General Block Diagram of Bottoming Cycle
- Bottoming cycle of co-generation system is that in which high temperature heat energy is produced using primary fuels.
- This heat produced is mainly used for other processes except generation of electricity. The waste or rejected heat from the process is utilized to generate electricity.
- The rejected heat is taken from a recovery boiler and it is then applied to the turbine connected to the generator to produce electricity.
- From manufacturing process of some products heat at high temperature is required in furnaces and kilns.
- After the manufacturing process, the heat rejected or not utilized is also at significantly high temperatures which can not be neglected.
- And if neglected it will reduce the overall efficiency of the system greatly.
- Hence, bottoming cycles are suitable for such type of cases. The typical areas of application of such bottoming cycles of co-generation are cement factories, steel manufacturing industries, ceramic factories gas and petrochemical industries etc.
- A typical bottoming cycle co generation plant is shown in Figure A.
- Here the fuel is burnt in a furnace and heat energy is applied to the main product process.
- The heat in the waste or rejected gases from gas turbine is applied to the boiler in which water is converted into high pressure steam, which is further applied to a steam turbine connected to the generator i.e. mechanical power generated by steam turbine is converted into electrical power by generator.
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| Figure A |
Combined Cycle of Topping System
- In the initial stages, combustion gases are produced in the combustion chamber by burning fuel and supplying air for combustion.
- Mechanical and then electrical power is conventionally produced.
- Now after this process, co – generation cycle starts as shown by the following diagram.
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| Figure B |
Topping System (Steam Turbine)
- High pressure drives the steam turbine which drives the generator. From the generator, Boiler HP useful electrical energy is produced.
- Now doing the work by high pressure steam in the turbine, steam outlet from turbine is a steam at low pressure. It can be used other purposes.
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| Figure C |
Gas Turbine Topping System
- Gas turbine requires gas at high pressure. This gas is obtained by firing the fuel in combustion chamber in which fuel is fired and air intake is supplied in the chamber.
- High pressure gas from the combustion chamber enters in the gas turbine. Turbine shaft rotates which in turn rotates the generator to produce useful electrical energy output. After doing the useful work in the turbine, gas is out at low pressure.
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| Figure D |
Heat Recovery Topping System
- This is the system of diesel engine which is used to produce electrical energy. After doing the necessary work of driving the generator, the heat is exhausted from the engine and its heat energy is reused for jacket cooling water water and in heat recovery boiler.
- In this way, co-generation is met with and it helps in energy conservation.
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| Figure E |
Bottoming Cycle of Co-generation
The main equipments are :
(1) Turbine as a prime mover of generator
(2) AC generator
(3) Heat recovery boiler
(4) Exhaust gas carrying arrangement
(5) Plant processor
- Useful heat is produced by fuel firing.
- Heat recovery boiler recovers the rejected heat and fed to turbine. Mechanical energy of turbine rotates the generator and electrical energy is produced.
- A big plant has left over gases which are recovered and used for co-generation.
- The process of co-generation is shown in Figure F.
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| Figure F |