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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How do you calculate the Deaerator & HP heaters steam consumption???

DEAERATORS:

What is the function of Deaerator?

Functions of deaerator

To remove the dissolved oxygen from deaerator & bringing down to <0.005 ppm

To heat the feed water

To provide NPSH to Boiler feed pumps

Acts as surge for feed water storage device

2-Which steam is used for deaerator?

Saturated steam having pressure from 0.3 to 3 kg/cm2 and temperature 130 deg C to 200 deg C

3-What are the pipe lines connected to Deaerator & storage tank?

Inlet lines:

• Make up water line
• Condensate water line from CEP
• Return condensate water from process
• Recirculation water line from BFP
• Balance leak off water lines from BFP
• Condensate water from HP & LP heaters
• Steam line from turbine extraction or PRDSH

Out let lines:

• Feed water line to BFP suction
• Deaerator overflow line
• Deaerator storage tank drain line

Understanding with examples.

1-Calculate the steam consumption of deaerator based on following given data

Sl No.	Particular	UOM	Value
1	LP steam flow to deaerator at pressure 1.5kg/cm2G & temperature 135 deg C t	TPH	?
2	Feed water outlet temperature from deaerator	0C	105
3	Feed water outlet flow from deaerator	TPH	125
4	CEP water flow to deaerator at temperature 50 deg C flow	TPH	120
5	Make up water to deaerator at temperature 25 deg C	TPH	5

        

 

 

 

 

 

 

 

 

 

Solution:

Enthalpy of steam at pressure 1.5 kg/cm2 & temperature 135 deg C = 653 kcal/kg

Enthalpy of CEP water at temperature 50 deg C = 50 kcal/kg

Enthalpy of makeup water at temperature 25 deg C = 25 kcal/kg

Enthalpy of deaerator outlet feed water = 106 kcal/kg

Mass of steam = (Feed water flow X Enthalpy –CEP flow X Enthalpy-Makeup water X Enthalpy) / (Enthalpy of steam-Enthalpy of deaerator outlet water)

Mass of steam      = 125 X 106 -120 X 50 -5 X 25 / (653-106)

Mass of steam = 13.02 MT

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2-Calculate the steam consumption of deaerator based on following given data

Sl No.	Particular	UOM	Value
1	LP steam flow to deaerator at pressure 2.5 kg/cm2G & temperature 150 deg C t	TPH	?
2	Feed water outlet temperature from deaerator	0C	128
3	Feed water outlet flow from deaerator	TPH	210
4	CEP water flow to deaerator at temperature 45 deg C flow	TPH	30
5	Make up water to deaerator at temperature 25 deg C	TPH	20
6	Process return condensate at temperature 95 deg C	TPH	160

        

 

 

 

 

 

 

 

 

 

 

 Solution:

Enthalpy of steam at pressure 2.5 kg/cm2 & temperature 150 deg C = 658.4 kcal/kg

Enthalpy of CEP water at temperature 45 deg C = 45 kcal/kg

Enthalpy of makeup water at temperature 25 deg C = 25 kcal/kg

Enthalpy of deaerator outlet feed water = 129 kcal/kg

Enthalpy of return condensate water = 96 kcal/kg

Mass of steam = (Feed water flow X Enthalpy –CEP flow X Enthalpy-Makeup water X Enthalpy-Return condensate X Enthalpy) / (Enthalpy of steam-Enthalpy of deaerator outlet water)

Mass of steam      = (210 X 129 -30 X 45 -20 X 25-160 X 96) / (658.4-129)

Mass of steam = 18.66 MT

HP & LP heaters:

1-What are the applications of HP & LP heaters?

HP & LP heaters are used to heat the feed water & to improve the cycle efficiency

 

2-What are the various pipe lines connected to HP heaters?

Inlet lines:

1-Feed water inlet line

2-Bleed steam line from Turbine

Outlet lines:

1-Feed water outlet line

2-Condensate outlet line

Understanding with examples.

A HP heater is used to heat the 105 TPH feed water from 105 °C to 150 °C by using turbine bleed steam at 8 kg/cm2 and 200 °C. The condensate returning from heater is at 125 °C, calculate the quantity of steam used.

 Given that,

Qf = 105 TPH

Tf1 = 105 °C

Tf2 = 150 °C

Hg at pressure 8 kg/cm2 & temperature 200 °C =677 kcal/kg

Enthalpy of condensate water Hf = 126 kcal/kg

Heat lost by the steam = Heat gained by feed water

Ms X (Hg-Hf) = Mw X (Tf2-Tf1)

Ms X (677-126) = 105 X (150-105)

Ms = 8.57 TPH

 

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Power plant & Calculations

How to calculate the cost of steam???

 

In power plant, calculation of cost of steam is very vital in commercial point of view. Following are the parameters which affect the cost of steam.

1. Steam pressure
2. Steam temperature
3. GCV of fuel
4. Price of fuel
5. And Boiler efficiency

Following gives you the relation among steam cost & above all parameters & vice versa

• Steam cost increases as the enthalpy or heat content in steam increases and vice versa
• Steam cost increases as the GCV of fuel decreases and vice versa
• Steam cost increases as the fuel price increases & vice versa
• Steam cost increases as the Boiler efficiency decreases & vice versa

Understanding with examples.

1.Calculate the cost of steam per kg, which is been using for Steam turbine having pressure 121 kg/cm2 & temperature 550 deg C.The boiler of efficiency 75% uses coal of GCV 4200 kcal/kg to produce this steam.Cosnsider the price of coal is Rs 5000/MT

Enthalpy of steam at above pressure & temperature H = 830.43 kcal/kg

Boiler efficiency ⴖ_b= 75%

GCV of coal = 4200 kcal/kg

Now, cost of steam = Heat content in steam in kcal/kg  X Fuel price / (GCV of fuel in kcal/kg X Boiler efficiency ⴖ_b)

                                     = 830.43 X 5000 / (4500 X 0.75)

                                     = 1230.26 rupees / MT of steam or Rs 1.23 / kg of steam

 2.Calculate the cost of steam per kg, which is been using for chemical process plant having pressure 5 kg/cm2 & temperature 180 deg C.The boiler of efficiency 65% uses biomass of GCV 2800 kcal/kg to produce this steam.Cosnsider the price of biomass is Rs 2400/MT

Enthalpy of steam at above pressure & temperature H = 670 kcal/kg

Boiler efficiency ⴖ_b= 65%

GCV of coal = 2800 kcal/kg

Now, cost of steam = Heat content in steam in kcal/kg X Fuel price / (GCV of fuel in kcal/kg X Boiler efficiency ⴖ_b)

                                     = 670 X 2400 / (2800 X 0.65)

                                     = 883.51 rupees / MT of steam or Rs 0.88 / kg of steam

3. Calculate the cost of saturated steam given to sugar process for juice boiling plant having pressure 1.5 kg/cm2 & temperature 130 deg C.The boiler of efficiency 68% uses bagasse (biomass) of GCV 2200 kcal/kg to produce this steam.Cosnsider the price of biomass is Rs 2500/MT

Enthalpy of steam at above pressure & temperature H = 650 kcal/kg

Boiler efficiency ⴖ_b= 68%

GCV of coal = 2200 kcal/kg

Now, cost of steam = Heat content in steam in kcal/kg X Fuel price / (GCV of fuel in kcal/kg X Boiler efficiency ⴖ_b)

                                     = 650 X 2500 / (2200 X 0.68)

                                     = 1086.22 rupees / MT of steam or Rs 1.086 / kg of steam

 

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