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

70 Part II: Cogeneration experiences in Asia and elsewhere
2.2.2 Details of the cogeneration system
The gas turbines with an ISO rating of 25 MW are capable of producing 20.7 MW at the site.
Since the gas supplier could not guarantee lean gas supply, dual fuel configuration (lean gas
as well as high-speed diesel) was specified for the gas turbines. This was further altered to
allow simultaneous firing of liquid and gas in such a manner that the gas gets a preference
and the liquid fuel meets the balance requirement.
A 25 MW capacity steam turbine generator was selected with the option for extracting
medium pressure steam at 19 bar and low pressure steam at 3.5 bar. The condenser was
designed for generating up to 20 MW of power without any steam extraction. The heat
recovery steam generators (HRSG) have the option for auxiliary firing with multi-fuel option.
High-speed diesel is used as a start-up fuel and the lean gas is supplied as the main fuel with
low sulphur heavy stock as the alternate liquid fuel. By-products available from the gas
cracking unit such as pyrolysis gasoline and off gas can also be fired. In order to allow the
HRSG to operate as a conventional boiler when the associated gas turbine was not operating,
a forced draft fan for supplying combustion air is installed with suitable dampers and safety
protections so that the boiler can run without exhaust from the gas turbine. This change over
scheme was well designed and tested and works satisfactorily at present.
In order to maximize the heat extraction from the exhaust gases after economizer and to
increase the overall efficiency of the HRSG, a separate low-pressure water coil was installed
in exhaust gas path. Such an arrangement allowed to generate hot water which, when
flashed, gives low-pressure steam that is used for deaeration of boiler feed water. This
feature helps to reduce the steam demand for the deaerator by 4 ton/hour.
2.3 Cogeneration in a Textile Mill
Encouraged by the Thai Government policy on industrial cogeneration and sale of excess
electricity to the utility grid, a synthetic fibre manufacturing industry decided to explore the
opportunity for cogeneration. The factory was particularly susceptible to any unintended
shutdown due to power interruption while led to high restarting costs. In addition, the factory
had a generating capacity to meet only 15 per cent of its demand and the existing diesel
generators were over 20 years old and were expensive to maintain. A techno-economic
feasibility study was first undertaken to identify the best cogeneration scheme in line with the
Government’s newly announced power buy-back option. 2
2.3.1 Existing energy situation of the factory
The production processes in the factory required steam at two different pressures, 56 bar and
12 bar, respectively. The total demand of steam was 101,120 tons of steam per annum,
giving an average of about 11.5 ton/hour, though the maximum and minimum demands were
of the order of 17 and 9 tons/hour, respectively. To meet these demands, 4 boilers were
employed with the following capacities:
- two boilers producing steam at 60 bar, each with a generating capacity of 7 tons/hour,
- two others operating at 12 bar and generating 15 tons of steam per hour each.
Heavy fuel oil used as fuel in the boiler was purchased at a price of US$ 0.12/litre.
2 P. Srisovanna, “Case study of cogeneration in textile sector”, ESCAP South-East Asia Sub-regional
Seminar on Promotion of Energy Efficiency and Pollution Control through Cogeneration, Hanoi, 10-11
November 1998.