Fossils as Energy Stores

Fossils are simply store of orderliness that was saved by plants and animal millions of years ago and therefore source of energy. This energy is converted to heat (high-entropy) which in turn is converted to mechanical and electrical energy (low entropy) which will do work.

Coalification

• Coal is simply old plants buried inside earth for millions of years to become a rock (sedimentary). The elemental composition of the plants, which includes carbon, hydrogen, and oxygen, transforms into coal over time. This transformation occurs in the absence of oxygen.
• With time, plant loses its oxygen and water content. The longer the coal is underground, the more concentrated the hydrocarbons become, resulting in a higher quality fuel.
• Accordingly, coal is classified peat, lignite, bituminous, and anthracite, with anthracite being the highest quality and most valuable type of coal. See table below for details.

Plant	Peat	Lignite	Sub-bituminous	Bituminous	Anthracite
Carbon Content		25-35	35-45%	45-85	85-96	Increasing Heating Value
Moisture				High	Low	Decreasing Moisture Content
Ash		10-40%	<10%	3-12%	10-20%
				Soft Coal	Hard Coal
				Most commonly found Used for power generation	Formed in Mountains Used mainly for manufacturing Coke Low smoke content Burns slowly
Sulphur				0.7-4%	<1%

Composition

• Coal or any hydrocarbon is primarily made of carbon, hydrogen oxygen. In addition, since they are just old plant matter, they should contain some nitrogen, sulphur and phosphorous. (remember all life is made of CNOS majorly)
• In addition, there is some moisture, heavy elements as they are buried deep.
• Carbon, hydrogen and oxygen are all combustible. Meaning when they mix with oxygen, they release some heat. This is the heat we are all interested in to do work in the powerplant.
• Note that even oxygen is combustible, but the proportion is so low we don’t bother.
• Note its carbon and hydrogen that decides the amount of heat we can generate.  If you look at chemical composition, it is C137H97O9NS for bituminous coal and C240H90O4NS for anthracite. (You don’t need to remember the numbers its only for you to know why anthracite is better)

Burning and Combustion

• What do mean by burning and combustion?
• Burning of any fuel simply means you are adding heat to break the bonds between carbon and hydrogen.
• Once you do that you add oxygen(air) in order for carbon and hydrogen to combine with. This gives oxides of carbon and hydrogen and in the process give out heat. (you always add heat to break the bonds and you always get heat when bonds are made)

Burning of Coal: Steps

• Coal when burnt, 1st thing that comes out is water.  As dug from the ground coal has some amount of moisture.
• At slightly above 1000C (boiling point of water) causes water to evaporate leading to loss of weight.
• The left-over dry coal is further heated but now in the absence of air (we don’t want burning). The matter that comes out is called volatile matter.
• Note that volatile matter is also hydrocarbon (aromatic rings) that evaporates when heated in the absence of oxygen.
• The left-over coal is called fixed carbon which is what we “burn” in coal plants.
• The carbon content now ranges from 50% to about 95%.
• Now we burn this fixed carbon coal in presence of oxygen to derive useful heat.
• After all carbon and hydrogen is ‘burnt’, the left-over non-combustible solid matter is called ash.
• Ash is simply left-over incombustible solid made of inorganic contents. This is the main useless and hence undesirable thing in solid fuels in general. (more on this later)

Process Of Burning

• In coal-fired plant fuel is coal and source of oxygen is air
• Complete combustion (allow all coal to burn in air)
• Coal + Oxygen —–> Carbon dioxide + other gases (mixture is called flue gas)
• 1 gram of Carbon —–> 3.6 gram of CO2 and 9 gm of N2
• Incomplete combustion (don’t allow all coal to burn by restricting the supply of air)
• Carbon + Oxygen —–> Carbon Monoxide (if you add more air it can ‘combust’)
• 1 gram of Carbon —-> 4.5 g of CO and 5.5 g of N2 

Challenges Related To Burning Of Coal

Thermal pollution

• The efficiency of coal-fired power plants is around 35%, meaning only 35% heat that is generated out of coal-burning is usable. Rest will go out to the atmosphere. In other words, for every 1000 MW of power produced 1500 MW of heat is wasted.

Carbon Emissions

• 1 gm of carbon burnt produces 3.5 g of CO2. A coal plant that produces 1GW of electricity emits 1 ton of carbon dioxide every 2 seconds.
• Solution: Carbon Capture and Storage

Air pollutants

Three potential pollutants are formed when coal is burnt; sulphur oxides, nitrogen oxides and fly ash.

Nitrogen

• Normally nitrogen is very stable and burns only at high temperatures. At high temperatures when nitrogen reacts with oxygen it forms oxides of nitrogen. This is very harmful as it acts as nuclei for fine dust causing PM pollution. 
• Solution: Remove nitrogen, reduce temperature of combustion

Sulphur

• Sulphur is of special interest again because it is a potential air pollutant. In addition, oxides of sulphur act as condensation nuclei during cloud formation leading to acid rain.
• Solution: desulphurization

Ash, mainly fly ash

• Left-over solid inorganic matter after burning of coal. (highest is bituminous)
• It can be collected at the bottom from where it can be removed.
• However, the problem is when the turbulent stream of gases in the boiler sweeps some of the ash out of the boiler along with flue gas. This is called fly ash.
• A 2016 report by IIT Kanpur says about 37% of PM 10 and 26% of PM 2.5 is caused by fly ash from coal plants.

