ಉಷ್ಣ ಬೇಸಿಕ್ಸ್

1. Thermal Power Plants: An Introduction

Electricity is generated using different sources of energy like coal, oil, hydro, nuclear, solar, biomass, etc. Coal, gas, diesel and naphtha are called thermal resources  and  the  plants  that  operate  on  them are known as Thermal Power Plants (TPPs). It is understood that even as Renewable Energy (RE) will gain  ground in the coming years and dependence on thermal sources - especially coal – must be reduced, they will play an important role in meeting the energy needs in the near future. How TPPs are set up and operated will have great environmental consequences.

2. Growth of Thermal Power Plants in India

As of 2013, the net power generation capacity in India is 2,25,793.10 MW. A major share in power generation is from thermal sources, with an installed capacity of 153847.99

MW (68%). The installed capacity of hydropower is 39,623.40 MW (18%) and installed nuclear power capacity is 4,780.00 MW (2%). The installed contribution from RE is around 27,541.71 MW.

India’s reliance on coal for power is a serious environmental concern because both the mining and usage of coal have serious environmental impacts. Coal-fired power plants are the biggest source of manmade-CO ,  Sulphur  and  Mercury  emissions. If adequate corrective measures are not taken, coal plants will pollute the water, air and soil, affect biodiversity and livelihoods. An assessment of the death and disease caused by coal-fired power plants in India by Conservation Action Trust, Greenpeace India and Urban Emissions has found that particulate emissions from these plants in 2011-12 resulted in an estimated 80,000 to 115,000 premature deaths and more than 20 million asthma cases, which cost the public and the government an estimated Rs 16,000 – Rs 23,000 crores ($3500- $3833 million).

 

While the Environmental Impact Assessment (EIA) Process is common to power plants fired by all fuels, the impact mitigation measures recommended and inputs for better public participation, in this handbook, will focus on power plants fired by coal, which is

considered the dirtiest fuel.

 

(Explore various Thermal Power Plants in India (TPP Map))

https://www.thermalwatch.org.in/content/tpp-map-and-database // basic page

 

// maybe this should not be here.

3. Coal Based Thermal Power Plants

1.2 C o al and C o al -B a sed  P o wer Plant s

1.2.1 Coal & its Qualities

1.2.2 Power Generation in Coal-based Plants

1.2.3 Environmental & Health Impacts of Coal-based Plants

 

1.2.1 Coal & its Qualities

Coal is categorized into three main types, namely anthracite, bituminous and lignite based on the state or condition of the coal. Anthracite is the most aged state of coal. Bituminous is a soft coal, higher in quality than lignite and inferior in quality than anthracite. Lignite is the least aged coal. While anthracite is a hard coal and has carbon with little volatile content and no moisture, lignite is a soft coal with low carbon high in volatile matter

and in moisture content.

 

Various Constituents in Coal and their implications

 

a)   Fixed Carbon

Fixed carbon is the solid combustible residue that remains after a coal particle is heated and the volatile matter is expelled. The fixed carbon content of a coal is determined by subtracting the percentages of moisture, volatile matter and ash from a sample. Fixed

carbon is a guesstimate of calorific value of coal after volatile matter is burnt.

 

b)   Volatile Matter

Volatile Matter in the coal is the combustible gases like methane, hydrocarbons, hydrogen and carbon monoxide, and some incombustible gases like carbon dioxide and nitrogen. More of volatile matter means easier combustion of coal as it increases the flames.

 

c)   Ash

Ash cannot be burnt and ash reduces the burning capacity and affects the combustion and boiler efficiency. It results in clinker formation in the boiler and also increases the handling cost of coal. Therefore, high ash content in coal is undesirable.

 

d)   Moisture Content

Moisture present in coal reduces the per kg heat content (Kcal/kg) of coal as it reduces combustible components. Therefore, moisture content should be in an optimum range.

 

e)   Sulphur

 

Sulphur in coal causes corrosion of the chimney, air heaters and economisers. This will reduce the life of the equipment and is also undesirable.

Coal Quality and Availability in India

Based on its calorific value, coal is classified as Coking Coal & Non-coking Coal. Coking coal is mainly used in metallurgy, steel, cement and sponge iron industries. The largest part of the coal resources of the country is non-coking coal. Coal is classified into seven grades from A to G. A, B and C categories are high quality coal and are used in cement, fertilizer and sponge iron industries. Most of the coalfields in India have D, E, F and G grade coal that are inferior in quality and used often in TPPs.

