Case Studies - Energy & Environment

In the competitive power generation market, environmental legislation is increasingly governing the way companies do
business. Gas turbine manufacturers, in particular, are facing
tighter limitations on combustion emissions.

Background
New legislation is being driven by rising levels of pollution in some parts of the world, such as California, USA which has seen a dramatic increase in smog levels in recent years. Across the globe, legislative limits of <25 parts per million by volume (ppmv) nitrogen oxide emissions are now commonplace and there is an increasing trend towards even lower emissions – for example some US states are legislating <3 ppmv emissions.

Siemens Power Generation is a world leader in the energy sector, with around 31,000 employees and sales of €7.5 billion last year alone. The company’s Industrial Turbomachinery division specialises in the construction of power plants, components and systems, plant services and small-medium-sized gas turbines for the power
generation market.

As new environmental legislation comes into force, Siemens is looking for the next generation of gas turbine combustion systems that can meet future legislative limits and help them maintain a competitive edge in the marketplace, by ensuring cost-competitive solutions.

Failure to meet new legislative limits comes at a high price. Siemens, like other manufacturers, could lose sales and face hefty financial penalties if an in-service engine does not meet its performance guarantees. These payments can run into millions of pounds across a whole fleet of gas turbines.

Challenge
Current low emissions combustion technology can suffer from combustion-generated noise and acoustic phenomena – sometimes known as ‘humming’. Combustion oscillations can lead to pressure fluctuations at high power levels, which can lead to mechanical failure of the combustion chamber or engine components. In order to avoid such damage, often emissions are compromised, since pressure fluctuations can be effectively controlled by modifying the level of fuel/air inhomogeneity within the primary combustion zone. As low emissions combustors have evolved, design concepts and practices have developed such that pressure fluctuations can be controlled whilst maintaining low emissions. For example, Siemens has evolved a dual fuel dry low emission combustor that has seen >3 million hours of operation, and guarantees Nitrogen Oxide emissions down to 10 ppmv.

As well as a need to continually reduce emissions, commercial pressures require reduced costs and increased availability. For the combustor and related components this requires longer lives. The impact of combustiondriven flow oscillations on the long-term life of the combustor is not fully understood and is likely to have a greater significance as emissions limits are reduced.

In 2004, Siemens approached QinetiQ to investigate this problem and develop improved design techniques in a joint programme funded by the DTI.