Operational stability of lean premixed combustion in gas turbines : an experimental study on gaseous alternative fuels

Författare

Parisa Sayad

Summary, in English

World electricity consumption is drastically increasing. One of the most common ways of producing electricity is to use the chemical energy of fossil fuels. This can be done in thermal power plants in which the chemical energy of fossil fuels such as natural gas is converted to mechanical energy and finally to electricity. Extracting the chemical energy of fuels is done through combustion of the fuel with air. Combustion produces heat, water and carbon dioxide as its main products. The produced heat can be converted to mechanical energy in different ways. In gas turbines, the hot combustion products are directly used to move turbine blades and produce mechanical energy, which is then converted to electricity by means of an electric generator. However, one should bear in mind that electricity is not the only outcome of this process. During this process, we are consuming the very limited reserves of fossil fuels, we are producing pollutants and we are negatively contributing in the climate change by producing carbon dioxide.
These negative consequences are becoming increasingly alarming. These concerns have led to a growing interest in the use of alternative fuels such as bio- and electro-fuels with reduced environmental impact for electricity production. Using bio- and electro-fuels in gas turbines provide reliable production of heat and electricity while decreasing the dependency on fossil fuels and contributing to the reduction of greenhouse gas emissions.
One of the most promising gaseous bio-fuels for gas turbines is digestion gas or ‘biogas. Biogas contains varying amounts of CH4 and CO2 as its major components. Another alternative fuel that can be considered for gas turbine combustors is synthesis gas (syngas) fuels that can be produced from renewable sources such as lignocellulosic biomass. Syngas may contain H2, CO, and CH4, as well as CO2, N2, H2O, and small amounts of higher hydrocarbons. The composition of these alternative fuels differs from natural gas, which has CH4 as its main component. This means that these fuels have different chemical and physical properties and therefore different combustion properties than natural gas. Therefore utilizing such fuels as the main or part of the fuel mixture in gas turbine combustors may substantially affect their efficiency, operability and emission characteristics. It is thus important to understand and quantify their operational characteristics to make their use in gas turbines viable.
One of the most important aspects of combustion that has to be considered in gas turbines when using alternative fuels is operational stability. It means that the combustion needs to take place in the combustor and in a smooth, reliable manner. In other words, the combustion needs to be sustained under all operating conditions. This is particularly important in modern gas turbines, referred to as lean premixed combustors, where fuel and air are mixed before entering the combustor. There are several operability risks that can occur and should be avoided in a lean premixed gas turbine combustor such as: lean blowout (the flame can extinguish due to reactions taking place too slowly), flashback (the flame can travel in to the premixing section), and autoignition (the fuel/air mixture can autoignite in the premixing section and before entering the combustor).
In this work, an experimental approach was used to investigate and understand the combustion of various fuel mixtures that can replace natural gas in gas turbines. A model combustor was designed and built that can mimic a real gas turbine combustor. The focus of the experiments was to investigate the combustion stability in the combustor when burning fuels comprising H2, CO and CO2. The combustor featured a quartz glass tube that provided optical access to the flame. Different experimental techniques were used to shed light on how the combustion behavior and operational stability of such fuels differs from natural gas. Various operating conditions and burner characteristics were examined in order to explore the possibility of reaching a fuel-flexible combustor.

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