 

Lignite and bituminous coal are largely used in Indian TPPs due to their availability.2

Lignite is considered suitable for power generation due to its low ash content. Coal India Ltd., a public sector company, provides coal for TPPs from national mines on a contractual basis.

Grades of coal used in India based on calorific value

 

 

Usually grade D, E and F coal are available to the Indian power plants through Coal India

 

ltd.

 

Indian coal is a low quality coal with low calorific value and high content of ash.

 

•     The ash content ranges from 40 – 50%

 

•     The moisture content ranges from 4 – 20%

 

•     The sulphur content ranges from 0.2 – 0.7%

 

•     The gross calorific value is between 2500 – 5000 kcal/kg

 

•     The volatile matter content between 18 – 25%

 

The above factors indicate that Indian coal is of inferior quality. The inefficiency of coal  

is the reason Indian power plants consume more coal than Australia and US for the same output.3

 

4. Working of a thermal Power Plant

Power Generation in Coal-Based Plants

 

 

Conversion of coal to electricity takes place in three stages:

Boiler Furnace -> Coal is burnt in a furnace to produce heat. Due to the presence of carbon in the coal and oxygen in the air, carbon dioxide (CO ) is produced

Boiler -> Water present in the boiler is turned to steam due to the production of heat. This happens at high pressure and temperature. Converting water to steam in a closed vessel will increase the pressure

 

Turbine and Generator -> Steam is sent to the turbine and rotates the blades in it, producing a mechanical force. The steam is then condensed and sent back to the boiler to start the cycle. Rotation of the turbine rotates the generator rotor to produce electricity.

 

Brief Description of Process

 

Coal is burnt in a boiler in a coal power plant to generate heat that is used to boil the water present to produce steam. The steam rotates the motor of the turbines that in turn rotates the generation rotor to produce electricity.

 

This process requires the preparation of coal, ensuring adequate air supply for combustion of coal, steam generation, reusing the cooled steam and the removal of residues (ash and flue gases).

Preparation of Coal

 

Coal yard à coal silo (storage) àcoal pulveriser

 

In the coal pulveriser, coal is ground to a very fine powder for efficient and easy burning.

 

3        IL & FS manual on TPPs

 

 

To dry the moisture present in the coal, hot air is generated by fans in the pulveriser. The generated air also carries the dry coal powder to the boiler furnace, where it is burnt.

Inside the Boiler

 

The boiler is one of the main equipment in a coal power plant. It burns coal to generate heat that is used to boil the water present to produce steam.

 

Process 1 – Combustion:

 

In a boiler, combustion of coal takes place with the help of air that comes through an air system. A forced draft fan sucks the air from the atmosphere and blows it into the furnace.  Heaters heat the air before it enters the furnace to increase the efficiency of combustion.

 

Heat, Ash and Flue Gas are generated in the combustion process. The heat converts the water to steam to turn the turbines. The flue gas is a residue and is drawn by an induced draft fan from the furnace and is sent to the electrostatic precipitator that captures the fly ash. The flue gas, without fly ash, is then released into the atmosphere through a stack/ chimney. Stacks are of great height so that the emission does not affect the ground level concentrations. Stack heights for large TPPs are around 250 to 280 meters. Another residue from the combustion is ash that is from inert matter present in the coal. In the furnace, 15 % of ash is collected at the bottom and the rest is captured in the electrostatic precipitator. The former is called Bottom Ash and the latter, Fly Ash.

 

Process 2 – Steam Generation:

 

Water at high pressure is constantly supplied to the boiler through a feed water pump through the use of recirculated and pre-heated feed water. This increases the efficiency of conversion of water to steam and also reduces the requirement of heat through combustion of coal. An economiser is a device to pre-heat the feed water. Therefore, the recovered hot gases, exhausted from the boiler, are taken by the economiser to heat the water. This hot water is sent to the water walls surrounding the furnace where the water is circulated and converted into steam.

 

The steam is collected in a drum. From here, it is passed through super heater coils. The temperature and pressure of the steam are increased here. This super-heated steam is what finally goes to the turbine.