC unit, whose turbine unit generates 1.3 MWe to boost the plant electrical output by 8 per cent.