part 1: overview of cogeneration and its status in asia - Fire

Examples of cogeneration projects implemented in Asia 71
The total electricity demand of the factory was 59,000 MWh/year, with an average demand of
around 6.7 MW. The actual demand varied between a minimum of 5.9 MW and a maximum of
8.9 MW. About 1 MW of electricity representing 15 per cent of the total demand was selfgenerated,
using more than 20 years old diesel generators.
Four alternatives were considered during the feasibility study and compared with the existing
situation: (1) Back pressure steam turbine, (2) Gas turbine, (3) Combined cycle, (4) Diesel
engine. In all cases, the criteria set was to meet the peak steam demand of the factory, i.e.,
17 tons/hour.
2.3.2 Option 1: back pressure steam turbine
The proposed option is schematically shown in Figure 2.2. This option was found to be not
attractive due to the need for extracting steam at two different pressures. The varying demand
of steam at these pressures will lead to quite unfavourable steam turbine operation. In steam
matching option, the net output would be only 0.8 MW, which is less than the current standby
needs.
Moreover, the unavailability of a suitable standard turbine will lead to high installation cost and
will be more difficult to operate in practice. Considering 40 per cent of custom duty and tax,
the investment was calculated as US$ 7,500/kW. The annual maintenance cost was
estimated as 3 per cent of the investment, i.e., US$ 180,000/year.
130 c C, 12.73 t/h, (1.93 MW)
Fuel
10.6 MW
Water: 70 o C
11.5 t/h,
(0.94 MW)
Boiler
η= 90%
Figure 2.2 Steam turbine cogeneration option for the textile mill
2.3.3 Option 2: gas turbine
Steam: 100 bar/450 o C
12.73 t/h (11.47 MW)
Steam: 12 bar/237 o C
1.23 t/h (0.99 MW)
Electricity
800 kW
Steam to Process
12 bar/237 o C
6 t/h (4.84 MW)
56 bar/380 o C
5.5 t/h (4.79 MW)
The schematic diagram of this option is shown in Figure 2.3. The system included a diesel
fired gas turbine with heat recovery steam boiler and an option for auxiliary firing to meet the
varying steam demands. A boiler bypass would allow the gas turbine to run at full load, and
the auxiliary firing option with heavy fuel oil will let the boiler run at full load even when the gas
turbine is shut down. The net output of the alternator would be 4.7 MW, and assuming a 90
per cent availability factor, the cogeneration plant was capable of providing 58 per cent of the
power needs of the factory, the rest being purchased from the utility grid.
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