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

Examples of cogeneration projects implemented in Asia 75
2.4.2 Economic evaluation of cogeneration options
Four different options were considered for comparing with the present case, as follows:
1. Use of low pressure boilers for process steam only , and no power generation on site;
2. Use of a high pressure boiler and a back pressure turbine to meet 30-40 per cent of the
power demand;
3. Use of a high pressure boiler of a higher capacity, a back pressure turbine and an
additional condensing turbine, or a single extraction-condensing turbine to meet 60-70 per
cent of the power demand;
4. The same as (2), but all the power needs of the factory are met in this option.
2.4.3 No power generation
To meet the increased steam demand of digesters and for availing stand-by capacity, it was
proposed in this case to replace an old boiler by a new fluidized bed combustion boiler having
a capacity to produce 10 tons of dry saturated steam per hour at 10.5 bar. Entire power
requirement was to be met by the purchase of power from the utility grid, the diesel generator
continuing to provide the back up in case of power outages.
2.4.4 30-40 per cent power generation
The erratic power supply of the utility makes it absolutely necessary to have at least a
capacity to self-generate 30-40 per cent of the power need (600-700 kW) to avoid production
losses. Though a diesel generator is available, the power generated from this unit is quite
expensive and the maintenance cost of this unit is expected to mount with time.
As there was a need to acquire a new boiler, this option considered the option of generating
steam at 42 bar and 440°C. The steam could be supplied to a back pressure turbine to
generate around 30-40 per cent of the power demand of the factory, and the steam leaving
the turbine at a pressure of 4 Bar can be sent to fulfil process heating needs.
The initial investment as well as the operating cost of this system was found to be lower than
a diesel engine. The fuel used in the boiler is cheap and available in abundance. Moreover,
only the incremental cost of fuel required generating the same quantity of steam at higher
pressure and temperature was considered, which is only 20 per cent higher. The cost of
power generation worked out to be 36 per cent lower than that with the diesel generator.
From the practical side, a smaller size would mean the use of inefficient single stage turbine
and low voltage generator. This may lead to large imbalance in the system due to variations in
the process steam and power demands. The system balance can be achieved only by
operating the system at low plant load factor, thereby compromising the overall efficiency and
productivity of the factory.
2.4.5 60-70 per cent of power generation
At this level of power generation, higher productivity can be guaranteed with practically no
production losses. Installation of a higher capacity (14 tons/hour) and higher pressure (42 bar
and 445°C) boiler was considered. As much as 6-7 tons/hour of steam could be used in the
back pressure turbine and match the process steam demand. The remaining high-pressure
steam can be sent to a condensing turbine for additional power generation. The latter will also
assure to absorb the fluctuations in the process steam demand, without affecting the power
output adversely. Further, the use of a single multistage backpressure cum condensing
turbine will assure increased power output and higher system efficiency.