Generation of electricity in a micro gas turbine

Micro gas turbines are small high-speed gas turbines with low combustion chamber pressures and temperatures. They are designed to deliver up to ca. 200 kWei electrical power.

Nearly all micro gas turbine manufacturers offer turbines of radial design with combustion air compressor, combustion chamber, generator, and heat exchanger (Figure 5.13). Micro gas turbines are characterized by a single shaft on which the compressor, the turbine, and the generator are fixed. The turbine propels the compressor, which compresses the combustion air and at the same time the generator. Thus radial forces to the bearings and to the shaft are avoided, which allows a simple design; e.g., the bearings can be "gas-lubricated" because of the low load. The gas lubrication can be accomplished by passing compressed air through the bearings. Oil changes as required for normal turbines are not necessary because of the oil-free running of the micro gas turbine.

For normal operation, the turbine sucks in the combustion air. The fuel is normally supplied to the combustion air in the combustion chamber. When biogas with a low calorific value is used it can also be mixed with the combustion air before the turbine. In the latter case, a little biogas has only to be supplied directly to the combustion chamber for fine adjustment.

The up to 100000rpm rotating generator produces high-frequency alternating current, which is converted in an electronic device (Figure 5.14), so that it can be fed synchronously into the power network.

The electrical efficiency of 15-25% of today' s micro gas turbines is still unsatisfactorily low. An attempt to increase the efficiency has been made by preheating the combustion air in heat exchange with the hot turbine exhaust gases. But great improvements are still necessary before micro gas turbines will penetrate the market of industrial biogas plants. However, already today the coupling of a micro gas turbine with a micro steam turbine to form a micro gas and steam turbine seems interesting and economical because of its high electrical efficiency (ca. 50%).

Micro gas-turbines are regulated only by varying the fuel supply.

Biogas Gas Turbine
Figure 5.13 Micro gas turbine: scheme (left), thermodynamical process (right).

Figure 5.14 Electronic gear.49)

Table 5.4 Some worldwide suppl

ers of micro

gas turbines.

Name

Country

Electrical capacity [kW]

Type of turbine

Allied Signal

USA

75

Radial one - stage

Bowman/G E/Elliott

USA

45, 60, 80

Radial one - stage

Capstone

USA

30, 60

Radial one - stage

Honeywell Power Systems

USA

75

NREC4

USA

70

Radial two - stage

Turbec

Sweden

Ca. 100

Radial one - stage

The maintenance interval can be as long as 2000-8000 h. It can be longer when the operating temperature is increased above the normal operating temperature of ca. 10 °C.

In the United States, many micro gas turbines are in operation for the generation of electricity from natural gas. The machines are well approved for low capacities. Some experiments on the supply of biogas to micro gas-turbines have been made in Sweden (Table 5.4).

Turbec50'

Combined heat and power generation can be achieved by micro gas turbines ready for installation like the turbine module Turbec T100. In this, the combustion air bypasses the generator in order to provide heat exchange between exhaust gas and combustion air.

The Turbec T100 turbine runs at 70000rpm. The shaft with the centrifugal compressor, the radial -flow turbine, and the four-pole permanent magnet ofthe generator are pivoted on both sides of the permanent magnet. As compressor, a Scroll compressor is installed with a suction head of 0.02-1.0 bar. In the combus-

tion chamber, a pressure of 4.5 bar is maintained. The Turbec T100 is controlled by a fully automatic module controller (power module controller PMC), which also regulates the start- up.

When fueling with biogas, a gas pressure of 6-8.5 bar, a gas temperature of 0-60 °C before compression, and a calorific value of the gas of 38-50MJkg-1 are required.

The fast-rotating generator supplies alternating current of high frequency. This is transformed in a static frequency changer into normal current of 50 cycles per second. Thus, the number of revolutions of the turbine is decoupled from the frequency. The micro gas turbine can therefore be operated with a variable rotation rate nearly without any loss of efficiency. The performance data of the Turbec T100 are: Electrical power capacity: 100 KW Nominal electrical efficiency: 33% Overall efficiency: 77%

Downsteam from the Turbec micro gas -turbine, a cross-flow heat exchanger transfers the residual heat of the exhaust gas to water, which can be used anywhere in the plant for heating purposes. With it, an exhaust gas temperature of 85 °C can be reached at an exhaust gas flow rate of 0.80 kgs-1.

The limiting values of the CARB (Californian Air Resource Board) can be achieved.

Capstone

Another type of micro gas turbine is the Capstone micro turbine, which could perhaps also be used for biogas supply. Its generator is cooled by the air that is sucked in. The compressor works radially.

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    How to build Electricity gas turbine?
    7 years ago

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