Working of Coal Based Thermal Power Plant in India

Working of Coal Based Thermal Power Plant in India

Working of Coal Based Thermal Power Plant in India

In coal based thermal power plant, coal is transported from coal mines to the generaing station to generate electricity.

Following energy conversion processes takes place in coal based thermal power plant
  1. Conversion of heat energy in to production of steam in boiler.
  2. Conversion of kinetic energy in steam in to mechanical energy by rotating the steam turbine.
  3. Converting this mechanical energy of rotation of steam turbine in to electrical energy by rotating the generator by steam turbine. 
Working Principle of Coal Based Thermal Power Plant

Thermal power station works on the principle of Rankine cycle which is explained as below along with methods for improving the efficiency of Rankine cycle. 

In Rankine cycle working fluid is vaporized to do the work by expansion,  after expansion heat is removed to convert it back in to liquid form so that same fluid can be re-circulated in closed cycle.
 
In coal based thermal power plant, water is used as a working fluid because of its low cost and relatively large value of enthalpy of vaporization
Water at state 1 is pumped in boiler at state point 2 but note that it is not at the saturation temperature corresponding to boiler pressure
Heat must be added to change water at 2 to saturated water at ‘ a ‘( sensible heat)
 
Then further heat is added to convert it from liquid state to gaseous state i.e. to convert water in to steam from ‘a ‘to state 3 (Latent heat)
This steam at state 3 is passed through turbine where steam expands and rotates the turbine and comes to state 4 in the steam form
To convert back this steam at state 4 to water at State 1, heat from the steam is removed in condenser so that the water could be re-circulated. This process leads to heat loss.
 
Efficiency = output / input. Here output is work done by steam in turbine and input is heat added in boiler
 
Methods to Improve Rankine Cycle Efficiency (Thermal Power Plant Efficiency)
  • Super heating the steam by addition of super heater in boiler and increasing the boiler pressure
  • Reducing the pressure of condenser to reduce the heat loss
  • Addition of re heater in boiler and addition of intermediate pressure turbine and low pressure turbine
  • Addition of low pressure heaters in condensate cycle
  •  Addition of high pressure heaters in the boiler feed water cycle

Ideal Rankine Cycle
Too expensive to build
Requires multiple reheat and regeneration cycles
 
Land, Water, Coal Requirement For Setting Up Coal Based Thermal Plant
Land Requirement:
Land is precious and in view of large capacity addition program decided by Ministry of power for 11th and 12th five year plan need was felt to optimize land requirement for coal based thermal plant and CEA set up expert committee to give its recommendations for land requirement for setting up of power plants of 500, 660, 800 MW size units. 
 
Recommendations are as below:
           PER/MW              1.42            1.03        0.86            0.82         0.765          0.692
From above it will be observed that it is better to plan multiple units of higher size at one place to economize land requirement as well as to get benefit of economics of scale. With this view ultra mega power plant of 5x800 MW is installed at Mundra in Gujrat and 6x660 at Sasan in M.P. is under progress.
 
Water Requirement:
Water is one of the key requirements of coal based thermal power generation.

Water requirement in coal based thermal power plant is for following:
  1. Cooling water system for condenser and plant auxiliaries
  2. Power cycle make up
  3. Potable water
  4. Service water
  5. To make up reservoir evaporation losses
  6. Bottom ash and fly ash disposal system
  7. Firefighting system
  8. Coal dust suppression system
  9. Gardening
For power plant, raw water is generally drawn from a fresh water source such as river, barrage, lake, canal
Plant water requirement is governed by the number of factors such as quality of raw water, type of condenser cooling, quality of coal, ash utilization, type of ash disposal, wastewater management.
In recent past power plants have been designed with water requirement of 3.5 to 4 m3/ hr per MW i.e. 2x500MW unit with 4000 m3/hr water requirement.

 
Considering the water scarcity and to optimize the water requirement CEA has set up an expert committee for this and they have recommended water requirement as follows for 2x500 mw plant:
Note: Above assessment involves following considerations
  1. Raw water requirement shall be maximum up to 3600m3/hr if fly ash is disposed in wet slurry form without ash water recovery and after commencement of every system 3000m3/hr. will be sufficient. If HCSD system is used 3000 m3 will be sufficient from beginning
  2. Water requirement is worked out with COC of CW system as 5. If permissible COC becomes less than 5 due to water quality, water requirement shall be higher. If FGD is required to be provided at later date water requirement shall be higher.
  3. Efficiency with a dry condenser cooling system shall be 7% less.

From above it is observed that minimum water required per/MW is 3m3/hr. with wet cooling tower and 0.55m3/mw/hr for dry condenser cooling system but efficiency is 7% less which is ill affordable hence not in much use. 

Coal Requirement:

In coal based thermal power station coal is the primary fuel which is burned and heat energy released is converted into electrical energy. In the cost of power generated about 80% is the cost of coal.
Coal requirement depends on quality of coal which is going to be burned. Normally considering the cost and availability, inferior quality of coal having a calorific value of around 3500 kcal/kg and ash contained 34 %is allotted for power sector use.
 Gross heat rate of 500MW unit is 2200 kcal/kWh.

