What do you mean by Regenerative system in power plants???

 

Thermal power plants efficiency is in the range of 30 to 40%, the improvement of thermal cycle efficiency can be done either by increasing the inlet steam pressure & temperature or decreasing the turbine exhaust pressure. But improvement of Boiler parameters & decreasing exhaust pressure lead to increase in cost & also have limitations related to metallurgy & risk.

Another way is to increase the thermal efficiency is by using regenerative cycle. This cycle is more efficient than Rankine cycle.

What do you mean by Regenerative process or cycle???

In this process, steam is extracted from turbine at one or more points during steam expansion. This pressure is high, medium & low. Steam after some expansion cycle gets feed water heating ability. Such extracted steam is utilized to heat the feed water going to the Boilers nearer to its saturation temperature. Heating the Boiler feed water temperature ultimately increases the overall thermal efficiency.

How does the increase in feed water temperature increase the overall thermal efficiency??

Increase in feed water temperature at economizer inlet reduces the work done by boiler to generate the steam & hence consumes less fuel. As a thumb rule, on every 6-7 degree C rise in feed water temperature reduces Boiler fuel consumption by 1%.

What are the advantages of Regenerative cycle???

Regenerative cycle helps to increase in power plant thermal efficiency

Amount of steam condensed in steam condenser per KW decreases

Cooling water consumption decreases

Condenser size reduces

Auxiliary power consumption of cooling system reduces

Heat rate of the plant drastically reduces

Due to less fuel consumption load on fuel handling, fuel feeding & Boiler fans decreases & hence saving in plant auxiliary power consumption.

What are the major disadvantages of Regenerative cycle or process?

There are no much disadvantages except the requirement of HP, LP heaters, relating piping & controlling equipments.

What are the major parts of Regenerative cycle?

HP heaters, LP heaters & Deaerator.

What are the HP heaters?

HP heaters are the shell & tube type of heat exchangers situated between Boiler feed pumps & economizer.

The main design purpose of the HP heater is to heat the feed water coming from Boiler feed pump.

 

Why the name HP heater?

Because it I situated in high pressure zone that is at Boiler feed pump discharge feed water circuit

What are the heat transfer areas present in HP heaters?

Main heat transfer zones are

De-super heating zone

Condensing zone

Sub-cooling zone

What is the function of de-super heating zone in HP heaters?

It is separate heat exchanger placed within the shell, its main function is to remove the super heat from extracted steam

What is the function of sub-cooling zone in HP heaters?

It is another separate counter flow heat exchanger placed within the HP heater shell, its main function is to sub cool the condensate formed in condensing zone

What are LP heaters & where they are placed?

The condensate formed in the surface condenser is pre heated to elevated temperature before it goes to deaerator are called LP heaters.

LP heaters are placed between deaerator & ejector or CEP

LP heaters are also having 3 zones as like HP heaters

Why the name LP heater?

LP heaters are placed at low pressure zone from CEP to deaerator hence called LP heaters

Schematic diagram of power plant regenerative system

 

How do you prove that, regenerative cycle will increase the thermal power plant efficiency?

This can be explained by taking an example

Sl No.	Particular	UOM	Boiler-1	Boiler-2
1	Boiler steam generation	TPH	100	100
2	Steam pressure	Kg/cm2	87	87
3	Steam temperature	Deg C	515	515
4	Boiler efficiency	%	69	69
5	Fuel GCV	Kcal/kg	4300	4300
6	HP heater available	YES/NO	Yes	No
7	Feed water temperature at economizer inlet	Deg C	160	110

 

Looking at the above example, both boilers seem to be safe, except Boiler-1 has HP heater that is feed water heater & Boiler-2 has no HP heater.

Based on this we shall calculate fuel consumption of both the Boilers.

Enthalpy of steam at above parameters Hg = 818 kcal/kg

Enthalpy of feed water at temperature 110 deg C, Hf1 = 111kcal/kg

Enthalpy of feed water at temperature 160 deg C, Hf2 = 162kcal/kg

Now we shall calculate the fuel consumption of Boiler-1

Boiler1 = Steam flow X (Steam enthalpy-Feed water enthalpy) / (Fuel GCV X Boiler efficiency)

Boiler 1 =100 X (818-160) / (4300 X 0.69)

Boiler 1 fuel consumption = 22.17 TPH

Boiler2 = Steam flow X (Steam enthalpy-Feed water enthalpy) / (Fuel GCV X Boiler efficiency)

Boiler 2=100 X (818-110) / (4300 X 0.69)

Boiler 2 fuel consumption = 23.86 TPH

Looking at the fuel consumption of both the Boilers, Boiler 1 consumes less fuel as it has HP heater & hence more feed water temperature than Boiler-2

 Less fuel consumption in the sense less heat rate & less heat rate is nothing but more efficiency of the plant

 

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