Coke (Carbon + Ash)

• Solid carbonaceous residue derived from low-ash, low-sulfur bituminous coal from which the volatile constituents are driven off by baking in an oven without oxygen at temperatures as high as 1,000 °C so that the fixed carbon & residual ash are fused together.
• Coke is used as a fuel & as a reducing agent in smelting iron ore in a blast furnace.
• Coke from coal is grey, hard, & porous & has a heating value of 29.6 MJ/kg
• Byproducts of this conversion of coal to coke include coal tar, ammonia, light oils, and “coal-gas“.

Ways To Clean Coal

Fly ash Electrostatic precipitators

• Fly Ash at exhaust can be dealt with using electrostatic precipitators. Simply charge metal plates to high voltage. The negatively charged plate will attract the particles and remove them from flue gas.
• Bottom ash
• The ash so collected are usually mixed with water and left to ponds nearby. But the problem with this is
• In wet season the soluble contents of ash seeps into the ground and pollutes ground water.
• In dry season, ash ponds dry up and ash flies off as fly ash.
• Solution:
• Use bottom ash to make bricks.

Flue-Gas desulphurisation

• Sulphur dioxide is acidic in nature, thus spray the exhaust gas(flue gas) with limestone/soap or anything that is basic in nature (sodium hydroxide, lime, sodium sulfite, ammonia).
• In 2015, Ministry of Environment, Forest and Climate Change issued notification to regulate emissions from coal-based power plants and set deadline of 2017 for flue-gas desulphurisation.
• However, the deadlines have been missed repeatedly leading to pushing of deadlines to 2024 for coal plants near Delhi, 2026 for other coal plants.

Coal Conversions

Need for coal conversion

• Coal is basically a complex molecule that is made of long chains of hydrocarbons arranged in the form of rings. (see figure).

• As a result, it takes a lot of heat to burn coal and in-turn lot of heat is released when carbon in coal oxidizes with air.
• As a result, the temperature in a coal power plant is extremely high which leads to two problems a) thermal pollution and b) oxides of nitrogen are formed. (remember otherwise stable nitrogen, oxidizes at very high temperatures)
• In addition, burning of coal as in case of any solid fuel leaves solid residue like ash.
• In order to tackle all these issues coal conversion is resorted to.
• Coal conversion simply means converting combustible solids (coal) to combustible liquids or combustible gases.
• Accordingly, coal conversions include liquefaction and gasification.

Gasification Of Coal

• While the goal of combustion is to produce the maximum amount of heat possible by oxidizing all the combustible material, the goal of gasification is to convert most of the combustible solids into combustible gases such as carbon monoxide, hydrogen, and methane.
• See how this works.

Coal gas/Producer gas

• Burn coal with air: complete combustion —-> CO₂
• Burn coal with controlled oxygen: incomplete combustion —-> [CO + N₂]. This is called coal gas or producer gas.

Advantage

• Fly Ash is absent.
• Less NO_x formation due to low temperature of incomplete burning
• Carbon sequestration is easy (easy to remove nitrogen from flue gas as compared to oxides of nitrogen)

Syn Gas

• Alternately burn Coal with Steam —–> CO + H_{2 (}Synthesis Gas/Syngas)
• Aptly this is called steam reforming.
• You can make variety of things using Syngas like Methanol, Hydrogen, Methane (methanation).
• Syngas can be used to make ammonia-based fertilizer.
• N2(g) + 3 H2(g) —–> 2 NH3(g)

Advantage

• Reduced CO₂ emissions.
• Syngas is combustible.
• NOx is not formed.
• Fly Ash is absent.

Syngas can be used to make methane in a process called Methanation/ Hydrogasification

• CO+3H₂—–>CH₄ + H₂O

Advantage

• CH4 is combustible.
• CO2 is not produced.

Liquefaction Of Coal

• In liquefaction the trick is to break the long chains of hydrocarbon rings and add hydrogen at high pressure.
• Depending on the type of coal used we get various coal liquids like gas oil, gasoline, kerosene.
• Note that the main difference between coal and petroleum products is the size of hydrocarbon molecule. (coal is long chain with 140-250 carbon atoms, petrol has 4-10, diesel has 8-14)
• The main advantage is low-quality coal is easy to liquify.

Advantage

• Heating value is doubled (because C-H ratio is increased)
• CO2 emissions are reduced.
• Fly Ash is absent.
• Since Indian coal is low in sulphur, methanol when used in vehicles can reduce SOX and NOX emissions and hence low PM pollution,

Methanol

• Another approach to make liquid fuel is compress the syn gas obtained from gasification process.  The mixture of CO and H2 is adjusted in its pressure and temperature to form methanol. (CH3OH) 
• CO(g) + 2 H₂(g) —-> CH₃OH(l) 
• Methanol is also called wood alcohol. (More on methanol in Gas-to-liquid alternative under transportation fuels)