Considering the above coal requirement is calculated as below for 1000MW (2x500) station:

1000 MW will generate 24 MU per day
2200kcal are required to generate 1 KWH power so to generate 1 KWH power 2200/3500= 0.6285 kg of coal will be required.
So to generate 24MU x 0.628=15084 tons of coal will be required.
Considering PLF of about 80% coal requirement per year will be about 44 Lakhs of ton or 4.4 million tons.
 
Land Acquisition and R & R Policy
Land for coal based thermal power plant needs to be acquired as per land acquisition act of 1894. Once the location is decided to set up coal based thermal power plant following steps in a sequence are involved in the land acquisition process.
  1. Filing of application to the district collector
  2. District collector forms a survey team for verification and valuation of land and property
  3. Special land acquisition officer (SLAO) issues a letter to deposit 10% of land value and 10% advance
  4. 20% amount needs to be deposited
  5. Notification is to be published in newspapers as per section 4 of land acquisition act and public notice is to be given to the villagers where land is to be acquired
  6. SLAO has to take note of objections raised by villagers and needs to be resolved
  7. 80% compensation of land value needs to be deposited with SLAO
  8. Rehabilitation and Resettlement scheme needs to be submitted and get it approved and compensation amount towards this needs to be deposited
  9. As per section 6 of the act, notification is to be published in newspapers and public notice is to be given to villagers
  10. Application as per section 6 as per act needs to be submitted
  11. Resurvey is to be done by SLAO to find out any defects/missing properties
  12. Objections from villagers are to be received by SLAO and he has to sort out them as per section 9(1) and 9(3)
  13. Collector has to call a meeting for crop compensation and decide the amount
  14. Crop compensation amount needs to be deposited
  15. Collector has to send the proposed award to Divisional Commissioner for approval
  16. Award is to be granted by the commissioner and declared by SLAO
  17. Compensation distribution is to be done by SLAO
  18. Land is finally allotted possession is given for setting up of power plant
National R & R Policy:
Objective: 
  1. Minimum displacement and promotion of least displacing alternatives
  2. Ensuring adequate rehabilitation package effective implementation
  3. Active participation of affected families
  4. Protecting rights of weaker section
  5. Treatment with sensitivity
  6. Providing a better standard of living
  7. Integration of rehabilitation concerns into development planning and implementation process
  8. Harmonious relationship between the project proponent and affected families
Social Impact Assessment:
Keeping R & R objectives in view social impact assessment of a project is done.
Consideration of impact on public and community properties, assets and infrastructure particularly roads, public transport, drainage system, sanitation, source of safe drinking water etc. should be kept in mind.
Village wise information of affected families/ labourers is to be kept for their rehabilitation and resettlement
 
Rehabilitation Action Plan
Company acquiring land has to safeguard that project affected persons improve or at least regain their former standard of living
Services of NGO having an impressive record of integrity and performance and in whom PAPs have faith shall be obtained and in association, with them, Socio economic study shall be carried out to generate baseline data which will be used to formulate a viable and practical rehabilitation action plan (RAP)
Employment to PAP or one-time additional monitory benefit shall be given
A subsistence allowance shall be paid as per government rules to all PAPs
Company shall have to provide at resettlement site the school, road with street lights, drainage system, public transport arrangement, pond, dug well and/or tube well for drinking water, community center, place of worship, dispensary, grazing land for cattle's and playground
 
Efficiency of Coal Based Thermal Power Plant
Efficiency of Coal based thermal power plant is quite low due to the following reasons.

The first process of energy conversion is combustion where chemical energy in coal is converted into heat energy which is transferred to water in boiler. Boiler efficiency is around 87% due to following losses.

Coal contains moisture and also small % of hydrogen which gets converted into moisture. In the furnace, moisture vaporizes taking latent heat from combustion heat and exit boiler along with hot gases.

Improper combustion of coal, hot ash discharge from boiler, radiation losses, leakages also affects the boiler efficiency.

All heat produced is not transferred to water and some heat is lost to the atmosphere as hot gases.

The second stage of conversion is a thermodynamic stage. The energy of steam is converted to mechanical rotation of turbine. The steam is then condensed to water and pumped back into the boiler for reuse. During this process, latent heat of condensation is lost to a cooling tower. This is a major loss and is about 40%. Losses in turbine control valve throttling, turbine seals, turbine blades and exit losses are some other losses.  

The third stage of conversion is a conversion of rotational energy of steam turbine into electrical energy in the generator. Here also some losses such as copper, magnetic, mechanical losses about 2% take place.

To operate the power plant about 7% power is required to run the various auxiliaries of the power plant. This also needs to be taken in to account.

This brings overall efficiency to about 33%.

Steps Taken to Improve the Over All Efficiency in India
Evolution of various capacities Unit
 
Turbine cycle heat rate means external heat energy input to turbine cycle required to generate 1 kWh of electrical energy at the generator terminal. 
Advantages of Supercritical 660Mw Over 500Mw Sub Critical Unit
  1. Reduced coal consumption for per unit generation hence reduced coal cost. (68800 tons/year coal saving.
  2. Reduced CO2 emission and SO2 emission (88270 tons/year CO2 and 385tons/years)
  3. Saving in per MW capital cost for installation.
  4. Better part load efficiencies 2% less as against 4% at 75% load and 5.5% less as against 11% at 50% load.    
Further Use of Very High Parameters is Restricted By:
  1. Metallurgy
  2. Reliability
  3. Capital Cost

Working of Coal Based Thermal Power Plant in India | Thermal power plant efficiency | coal based thermal power plants 

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