Recently, mineral gas deposits were disposed in the Southern Region of Tanzania. To date the gas has been piped to Tanzania’s capital city, Dares salaam where it is used to power a cement Factory and Power Generating Plant. Kenya’s dream of turning to Gas to replace the traditional Liquid fuels to generate power may not be too far in the distant future. What is a Gas Turbine?

In simple terms, a Gas Turbine is a machine that converts energy generated by Combustion fuel which is mixed with compressed air to drive a turbine which in turn produces work. The energy generated through combustion of the Fuel drives both the compressor and turbine. Heavy duty Gas Turbines are used for power generation whereas aeroderivates are preferably used in the Aircraft Industry. However, the design principle is basically the same. A flow diagram of a simple cycle, single-shaft Gas Turbine is shown below:

Air enters the axial flow compressor at point 1 at ambient conditions. The standard conditions used by the Gas Turbine Industry are 59F/15C, 14.7 psi/1.013 bar and 60% relative humidity, which are established by the International Standards Organization (ISO). Air entering the compressor at point 1 is compressed to some higher pressure. No heat is added, however, compression raises the air temperature so that the air at the discharge of the compressor is at a higher temperature and pressure.

The function of the axial flow compressor is to furnish a large volume of high-pressure air to the combustion chambers for the production of the hot gases necessary to operate the turbine. Only a portion of this air is used for combustion, the remainder is used as dilution air to lower the temperature of the products of combustion and also serves as a source of cooling air for the turbine nozzles, turbine wheels and other portions of the hot gas path. Compressor consists of stages of rotating row of blades that adds velocity energy to the air, followed by a stationery row of blades that converts the velocity energy to a pressure increase. On average, the final pressure ratio is 12:1 for heavy-duty turbines. The air which continuously discharges from the compressor will occupy a smaller volume at the compressor discharge than at the inlet and, due to heating during compression, will have a temperature of 2880C to 3150C.

Upon leaving the Compressor, air enters the combustion system at point 2, where fuel is injected and combustion occurs. The combustion process occurs at essentially constant pressure. Although high local temperatures are reached within the primary combustion zone (approaching stoichiometric conditions), the combustion system is designed to provide mixture, burning, dilution and cooling. Thus by the time the combustion mixture leaves the combustion system and enters the turbine at point 3, it is at a mixed average temperature.

The stationery nozzles of the Turbine have a high-pressure drop across them that convert the high-pressure gases from the combustion system into high velocity jets that impinge against the turbine blades (buckets) that are attached to the turbine rotor. The kinetic energy of the hot gases is converted into useful rotational, mechanical energy by the turbine buckets. This produces the power necessary to meet the load requirements and to drive the axial-flow compressor.

The exhaust system is that portion of the Turbine in which the gases used to power the Turbine wheels are directed to the exhaust stack or HRSG and released to the atmosphere at point 4. On average, exhaust temperatures reach 5000C.

COMBINED CYCLE

In a simple open cycle Systems, the exhaust gases are released to the atmosphere. A typical simple-cycle gas turbine will convert 30% to 40% of the fuel input into shaft output. The combined cycle is generally defined as one with heat-recovery steam generators (HRSG) in the exhaust, producing steam for a steam turbine generator (see figure below).

Thermal efficiencies of 50% to 60% are achievable on combined cycle systems producing electrical power only.

THE BRAYTON CYCLE

The thermodynamic cycle which all gas turbines operate is called the Brayton cycle. The pressure - volume (PV) and temperature - Enthropy (TS) diagrams are shown below: -

The numbers on this diagram correspond to the numbers also used in figure 1. Path 1 and 2 represents the compression occurring in the compressor, path 2 to 3 represents the constant – pressure addition of heat in the combustion systems, and path 3 to 4 represents the expansion occurring in the turbine. The path from 4 back to 1 on the Brayton cycle diagrams indicates a constant pressure cooling process. In the gas turbine, the cooling is done by the atmosphere, which provides fresh, cool air at point 1 on a continuous basis in exchange for the hot gases exhausted to the atmosphere at point 4.

Eng. Abel Rotich
Vice Chairman, IEK Mombasa Branch
Ag. Thermal Operations Manager KenGen Co. Ltd