Last week we finished the tutorial on electricity generation with the steam power process in coal-fired power plants. Now, we focus on another fossil energy carrier, natural gas, and its utilization in power plant technologies.

kraftwerk gersteinwerk luftbild - rwe pressebilder
© rwe pressebilder

6. Gas turbines

In contrast to the steam power process, the gas turbine process is an open process.  A gas turbine consists of a compressor, a combustion chamber and an expander. The condenser is not part of it. The working fluid is the ambient air, respectively the hot flue gas. In modern gas turbines, the main parts (compressor, combustion chamber and expander) are physically connected to one shaft.

While in operation the compressor draws in ambient air and thus oxygen. The air is then compressed and led into the combustion chamber. In the combustion chamber, natural gas or diesel oil is added to the compressed air. High temperatures result when burning the fuels. Then, the hot flue gas is directed through the expander and thus expanded. During this process work is being done, which results in the rotary movement of the turbine. With the rotational energy of the shaft, the compressor (two-thirds of the energy) and the generator (one-third of the energy) are powered.

During the expansion the hot gas is cooled down. But even after being released from the expander it is still hot and is released into the environment via a chimney.

Field of application of gas turbines

Power plants based on gas turbines have lower investment costs and faster start-up times than steam power plants.  Such a gas turbine power plant basically consists of a gas turbine, the generator and a chimney for the flue gases. As the combustion of natural gas is very clean, subsequent flue gas treatment is in most cases not necessary.
Those conditions render gas turbines to be ideal peak load power plants. In addition, they have the capacity for black starts. Hence, they can be used to start-up regular steam power plants, as these require start-up power, before being able to generate electricity by themselves.

The low investment costs are however faced with high operating costs, as the only possible fuels are high-grade fuels such as natural gas or diesel oil. Firing such a plant with coal is not possible, as the combustion of coal results in ash. The ash would damage and destroy the turbine.

Due to the high temperatures of the flue gas, the efficiency of a gas turbine varies from 30 to 40 % (depending on the size). Losses also result from the gap between the turbine blades and the casing, which to be sure, has to be present. The bigger the turbine is, the smaller the gap is, when compared to the entire turbine diameter. Thus, a bigger turbine can reach higher efficiencies.